"Stretch or not strech" of aircraft designs

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P-51 was the ultimate stretch, getting a two stage engine and 85 gallons more fuel.

P-39 could have been stretched. Substitute 50gal fuel for the wing .30s and use the V1710-93 two stage engine available from April '43. Would have weighed around 8000# as compared to a P-63 at 8950# or a P-51B at 9600#.
 

Hello P-39 Expert,

I believe that by the time all the that would result from the substitution were corrected, the aircraft would greatly resemble a P-63 King Cobra. Where would you put an intercooler? Where would the additional pieces of the supercharger go? The cooling system isn't really adequate for the non two-stage supercharged engines. Is anyone other than the Russians willing to accept a frontline fighter with just a 37 mm and two cowl mounted .50 cal?

- Ivan.
 

Actually, it went the other way - the original engine was larger & heavier than its replacements.

The A-4's original engine (A-4A/B/C) was the Wright J65-4/16/20 (license-built Armstrong-Siddeley Sapphire 100 series).
That engine had a length of 130", a diameter of 37.7", and a weight of 2,750 lb.
Thrust was 7,650 lb.s.t. A-4A (-4); 7,700 lb.s.t. A-4B (-16); & 8,400 lb.s.t. A-4C (-20).
Fuel consumption was .90 lb fuel/lb thrust/hr

Starting with the A-4E, the engine was the Pratt & Whitney J52-6/8.
That engine had a length of 117", a diameter of 30.2", and a weight of 2,100 lb.
Thrust was 8,500 lb.s.t A-4E (-6); 9,300 lb.s.t. A-4E/F/G/H/J/K (-8).
Fuel consumption was .82 lb fuel/lb thrust/hr

The "Super-Skyhawk" (A-4M/N/KU) had the J52-408.
That engine had a length of 118.9", a diameter of 32.1", and a weight of 2,318 lb.
Thrust was 11,200 lb.s.t
Fuel consumption was .79 lb fuel/lb thrust/hr

Singapore rebuilt its used A-4B/Cs with the F404-100D (A-4S-1/SU).
That engine had a length of 94", a diameter of 35", and a weight of 1,830 lb.
Thrust was 11,000 lb.s.t
Fuel consumption was .80 lb fuel/lb thrust/hr
 
Interesting discussion! I had a few thoughts nobody will probably care about but I will post them anyway....

On the point in the OP
  • The OP has a good point. I think 'stretch' was a fairly common tactic both in substantial revisions of old designs and in new ones. Not universal but by no means unusual. Even the Spitfire was stretched.
  • I think the main reason for the stretch however is for improved stability, which became needed with increased speed and power, such as during takeoff and also hard maneuvering.
  • Fuel tanks seemed to do best when placed between engine and cockpit, in the central part of the wings, or beneath the cockpit in roughly that order. Fuel tanks behind the pilot seemed to frequently cause stability problems when full (though they could still be used anyway for long range flights).
Conversely I believe a lot of very short (and often overly fat) aircraft, particularly early war fighters, suffered some stability issues as a result. The I-15 / I-16 series are one good example. The Bloch 152 is I think another as is the Brewster F2A Buffalo. However the trend of some shorter and also smaller planes did also exist as previously mentioned (Bearcat etc.)

Since guns and fuel were best put between the engine and cockpit there was a limit to how far you could push the cockpit backward that limited how much 'stretch' could help. Some, when it came to adding a bigger supercharger or for ammunition storage for example. But not too much.

Limitations of Japanese fighters
I think the biggest problems with Japanese fighters and aircraft more generally was with engines. I believe like the Russians and Italians and all the lesser powers, they struggled with engine design most of all. At the end of the 30's nearly every advanced nation proved capable of making the modern standard of fighter planes: single-engined, closed cockpit, cantilever winged monoplanes with at least partly stressed metal skin*, closed cockpit, and retractable landing gear. Small countries like Czechoslovakia, Lithuania and Romania made reasonably good planes - engines were the single most difficult stumbling block for most. When you read the design history of countless planes problems with the new or anticipated engines were so routine as to be almost inevitable - even in the UK, Germany or the US but especially in every other country.

