B-29: Question: Auxiliary exhaust on post war

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Juanita

Airman
68
39
Apr 7, 2014
Checking over graveyard photos of B-29s I notice that some had an unusual black triangle on the rear fuselage between the rear upper hatch and the tailplane.
On closer inspection is appear to be an exhaust with a black triangular heat/exhaust shield.
Some aircraft have them fitted, but others don't.
I'm wondering it it might have been related to the target towing modification that was made to a number of B-29s.

Have included a photo...any information appreciated

Juanita
 

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Let me offer an alternative. The B-29 was a pressurized aircraft. The cockpit and aft section (crew compartment) were pressurized. Like all pressurized aircraft the airplane body (fuselage) was capable of withstanding a fair amount of differential air pressure. In theory, we could seal the bottle so, as the airplane climbs, the interior air pressure would stay the same. While possible it would be VERY hard to perfectly seal a huge airplane fuselage and in a sealed system the crew would quickly use up the available oxygen. To solve the problem, pressurization systems constantly pump fresh, outside air into the fuselage. To control the interior pressure, and allow old air to exit, there is a motorized door called an outflow valve located near the tail of the aircraft. It's about the size of a briefcase and located on the side or bottom of the fuselage. Larger aircraft often have two outflow valves. The valves are automatically controlled by the aircraft's pressurization system. If higher pressure is needed inside the cabin, the door closes. To reduce cabin pressure, the door slowly opens, allowing more air to escape. It's one of the simplest systems on an aircraft.
 
Thanks for those replies.
Mike: Wouldn't this system be fitted to every B-29? Right from the beginning they were pressurized....however this particular triangular shapes detail is only visible on post-war aircraft. Its not on Korea war examples either.

Thanks also pbehn: I suppose its possible that it was a different type of putt putt fitted on some B-29s towards that end of the types service.
 
Juanita said:
Thanks for those replies.
Mike: Wouldn't this system be fitted to every B-29? Right from the beginning they were pressurized....however this particular triangular shapes detail is only visible on post-war aircraft. Its not on Korea war examples either.

Thanks also pbehn: I suppose its possible that it was a different type of putt putt fitted on some B-29s towards that end of the types service.
Click to expand...
I only started reading about these as a result of your post. But reading the links I attached, there are all sorts of pictures of "putt putts" and one B29 picture that has an exhaust in a similar but different location, it looks higher up.
 
Thank you, I see on the The 330th Bomb Group link there is a photo of the external exhaust...and it is exactly where the triangle on the graveyard examples is located. In fact the exhaust hole looks the same shape but perhaps a little bigger.
Its beginning to look like its a safe to bet that a replacement APU units fitted to these aircraft. Interesting that they felt the need to paint the area around it black, but this hadn't been necessary in the past.
 
Interesting...perhaps just a post war thing to paint the exhaust area black.
 
Not an expert by any standard but I'm gonna stick with outflow valve. The B-29 was the most expensive weapons system every undertaken by the US. Development of the B-29 exceeded the cost of the Manhattan Project by almost 2 billion USD. The first prototype flew on 21 September 1942 and the second prototype on 30 December 1942 when an engine fire terminated the flight. The second prototype flew again on 18 February 1943 when an extensive engine caused the aircraft to crash into a Seattle Meat packing plant. The 10 - man crew died plus 20 workers at the plant and a Seattle firefighter. Problems continued and by late 1943 100 aircraft had been built but only 15 were airworthy. By early 1944 brand new B-29s were being flown from their 4 main assembly plants directly to numerous modification depots for extensive rebuilds. Those early Wright R-3350 engines were constantly overheating, throwing valves and starting fires. The upper 5 cylinders were replaced every 25 hours and the entire engine every 75 hours.
The change to the P & W R-4360 Wasps mostly fixed the overheating but even today the two flyable B-29s "Doc" and "Fifi" must be gotten up to speed as quickly as possible to cool the engines. Gaining altitude becomes a secondary problem
As one of the first pressurized aircraft I suspect that the initial aircraft leaked enough air so as not to require an actual outflow valve.
After the war with more time to do a better build those leaks were eliminated and the outflow valve installed.
 
Appreciate what you're saying Mike.
Think in this instant its the APU...as shown in the extract from the manual posted by Funbar57

Here is my photo again, but this time with the detail circled in red.
Perfect match up with the APU material posted by Funbar57.
As an asided: look at the 4 'handles' fitted at each corner of the upper hatch...interesting....Who said post war B-29s were boring?
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