P-51 crit Mach-figure?

[wuzak]

The picture of the spitfire that actually did mach 0.9 (or there abouts) is not good viewing. View attachment 541921

Several test flights were undertaken to 0.9M, this was the result of one of them.

There were successful flights to 0.9M and some where other failures occurred, such as when the supercharger blew up. In the above picture it was a reduction gear housing failure which led to the propeller parting ways with the fuselage, after which the aircraft experienced some violent manoeuvres.

 

[wuzak]

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)?

Biff, while the wing profile and the radiator installation were big factors, overall it was a combination of many factors that saw the P-51 have superior speed by 20-40mph than the Spitfire IX.

An example is that the 20mm cannon barrels protruding from the leading edge of the Spitfire's wing cost as much as 8mph.

Fit and finish was certainly a factor, as was the cockpit canopy (Spitfire's was steeper).

The Spitfire's radiator(s) could have worked better, even in the historical location under the wings (Supermarine did propose an under fuselage radiator).

 

[pbehn]

Several test flights were undertaken to 0.9M, this was the result of one of them.

There were successful flights to 0.9M and some where other failures occurred, such as when the supercharger blew up. In the above picture it was a reduction gear housing failure which led to the propeller parting ways with the fuselage, after which the aircraft experienced some violent manoeuvres.

Some accounts say in addition to losing the prop the wings were distorted backwards. My point was that the discussion is mainly a theoretical one about air flow. When you actually test it physically all sorts of things happen because the air frame is beyond what it was designed for. It may pop some rivets, or bend something or something fall off. To get a plane to go to M0.9 and guarantee being able to land with no damage would need a complete re design of almost everything.

 

[tomo pauk]

Biff, while the wing profile and the radiator installation were big factors, overall it was a combination of many factors that saw the P-51 have superior speed by 20-40mph than the Spitfire IX.

An example is that the 20mm cannon barrels protruding from the leading edge of the Spitfire's wing cost as much as 8mph.

Fit and finish was certainly a factor, as was the cockpit canopy (Spitfire's was steeper).

The Spitfire's radiator(s) could have worked better, even in the historical location under the wings (Supermarine did propose an under fuselage radiator).

We can also recall that wartime Spitfires were mostly with uncovered main wheels wells, plus there is rear view mirror to steal a few mph.

 

[DarrenW]

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.

At first glance one wouldn't believe that the FW 190A had less parasitic drag than the Spitfire, giving it less total drag with both having roughly the same induced drag. Greg (not GregP but the one on YouTube) however performed calculations which proved that this was indeed the case.

 

[drgondog]

At first glance one wouldn't believe that the FW 190A had less parasitic drag than the Spitfire, giving it less total drag with both having roughly the same induced drag. Greg (not GregP but the one on YouTube) however performed calculations which proved that this was indeed the case.

I listened to the entire presentation. Several of Greg's assumptions were well reasoned but collectively made his conclusion subject to questions.

Further, he didn't quite grasp what Lednicer's methodology of CFD yielded with respect his drag analysis. Nor was it particularly elegant to make assumptions about Exhaust Gas Thrust -

but I can't disagree his conclusions that the FW 190A had less total drag than the Spit IX at medium to high speeds where parasite drag dominates.

The major difference maker was the Cooling System drag arrangement of the Spit IX attempting a Meridith System design to reduce the drag of the inlet and arrangement of the oil/glycol radiators.

The Wing of the Spit IX (save the cooling system and 'open' wheel well) should have less profile drag due to thinner wing as well as significantly less Induced Drag. I suspect that the Spit IX may have less total drag at speeds up to optimal cruising speed for distance, but because the oswald efficiency factor should be much higher for elliptical wing Spit, the Induced drag must be much lower for the Spit - But Drag total = Drag (parasite , profile) + Drag (induced). For given speeds equal to each other, this alone strongly favors the Fw 190A as having less Parasite/profile drag if their total drag values are nearly equal for a given speed and altitude.


Max L/D= CL/CD, is at the Velocity for which CDtot=CDi. The drag polars should have notable difference for minimum velocity required for level flight (lower for Spit)

 

[DarrenW]

I listened to the entire presentation. Several of Greg's assumptions were well reasoned but collectively made his conclusion subject to questions.

Further, he didn't quite grasp what Lednicer's methodology of CFD yielded with respect his drag analysis. Nor was it particularly elegant to make assumptions about Exhaust Gas Thrust -

but I can't disagree his conclusions that the FW 190A had less total drag than the Spit IX at medium to high speeds where parasite drag dominates.

The major difference maker was the Cooling System drag arrangement of the Spit IX attempting a Meridith System design to reduce the drag of the inlet and arrangement of the oil/glycol radiators.

The Wing of the Spit IX (save the cooling system and 'open' wheel well) should have less profile drag due to thinner wing as well as significantly less Induced Drag. I suspect that the Spit IX may have less total drag at speeds up to optimal cruising speed for distance, but because the oswald efficiency factor should be much higher for elliptical wing Spit, the Induced drag must be much lower for the Spit - But Drag total = Drag (parasite , profile) + Drag (induced). For given speeds equal to each other, this alone strongly favors the Fw 190A as having less Parasite/profile drag if their total drag values are nearly equal for a given speed and altitude.


Max L/D= CL/CD, is at the Velocity for which CDtot=CDi. The drag polars should have notable difference for minimum velocity required for level flight (lower for Spit)

Thanks Bill for chiming on on this. I knew you could help dissect this further and give credence for or against Greg's hypothesis.

 

[Snautzer01]

 

[Snautzer01]

 

[drgondog]

View attachment 657234

I had Hoerner's Fluid Dynamic Drag as one of my Aero Textbooks in undergrad. Still have the book.

Clark Millikan PhD at CalTech (along with Von Karman) had more influence in the development of assumptions and calculations for Performance Estimates at NAA, largely because of his 'disciple' Ed Horkey who was hired right out of CalTech..

I still occasionally draw on both Hoerner and Millikan's "Aerodynamics of the Airplane' when I am pondering what I have forgotten over 55 years.

Millikan was a key advisor on Arnold's technology Board and frequently served as a conduit to Arnold.

 

[Snautzer01]

Both are pdf so if anybody has use for it ? Wurger please advise if it is ok?. P-51 is from 1945 ( should be good Hoerner 1965 i am not sure about.

 

[Wurger]

Snautzer01 both pics work. I can see them if you meant that.

 

[Snautzer01]

Snautzer01 both pics work. I can see them if you meant that.

No full books as pdf i can put up.

 

[Wurger]

Due to the possible copyrights I wouldn't post them here.

 

[drgondog]

Both are pdf so if anybody has use for it ? Wurger please advise if it is ok?. P-51 is from 1945 ( should be good Hoerner 1965 i am not sure about.

The NACA report should be reproduceable but credited.

 

[tomo pauk]

That NACA report for the P-51B is here: link

 

[msxyz]

An interesting question regarding the reports mentioned in this thread. Were the speeds measured with 'modern' probes? The instruments available at the time gave incorrect readings once the speed climbed above 500mph/800kmh; a typical example was the diving speed of many Italians planes that the tester pilots enthusiastically reported. Ing. Castoldi (of Macchi fame) was one of the first that desumed that the probes of the time ceased to report accurate airspeeds above Mach 0.75-0.8 (hence the 'true' airspeed was somewhat lower than what the instrument would show)