# Ram Air and Its Real Effect on Performance



## DarrenW (Feb 16, 2022)

Corky Meyer famously said that the reason for the Corsair being faster than the Hellcat in neutral blower was that it used ram air and the Hellcat did not. The Corsair pilot COULD however select between DIRECT (ram air) or ALTERNATE, which allowed warm air to be taken from the engine compartment.







The Corsair flight manual states that the pilot would only select ALTERNATE in situations where carburetor icing was possible so I'm assuming that ram air was used in neutral blower during most Corsair operations. Knowing this, I examined multiple Corsair test reports to see what engine power levels were actually available in neutral blower and then compared this to what can be found in Hellcat test reports. Astonishingly, I calculated that the Corsair had on average less than 50 extra horsepower, but still managed to be on average 23 mph faster (S/L to 5,000 feet). From test data I calculated that it took roughly 20 extra horsepower to improve the maximum level speed of the Hellcat at sea level by just 1 mph. Added to this, engine charts show the R-2800-8 developing the exact same horsepower at each critical altitude with or without ram. All of this tells me that ram air had little to do with the Corsair's deciding speed advantage in main stage so it must be related mostly to aerodynamics.

My take is that the narrower fuselage cross section and more importantly the unique wing design were the primary attributes which made the Corsair faster than the Hellcat. I have read that the gull-wing had a positive effect on interference drag normally found at the wing root, another reason why Vought decided on the layout. Granted, the drag coefficients of the two were very close in NACA reports but these were calculated at 100 mph so it's my belief that the difference in overall drag between the two only grew as speeds increased. This is probably why pilots have claimed that the two could often hold cruise formation together at basically the same engine settings. The difference in speed also seems to be more apparent at lower altitudes, probably due to the denser air where airframe drag has the most effect on an aircraft's overall speed (assuming that induced drag between the two aircraft remain fairly close as altitude increases).

Agree or disagree?

I've also noticed that using ram air at various engine settings can also raise the critical altitude (as seen in the Corsair flight manuals). Could someone explain in easy terms why this is so? I'm assuming it's connected to the cooling effect ram air provides which raises manifold pressure but I don't know the exact science behind it all.

Reactions: Like Like:
1 | Like List reactions


----------



## tomo pauk (Feb 16, 2022)

DarrenW said:


> Agree or disagree?
> 
> I've also noticed that using ram air at various engine settings can also raise the critical altitude (as seen in the Corsair flight manuals). Could someone explain in easy terms why this is so? I'm assuming it's connected to the cooling effect ram air provides which raises manifold pressure but I don't know the exact science behind it all.



End users were in agreement that Corsair was faster at any altitude.
If the ram air intake was well executed (that was not always the case, eg. on Fw 190A or Spifires with ice guard and float carb), it's main benefit was that it 'elevated' the rated altitude by thousands of feet. Eg. on the Merlin Mustangs gain of 5000+ ft vs. non-ram (ie. engine is on test bench so the supercharger does all the work) can be read on the engine data sheet. Faster forward speed was also crucial - the air was rammed in the intake by a greater force than it was the case when flight was slow.
Some data sheets, especially for the German engines, also give a bit of power gain along with the gain wrt. the rated altitude. We can note that German, French and Soviet V12 engines didn't have the carburetor before the impeller, this might help a tad, too?

The gain was far less pronounced with engines that have had turbochargers to help out, talk 1000-1500 ft.

Reactions: Like Like:
1 | Agree Agree:
1 | Informative Informative:
1 | Like List reactions


----------



## DarrenW (Feb 16, 2022)

tomo pauk said:


> End users were in agreement that Corsair was faster at any altitude.


Yes I do realize this. I was only concentrating on speeds achieved using main stage blower as the Hellcat could also be aided by the ram effect while in auxiliary blower. To me ram air didn't help the Corsair much at lower altitudes, save for a possible increase in acceleration (which is debatable).

I also feel that Grumman would have included a selection of ram air in main stage just like Vought if the performance drop was indeed significant without it, which I believe wasn't the case. They most likely figured that simplifying the process for the sake of safety wouldn't effect overall performance very much, if at all.


----------



## Snowygrouch (Feb 22, 2022)

Hookers WW2 pamphlet "The performance of a supercharged aero engine" - from Rolls-Royce Heritage Trust contains pretty decent calculations on ram air and how
to determine the impact on speed from first principles. You`d have a lot more insight playing with that, than you will trying to gauge its effect from looking at the performance of two different aircraft - where its impossible to isolate the effects of any one system from the other.

Most of it is not terribly difficult and can be easily put into an EXCEL sheet, although there are a few annoying typos and missing units scattered about.

Reactions: Like Like:
1 | Agree Agree:
1 | Informative Informative:
2 | Like List reactions


----------



## DarrenW (Feb 22, 2022)

Snowygrouch said:


> Hookers WW2 pamphlet "The performance of a supercharged aero engine" - from Rolls-Royce Heritage Trust contains pretty decent calculations on ram air and how
> to determine the impact on speed from first principles. You`d have a lot more insight playing with that, than you will trying to gauge its effect from looking at the performance of two different aircraft - where its impossible to isolate the effects of any one system from the other.
> 
> Most of it is not terribly difficult and can be easily put into an EXCEL sheet, although there are a few annoying typos and missing units scattered about.


Thanks Calum I was actually hoping to hear from you at one point.


----------



## Snowygrouch (Feb 22, 2022)

DarrenW said:


> Thanks Calum I was actually hoping to hear from you at one point.


Being old, its also in "proper units" for our American friends too....

Reactions: Like Like:
1 | Funny Funny:
3 | Winner Winner:
1 | Like List reactions


----------



## Metrallaroja (Jun 30, 2022)

The biggest impact on performance of ram that I ever saw is in F8F-1, F7F-3/1 and AU-1, the three use the same light weight low alt R2800.
An impact in the crit alt of 6000 feet at 420mph, this allows this aircrafts to have 2800HP at SL and 2350HP up to 12000 feet at high speed.
From AEL-999 Combat Power Endurance Test of Pratt & Whitney Model R-2800-34W Engine.


----------



## Metrallaroja (Jul 3, 2022)

Metrallaroja said:


> The biggest impact on performance of ram that I ever saw is in F8F-1, F7F-3/1 and AU-1, the three use the same light weight low alt R2800.
> An impact in the crit alt of 6000 feet at 420mph, this allows this aircrafts to have 2800HP at SL and 2350HP up to 12000 feet at high speed.
> From AEL-999 Combat Power Endurance Test of Pratt & Whitney Model R-2800-34W Engine.
> View attachment 675649
> View attachment 675650



N
 Neil Stirling
I think you will quite like this, I saw you investigated F8F-1 power in another thread.
And yes you were right, it was RAM effect.

Reactions: Like Like:
1 | Like List reactions


----------



## Neil Stirling (Jul 3, 2022)

Thank you.

Reactions: Friendly Friendly:
1 | Like List reactions


----------

