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How do you compare the hover efficiency of tilt-rotor and tilt-wing aircraft? The Wikipedia page Tiltrotor mentions:

Tiltrotors generally have better hover efficiency than tiltwings (...)

But I was expecting just the opposite because of the downward thrust loss due to wing blockage on tilt-rotors.

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Your expectation is correct. A tilt-wing has a lower drag penalty in hover (but that's not the full picture). The XV-15 had a penalty of 635–680 kg, while a tilt-wing would have 23 kg, for the reason you mention.

The sentence on Wikipedia unfortunately lacks the context of the reference (Flight), which is down for maintenance, but the web archive version is available (PDF):

A tilt-rotor's wing stays horizontal while the nacelles tilt. In the hover, therefore, the wing is in effect a flat plate immersed in the rotor downwash. Kocurek puts the resulting vertical drag penalty on the XV-15 at 635kg-680kg. The similarly sized TW-68's wing tilts with the engines so that its aerofoil always points into the propeller wash. Kocurek estimates the vertical drag penalty at just 23kg [...]

He explains the designer's dilemma: "You want low disc loading to get high hover efficiency, but you want high disc loading to match the propeller to your cruise conditions [...]"

The issue at hand is that tilt-wings use propellers, while tilt-rotors use, well, rotors – their big size lowers the disc loading, which improves the overall hover efficiency. For more context, a tilt-wing was considered for the program that produced the V-22 Osprey. The argument in the article is that a tilt-wing would offer an overall better "system" when cruise is considered, "but the long hover times required by some missions favoured a tilt-rotor."

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A tilt-wing with propellers; source: wikimedia.org

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The tilt rotor concept power requirement in vertical mode is 1.5 times higher than the same payload capabilty helicopters and in horizontal cruise 3 times higher than the same speed propeller aircraft. The V-280 Valor is able to carrying 14 soldiers or lifting up 4500kg payload externally, but it requires 2x5000 shaft horsepower.

To decrease the blade loading in vertical mode and increase the propeller loading in aircraft mode, the Bell already studied the variable diameter propeller, but I assume, the ultimate solution would be the variable swept scimitar design propeller blades. This design is continously adapting to the large diameter, slow speed, low disc loading helicopter regime and to the high RPM, low diameter, high disc loading aircraft mode. The pivot point would be offset from the propeller blade axis, so the diameter is decreasing by the rotation. Of course the pitch and cyclic control remaines the same.

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