The Boeing XB-39 Superfortress: Ninety-six cylinders of American muscle

"ttyymmnn" (ttyymmnn)
08/05/2019 at 12:35 • Filed to: wingspan, planes you've (probably) never heard of, Planelopnik

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From the Planes (and Perhaps Engines) You’ve (Probably) Never Heard Of Department of Wingspan, we bring you the Boeing XB-39 Superfortress.

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When the !!!error: Indecipherable SUB-paragraph formatting!!! took its maiden flight on September 21, 1942, it was one of the largest and most technologically advanced heavy bombers of its day. It would go on to be one of the most effective bombers of the war, and enjoy a successful postwar career where it served as the basis for numerous other aircraft, both military and civilian. But early problems with the reliability of its radial engines caused major headaches for the Army Air Forces, and they began looking at other possible engine solutions. They also wanted a fallback in case the supply of radial engines was interrupted. So, like the !!!error: Indecipherable SUB-paragraph formatting!!! before it that experimented with inline rather than radial engines, a radical new inline engine was also tested for the B-29.

Allison V-1720 liquid-cooled V-12 engine (US Air Force)

Liquid-cooled V-12 engines were all the rage in Europe, but the United States fielded only a single V-12 aero engine during WWII, the !!!error: Indecipherable SUB-paragraph formatting!!! (a number which reflects the engine’s !!!error: Indecipherable SUB-paragraph formatting!!! ). This engine appeared in a number of iconic warbirds, including the !!!error: Indecipherable SUB-paragraph formatting!!! , !!!error: Indecipherable SUB-paragraph formatting!!! , and !!!error: Indecipherable SUB-paragraph formatting!!! . But if 12 cylinders are good, wouldn’t 24 cylinders be even better? The result of that thinking was the !!!error: Indecipherable SUB-paragraph formatting!!! 24-cylinder engine.

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At its most basic level, the V-3420 is two V-1710s placed side-by-side (because 1710 x 2 = 3420), with the two crankshafts joined through a common crankcase. The “double-v” configuration resulted in what was known as a W engine (because, after all, a W is two V’s, not two U’s). Fitted with both a gear-driven single-stage supercharger and a turbocharger, the V-3420 produced 2,600hp at full military power while turning at 3,000 rpm, and 1,575hp while cruising at 25,000 feet. Not only was the experimental engine fitted to the XB-39, it was also used in other experimental aircraft such as the !!!error: Indecipherable SUB-paragraph formatting!!! , !!!error: Indecipherable SUB-paragraph formatting!!! and !!!error: Indecipherable SUB-paragraph formatting!!! .

The first pre-production YB-29 (41-36954), with its original Wright Cyclone radial engines, awaits its maiden flight at Boeing’s Wichita facility. Note the green paint that was dropped on production aircraft, as well as the three-bladed propeller which later gave way to a four-bladed prop. (Author unknown)

Boeing loaned the first pre-production YB-29 (41-36954) to General Motors, who undertook the task of mating the massive V-3420s to the bomber. With the increased power, the XB-39, named Spirit of Lincoln , reached a top speed of 405 mph at 25,000 feet—about 50 mph faster than the radial engined B-29 and just 35 mph slower than the top speed of the Mustang. It could top out at 35,000 feet, about 3,000 feet higher than the B-29. Cruising speed was slightly slower than the B-29, but the maximum range was more than 6,000 miles.

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The XB-39 took its maiden flight on December 9, 1944 and, by all accounts, the Allison engines performed well and the overall performance was impressive. However, despite continuing problems with the radial engines of the production B-29s that were already in service, the Army Air Forces decided that the bombers fighting in the Pacific were doing well enough that it made little sense to completely overhaul the production line to introduce the new engine. And with so much attention being paid to the development of the XP-75 Eagle and its V-3240 engine, there was little time to spend perfecting the engines for use on the XB-39. Only a single XB-39 was built, and its final disposition is unknown.

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Much more information about the Allison V-3420 can be found !!!error: Indecipherable SUB-paragraph formatting!!! .

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Connecting Flights

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For more stories about aviation, aviation history, and aviators, visit !!!error: Indecipherable SUB-paragraph formatting!!! . For more aircraft oddities, visit !!!error: Indecipherable SUB-paragraph formatting!!! .

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DISCUSSION (22)

Highlander-Datsuns are Forever > ttyymmnn
08/05/2019 at 13:00

So what is the firing order?

ttyymmnn > Highlander-Datsuns are Forever
08/05/2019 at 13:04

No idea. But I found this, which is interesting.

An interesting episode in the development of the Allison V-3420-B10 engine (which powered the Fisher P-75) was its initial inability to meet the contract guarantee for rated power and how it was resolved.