The Soviets struggled with derivations of the Hispano Suiza 12Y or whatever and until nearly the end of the war their inline engines were rarely capable of more than 1,200 or 1,300 HP so to improve performance they had to keep their fighters very light and therefore small. Only their Shvestov M-82 radial enabled them to break out into heavy duty horsepower (which is perhaps why they loved the La 5 series so much). The Italians just had to adopt German DB 600 series engines.

In Japan they did make much more powerful engines than the 800 or 900 hp motors they started the war with (up to 2,000 hp and more eventually), they just couldn't make more powerful engines that could reliably work and be maintained onboard aircraft carriers or dusty, humid, squalid island revetments. So to speak only of fighters the Ki-44 (early 1942), J2M (Dec 1942), and Ki-61 (early 1943) did arrive in time to potentially make a difference in the war. They were not available in huge numbers but many of the key battles in the Pacific were fought with surprisingly few aircraft. The float plane version of the N1K1 also arrived in 1943, and there were some innovative and highly capable Japanese bomber types arriving around the same time as well. But problems with their engines, both on the production side and in terms of maintenance in the field were so crippling that these aircraft were produced at a slow rate and had a very low availability ratio. So if you have 100 new planes that are pretty good, but can only field 20 at a time and then half of them have to turn around en-route to the target, you really have no benefit.

Furthermore trying to fix teething problems with engines slowed down both engine and aircraft production & development and is a large part of the reason why so many more very promising Japanese aircraft which flew in say 1941 or 1942 didn't make into combat until late 1944 or 1945 (and then only in small numbers), when it was clearly too late.

Japanese aircraft and armor
My understanding is that the Japanese fighters and some of the bombers arriving mid-war did have armor and self-sealing fuel tanks. Can somebody clarify about the N1K1? I thought it had armor too. The problem was that the Japanese didn't have a robust system for pilot rescue and recovery. Armor in an aircraft served two purposes. One was Tactical - to enable the aircraft to survive being hit and keep fighting- maybe still win in spite of a few bullet holes. Japanese fighters such as the A6M5 had that. The other was Strategic / Attritional: for the aircraft though basically smashed up, to remain flyable long enough for the pilot to bail out, ditch or crash-land. Read the personal histories of German or Russian aces in WW2, or any Ace who fought for a very long period of time - you will find almost inevitably that they were shot down multiple times, but survived to return to base and fight again and again. On the Russian Front for both sides this was accomplished mainly by the air combat being right next to the front line and a relatively lucky pilot being able to make his way to friendly troops. In the West you had the Resistance plus air-sea rescue.

In the Pacific though due to the vast distances over water you needed a very robust dedicated air-sea rescue system. Even a moderately damaged aircraft often faced a very long flight back to base, often through dangerous weather, in a challenging navigational environment and near the limits of their fuel / range. The Americans and ANZAC forces had life jackets, they had dinghy's in their combat planes, most importantly they had fleets of PBY, OS2U, Lockheed Hudsons, Sunderlands, later PB4Y as well as submarines and PT boats and the like assigned to rescue and recovery duties. The Japanese never really had this in place as a system (even though they had submarines and some great seaplanes and flying boats that could do the job), they famously had a hard time even getting pilots to wear parachutes. The notion of a pilot who was defeated still being worth keeping as a pilot was also slow to realization for the IJA and IJN.

So without a recovery system in other words most shot down Japanese pilots, even if they survived the destruction or crippling of their aircraft, ended up POW, MIA, or KIA, whereas a large number of Anglo-American / ANZAC pilots survived being shot down (sometimes two and three times) and were able to keep fighting. In part due to the armor and self-sealing fuel tanks on their aircraft, but in part due to that air-sea rescue system. Even when the Japanese caught up in this area, they still lacked the pilot recovery infrastructure or system to get the greatest benefit out of it in terms of attrition warfare.


