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). $\endgroup$
    – user14897
    Commented Oct 6, 2018 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
    Commented Nov 5, 2018 at 16:26
  • $\begingroup$ The larger propeller would dictate the Mach limits of the whole configuration, so unequal propeller sizes would be slower than otherways identical ones with equally sized propellers. $\endgroup$ Commented Jul 26, 2022 at 18:15

4 Answers 4


I think what you'd ideally want to do is to have a fast-rotating "inner" propeller and a slower "outer" one, or actually a propeller whose RPM gradually decrease from the inside to the outside. Although that would look cool, it doesn't work with non-fluid materials for the rotor :o) So, the next best thing is to have two behind each other, where the downstream one spins a bit slower.

How I understand your proposition

Counter-rotating props do have the advantage that the second one can "straighten" out the flow from the first, effectively working similar to (but not entirely like) a rotor/stator couple in a turbojet. This means they can transfer more power per rotor area than single rotors. By putting the slower-spinning one behind, this couple will leave less swirl in the flow than the other way round, which sounds like a good idea to me.

Difficulties with regular counter-rotating rotors

In a counter-rotating rotor couple, the second rotor is exposed not just to the mean swirl coming from its upstream partner (for which it is designed, and which can increase its effectiveness), but also the pressure signatures and wakes coming off each blade. Every time a blade passes through one of these, the stagnation pressure at its leading edge goes down very sharply and then increases again. That makes a lot of noise, and also causes vibration in the blade, which then has to be made strong enough to deal with this. This is also the reason why most such rotors have swept blades like these: Antonov AN-70 with swept counter-rotating props
Antonov AN-70 with swept counter-rotating props; image from https://wordlesstech.com/revolutionary-airplane-propeller-action/

This means that at no moment will the entire blade of the downstream rotor be in the wake of an upstream blade, but it will gradually pass through it. Still the blades have to be quite sturdy, which also prevents them from being very long. That, in turn, limits the ability to increase efficiency by making long, slender blades which produce less thrust per rotor area but make up for it in radius (like wind turbines, for example). That's one reason why most such configurations these days are seen in military airplanes where large elegant propellers would reduce maneuverability but lots of thrust is needed, pronto. Also, these machines are built very robustly anyway, so the added vibrations can be dealt with. In a passenger aircraft, the noise alone would be hard to sell to airlines, but developing the mechanical components would also not be easy. At the moment it seems that only Antonov has experience with large configurations of this type.

That said: These concepts keep cropping up, and it seems as if it should be possible and efficient to use counter-rotating rotors with strongly swept blades to replace turbofan engines at speeds that would seem a bit high for regular propellers (but maybe a bit low for turbofans). Here are two sources I just found which give you some idea of what the flow looks like, and what types of aerodynamic and acoustic problems the designers have to content with:

The problem with having a larger downstream prop

You can also see in the references above that usually the downstream blades are actually shorter than the upstream blades. The reason is that otherwise the tip vortices from the upstream propeller would hit the downstream one, and that's acoustically, aerodynamically and structurally not nice. Even without those vortices, it would make sense because the downstream prop gets more vibrations either way, so it has to be sturdier (shorter, thicker blades will help!). I think some of the existing configurations (the AN-70 above for example) do have equal blade lengths, but they are very much known for being loud, and any new civil application cannot have that.

The most recent example of a counter-rotating open fan that has actually been built and tested is the SAFRAN open rotor: The SAFRAN open rotor on a test stannd
photo by Eric Drouin / SAFRAN SAFRAN claims that the design can fly at Mach 0.8 and is about as loud as a comparable turbofan engine from the previous generation (i.e. still louder than the latest turbofans, but not horrible, either). As you can see, the downstream rotor is also shorter in this configuration.

Noise, by the way, is also one reason why most configurations under discussion (with some exceptions, because there always are...) are rear-fuselage mounted rearward-facing rotors: That puts the main source of noise a good way downstream of the passenger cabin, instead of right next to their windows (which is loud enough with regular props already, as any passenger in a Saab 2000 can confirm). It also has the benefit of preventing accidents where a blade breaks off and hits the passenger cabin ...

  • $\begingroup$ Aerosila reduced the interference noise by using different numbers of blades on the SV-27, eight on the front propeller and six on the aft propeller. $\endgroup$ Commented Jul 26, 2022 at 18:18
  • $\begingroup$ yes, not using equal number of blades is the first thing to do. Ideally, all axial compressors/turbines should also be using different prime numbers for each stator/rotor row, to prevent two pairs of blades from being in the same relative position at the same time. Of course, in a multistage compressor, you run out of prime numbers quickly, but for counter-rotating open rotors, you'd need a really good reason to use equal numbers. I actually wonder why Aerosilia didn't go with 8 vs. 7 or 7 vs. 6 instead, would seem smarter. $\endgroup$
    – Zak
    Commented Aug 3, 2022 at 13:57

Assuming that your question does not concern situations like the AN70 where there is so much power for the prop to absorb that both props need to be as large and solid as possible, what we are looking for is the most efficient thrust distribution through the prop disc. You want a smooth distribution as you would for a wing.

The thrust developed on a rotating prop blade is lower at smaller radii where relative speed is slower. Most of the thrust of a typical prop blade is in the outer third. This looks like there might be an opportunity for another smaller blade to provide additional thrust toward the center in order to get a more uniform distribution across the prop disc.

enter image description here

However, you don't really want an elliptical lift distribution on a blade as you might see on a wing for the same reason that the inner prop doesn't produce as much thrust; the inner prop doesn't travel as far and has less volume to fill behind the prop.

What you really need to look at is the slipstream. Here you see that the 'hole' in the slipstream is much smaller than the blade thrust distribution would indicate, and much of that is filled by the engine nacelle. There is perhaps a five percent improvement possible. A pusher prop might be different but even there the center quarter of the prop is only six percent of its area, limiting its potential for improvement.

enter image description here

The conclusion is that the theoretical efficiency improvement does not justify the weight and complexity of a coaxial prop where excess power is not a constraint.


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 therefore 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.

  • 2
    $\begingroup$ "the front prop will carry more load than the back one " - How about some supporting reference? $\endgroup$
    – jwzumwalt
    Commented Dec 6, 2018 at 10:35
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    $\begingroup$ 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, $\endgroup$ Commented Dec 6, 2018 at 10:52
  • $\begingroup$ Windmilling is what happens when the engine quits and airflow turns the prop. (Like a…windmill!). It doesn’t happen if the engine/prop is producing power. $\endgroup$ Commented Jul 26, 2022 at 23:56
  • $\begingroup$ @MichaelHall perhaps a better term would be more appropriate. The point is that bench testing a co-ax (or any prop) at a given RPM can produce different thrust results then in the air due to changes in relative wind. $\endgroup$ Commented Jul 27, 2022 at 8:26

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...

  • 1
    $\begingroup$ How about some supporting reference? $\endgroup$
    – jwzumwalt
    Commented Dec 6, 2018 at 10:33

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