P-51 crit Mach-figure?

[delcyros]

Despite having read elsewhere that the P-51 Mustang could exceed Mach 0.8 I found those comments of an encounter report:

Pilot: Lt.Col. Thomas L. Hayes
date: 28th of may 1944
unit: 364 Fighter Squadron, 357th Fighter Group
time: 14:00 DST
area: NW of Magdeburg, Germany
Weather: Clear
Circumstances: Dogfight

quote:"(...)Taking up pursuit again, I was able to get on one of the Me-109, which now began to dive (now following details of the kill...) I WENT OUT OF
CONTROLL INDICATING 500 MPH @ 20.000ft. AND SO DID MY WINGMAN"

At 20.000ft altitude, Mach 1.0 is normally encountered with 706.6 mp/h
(699.5 Mp/h with temps. 5 degrees below normal and 713.7 mp/h with temps 5 degrees above normal), so both Mustangs went out of controll at Mach 0.71 instead of the often quoted 0.82 figure.
Altough I also read several accounts of Mustang Pilots swearing they hit 650 mp/h at low altitude, implying Mach 0.85.
Any ideas anyone?
Thanks in advance,

 

[prentice672]

I have the flight manual for the P-51H (1954 version) and it lists 0.75 as the limiting Mach. The "H" was a little cleaner than the earlier versions but I seem to recall the limiting Mach was about the same.

Ron

 

[mad_max]

According to my P-51D Handbook limiting max speeds:

All speeds are IAS.

40,000 ft. = 260 mph
30,000 ft. = 300 mph
20,000 ft. = 400 mph
10,000 ft. = 480 mph
5,000 ft. = 505 mph

From Robert Gruenhagen's book on the Mustang.

Maximum diving speed is 505 mph under 7,000 ft. and that the max. mach
number as .75.

Hess's book states dives were done to 650 mph; but most likely once compressibility
started to show it's ugly head, the pitot tube would be varying wildly showing wrong airspeeds.

I also understand that out of trim airframes and even how a pilot reacts to his airframe
will also have effects on max dive speeds.

Hope this helps.

 

[Mike Williams]

Hello "delcyros":

Please see Hayes' Encounter Report for 28 May 1944. Here's the relevant section:

Taking up pursuit again I was able to get on one of the Me-109's which now began to dive. I fired three short bursts. After the first burst he skidded, I suppose to look back. I fired again seeing debris and canopy come off. Just as the third burst was fired it looked like the pilot started out, however at that instant strikes were noticed on and around what looked like the pilot. Then the ship actually disintegrated. I went out of control indicating 500 mph. at 20,000 ft. and so did my wing man. I picked up my element leader, Lt. Howell, quickly and covered him as he nailed a Me-109 with the pilot parachuting. We climbed back to the bombers from 12,000 ft. and continued the escort. How the enemy pilot came out of his plane at 700 mph is beyond me.

Please take careful note that Hayes records his speed as indicating 500 mph. He then concludes his account of this combat by noting that the enemy pilot came out of his plane at 700 mph. Now it just so happens that 500 mph indicated at 20,000 feet is approximately 700 mph TAS. 700 mph TAS at 20,000 is approximately Mach .989. It's not plausible that the dives actually reached those speeds. One possible explanation is that Hayes failed to account for instrument and compressibility errors when he did his quick calculation to arrive at 700 mph TAS from 500 IAS. Certainly Hayes and his wing man exceeded the P-51's recommended dive limits, and fortunately for them, their aircraft apparently were no worse from it as they went on to complete their escort mission. The Me 109 wasn't as lucky since it "actually disintegrated" during the dive. We'll never know if it was the bullets or structural failure.

Please see Encounter Reports of P-51 Mustang Pilots, in particular the section titled Dive, for more comments from Mustang Pilots relating to the speeds recorded during a dive while engaged in combat.

Please see also Army Air Forces, Air Technical Command report on Dive Tests on P-51D. The report records the highest speed obtained during a test dive was a maximum true Mach Number of 0.85; concluded that the standard P-51D airplane may be safely flown to a Mach Number of 0.80; and in extreme war emergency the airplane can be dived to a Mach Number of 0.83. Read the full report for all the details.

Mike Williams

 

[delcyros]

Thanks all, particularely Mike for the source!
A crit Mach figure of 0.75 is also closer to my equitation table.
I also expect that the racks for the underwing racks do imply some slightly lower crit Mach figure for the P-51 as used over Berlin.
505 mp/h at 20.000ft altitude imply something between 550 and 565 mp/h TAS (recalculated, factoring compressability effects), which also very well fit to Mach 0.8.

again thanks for Your comments.

