exhaust stacks & thrust

[agent 009]

This might be a no brainer, but. I did read about DB 605 motor having exhaust exits designed for extra thrust. I thought about it & went, hmmm don't all of em point backwards, so creating thrust?

& does it make any difference if the stack is on the bottom like 109, or on top like Spit & Mustang?

I've looked pretty close & they all look fairly similiar in direction. Some angled a bit more "out" than others versus straight backwards.


Just curious.

[Krusty]

Well from what I understand the stacks on the spit (or hte hurricane can't remember) added +3 mph alone.

Maybe they only add thrust if they are constricted, and forced out at a higher speed? Or maybe if they take turns before releasing the exhaust it lessens the thrust? I don't know, but those are some ideas.

[Tails]

Originally posted by Krusty
Well from what I understand the stacks on the spit (or hte hurricane can't remember) added +3 mph alone.

Maybe they only add thrust if they are constricted, and forced out at a higher speed? Or maybe if they take turns before releasing the exhaust it lessens the thrust? I don't know, but those are some ideas.

Well, there is a system called an 'Augmentor' that is designed to create a few pounds of jet-thrust from recip engine exhaust. Basicly is collects all the exhaust stacks to a single pipe pointed rearwards. Just past this common exhaust pipe is another tube of larger diameter, positioned a scant fraction of an inch from the end of the exhaust pipe. The fast moving, hot, exhaust gas from the common pipe draw cooler ambient air with it into this second tube, and this cooling mix of gases expands on its way down the tube, and when leaving the other end.

Like I said, it isn't much, only a couple pounds of jet thrust, but it helps. I dont know of any other designs that create jet thrust from recip exhaust, but that doesn't mean they dont exist either. I will say that short-stack exhaust, like what the Spit's seem to have (and not having poked under the cowl on a Supermarine product, I cant say for certain thats what they are) will not add much, if anything, in the way of jet-thrust.

[Guppy35]

Originally posted by Tails
Well, there is a system called an 'Augmentor' that is designed to create a few pounds of jet-thrust from recip engine exhaust. Basicly is collects all the exhaust stacks to a single pipe pointed rearwards. Just past this common exhaust pipe is another tube of larger diameter, positioned a scant fraction of an inch from the end of the exhaust pipe. The fast moving, hot, exhaust gas from the common pipe draw cooler ambient air with it into this second tube, and this cooling mix of gases expands on its way down the tube, and when leaving the other end.

Like I said, it isn't much, only a couple pounds of jet thrust, but it helps. I dont know of any other designs that create jet thrust from recip exhaust, but that doesn't mean they dont exist either. I will say that short-stack exhaust, like what the Spit's seem to have (and not having poked under the cowl on a Supermarine product, I cant say for certain thats what they are) will not add much, if anything, in the way of jet-thrust.

They tested a Spit Vb in 1943 to see what could be done to increase speed.  Initially the speed was 357 mph.

Replacing the regular' fishtail' exhaust with the multiple ejector exhausts gave an increase of 7 mph.

Removing the carb intake ice guard gave an extra 8 mph.

Changing the mirror from the flat rectangle, to the rounded cupped mirror added 3 mph.

Adding a whip aerial after removing the standard mast gave 1/2 mph.

Cutting the cartridge case link ejector chutes flush with the wing gave 1 mph.

Sealing all the cracks, rubbing down, painting and polishing the leading edge of the wing gave and extra 6 mph.

Polishing the rest of the aircraft gave an extra 3mph.

So they were able to increase the all out speed on that one Spit Vb from 357 to 385 1/2 mph.

Time to start bribing your crew chief

[Tails]

Originally posted by Guppy35
They tested a Spit Vb in 1943 to see what could be done to increase speed.  Initially the speed was 357 mph.

Replacing the regular' fishtail' exhaust with the multiple ejector exhausts gave an increase of 7 mph.

Removing the carb intake ice guard gave an extra 8 mph.

Changing the mirror from the flat rectangle, to the rounded cupped mirror added 3 mph.

Adding a whip aerial after removing the standard mast gave 1/2 mph.

Cutting the cartridge case link ejector chutes flush with the wing gave 1 mph.

Sealing all the cracks, rubbing down, painting and polishing the leading edge of the wing gave and extra 6 mph.

