P-51 crit Mach-figure?

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tomo pauk said:
In one corner we have recollections of a company's test pilot, and we can accept them in good faith. However, in another corner we have test results and official data sheets issued by two main users - USN and RN/FAA. Whenever a simmilar situation arises, I tend to believe the 1st hand data, here it will be the papers issued by USN and the British. Those papers do show 20 mph (give or take) speed advantage for Corsair at all altitudes.
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That certainly makes sense. I'm certainly no aerodynamics expert but just eyeballing the two planes it's hard to imagine speed parity between them with the same power.
The corsair, at least in appearance, is much more streamlined.
 
While air is not a perfect gas, it is quite close; speed of sound in air is predominantly affected by temperature: speed of sound (in meters per second) is 20.05 times the square root of the temperature (in kelvin). It is affected by humidity (mostly as water vapor has a lower molecular weight than most of the other gases that make up air); the affect of pressure is probably measurable only with really good equipment at the pressures WW2 aircraft could operate.

In warmer climates, the P-38 would be operating at lower Mach numbers.
 
critical mach spit mustang.png
 
llemon said:
View attachment 541632
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This is a good representation of the individual CD vs Mach, but one must also take into account that the Spit IX CDo was about .025 to Mustang .019 at the same low RN speed of 100mph... Additionally, the CD to RN slope is steeper for the Mustang - which is why it was 20-40mph faster at all altitudes with the same engine, including up in the 0.65M range The above plot represents the CDm/CD drag factor to account for compressibility as shown below.

The equation kinda looks like CD = (CDp + CDcl +CDcool) CDm/CD + CDind. The stuff in between the ( ) is much lower for the Mustang, the Induced Drag CDind is lower for the Spit. At high speed CDcool =0 for the Mustang and varies in level flight for the Spit at ~ 0.0040.

So, one needs to take the Entire drag relationships into context.
 
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drgondog said:
This is a good representation of the individual CD vs Mach, but one must also take into account that the Spit IX CDo was about .025 to Mustang .019 at the same low RN speed of 100mph... Additionally, the CD to RN slope is steeper for the Mustang - which is why it was 20-40mph faster at all altitudes with the same engine, including up in the 0.65M range The above plot represents the CDm/CD drag factor to account for compressibility as shown below.

The equation kinda looks like CD = (CDp + CDcl +CDcool) CDm/CD + CDind. The stuff in between the ( ) is much lower for the Mustang, the Induced Drag CDind is lower for the Spit. At high speed CDcool =0 for the Mustang and varies in level flight for the Spit at ~ 0.0040.

So, one needs to take the Entire drag relationships into context.
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Bill,

I'm trying to gonkulate your reply into layman / Biff terms. The Mustang had lower drag at "lower" Mach numbers but the drag rise eventually becomes steeper at higher Mach than the Spit? Or, why did the Mustang have a lower Mach crit than the Spit?

The Mustang is lauded as having one of the or best "mileage" numbers of WW2 piston fighters. With that in mind was it mostly contributed to the wing and the radiator "scoop" / housing being so efficient or is it more a combination of details large and small (wing, radiator installation, fit and finish)?

Cheers,
Biff
 
drgondog said:
This is a good representation of the individual CD vs Mach, but one must also take into account that the Spit IX CDo was about .025 to Mustang .019 at the same low RN speed of 100mph... Additionally, the CD to RN slope is steeper for the Mustang - which is why it was 20-40mph faster at all altitudes with the same engine, including up in the 0.65M range The above plot represents the CDm/CD drag factor to account for compressibility as shown below.

The equation kinda looks like CD = (CDp + CDcl +CDcool) CDm/CD + CDind. The stuff in between the ( ) is much lower for the Mustang, the Induced Drag CDind is lower for the Spit. At high speed CDcool =0 for the Mustang and varies in level flight for the Spit at ~ 0.0040.

So, one needs to take the Entire drag relationships into context.
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Geez Bill...

My head hurts now...
 
BiffF15 said:
Bill,

I'm trying to gonkulate your reply into layman / Biff terms. The Mustang had lower drag at "lower" Mach numbers but the drag rise eventually becomes steeper at higher Mach than the Spit? Or, why did the Mustang have a lower Mach crit than the Spit?

