Aces High Bulletin Board
General Forums => Aircraft and Vehicles => Topic started by: Squire on December 06, 2005, 03:15:38 AM
-
I was going through my book "Hellcat" by David Handerton, and I came across a passage regarding the engine on the F6F-5.
To paraphrase, it said quite simply that many of the water injection units were removed in the field for ease of maintenance.
If thats true it explains the two sets of top speed #s we get sometimes, 380mph and 409 mph.
TAIC probably tested a F6F-5 with W, as opposed to without.
Its also the sort of thing that throws monkey works into research, and doesnt always make its way into the history books.
Hmm.
Btw, I am not a anti-Hellcat guy, my old AH squadron was an allied navy unit (check my avatar).
-
I always thought that the slower recorded speed of the Hellcat was caused by a misplaced pitot or something. Widewing will know.
-
It was a missplaced pitot tube.
The Hellcat was right about a 400MPH bird at 20K in clean condition. Keep in mind that this disparity in speed was discovered in early 1944. Most of the F6F-5 series had the correct placement.
I have a 1948 pilot Handbook with the CAS chart for both airplanes along with performace diagrams.
The 409MPH figure came from the TAIC flight test. That specific A/C had a very unusual speed curve at low altitude like it was using rammed air (which it did not have) and I cannot explain it.
The F6F is an enigima in it's performance why I do not know.
-
Its clearly part of a vast anti-Allied conspiracy.
-
Be that as it may, that does not mean the F6F-5s water injection was not sometimes removed though.
An F6F-3 and an F6F-5 would be very close in tops speeds without water injection, 376-386 range. The F6F-5 would be a bit faster as it had a slightly better cowling and a few othe minor tweaks for air flow, both having the same hp engine, 2000hp.
Im not disputing the fact the F6F-5 went 409mph with it installed, as the TAIC report seems to indicate that it does (test vs the A6M5).
Its a very good book, and it has a lot of technical detail on the Hellcat.
-
When they revisit the F6F and F4U-1, they should make a clear distinction between the "wet" ratings of the F4U-1D, F4U-4 and F6F-5, and the dry ratings of the early F4U-1 Birdcage model (which had no water injection), with corresponding Hg levels.
I could also see an F6F-3 in the mix at some point too.
-
That's why I'd like to see the -1A. Same powerplant as the -1C/D without the extra weight would make for a SWEET ride.
-
I think we *have* the -1A. I think it's just got the wrong name, like the p47d40 did.
-
Maybe posted before, but here is Corky Meyer's writing in Flight Journal:
"To simplify the evaluation and reduce data, we decided to test-fly the Hellcat and the Corsair in close formation. Instead of comparing complex calculations, performance could then be compared directly at the critical altitudes of the main stage, high and low blower altitudes of the engine's superchargers, and from cruise to high-speed, level flight with water injection. We also included some formation dives to learn which airplane was the slickest.
Except for the Corsair being 20 knots faster than the Hellcat in the main, sea-level, supercharger stage, both fighters had almost exactly the same speed at the low and high blower stages from 5,000 feet altitude up to service ceiling! In essence, they had the same performance. Our formation flights showed that both airplanes (with similar power settings) were in closely stabilized formation at all altitudes tested above 5,000 feet Sometimes, the Corsair would slowly gain a lead of 100 to 200 feet after five minutes of stabilized power flight, and sometimes, the Hellcat would do the same. Considering that both airplanes had the same engine, propeller, gross weight, wingspan, etc., they should have had about the same performance. We did notice that during these runs, the Corsair always had about a 20-knot indicated airspeed (IAS) advantage! We didn't realize just how embarrassing it would be to solve that dilemma.
The reason the Corsair was faster in the main stage blower was that its engine and carburetor were provided with ram air coming in directly from the forward-facing wing duct, whereas the Hellcat had the carburetor air coming in from the accessory compartment of the fuselage just behind the engine, with no ram air effect Our airplane was getting carburetor air at the same pressure as it would have were it motionless on the ground, and the Corsair was getting carburetor air supercharged by the speed of the airplane giving it more power (speed) in the main stage blower. In both aircraft, however, the designs were similar in that they provided ram air to the low and high blower stages. Our engineering department defended its position because taking the warmer air for the main stage blower would prevent inadvertent carburetor icing engine failures. Many Wildcats that had ram air in the main stage like the Corsair were lost because pilots failed to take precautions in time to avert this type of disaster. The Hellcat design was reviewed and approved by the Navy. I had had a carburetor icing accident during final approach on my first flight in a Wildcat a few months previously; it resulted in my first deadstick landing and a vertical ground loop. I therefore heartily agreed with the Navy's decision.
After noting the 20 knots indicated airspeed difference that had caused all the "lower performance" ruckus for our Hellcat, we eagerly decided to change the airspeed system so that it would read evenly with the Corsair when they were in formation. We had taken a lot of flak from all who had flown both airplanes (but not in formation) and, therefore, everybody 'mew' that the Hellcat was inferior in high-speed performance. We liked our simple and less complicated airspeed system with the static and dynamic orifices on the same boom, but we decided to go whole hog and put the static orifice on the fuselage (like the Corsair) to tailor the system to read 20 knots higher. We tried several orifice locations to get the required reading. After I had done a thorough testing of the final system over the entire flight envelope-or so I thought-I proudly flew the airplane to the Naval Air Test Center at Patuxent, Maryland for an evaluation. We soon found out that we had not purloined the Corsair airspeed system design thoroughly enough.
We soon received the Navy's glowing report of the new system; and it went on to say that the Air Test Center had never tested an airplane with such remarkable low-speed performance in its entire history. They found that in a left side slip with the wheels and flaps extended, the Hellcat could fly at zero airspeed. Wonder of wonders! Grumman led the industry again! Upon re-evaluation, we found that the engineers, inexperienced with flush static airspeed systems, had designed ours with only one orifice on the left side of the airplane, and it was very unbalanced with the flaps down. As the senior engineering test pilot, I was in deep doo-doo for not testing the new system in all side-slip conditions. A dualorifice system way behind the lowered flaps (similar to the Corsair's) finally provided a satisfactory means to give the Hellcat a cockpit indicated airspeed reading comparable to the vaunted Corsair's. That was the last we heard of the Hellcat's performance gap with the Corsair. Performance case closed. "
-
Originally posted by Krusty
I think we *have* the -1A. I think it's just got the wrong name, like the p47d40 did.
F4U-1A had introduction of Bubble Canopy vs the F4u-1 Birdcage.......if I am not mistaken.
read about the F6f vs F4u speed / flight comparison long ago roughly 10 years or so, still lots of disbeleivers out there that say it isn't so...........and still most every WWII flight simulation game has the F4Us faster than the F6fs
-
Originally posted by Squire
I could also see an F6F-3 in the mix at some point too.
now that would be something to look forward tooooo :aok
-
TimRas/Tequila,
Consider this.
1. The discovery of this error was in early to mid 1944. Prior to the Pax River JFC.
2. In the Dec 1947 POH for the F6F-3/5 the performance Appendix (Navy Flight Test) dated July 19 1945 list the top speed at sea level as 285Knots (328MPH) and at just under 20,000ft at 339Knots (390MPH).
This flight test was in the "clean" condition at 11,700LBS at combat power.
3.Also consider that the F4U had a 314SQ ft of wing area while the F6F had 334SQ ft of wing area as well as a lerger frontal area and higher drag coefficient.
If you add these things up along with the numerous side by side flight test it is unlikely that the two A/C were ever equal.
By contrast the F4U-1D manual from 1952 the performance index shows the sea level speed as 313Knots (360MPH) and the 20,000FT speed as 360Knots ( 414MPH).
This test was done in the clean condition at 11,500LBS at Combat power.
Also if you look at the A6M5 test numbers closely the F6F-5 performance is at it's highest however speeds are listed as being faster than the A6M5
F6F-5 F4U-1D
Sea level 41MPH 48MPH
5000FT 28MPH 42MPH
10,000FT 45MPH 58MPH
15,000FT 62MPH 70MPH
20,000FT 69MPH 78MPH
25,000FT 75MPH 80MPH
30,000FT 66MPH 74MPH
Close but not a match. Notice also the close speed at sea level. This seems out of order but I don't know why.
-
The TAIC report does indicate 409mph achieved for the F6F-5 though F4UDOA in the test quoted above, how do you explain the discrepancy? water over no water, or?...
-
ok F4UDOA,
so you saying all them test done by the Navy on the F6f-5 had the pitot tube in its factory installed place verses the same location as the F4U had it's own pitot tube located? so all test and data shown is like it is showing these numbers. If they had relocated the pitot tube in every test then the numbers would be higher or not?
anyhow, some say it was just as fast some say it wasn't, and in this game it will never be.......... :D
TC
-
The TAIC A6M5 report does say that WEP was used on the F4U-1D, F6F-5 and FM-2.
Also the FW190 TAIC test on the F6F-3 shows WEP at 60" MAP.
Again the same use of WEP in the NAVAIR F6F-5 chart as well as the POH I am referencing.
I have never seen a flight test for the F6F that did not include the use of water so I can't really explain what I haven't seen and I have NAVIR, TAIC, Royal Navy, USN, Grumman and Vought test.
I don't doubt the F6F is a 400MPH bird but more than that is optimistic.
-
Tequila,
Funny you should ask. I have the manuals next to me as I am typing.
I am looking at the CAS correction chart for both A/C F4U and F6F. The F6F has two charts, one for each Pitot tube location.
The F4U-1 shows an error at 300knots of add 8 knots. So you need to add 8 Knots to 300Knots to get the CAS (calibrated Airspeed) of 308Knots
The F6F shows two Pitot positions as follows.
Pitot at the Fuselage.
300Knots deduct 2 Knots = 298Knots (This is the later version)
Pitot at wing tip
300Knots Add 11.5 Knots= 311.5Knots.
What does this mean? Even at it's worse the F6F was only 3.5knots worse than the F4U in CAS adjustment.
Also the F6F in AH is faster than you think. Go to 21K and check the top speed on the E6B. It IS 400MPH. It seems HT and Pyro modeled the CAS error as well. I'm not kidding.
Also I am not saying that they moved all the pitot tubes just that they knew about the error.
-
Originally posted by F4UDOA
Also the F6F in AH is faster than you think. Go to 21K and check the top speed on the E6B. It IS 400MPH. It seems HT and Pyro modeled the CAS error as well. I'm not kidding.
Measured max speed for F6F-5 in WEP with 25% fuel, using E6B:
22,000 feet: 382 mph
21,000 feet: 385 mph
20,000 feet: 386 mph
With 100% fuel:
22,000 feet: 378 mph
21,000 feet: 381 mph
20,000 feet: 382 mph
The AH2 F6F-5 is nowhere near being a 400 mph fighter. Our current F6F-5 does not conform to either the published standard Navy speed curve or that of the manufacturer. It's 18 mph less than factory data, and 27 mph slower than TAIC flight test data. What we have if an F6F-5 with F6F-3 performance. This needs to be fixed. It's been established beyond question that the F6F-5 was a 400+ mph fighter. Grumman's lead experimental test pilot who performed almost all of the F6F-5 flight testing is still with us and states this categorically. So, HTC absolutely needs to look at this when the Hellcat comes around for updating.
My regards,
Widewing
-
Widewing,
It has been around a year since I tested but I am sure I got 399MPH out of that thing.
