Aces High Bulletin Board
General Forums => Aircraft and Vehicles => Topic started by: F4UDOA on October 25, 2002, 11:14:08 AM
-
Well I have argued that these two areas are not completely linear for a long time and have been shouted down many times.
But I will be short and sweet with this expanation.
The F4U has RAM AIR effect in the main blower stage under 8K and the F6F does not. Why compare to the F6F? Well they have the same engine with the same rated HP, MAP and RPM but the F6F climbs better in AH and therefore accelerates better because as everyone here knows climb and accleration are the same.
Except for Ram Air.
It only affects the F4U in level flight and provides greater engine efficiency at the main Blower stage and should provide for greater acceleration because the engine requires far less effort to reach max map and RPM and HP leading for higher top speed and greater acceleration.
Any comments?
-
Except for Ram Air.
It only affects the F4U in level flight
And why would this be true?
HiTech
-
It's not actually. It does affect it in climb but the affect reduces very quickly as the climb is prolonged.
Look at the F4U-1D climb chart and the spike it the climb at sea level in main blower. Niklas mntion he thought that was a Ram Air effect a long time ago but I didn't realize what he meant.
(http://www.hitechcreations.com/ahhelp/models/charts/f4u1dclimb.gif)
The first 3K are the effect of Ram Air.
Then the F4U reaches 160MPH and stops accelerating and sustains climb at 60" of MAP 2700RPM and 2250HP the same as the Hellcat. Remember the F4U-1D and F6F-5 have the same exact prop blade and number of blades. 6501A-0 for both.
The RAM air effect helps you get there more quickly and efficiently. Once you have achieved your max MAP the advantage is only in engine efficiency.
BTW. I have no source for your Hellcat performance. And I have many. I have requested that you guys share the wealth with some only documentation but I have not heard anything back.
-
Hitech,
Here is a quote from Corkey Meyer (Grumman test pilot) on the effect of Ram Air in the two A/C.
The reason that the Corsair was faster in the Main stage blower was because the engine and carburator were provided with Ram Air coming in directly from the forward facing airduct in the wing, where as the Hellcat had the carburator air coming in from the accessory compartment of the fusalauge just behind the engine, with no Ram Air affect. Our airplane was getting carburator air at the same pressure as if it were motionless on the ground and the Corsair was getting supercharged air from the speed of the aircraft giving it more power (speed) in the main stage blower
If both A/C have the same MAP, Prop, HP and RPM then what effect do you believe this would have?
-
Originally posted by F4UDOA
If both A/C have the same MAP, Prop, HP and RPM then what effect do you believe this would have?
Didn't you read Shiva's post? The F4U will exceed rated power under those conditions.
-
F4UDOA it seems you are still having trouble understanding excess power and how it relates to climb and accelerations. You seem like a smart guy, so I can only assume that our explanations have been inadequate.
This book (http://search.barnesandnoble.com/textbooks/booksearch/isbnInquiry.asp?isbn=0471575062) explains it very well and I suggest you buy it and read the relevant chapters, instead of repeatedly banging your head against the wall in here.
PS, I'm serious, not being a smart-ass.
PPS, It's overpriced on that site. I think I paid about $50 for a used copy.
-
dang, I thought he was talking about RAM.
Hair on me neck stood up seeing F4UDOA quoting our beloved RAM as a source!
pssst.. HT.. look at the lil' dive flaps on ze 38 pls? I know you can fix them.. umm... take the laptop with you when you in the toilet, I hear closet coding is a karmic experience. ;)
-
Funked,
I didn't see where Shiva said anything about overboost. I whish he did. Then I would say overboost all of the R2800 in AH. But what I understood was that it required less effort to acheive the same boost. Therefore reaching desired MAP faster and with less parasitic loss of power to the engine and prop.
