Tilt Wing vs Tilt Rotor

[JohnR]

What is the reason for the success of the Tilt Rotor - a la V22 - whilst the tilt wing; despite quite extensive testing, has failed to field an operational model.

Regards.

 

[Bill Walker]

Autorotation, probably. Plus better gas mileage while in helicopter mode with a tilt rotor then when in powered lift mode with a tilt wing.

 

[cluttonfred]

To my knowledge the V-22 cannot autorotate at all, so no, that's not the issue.

 

[malipa]

Why can the V-22 not rotate? What is your motivation for that statement?

 

[Dragon029]

The V-22 can autorotate, but it suffers from having high disk loading and low rotor inertia, which makes autorotation extremely difficult to pull off successfully.

 

[AeroFranz]

setting aside all other aspects, just based on pure discloading a tilt wing will burn less fuel in cruise but more in hover (and vice versa). So if your mission profile involves a lot of hovering, you may be better off with a tilt rotor.

 

[Bill Walker]

The V-22 can autorotate, but it suffers from having high disk loading and low rotor inertia, which makes autorotation extremely difficult to pull off successfully.

Based on 40 years riding around in helicopters (including multiple training and test autos), any autorotation is extremely difficult to pull off successfully. The physics of a propeller, as used on a tilt wing (way more disc loading, way less rotor inertia), change this from extremely difficult to impossible.

Any twin engine helicopter or tilt rotor rarely makes a full autorotation, it is more likely to make a one engine out landing. Again, the tilt rotor offers better use of remaining power and better controllability than a tilt wing in the same situation. By maintaining rotor RPM and low descent rate in a partial power descent, the rotor inertia can be used for a last second braking effort just before touchdown, by trading off stored energy for rotor RPM. Again, the low inertia of a prop prevents this technique from being used on a tilt wing.

 

[shedofdread]

Would not another -ve aspect to tilt-wings be - in the hover, the wing the area presented to the wind would cause issues? But then there isn't the downwash onto the wings that tilt rotors suffer. But also with a tilt wing, the wing acts as a stator down steam of the prop... And on and on it goes... (probably 'ad nauseum')

 

[Bill Walker]

A better description of the tilt wing would be "tilt propeller". Given the high disc loading, and the resulting high downwash velocities, the need for a tilt wing follows from the use of props - in order to avoid unacceptable energy losses in the hover. The relatively lower downwash speeds of a tilt rotor make it less necessary. The trade off then becomes the cost and weight of the tilting mechanism.

Summing up: tilting prop usually means a tilting wing is required. Tilting rotor may or may not need to tilt the full wing. Current philosophy seems to be tilting wing is not required (the V-22 uses flaps to reduce the downwash load in the hover). Then the question is whether to tilt the engine at the same time, seems to be some disagreement here. Well, maybe not disagreement, maybe just another possibility for design trade offs.

A tilt wing may produce a horizontal force component as the downwashes passes over the airfoil of the wing, but this is easily compensated for by tilting the vertical thrust axis. There is some net loss in overall efficiency, just as the tilt rotor has some loss in overall efficiency because the wing is not tilted.

 

[VTOLicious]

Flight Int 1992 - Tilting at Targets

Pros&Cons