Design Analysis of the Bell Airacobra from Cannon to Tail (1943)

[Jetman 33]

One more point that is missing is the Russians operated the Cobra at full throttle, not the book 42 "mp. The engine lasted about 20 hours but they had spare engines on lend-lease or damaged aircraft.
Pokryshkin said "you would have to be crazy to run it at less power".

 

[FLYBOYJ]

You could stand some research too. The Brits did need aircraft badly but not a low altitude fighter so they passed them on to the Russians who did need them. They preferred the Cobra to the Spitfire and the Mustang and even their LA-5 and LA-7. In spite of that the British later bought the low altidude, Allison powered Mustang. They did evaluate the P-39 and found it to out perform the current Spitfire Mk V below about 14,000 ft.

The British gave up their P-39s because they were inferior to what they were operating at the time, and did not meet promised performance requirements.

"These aircraft were soon assembled, and on 30 July 1941 tests began at the Fighter Development Unit at Duxford. These tests revealed that the Airacobra could out-turn and out-dive the Bf 109E at up to 15,000ft, but was "utterly useless" above 20,000ft. The top speed of the Airacobra also disappointed.

Only one RAF squadron ever received the Airacobra. No.601 "City of London" Squadron swapped its Hurricane IICs for Airacobras in August 1941, just in time to see the aircraft withdrawn to have twenty-five modifications made to the fuselage. The first four aircraft were finally declared operational in October 1941."

Bell Airacobra I in British Service

The Bell Airacobra first entered service with the RAF, in October 1941, but only flew a handful of sorties before it was withdrawn from the front line.

www.historyofwar.org

And from Wiki -

British expectations had been set by performance figures established by the unarmed and unarmored XP-39 prototype. The British production contract stated that a maximum speed of 394 mph (634 km/h) +/- 4% was required at rated altitude. In acceptance testing, actual production aircraft were found to be capable of only 371 mph (597 km/h) at 14,090 ft (4,290 m). To enable the aircraft to make the guarantee speed, a variety of drag-reduction modifications were developed by Bell. The areas of the elevator and rudder were reduced by 14.2% and 25.2% respectively. Modified fillets were installed in the tail area. The canopy glass was faired to its frame with putty. The gun access doors on the wing had been seen to bulge in flight, so they were replaced with thicker aluminum sheet. Similarly, the landing gear doors deflected open by as much as two inches at maximum speed, so a stronger linkage was installed to hold them flush. The cooling air exit from the oil and coolant radiators was reduced in area to match the exit velocity to the local flow. New engine exhaust stacks, deflected to match the local flow and with nozzles to increase thrust augmentation, were installed. The machine gun ports were faired over, the antenna mast was removed, a single-piece engine cowling was installed and an exhaust stack fairing was added.

The airframe was painted with 20 coats of primer, with extensive sanding between coats. Standard camouflage was applied and sanded to remove the edges between the colors. Additionally, about 200 lb (91 kg) of weight was removed, making it lighter than normal (7,466 lb (3,387 kg) gross). After these modifications, the second production aircraft (serial AH 571) reached a speed of 391 mph (629 km/h) at 14,400 ft (4,400 m) in flight test. As this speed was within 1% of the guarantee, the aircraft was declared to have satisfied contractual obligations. Despite the success of these modifications, none were applied to other production P-39s. Later testing of a standard production P-400 by the British Aeroplane and Armament Experimental Establishment (A&AEE) revealed a top speed of only 359 mph (578 km/h).

The Soviets were a different story, they operated the P-39 in an environment where it excelled, I think you're misinterpreting my posts, agree they served well with the Soviets!!!

You dismiss the Cobras 1946-47 National Air Race performance where it had minimum airframe mods and mainly a horsepower boost equal to the Mustangs it was running against. A Merlin in the Cobra would likely have given a boost close to the original Mustang, Much of the later Spitfire and Mustang performance boost came from higher fuel octane allowing higher manifold pressure. That helped everyone except the P-39. It never got the engine improvements, just 1200 h.p..

I never dismissed the Cobras from the 1946 air races, in fact they were the best prepared and modified aircraft in the lot, although they had big bucks backing them up! I clearly showed what was modified on the aircraft, this indicated in the book "The Air Racer" by Mendenhall.

 

[FLYBOYJ]

One more point that is missing is the Russians operated the Cobra at full throttle, not the book 42 "mp. The engine lasted about 20 hours but they had spare engines on lend-lease or damaged aircraft.
Pokryshkin said "you would have to be crazy to run it at less power".

