Why was the F4u faster than the F6f?

[hitech]

Hi HoHun

Ram effect is beneficial above full throttle height only.

Have been rolling this statement over in my head. And debating the accuracy.

I agree ram air is more benificial above critical alt. Just wondering if the higher inlet pressure do to ram air (I.E. inlet to Super charger) has the effect of lowering torque requirements of the super charger and hence more brake horse power?


HiTech

[HoHun]

Hi Hitech,

>Just wondering if the higher inlet pressure do to ram air (I.E. inlet to Super charger) has the effect of lowering torque requirements of the super charger and hence more brake horse power?

I have to admit that I'm not fully comfortable either with any of the explanations I have read for the drop of power with incrased supercharger intake pressure below full throttle height.

I tend to believe it's all accounted for in the volumetric efficiency value, but that's more of a black box approach.

However, looking at the power over altitude engine curves, it's clear that mechanically-driven superchargers of the centrifugal type lose power with increasing intake pressure below full throttle height.

The ram effect yields a slightly different temperature/pressure combination than just decreasing altitude, but the overall effect should be very close.

How the force/torque balance changes for the individual components of the system, I'm not certain.

The relation I postulated is not entirely universal, either, as the Jumo 213 (for example) contradicts it by the use of the spin throttle and the constant charge mass control (instead of the usual constant boost control).

Regards,

Henning (HoHun)

[hitech]

Hi HoHun,

The ram effect yields a slightly different temperature/pressure combination than just decreasing altitude, but the overall effect should be very close.

This is very close to how we model it.

I have pretty much concluded that the loss in power is do to increase in temp.  more than the pressure effiency gained by the charger.


HiTech

[joeblogs]

HoHun

Originally posted by HoHun
Hi Joe,

Have you ever seen a diagram of the supercharger intake arrangement? I'm not quite certain on how the description translates into a system.  

The diagrams are in White's new book called R2800.

Originally posted by HoHun

>This anomally explains why a Corsair handily outperforms a Hellcat at lower altitudes. The Corsair's main stage was fed ram air and thus gained a significant amount of power.

Well, I don't believe this is a correct intepretation. Below full throttle height, you actually lose power to ram effect because the throttle has to be closed further to avoid overboosting, deteriorating volumentric efficiency.

At 12k, you are at full throttle height for the first stage of the supercharger. remember the 2nd stage is not engaged. If the pilot engages the second stage, he MUST throttle back or he will overboost the engine.

Originally posted by HoHun


The reason for the Corsair's superior speed is its smaller frontal area (there's little to chose between the wings), made possible by embedding the air intakes into the wing root. As mentioned above, the gull-wing arrangement helps to reduce fuselage-wing interference drag, too..

We are all familiar with the other arguments that explain the disparity in Vmax between these two planes. White's explanation suggests the disparity should be greater at lower altitudes. That should be pretty easy to check, but I am afraid I can't do it right now.


-Blogs

[joeblogs]

F4uDOA

Greetings -

I agree with most of your points, but I'm a bit confused about your horsepower numbers.

The r2800-8W & r2800-10W are B series engines. Even with 100/125 PN gas, these are rated for no more than 2100 HP with ADI. Now the r2800-18W is a C series which will reach 2,300 HP with ADI.

So it sounds like you are comparing the output of a C series engine on very late model Corsairs to the F6f, which never had such an engine.

-Blogs

Originally posted by F4UDOA
...However the big difference at low altitude was the RAM air capability of the F4U. In fact the later flight manuals of the F4U and F6F despite having the same engine rate the F4U-1 R2800-8W with 2300HP at sea level while the F6F R2800-10W remains at 2250HP rating no doubt due to RAM air.

Also the later F4U-1's were cleared for 2800RPM on takeoff.

[joeblogs]

Hitech:

You know how to reach him or Kimball McCutcheon. Why not ask them while you can?

-Blogs

Originally posted by hitech
Hi HoHun,

 

This is very close to how we model it.

I have pretty much concluded that the loss in power is do to increase in temp.  more than the pressure effiency gained by the charger.


HiTech

[joeblogs]

I added a note to your thread on turbosupercharging on the P47.

White documents the evolution of engine changes within and across the plane model numbers. So it is possible to trace when the GE turbos change and what impact there is on performance.

-Blogs

[Pongo]

I thought it was slower because the pitot tube location reported speed differently, and they are really close in speed.
Might just be nonsense I read here.

[bozon]

I'm not sure about the larger frontal area being the F6F's problem.

Large frontal area does not automatically mean more drag. Radials have suprisingly low drag compared with the pointed nosed liquid cooled inlines. What liquid cooling saves in a streamlined shape it looses to the need to stick a radiator into the wind. The P51's low drag is said to be due to a good design of the radiator.