I will retest tonight.
I have no doubt that it was a 400MPH bird. It is when the bar is raised to F4U-1A/D 415MPH catogory that I have a problem with because there is no test record to support it and the drag condition would make it improbable.
Even the F6F-3 reached 391MPH when tested against the FW190.
-
Corky Meyers originally published:
Except for the Corsair being 20 knots faster than the Hellcat in the main, sea-level, supercharger stage, both fighters had almost exactly the same speed at the low and high blower stages from 5,000 feet altitude up to service ceiling! In essence, they had the same performance. Our formation flights showed that both airplanes (with similar power settings) were in closely stabilized formation at all altitudes tested above 5,000 feet Sometimes, the Corsair would slowly gain a lead of 100 to 200 feet after five minutes of stabilized power flight, and sometimes, the Hellcat would do the same.
This physical test of flying in formation was not an official "Approved" test so you got what you got. A F6f that is as fast as a F4U in real life even though the IAS showed it on a guage to be slower, so hey , it is slower. The guages &/or paperwork does not lie. ;)
either Grumman messed up on the location of the pitot tube and design, or Vought found a way to cheat 20 more knots out of what it could really do and got the contract to build by showing it to be faster than a Hellcat :D
( got this theory after watching "the Aviator", ROFL )
-
Hi Timras,
>The reason the Corsair was faster in the main stage blower was that its engine and carburetor were provided with ram air coming in directly from the forward-facing wing duct, whereas the Hellcat had the carburetor air coming in from the accessory compartment of the fuselage just behind the engine, with no ram air effect Our airplane was getting carburetor air at the same pressure as it would have were it motionless on the ground, and the Corsair was getting carburetor air supercharged by the speed of the airplane giving it more power (speed) in the main stage blower. In both aircraft, however, the designs were similar in that they provided ram air to the low and high blower stages. Our engineering department defended its position because taking the warmer air for the main stage blower would prevent inadvertent carburetor icing engine failures.
Thanks a lot, that is highly interesting! :-) The lack of ram effect would not be the only reason for the power loss, the warmer air would also lead to a lower charge weight, decreasing power further. I guess this effect might not be as important as the loss of ram, though.
The Grumman solution operationally makes very good sense, but I don't quite understand why they didn't switch between rammed/unrammed using some other means than the supercharger gear, such as flap or landing gear extension. (I'd expect the landing to be the only situation where carburetor icing was a danger.)
Anyway, I had been puzzled by the BuAer data for the F6F-5 which has a definite anomaly regarding the switch from MIL to Combat Power.
My "old" interpretation: Low gear is not used because its critical altitude drops below sea level at increased boost, main stage is used from ground up to a certain altitude, above which high gear is used. In this interpretation, high gear inexplicably has a lower full throttle height at Combat Power than at MIL.
My "new" interpretation based on the information you posted: Low gear is used from the ground up because it's superior due to exploiting the ram effect. Above low gear full throttle height, main stage is used with a smaller power increase than low gear and about the same full throttle height as on MIL (these two points still seem anomal), and high gear does not get any benefit from Combat Power at all.
I like the new interpretation better, but it still seems to raise as many questions as it answers :-)
Regards,
Henning (HoHun)
-
Originally posted by F4UDOA
Widewing,
It has been around a year since I tested but I am sure I got 399MPH out of that thing.
I will retest tonight.
I have no doubt that it was a 400MPH bird. It is when the bar is raised to F4U-1A/D 415MPH catogory that I have a problem with because there is no test record to support it and the drag condition would make it improbable.
Even the F6F-3 reached 391MPH when tested against the FW190.
I ran another test tonight to check MAP at various altitudes. In the process, I checked speed at 18k. I managed 382 mph on 25% gas.
Checking MAP produced a couple of errors; one very minor, the second more significant.
At 22k I should have seen 52.5 in/hg. 52.0 was the best I could get. This is generally insignificant. Down at 20k I should have seen 59.5 in/hg, but could only manage 57 inches. Not huge, but worth a few mph. I did get the correct 59.5 inches at 15k. If full MAP was available, the F6F-5 might have reached 390 mph at 20k.
My regards,
Widewing
-
HoHun,
My "new" interpretation based on the information you posted: Low gear is used from the ground up because it's superior due to exploiting the ram effect. Above low gear full throttle height, main stage is used with a smaller power increase than low gear and about the same full throttle height as on MIL (these two points still seem anomal), and high gear does not get any benefit from Combat Power at all.
I don’t think I follow what you are saying. When you say “gear,” are you referring to blower gear? As in Neutral, Low and High?
My understanding is the following:
Neutral blower was used from sea level up its critical altitude where power then began to drop. Once power dropped off to a certain degree, the supercharger was switched from neutral to low blower. Low blower was used up to its critical altitude where power then began to drop off. Again, once it dropped off to a certain degree, low blower was switched to high blower. High blower maintained power up to its critical altitude after which even high blower was no longer sufficient to maintain rated power and it began to fall off.
Starting off in low blower (gear) at sea level would result in poorer performance than starting off in neutral blower - up to the point where the shift from neutral to low blower would have been. There does not appear to have been any benefit to running in low blower at sea-level (or up to neutral blower’s critical altitude) because it sapped more power than it produced. The R-2800-10 could make 2,000 hp (at Military Power) at sea-level in neutral blower. The most it could obtain at sea-level in low blower would have been around 1,800 hp (according to the charts Characteristics of R2800-10 Engine – Neutral Gear, Low Gear, High Gear).
Now, the engine power charts I’m referring to don’t make mention of ram effect.
I’m not saying it’s impossible, but I find it hard to believe that flying in lower blower in the Hellcat at sea level would produce enough ram effect to more than compensate for the power it took to run the supercharger in low blower. If this could be done, there would have been no need for neutral blower and pilots would have just flown around in low or high blower. In other words, for flying in low blower at sea level to have been more beneficial than flying in neutral blower, the ram effect alone would have had to add more than 200 hp – so that the engine was developing more HP in low blower at sea level than it was in neutral blower. I’m not sure it could do that, but I’m willing to change my mind if someone can show me.
All the speed and climb charts I’ve seen for the Hellcat, and Corsair (with R-2800-8) show the planes starting out at sea-level in neutral blower. All show that performance would have been worse at sea level if low blower was used.
If someone has a climb or speed chart for the F6F or F4U that takes into account ram, I’d love to see it. Again, I’m open to altering my position.
-
I got 386 out of it at 21k. Clean 50 fuel, from a dive and held with wep. The AH charts show it as that as well, and there has been no FM change to it.
-
Shorty,
The F6F vs F4U RAM issue is allready discused in this (http://www.hitechcreations.com/forums/showthread.php?s=&threadid=155142) thread (see my post in the second page). The speed charts from F4UDOA's docs shows this phenomena clearly.
gripen
-
(http://hitechcreations.com/ahhelp/models/charts/f4u1dspeed.gif)
The RAM affect is the sea level in WEP. Both R-2800's have WEP but only the F4U can utilize RAM in the Main blower stage so the F4U can really haul on the deck. But as it gains altitude the main blower looses efficiency and is quickly shifted to Low Blower.
(http://hitechcreations.com/ahhelp/models/charts/f6f5speed.gif)
-
I think the larger part of the speed differential is being missed. The reason for the speed adavantage other than the main blower stage is simply drag. The F6F has a larger frontal area as well as a larger wing area that gives it a higher drag coefficient. Both A/C are producing the same HP above sea level so the A/C with lower drag will be faster.
Take a Look
(http://mywebpages.comcast.net/markw4/HellcatAndCorsair.jpg)
-
IIRC, even the F4U's wing configuration had the then-unexpected effect of further reducing drag on the aircraft.
-
How can the F6F-5 have a higher FTH for Neutral blower than the F4U-1D, if the Hellcat has no ram air and yet the Corsair does? It doesn't follow.
-
Justin,
That is only in WEP. I think what happens is the F4U throttle is adjusted for FTH at sea level to reach about 60" MAP in the main stage. It then looses efficiency and is shifted to Low. The F6F cannot attain reach max efficiency at sea level because of lack of RAM so the slow increase until FTH is reached.
I don't understand two things.
1. What happens if you advance to full throttle below FTH? Will it overboost?
2. The F6F consistantly produces higher MAP than the F4U. In all AFDU test the F4U rarely reaches beyond 57.5" and the F6F reaches 59" and yet the F4U still has higher power. I guess because of RAM the same power can be reached with better efficiancy and less parisitic loss of power.
-
Hi Shorty,
>I don’t think I follow what you are saying. When you say “gear,” are you referring to blower gear? As in Neutral, Low and High?
Yes. In my above post, I tried to use the same terminology as found in the Corky Meyer quote, which was a bit different than the one I was used to.
The superchanger sequence "Neutral Gear, Low Gear, High Gear" you pointed out is what I'm familiar with, too, and the Corky Meyer quote seems to use "Low Gear, Main Stage, High Gear" instead. (Maybe I misunderstood that.)
>Neutral blower was used from sea level up its critical altitude where power then began to drop. ...
Yes that's the normal sequence. However, it seems that this is not what happens in the BuAer data set for the F6F-5:
http://www.history.navy.mil/branches/hist-ac/fighter.htm
>I’m not saying it’s impossible, but I find it hard to believe that flying in lower blower in the Hellcat at sea level would produce enough ram effect to more than compensate for the power it took to run the supercharger in low blower.
The problems I have with the BuAer graph can maybe explained best using the climb graph.
You can see that at MIL, the full throttle height of Neutral Gear is just below 2000 ft. At the higher boost at Combat Power, full throttle height would be about -500 ft, so Neutral Gear is not used in climb at Combat Power.
However, the rest of the Combat Power climb graph appears to be about 800 ft depressed compared to the MIL graph, and I have no explanation for that. This would indicate lower rpm, but it would be unusual to lower rpm at a higher power setting. Sometimes, a power loss is experienced when rpm is increased at a higher power setting due to propeller tip Mach losses, but that should not result in a lower throttle height, but rather in a higher one. Anyway, the rpm for MIL and Combat Power is given as 2800 rpm alike, so what's happening there?
Both my attempts fail to provide a satisfactory explanation for that curve (and the speed curve does not make it any clearer).
Note that the F6F speed curve F4UDOA has provided from HTC's overviews does not shows these problems. High Gear and Low gear speeds are exactly what I'd expect.
There is one peculiarity in the HTC graph, too, and that's that in Neutral Gear, there seems to be too much boost available. Extending the MIL curve section from 6000 ft to 3000 ft down to 0 ft, it seems that the sea level speed of roughly 330 mph would be achieved at Neutral Gear full throttle height, and speed would remain about constant up to 3000 ft when switch over to Low Gear should occur.
(The BuAer curve according to my "old" interpretation does not show use of Neutral Gear at Combat Power at all, which makes a visible difference to the HTC graph.)
So I have a qualitative problem with understanding the sequence of gear changes here. I'd like to solve that before taking on the quantitative aspect of the analysis :-)
>If someone has a climb or speed chart for the F6F or F4U that takes into account ram, I’d love to see it.
Hm, do you mean a power chart? Ram would be automatically taken into account for flight test data.
Regards,
Henning (HoHun)
-
Yes. In my above post, I tried to use the same terminology as found in the Corky Meyer quote, which was a bit different than the one I was used to.