Shiva said
What ram air does is increase the pressure in the intake, allowing the blower to raise the pressure with less effort, so that more of the engine's power goes to the propellor. If an F6F and an F4U are at 18,000 feet, the intake blower on the F6F would have to add 38 inches of pressure to the incoming air to bring the engine to its maximum rated manifold pressure. The F4U, however, at the same speed as the F6F, would be getting, say, 15 inches of pressure from ram air, meaning that the intake blower would only have to raise the pressure 23 inches to get to the same maximum manifold pressure. This leaves more torque to be applied to the F4U's propellor.
-
I didn't see where Shiva said anything about overboost. I whish he did. Then I would say overboost all of the R2800 in AH. But what I understood was that it required less effort to acheive the same boost. Therefore reaching desired MAP faster and with less parasitic loss of power to the engine and prop.
Yes that's right, and the "less parasitic loss of power" means there will be increased power output at the crankshaft. Which means the plane goes faster.
-
Hi F4UDOA,
>The first 3K are the effect of Ram Air.
Actually, they're the result of using "neutral blower". The ram effect at climb speed is much smaller than the ram effect at top speed, anyway.
Regards,
Henning (HoHun)
-
Funked,
I thought you meant overboosted.
HoHun,
The Neutral Blower stage or main stage is the only one of the three blower stages where the Ram Air effect is different between the F4U and F6F.
That's why it looks somewhat different on the charts at sea level between the two A/C in both speed and climb.
Notice the speed chart shows a clear spike at sea level.
(http://www.hitechcreations.com/ahhelp/models/charts/f4u1dspeed.gif)
And climb
(http://www.hitechcreations.com/ahhelp/models/charts/f4u1dclimb.gif)
Surely this also carries over into acceleration.
-
Hi F4UDOA,
>Notice the speed chart shows a clear spike at sea level.
That has nothing to do with ram effect.
You can see the ram effect in these charts, but it's very different from what you're suggesting:
In high-speed level flight with full ram effect, the full pressure altitude of the high blower is at 20000 ft (see speed graph).
In low-speed climbing flight with very much diminished ram effect, the full pressure altitude of the high blower is down at 17000 ft (see climb graph).
The ram effect applies to the entire speed graph uniformly and will leave no trace in form of a spike or something because the aircraft is in a high-speed flight condition all of the time.
Regards,
Henning (HoHun)
-
ok i will try to explain it
to accelerate or climb your airplane has to have work performed on it, conservation of energy.
so when this work is being done, it can either work against the earth's gravitational force to add potential energy to the system, or accelerate the airplane and add kinetic energy.
-
HoHun,
So your saying the faster the A/C goes the more Ram Air helps the engine.
So at speeds above the best climb speed Ram Air helps the A/C accelerate correct.
Am I missing something here?
-
Hi F4UDOA,
>So your saying the faster the A/C goes the more Ram Air helps the engine.
>So at speeds above the best climb speed Ram Air helps the A/C accelerate correct.
Yes, that's right.
The ram effect gives some power at any speed, helping the aircraft to accelerate, but the gain is much larger at high speeds than at the speed of best climb.
Regards,
Henning (HoHun)
-
Interesting thread ! (S!)
SO; If I understand correctly, the RAM AIR effect, (as it relates to Angle of Attack), could be demonstrated by tipping a beer bottle up, to the horizontal, and blowing into the opening.
The amount of beer that remains in the bottle is directly proportional to the density, and velocity of the air pushing against it at the opening.
However, as the angle of attack increases, So does the pitch of the sound made at the opening, indicating that more and more air is moving ACROSS the opening, than INTO it. Thereby reducing the effect of RAM AIR, (which is no longer needed to keep your lap dry).
Yeah.... THAT'S the ticket!
Thanks!
S!
SC-"Mutt"
CO
The Skeleton Crew
"Fly with Honour!"
-
I wonder if any of this has anything to do with the fact that I get roughly the same climbrate in my -1 at the default climbspeed (160?) as I do at 200mph.
-
SC-Mutt,
I think you just said what I have been trying to say for the last several post but have been unable to articulate.
