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Thrust from model engine exhaust

  1. Aug 25, 2014 #1
    I’m interested in your views on whether any significant thrust could be obtained using the cooling air and exhaust gases from a model aircraft engine. International class control line speed models (known as F2A) achieve speeds of around 300 km/hr using 2.5cc engines and tuned pipe exhausts that allow the engines to generate around 1.5 KW of shaft power at 40,000 rpm. Assuming about 20% efficiency, that means there is at least 6 KW of heat energy being added to the exhaust air from engine cooling and exhaust gases. The latter leave the tuned pipe at around 350 C. Photos and drawimgs of typical models can be found at http://www.f2aspeed.org/gallery

    I’m thinking that if the tuned pipe were totally enclosed in a cowling, and the heated air mixed with exhaust gases then exited through a nozzle, some net thrust might be achievable. Any thoughts on this would be appreciated.

    Last edited: Aug 25, 2014
  2. jcsd
  3. Aug 25, 2014 #2


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    It's most likely that any gain that involves restricting the exhaust flow such as the cowling, would be offset by a loss in overall power.
  4. Aug 26, 2014 #3
    Restricting the exhaust flow may not be a problem. The tuned pipe needs back pressure to operate efficiently, the more the better up to the point where the engine runs too hot. A typical pipe has a cross section area of 100 sq mm at the engine end and 16 sq mm at the "stinger" (outlet) end.
  5. Aug 27, 2014 #4


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    A tuned pipe relies on sound waves reflecting back at the exhaust port at the proper time (for a given rpm) to prevent fuel + air mixture from escaping out the exhause port while it's exposed. In an ideal situation, most of the "back pressure" is from the sound waves.

    As far as thrust from the tuned pipe, isn't already generating thrust without having to be routed through the cowling. This is assuming that the speed of the exhaust is greater than the speed of the prop wash that is flowing past the exhaust tip.
  6. Aug 27, 2014 #5
    I had not intended to start a discussion about tuned pipes. The question is: if we can add 6KW of heat energy to the total exhausted cooling air plus engine exhaust, is it possible to generate net thrust - or even just reduce the drag from the cooling air flow?
  7. Aug 27, 2014 #6


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    Engine efficiency could be improved to harness more of the heat energy, but it seems to me that in this case, fuel efficiency isn't the goal, but instead maximum power at some specific rpm, so a pipe tuned for that rpm is used. Streamlining the fuselage and any exposed parts would reduce drag, but I don't think the exhaust flow could be made faster than the prop wash blowing back over the model without sacrficing engine power. I'm assuming that forced induction like mini turbo charger or mini super charger are not allowed.
  8. Aug 27, 2014 #7


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    It is a very remote analogy, but there was substantial work done in the 1960s to try to get improved thrust by entraining ambient air with the jet exhaust. Your question seems to me to have a similar goal, how to get some incremental work from the engine waste heat. Unfortunately, getting useful work from low grade heat is hard and all approaches I can think of cost too much in weight, drag or both to be beneficial for a competition design.
    The auto industry is looking at thermocouples to extract exhaust energy from the turbo exhaust, so the dream of getting a bit more is still getting attention.
  9. Aug 27, 2014 #8


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    I don't know how this would relate to your model, but with the Merlin engine in WWII somebody realized that the simple modification of directing the exhaust backwards, instead of a "straight" exit sideways relative the the plane, was worth about a 5% increase in engine power, and 10 knots on the top speed of planes like the Spitfire.

    Isn't the important quantity the change in momentum, rather than the exhaust velocity?

  10. Aug 27, 2014 #9


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    My guess is that with the induced prop wash flowing past the exhaust, the end result was partial vacuum at the exhaust, creating a scavenging effect. A change in momentum would normally not result in an increase in power, only a reduction in pressure at the exhaust ports of the engine.

    In the case of super charged nitro methane burning drag cars, the pipes are relatively short and turned upwards and a bit backwards, and the exhaust generates some downwforce helping to increase grip.
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