* almost always some kind of duralumin or the equivalent in some kind of wartime-expedient advanced plywood
 
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The big advantage of the P-39 with the -93 engine is it would have been available 6 months before the first P-63 appeared. And the P-39 was already in full production while the P-63 was just entering production. No intercooler needed. P-63 didn't have on but used water injection instead. Or don't even use water injection, just don't utilize WEP (war emergency power) which was only good below the critical altitude and the P-39 already had excellent performance at low/medium altitude. Oil cooling radiators could have been enlarged somewhat by better utilizing the space allotted. A 37mm cannon with two .50s was more than adequate considering the AAF and Navy had newer fighters armed with only 4 x .50 caliber MGs (P-51A/B/C, FM-2 and F8F).
 

Hello P-39 Expert,

My mistake regarding the two-stage Allison engine.
Without a lot more engine power or a much greater critical altitude, performance would still be somewhat lacking compared to contemporaries.
I don't believe there actually was the capability to increase the cooling capacity in the available space or that would have been done because cooling was always somewhat inadequate in the Airacobra.
I think you have much greater faith in the air-to-air capability of the 37 mm cannon than I do. Ballistics are lousy, firing rate is too slow and ammunition capacity is too low.
The other 4 gun fighters you mentioned had free-firing, not synchronized .50 cal MG. The Bearcat also was using the M3 HMG with a much higher firing rate.
If one HAD to keep the P-39 in service, I believe it would have made more sense to stretch the wings and possibly the horizontal tail to adjust the center of lift to cure the aft CoG problem and use a more modern airfoil. A symmetrical airfoil just didn't make sense in an aircraft that was not intended to sustain inverted flight.
From various descriptions, the migration of the CoG did not seem particularly great or very far aft but the Airacobra seemed to be affected much more than other fighters. Perhaps a new wing design would cure that problem.

Schweik said:
The J2M looks like a good candidate for a 'stretch' by the way... maybe the Ki-44 too...
Click to expand...

Hello Schweik,

Out of curiosity, how would you stretch the J2M?

- Ivan.
 


How? I think I would just push the tail about two feet back, if nothing else for aesthetic reasons.

I wonder if they made it so short to fit on aircraft carrier elevators maybe? Wouldn't be the first design messed up for that reason.

I really don't know but would expect a slightly longer fuselage with the tail 2 or 3 feet back might improve stability a little bit without adding much weight. No other reason.
 
The fuel tanks of a plane must be as close to the centre of gravity as possible because they change in weight as the plane flies. The performance envelope of the Spitfire in its service life was stretched massively, its power and weight doubled however the wing plan hardly changed at all, and the fuselage became longer at the front because of bigger engines but hardly at all at the rear, instead they used balance weights. Literally stretching an airframe, making it longer makes it more stable, that isn't always desirable in a fighter.
 

Hello Schweik,

The J2M was never intended to be flown from carriers. The J designation meant that it was a land based fighter as opposed to A for a carrier fighter or N for a seaplane fighter.
The length was 9.695 Meters and wing span was 10.800 meters, so it wasn't really that short at 31,8 feet.
The pilot report actually commented that stability was excellent, so there wasn't really anything to fix in that area.

- Ivan.

 


-93 engine, available how and at what ratings? It didn't pass it's type test until Nov 27th 1943. P-63s built and equipped with the -93 engine were flying with restrictions.