So the answer to Hitechs original question would be, what he said . Which is.....
the RAM AIR effect, (as it relates to Angle of Attack), could be demonstrated by tipping a beer bottle up, to the horizontal, and blowing into the opening.
So basically that is why RAM would not be as effective in climb as it would be in acceleration. And that is why it is possible to accelerate better than you climb.
Kapish??
-
Originally posted by F4UDOA
SC-Mutt,
I think you just said what I have been trying to say for the last several post but have been unable to articulate.
So the answer to Hitechs original question would be, what he said . Which is.....
So basically that is why RAM would not be as effective in climb as it would be in acceleration. And that is why it is possible to accelerate better than you climb.
Kapish??
I would agree if it was the Air that was being moved and the Aircraft was stationary, but in the case of flight we are moving an object through the air so I wouldn't think that it would matter what angle the Aircraft was at as long as the RAM AIR intakes are always facing the direction of motion. I wouldnt think the angle of a climb would effect the air flow too much because we are moving in that direction and the intakes are much larger than the opening of a beer bottle. However, I could be crazy. :p
-
F4UDOA: Your AOA is less in climb than in level flight for the same speed and throttle settings. There for if you wish to adjust the ram air it would be just the oposit effect you are trying to prove.
HiTech
-
HT,
How is it possible for the AOA to be less in climb than in level acceleration?
I can't even imagine how that is possible to reply to your comment:confused:
In anycase what I have gathered from the post of other contributors is that the Ram Air effect increases with speed and therefore would increase acceleration beyond the linear corresponding climb rate with every mile per hour beyond best climb speed. How much more I do not know.
-
Hi F4UDOA,
>How is it possible for the AOA to be less in climb than in level acceleration?
Extreme example: Think of a vertical climb. The G meter has to read zero in such a climb - you don't need any aerodynamic lift to counteract gravity. (What you need is thrust :-) No lift means zero angle-of-attack, at least as long as our example is based on a symmetrical airfoil.
>therefore would increase acceleration beyond the linear corresponding climb rate with every mile per hour beyond best climb speed.
Could it be that you're thinking that acceleration ability at all speeds is tied to climb rate at best climb speed? In that case, you'd indeed have uncovered a contradiction, so I'll better explain it here:
Acceleration and climb are linearly connected only if you examine both at the same speed. (And it can be quite difficult to deduce the climb rate and acceleration at any specific speed from the climb rate at speed of best climb.)
Thanks to the ram effect, the Corsair does indeed gain acceleration capability in comparison to the F6F without ram when both aircraft are flying at the same high speed.
The good news for you is that because climb is linearly tied to acceleration as long as you look at both of them at the same speed, the Corsair also gains in climbing capability! :-)
That really means that if you go fast enough, the Corsair will not only out-accelerate, but also outclimb the Hellcat! Of course, a very shallow high-speed climb is required, but there's no way the Hellcat can stay with the Corsair in such a situation.
Regards,
Henning (HoHun)
-
F4UDOA:
AOA is not the same as pitch angle of the nose. Comparing same airspeeds, in level flight you will need more AOA in order to satisfy the Lift=Weight equation than you will in a climb.
Why? In level flight you have pretty much L=W (lift and weight directly oppose each other). In a climb, 100% of the weight doesn't oppose lift therefore less lift is needed to maintain flight hence lower AOA. In constant speed climbs this difference is neglegible.
Tango, XO
412th FS Braunco Mustangs
-
Originally posted by HoHun
Hi F4UDOA,
Thanks to the ram effect, the Corsair does indeed gain acceleration capability in comparison to the F6F without ram when both aircraft are flying at the same high speed.
Henning (HoHun)
Your example about the vertical tail stand was right, but i disagree with the influence of ram effects. I already said in the other thread that ram effects will only help you when the supercharger canīt build up desired MAP.
If the supercharger does build up desired MAP even without RAM effects, it has actually a negative influence.
niklas
-
Niklas,
Could you explain that part about having a negative affect if the engine can acheive Max MAP without it??
