Mustang XP-51F/G/J prototypes

[drgondog]

Hi Bill,



Are we positive that Chilton talked about top level speed and a true air speed figure?

If he was actually talking about 491 mph indicated air speed in a dive, that would be in line with a dive into the region of the P-51D manual's Mach limit, or even beyond it. Validating the diving capabilities of the XP-51F probably would have made good sense too, considering how much of a headache compressiblity could be at the time.

Just my speculative thoughts based on the absence of actual source information!

Regards,

Henning (HoHun)

No, HoHun - a fair question. That said, 0.75M was the Placard limit. The fundamental question was the altitude at which the 491mph was recorded. For 491mph at 0,75M, the M=1 is at ~ 655 mph TAS. The altitude temp in Rankin would be in the 388-390 degrees Rankin around 33,000 feet and above.

Further complicating the question is that the ram delta over bench HP for the 1650-3 was 4500 feet, in the 29,000 feet range, to deliver 1200-1250 Hp @61".

That said, the XP-51F was only very slightly less than P-51D in Parasite/profile drag.

I need to ponder this more. The Absolute dive speed limit however was 0.8M but I'm sure the 491mph is not for attained speed in a dive. The NA-73 was capable of that.

 

[HoHun]

Hi Bill,

No, HoHun - a fair question. That said, 0.75M was the Placard limit. The fundamental question was the altitude at which the 491mph was recorded. For 491mph at 0,75M, the M=1 is at ~ 655 mph TAS. The altitude temp in Rankin would be in the 388-390 degrees Rankin around 33,000 feet and above.

Agreed, and we'd probably also need to know a lot of the details of the airspeed indication system to convert it to an accurate Mach number.

I was merely going by the attached page from the P-51 pilot's handbook, which shows that the maximum allowable speed expressed in IAS exceeds 491 mph only at very low altitudes, where in the interest of safety you'd probably want to pull out.

Thus my conclusion that Chilton's figure would be consistent with a very fast dive to the edge of the envelope.

That might not have been anything new for the Mustang airframe, but with the XP-51F being the product of some significant redesign, I imagine it would still have been good news that the changes did not cause unforeseen trouble at high Mach numbers, and particularly, that it would not be necessary to include dive recovery flaps with their added weight and complexity to a light-weight fighter design. These definitely were on the menu at the time, and fitted to the P-38 and P-47. So I'd guess there might have been a good reason to validate this particular data point.

Regards,

Henning (HoHun)

 

[drgondog]

Hi Bill,



Agreed, and we'd probably also need to know a lot of the details of the airspeed indication system to convert it to an accurate Mach number.

I was merely going by the attached page from the P-51 pilot's handbook, which shows that the maximum allowable speed expressed in IAS exceeds 491 mph only at very low altitudes, where in the interest of safety you'd probably want to pull out.

Thus my conclusion that Chilton's figure would be consistent with a very fast dive to the edge of the envelope.

That might not have been anything new for the Mustang airframe, but with the XP-51F being the product of some significant redesign, I imagine it would still have been good news that the changes did not cause unforeseen trouble at high Mach numbers, and particularly, that it would not be necessary to include dive recovery flaps with their added weight and complexity to a light-weight fighter design. These definitely were on the menu at the time, and fitted to the P-38 and P-47. So I'd guess there might have been a good reason to validate this particular data point.

Regards,

Henning (HoHun)

I'll do a HP required analysis using P-51H drag values for parasite drag - same wing, approx. same meredith effect benefits - over the next couple of days. I'll also start with M=.75 for compressibility multiplier over wind tunnel data.

Probably have to iterate a few passes by guessing altitude with first start at 29K.

 

[Scott Kenny]

Following for interest, I love the bubble top Mustangs...

 

[drgondog]

HoHun - for 6200#GW, 491mph at 22000 feet, I get 1998Hp Required.
Assumptions - P-51H Base Drag of CDP1 =0.01431 @RN=9.0 x10^6, plus Delta 0.00105 for gun ports, airspeed mast, surfrace roughness and leaks. Zero Cooling Drag (internal pressure drag plus friction), due to high speed cruise in level flight.

