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Drag of exhaust ?

All too often, engine exhaust tubes stick out at an odd angle with the gases directed somewhat down and backwards but typically not aligned with local flow on the fuselage.

I did a quick guesstimate of the speed of theses gases. the value is between 20 and 30 meters per second (about 50 knots). I wonder how much distance such a powerful stream of hot air travels before it stops disturbing the local flow. I would love to see a CFD simulation. Any pointers much appreciated.
My guess is that the initial cylindrical section of the exhaust gas stream will expand in a more or less conical shape while being simultaneously ripped apart and bent backward by the local slipstream, creating an immense tridimensional bubble of turbulent air alongside the fuselage.

Has anyone done some investigation of the effect of aligning exhaust gases with the local flow? Why do our planes not use exhaust augmentation to tame and make use of the enormous “air pump” instead of treating it as a necessary evil ? Exhaust augmentation is know in the experimental world and used in air racing to produce low cost suction for cooling.

Last Edited by Flyingfish at 02 Jul 09:29
LSGG, LFEY, Switzerland

Not only experimental world, old Czech twin – L200 Morava uses so called “ejector cooling” where exhaust gases help to create engine cooling airflow.

In case of turboprop aircraft, the exhaust gases have quite significant effect on the propulsion of the airframe, I was told it accounts for some 5-10% of the total thrust…

LKHK, Czech Republic

I would guess it’s one of those things that makes no sense individually. For a decent increase in performance you also have to polish the aircraft every day, you have to fine tune the prop, fine tune the cooling, and dozens of other small things that by themselves hardly makes a difference. You have to do it “all” to see a real improvement. Doing it all becomes so expensive and so unpractical that you would only do it if performance was the one and only goal. It’s like a bicycle. Up to € 2-3k you get real improvements. From 2-3k and up (to 10-20k), the only improvement is weight. To win races at the highest levels, this weight is important, but for 40-50 year olds with 10-20 kg of belly fat it makes no sense whatsoever. But, 40-50 year olds have lots of money, so they gladly pay 10k for a bike anyway

The elephant is the circulation

Didn’t the P51 Mustang have a specially designed outlet for the exhaust gasses, effectively creating a jet-like boost? Running at 75" MP it must have been worth something…

EPKP - Kraków, Poland

Augmentors were often used on US-built production twins like the Twin Bonanza and Cessna 310.

The P-51 utilized heated radiator exhaust air for propulsion.

There were a lot of these ideas around in the pre-jet era!

Last Edited by Silvaire at 02 Jul 14:20

According to Wikipedia the Rolls Royce Merlin with ‘ejector exhausts’ turned backwards instead of outwards gained the equivalent of 70hp of thrust or 10mph of top speed

EGHO-LFQF-KCLW, United Kingdom

Yes the P51 is a textbook example of using the energy of COOLING to produce thrust. Exhaust is a similar but different discussion. Basically you use the speed of the outgoing gases to suck more gases out around it. Somme googling found this. It is a turboprop but very interesting read on the matter of exhaust gas augmentation



LSGG, LFEY, Switzerland

All too often, engine exhaust tubes stick out at an odd angle with the gases directed somewhat down and backwards but typically not aligned with local flow on the fuselage.
There have been fatalities in the U.S. where people died of CO because they shortened the exhaust pipes of their planes and CO got into the boundary layer, which moves very slow in comparison to the airspeed, creeping even “forward” (in flight direction) and entering the aircraft cabin …
Exhaust tubes should be long enough or otherwise constructed to prevent the a.m. – do it yourself shortening is a bad idea.

EDxx, Germany

My Commander 520 had augmenter tubes. It’s what made it sound like a pack of Harley’s when one flew by.

No intention to shorten the exhaust – quite the opposite. Augmenters typically extend the total length
Additional noise is a factor i had not considered !!! The Germans will eat me alive with their noise level based landing fees.
There must be some reason for setting the exhaust pipe at a sharp angle to the fuselage. If the only intent is to keep the soot away then it is a failure. Turbulence caused by the exhaust tube sticking out sticks the soot back all over the place…

LSGG, LFEY, Switzerland
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