USAAF Attack & Light Bombers: Needs & Desires

[pbehn]

C CORSNING S Shortround6 T ThomasP

I was thinking of something about a WWII-era design achieving a combination of high speed and good range with one engine and I was thinking about the power output of the R-4360 (3400 hp with WEP, 3000 hp with military rated power, and 2550-2700 hp with normal rated power).

That power output is about twice the output of some RR Merlin variants, and that got me thinking of the De Havilland Mosquito with the RR Merlins removed and an R-4360 put up front (and this is, of course, grossly over-simplified): While the Mosquito isn't capable of the normal-rated g-loads demanded by either the USAAF (8-9) or USN (7-8), I figure something with the strength of the Tempest scaled-up to the Mosquito's size with a R-4360 could probably deliver some remarkable performance in terms of both speed and range.

The success of the Mosquito as a design was its high speed, two man crew and useful but not huge internal load capacity. Depending on use it could have huge range, or good range with bombs or cannon and all sorts of combinations. It was never developed to the fullest of any performance because fitting Griffons would improve some parts of performance and lose on others, because the Griffon is heavier and uses more fuel. By the end of its life the Merlin was producing 2,000HP so two would be 4,000HP and the Griffon was circa 2,500HP so 5,000 in an uprated Griffon Mosquito. The thing is, whether you use one or two engines or four engines to produce 5,000 HP you still consume similar amounts of fuel, so you have to put fuel somewhere.

 

[ThomasP]

Hey Zipper730,

The only maybe I can think of is a late-war Sabre powered single-engine detail improved kinda-Mosquito (wood or metal). By the late-war the Sabre was reliably putting out around 2x the power of the mid-war Merlin (non-WEP in both types). 1x Sabre installation would incur more drag than 1x Merlin, but less drag than 2x Merlin. And since you could get rid of the drag producing engine nacelles, you would be able to significantly reduce drag even further. Or maybe redesign the nacelles for landing gear only, in a more streamlined form? If IIRC the very late Sabre's sustained (Normal) power was 2x the Merlin sustained (normal). I do not know what could have been done relative to the development of higher altitude rated models, in terms of practicality. The immediate post-war Sabre model being worked on was putting out 3500 BHP with WEP and water injection.

As has been mentioned up-thread, there were other engines with similar power output, but all of them would have had significantly higher drag installations (I think). Also the Sabre installation should have greater fuel efficiency than the other similar power engines (I think) - although as SR6 and pbehn pointed out above, higher power incurs higher fuel usage in similar proportion.

 

[Zipper730]

C CORSNING , P pbehn , S Shortround6 , T ThomasP

I want to make sure that everybody understands that I'm not literally saying that the idea I was thinking of was exactly a De Havilland Mosquito with an R-4360 in the nose: It was merely the basic idea from which everything else would come out of. After all the aircraft has a number of redeeming qualities from an aerodynamic standpoint.

1. The wing-cross section and overall planform was very efficient and played a role in the aircraft able to achieve it's high speeds. It's maximum mach number also seemed to be pretty good too, though I'd have to check that out. By the time the universal wing was adopted, the stations used to mount drop-tanks and bombs seemed to do a pretty good job of blending in the tank or the bomb with the wing.​

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2. The aircraft had a decent overall internal volume which makes it possible to store a considerable amount of fuel internally, and carry a decent internal load (typically 4 x 500 pound bombs), while still achieving a respectable range in the process.​

The most basic changes would be that

1. It would be constructed out of metal instead of wood: This might actually free up more internal volume, but weight would probably be the same for the same g-load.​

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2. The R-4360 would eliminate the need for radiators: Oil coolers would need to remain, and they'd probably remain in basically the same place as before. To maximize performance, I figure you'd need a tight cowl with as much a bell-mouthed shape as possible.​

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3. The aircraft would only have a single crew: The canopy could be made smaller, and that would improve aerodynamics. There'd also be a weight reduction of around 200 pounds.​

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[Zipper730]

M MIflyer ,

Two issues, the first is the A-26: From what you said, it was felt to be about as fast at low altitude as the P-47, but nobody realized it's larger size would make it easier to hit. I assume this was before it entered operational service?