If so why would Grumman Test Pilot Corkey Meyer say this??
The reason that the Corsair was faster in the Main stage blower was because the engine and carburator were provided with Ram Air coming in directly from the forward facing airduct in the wing, where as the Hellcat had the carburator air coming in from the accessory compartment of the fusalauge just behind the engine, with no Ram Air affect. Our airplane was getting carburator air at the same pressure as if it were motionless on the ground and the Corsair was getting supercharged air from the speed of the aircraft giving it more power (speed) in the main stage blower
Could it be that there is no supercharging used in the Neutral blower stage.
Dtango,
I'am just going to have to push the accept button and move on with this AOA thing. Unless you type really slowly I may not get it.
However just to explain my thought. I am looking at this in terms of pressure in the main supercharger stage. I don't see that the A/C is flying at a true 45 degree angle and that it is really slipping forward as it climbs and therefore looses much of it's Ram Air effect. I just don't think that WW2 fighters had the power to do that unless in Zoom climb. So what would happen is airflow across the air intake instead of directly in as in level flight reducing pressure as in a Venturi rather than "Ramming" the air. Again this is my vision and not necessarily reality. If indeed the A/C truely flew straight even when climbing I believe evn I would understand that the only variable would be speed, not AOA.
HoHun,
No I don't think that acceleration is tied to best climb speed but that they are linear at corresponding speeds. My point being that if Ram Air provides any assist at all then it provides greater assistance at moderate to higher speeds rather than lower speeds. For instance if it provides 1% greater power at 100MPH then 5% at 200MPH and 10% at 300MPH. I am making these numbers up of course. However if Ram Air provides any additional power in level flight than in climb then that advantage would be greatest at higher speeds.
-
Hi Niklas,
>Your example about the vertical tail stand was right, but i disagree with the influence of ram effects.
You're correct that it depends on the exact supercharger configuration, and it depends on the speed and altitude as well, but for the sake of the example, I just assumed ram effect was always beneficial :-)
(Another factor not usually evident from power charts is exhaust thrust, which also gives more power at higher speeds so that it's somewhat similar to ram effect in that aspect.)
Regards,
Henning (HoHun)
-
Hi F4UDOA,
>However if Ram Air provides any additional power in level flight than in climb then that advantage would be greatest at higher speeds.
Yes, exactly, that's the point :-)
Here's a quote from the FM-2 manual:
"12. CLIMB
a. DISCUSSION.-The best climbing airspeed for this airplane is about 125 knots indicated. Speeds slightly higher than this have very little effect on the rate of climb but result in better cooling. At these relatively low speeds it should be noted that the resulting reduction in Ram brings about a lower airplane critical altitude. [...]"
Regards,
Henning (HoHun)
-
F4U:
Yes, I understand what your logic is which is increased AOA leads to reduced ram-air effect.
What I'm saying is that AOA is lower (not higher) in a climb vs. in level flight. This is what HT is saying. Using your logic this would mean increased ram-air effect in a climb vs. decreased.
As for ram-air effect I'm not an expert on that topic and need to do just a little research on it to make sure I understand the impact to WW2 piston engine performance. From what I can gather though ram effect has negligible impact when you're talking about the same altitude and just minor impact when comparing different altitudes (in higher alts ram effect will lead to a slight increase in available horsepower due to lower air temperatures).
Performance differences for rate of climb / acceleration characteristics is predominantly a function of understanding and analyzing "excess power" and the major variables contributing to the relationship.
Tango, XO
412th FS Braunco Mustangs
-
Originally posted by F4UDOA
Niklas,
Could you explain that part about having a negative affect if the engine can acheive Max MAP without it??
If so why would Grumman Test Pilot Corkey Meyer say this??
[(QUOTE]
I already explained it in the other thread.
And the test pilot was wrong.