OK.
The superchanger sequence "Neutral Gear, Low Gear, High Gear" you pointed out is what I'm familiar with, too, and the Corky Meyer quote seems to use "Low Gear, Main Stage, High Gear" instead. (Maybe I misunderstood that.)
I haven’t seen the quote to which you refer. But frankly, this doesn’t make any sense. It would be a very inefficient to fly the plane in this matter. Maybe it was a mis-speak, or an error.
The R-2800-8 makes more power at sea-level in neutral blower than in low-blower. And it makes more power at medium altitude in low-blower than it would in neutral. On top of that, at medium altitude, in neutral, there would be no ram effect.
Yes that's the normal sequence. However, it seems that this is not what happens in the BuAer data set for the F6F-5:
http://www.history.navy.mil/branche...-ac/fighter.htm
That’s exactly what happens. Neutral, Low, and High are all represented at both Military Power and Combat Power on both the climb and speed charts – in the correct order of use. You’d get a wild looking chart if they were represented in the “low-neutral-high” sequence.
The problems I have with the BuAer graph can maybe explained best using the climb graph.
You can see that at MIL, the full throttle height of Neutral Gear is just below 2000 ft.
The “full throttle” term is mucking things up at this point and perhaps it’s a point of confusion..
That point to which you refer at the 2,000 foot mark in Military Power is the critical altitude of the engine in neutral blower at Military Power. The plane would not be at full throttle at that point. The throttle would be at the military power setting. The full throttle position would have made put it at combat power.
At the higher boost at Combat Power, full throttle height would be about -500 ft, so Neutral Gear is not used in climb at Combat Power.
No, neutral blower is used in the combat power climb. Both the Military Power and the Combat Power climb lines are in three clear stages, and that represents the three blower stages. Have a look:
(http://[url]http://members.cox.net/us.fighters/blower.JPG[/url])
The neutral blower (the red line) is used in the initial climb stages at both military power and combat power. They are used up to their critical altitudes. The critical altitude of the engine in neutral blower is higher at combat power than it is at military power. Once the critical altitude is reached, power begins to drop off. That drop-off is represented by the first bend and the more horizontal portion of the red line. Once it reaches the point where low-blower can develop maximum power, the blower is shifted from neutral to low (green line). That shift is represented by the upward turn in the line (the point where the line turns from red to green).
It starts all over again. Low blower (the green line) is used up to its critical altitude (represented by the bend in the green line). At that point, power begins to drop off. Once a point is reached where high blower can develop maximum power, low blower is shifted (where the green line meets the blue line) to high blower (the blue line).
At Military Power, the shift from low to high occurs at about 18,000 feet. At Combat Power, the shift occurs at about 20,000 feet.
Once the plane is in high blower, it continues to climb until its critical altitude is reached – about 21,000 feet according to the chart. As you can see from the chart, once the blower is shifted to high, the military power line and the combat power line quickly converge – at about 20,000 feet.
(Note: critical altitudes for military power and for combat power in low and high blower are achieved at the same altitudes)
The entire Combat Power climb is pretty much a “full throttle climb.” The Military power climb is not a “full throttle” climb until an altitude is reached where full throttle is necessary to develop military power. That’s where the top and longest portion of the blue lines comes in.
When the high-blower combat power critical altitude is reached, the throttle is as far forward as it will go. That is the highest point where combat power can be maintained. After that, combat power can no longer be maintained and power begins to drop. And there is nothing the pilot can do about it. Sooner or later, power drops off to the point where even full throttle will not maintain Military power.
That’s where this chart is a little confusing. The military power critical altitudes in low and high blower should be a little higher than the combat power critical altitudes. That’s because you can maintain a lower power at higher altitude when you may not be able to maintain high power at that same altitude.
Hm, do you mean a power chart? Ram would be automatically taken into account for flight test data.
Regards,
Henning (HoHun)
No, I have the power charts for the R-2800-8, the engines used in the Hellcat. I don’t think ram is taken into consideration. These things were usually tested in chambers where atmospheric pressure could be altered to simulate a particular altitude. No ram was possible in the chambers, so many engine power charts don’t take ram into consideration. Some charts are “corrected” to take ram into consideration, but I don’t think mine are. If mine do represent power with ram, then I’d have to say emphatically that there is no benefit to using low blower at sea-level. Not only is there no benefit, it would be detrimental to performance.
-
Originally posted by F4UDOA
1. What happens if you advance to full throttle below FTH? Will it overboost?
It depends.
AFAIK, the Hellcat engine had automatic boost control. On the Hellcat, you set the throttle to combat power setting. At combat power, there really was no FTH. It was FTH from the ground up. The boost control made sure the plane’s supercharger didn’t overboost down low. At critical altitude and above, the boost control didn’t do anything, because it took full throttle to achieve combat power at critical altitude, and above critical altitude, there was no way to achieve combat power, even at full throttle.
The achieve military power, you set the throttle to that position as well. If you advance the throttle below FTH, then you would simply increase the engine power. But the boost control made sure you didn’t boost it above combat power.
On planes that didn’t have that type of boost control, like early P-39s, it was, indeed, possible to over boost the engine. On the ground, fire-walling the throttle would boost the engine past combat power MAP and damage could easily occur.
-
"Full throttle height" (FTH) is an English term; it refers to the throttle valve in the intake, not the lever in the cockpit. FTH is reached when the throttle must be fully open to reach the desired boost(MAP), ie: Critical altitude.
-
Originally posted by justin_g
"Full throttle height" (FTH) is an English term; it refers to the throttle valve in the intake, not the lever in the cockpit. FTH is reached when the throttle must be fully open to reach the desired boost(MAP), ie: Critical altitude.
OK thanks. That term was confusing because its not a common US term.
-
Your assessment of the BuAer chart for the F6F-5 is incorrect re: blower gear in use.
In Combat power setting Neutral gear is not used.
On the speed chart; Low blower is used from sea level(critical altitude is 13,100ft) until 15,700ft - when High blower is engaged, which reaches its critical altitude at 18,000ft.
This can be confirmed by the engine powers listed for Combat power:
2030hp@sea level -> 2110hp@13,100ft(Low blower)
1930hp@15,700ft -> 1940hp@18,000ft(High blower)
For Military power, only power at critical altitude is listed:
1960hp@3,400ft(Neutral blower)
1840hp@18,200ft(Low blower)
1670hp@23,400ft(High blower)
There is some confusion because the Combat power curve doesn't fall back to meet the Military curve at it's critical altitude. If you examine the F4U-1 chart found here (http://www.geocities.com/slakergmb/id67.htm) you can see that neutral blower is not used a Combat power, but the curve falls back until it matches the Military power curve at it's critical altitude. The F4U-1D curves here (http://www.geocities.com/slakergmb/id77.htm) show the use of Neutral blower with Combat power below 2000ft only.
The BuAer F6F-5 chart shows incorrect critical altitudes at Combat power(2000ft too low).
The only clue I see as to why is in the power charts: "Performance based on engine power determined in flight test as follows"
The engine powers listed for Combat power are most likely climb powers - the critical altitudes listed(13k,18k) are a match for those seen in F4U climb charts(same engine), while the Military and Normal power settings have the correct critical altitudes for level flight.
My theory is that the performance figures shown on the graphs were calculated - using the engine powers listed(which were obtained in seperate flight tests and somehow got mixed up) - hence the incorrect, lower critical altitudes shown for Combat power.
-
Originally posted by justin_g
How can the F6F-5 have a higher FTH for Neutral blower than the F4U-1D, if the Hellcat has no ram air and yet the Corsair does? It doesn't follow.
If we assume same MAP, the engine stage (Neutral blower) FTHs for the AH F4U-1D and F6F-5 seem to be wrong. The F4U should have about 2-3k higher FTH for the neutral blower at given MAP than the F6F (or the altitude to switch auxilary stage on).
Charts here (http://mywebpages.comcast.net/markw4/MSWF4UDATA.pdf) show logical FTHs (as an example chart in page 4).
gripen
-
Hi Shorty,
>But frankly, this doesn’t make any sense. It would be a very inefficient to fly the plane in this matter. Maybe it was a mis-speak, or an error.
OK, I agree on that, let's stay with the normal sequence for our analysis. I didn't actually think the quote changed the order of gears, just the order of names given for them :-/
>The “full throttle” term is mucking things up at this point and perhaps it’s a point of confusion..
Argh, it was! Thanks to Justin for the clarification :-)
>No, neutral blower is used in the combat power climb. Both the Military Power and the Combat Power climb lines are in three clear stages, and that represents the three blower stages. Have a look:
(http://members.cox.net/us.fighters/blower.JPG)
Good idea to mark it in the chart! The colours shows exactly what I've been trying to explain. I believe that there is no doubt about the MIL power being used exactly as you have marked them.
I'm not sure about Combat Power use, though. The marking you provided is the same as I tried to line out in my "new" explanation, here is my "old" explanation (which might be more accurate after all):
(http://hometown.aol.de/HoHunKhan/blower.JPG)
>(Note: critical altitudes for military power and for combat power in low and high blower are achieved at the same altitudes)
That would be somewhat unusual if you keep rpm constant between those settings, as it seems to be the case for the R-2800. The blue lines I have added to the chart are lines of constant rpm, maximum boost at the three supercharger gear changes. Full Throttle Height of each gear should normally be a point on this chart, the further down/to the right the higher the boost.
No matter which of the two sequences we consider correct, this criterium is not met by the BuAer chart :-(
You can see from the chart that if the lowest "step" at Combat Power would be neutral gear, it would operate far above the full throttle height one should expect from it, which is the reason I'm preferring the "old" explanation at the moment.
>No, I have the power charts for the R-2800-8, the engines used in the Hellcat.
I agree that these would not include ram. (British power charts sometimes do, but as far as I know US charts don't.)
>If mine do represent power with ram, then I’d have to say emphatically that there is no benefit to using low blower at sea-level. Not only is there no benefit, it would be detrimental to performance.
Hm, that was a terminology misunderstanding anyway. However, does the chart have power settings for water injection, too? At a higher power, full throttle height might drop below sea level, making it unavailable. That's what I tried to point out with my blue lines in that graph above :-)
Regards,
Henning (HoHun)
-
Originally posted by justin_g
Your assessment of the BuAer chart for the F6F-5 is incorrect re: blower gear in use.
In Combat power setting Neutral gear is not used.
Yes, it is.
If neutral blower was not used, there would have been only two stages to the climb line.
On the speed chart; Low blower is used from sea level(critical altitude is 13,100ft) until 15,700ft - when High blower is engaged, which reaches its critical altitude at 18,000ft.
This can be confirmed by the engine powers listed for Combat power:
2030hp@sea level -> 2110hp@13,100ft(Low blower)
1930hp@15,700ft -> 1940hp@18,000ft(High blower)
For Military power, only power at critical altitude is listed:
1960hp@3,400ft(Neutral blower)
1840hp@18,200ft(Low blower)
1670hp@23,400ft(High blower)
There is some confusion because the Combat power curve doesn't fall back to meet the Military curve at it's critical altitude. If you examine the F4U-1 chart found here (http://www.geocities.com/slakergmb/id67.htm) you can see that neutral blower is not used a Combat power, but the curve falls back until it matches the Military power curve at it's critical altitude. The F4U-1D curves here (http://www.geocities.com/slakergmb/id77.htm) show the use of Neutral blower with Combat power below 2000ft only.