The second is the 37mm M9 cannon: While I remember hearing the length was 104" and weighed 405 pounds, was there any figures for the width and height of the gun? This has to do with the XA-41's design as it had really thick wings.

S Shortround6 , given that you seem to be the resident gun expert, I figured I should tag you on this one.

 

[MIflyer]

They found out about the speed and vulnerability of the A-26 when they actually introduced it into service in the ETO. They mainly switched A-20 units over to the A-26, the A-20's being flown to Scotland and run off a cliff, but I read of one B-26 unit going over to A-26 as well. I think they retained one of their B-26's for use as a transport.

 

[Zipper730]

They found out about the speed and vulnerability of the A-26 when they actually introduced it into service in the ETO.

So, it was kind of a retroactive detail they learned after the fact? I'm surprised nobody noticed this issue with the A-20's (they were used in the ETO right)

 

[MIflyer]

A-20's were not used as CAS, as far as I know. They were used on more medium bomber style missions in the ETO.

 

[Zipper730]

A-20's were not used as CAS, as far as I know. They were used on more medium bomber style missions in the ETO.

So, the A-26's were to be used for CAS & Interdiction?

 

[MIflyer]

I think that by then the USAAF's concept for light and medium bombers had changed. They cancelled the high altitude B-28 medium and in the Pacific the B-25 and A-20 had proved to be mainly valuable for low altitude attacks. In contrast the mediums seemed to have done almost no strafing in the ETO; many of the B-26's had four a package guns aside the nose and almost never used them - and the nose gun for the bombardier could not be used without first removing the Norden bomb sight. The A-26 was supposed to replace the A-20, B-25, and B-26, and with its interchangable noses could be configured either for level bombing or low altitude attack.

The concept they came up for the A-26 apparently was it would be used like a fighter bomber with a much greater load of ordnance, and then they found out it was so much bigger it was easy for ground fire to hit. I think the A/B-26s in Korea were used mainly at night and I have not read of them being used for CAS. I was told they fired 5 inch HVAR rockets at night, too.

I read of one Douglas B-26 in Korea chasing a N Korean PO-2 down a river bed, the bombardier yelling directions to the pilot, since he did not have a gun sight that was effective at night and thus they had to make an "X" with the nose while firing the wing guns in hope of hitting the target. Later the pilot of that same 26 decided to sneak up behind about 50 Yak-9's in daylight and pick a few off, which sounded like a really dumb idea to the bombardier (and I tend to agree with him). The Yaks spotted them before they could fire and scattered wildly.

 

[Zipper730]

I think that by then the USAAF's concept for light and medium bombers had changed. They cancelled the high altitude B-28 medium and in the Pacific the B-25 and A-20 had proved to be mainly valuable for low altitude attacks. In contrast the mediums seemed to have done almost no strafing in the ETO; many of the B-26's had four a package guns aside the nose and almost never used them - and the nose gun for the bombardier could not be used without first removing the Norden bomb sight. The A-26 was supposed to replace the A-20, B-25, and B-26, and with its interchangable noses could be configured either for level bombing or low altitude attack.

The concept they came up for the A-26 apparently was it would be used like a fighter bomber with a much greater load of ordnance, and then they found out it was so much bigger it was easy for ground fire to hit. I think the A/B-26s in Korea were used mainly at night and I have not read of them being used for CAS. I was told they fired 5 inch HVAR rockets at night, too.

I read of one Douglas B-26 in Korea chasing a N Korean PO-2 down a river bed, the bombardier yelling directions to the pilot, since he did not have a gun sight that was effective at night and thus they had to make an "X" with the nose while firing the wing guns in hope of hitting the target. Later the pilot of that same 26 decided to sneak up behind about 50 Yak-9's in daylight and pick a few off, which sounded like a really dumb idea to the bombardier (and I tend to agree with him). The Yaks spotted them before they could fire and scattered wildly.