Is this "20 hours" based on what you read in Pokryshkin's book or another source? As I posted yesterday, another Soviet Ace N. Golodnikov stated;

We had Allison engines. They were powerful, but . . . the engines in the Cobras were unreliable, especially early on. These were on the English variants, the Q-1 and Q-2. Their engines were weaker. After the first three or four air combats, all ten Cobras were laid up for engine repairs.

These first Allisons did not deliver even one-half of the recommended engine hours. 50 hours was its limit, and frequently less. Normally 10—15 sorties if they were in combat. They seized, the bearings melted; this happened to me once. I sat out for a while with no engine. They monitored these engines closely. As soon as any metal showed up in the oil, they changed out the engine.

And he went on to say:

Now regarding power settings. In principle the RPMs were regulated by a conventional throttle. In the Cobras there were two regimes of throttle operation, "normal" and "war emergency", which was characterized by increased manifold pressure. The throttle quadrant was mounted in the [left side of the] cockpit and the pilot controlled it. The "war emergency" regime had a lever position that we called "51 inches and 57 inches of boost". If we were flying on Soviet B-95 fuel, then "war emergency power" was set at 51 inches. If we were using American B-100 fuel, then "war emergency power" was set at 57inches. Although it was mounted in the cockpit, on the throttle quadrant, the pilot did not adjust this setting. The position of the "war emergency power" selector was controlled by a piece of wire that could be broken easily with greater forward pressure on the throttle quadrant.

Conversations with N.Golodnikov. Part Three. P-39 Airacobra and Yaks – Lend-Lease

 

[pbehn]

Too bad the Brits rejected it after they found that It would out perform their Spitfire V down low, even out turn it. If they had put the same Merlin in it that was used in the first Mustang what would it have been? I think it would have equaled the 1946 US National Air Race champion!

I almost succumbed to the dark side.

 

[pbehn]

You could stand some research too. The Brits did need aircraft badly but not a low altitude fighter so they passed them on to the Russians who did need them. They preferred the Cobra to the Spitfire and the Mustang and even their LA-5 and LA-7. In spite of that the British later bought the low altidude, Allison powered Mustang. They did evaluate the P-39 and found it to out perform the current Spitfire Mk V below about 14,000 ft. You dismiss the Cobras 1946-47 National Air Race performance where it had minimum airframe mods and mainly a horsepower boost equal to the Mustangs it was running against. A Merlin in the Cobra would likely have given a boost close to the original Mustang, Much of the later Spitfire and Mustang performance boost came from higher fuel octane allowing higher manifold pressure. That helped everyone except the P-39. It never got the engine improvements, just 1200 h.p..

Some of the issues around the P-39 were covered here, its always great to have new angles on it though.

Groundhog Thread Part Deux - P-39 Fantasy and Fetish - The Never Ending Story (Mods take no responsibility for head against wall injuries sustained)

Hello! I was intrigued by reading of this interview with Golodnikov, especially by his mention about true performances of German fighters (color plus some correction of the text by me): "A. S. Nikolay Gerasimovich, if you look at any reference book, the superiority in speed of German aircraft...

ww2aircraft.net

 

[special ed]

The evil twin of P-39 Expert.

 

[GrauGeist]

The 1946 racer had more than a "few" modifications.
First off, it had ALL of it's military equipment removed.
This would include: armor plating (including nose armor), armored windscreen, cannon, machine guns, radio equipment (including IFF), self-sealing tanks and so on.
The engine was modified, the prop was modified and the airframe was lightened to a degree.
Even the pitot tube was relocated from the port wing to the prop hub (where the cannon used to be).

These "few" modifications put the racer into a class of performance that even Bell never imagined.

 

[MIflyer]

I recall reading somewhere - do not know where - that an NACA engineer was given the job of improving the V-1710. He complained bitterly, "What's the use of trying to improve this piece of junk?"

In reality, the simple addition of a second speed for the supercharger - like everyone else did for their engines - would have helped the altitude performance of the Mustang Mk 1, P-39, P-40, P-51A and A-36A. As it was, setting the supercharger for only one gear ratio was like trying to drive a car only in 2nd gear. And, by the way, only the A-36A was optimized for "low altitude", with max performance at 5,000 ft; the rest were set up for about 15,000 ft and the P-51A for 20,000 ft. Admittedly, then Allison would have needed to build a second model of rear accessory case rather than using the same one for everything.