Wing same between XP-51F/G/J and P-51H but P-51H has about 5% more wetted area so the friction drag should be conservative in the example.

@491mph, c=701mph so @491mph I have M=.699; 1/(1-M^2) --->1.39 compressibility factor applied tp Parasite drag
@491 at 22,000 feet RN2=17.02x10^6 and CDP2 @22000ft for RN2, = 0.9323CDP1. CDP2= 0.01334

CL = 0.086; for CDi ---> (CL)^2/(PixAR); CDi = .0004;

CDtotal = {CDP2+DeltaCD}*Compressibility factor + CDi

CDtotal =[0.01334+0.00105]*1.39 + 0.0004 = .02000+0.0004 = 0.0204

HP = W*(CD/CL)*V/375 = 1925HP

Doable with 14 S.M.,but not 1650-3.

From Gruenhagen - "Dive limit of 475mph imposed, then reduced to 420 due to yaw instability"; 'Top speed 466mph at 29000 feet"

I'llrun 466 at 29K and deliver HP req'd next

 

[drgondog]

for 466mph @29000 feet, GW=7265.
RN2 = 13.276x10^6; CDP2= 0.01376; delta CDP =0.00105 (same); CDi = 0. 0148; CDtotal = 0.02029; CDt/CL=0.1327

HPreq=W(CDt/CL)V/375
HP req=7265(0.1327)466/375 =1198.4
Believeable.for XP-51F and conformal to flight test results.

PS the figures on the 491mph (earlier post) are slighly off because I used Aspect Ratio of 5.89 for P-51D vs 5.82 for H (and XP-51F) by mistake - that will affect the RN2 value which will be very slightly lower.

The corrected HPreq=1935 for 491 at 29000 feet.

 

[HoHun]

Hi Bill,

HP req=7265(0.1327)466/375 =1198.4
Believeable.for XP-51F and conformal to flight test results.

Sounds perfectly sensible, and also matches the (estimated) performance chart from Mike's site:

http://www.wwiiaircraftperformance.org/mustang/xp-51f-chart.jpg

From Gruenhagen - "Dive limit of 475mph imposed, then reduced to 420 due to yaw instability"

Hm, that throws a spanner in the works of my "491 mph must be a dive speed" line of thought.

Not sure what that figure could be then, if we rule out a simple typo.

Regards,

Henning (HoHun)

 

[BarnOwlLover2]

That's indicated air speed. Taking into compressibility the actual limit would be significantly higher.

 

[GregoryUSSR]

How different sometimes is the perception of the same thing by Americans (or the Pentagon) and the rest of the world. I know that for Americans, the P-51 is almost the best fighter of World War 2... Somewhat in the shadow of the Mustang is the P-47 Thunderbolt. We considered them so-so planes. Average level. These are good interceptors, but medium fighters. A really cool fighter was the P-39 Airacobra. Of course, he was greatly respected.

 

[drgondog]

Hi Bill,



Sounds perfectly sensible, and also matches the (estimated) performance chart from Mike's site:

http://www.wwiiaircraftperformance.org/mustang/xp-51f-chart.jpg

Hm, that throws a spanner in the works of my "491 mph must be a dive speed" line of thought.

Not sure what that figure could be then, if we rule out a simple typo.

Regards,

Henning (HoHun)

The first XP-51F had a production P-51D empennage for the first flight, (decided upon because?) but in any case the next two plus the two G/J's had the larger empennage for better yaw stability. The reverse rudder boost tab was not installed.

The '491' figure was a brain fart/typo. Simply I pulled a number from only God knows where

Anecdotally, my father told me there was a 'tangible but not metrically definable' diminished roll authority in the B/D after boost tab installed. He flew both B and D w/o boost before the last block D-5.