The USAAF really liked things big. While the A-26 wasn't trivial in terms of maneuverability, it wasn't able to pull the g-load a fighter could, and would likely be less accurate than an P-47 or A-36

 

[MIflyer]

The USAAF really liked things big. While the A-26 wasn't trivial in terms of maneuverability, it wasn't able to pull the g-load a fighter could, and would likely be less accurate than an P-47 or A-36

They did use them for CAS in Vietenam and in the early days there tried what amounted to dive bombing with them - and pulled the wings off a few. That resulted in the B-26K Counter-Invader.

 

[Zipper730]

They did use them for CAS in Vietenam and in the early days there tried what amounted to dive bombing with them - and pulled the wings off a few.

Yeah, that's what happens when you build up too much speed and then pull up too abruptly. The plane wasn't really built for dive-bombing. It seemed to be built for about 30-degree glide-bombing, maybe 45-degrees.

That resulted in the B-26K Counter-Invader

They strengthened the wings?

 

[Shortround6]

Yeah, that's what happens when you build up too much speed and then pull up too abruptly. The plane wasn't really built for dive-bombing. It seemed to be built for about 30-degree glide-bombing, maybe 45-degrees.
They strengthened the wings?

From Joe Baughers website.
"The B-26K could carry 4000 pounds of bombs internally, plus up to 8000 pounds on the underwing racks. Besides the fixed wingtip tanks, two 230-gallon drop tanks or a 675-gallon bay tank could be carried. These changes increased the maximum cruising speed from 240 to 265 knots, the combat radius from 210 to 500 nautical miles, and increased the armament load from 7500 to 12,000 pounds."

yes they strengthened the wings.

"Unfortunately, these B-26s began to suffer frequent wing failures, forcing them out of service. Those that remained were provided with a strengthening wing strap along the bottom of the wing spars to prolong service life. The success of these modifications led the USAF to order a remanufactured version of the Invader from the On Mark Engineering Company of Van Nuys, California that would be specifically adapted to the counterinsurgency role."

"The Counter Invader was powered by a pair of 2500 hp Pratt & Whitney R-2800-103W water-injected engines driving a set of fully-reversible automatic feathering propellers. The wings were entirely rebuilt and strengthened by the installation of steel straps on the top and bottom of the spars. The rudder was enlarged to improve single-engine handling. Permanent 165 US gallon wingtip fuel tanks were installed. An anti-skid wheel braking system was adopted. Deicer boots and anti-icing equipment was added. The instrument panel was revised and provision for dual controls was made. New electronic equipment was adopted. Eight new underwing pylons were added for a variety of external stores. The dorsal and ventral defensive turrets were eliminated, and fixed armament consisted of a set of eight 0.50-inch forward-firing machine guns in the nose. Alternatively, the aircraft could be fitted with a glazed nose for photographic reconnaissance."

The P & W R-2800-103W engines were not used on the production versions. The 2500hp R-2800-52W engines were substituted.

 

[MIflyer]

A friend of mine that got to fly them in Thailand and Laos said the B-26K flew significantly different than the original models. The CG had changed. It felt different and was a bit harder to trim up. Note the "butter paddle" props.

 

[Zipper730]

yes they strengthened the wings.

Do you have any idea how much, if it's not classified?

 

[MIflyer]

The friend described the spar mod and I'll ask him about it if I see him today. The book "Foreign Invaders" says the 40 B-26K's had the fuselage re-manufactured with a larger rudder and the wings replaced, using only some of the original spar. They installed new brakes using KC-135 parts and 2500 HP water injected engines as well as all new weapons pylons built by Baldwin Locomotive.

 

[Zipper730]

While I should have covered this earlier...​

in the Pacific the B-25 and A-20 had proved to be mainly valuable for low altitude attacks. In contrast the mediums seemed to have done almost no strafing in the ETO

While I assume low-altitude attacks over the ETO (in particular, Germany) would have been more dangerous than over the PTO judging by what you wrote, though I'm still surprised they wouldn't have realized a bigger plane would be easier to hit.

While this might sound like a strange question: Did the USAAF have the same guys issuing requirements for fighters, attack planes, and bombers? Or were there different guys for fighters (or fighter & attack), attack planes, and bombers? I'm also curious if there was much communication between them as to what would and would not work (i.e. I know during the pre-war years, there was a hostile relationship between the fighter and bomber guys) during the course of the war itself.

