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?
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asked Sep 17 '19 at 15:22
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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.
answered Sep 17 '19 at 15:43
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$\begingroup$ 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
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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.
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$\begingroup$ 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
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$\begingroup$ @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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2$\begingroup$ 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
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$\begingroup$ 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
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No, not all contra-rotating propellers turn at the same rpm.
Take two russian (very famous) examples, the Tupolev TU-95 Bear, and Antonov AN-22 Cock (go figure...). Both use versions of NK-12 turboprop engine, and as can easily be seen in many videos available on the internet, their propellers are not geared in a way that would make them turn in equal rpm.
TU-95 propellers turning ratio is such, that for each full turn of the front propeller, the one behind turns a little more than half a turn: TU-95 engine start on Youtube.
For AN-22 the ratio is one full turn for front prop, and about 3/4 for the rear one: AN-22 engine start on Youtube.
As for the reason of this, I have no knowledge about that. My more or less civilized guess would be that it has to do with load distribution between propellers, and maybe something to do with vibrations and resonances of the powertrain.
The aforementioned ratios can be seen during engine startups, but if the propellers are geared together, the ration will of course be fixed througout the rpm range. There has been speculation that the props on NK-12 are driven by separate turbine shafts (not making their rpm's physically connected at all), but I have not been unable to prove this wrong or right.
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$\begingroup$ "TU-95 propellers turning ratio is such, that for each full turn of the front propeller, the one behind turns a little more than half a turn: TU-95 engine start on Youtube." During initial startup, yes, but this disappears once the engine core reaches operating speeds. You would see interference patters in the props and create nauseating vibrations and noise pulses if it did not. $\endgroup$ – Carlo Felicione Feb 21 at 23:34
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$\begingroup$ Have you whatched the videos? There are all kinds of interference patterns during and after startup, it actually has more to do with the camera used than anything else... If the propellers are geared together, how would the ratio be changed after start? And more importantly, why? $\endgroup$ – Jpe61 Feb 21 at 23:39
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$\begingroup$ The props may not be geared together, but make use of independent propeller govenors to maintain a constant, equal speed between them during operation. Yes I watched the videos, but as I said in the first comment, watch for interference patterns once the airplane is ready to taxi. You won't see them. $\endgroup$ – Carlo Felicione Feb 21 at 23:44
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$\begingroup$ And as I said, the interference patterns are dependent on the camera. They may not be visible because of the frequency being such that the camera is not able to capture it. I would gladly accept being proven wrong here, if undisputable evidence was presented. Is it possible to drive two propellers from a single shaft with variable rpm ratio, if yes, how? If not, why? $\endgroup$ – Jpe61 Feb 21 at 23:50
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$\begingroup$ You could but why would you want to? See my answer below. $\endgroup$ – Carlo Felicione Feb 22 at 0:03
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For the aircraft in the picture ie a heavily modified P-51 unlimited racer driving a Rotol Contra Rotating propeller, the answer is yes. Both propellers are geared off the same crankshaft and turn at the same speed.
All contra rotating propellers turn at the same speeds during operation. If not, it would negate the purpose of the setup, that is to eliminate a net P-Factor, spiral slipstream and engine torque effects. They would also cause nauseating pulsing noises in the cabin which become very uncomfortable after long periods.
answered Feb 21 at 23:45
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$\begingroup$ The second propeller encounters airflow affected by the first propeller. For the P-factor to be balanced out, the propellers would have to be turning in same rpm, pitch and airflow. Change any of these, you have to modify something else too, like rpm for instance. And by the way, TU-95 is famous for its noise and vibrations. $\endgroup$ – Jpe61 Feb 22 at 0:26
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$\begingroup$ Properly designed a good contra rotating propeller will cancel out P-factor, or significantly reduce it. I’m not buying that Tu-95 operates with one of the propellers turning faster than the other one is. This is a colossally stupid design and extremely wasteful in terms of additional drag. That fact that makes noise and vibrates is irrelevant. Soviet military hardware was brutally functional. $\endgroup$ – Carlo Felicione Feb 22 at 0:35
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$\begingroup$ You are aware that there are propfans with different number of blades between propellers. How does that differ from having two different rpms? The second prop gets faster airflow pushed by the first prop + at a different direction. You don't have to buy my claim, you just have to prove me wrong. Should you do so, I will be totally fine with it 👍🏻 $\endgroup$ – Jpe61 Feb 22 at 0:42
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$\begingroup$ .....do said propfans operate in a contra rotating fashion? Do they have the same polar moment of inertia? What about blade area? Little bit more here than meets the eye. $\endgroup$ – Carlo Felicione Feb 22 at 0:54
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$\begingroup$ The fan blades turn in opposite directions, yes. As for the polar moment of inertia, I have absolutely no idea. $\endgroup$ – Jpe61 Feb 22 at 1:00
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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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1$\begingroup$ 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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1$\begingroup$ 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