 

[Soren]

Robert C.Curtis, American P-51 pilot:
"My flight chased 12 109s south of Vienna. They climbed and we followed, unable to close on them. At 38,000 feet I fired a long burst at one of them from at least a 1000 yards, and saw some strikes. It rolled over and dived and I followed but soon reached compressibility with severe buffeting of the tail and loss of elevator control. I slowed my plane and regained control, but the 109 got away.
On two other occasions ME 109s got away from me because the P 51d could not stay with them in a high-speed dive. At 525-550 mph the plane would start to porpoise uncontrollably and had to be slowed to regain control. The P 51 was redlined at 505 mph, meaning that this speed should not be exceeded. But when chasing 109s or 190s in a dive from 25-26,000 it often was exceeded, if you wanted to keep up with those enemy planes. The P 51b, and c, could stay with those planes in a dive. The P 51d had a thicker wing and a bubble canopy which changed the airflow and brought on compressibility at lower speeds."



Thomas L. Hayes, Jr., American P-51 ace, 357th Fighter Group, 8 1/2 victories:
"Thomas L. Hayes, Jr. recalled diving after a fleeing Me-109G until both aircraft neared the sound barrier and their controls locked. Both pilots took measures to slow down, but to Hayes' astonishment, the Me-109 was the first to pull out of its dive. As he belatedly regained control of his Mustang, Hayes was grateful that the German pilot chose to quit while he was ahead and fly home instead of taking advantage of Hayes' momentary helplessness. Hayes also stated that while he saw several Fw-190s stall and even crash during dogfights, he never saw an Me-109 go out of control."

 

[Crumpp]

Measuring speeds in the transonic realm in the late 1940's was problematic at best. I would be highly suspect of any propeller driven aircraft achieving anything higher than Mach .8.

All the best,

Crumpp

 

[delcyros]

Measuring speeds in the transonic realm in the late 1940's was problematic at best. I would be highly suspect of any propeller driven aircraft achieving anything higher than Mach .8.

That´s exactly, what I think, too, Crumpp! Altough I would rather add "...other than in terminal dive" even in case this was more a problem for jet driven aircraft in this timeframe.

 

[HealzDevo]

I think this is questionable, however the only way to really prove one way or the other would be to try to design replicas using the original methods install the computer equipment in a way that doesn't change overall weight and then conduct the testing. I still remain sceptical though that the resultant force wouldn't have ripped off the wings, and tail...

 

[Zipper730]

Please see Encounter Reports of P-51 Mustang Pilots, in particular the section titled Dive, for more comments from Mustang Pilots relating to the speeds recorded during a dive while engaged in combat.

Please see also Army Air Forces, Air Technical Command report on Dive Tests on P-51D. The report records the highest speed obtained during a test dive was a maximum true Mach Number of 0.85; concluded that the standard P-51D airplane may be safely flown to a Mach Number of 0.80

So 0.75 is like maximum safe speed in peacetime, 0.80 in wartime, 0.83 if you're in for the fight of your life and gunfire is a more certain threat to your life than the high-risk of losing your wings from high speed (a gradual pull out was needed, though), with mach 0.84 being the fastest speed that the test pilots dared go.

 

[soulezoo]

IIRC, a KC-10 cruises at .87 mach. It has thick wings too...

Nice necro post!

 

[drgondog]

Despite having read elsewhere that the P-51 Mustang could exceed Mach 0.8 I found those comments of an encounter report:

Pilot: Lt.Col. Thomas L. Hayes
date: 28th of may 1944
unit: 364 Fighter Squadron, 357th Fighter Group
time: 14:00 DST
area: NW of Magdeburg, Germany
Weather: Clear
Circumstances: Dogfight

quote:"(...)Taking up pursuit again, I was able to get on one of the Me-109, which now began to dive (now following details of the kill...) I WENT OUT OF
CONTROLL INDICATING 500 MPH @ 20.000ft. AND SO DID MY WINGMAN"

At 20.000ft altitude, Mach 1.0 is normally encountered with 706.6 mp/h
(699.5 Mp/h with temps. 5 degrees below normal and 713.7 mp/h with temps 5 degrees above normal), so both Mustangs went out of controll at Mach 0.71 instead of the often quoted 0.82 figure.
Altough I also read several accounts of Mustang Pilots swearing they hit 650 mp/h at low altitude, implying Mach 0.85.
Any ideas anyone?
Thanks in advance,

BIG difference between IAS and TAS - both for normal reasons dealing with pressure altitude and temperatures as well as Mach corrections.

