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...