A contra-rotating propeller
Image source

Here is a contra-rotating propeller. The front propeller rotates counterclockwise while the back propeller rotates clockwise. My questions are:

  • Do they rotate at the same speed (RPM)?
  • If not, which one is rotate faster, and why?

Most if not all contra-rotating propeller systems are geared together so that both sections turn at the same speed. This is done so that as well as allowing effective conversion of very high power into thrust, torque and P-factor are cancelled, making the aircraft easier to fly (especially in a single-engine or two-into-one installation).

This is why you see the same diameter propeller in front as in the rear -- if they were turning at different rates, the slower one could be larger diameter (a limitation on diameter is transonic conditions at the tips) -- but you always see the same diameter on front and rear.

  • $\begingroup$ Vibration would be an issue too. $\endgroup$ – ymb1 Sep 17 '19 at 17:53
  • $\begingroup$ Thank you for the explanation. It make sense if one is longer than another, then the longer one must be slower. $\endgroup$ – AirCraft Lover Sep 18 '19 at 0:05

It depends on the power source.

  • a single piston engine driving both props (as shown in the image in the question): they're geared together so rpm will be the same.
  • two turbines driving one prop each (e.g. Double Mamba): no coupling between the engines, so prop speeds may be different. You could even shut down one engine in flight (for low-speed, fuel-efficient cruise). The speed of each prop depends on the throttle setting of each engine, so the pilot can run either the front or rear prop faster, or have them both at approximately the same rpm.
  • Compound engine driving both props: the Napier Nomad drove one prop from the crankshaft, and the other from a turbine. While RPM would be related, they wouldn't necessarily be identical.
  • $\begingroup$ The Russian Bear bomber has one turbine driving each propeller set, as I recall -- and works like a single piston engine, gears to drive the two props off the same output shaft. $\endgroup$ – Zeiss Ikon Sep 18 '19 at 12:05
  • $\begingroup$ TU 95 Bear Propellers are not geared together, as can be seen from this video where engine number 2 is shut down in flight, and the feathered propellers rotate at "random" rpm: youtu.be/UZgBJOUcK3U?t=102 $\endgroup$ – Jpe61 Sep 18 '19 at 12:34
  • $\begingroup$ @hobbes, I am quite confuse to your answer. Just back to my question: Do they rotate at the same speed (RPM)? and If not, which one is rotate faster, and why? MY question is NOT about how the work. $\endgroup$ – AirCraft Lover Sep 18 '19 at 13:41
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    $\begingroup$ My answer is that the rpm difference (if any) depends on the aircraft you're looking at. On some aircraft, the props are geared together so they always run at the same speed, on other aircraft the props are independent so prop speeds are up to the pilot. For the aircraft you've shown in the question, the props are geared together so they run at the same speed. $\endgroup$ – Hobbes Sep 18 '19 at 13:48
  • $\begingroup$ That nice that they propellers are not rotating at the same RPM. But which one is rotate faster in the case they are not rotate at the same RPM and why? That is my second question. $\endgroup$ – AirCraft Lover Sep 18 '19 at 13:53

From efficiency point of view, counter-rotating propellers are worse than a single propeller. Exception to that is counter-rotating rotors on some helicopters where there is enough vertical separation so the upper rotor stream contracts and some fresh air is feed to the bottom rotor. As airplane propellers would work most of the time at high speed, the influx of fresh air is lost, the second propeller is operating in the stream of increased velocity, and so the efficiency is less. To answer your question I'll need more information, as what you want to achieve. For example if the goal is to eliminate torque, the rear propeller needs to rotate faster. If the goal is to reduce fuel burn then one propeller needs to be stopped and feathered.

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    $\begingroup$ What is your source for counter-rotating propellers being less efficient than single propellers? This paper cites two studies showing at least 6% better efficiency for CRP: rcgroups.com/forums/showatt.php?attachmentid=2815700 $\endgroup$ – Jpe61 Sep 18 '19 at 16:00
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    $\begingroup$ Turbulence from the front blades may reduce efficiency of the rear blades, but in cases where you have too much power for a single propeller to handle at RPM that gives subsonic tips, a contra-rotating setup is a lot more efficient (and quieter,and more durable) than a single propeller with supersonic tips. $\endgroup$ – Zeiss Ikon Sep 18 '19 at 18:59

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