Reason for F6F-5 speed discrepancy?

[F4UDOA]

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.

[Widewing]

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

[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.

[TequilaChaser]

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
( got this theory after watching "the Aviator",  ROFL )

[HoHun]

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)

[Widewing]

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

[ShortyDoowap]

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.

[Squire]

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.

[gripen]

Shorty,
The F6F vs F4U RAM issue is allready discused in this thread (see my post in the second page). The speed charts from F4UDOA's docs shows this phenomena clearly.

gripen

[F4UDOA]

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.

[F4UDOA]

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

[Saxman]

IIRC, even the F4U's wing configuration had the then-unexpected effect of further reducing drag on the aircraft.

[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.

[F4UDOA]

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.

[HoHun]

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)