The BuAer F6F-5 chart shows incorrect critical altitudes at Combat power(2000ft too low).
The only clue I see as to why is in the power charts: "Performance based on engine power determined in flight test as follows"
The engine powers listed for Combat power are most likely climb powers - the critical altitudes listed(13k,18k) are a match for those seen in F4U climb charts(same engine), while the Military and Normal power settings have the correct critical altitudes for level flight.
My theory is that the performance figures shown on the graphs were calculated - using the engine powers listed(which were obtained in seperate flight tests and somehow got mixed up) - hence the incorrect, lower critical altitudes shown for Combat power. [/B]
Sorry, you are simply wrong on this. I’ll show you in a new chart.
-
Originally posted by HoHun
(http://hometown.aol.de/HoHunKhan/blower.JPG)
Henning (HoHun) [/B]
HoHun, you are extending the wrong lines. That’s throwing things off. On the speed chart and the climb chart you need to extend the more vertical portions of the lines, not the more horizontal portions.
The vertical portions represent climb at below the critical altitude of that blower stage – when the engine is generating rated power in that blower stage. The horizontal portion shows the climb above the critical altitude of that blower stage – when power has begun to drop off.
Here’s how it should look:
(http:// [url]http://members.cox.net/us.fighters/blower2.jpg[/url])
Here are the engine charts for the R-2800-8 engine:
(http:// [url]http://members.cox.net/us.fighters/neutral.jpg[/url])
(http:// [url]http://members.cox.net/us.fighters/low.jpg[/url])
(http:// [url]http://members.cox.net/us.fighters/high.jpg[/url])
Even though these are military power charts, they illustrate the point nicely. Maximum power is achieved in neutral blower at sea level up to critical altitude – 2,000 hp.
The most power that can be achieve at sea level at low blower is 1,800 hp.
The maximum power that can be achieved at sea level in high blower is 1,650 hp.
You simply cannot get better performance at sea level in low blower than you can in neutral blower. The point where low blower become more effective than neutral is the altitude where neutral blower can no longer even maintain 1,800 hp. That point is above the critical altitude, where power begins to fall off. Once it falls of to 1,800 hp, the supercharger is shifted from neutral to low where low can maintain 1,800 hp up to its critical altitude.
In those BuAer climb charts, neutral blower is absolutely used in both the military power and combat power climbs.
-
BTW, my comments above are with respect to the BuAer Hellcat charts only.
I examined the F4U-1 chart, and the combat power climb does not use neutral blower, it used low from sea-level on up. That is not the case with the Hellcat chart, which shows neutral blower in the initial stage of the climb.
I don't have R-2800-10 engine power charts. At least I don't think so. I'll check and see.
I'm sure is has something to do with ram. Low blower was probbaly able to achieve at-least equal power down low using ram as neutral blower was. As I stated previously, that's the only way using low blower down low would make any sense.
To me, the F4U-1 charts are more confusing than the Hellcat charts. The Hellcat used all blower stages. So did the -1D Corsair. The -1 Corsair did not. But we can only speculate as to why. Ram and water injection, which the -1D had, and the -1 did not likely have a lot to do with it - even thought he -1 chart shows water useage. Hmmmm.
-
Consider this. You have marked the BuAer graph as using Neutral blower with Combat power up to a critical altitude of 10,000ft when climbing.
According to the R2800-10 charts you have provided - at 10,000ft and 2700RPM - Neutral blower is only capable of 34" MAP and just under 1500hp. MAP has fallen well below the required 60" for Combat power because this altitude is far above the critical altitude of Neutral blower!
At the same altitude in Low blower, output is 1600hp with only Normal power rating(2550RPM & 49.5" MAP*). At Military power(2700RPM & 53" MAP*) the output would be about 1750hp. Combat power(2700RPM & 60" MAP*) would yield around 2000hp. The BuAer document tells us that Combat power produces something between 2030-2110hp at this altitude.
At altitudes below 2000ft or so, Neutral blower would certainly make more power than Low blower - but for reasons unknown, this was not used for the graphs in the F6F-5 document.
* power settings come from SEFC at http://www.zenoswarbirdvideos.com/More_F6F_Stuff.html
-
Hi Shorty,
>HoHun, you are extending the wrong lines.
Hm, actually my extensions ask a different question than your extensions :-)
Your chart asks "What if I use each supercharger below its full throttle height at the same boost as before?"
(This is useful for sorting out the question of the supercharger sequence. I now recognize that I had confused "red" and "green" as depicted in your first chart! :-)
My chart asks "What if I use each supercharger gear with increased boost?"
That's useful for sorting out the Combat Power case. You can't get above the blue line for each supercharger gear because the supercharger doesn't provide enough air for that. How far to the right you can slide on that graph depends on the amount of boost you allow.
(http://members.cox.net/us.fighters/blower2.jpg)
(The image shows up immediately if you remove the URL tags, by the way.)
Your lines are lines for constant boost, with the automatic boost regulator throttling down as required.
My lines are lines for fully-open throttle, with supercharger capacity limiting the power.
>Here are the engine charts for the R-2800-8 engine:
Thanks, that's great stuff! :-) I'll try to generate some generic speed charts from that one. The chart only goes up to MIL power, though - do you also have the Specific Engine Flight Chart mentioned on the bottom of the diagram? These usually include the take-off and emergency power limits as well. (The BuAer chart unfortunately does not provide boost levels.)
>In those BuAer climb charts, neutral blower is absolutely used in both the military power and combat power climbs.
Hm, I think Justin is right here because the supercharger would not be able to provide enough air in neutral gear at combat power. One could say that it's not possible to cross the blue line without changing supercharger gears.
Regards,
Henning (HoHun)
-
Link to a SEFC is in my post above, it's listed as "F6F Flight Operation Instruction Chart". Has a weird rating for Military power/High blower though...
-
Hi Justin,
>Link to a SEFC is in my post above, it's listed as "F6F Flight Operation Instruction Chart". Has a weird rating for Military power/High blower though...
Thanks a lot! :-)
After a preliminary analysis of the power data provided by Shorty, I'm pretty certain that the weird rating you pointed out was in fact used for the BuAer F6F-5.
Regards,
Henning (HoHun)
-
Hi again,
>After a preliminary analysis of the power data provided by Shorty, I'm pretty certain that the weird rating you pointed out was in fact used for the BuAer F6F-5.
Disregard that, I just found a mistake in my analysis.
Regards,
Henning (HoHun)
-
Originally posted by justin_g
Consider this. You have marked the BuAer graph as using Neutral blower with Combat power up to a critical altitude of 10,000ft when climbing.
According to the R2800-10 charts you have provided - at 10,000ft and 2700RPM - Neutral blower is only capable of 34" MAP and just under 1500hp. MAP has fallen well below the required 60" for Combat power because this altitude is far above the critical altitude of Neutral blower!
I didn’t provide R-2800-10 charts. The charts are for the R-2800-8. And they show power at military power.
At the same altitude in Low blower, output is 1600hp with only Normal power rating(2550RPM & 49.5" MAP*). At Military power(2700RPM & 53" MAP*) the output would be about 1750hp. Combat power(2700RPM & 60" MAP*) would yield around 2000hp. The BuAer document tells us that Combat power produces something between 2030-2110hp at this altitude.
At altitudes below 2000ft or so, Neutral blower would certainly make more power than Low blower - but for reasons unknown, this was not used for the graphs in the F6F-5 document.
* power settings come from SEFC at http://www.zenoswarbirdvideos.com/More_F6F_Stuff.html [/B]
[/quote]
If there is a discrepancy between the charts I provided and the BuAer documents, then consider the fact that we are talking about different engines. The -5 Hellcat used the R-28800-8W with water injection. It should produce more power at Combat Power than the -8 engine.
-
HoHun,
The Combat Power climb and Combat Power speed chart show highest possible performance. (I know about other sources, let's just concentrate on the BuAer -5 Hellcat chart.) Combat power represents maximum boost.
You cannot get more boost than Combat Power (unless your plane is tweaked, or you you exceed maximum allowable MAP in neutral - which I am not sure is possible, unless your plane is tweaked). And you cannot get better performance by shifting the gears differently. By shifting differently, you can only hurt performance.
If you shift too soon, then you shift out of a higher horsepower than the next stage can develope. In other words, best performance is achieved by climbing to critical altitude then letting the power bleed off until it is equal to the maximum power the next stage can develope. The trick is to maintain highest possible power at all times. If you shift too soon, you may be shifting from 2,000 directly to 1,800 hp when you could have let the power simply bleed off from 2,000 to 1,800 thereby flying at 1,995hp, 1,900hp, 1,850hp down to 1,800hp.
Likewise, if you shift too late, then you've allowed the horsepower to fall below that which can be developed by the next gear. In other word, if you stay in neutral too long, you may let your horsepower slip to 1,700 hp. Then when you shift to low, you have to wait for the horsepower to build back yup to 1,800 hp.
There is no benefit, only detriment, to shifting at other times.
-
Hi Shorty,
>I didn’t provide R-2800-10 charts. The charts are for the R-2800-8. And they show power at military power.
Hm, the charts you posted are labeled "Altitude Characteristics of P & W R2800 - 10 Engine". The BuAer data quotes the R-2800-10W for the F6F-5, too, so we seem to have a good match-up here. (Of course, R-2800-8 charts would be just as welcome :-)
Regards,
Henning (HoHun)
-
Argh, your right. I was boogering around thru my stuff looking for -8 charts when I read that and had that on my mind when I posted. Never mind what I said.
Yes, those are R2800-10 charts, not R2800-8 chart as I wrote. Those charts come from the F6F manual.
Nevertheless, the BuAer chart is for a F6F-5 with R-2800-10W with water. The engine charts I posted are for the R-2800-10 without water.
-
Hi Shorty,
>Combat power represents maximum boost.
Combat Power represents maximum permissible boost. That's the vertical line.
The horizontal lines represent maximum achievable boost, regardless of whether its permissable or not.
Anything above the respective gear's horizontal line can only be achieved in a higher gear.
(http://hometown.aol.de/HoHunKhan/blower.JPG)
The BuAer diagram is confusing because somehow, Combat Power brings a slight downward jump in full throttle height.
However, there can be no doubt that the indicated climb graph is achieved in low blower, not in neutral blower.
Neutral Blower has a full throttle height that is below sea level for the high boost level of Combat Power. You can verify this in the graph you posted: Just extend the 2700 rpm line of the Neutral Blower chart to 60 "Hg. With a full throttle height of 2800 ft for 50" Hg, and 1800 ft for 52" Hg, increasing boost by 2 "Hg decreases full throttle height by 1000 ft. At 60" Hg, we end up with a full throttle height 4000 ft below that for 52" Hg, or about 2200 ft below sea level.
So at sea level - above its full throttle height - Neutral Blower power has dropped so far that it does not yield much of an improvement over Low Blower, and thus it's left out of the BuAer chart.
Regards,
Henning (HoHun)
-
Originally posted by HoHun
However, there can be no doubt that the indicated climb graph is achieved in low blower, not in neutral blower.
Henning (HoHun) [/B]
No, HoHun, this is wrong. Leave the manifold pressures out of it. They confuse things. Manifold pressures are incidental to all this.