How different sometimes is the perception of the same thing by Americans (or the Pentagon) and the rest of the world. I know that for Americans, the P-51 is almost the best fighter of World War 2... Somewhat in the shadow of the Mustang is the P-47 Thunderbolt. We considered them so-so planes. Average level. These are good interceptors, but medium fighters. A really cool fighter was the P-39 Airacobra. Of course, he was greatly respected.

I have a different POV.
The very best VVS fighter was a better dog fighter than any US fighter, but range and ceiling were far under US standards for USAAF war strategy. They would have been more effective versus USN.

If the Spit IX, XIV had the range of the P-51/P-47 they would have to be judged as clear best along with perhaps Ta 152 (with near equal range) as best al around fighters.

The P-39 was competitive but still inferior to Yak 9 and LaaG 7, both being short range low/medium altitude battlefield air superiority/interceptor types.

IMO neither the Yak 9 or Laag 7 would have been as good against US heavy bombers as Bf 109 and FW 190. The manueverability distinction between Yak 9 and P-51D disappeared above 20,000 feet. Had war existed between USSR and US just after WWII, the US Strategic Air Force would have been unstoppable (IMO) vs B-17/B-29 escorted by either P-51 or P-47. There was no role for VVS fighters in US battle doctrine. The closest was P-63, which was still inferior to P-51 and P-47.

The introduction of the F8F, P-47M and P-51H may have altered the 'perception' of lacking manueverabilty vs VVS fighters in every area save turn. Available Horsepower vs HP required is a great equalizer to greater Lift loading in climb and turn.

'Best Fighter' is entirely subjective depending on selection and weighing/evaluation criteria. That said Speed, dive speed, zoom climb and range will dominate over manueverability, and all western designers prized top speed for the ability to extend or engage in combat. Good pilots flying aircraft with significant top speed and dive capability didn't fight in the horizontal unless no other choice was available.

 

[HoHun]

Hi Bill,

The first XP-51F had a production P-51D empennage for the first flight, (decided upon because?) but in any case the next two plus the two G/J's had the larger empennage for better yaw stability. The reverse rudder boost tab was not installed.

The '491' figure was a brain fart/typo. Simply I pulled a number from only God knows where

Actually, I think you were pretty accurate ... in this article it's 493 mph, but connected to Chilton just like you stated:

https://www.secretprojects.co.uk/threads/post-ww-ii-fighters-ki-84-hei-vs-ta-152h-vs-f8f-2.38157/#post-503555

In the full context of the quote, it really sounds as if it's about diving, though it's not expressly stated.

Regards,

Henning (HoHun)

 

[GregoryUSSR]

Had war existed between USSR and US just after WWII, the US Strategic Air Force would have been unstoppable (IMO) vs B-17/B-29 escorted by either P-51 or P-47.

It is better not to think about the war between the USSR and the USA, even immediately after World War 2. We wouldn't be with you right now. It would have inevitably been nuclear-chemical even then.
And what is so good about Messerschmitt against flying fortresses? When we bombed Berlin with TB-7 strategic bombers with 5-ton bombs, we somehow did not notice its outstanding qualities. Focke-Wulf 190 he still yes, could that. In general, to fight strategic bombers, you need something like the Yak-9T, Yak-9K, IL-2NS-37 and the like.

 

[Justo Miranda]

In my opinion, the Mustang was the best American fighter used during World War Two because it managed to do a very difficult job over the Reich in which all British fighters, the Thunderbolt and the Lightning, had already failed. In addition, its large range of action allowed it to operate against Japan at a time when B-29 bombers desperately needed any kind of escort to defend against the Ki.45 and Ki.61 rammers of the IJA.

 

[Justo Miranda]

The 37- and 45-millimeter guns of the Yak-9T and 9K would have been very effective against the B-29 bombers, but their excessive weight would have penalized the climb rate. In my opinion these fighters would have needed too long to reach the flight height of the bomber stream, also the Russian pilots would have had difficulty controlling their small fighters at that altitude, because of their small wingspan. The problem continued during the clashes between the MiGs 15/17 and the B-36s during the Cold War.