Performance guarantees had been based on testing using an engine configured as a V-3420A to enable its running on the available Allison altitude test stand. So configured, the test engine used the standard V-3420A firing order with each of the 24 cylinders firing 30 degrees apart.

As designed and delivered the B-10 engine used 60 degree firing, i.e. simultaneous firing of two cylinders (one in each vee), referred to as “zero” phasing by Allison. When the engines were run in the altitude dynamometer it was found that the B-10 was about 3 percent low on power. In an effort to restore this power Allison adopted an alternate firing scheme known as 150 degree phasing and utilized a 30 degree interval. Historically, the details of how they accomplished this have been a mystery. Allison Competition Engine s recently had an opportunity to teardown a B-10 known to have been modified with the 150 degree phasing. By taking careful notes of the as-found configuration we are now able to provide the details.

https://www.456fis.org/ALLISON_V-3420_ENGINE.htm

Rusty Vandura - www.tinyurl.com/keepoppo > ttyymmnn
08/05/2019 at 13:04

B-36 Peacemaker = 336 spark plugs and 22,800 horsepower

Wright Cyclone 4360, 28 cylinders. But I assume you already knew that.

Next time you’re in California, let’s drive out to Merced / Atwater and see the Castle AFB Museum, or whatever it’s called nowadays. They have an excellent collection there. Not sure what shape it’s in since the AFB’s been closed several years.

ttyymmnn > Highlander-Datsuns are Forever
08/05/2019 at 13:06

Here’s the firing order for the V-1710:

RH turning: 1L-2R-5L-4R-3L-1R-6L-5R-2L-3R-4L-6R

LH turning: 1L-6R-5L-2R-3L-4R-6L-1R-2L-5R-4L-3R

Highlander-Datsuns are Forever > ttyymmnn
08/05/2019 at 13:10

30 degrees makes sense, that’s 12 cylinders firing per revolution, wow.

McMike > Highlander-Datsuns are Forever
08/05/2019 at 15:01

Since they were using two different cranks, they retarded the timing of the second engine 15 degrees, splitting the difference so the whole unit would fire every 15 instead of 30.

1-13-12-24-4-16-9-21-2-14-11-23-6-18-7-19-3-15-10-22-5-17-8-20

I just made this up.

ttyymmnn > McMike
08/05/2019 at 15:25

That’s retarded.

wafflesnfalafel > McMike
08/05/2019 at 23:56

and 17 fires twice each cycle just for good measure

wafflesnfalafel > ttyymmnn
08/05/2019 at 23:59

T hat is really interesting - knew the radials had problems but never heard of these motors much less hanging them off a 29. . Those engine nacelles are gigantic.

ttyymmnn > wafflesnfalafel
08/06/2019 at 00:44

Thanks for reading. Kind of a shame that none of the applications for these engines (see Connecting Flights) ever entered production. It’s amazing just how for piston technology came during the war. The Eagle has always been one of my favorites, and I got to see it at the USAF Museum a couple of years ago. It’s a big bird.

Kinja'd!!!

Full of the sound of the Gran Fury, signifying nothing. > ttyymmnn
08/06/2019 at 01:23

I thought I knew a great deal about aviation history, but maybe my specialty is more on the commercial side of the house. You never fail to amaze me with the odd and obscure aircraft you detail in these posts. Keep up the good work!

ttyymmnn > Full of the sound of the Gran Fury, signifying nothing.
08/06/2019 at 07:43

Thank you very much. I appreciate that. I do spend a rather inordinate amount of time trolling through collections of old photographs and books looking for oddities. In many ways, I’m more interested by the aircraft that failed to catch on than the ones that did, and WWII and postwar period are by far my favorite eras. Before the computer, it was much more of a, “Let’s see of this works” sort of thing. Thanks again.

benjrblant > ttyymmnn
08/06/2019 at 09:49

Where were the heat exchangers for these water-cooled engines? Did they live in the nacelles too?

ttyymmnn > benjrblant
08/06/2019 at 10:00

That’s a good question, and one that I don’t have a ready answer for. I did find this photo, though.

Kinja'd!!!

On the XP-58, the radiator was back in the tail boom inside the air intake. I’m more of a historian than an engineer, so I would guess that the radiator on the XB-39 was underneath the engine behind the intake below the spinner.

Kinja'd!!!

ttyymmnn > benjrblant
08/06/2019 at 10:02

Well, if I had bothered to read....

The source of that photo has this in the caption:

The V-3420’s radiator, oil cooler, turbosupercharger, and intercooler were all mounted in the nacelle, under the engine. This configuration prevented the need for heavily modifying the aircraft.

You can find tons more technical stuff about the engine here.

https://oldmachinepress.com/2017/04/20/allison-v-3420-24-cylinder-aircraft-engine/

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benjrblant > ttyymmnn
08/06/2019 at 11:48

Awesome! Thank you.