Adding a Stanley Hooker style liquid intercooler/aftercooler and second stage of supercharging would have made the V-1710 fully competitive with the two stage Merlins. For the P-63 and F-82, Allison added an auxiliary stage supercharger in its own case, coupled to the starter drive, but with no intercooler. The F-82 almost had to be abandoned because the engine would not work well.

If that NACA feather merchant had been the least bit enthusiastic about his job, the V-1710 could have been as good as anything else. I guess maybe he was P.O.ed over not getting to work on jets.

 

[pbehn]

I recall reading somewhere - do not know where - that an NACA engineer was given the job of improving the V-1710. He complained bitterly, "What's the use of trying to improve this piece of junk?"

In reality, the simple addition of a second speed for the supercharger - like everyone else did for their engines - would have helped the altitude performance of the Mustang Mk 1, P-39, P-40, P-51A and A-36A. As it was, setting the supercharger for only one gear ratio was like trying to drive a car only in 2nd gear. And, by the way, only the A-36A was optimized for "low altitude", with max performance at 5,000 ft; the rest were set up for about 15,000 ft and the P-51A for 20,000 ft. Admittedly, then Allison would have needed to build a second model of rear accessory case rather than using the same one for everything.

Adding a Stanley Hooker style liquid intercooler/aftercooler and second stage of supercharging would have made the V-1710 fully competitive with the two stage Merlins. For the P-63 and F-82, Allison added an auxiliary stage supercharger in its own case, coupled to the starter drive, but with no intercooler. The F-82 almost had to be abandoned because the engine would not work well.

If that NACA feather merchant had been the least bit enthusiastic about his job, the V-1710 could have been as good as anything else. I guess maybe he was P.O.ed over not getting to work on jets.

That is all true, but the RAF would have operated the Mustang Mk I as it was until the end of the war, in the role of tactical recon, a small but important niche it was the best. The P-39 was not "let down" by its engine, it just want very good and was too late in the game by a country mile.

 

[MIflyer]

I believe that the RAF modded the Mustang Mk1/1A/II engine for more optimum performance at low altitudes? They had found through experience that the V-1710 could take a lot more boost than the manufacturer's recommendations. They borrowed an A-36 in the Med for Tac Recon and provided a Spit escort. The Spits could not keep up at the speed and rage the A-36 had.

 

[Aeroweanie]

Attached is a technical paper I wrote about the P-39 & P-63 a while back.

One of the biggest points of contention regarding the P-39 is the year of the first flight. I personally believe Birch Matthew's research, supported by USAAC documents, which says 1939. Most other publications echo one another and say 1938.

 

[Thumpalumpacus]

The Spits could not keep up at the speed and rage the A-36 had.

Probably a typo, but I'll put in an early nomination for best typo of the year thus far.

 

[GrauGeist]

Attached is a technical paper I wrote about the P-39 & P-63 a while back.

One of the biggest points of contention regarding the P-39 is the year of the first flight. I personally believe Birch Matthew's research, supported by USAAC documents, which says 1939. Most other publications echo one another and say 1938.

I agree - the USAAC issued the X-609 Proposal in February '37, barely over a year prior to the April '38 date that keeps popping up.

 

[Aeroweanie]

I agree - the USAAC issued the X-609 Proposal in February '37, barely over a year prior to the April '38 date that keeps popping up.

The trouble is, the error keeps getting repeated and the Wikipedia czars use that as a preponderance of evidence.

 

[Greg Boeser]

If your research contradicts the prevailing wisdom, you must be wrong. Just ask Galileo.

 

[FLYBOYJ]

If your research contradicts the prevailing wisdom, you must be wrong. Just ask Galileo.

Or the P-38 Expert!

 

[GrauGeist]

The P-38, which was selected as a response to the X-608 Proposal, first flew in January 1939 - which is a realistic timeline from drawing board to prototype.

The XP-39 prototype (s/n 38-326) first flew from Wright field on 6 April 1939 and it's this actual date, that also follows a realistic timeline from drawing board to prototype. Especially since the P-39's design was unconventional (much like the P-38).

I suspect that some author typoed the date and before it could be corrected, became gospel.

 

[Thumpalumpacus]

If your research contradicts the prevailing wisdom, you must be wrong. Just ask Galileo.

Copernicus would like a word with you ...

 

[Greg Boeser]

Crap! I thought he was dead.

 

[GrauGeist]