There is no question neutral, low and high blowers are used in both the military power and combat power climbs.
http://members.cox.net/us.fighters/blower3.jpg
-
Look at it this way. In order to get three distinct stages to each speed and climb line, three distinct supercharger gears had to be used.
See:
http://members.cox.net/us.fighters/blower4.jpg
The red line represents the stage of the climb where neutral gear was used.
The green line represents the second stage of the climb where low gear was used.
The purple line represents the last stage of the climb where high gear was used.
If only low and high gears were used, there would only be two stages to the lines.
-
Hi Shorty,
>No, HoHun, this is wrong. Leave the manifold pressures out of it. They confuse things.
Engines are powered by combustible fuel, mixed with air that provides the oxygen for combustion. How much air you get into the cylinder is determined by the boost pressure.
You can't just magically increase the neutral blower's full throttle height by calling it "Combat Power". If you keep the speed of the engine constant and increase boost pressure, full throttle height will drop.
Your graph shows a neutral blower full throttle height that does not drop, but actually jump up by thousands of feet when you increase boost pressure. This is not going to happen.
The BuAer chart is weird, and it's easy to get confused. However, armed with the chart you posted, I can tell you for certain that neutral blower is not used on the Combat Power branch of the BuAer chart.
The engine working point on the full throttle height of that branch is 2700 rpm, 60" Hg. All you have to do is to find that working point on the neutral blower chart.
It's 2470 HP @ -2200 ft - safely subterranean :-)
Regards,
Henning (HoHun)
-
You have a misunderstanding and I don't think I can explain it any better.
Forget the "full throttle height" terminology. TIf the supercharger is used correctly, there is no full throttle height at military power until above the high-blower critical altitude - when it takes full throttle to maintain military power. The only way you have a full throttle height situation below the high-blower's critical altitude is if you misuse the supercharger and fail to shift at the right time. You MUST shift the supercharger at the right time to avoid this. Failure to do so will result in a loss of performance.
And the entire combat power climb is a "Full throttle" climb as max boost is being generated at all stages of the climb.
The BuAer chart shows neutral is used in both military climb and combat climb (and speed). There is simply NO question about it. I've shown several times that it is is used. The engine charts I posted also confirm that low blower simply cannot generate the same power as nuetral blower below low blower's critical altitude. Using low blower ANYWHERE below neutral blower's critical altitude will result in a loss of power and performance.
If you want to see what a climb line looks like using just low and high blower, look at the chart for the F4U-1 (NOT F4U-1D) Justin posted a link to.
-
Hi Shorty,
>The only way you have a full throttle height situation below the high-blower's critical altitude [...]
Each supercharger gear stage has its own full throttle height. Remember, full throttle height is the same as critical altitude.
Neutral gear critical altitude is below the ground. You would see three steps in the climb graph just as you expect if the graphs were extended below the ground to show performance in a sufficiently deep valley (think "Dead Sea").
>I've shown several times that it is is used.
You have painted red lines. But I have painted blue lines ;-)
>Using low blower ANYWHERE below neutral blower's critical altitude will result in a loss of power and performance.
We agree on that :-) However, the reason neutral blower is not used on the BuAer chart in combat power is that its critical altitude is below sea level.
Regards,
Henning (HoHun)
-
Originally posted by HoHun
Each supercharger gear stage has its own full throttle height. Remember, full throttle height is the same as critical altitude.
Using that term, which I don’t like, isn’t really American terminology, and which I think mucks things up, I’ll explain it using the definition of FTH provided earlier.
FTH IS NOT the critical altitude at anything under combat power. It would be ABOVE critical altitude at power below combat power.
For instance, at military power. The throttle is partially open from sea level up to the critical altitude. The throttle is no more open at critical altitude than it was at sea-level. Critical altitude is the highest altitude at which rated power can be maintained using that blower setting. ABOVE THAT POINT, power begins to fall off. If you are climbing at Military Power, and you climb above critical altitude, your power will begin to fall off. If you don’t have another supercharger stage to shift to maintain power, the only thing you could do to maintain power would be to increase throttle. Sooner or later, the throttle would be wide open. At the point where the throttle has to be wide open to achieve military power – THAT would be full throttle height. THAT is ABOVE critical altitude.
Neutral gear critical altitude is below the ground. You would see three steps in the climb graph just as you expect if the graphs were extended below the ground to show performance in a sufficiently deep valley (think "Dead Sea").
Not for the R-2800-10. Critical altitude in neutral blower is above seal level. It’s proven by the BuAer Charts.
I finally found a R-2800-10W (water injection) power chart. This is from America’s Hundred Thousand:
(http://members.cox.net/us.fighters/blower5.jpg)
For the R-2800-10W at Combat Power, Critical Altitude for neutral blower IS at or below sea-level. BUT THAT DOESN’T MEAN IT WASN’T USED.
REMEMBER – above critical altitude, power begins to DROP. Once critical altitude is reached, power in neutral doesn't just end, it gradually drops.
Look at the chart. Look at the Combat Power line. You can see that EVEN THOUGH the critical altitude for neutral blower was at or below sea level, it was still used. The red line represents the neutral blower stage. At combat power in neutral blower, power began to drop immediately above seal level. But it didn’t drop to below low-blower power until about 5,000 feet. It would not have made any sense to use low-blower at sea level because it couldn’t generate the same power neutral blower did.
ONLY WHEN NEUTRAL BLOWER COULD NOT MAINTAIN THE MAXIMUM POWER THAT LOW BLOWER COULD DID THE SUPERCHARGER GET SHIFTED FROM NEUTRAL TO LOW.
Neutral blower at Combat Power could achieve around 2,300 hp at sea level. Low blower could not even make 2,000 hp at sea level. Therefore, it only makes sense to use neutral blower from sea-level on up until it couldn’t maintain low-blower sea-level power.
--
editted because I keep saying R-2800-8
-
(http://members.cox.net/us.fighters/blower6.JPG)
That secon sentence should say "...and get more power."
-
But you have marked the BuAer chart as using Neutral blower up to 10,000ft in Combat power - this is simply not possible - neutral blower is only capable of 1500hp at 10,000ft! Low blower has over 2100hp at 10,000ft with Combat power!
The BuAer F6F-5 charts do not use Neutral blower with Combat power - if they did, then they would look more like the F4U-1D chart - with speed initially falling from s/l up to ~2000ft as Neutral blower falls of in power from about 2250hp down to 2000hp, then speed is increasing again after Low blower is engaged, giving 2000-2100hp up to 15,000ft.
-
Justin, you are wrong. You need to study the charts some more.
Again, the BuAer chart showing the combat power climb of the F6F-5 shows three DISTINCT stages. EACH stage represents a different blower stage. There are three blower stages - neutral, low and high. You CANNOT have three distinct stages if three distinct blower stages are not used. There are two distinct shift points, from neutral to low, and from low to high. If neutral was not use, what do you think that last shift was to? From "high" to "higher?"
Now, look at the combat power climb for the F4U-1. That is using TWO stages. Just two. Low and high blower stages. There is ONE distinct shift - from low to high.
All this talk about FTH and MAP numbers confuses the issue.
The BuAer F6F-5 climb and speed charts at combat power, DO, without question, show the use of neutral blower.
-
Originally posted by justin_g
But you have marked the BuAer chart as using Neutral blower up to 10,000ft in Combat power - this is simply not possible - neutral blower is only capable of 1500hp at 10,000ft! Low blower has over 2100hp at 10,000ft with Combat power!
One: I didn't make the BuAer charts. The charts speak for themselves. I colored the lines on the chart to help HoHun understand them. But it is the BuAer chart, not me, that indicates neutral was used in the combat power climb.
Two: the terms COMBAT POWER and MILITARY power ARE NOT defined by the amount of horsepower made. Horsepower is INCIDENTAL to these terms.
Combat Power and Military Power are defined by specific manifold pressures and prop RPMs. Horsepower is, again, incidental.
A plane at COMBAT POWER in neutral blower at sea level may be at 60" hga and 2800 rpm. It MAY be making 2,300 hp. At its critical altitude, it will still be 60" hga and 2800 rpm. But it may only be making 1,800 hp. Neverthless, it is still at combat power - because it's still at the settings that define Combat Power - 60" hga and 2,800 rpm.
Above critical altitude, hga will begin to fall. Once it falls to point where the actual hp is equal to that which can be generated by low blower, the supercharger stage is changed. Low blower raises the hga back up to 60" hga - though HP at this point will still be lower than it was at sea level.
The point of the shift is set in the flight manual. It was determined through testing and calculation. There is no horsepower guage in a plane. The pilot knows when to shift because his manual tells him at what altitude to shift at a particular blower stage and power setting.
Again, Combat Power and Military Power (as well as Normal and Cruise) have nothing really to do with the amount of HP being made. Those settings are defined by the MAP and RPM. Your plane would be at Combat Power at 60" hga and 2,800 rpm whether it was churning out 2,300 hp or 500 hp. As long as the MAP and rpm are at the defined points, you are at combat power.
-
Look, from the Hellcat's manual. Not only does it show that the Neutral is used at combat power, it shows when the shift should take place:
(http://members.cox.net/us.fighters/chart.jpg)
note: Adobe has superposed some number errors, but the proof remains still
-
Yes, it says that at Combat power(WEP) to shift to Low blower at 3,000ft - and then to High blower at 16,000ft.
You have indicated on the BuAer charts that Neutral blower is used up to 13,000ft before shifting to Low blower, which is maintained until 22,000ft!
The reason there are 3 steps in the Combat power graph is because of AN ERROR in the making of said graphs.
It appears that the engine powers used to calculate the speed graph at Combat power were for climbing and not level flight, thus the critical altitudes displayed are 2000ft too low. The first critical altitude should be at 15,000ft and not 13,100ft, and the second at 20,000ft and not 18,000ft as displayed on the chart. See next post.
-
Originally posted by HoHun
The BuAer chart is weird, and it's easy to get confused. However, armed with the chart you posted, I can tell you for certain that neutral blower is not used on the Combat Power branch of the BuAer chart.
I think I know why the BuAer chart has such a weird shape.
The BuAer F6F-5 chart has incorrect critical altitudes at Combat power(2000ft too low, they should be the same as F4U-1, ie: 15,000ft & 20,000ft).
The only clue I see as to why is in the power charts:
(http://i22.photobucket.com/albums/b311/qwejibo/F6F5power.gif)
The engine powers listed for Combat power are climb powers - the critical altitudes listed(13,100ft & 18,000ft) are a close match for those seen in F4U-1 climbrate charts, while the Military power and Normal power settings have the correct critical altitudes for level flight.
My theory is that the performance figures shown on the graphs were calculated - using the engine powers listed(which were obtained in seperate flight tests and then somehow got mixed up, re: climb/level speed powers) - hence the incorrect, lower critical altitudes shown at Combat power, which make the graph curves so confusing.
Here is my interpretation of how the graphs should appear:
(http://i22.photobucket.com/albums/b311/qwejibo/F6F5perf.gif)
Note that the critical altitudes on the speed chart have been raised to 15,000ft for Low blower and 20,000ft for High blower, and the effect of Neutral blower has been added below 2000ft.
-
Hi Shorty,
>FTH IS NOT the critical altitude at anything under combat power. It would be ABOVE critical altitude at power below combat power.
It is exactly the lowest altitude at which the throttle is fully open, in any gear, at any power setting. Thus, full throttle height varies with engine speed as well as with boost pressure.