The Mustang is lauded as having one of the or best "mileage" numbers of WW2 piston fighters. With that in mind was it mostly contributed to the wing and the radiator "scoop" / housing being so efficient or is it more a combination of details large and small (wing, radiator installation, fit and finish)?

Cheers,
Biff
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Hi Biff - The primary contribution to onset drag rise leading to Mcr is the wing for both airplanes. The Spitfire delayed Mc due to thinner wing, the Mustang delayed Mcr due to the Low Drag/High Speed profile even though the wing T/C > Spit. The actual increase in the Ration of CDm/CD (incompressible flow) favored the Spit as shown in the plot above. That said, the Parasite Drag of the Mustang a.) started at a much lower value than the Spit for the same Reynolds Number, and b.) as a result would cross over around 0.83-.85M where the Mustang was in dangerous territory due to Q loads. Recall that the Mustang did Not fail in tests up to .85 Mach but suffered significant damage to flap and some rivet popping/skin buckling. At .9M the Spit lost its prop and suffered major damage to cowl for a dead stick landing.

The Plot in context is strictly the RATIO of Drag Rise over Incompressible Speed range to account for incompressibility. The Rate of CD increase was greater for the Mustang but the cumulative Total Drag was lower than the Spit until the 0.8-0.85M envelope.
 
I have discovered I am left brained in logic and organization but right brained in art and possibly language. I have always envied those who could look at an equation and come up with the answer in their head. An example was my last roomie at USAFA. The graded reviews lasted 4 hours and after returning, having only answered about 2/3rds , my roomie is resting on his bunk, staring at the ceiling. I asked," How did you do?" He answered, 'You remember question 38? I think I put the decimal point in the wrong place."
 
special ed said:
This kind of math is exactly why I don't have a degree today.
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I was always told by my maths teacher that maths is a language, a beautiful language. To me it is a second language so I have to translate the expressions back to English. If you transpose the mathematical expressions for an English description you get a beautiful and elegant explanation but without the precision or absolute truth that mathematics has. To me the discussion based on the graph is misleading to laymen like me, it took me a long time to realise that it is largely theoretical, but discussions in mathematics always tend towards limits, calculus is used to find and define limits. Mach 0.7 is 532 MPH at sea level, below that the P-51 held all the aces not only for the wings but also cooling drag, drag on the fuselage/cockpit and actual fit finish of the non theoretical plane in service. Not shown on the graph, which is about the limits of speed, faster even than the speed in most combats, is that at any given speed the P-51 used less fuel, and for any cruise setting it went faster and therefore further for a given amount of fuel. That is the area of the graph below M0.6
 
michael rauls said:
That certainly makes sense. I'm certainly no aerodynamics expert but just eyeballing the two planes it's hard to imagine speed parity between them with the same power.
The corsair, at least in appearance, is much more streamlined.
Click to expand...
The F4U-1 in 'as delivered/unsealed' condition to Langley for full scale wind tunnel testing had a CDo of 0.0284, the F6F-3 had a CDo of 0.0293 (Compared to P-51B at 0.020. Notably the Mustang CDo is overstated due to zero Meredith effect of the exiting heated radiator heat in a contracting plenum.

For the purposes of the Plot above of P-51 CDo vs Spitfire, if you peek back in the .1 to .5M range the CDo (scale model wind tunnel results) of the Mustang is 0.017 to the Spit value of ~0.023
 
drgondog said:
The F4U-1 in 'as delivered/unsealed' condition to Langley for full scale wind tunnel testing had a CDo of 0.0284, the F6F-3 had a CDo of 0.0293 (Compared to P-51B at 0.020. Notably the Mustang CDo is overstated due to zero Meredith effect of the exiting heated radiator heat in a contracting plenum.

For the purposes of the Plot above of P-51 CDo vs Spitfire, if you peek back in the .1 to .5M range the CDo (scale model wind tunnel results) of the Mustang is 0.017 to the Spit value of ~0.023
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See... there you go again...


ASPIRIN... STAT!


Wait! Some dim light clicks on in my noggin... Mustang slippery, Spit, not AS slippery, Mustang use less fuel, go farther faster with same fuel, maybe even able to go faster with same power... grunt... Peter begin to understand... also forget use pronouns...
 
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