I’m building a MustangII (P-51A in RAF service), and one question
just keeps nagging at me. Hopefully some of the mustang experts
around here can help me with the answer.
It’s common knowledge that the Allison Mustangs were out-performed
at high altitudes by their Merlin-powered brothers, but I’ve been told
that Allsion mustangs were faster on the deck. Is this true?
MustangsMustangs.com claims the 51A could make 412mph at 10,000ft.
That’s a pretty damn impressive speed. The same webpage says the
Merlin MustangIII (with boosted manifold presure) could reach 417mph
at 2,000ft. So that’s only 5mph faster, in a much more oxygen-rich,
engine preferred atmosphere. Granted, there would also be more air
resistance on the airframe from the more-pressurized air at 2,000ft,
so it’s not clear how to compare these metrics.
Instinctively, it seems to me that if these numbers are accurate, the
Allison mustangs could at least closely match the Merlin stangs at
low altitude. You comments please.
I have nothing to confirm it with but it doesn’t suprise me. The allison mustangs did perform well at lower altitude. The two stage supercharger that Merlin used was only needed at altitude where the air was less dense, therefore artificially duplicating the atmosphereic conditions of a lower altitude.
As far as the allison engined plane being faster than the merlin at lower altitude, well, maybe, maybe not. I have no way of confirming.
There is a table in Robert Gruenhagen’s book on the Mustang which has test results for most Mustang variants, but not all tests were done at the same altitudes, so there is no definitive answer there, except between the Mustang I and Mustang III, but even those results are not of much use, as the elevation was at ground level on one and at 1,000 feet for the other, and the Mustang III was using 80" of manifold pressure which is considerably higher than operational aircraft were flown. The Mustang II tested at 381 mph at ground level and 80"/3,000 rpm, the Mustang I 328 mph at 1,000 feet, but the Allison was only boosted to 44.5" and 3,000 rpm, compared to 52" mp/3,000 rpm on the A-36, which did 366 mph at 1,000 feet.
It would seem that in operational use the Allison powered airplane might have a slight advantage at very low altitude, but individual airplanes might be on either side of that line.
The P-51B airframe was modified to take advantage of the extra engine power. The -B engine in the -A airframe would have resulted in more impressive specs. Also, note that you don’t know what WEP corresponds to in MP…the Allison engine did have a supercharger, just not as efficient as the Merlin/Packard.
The Allison Mustang did indeed have superior performance over both allied and axis fighters at low altitude. The Air Ministry/RAF wanted to order more but the request was refused probably because it was not feasible to restart production. I quote Rick Kent from an email he sent me last year.
“…The much later photo of a Mk. I with the code letters is also of great interest: the upper grey by this time is the proper Ocean Grey but, most particularly the squadron codes are applied. Throughout most of the service life of the Allison Mustangs they did not carry unit codes, just the single letter. As they only operated singly or in pairs the unit code was not really necessary. The very early squadrons applied them for a short time. I have only previously seen the aircraft of one squadron with the codes applied late in the war, namely No 26 Sqn that reverted to Mk. I Mustangs late in 1944 and applied the new codes XC. 26 Sqn had been the very first one to equip with Mustangs anywhere and used the code RM for a short while, but they were re-equipped with tactical reconnaissance Spitfires of various marks from March 1944 to the reintroduction of Mustangs in December 1944. By the end of the war they were the only RAF squadron still flying the Allison-engined version; so they were the first and the last. The Allison version simply ran out due to attrition and having been out of production for a long while, not because the types which replaced it (mainly Spitfire FR. IX’s and Typhoon FR. IB’s) were better. As far as the fact of it and more especially the pilots were concerned the replacements were nowhere near as good for the job they did. Even by 1945 the Allison Mustang was still the fastest fighter type below 4,000 feet in Northern Europe, which was where the tactical reconnaissance types operated.”
The type of Merlin used on Mustangs was fitted with a two-stage supercharger originally developed for a high-altitude version of the Wellington with pressurised compartments. There were also versions of the Merlin optimised for lower altitudes used in LF Spitfires but nobody criticises a Spitfire LF V for its poor performance at altitude.
Top speed is only one part of it anyway. Acceleration, ability to maintain speed in a turn, and the ability to climb at a higher rate and speed than an adversary are also factors to consider. A fast top speed does no good if it takes too long to get to that speed, or if you use more fuel to go that fast than your adversary does, or too much fuel at that speed such that range is compromised. What the pilots thought that flew the airplane is sometimes the best guide, because they had to balance all those factors on a daily basis, and you could say their lives depended on it. The Allison was a very robust and reliable engine according to what I have read and heard, more so than the Merlin, though the Merlin was no slouch. If Allison had had as good a group of people doing supercharger design as RR did, I think it would have been the equal of the Merlin at high altitude.
Are you saying that it’s all about sales and profits - that Packard need not have tooled up for Merlin production if they could have built the Allison engine under license with a RR-licenced supercharger? [:-^]
Don’t forget that Allisons were supercharged / turbocharged on P-38, P-39 (prototype) etc. Maybe it’s more that the pistons/conrods/gudgeon pins/crankshaft/ other engine bits couldn’t handle the boost that a Merlin could than the quality of the supercharger engineering team . I don’t know - just taking a stab at it. Far too complex for my simple mind.
I have a friend that flys warbirds and owns a T-6 converted to NA-50 specs. While we were doing an anual on the plane last year we were talking about engines when he made a comment.
He said “The Allison is a great engine but you can’t give me a Merlin. Those things will kill ya!”
He knows much more than me so I can only guess he knows what he’s talking about. I never got him to elaborate as to why he felt that way.
A good source for background on this subject is Ronald Harker’s book “The Engines were Rolls Royce”. The same individuals that were responsible for supercharger development at Rolls were called upon to develop the first allied turbine engines past the Whittle/Power Jets work. They worked on the project with GE who contributed their work with turbochargers with the centrifugal supercharger of Rolls Royce development to get the jet engine jump started. Harker was a Rolls test pilot, if the memory is correct he was the originator of the idea of pairing the Merlin with the Mustang.
Sheesh, look at that, post 1,501. Methinks I talk too much!