As for no intercooler needed?
The earlier E9 engine(alias -47, the -93 was the E11) was supposed to use an intercooler. As things developed this was changed to an after cooler. In the end in Dec of 1943 the aftercooler was canceled due to poor performance of the unit supplied by the Harrison Radiator division of General Motors and also the large amount of mechanical troubles with the set up. (page 256 Vees for Victory)

The -47 was rated at 1150hp at 21,000ft and the -93 was rated at 1150hp at 22,400ft. A Merlin 61 was supposed to give 1390hp at 23,500ft.
Since the Merlin 61 was over 1 year earlier in timing (vs the -93) one can see why the two stage Allison didn't exactly take the Allied aviation world by storm.

Not using WEP power would have simplified things and speeded up the engines introduction as much of the summer of 1943 was spent trying to get the engine to stand up to the WEP ratings ( Better pistons and rings for example).
The water injection was a fall back position. The two stage engines were around 165lbs heavier than the single stage engines so does the added weight really pay off without the performance the wep settings give?

Please note that several thousand of the -47 engine were on order at one point for the P-39E (2000 engines, 270 engines as spare parts and 430 engines without reduction gears) , with the contract amended to add another 2300 engines (and 700 spares) added in July of 1942.
Since we are talking about aircraft stretch consider the P-39E
" The XP-39E bore the same armament as the P-39D but featured a new wing with square-cut tips. Wing span and gross area were increased to 35 feet 10 inches and 236 square feet........ The carburetor air intake was relocated and the wing-root radiator intakes were enlarged. The fuselage was lengthened by 1.75 feet to accommodate the longer -47 engine.

Empty and loaded weights were 6936 lbs and 8918 lbs respectively, making the XP-39E the heaviest of all Airacobra variants. During tests, a maximum speed of 386 mph at 21,680 feet was attained, which was much better high-altitude performance than other Airacobra variants. "

From Joe Baugher's website.
The P-39E was redesignated the P-76 and the then the whole things was canceled in favor of the P-63.
 
The whole point of putting the -93 in the P-39 was to have a two stage P-39 earlier than the P-63. No WEP speeds up the whole process (no intercooler, no water injection etc).

The two stage -93 was about 165# heavier and the extra horsepower would have dictated a four blade prop which would have weighed about 100# more, so 265# extra weight. But an extra 300HP over the single stage -85 at 22400' would have made it all worth the trouble. The heavier prop somewhat offset the weight of the second stage for balance. The two stage P-39 would have weighed in the neighborhood of 8000#, quite an improvement over the 8918# for the XP-39E and the 8950# for the P-63. And that's with 170gal fuel internal.

Merlin 61 made more power but would not fit into a P-39. The -93 would fit and was available from April '43 for a plane that was already is series production.
 
Hello P-39 Expert,

Let's accept for a moment that there are no issues with an engine swap and no additional cooling requirements for the higher powered engine (which I believe to be unlikely).
How much performance improvement do you believe would result from adding 300 HP at 22,400 feet?
The P-39N with the V-1710-85 was tested at
398.5 MPH @ 9700 feet and
389.5 MPH @ 16100 feet.

Would it even break 400 MPH at altitude?
Would the performance level be more suited to a typical ETO fighter from 1942?
The armament of a single big 37mm and a couple synchronized .50 cal would still be less than adequate.

- Ivan.
 
Maybe this is a topic for it's own thread but speaking of the p39 I've often wondered why it was not that effective aside from the Russian front(at least at medium to low altitudes like the p40) as it looks pretty good on paper. Good speed, good climb, and at least descent in the moaenuverability department.
Kind of a head scratcher.
 

You have a point - instability can be used to an advantage, and sometimes seems to correlate with agility. The I-16, which I mentioned, was notoriously twitchy but also had a great roll rate and was considered very agile... which you could take full advantage of if you were a very skilled pilot. However instability was also a problem, particularly if the fighter isn't super fast for it's time. The LaGG-1/3 and MiG-3 were also quite 'twitchy' or unstable but this was not considered an asset. Instability was especially a problem for less experienced pilots during takeoff and landing when many accidents, including often fatal ones, occurred.