As an additional matter: I was thinking about g-load figures for aircraft that had been employed in dive-bombing attacks and, unfortunately there's not a lot of figures I have, so there's a bit of guesses from various sources, and I'm not sure all of them are accurate

.....Aircraft	Normal Rated Load Factor​	Ultimate Rated Load Factor​
.....Douglas SBD	6g​	9g​
.....Junkers Ju 87	7.2g​	10.8g​
.....Vultee Vengeance	9g @ 11040 lb.​	13.5g @ 11040 lb.​
.....Fairey Barracuda	6g @ 12000 lb.​	9g @ 12000 lb.​
.....Curtiss SB2C	9g​	13.5g​
.....Bristol Beaufort	6.27g @ 17000 lb.​	9.4g @ 17000 lb.​
.....Bristol Beaufighter	5.67g @ 21000-22100 lb.​	8.5g @ 21000-22100 lb.​
.....Junkers Ju 88	4.95 - 5.4g​	8.1g​
.....Heinkel He 177	4.8g​	7.2g​

Regardless, the figures for the A-26 indicate a normal rated maximum g-load of 4.27g @ 26000 lb. which is below the combat weight and bombs-off. Even if that included a small bomb-load with fuel burned down a lot, it'd still be less than the He 177 which is the lowest figure in the group. The fact that the Ju 88 were sometimes fitted with dive-brakes for dive-bombing missions (some Beaufighters also) also seem to work against the A-26 as well.

While the He 177 was tested by Captain Brown who executed dive attacks in tests, it had a higher maximum g-load than the A-26 as well, and the fact that the probable cruise and maximum speeds would likely exceed the He 177, Ju 88 and Beaufighter, I figure it'd pick up speed faster in a dive which would make it harder to execute the maneuver.


Notes

• G-load figures for the SBD & Ju-87 were based on the assumption that the ability to routinely execute 6g pull-outs might indicate this to be the normal rated maximum load factor: With the Germans using an ultimate load factor being 180% the normal rated load-factor, the 6g figure would become 7.2g if corrected for the 150% safety factor used by the Allies.
• G-load figures for the SB2C were based on a statement that the maximum load the pilot could take was 9g, and the plane was built around that.
• G-load figures for the Vultee Vengeance, Fairey Barracuda, Bristol Beaufort, and Bristol Beaufighter are based on this chart, which unfortunately has a number of errors, but is the best information I have available (A flight manual on covers multiple variants (VI, X & XI) and states the plane should not be violently maneuvered above 21000-22100 lb. depending on modifications implemented).
• Figures for the Junkers Ju 88 came from here, with figures from the He 177's figures were from the book "Heinkel: He 177, 277, 274" by Manfred Griehl & Joachim Dressel: The Ju-88 was listed as having a normal rated load of 4.5g with the He 177 being rated for 4g with both rated for safety factors of 180% the listed numbers which, when corrected for allied safety figures produce the specified numbers. The discrepancy with the Ju-88 have to do with the fact that failure occurs at 180% (8.1g) with skin dimpling occurring at 110% (4.95g).

 

[Zipper730]

Would it have been hard to have designed the A-26 with the ability to pull g-loads similar to the Ju-88 or Beaufighter? This assumes the strength was built into the design from the start, not grafted on.

 

[MIflyer]

I recall reading that the A-26 was the first US airplane designed with the armor built in as part of thee structure rather than tacked on afterwards.

But it was not designed as dive bomber but as a level-in-formation bomber or a low altitude strafer and bomber, In Vietnam they were trying to use it as a dive bomber and that's when the wings started coming off.

 

[Zipper730]

I recall reading that the A-26 was the first US airplane designed with the armor built in as part of thee structure rather than tacked on afterwards.

I never knew it had integral armor in its construction. Since I don't know how strong the modified A-26's used in Vietnam were, I couldn't determine what could be done then or earlier, but I figured if they won't build a small single-engined bomber, at least they could build a twin that was tough enough to have some dive-capability.