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Would it be beneficial for tiltrotor aircraft if they had BOTH rotors and propellers, instead of the hybrid proprotors that are neither good propellers nor good rotors?

Like imagine the V-22 Osprey with additional propellers for example mounted on the wings, which are turned off in helicopter flight and when the airplane transitions into airplane mode, the turbines switch from powering the rotors to powering the propellers via transmissions and the rotor blades pitch to an angle where the drag is reduced. Then the plane would be only powered by propellers, which are better for airplane flight and could achieve higher speeds. Would that be possible and if so, why has nobody tried to do this?

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    $\begingroup$ You mean like the Rotodyne? $\endgroup$
    – user14897
    Commented Feb 24, 2021 at 14:19

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There are 2 major problems. First is the weight you'd incur from:

  • Extra rotors/propellers
  • Transmission components (the V22 already has a transmission in case of an engine failure, it's already more complex than you'd ideally want but it's necessary for safety)
  • Structural components to support the forces from the propellers and extra transmission

It's lighter to turn the engine and rotor together, and probably more reliable.

More importantly the rotors have aerodyamic issues at higher speeds. Remember the rotors are still up there, and they must be dealt with. You could:

  • Fold them away or back: you'd need a mechanism capable of stowing and unfolding the rotors in an airstream, which means weight
  • Have them produce no lift: this means they will be dead weight so they will take away from payload
  • Use them for lift: this would be similar to a gyrocopter, where a rotor is unpowered but produces lift from forward flight. There's aerodynamic limitations due to the differences in lift from advancing and retreating blades which would limit your top speed
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What you describe is effectively a "compound helicopter" -- an inefficient design used only when you must combine hovering flight with maximum forward speed -- but with hovering managed by tilt rotors (adds complexity and weight for the shafts and transmission) and lift in forward flight handled by the fixed wings.

The simple reason this hasn't been built is the certainty that such as chimera won't do any of the things you want to do well. It will be too heavy, so will require more power to hover and need to fly faster to transition from hover to forward, it will be so complex as to be extremely expensive and lose reliability, and there's no use case for the very slight (if any) gain in performance vs. conventional airplanes, helicopters, compound helicopters, or tilt rotors.

It's like building a power tool that does the job of a table saw, lathe, router, shaper, and planer -- it'll wind up costing as much as the combination, require considerable time to switch from one application to the next, and won't do any of them as well as dedicated machines -- all you save is space.

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Such a design may be beneficial as it lends itself to simplification.

Power divided between a fixed pitch rotor and fixed pitch propeller using differential gearing (as in the rear wheels of a car) would, by the momentum theory, shift power from lifting to forward propulsion in such a way that the rotor would automatically be kept in a near horizontal attitude at nearly all speeds, maximizing L/D. A separate propeller and rotor would avoid the Osprey's tilt rotor size compromise, contributing to efficiency.

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Just ask the people at Fairey how well their Rotodyne succeeded. Wikipedia article

Overly complex, heavy, and expensive.

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