>The throttle is no more open at critical altitude than it was at sea-level.
Quite wrong. Don't think of the throttle lever, think of the actual throttle valve which is operated by the automatic boost regulator even if all the pilot does is to push the level to the firewall and leaving it there.
>If you are climbing at Military Power, and you climb above critical altitude, your power will begin to fall off. If you don’t have another supercharger stage to shift to maintain power, the only thing you could do to maintain power would be to increase throttle. Sooner or later, the throttle would be wide open. At the point where the throttle has to be wide open to achieve military power – THAT would be full throttle height. THAT is ABOVE critical altitude.
Hm, why do you think the power falls off with increasing altitude in the first place? The reason is that the supercharger is unable to provide enough air to the engine to maintain the boost. Power begins to fall off exactly at that altitude where the throttle is fully opened for the first time.
Critical altitude and full throttle height always coincede.
>Not for the R-2800-10. Critical altitude in neutral blower is above seal level. It’s proven by the BuAer Charts.
By what aspect of the BuAer charts? The step from MIL to Combat Power certainly contradicts that opinion.
>Neutral blower at Combat Power could achieve around 2,300 hp at sea level. Low blower could not even make 2,000 hp at sea level. Therefore, it only makes sense to use neutral blower from sea-level on up until it couldn’t maintain low-blower sea-level power.
Even the AHT chart (which is rather inconsistent in several points) shows neutral blower full throttle height as subterranean. Your interpretation shows neutral blower full throttle height as 10000 ft. There is no way to resolve this contradiction.
Regards,
Henning (HoHun)
-
Hi Shorty
>There are two distinct shift points, from neutral to low, and from low to high. If neutral was not use, what do you think that last shift was to? From "high" to "higher?"
The third jump is what makes the analysis so difficult. However, from the analysis of full throttle heights (blue lines), it's certain that the last jump in the graph was not a supercharger gear change.
>All this talk about FTH and MAP numbers confuses the issue.
Actually, it's the key to the solution - provided that if you know enough about engines to understand that there is a real physical background behind the curves.
Regards,
Henning (HoHun)
-
Hi Justin,
>Yes, it says that at Combat power(WEP) to shift to Low blower at 3,000ft - and then to High blower at 16,000ft.
>You have indicated on the BuAer charts that Neutral blower is used up to 13,000ft before shifting to Low blower, which is maintained until 22,000ft!
You have nailed the critical point here :-)
Regards,
Henning (HoHun)
-
Hi Justin,
>The only clue I see as to why is in the power charts:
Hm, these power charts don't match the ones provided by Shorty all too well, regardless of the power setting.
(Of course, I'd like to find a physical explanation for the weirdness of the BuAer graphs, but currently I have no idea at all.)
>Here is my interpretation of how the graphs should appear:
That's a good way to remove the weirdness :-) However, it is still unexplained.
Regards,
Henning (HoHun)
-
Originally posted by HoHun
Hi Justin,
>The only clue I see as to why is in the power charts:
Hm, these power charts don't match the ones provided by Shorty all too well, regardless of the power setting.
(Of course, I'd like to find a physical explanation for the weirdness of the BuAer graphs, but currently I have no idea at all.)
>Here is my interpretation of how the graphs should appear:
That's a good way to remove the weirdness :-) However, it is still unexplained.
Regards,
Henning (HoHun)
The engine power charts posted by Shorty are without ram effect. The powers listed in the BuAer document were collected by actual flight test, and vary for several reasons. The most important difference is that the figures listed of course include the effect of ram air, which raises the critical altitudes.
The engine power chart from the Vought documents below shows Combat power critical altitudes of 13,700ft & 18,300ft in climb, and 15,300ft & 20,100ft in level flight for the F6F-5. The the AHT R2800-10W engine graph posted previously shows critical altitudes of 15,000ft & 20,000ft for Combat power(which would be for level flight too). Every chart for the F4U-1 shows critical altitudes in level flight of around 15,000ft & 20,000ft at Combat power.
Why does the BuAer chart for the F6F-5 level speed at Combat power have critical altitudes of 13,100ft & 18,000ft then? Because they calculated the Combat power level speeds using engine powers achieved in a climb - and not the level flight powers that they should have used(which they did use correctly for the other power settings).
Vought chart:
(http://i22.photobucket.com/albums/b311/qwejibo/f6fpowers.gif)
-
Hi again,
Here is an analysis based on the power charts posted by Shorty:
(http://hometown.aol.de/HoHunKhan/f6f.png)
I have calibrated F6F-5 drag to match the speed at low gear full throttle height.
Power settings are from the SEFC provided by Justin.
I'm a bit suspicious against the way power drops off below full throttle height, resulting in exaggerated sea level speeds. Maybe it would be necessary to apply a temperature-modified pressure altitude to the engine calibration chart - I have seen something similar in the B-29 manual. That's only a thought, of course.
Please consider the entire analysis preliminary :-)
Regards,
Henning (HoHun)
-
Could you do one for WEP?
-
justin_g,
Use the chart from page 4 of the Vought doc. It shows speeds and FTHs at same power in high speed flight. The chart is a bit unclear but you can roughly read:
F6F-5
Neutral gear FTH 2k
Low gear FTH 16k
High gear FTH 20k
F4U-1
Neutral gear FTH 4k
Low gear FTH 16k
High gear FTH 20k
The chart correctly shows higher neutral gear FTH for the F4U-1 due to RAM effect while low and high gear FTHs are about the same for F6F-1 and F4U-1.
The speed values itself are quite unrelevant but the point here is that the AH F6F-5 has higher neutral gear FTH than the F4U-1D. In the real life the F4U had higher FTH for the neutral gear due to RAM at given MAP.
gripen
-
Engine power chart on page 2 shows the same thing(see above) - but what I'm mainly trying to prove is that the BuAer charts for the F6F-5 are porked for Combat power due to incorrect hp figures used in the calculations.
-
Originally posted by HoHun
Hi Shorty,
>The throttle is no more open at critical altitude than it was at sea-level.
Quite wrong. Don't think of the throttle lever, think of the actual throttle valve which is operated by the automatic boost regulator even if all the pilot does is to push the level to the firewall and leaving it there.
Please understand that I am speaking in terms of that BuAer F6F-5 chart – that’s all.
The reason I assert what I do is because Military Power and Combat Power in Neutral, Low and High have different critical altitudes. Military power will always have a higher critical altitude than Combat Power. But the BuAer chart does not reflect that – and that’s where I think it’s screwed up.
The Combat Power critical altitude is FTH. If the chart is showing Military Power critical altitude as the SAME height at the same blower mode, then it’s wrong in that regard. If Combat Power critical altitude is a FTH, it wouldn’t be a FTH for Military power, it would be above that. THAT’S what I am trying to say.
So, I agree there is something fishy about the chart. Nevertheless, the chart itself shows neutral was used in both Military and Combat Power.
Here is the BuAer chart on the F4U-1D – powered by the R2800-8W engine – essentially the same as the Hellcat’s -10W engine. In fact, I think they have exactly the same ratings.
(http://members.cox.net/us.fighters/corsair.jpg)
Now this chart DOES reflect the FTH’s of both Combat Power and Military Power in the 3 blower modes in the way you describe. In this case, it shows the critical altitude of military power in neutral, low and high as higher than the CSA’s of combat power in the same blower modes. On this chart, the CA of military power would be a FTH. There would be no way of obtaining combat power in neutral blower at Military Power/Neutral Blower CA.
What I’m getting at is that on the F6F-5 chart, the FTHs (and I hate that term) the critical altitudes of Miltiary Power and Combat Power are shown as the same (in low and high). If it’s FTH for Combat Power, IT’S NOT a FTH for Military Power, it would be above the CA reflected on that chart.
You guys are arguing numbers – I’m not. I’m simply arguing what that chart shows. I think it’s screwed up to. But nevertheless, the chart shows neutral was used. In reality, it may not have been used to the extent that chart reflects, but all other evidence I’ve seen shows it should have been used. Again, it would make no sense at all to use low blower from sea-level when neutral blower generated more power for several thousand feet.
So, while I am not denying the chart is odd, I think it would have been silly to use low blower at sea level.
-
I have not really looked the BuAer data but the so called "combat" rating for the F6F-5 in the BuAer data seem to be based on lower MAP than "war emergency" rating for the F4U-1 and F6F-5 in the Vought doc.
gripen
-
Originally posted by justin_g
Engine power chart on page 2 shows the same thing(see above) - but what I'm mainly trying to prove is that the BuAer charts for the F6F-5 are porked for Combat power due to incorrect hp figures used in the calculations.
I agree with you to that extent, the chart is very odd. But I think HP has nothing to do with it. I think it has to do with incorrect critical altitudes being reflected on the combat power climb and speed lines. HP is incidental to the power rating.
Frankly, I think the Combat Power climb should have a line similar to the combat power line in the F4U-1D chart I posted above. The -8W and the -10W engines had the same ratings and CAs. So the lines should be similar, even if showing less performance for the F6F.
But I don't agree that Neutral wasn't used. It may not have been used up to 10,000 feet, but I think it's clear it was used. To get best performance, it would have had to be used. Foregoing the use of neutral at sea level would have hurt performance.
The "FTH" thing HoHum was arguing doesn't make any sense with respect to that F6F chart - and it's on that I base my arguments - solely. HoHun's arguments make perfect sense with repsect to the F4U-1D chart I posted. The F6F chart shows incorrect CAs for either Military power, or Combat power, or both. FTH for Combat power is not the the FTH for military power- Military Power FTH is above Combat Power FTH (if I'm using that term correctly). On the F4U-1D chart, the Critical Altitudes appear to be are properly marked.
I've clearly not made myself very understandable. The whole FTH term is new to me and maybe I'm not understanding how its used.
-
Shorty - "Full Throttle Height"(FTH) is just the British term for "Critical Altitude" - they are the same thing.
I agree with you to that extent, the chart is very odd. But I think HP has nothing to do with it. I think it has to do with incorrect critical altitudes being reflected on the combat power climb and speed lines. HP is incidental to the power rating.
That's basically what im trying to say.
The graphs in the BuAer data are calculated, using engine power outputs which were obtained in flight tests - they are listed on the last page of the document(link (http://i22.photobucket.com/albums/b311/qwejibo/F6F5power.gif)).
If you compare the critical altitudes listed(13,100ft & 18,000ft) to those in the Vought document(link (http://i22.photobucket.com/albums/b311/qwejibo/f6fpowers.gif)) you can see that they match the critical altitudes for CLIMB POWERS. The critical altitudes for HIGH SPEED POWERS are about 2000ft higher(more ram air) and correspond to any F4U-1 chart at Combat power, with critical altitudes around 15,000ft & 20,000ft.
Frankly, I think the Combat Power climb should have a line similar to the combat power line in the F4U-1D chart I posted above. The -8W and the -10W engines had the same ratings and CAs. So the lines should be similar, even if showing less performance for the F6F.
If the BuAer had used engine power data with the correct critical altitudes in their calculations, then the graphs might have looked something like this:
(http://i22.photobucket.com/albums/b311/qwejibo/F6F5perf.gif)
But I don't agree that Neutral wasn't used. It may not have been used up to 10,000 feet, but I think it's clear it was used. To get best performance, it would have had to be used. Foregoing the use of neutral at sea level would have hurt performance.
