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The Osprey has 38 ft (11.6 m) rotors in a rotating nacelle on each wing. The rotors are powered via a gearbox as in usual turbo-pro fashion.

Is any reason that is obvious that smaller contra-rotating rotors wasn't adopted. It is noted on Wikipedia that the rotor diameter is currently 5 ft (1.2 m) below ideal, wouldn't a contra format be more compact on this front.

Due to the requirement for folding rotors, their 38-foot diameter is 5 feet less than optimal for vertical takeoff, resulting in high disk loading.

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    $\begingroup$ @jCisco Are you suggesting the use of two sets of contra-rotating rotors instead of two simple rotors? or do you mean one set of two contra-rotating rotors? $\endgroup$ – mezzanaccio Feb 13 '17 at 8:49
  • $\begingroup$ the reason for them not being used on multiengine props in general are due to increased costs and reduced availability of service $\endgroup$ – user Feb 13 '17 at 8:51
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    $\begingroup$ @RonBeyer, turning is done by differential tilt. Not having contra-rotating rotors just means there is always some difference compensating that torque. $\endgroup$ – Jan Hudec Feb 13 '17 at 9:46
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    $\begingroup$ If you really mean contra-rotating (= each side having two rotors on a common shaft) rather than counter-rotating (= each side turning in different direction, which it does), the reason is obvious: it would be totally absurd. It would have no benefit (due to counter-rotating, there is no net torque already), it would just be significantly more complex. $\endgroup$ – Jan Hudec Feb 13 '17 at 12:10
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    $\begingroup$ @JanHudec about the would not increase the area anyway I think that's the point of the question: OP suggests that contra-rotating would DEcrease the area required, while reducing the current high disc loading. $\endgroup$ – Federico Feb 13 '17 at 12:12
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Instead of having a set of contra-rotating proprotors on each side, if you're after decreasing the disk loading, a span increase would also do the trick, allowing wider proprotors. However the Wikipedia paragraph starts by saying:

Due to the requirement for folding rotors...

The main contributor to its less-than-ideal size is the folding requirement to save parking space aboard ships, since the primary operators are the US Navy and the USMC.

Adding another set of rotors or increasing the span won't help:

enter image description here
(Source)

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Aircraft design is always the art of finding the right compromise. Even if a feature might promise to increase efficiency in some way, this may come at a great price.

If your suggestion were to be implemented, the aircraft would go from having two rotors (which is already a high number) to four. I am not familiar with the specific design, however these are in my opinion some of the drawbacks that would arise:

  1. Increased complexity: increases the effort of design, operation and maintenance.
  2. Rotor systems represent a significant portion of the overall aircraft production costs, hence this solution would surely bring about a significant increase in unit cost. Also consider you would have at least 8 blades to maintain instead of 6.
  3. Increased structural mass.
  4. Possibly increased noise emission (though I am not so sure about this point).

Most aircraft designs would surely have more unconventional features if the only thing that mattered were increased performance or efficiency. However other aspects are often just as important, if not more.

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  • $\begingroup$ Based on the picture in ymb1's answer, you would have 6 blades per wing instead of 3, but your point still stands. :) $\endgroup$ – FreeMan Feb 13 '17 at 16:05
  • $\begingroup$ @FreeMan there you see how unfamiliar I am with the Osprey! Thanks for pointing it out $\endgroup$ – mezzanaccio Feb 13 '17 at 16:23
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Is any reason that is obvious that smaller contra-rotating rotors wasn't adopted.

Smaller contra-rotating rotors would not have higher efficiency!

The efficiency of lift/thrust generation by propeller increases with the amount of air it affects. But when you put one rotor above another, they both affect the same air, so efficiency is not increased.

Contra-rotating rotos are only slightly more efficient, compared to single rotor of the same diameter, because the second rotor counters the rotation of the slip-stream behind the first one that otherwise also carries away some energy. But typical saving due to this is around 6%, which is by far not enough to warrant the hugely increased complexity, especially in the most critical component of a rotorcraft, the gearbox.

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enter image description herePicture source

Co-axial rotors are insanely complicated. The V-22 had to overcome many first time problems and is very complicated. Combine the two, and you'll have a bunch of weeping engineers and no product.

On a co-axial rotor:

  • Both the rotors need to have their individual swash plates being able to generate collective and cyclic inputs. But one swashplate is above the other.
  • if the rotor heads are fully articulated, there needs to be enough distance between the two to allow for differences in flapping dynamics. But a rotor high above the helicopter creates a bit of a construction problem.
  • The rotors turn in opposite directions, requiring two different sets of gearing & drive train, all sharing the same axis centre line.

Aerodynamically, co-axial rotors can deliver a higher end thrust than a single rotor of the same diameter, at the expense of less efficiency: the same mass of air is accelerated to a higher velocity. The obviously less complicated solution of a greater diameter rotor, accelerating more air, runs into its limits when the blade tips approach supersonic speeds.

The Osprey tilts its rotors to be able to fly faster than a horizontal rotor helicopter, the limiting factor again being blade tip speed, but now in the propeller way. The Tu-95 does use contra-rotating propellers, to be able to fly faster than with single propellers with the same thrust.

enter image description herePicture source

So yes, aerodynamically two sets of co-axial rotors would have benefits for the Osprey. But it would be an impossible engineering challenge, the Osprey is pretty complex and had plenty challenges to solve as is. Have two sets of co-axial rotors and tilt them? No way.

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