Looking at various propeller designs, I started wondering about these limitations:

  • Large propellers at high speeds have tips going supersonic
  • Scimitar props attempt to keep prop tip from going supersonic
  • Counter-rotating props absorb more power at smaller sizes

This made me wonder if a coaxial prop had a larger prop spun at slower speeds and a smaller prop spun at high speeds would increase thrust better, or do other designs already deal with that problem?

  • $\begingroup$ The only research I found is for ship propellers, that's why this is a comment. They discuss the question you pose; results have shown a 2.5% increase in efficiency over equal-size tandem propellers. However, it was worse in vibration (pressure pulsations). $\endgroup$ – ymb1 Oct 6 '18 at 15:56
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    $\begingroup$ This answer was proposed for a different question, but it seems to have some useful crossover: aviation.stackexchange.com/questions/53422/… $\endgroup$ – Marius Nov 5 '18 at 16:26

Coaxial, contrarotating props are one way to improve engine efficiency over a single propeller. When doing static or bench testing with no airflow into the props, the front prop will carry more load than the back one (which may have lead to this question).

Fixed pitch propellers "unload" a bit once the plane gets moving. A distinct increase in rpm can be heard with models. This effect is also called "windmilling".

With a contra rotating prop, the loads on the front and back props will there for even out more in flight.

An increase in weight and complexity of the gearbox has caused many to shy away from this design, but it is an interesting study as powerful electric motors with much higher rpm ranges are coming into use.

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    $\begingroup$ "the front prop will carry more load than the back one " - How about some supporting reference? $\endgroup$ – jwzumwalt Dec 6 '18 at 10:35
  • $\begingroup$ Increased load of leading prop on static bench test may be due to increased drag/ AoA effects. A coaxial prop could be load tested in a wind tunnel to give power draw curves of both props at flying speeds. Individually powered electrics mounted in a coaxial configuration (of varying distances apart on varying airframes (such as Dornier DO 335 "Pfeil")) would make a great study, $\endgroup$ – Robert DiGiovanni Dec 6 '18 at 10:52

Contrarotating propellers may have a better efficiency than a single propeller, since the propwash is 'straightened out', the tangential component of the airflow being much reduced. Besides, and for very high engine powers, those props have also the advantage of being able to absorb a much higher power. But that's all...

  • $\begingroup$ How about some supporting reference? $\endgroup$ – jwzumwalt Dec 6 '18 at 10:33

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