O/T, RR Merlin exhausts

One fell off :-)

I'll get my coat .

John

In message , John writes

The Merlins on some Mosquitos, including the BAe-owned T. Mk. 3 RR299 (sadly lost some years ago) had 5 exhaust stubs per side; the rear two cylinders shared a stub. I don't know why this was done; perhaps there were clearance issues. Many other Mossies had the full six stubs per side, though.

I have a couple of books on the Mosquito - when I get a tuit of the circular geometry, I'll have a browse through them and see whether there are any comments about the 5-stub arrangement.

The only think that springs to mind is if they were for the night fighter or pathfinder bomber - designed to cut down on the exhaust glare which made the planes easily visible at night and wouldn't help with the pilots' vision. Why use both types, not sure really, maybe supply shortages during the war?

Julian.

I didn't think any Mosquitoes had exhaust stubs? Surely by then they were all using ejector exhausts, where the stub is longer and turns backwards, to give some additional thrust

The ejectors seem to have come in every shape and combination, especially those to do with flame suppression for use at night. Unlike the German designs (huge things, seemingly made from old woodstoves) the designers seem to have managed to achieve low visibility without generating huge drag.

Here

is a picture of RR299 showing the 5-exhaust arrangement. They are swept back and downwards at about 45 degrees; they might be described as ejectors. I'll have to dig through the Mossie books on my shelf to see whether there's any more info.

I've found a bit about the exhausts in 'Mosquito - the original multi-role aircraft' by Graham M. Simons. Basically it says that Mossies fitted with 'saxophone' shrouded flame damping exhausts (my note: these were mostly night fighters, but some other variants had them too) had exhaust shroud overheating problems. Open (round-ended) stubs were tried, but there was too much glare for night landings, and the aircraft became too visible to the enemy at night. The stub ends were given an oval section, which reduced the glare and also gave a speed increase of

13mph maximum, depending on altitude.

In message , Julian writes

I've had a quick look through the two Mossie books I've got, and found a bit of info which I've put in my reply to Andy Dingley's post. It seems the oval-ended stubs/ejectors were successfully developed after first trying open-ended stubs as a possible answer to the overheating problems with the shrouded 'saxophone' anti-glare exhausts fitted to night fighter and some bomber variants. There's no comment there, though, about the 5- versus 6-stub/ejector alternatives.

At a guess, looking at that picture, I'd say the idea there is to keep the (hot) exhaust gasses away from the wing leading edge - I guess it would be plywood?

Julian.

Looking at that Mossie picture that got posted, I'd hazard a guess (coupled with the 45 degree sweep down) that the rear cylinders shared an outlet which is well forward and thus gained clearance from the wooden wing structure.

Julian.

The answer is in this months "Aeroplane" courtesy of the RRHT. "The exhaust from no.6 cyl of Merlin XX engines was too close to the wing leading edge. so 5 and 6 were siamesed. This was not necessary on later Mosquitoes having 60 and 100 series Merlins which were 7" longer at the rear".

ttfn Roland

John

Ah, the Merlin 100 - the ultimate military engine!

Regards,

Kim Siddorn

Thanks Roland. It's all very obvious when you read it but it's nice to have it confirmed.

John