At the end the Corsair was faster because it had a better aerodynamics. It was maybe the first american design with radial engine where they didnīt make the fuselage even larger after the huge engine.
Oh btw, air inlet ducts are pointed are bit downwards, to have optimum effects at certain AoA. It is a design question, and NACA did some tests.
Anyway it wonīt help you at full MAP.
And imo F6F climbs too good in AH, and it isnt the F4U that climbs too bad.
niklas
-
Niklas, if you increase inlet pressure to a compressor, and hold outlet pressure and flow rate constant, it requires less power to run the compressor.
-
But outlet pressure isnīt constant of the supercharger. It runs with constant RPM. Today they change the AoA of the blades in the turbos, but that was not made 60years ago. You have to reduce afterwards pressure. In front of the boost limiter pressure can be quite high.
Because you have to "destroy" more energy afterwards your air gets a bit more hot, and this means you get less filling in the cylinder.
niklas
-
Niklas,
But doesn't that mean you get to acheive higher boost faster??
-
No F4UDOA, the pressure and work done are messured at different parts of the compresor system.
It would be the same as saying you should be able to run at higher pressures at lower alts by shifting the compresor gear.
The easyest way to view ram air is just to think of it as lowering your alt a few 1000 feet. The compresor is producing more MP then you can use.
-
F4UDOA, "faster", why faster?
RAM effects helps you when your supercharger canīt build up desired MAP anymore.
There exists an altitude where the compressed pressurer is equal to desired boost. Below that altitude, the supercharged air has a higher pressure - it will be limited to MAP. Above that altitude, supercharged pressure is below your desired MAP. The higher you fly over that alitutde, the lower your MAP will become, because atmospheric pressure drops quickly.
This altitude is called critical altiude afaik (Volldruckhöhe).
Now you can do simple math again to understand whats going on:
Image you have for slow flights without RAM a critical altitude of 6km for 2bar MAP. In 6km atmospheric pressure has dropped to approx. 0.5bar. This means your supercharger must have a compression ratio of 4:1.
In 2km atmospheric pressure is approx. 0.8bar, so after your supercharger you have approx. 3.2bar. This has to be limited down to 2bar.
In 7km atmospheric pressure is approx. 0.45, so you get maybe only 1.8bar. Less than full desired MAP, so your engine power is already less than full power.
Now if u assume that in a fast flight you have a RAM effect equal to, letīs say, 0.2bar, then u get 0.8*4 +0.2 = 3.4bar in 2km.
That doesnīt help you, because your boost limited wonīt allow more than 2bar in any case.
But in 7km, you get now 0.45*4 +0.2 = 2.0bar. This is equal to full MAP, and because itīs equal it is also your new critical altitude. 1km higher than for climb power in 7km (1.8bar)
Now it did some serious mistakes here, because a supercharger wonīt work like a compression "ratio", so the differences in critical altitudes maybe not 100% correct. Nevertheless you can see the most visible effects of RAM pressure: It shifts your critical altitude upwards, to higher altitudes.
Just compare the climb and speed charts in AH, the critical altitudes for speed are always higher than for climbs. And actually you can take a P51 in AH, climb until you see in ~10k feet the MAP pressure drop. Then level out and watch the MAP gauge. With more speed it will raise, now u get more MAP due to RAM effects.
Now i try to make visible the "negative" influence of RAM below critical altitude:
if u look at engine performance curves, youīll see for engines with fixed gear ratios usually a power gain with more altitude, until u reach the critical altitude. For example the ouput at sealevel of a V-1710 without RAM was afaik 1650PS, in 10k feet 1710PS. Imagine the power for climb at -1km (just imagine u can fly in hole). It would be less than at sealevel logically, following the trend downwards, letīs say, 1620PS. Now shift the performance curve upwards for a gain in critical altiude of 1km, and u get 1620PS for the engine with RAM effects at sealevel. LESS than without RAM!!!(ok, ok, very simple assumption, but i hope you understand what i mean...oh well, probably i confused you now completly:) )
niklas