Dive Bombing: USN vs USAAF/USAF

[KJ_Lesnick]

Wuzak

I suppose you could compare with other designs

I suppose that makes enough sense.  Admittedly if I knew the airplane's critical altitude was higher than the stated limit of the engine, that would indirectly give me ram I suppose.

I originally said to do this to see how drag varied with altitude.

So the idea would be to determine what level of supercharging, cooling, and aerodynamics are needed, correct?

Interesting that the drag is higher at 8,250ft than 7,400ft.

I noticed that also...

I'd suggest, though, that the power isn't the same at both altitudes.

Seems logical, but the manual seems to indicate low-blower for both and the same power: It could be wrong of course.  The only things I can think of that would change this would be the following

Trim drag: Usually that goes down with speed, not up

Mach number: Goes down with altitude -- doesn't seem like it would be very significant from 7400 to 8250 feet

Cowl-flap position: I didn't check on that but if I recall cowl-flap positions are changed at various points in the flight.  I don't know why you'd do it with the same power setting at 850 feet of altitude change at the same power setting

[wuzak]

Mach number goes up with altitude, as the speed of sound reduces with altitude.

[KJ_Lesnick]

Mistyped...

[jcf]

No, Mach number stays the same regardless of altitude, the actual airspeed that is equivalent to Mach, 1, 2 etc
varies by altitude.

[KJ_Lesnick]

Good catch

[wuzak]

No, Mach number stays the same regardless of altitude, the actual airspeed that is equivalent to Mach, 1, 2 etc
varies by altitude.

What I meant was that for a given air speed the Mach number increases with altitude.

[KJ_Lesnick]

Wuzak

Some of the performance testing I checked were very well written (particularly the F4U stuff), and I've been careful to only look at documents that involve tests involving a non-modified airplane (i.e. combat trim).

That being said some of the documents aren't terribly readable.



Do you think trying to use photoshop to to create a new graph and overlay everything to make it readable would work?

[rickshaw]

I'd recommend recreating the graph in Excel or something similar.

[KJ_Lesnick]

rickshaw

Just to be clear the data on the left is manifold pressure, the center is BHP, and the one all the way to the right is the speed right?

[wuzak]

rickshaw

Just to be clear the data on the left is manifold pressure, the center is BHP, and the one all the way to the right is the speed right?

Yes.

[KJ_Lesnick]

wuzak

Yes

Thank you: I've actually been compiling a list of k-constants (By aircraft category: Fighter/Bomber; by aircraft-type: F4U, P-51) based on the data on the http://wwiiaircraftperformance.org page, and am planning on adding other figures like power-loadings across the altitude range as there's probably some useful research material in all this.

I've noted a couple of interesting things

• The US Navy tended to produce way better performance graphs than the US Army Air Force
  
• One has to carefully read through the data because in some cases they didn't use a stock aircraft: They put a different propeller on, used different cowl-settings, and I'm not sure how much of this was used on production aircraft, or just for experiments
  
• The RAF seems to not always list horsepower figures: They list boost pressure (which I can convert to MAP), and RPM.
  

It's quite fascinating to read through all of this.

I'm curious about two things

• Is there any ability to gauge HP from boost pressure or RPM if you know the engine type?  I'm sifting through the British designs and some only list RPM and boost pressure
  
• While I know the F4U and F6F had twin-stage, twin-speed superchargers with inter-cooling: Did the F7F have inter-cooling?
  

[wuzak]

Is there any ability to gauge HP from boost pressure or RPM if you know the engine type?  I'm sifting through the British designs and some only list RPM and boost pressure

You will need the engine chart.

Such as this

http://www.spitfireperformance.com/griffonhp_b.jpg

[KJ_Lesnick]

Wuzak

Thank you very much, I assume there are tables for the Merlins as well?

[wuzak]

Wuzak

Thank you very much, I assume there are tables for the Merlins as well?

Yes.

[tomo pauk]

...
While I know the F4U and F6F had twin-stage, twin-speed superchargers with inter-cooling: Did the F7F have inter-cooling?

Both F7F an F8F used single-stage supercharged engines, without intercoolers. All of those four USN fighters mentioned used water-alcohol injection, the Hellcat and Corsair starting from early 1944 on.