Polishing the rest of the aircraft gave an extra 3mph.

So they were able to increase the all out speed on that one Spit Vb from 357 to 385 1/2 mph.

Time to start bribing your crew chief

If a fishtail exhaust is a collector pipe, then I can see how they would get some extra speed going to the short-stacks. Aerodynamic cleaning for one, and a reduction in backpressure adding a horse or two to that Merlin for another.

Pretty amazing to see the kinds of effects one can have 'nickle and diming' their way to a cleaner aircraft, though

[agent 009]

I did read the 109 exhaust system was redesigned in 44. Wondered if this was what the reason was.

[frank3]

Many aircraft were designed to have extra thrust from the exhaust pipes, it was just a clever and easy thing to do

[gripen]

Originally posted by Tails
If a fishtail exhaust is a collector pipe, then I can see how they would get some extra speed going to the short-stacks. Aerodynamic cleaning for one, and a reduction in backpressure adding a horse or two to that Merlin for another.

The fishtail system had individual exhausts for all cylinders (one cylinder per one exhaust pipe) ie it was not a collector style. Early Spitfires had ejectors with two cylinders per one exhausts but that changed to one/one during the production of the Spitfire V. The DB 605A had one exhaust per one cylinder system.

gripen

[frank3]

That must explain why the 190's have 2 slides for exhaust pipes instead of the tubes! Thanks

[Badboy]

Originally posted by frank3
Many aircraft were designed to have extra thrust from the exhaust pipes, it was just a clever and easy thing to do

Yep, and some manufacturers were slower to catch on than others, so it wasn't until 1943 that the A6M5 recieved the Sakae 21 radial engine, modified with individual ejector exhaust stubs for greater thrust augmentation.

Badboy

[HoHun]

Hi agent,

>This might be a no brainer, but. I did read about DB 605 motor having exhaust exits designed for extra thrust. I thought about it & went, hmmm don't all of em point backwards, so creating thrust?

You are right, using exhaust thrust was a standard feature on WW2 fighters. The total contribution of exhaust thrust to propulsive power could be as high as 20% at top speed.

Usually, short individual stacks gave higher thrust than longer stacks combining the exhaust of several cylinders. As a rule of thumb, radial engines with exhaust collectors generate only 50% of the exhaust thrust they'd have with individual exhaust stubs.

The augmentor mentioned here is rather unusual. Similar designs were fashionable in the late 1930s, and Focke-Wulf tried something like that on their fighters in 1943 without success, and apparently the British augmentor didn't make it into service either.

>does it make any difference if the stack is on the bottom like 109, or on top like Spit & Mustang?

No. The difference is simply due to the location of the cylinder heads, which are on top for the Allied aircraft and on the bottom for the German aircraft.

Regards,

Henning (HoHun)

[Porta]

Originally posted by agent 009
I did read the 109 exhaust system was redesigned in 44. Wondered if this was what the reason was.

What's the source for this?.

Just curious, as I've read a pair of documents about improving the powerplant of the Me 109 in mid-43 and the exhaust systeme was in it, but nothing about it's final adoption.

[gripen]

AFAIK the first plane to utilize exhaust thrust from an radial engine was the I-16 but probably even some WWI planes benefited from it. Here in Finland, the VL Humu got quite similar stacks as the late Zeros while the original Brewster B-239 did not utilize exhaust thrust.

I don't know if any true augmentation systems reached service during the WWII but after war at least the Convair 340 had such system, some of them might be still flying.

gripen

[Schutt]

Exhaust stacks where also modified to include flame dampeners so the
Aircraft are less visible by night and the pilots got better view since they are not blinded by bright flames spouting through the exhaust.

So not all exhaust stack modifications where done for speed, some where also for flame dampening.

Not sure if there was any sound engineering with it like to deflect the sound to extend backwards/upwards rather than forward/down, but i guess it did go that way anyways.

[Crumpp]

The 1944 BMW801D2 output around 300lbs of exhaust thrust at Erhöhte Notleistung.

The Germans did the same thing and redesigned the exhaust system on the BMW-801 series begining in April 1943.  The design changes were incorporated in June 1943 serial production.

All the best,

Crumpp