Mcr for the NAA/NACA wing was ~.75 but in comparison with P-38/F6F/F4U/Fw 190 NACA 23-016, never achieved uncontrollable 'pitch down' control forces.

The highest M no. for dives was ~ .85M and the issue was the structural damage due to Q forces.

 

[grampi]

Robert C.Curtis, American P-51 pilot:
"My flight chased 12 109s south of Vienna. They climbed and we followed, unable to close on them. At 38,000 feet I fired a long burst at one of them from at least a 1000 yards, and saw some strikes. It rolled over and dived and I followed but soon reached compressibility with severe buffeting of the tail and loss of elevator control. I slowed my plane and regained control, but the 109 got away.
On two other occasions ME 109s got away from me because the P 51d could not stay with them in a high-speed dive.

I can't imagine a 109 being able to outdive a P-51...

 

[fliger747]

Critical Mach number would be when some part of the aircraft begins to generate shockwaves. This is different than the limiting Mach number. For instance with the B747-400 one could see shockwaves standing up about 10' inside the winglets at M .90 though the certified Mach was .92. A little light shaking at M .90. My understanding the aircraft was flight tested to plus M .99 and several Asian carriers have "field tested" it in service to plus M 1.0. A limiting Mach number would exist when adverse aerodynamic effects such as shift of CL begin to arise.

Just as a guess, the D model with the bubble canopy might begin to generate a shockwave off of the canopy at a lower speed than the B model.

 

[Joe Broady]

Please take careful note that Hayes records his speed as indicating 500 mph. He then concludes his account of this combat by noting that the enemy pilot came out of his plane at 700 mph. Now it just so happens that 500 mph indicated at 20,000 feet is approximately 700 mph TAS. 700 mph TAS at 20,000 is approximately Mach .989. It's not plausible that the dives actually reached those speeds. One possible explanation is that Hayes failed to account for instrument and compressibility errors when he did his quick calculation to arrive at 700 mph TAS from 500 IAS.

We can't make any accurate computation of the true airspeed or Mach at 500 mph indicated airspeed because we don't know the P-51 correction for "position error". That is the error due to the inaccuracies in the pitot and static pressures supplied to the airspeed indicator. Usually most of the error is in the static pressure, since it's difficult to find a place on the skin where a sample port experiences true freestream pressure.

An example was the Hellcat. At a moderate cruise speed of 140 knots indicated airspeed, the F6F-3 read 12 knots low and the F6F-5 reads 7 knots high with respect to calibrated airspeed (the speed you would see if the instrument were provided perfect samples of pitot and static pressure, and the instrument itself responded to those pressures exactly as designed). That's almost 20 knots discrepancy between two variants of the same basic airplane! The reason is that Grumman relocated the static pressure port on the -5. According to their long time test pilot Corky Meyer, that was to silence Navy complaints of the Hellcat's slow speed relative to the Corsair, and it worked. (See the book on the Hellcat he co-wrote with Steve Ginter. He says Grumman flew an F6F-3 and an F4U-1D on loan from the Navy in formation to compare top speed. It was practically a tie, but the Hellcat consistently indicated 20 knots lower, so Grumman found a new static port location which made the Hellcat match the Corsair reading.)

Getting back to the P-51, without knowledge of its position error at 500 mph we have no basis to convert IAS to CAS, the latter being the correct route to TAS. My Jeppesen CR-2 "whiz wheel" says 500 mph (435 kt) CAS and 20,000 feet pressure altitude = .92 Mach = 650 mph (565 kt) TAS.

On the other hand, the E-6B slide rule says 500 mph CAS at 20,000 feet = 685 mph TAS. That's 35 mph high because the CR-2 is designed for jet speed, while the basic design of the E-6B is from the WW2 era and ignores compressibility. The error is trifling until about .5 Mach.

 

[tomo pauk]

...
An example was the Hellcat. At a moderate cruise speed of 140 knots indicated airspeed, the F6F-3 read 12 knots low and the F6F-5 reads 7 knots high with respect to calibrated airspeed (the speed you would see if the instrument were provided perfect samples of pitot and static pressure, and the instrument itself responded to those pressures exactly as designed). That's almost 20 knots discrepancy between two variants of the same basic airplane! The reason is that Grumman relocated the static pressure port on the -5. According to their long time test pilot Corky Meyer, that was to silence Navy complaints of the Hellcat's slow speed relative to the Corsair, and it worked. (See the book on the Hellcat he co-wrote with Steve Ginter. He says Grumman flew an F6F-3 and an F4U-1D on loan from the Navy in formation to compare top speed. It was practically a tie, but the Hellcat consistently indicated 20 knots lower, so Grumman found a new static port location which made the Hellcat match the Corsair reading.)
...