 

[drgondog]

It is better not to think about the war between the USSR and the USA, even immediately after World War 2. We wouldn't be with you right now. It would have inevitably been nuclear-chemical even then.
And what is so good about Messerschmitt against flying fortresses? When we bombed Berlin with TB-7 strategic bombers with 5-ton bombs, we somehow did not notice its outstanding qualities. Focke-Wulf 190 he still yes, could that. In general, to fight strategic bombers, you need something like the Yak-9T, Yak-9K, IL-2NS-37 and the like.

The 109 without escort fighters harrasing it was very effective with gondola mounted 20mm to add to the rest of the armament. Candidly I'm not familiar with armament for Yak9T/K.

 

[GregoryUSSR]

I gave the Yak-9T and Yak-9K as an example. These are not very successful models. Yak-9UT (1 - 37mm and 2 - 20mm guns), La-7 (2 - 20mm guns), La-7R (2 - 20mm guns) with a motor-compressor jet engine were better.
But after the start of the jet era of aircraft construction, piston fighters we were quickly abandoned. The most advanced piston-engined Soviet fighters were in service with secondary areas until the end of the 40s at most. So only reactive. The high losses of US aviation in Korea were explained by the fact that you used a lot of outdated piston-engined aircraft from the of World War 2 there. Apparently remembering the successful fights with the clumsy Me-262. But purely aesthetically piston fighters of World War 2 are a real work of art. But I can't look at the MiG-15/17. It's a flying tube with wings. The MiG-19 is the first more or less normal jet fighter.

 

[Scott Kenny]

How different sometimes is the perception of the same thing by Americans (or the Pentagon) and the rest of the world. I know that for Americans, the P-51 is almost the best fighter of World War 2... Somewhat in the shadow of the Mustang is the P-47 Thunderbolt. We considered them so-so planes. Average level. These are good interceptors, but medium fighters. A really cool fighter was the P-39 Airacobra. Of course, he was greatly respected.

The USAF and Red AF had very different needs for their fighters, though.

Eastern Front flying was mostly under 3000m or so, where the P39 flew very well. Western Front flying was escorting bombers at high altitude, and it wasn't until the P-51s showed up that the bombers had escort fighters the entire way there and back. P-38s and P-47s didn't have the range to stay with the bombers, they'd have to set up relays to meet up with the bomber formations at specific times.

 

[P1127]

The first XP-51F had a production P-51D empennage for the first flight, (decided upon because?) but in any case the next two plus the two G/J's had the larger empennage for better yaw stability. The reverse rudder boost tab was not installed.

Coming in late, but - from pictures, at least the third XP-51F (Shipped to the UK as FR409) and the first -G seem to have had 'normal' tails.

In the photo above on post #20, the second G FR410/43-43336) seems to have had the taller tail as per the J and H - however I'm not convinced that the photo of it (in RAF markings) is completely genuine. The tall tail and 4 bladed prop don't match completely with what we know of the G, the antennae from the fin going to the rear of the canopy is strange (How does the canopy open?) And there appears to be a little retouching 'smudge' where it touches the fuselage.

https://www.secretprojects.co.uk/threads/mustang-xp-51f-g-j-prototypes.12426/#post-500755

The story of the second G is a little murky as whilst it does seem to have gone to the UK, it doesn't appear to have been tested anywhere.

The RAF H never left the US, and there are no known photos of it.

 

[Mugs914]

What was the rationale for the massive canopy? Were they planning on a second seat?

Looks horribly out of proportion to what is otherwise a very nice-looking airplane...

 

[Scott Kenny]

What was the rationale for the massive canopy? Were they planning on a second seat?

Looks horribly out of proportion to what is otherwise a very nice-looking airplane...

Supposedly less drag than the original -D bubble canopy.