Note that on the modified version I have included the use of Neutral blower only below 2000ft - as can also be seen on the BuAer charts for the F4U-1D, but NOT on those for the F4U-1.
-
Hi Shorty,
>If Combat Power critical altitude is a FTH, it wouldn’t be a FTH for Military power, it would be above that. THAT’S what I am trying to say.
Look at Justins "corrected" graph - it meets your requirement.
Justin made a 2000 ft correction to get a perfectly logical graph. You had to make a 12000 ft correction in neutral gear full throttle height and got a graph that is far from perfectly logical. That's the reason I go with Justin's interpretation :-)
>On this chart, the CA of military power would be a FTH.
Critical altitude always is a full throttle height. The reason I'm using the term "full throttle height" is that it is based on what really happens in the engine, while "critical altitude" is a term belonging to a rating system that does not reflect what's really going on in the engine (US system, constant rated power up to critical altitude).
>I’m simply arguing what that chart shows.
There are two interpretations, both of which have merit. After a thorough analysis, Justin's variant is much more logical than yours (which is one that I initially suggested, too).
Regards,
Henning (HoHun)
-
Hi Justin,
>Could you do one for WEP?
OK, here it is:
(http://hometown.aol.de/HoHunKhan/F6F-5%20WEP.png)
It's not particularly accurate as I had to extrapolate the power graphs though they weren't always linear. Escpecially the high gear is a bit of a guess. However, the result is still qualitatively accurate.
Note the break between MIL and WEP in high blower, with the higher rpm yielding a lower top speed!
This is caused by a propeller efficiency loss due to supersonic tip speeds. If you look at it, 2700 rpm only yields some extra 50 HP, and the higher tip speeds cost slightly more than that so that it's sensible to use 2550 rpm.
The break is not quite as large as in the BuAer chart, but it's there, and I believe propeller tip Mach effects are to blame for the BuAer chart's weirdness as well.
Regards,
Henning (HoHun)
-
Hi again,
On the other hand, it could be that my analysis is completely wrong due to a KTAS/MPH confusion.
With the power chart broken down into three parts, there is so much manual labour required in my analysis that I have too many opportunities to screw up somewhere in the process :-/
Regards,
Henning (HoHun)
-
The biggest confusion about the BuAer speed and climb graphs is that they are plotted inconsistently with each other.
Refering only to the Combat power lines:
On the speed chart 13,100 feet is plotted as the CA in neutral blower.
On the climb chart, 10,000 feet is plotted as the CA in neutral blower.
On the speed chart 15,700 feet is plotted as the shift point from neutral to low.
On the climb chart 13,100 feet is plotted as the shift point from neutral to low blower.
On the speed chart 18,000 feet is plotted as the CA for low blower.
On the climb chart 15,700 feet is plotted as the CA for low blower.
On the speed chart 21,500 feet is plotted as the shift point from low to high.
On the climb chart 20,000 feet is plotted as the shift point from low to high.
On the speed chart 23,000 feet is plotted as the CA for high blower.
On the climb chart 21,000 feet is plotted as the CA for high blower.
Regardless of what altitudes are used for each phase, they should be consistent between graphs - speed and climb should show the same CAs and shift points.
-
Here's anopther chart to ponder. Forget the Navy data on the chart, I think it comes directly from BuAer. The Grumman data is interesting:
(http://members.cox.net/us.fighters/hellcat.jpg)
-
Hi again,
>On the other hand, it could be that my analysis is completely wrong due to a KTAS/MPH confusion.
I found I accidentally used a higher drag coefficient for some portions of the graph. It's fixed now.
The slight jump is still there, but a bit smaller in magnitude.
Regards,
Henning (HoHun)
-
Hi Shorty,
>Regardless of what altitudes are used for each phase, they should be consistent between graphs - speed and climb should show the same CAs and shift points.
Not if there is any ram effect, which increase intake pressure and allows the supercharger to operate as it would at a lower altitude at a lower speed.
Regards,
Henning (HoHun)
-
Originally posted by HoHun
Not if there is any ram effect, which increase intake pressure and allows the supercharger to operate as it would at a lower altitude at a lower speed.
Regards,
Henning (HoHun) [/B]
OK.
Ram has the effect of raising critical altitude. Is there less ram effect when climbing then in straight lin speed? Now that I think about it, it think there would be. That explains why the critical altitude on the F4U-1D VMax graph are higher than those on the Climb graph.
But why, then, is the neutral-to-low blower shift point on the VMax graph at the same altitude as it is on the climb graph? With ram, which the Corsair used in neutral, it would have been able to maintain power higher than without, or with less, ram, so it seems the shift point on the VMax graph should be higher.
-
Hi Shorty,
>Ram has the effect of raising critical altitude. Is there less ram effect when climbing then in straight lin speed? Now that I think about it, it think there would be.
Yes, ram pressure increases to the square of the speed, so the difference between climb and level flight is marked.
>But why, then, is the neutral-to-low blower shift point on the VMax graph the same as it is on the climb graph. With ram, which the Corsair used in neutral, it would have been able to maintain power higher than without, or with less, ram, so it seems the shift point should be higher.
Hm, very good observation! That would indeed indicate the absence of ram pressure. Maybe we should search for a diagram of the F4U's intake system? (Zeno's site linked by Justin has a diagram of the F6F's system, and if I'd seen that earlier, I had not been as confused :-)
With regard to the gear change question: Did you notice that the Grumman graph you posted above features neutral gear on combat power? In the side by side comparison, it demonstrates that the BuAer data did not rely on neutral gear for combat power. The Grumman data looks a lot like the F4U-1D data, too, so I think it should finally solve the question.
The Grumman data also has the same "extra-step" weirdness between MIL and Combat Power! =8-O
With regard to neutral blower and ram pressure, here some data from my F6F-5 calculation:
Top speed at sea level, 60" Hg, neutral blower, no ram effect: 550 km/h
Top speed at sea level, 60" Hg, neutral blower, ram effect: 564 km/h
So I get 7 to 8 knots speed loss due to the lack of ram effect - far less than the 20 knots quoted by Corky Meyer. (Since I estimated ram effect based on the speed graph which might not perfectly match the power graph, there is a certain uncertainty about my number, but 20 knots seem a bit much.)
I believe my calculation currently over-estimates low-level speeds because it under-estimates high-altitude thrust. (Currently, it's based on shaft power rather than on indicated power because I took the quick and dirty route.)
To round off this post, I finally had a look at the different data sets we have for the F6F, and it seems that the BuAer set is the slowest of them all.
I have found:
- BuAer
- AHT graph (posted by Shorty)
- F4U-4 documentation (posted by F4UDOA)
- Fw 190 comparison report
- TAIC Zero report (posted by F4UDOA)
Do we have any more data? I'll try to prepare a graph showing all the figures combined.
Regards,
Henning (HoHun)
-
The Grumman and BuAer lines on the chart I posted from America's Hundred Thousand both show neutral gear useage in combat power climbs. Grumman data shows a MUCH earlier shift to low blower, about where I would expect it. It also shows neutral blower CA is higher than sea level.
I'm still not convince the BUAer climb did not use neutral blower. But I agree the Grumman line looks more reasonable.
-
Originally posted by HoHun
Note the break between MIL and WEP in high blower, with the higher rpm yielding a lower top speed!
This is caused by a propeller efficiency loss due to supersonic tip speeds. If you look at it, 2700 rpm only yields some extra 50 HP, and the higher tip speeds cost slightly more than that so that it's sensible to use 2550 rpm.
The break is not quite as large as in the BuAer chart, but it's there, and I believe propeller tip Mach effects are to blame for the BuAer chart's weirdness as well.
I don't think the "jump" on the BuAer chart is entirely due to this reason(It's a helluva lot bigger jump). However, I do agree that 2700rpm would cause a performance loss at high altitude - you can see this effect on the BuAer F4U-1 charts, where Normal power(2550rpm) is actually faster than Military & Combat powers(2700rpm) at altitudes over 25,000ft.
Originally posted by HoHun
Maybe we should search for a diagram of the F4U's intake system? (Zeno's site linked by Justin has a diagram of the F6F's system, and if I'd seen that earlier, I had not been as confused :-)
F4U-1 uses wing root ducts for intake air, so it is always getting ram effect. It did however have a switchable unrammed intake which (it appears)could only be used with the aux. blower in neutral gear(item G). Diagram from Zenos (http://www.zenoswarbirdvideos.com/Images/F4U/F4UIS.gif)
-
A few things.
1. In later Combat climb instructions in the POH pilots are instructed to climb at 2500RPM at high alt because 2700RPM the prop looses efficiency at 20K and above.
2. The R2800-8W put out more HP especially at sea level in low blower. But even above that altitude the R2800-8W out performs the R-2800-10W. I will scan and post the 1945&1946 engine calibration charts for both.
3. The POH for the F6F-5 mentions that despite the engine cal curve that the low blower stage produces more HP in the Neutral stage. I have a copy of the British AFDU for the F6F-5 and they use the Low blower stage from sea level as well.
-
Originally posted by HoHun
With regard to neutral blower and ram pressure, here some data from my F6F-5 calculation:
Top speed at sea level, 60" Hg, neutral blower, no ram effect: 550 km/h
Top speed at sea level, 60" Hg, neutral blower, ram effect: 564 km/h
So I get 7 to 8 knots speed loss due to the lack of ram effect - far less than the 20 knots quoted by Corky Meyer. (Since I estimated ram effect based on the speed graph which might not perfectly match the power graph, there is a certain uncertainty about my number, but 20 knots seem a bit much.)
[/b]
This has been an interesting diversion, I'll see if I can do a workup on both the F6F-5 and the F4U-1. But note there is a lot of conflicting information on the R-2800-8/8w, listing anywhere from 57.5 in to 60 in Hg for WEP, and from 2100 to about 2300 hp in neutral blower.
But this doesn't make any sense HoHun. 60 in Hg is 60 in Hg, the engine doesn't know it is rammed or not, ie as long as the engine can manage 60 in Hg unrammed it will actually get HIGHER speed than 60 in Hg rammed would provide. You have to take compression heating of the charge air into account.
Lets call it 340 mph, nice easy number. Ram heating is:
340/100 ^ 2 = 11.56 deg F
SL standard temp is 518.69 R so charge air temp = 518.69 + 11.56 = 530.25
Density is going to be sqrt (518.69 / 530.25) = .9984 so the engine will lose .16% power rammed, not much but definitely going to be slower rammed than unrammed.
Offsetting that is the fact the the F6F-5 charge air was heated as it passed through the engine compartment to the carb, but I don't even have any empirical figures for how much that would be.
The point is, I don't know where the claims of a significant speed loss at SL came from. As long as the engine is capable of the required manifold pressure unrammed then ram is not going to provide an increase in power and speed, quite the opposite in fact.
The only possibility I can see is that while the engine is capable of 60 in Hg in neutral blower on a test stand, there are pressure losses ahead of the blower as installed in the F6F-5 that get multiplied by the neutral blower pressure ratio. The blower has approx a 2:1 pressure ratio, ie 60 in Hg / 29.92 = 2.005:1, so 2 in Hg pressure loss ahead of the blower drops MAP by 4 inches which could cost about 140 hp or so going from 60 in Hg to 56 in Hg. That could certainly cause a significant loss of speed in neutral blower.