An informative post.
I do have a thing sticking out there, however - even when F6F-5s went through the USN and British tests, they were still ~20 mph slower than F4U-1 on same horsepower. We have USN and British data sheets where -3 and -5 are within a few mph true air speed when using same horsepower.
The XF6F-6 was slower by 20 mph than F4U-4, both used same engine and prop.

 

[fliger747]

Manufacturer's and service test data for WWII planes is usually at variance. In fact I generally suspect the number of significant figures one should trust. Much data was recorded on the test pilots knee board, subject to all sorts of error. Yes I could probably find a location for the static port on a Hellcat that would increase the IAS... However the point should be to produce accurate data. Individual planes varied a lot, especially ones pulled off the ramp at random.

 

[Joe Broady]

On the other hand, the E-6B slide rule says 500 mph CAS at 20,000 feet = 685 mph TAS. That's 35 mph high because the CR-2 is designed for jet speed, while the basic design of the E-6B is from the WW2 era and ignores compressibility. The error is trifling until about .5 Mach.

What I should have said was "... 500 mph EAS (equivalent air speed) ..." At light plane speeds EAS and CAS are practically the same, but at the speed we're talking about CAS is significantly greater than EAS, this "inflation" being due to compressibility. The more sophisticated CR-2 device compensates for that, but the E-6B doesn't. So to convert IAS to TAS with an E-6B when compressibility is significant: 1) look up IAS in the airplane's position error chart and apply the correction to obtain CAS, 2) subtract compressibility error from CAS to obtain EAS, 3) enter EAS and pressure altitude in the E-6B to obtain TAS.

USAF flight manuals from the jet era often include a graph of compressibility error. (This is purely a function of airflow physics and the same for all airplanes.) From one such chart I get a correction of 24 mph at 500 mph CAS and 20,000 feet. Thus EAS = 500 - 24 = 476 mph. When I enter that in the E-6B it says 655 mph TAS. With the CR-2 I got 650. That's better than 1% agreement, not bad for slide rules and a chart.

Someone expressed skepticism about Corky Meyer's claim of speed parity between the Hellcat and Corsair. I had the same reaction. However, his is a first hand account from a pilot who participated in the tests. Admittedly Grumman received only one exemplar of the Corsair. Meyer says it was brand new, but maybe it was a "runt of the litter"? One interesting thing I omitted was that he said the Corsair was definitely the faster at low altitude, when neutral blower was in use. The reason was that the Hellcat inhaled hot unrammed air from the accessory section in this mode, while the Corsair blower was fed cold rammed air. This was no oversight by Grumman. They chose to make the performance sacrifice in exchange for freedom from induction icing even in bad weather.

 

[tomo pauk]

...

Someone expressed skepticism about Corky Meyer's claim of speed parity between the Hellcat and Corsair. I had the same reaction. However, his is a first hand account from a pilot who participated in the tests. Admittedly Grumman received only one exemplar of the Corsair. Meyer says it was brand new, but maybe it was a "runt of the litter"? One interesting thing I omitted was that he said the Corsair was definitely the faster at low altitude, when neutral blower was in use. The reason was that the Hellcat inhaled hot unrammed air from the accessory section in this mode, while the Corsair blower was fed cold rammed air. This was no oversight by Grumman. They chose to make the performance sacrifice in exchange for freedom from induction icing even in bad weather.

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.

 

[michael rauls]

Another possible explanation for variations in the reported critical mach of the same type of aircraft might be that the speed of sound changes rather substantially not only with altitude but with temperature and humidity as well. A little while back I went poking round the internet about the speed of sound looking for an explanation for why in warmer/ dryer theaters the p38 seemed to have little trouble with compresability but in colder/ wetter ones compresability became a hindrance.
Three factors changed the speed of sound in the order of greatest to least; altitude( as I'm sure everyone here is already aware), temperature, and last and least influence humidity. While altitude had by far the greatest influence on the speed of sound the combination of hotter and drier or colder and more humid could change the speed of sound as much as about 25 m.p.h.( doing this from memory so that 25m.p.h might not be exactl but it's pretty close if not).
Add this to some of the other explanations others have posted like a change in pitot tube positioning and if all the variables were working in the same direction it doesn't seem like a stretch that you could have pilots reporting getting into trouble at 50 or 60 m.p.h mach reading differences at the same altitude in the same type of aircraft.