Generally speaking there is a kind of sweet-spot that designers are looking for, and the perception of these characteristics by a given group of pilots, can in part be a reflection of the culture or training regime. Too stable can definitely be a drawback, look at the Hurricane. Too twitchy without a doubt can also be a problem.

For example there is also mention here of the P-39. One of the great mysteries of WW2 is why the Soviets liked them so much and did so well with them, while the Americans, British, Australians, Free French, Italian Co-Belligerent forces and so on, did fair to very badly with them and tended to want to get rid of them as fast as possible.

One explanation is that the Soviets were lying and they didn't actually shoot down very many German planes. I find this hard to credit.

Another partial explanation is that Soviet pilots were used to planes prone to stalls such as the I-16, LaGG-3, MiG-3 and so on, and they knew how to fly them without going into a spin etc. So they weren't bothered by the potential instability of the P-39, it was just like a LaGG-3 only faster, better armored, equipped with a radio and a giant cannon... whereas the Americans were not accustomed to these characteristics and hated the plane which they called the "Iron Dog".



However as a general rule, I would say that if you start out with say an airframe of a given size, it makes sense to lengthen it somewhat as you increase your engine output to help with stability. You can offset your tail stabilizer, you can make one wing shorter than the other and there are other tricks, but lengthening works too.

As was done between the P-40C (31'8") to the P-40N (33' 4") or from the Spitfire Mk1 (Length 29' 11" - 1030 hp) to the Spitfire Mk XIV (30' 9") to the Spitfire Mk. 24 (2,035 hp, length 32' 8")
 
I've always wondered why Bell went with a symmetrical airfoil, given that practically all combat maneuvers of the day were positive G, with the exception of the pushover ("bunt" in Brit-speak). A carefully chosen positive G "lifting" airfoil could have given better L/D in all positive G maneuvering, as well as improved rate of climb and possibly slightly higher speed. The P39 does look better on paper than its reputation supports, and I think that's partially due to the "paper" airplane's numbers being a little more optimistic than the aluminum plane's. This, coupled with its unorthodoxy of design, its behavioral issues and the inappropriate tactical training its early pilots got, started it off on the wrong foot, from which it never recovered.
Cheers,
Wes
 

Hello Schweik,

I believe that part of the explanation here is the relative performance of the P-39 in each theatre.
On the Eastern Front, the P-39 was flying near ground level where its performance was at its peak.
The Soviets deleted the wing guns, reduced the fuel load, and seriously overboosted the engines without regard to the limitations in the manual. At low altitude, there was enough supercharger to give some extra boost.
They also typically loaded more ammunition for the cowl mounted .50 cal than the US or other allies did.
This would tend to push the CoG forward and increase the margins for stability.
The single 37 mm cannon and pair of cowl mounted .50 cal didn't bother them because that was pretty close to what other Soviet fighters used. 1 MG and 1 cannon was not an unusual armament for their late fighters.
Also, for the most part, Soviet fighters were not particularly fast, so the speed of a late model P-39 would have been very comparable.

On the Western Front, the battles are much higher and start off about where the P-39 runs out of steam.
The P-39 was also relatively slow at altitude, especially by the standards in the West.


In the case of the Spitfire, I believe the "real" length of the airframe never really changed. The distance from firewall to rudder hinge line was the same for all versions. The overall length changed because the engine dimensions forward of the firewall changed and the rudder aft of the hinge line changed.

In the case of the P-40 Hawk 87 series, the short tail aircraft had an offset fin that was straightened on the long tail aircraft. Part of the problem with offset surfaces is that they are really only balanced for a certain speed range. There isn't enough aerodynamic force below the speed and there is too much above the speed and both conditions need to be trimmed out to fly straight and level without control inputs.

- Ivan.
 
Schweik said:
One explanation is that the Soviets were lying and they didn't actually shoot down very many German planes. I find this hard to credit
Click to expand...

Not lying, but perhaps overly optimistic as to how many enemy aircraft that they did shoot down?
 
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