Greg Shaw
-
Originally posted by gwshaw
The point is, I don't know where the claims of a significant speed loss at SL came from. As long as the engine is capable of the required manifold pressure unrammed then ram is not going to provide an increase in power and speed, quite the opposite in fact.
IMHO Meyer and others mean overall low altitude performance, say below 10k; the FTH of the Neutral blower is very low with high ratings and the F4U could use it 2-3k higher (due to RAM effect) than F6F resulting better performance. Infact some F4U and F6F charts show that the neutral blower FTH is below sea level with high MAP (see here (http://www.geocities.com/slakergmb/id105.htm)). In that case the F4U has a clear advantage until it activates the auxilary stage.
gripen
-
For comparison
F6F-5 1945 R2800-10W
(http://home.comcast.net/~markw4/R2800-10W.jpg)
F4U-1D 1946 R2800-8W
(http://home.comcast.net/~markw4/R2800-8W.jpg)
Check out 6. A about the low blower developing more HP than neutral on the F6F-5
(http://home.comcast.net/~markw4/R2800-10lb.jpg)
-
Hi Shorty,
>The Grumman and BuAer lines on the chart I posted from America's Hundred Thousand both show neutral gear useage in combat power climbs.
There are only Grumman speed lines, no Grumman climb lines on that chart.
Compare the two speed lines, the manufacturers is much faster but it has one blower change more while otherwise following the BuAer chart exactly in shape. Unless Grumman uses a 4th supercharger gear, the BuAer Combat Power chart uses only 2 speeds :-)
Regards,
Henning (HoHun)
-
Hi Justin,
>I don't think the "jump" on the BuAer chart is entirely due to this reason(It's a helluva lot bigger jump).
The chart I posted is only a rough estimate, the jump might easily grow bigger once I enter more accurate data.
The jump is repeated in the Grumman data, by teh way.
>F4U-1 uses wing root ducts for intake air, so it is always getting ram effect. It did however have a switchable unrammed intake which (it appears)could only be used with the aux. blower in neutral gear(item G). Diagram from Zenos (http://www.zenoswarbirdvideos.com/Images/F4U/F4UIS.gif)
Ah, thanks. That means we don't know whether ram was used :-) We can probably figure it out from the power chart, though.
Regards,
Henning (HoHun)
-
Hi F4UDOA,
>3. The POH for the F6F-5 mentions that despite the engine cal curve that the low blower stage produces more HP in the Neutral stage. I have a copy of the British AFDU for the F6F-5 and they use the Low blower stage from sea level as well.
Does the British AFDU show top speed as well? I'd like to include it in the overview graph I'm about to prepare :-)
Regards,
Henning (HoHun)
-
Hi Greg,
All good points in your post! :-)
>The point is, I don't know where the claims of a significant speed loss at SL came from. As long as the engine is capable of the required manifold pressure unrammed then ram is not going to provide an increase in power and speed, quite the opposite in fact.
You nailed it :-) The engine is *not* capable of the required manifold pressure unrammed or rammed, it's operating above neutral gear full throttle height even at sea level.
That also explains the varying figures on boost pressure and power in neutral gear - the lower figures are what you can realistically expect, the higher figures are the engine's actual limits.
You can see it in the graphs I prepared - speed falls off from sea level upwards. That's the same effect we can see in the F4U-1D charts.
Regards,
Henning (HoHun)
-
Hi F4UDOA,
>Check out 6. A about the low blower developing more HP than neutral on the F6F-5
Hm. The paragraph about derichment seems to suggest the opposite effect of the "weirdness" we have noticed both in the BuAer and the Grumman chart. Doubly weird!
Regards,
Henning (HoHun)
-
Originally posted by HoHun
Hi Shorty,
>The Grumman and BuAer lines on the chart I posted from America's Hundred Thousand both show neutral gear useage in combat power climbs.
There are only Grumman speed lines, no Grumman climb lines on that chart.
Compare the two speed lines, the manufacturers is much faster but it has one blower change more while otherwise following the BuAer chart exactly in shape. Unless Grumman uses a 4th supercharger gear, the BuAer Combat Power chart uses only 2 speeds :-)
Regards,
Henning (HoHun)
Sorry, I meant speed lines.
Ok, I think I finally got what was obvious to you and Justin.
You are saying this is the how it really is:
(http://members.cox.net/us.fighters/2stages.JPG)
As opposed to the way I thought it was:
(http://members.cox.net/us.fighters/3stages.JPG)
I don't know why the hell I didn't see that earlier, it was clearly explained. And now that I look at it, it should have been obvious to me, too. I guess I just got a notion in my head and didn't let go ouf it, even though it didn't make any sense.
-
LOL, if only someone had drawn that picture at the start of the discussion - might have saved about a 1000 words! :D
-
Hi Shorty,
>I don't know why the hell I didn't see that earlier, it was clearly explained. And now that I look at it, it should have been obvious to me, too.
Hm, I'd say the critical point was the the virtual "fourth" stage which we now suspect as being due to propeller efficiency effects. Only the last chart you posted finally demonstrated its "virtual" nature, so you shouldn't be so critical with yourself :-) This was tough, and you found the decisive clue yourself!
Regards,
Henning (HoHun)
-
It was too simple.
My apologies to HoHun for thinking he was nuts, and especially to Justin, who I said was wrong when it was clearly I that was wrong.
Now that it is clear to me, all the stuff HoHun said about FTH and the other stuff Justin said makes perfect sense.
The whole thing was like looking at one of those pictures with the weird designs on it. Everyone tells you that if you just look at it right, you'll see a 3D dinosaur. But you keep looking and looking and all you see if weird designs. But they tell you it's right there, just look at it this way and you'll see it. But you tell them they're full of crap. And you are about to give up on it, you lay the picture down, get a Pepsi, walk past it again, the light hits it right, and that 3D dinosaur jumps out and pops you right on the pie-hole.
-
R 2800-8W and -10W Combat power(WEP) output compared(from F4UDOA's charts with thanks!)
(http://i22.photobucket.com/albums/b311/qwejibo/R2800-8W_10W_WEP.gif)
R 2800-8W and -10W Military power output compared
(http://i22.photobucket.com/albums/b311/qwejibo/R2800-8W_10W_MIL.gif)
-
Well, the question still remains why the AH F6F has neutral blower FTH roughly 2k at WEP? Apparently the MAP of the WEP at neutral gear is much less than 58" or the engine had a quite bit different SC gearing.
gripen
-
The Grumman data(see graph from AHT) has the same anomaly in neutral gear/WEP as the HTC version... it's very odd.
-
Hi Justin,
>The Grumman data(see graph from AHT) has the same anomaly in neutral gear/WEP as the HTC version... it's very odd.
Here is a comparison of the F6F data we have found so far:
(http://hometown.aol.de/HoHunKhan/F6Fcompare.png)
The Grumman data seems to have the highest low-level power.
The PTR 1107 report features data that's a bit strange - since it was only geathere during a rough side-by-side comparison, it might not be sufficiently accurate for a comparison of absolute numbers. The data points are marked individually since it seems to skip high gear full throttle height, and the continous line I provided there might be misleading.
The data from the Vought F4U-4 comparison report is odd, too, since the graph provides a sea level speed figure 10 mph lower than the tabulated data in the same report. Since the shape of the graph looks more sensible using the higher numbers, I have used the higher numbers near sea level.
The data point from the TAIC report sticks out, of course. It might be inaccurate for the same reason as the PTR 1107 data.
The BuAer data is the lowest performance data set. However, it's interesting that it seems to match the data provided by the AHT chart for the F6F-3.
Now I wonder if the BuAer might have used an F6F-3 using water injection as a basis for the F6F-5 data. (PTR 1107 suggests that there were F6F-3 aircraft with water injection, possibly retro-fitted.) That would in turn raise the question whether there were (aerodynamic) differences between the F6F-3 and the F6F-5 that would cause the newer version to be faster at the same power.
Regards,
Henning (HoHun)
-
Thank you for preparing the chart!
Here's some of my thoughts:
1. PTR 1107 report lists the speed reached after 2 mins at full power - "It should be noted that the runs were made for about two minutes only, during which time full speed was probably not developed but serve for the purpose of comparison." I also suspect that the figures may not have been corrected to standard conditions.
2. The graph in the Vought data looks like 330mph at s/l to me?
3. Never seen the TAIC report. I gather that it is a comparitive report, so maybe it was not corrected to standard conditions as well?
4. The BuAer data states 2 things: "Performance is based on flight test of F6F-3 and F6F-5 airplanes." and "Performance based on engine power determined in flight test as follows:" I guess the author of AHT took it as good for a late production F6F-3, at the time there probably weren't as many data sources available as now.
5. From the Navy pdf:
"Provisions for water injection to increase combat power were added in late 1943, as production -10W engines fitted for it became available. Wing stub racks were added for bombs, or for additional fuel tanks, to supplement the single centreline belly tank that had become a standard operational feature.
In April 1944, when necessary strengthening of the rear fuselage and horizontal stabilizer were incorporated, along with other improvements - including engine cowling changes to reduce drag and aileron spring tabs to reduce stick force in combat maneuvers - the designation of the production aircraft was changed to F6F-5. With the structural changes, dive speed and pull out restrictions on the Hellcat were removed."
Here's a picture on which I've noted the details of changes from F6F-3 to F6F-5(thankyou google image search!):
(http://i22.photobucket.com/albums/b311/qwejibo/F6f3_5.gif)
-
Hi Justin,
>1. PTR 1107 report lists the speed reached after 2 mins at full power - "It should be noted that the runs were made for about two minutes only, during which time full speed was probably not developed but serve for the purpose of comparison." I also suspect that the figures may not have been corrected to standard conditions.
I believe the F4U and Fw 190 speeds in that report are also slightly unusual, confirming the latter suspicion.
>2. The graph in the Vought data looks like 330mph at s/l to me?
Hm, you're right. I guess this proves it's possible to read off the same wrong value three times even when you're aided by a drawing program with a pixel readout, and considering the perceived value as suspicious! :-/
>3. Never seen the TAIC report. I gather that it is a comparitive report, so maybe it was not corrected to standard conditions as well?
F4UDOA linked it over in the A6M thread, but it's not very useful as far as the F6f is concerned. I share your suspicion.
>Here's a picture on which I've noted the details of changes from F6F-3 to F6F-5(thankyou google image search!):
Hm, interesting! While these seem small changes on an airframe as big as the Hellcat's, it certainly confirms the direction of the change - towards a faster F6F-5 :-)
Regards,
Henning (HoHun)
-
HoHun,
Drop me your email and I will send you the entire F6F-3/5 AFDU report.
-
Hi F4UDOA,
>Drop me your email and I will send you the entire F6F-3/5 AFDU report.
heruch at aol dot com
Thanks in advance! :-)
Regards,
Henning (HoHun)
-
Hi Justin,
>I guess the author of AHT took it as good for a late production F6F-3, at the time there probably weren't as many data sources available as now.
By the way, what where the boost limits for WEP on the F6F-3?
Regards,
Henning (HoHun)
-
Originally posted by HoHun
By the way, what where the boost limits for WEP on the F6F-3?
Regards,
Henning (HoHun) [/B]
Good question. I'm not sure there were any WEP ratings for the F6F-3 without water (-10 engine). I can't find a single source that shows a WEP rating for the F6F-3 without water injection. AFAIK, take-off and military power was the highest rated power - 54" hga.