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