This cooling drag is extremely dependent on the specific design on the intake the tunnels and the exit. I'm not an expert but I belive this drag is not a simple function of airspeed since higher speed -> higher engine output -> more air mixing needed for heat transport -> more drag depending on design.
Cowl flaps and radiator doors can be adjusted for the cooling needed and they affect the drag.

Bozon

[gwshaw]

Originally posted by joeblogs
F4uDOA

Greetings -

I agree with most of your points, but I'm a bit confused about your horsepower numbers.

The r2800-8W & r2800-10W are B series engines. Even with 100/125 PN gas, these are rated for no more than 2100 HP with ADI. Now the r2800-18W is a C series which will reach 2,300 HP with ADI.

So it sounds like you are comparing the output of a C series engine on very late model Corsairs to the F6f, which never had such an engine.

-Blogs

The -8W and -10W are generally credited with anywhere from 2130 to  2250 hp WEP @ 56-60 in Hg and 2700 rpm in neutral blower. The -18W running at the same map but 2800 rpm would do about 2375 hp, increase map to 65 in Hg would put power about 2450-2550 hp. Which matches the figures I have for -18W map fairly well.

Welcome to the wonderful world of trying to figure out exactly how much power the R-2800 was capable of, I have at least 3 different official figures for the -8/8W. They are all 2250 hp, but at anywhere from 57 in Hg to 60 in Hg.

As near as I can tell, all B-series R-2800 were 2000 hp @ 52 in Hg SL & 2700 rpm core engines. From there they varied according to whether they had a 2nd gear for the integral blower or what aux blower setup they had. IE the 2-speed aux blower setup in the -8/-10 or the exhaust driven supercharger in the P-47.

My guess is that the -8 was initially 52 in Hg 2000 hp mil, then was first allowed 56 in Hg for about 2150 hp. Then bumped to 58 in Hg and about 2230 hp, finally to 60 in Hg and approx 2250-2300 hp. While the -10W was cleared for 60 in Hg and about 2250-2300 hp initially. The lack of compression heating of the charge for the -10W could explain why it was allowed higher map than the -8W at first.

Besides which, the 360 mph at SL range for the F4U-1 is much more believable at 2250-2300 hp than it is at 2100 hp.

I need to pick up White's new book, but I take all his stuff with some reservations. I have found too many minor errors and inconsistencies in his other WWII Allied Piston Aircraft Engines book to trust him fully. I would really like to see a Whitney-esque treatment of the R-2800 like his opus on the V-1710.

Greg Shaw

[pasoleati]

White´s second book is much better than his first one but not on par with Whitney´s effort. One of White´s problems is his use of sources. E.g. his descriptions of individual aircraft type development is often taken from very dubious sources like Squadron/Signal in actions. Another problem is that he gives the power rating, but the tables don´t have any boost info.

[F4UDOA]

JoeB/GWShaw,

I haven't been on the boards in a while so this conversation is refreshing.

Here is my source. It is the 15 October 1945 POH for the F4U-1D. It is revised with updates until 1 December 1952 and it is very thorough with a full performance diagrams. It shows the R2800B block producing 2300HP at sealevel at 57.5"MAP, 2100HP at 58.5"MAP at 14500FT and 1925HP at 19,000FT all at WEP.

Both lower ratings are at 2700RPM and the High blower is at 2550RPM.

Also there is a column for with RAM and NO RAM that shows no difference for HP rating only in critical altitude.

I would show you the scan but my PC has been down for months and all my scans are trapped in a lifeless hunk of junk. If I can retrieve the doc I will post it ASAP.

[stantond]

I was looking at my copy of the AN 01-45HB-1 Pilots Handbook for the F4U-4, revised 15 December 1944 and noticed on the last page, which has a engine calibration curve, there is a statement that says "no allowance has been made for ram".  So, what exactly is ram?  Is that air taken in by the chin scoop on the F4U-4?  


Regards,

Malta

[gwshaw]

Originally posted by stantond
I was looking at my copy of the AN 01-45HB-1 Pilots Handbook for the F4U-4, revised 15 December 1944 and noticed on the last page, which has a engine calibration curve, there is a statement that says "no allowance has been made for ram".  So, what exactly is ram?  Is that air taken in by the chin scoop on the F4U-4?  


Regards,

Malta

Yes, ram is the pressure increase created by the forward movement of the aircraft. The faster you go.... the faster you go. It is the reason maximum climb performance is typically a bit above the rated altitude of the engine, and max speed performance is even more above engines rated altitude.

Greg Shaw

[stantond]

Thanks Greg, that's what I though.  

But, where is the ram air taken in for the F4U-1 models?  They have no chin scoop like the -4.  I don't think the oil coolers in the wing roots would work very well for carburettor air.  Would part of the engine cooling air be used?  If that is the case I see no reason why the F6F or P47 could not have done the same.  


Regards,

Malta