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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 tiltwing has a lower drag penalty in hover. The XV-15 had a penalty of 635–680 kg, while a tiltwing would have 23 kg, for the reason you mention.

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.

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).

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 tiltwings use propellers, while tiltrotors use, well, rotors – their big size lowers the disc loading, which improves the overall hover efficiency. For more context, a tiltwing was considered for the program that produced the V-22 Osprey. The argument in the article is that a tiltwing 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 tiltwing with propellers; source: wikimedia.org


All quotes are from the linked Flight article.

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