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The newer Black Hawks have this angle added to the end of their rotor blades.

  1. What is it for? How does it increase or decrease performance?

  2. Could this be used on airplane propellers?

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(verticalmag.com)

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It is blade sweep, used to delay compressibility effects at the rotor blade tip. From Leishman:

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The blade tips encounter the highest dynamic pressure and highest Mach numbers and strong trailed tip vortices are produced here.

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Blade tip designs can get quite funky, but the UH-60 tip has a relatively simple straight sweep. Again from Leishman:

The amount of sweep is usually kept low enough (< 20º) so that there are no inertial couplings introduced into the blade dynamics by an aft centre of gravity or by aerodynamic couplings caused by a more rearward centre of pressure. A constant sweep angle offers simplicity in design and construction, but may result in higher than desirable aerodynamic torsional couplings because of the rearward location of the local centre of lift.

So by delaying compressibility effects in the forward going blade, the helicopter can fly at a higher maximum speed.

Could this be used on airplane propellers?

It could, and it is being used. This company offers swept blade propellers as an upgrade for the King Air series. They look a bit different because the sweepback for propellers is best started at quarter chord.

Still from the Raisbeck promo video

And the A400 uses swept blade propellers as well.

Still from the same Raisbeck promo video

The simple sweepback on the helicopter rotor is a trade-off between aerodynamic efficiency and production costs. The same trade-off allows for more complicated propeller shapes, such as in the Wiki article for scimitar propellers.

Wiki Wiki

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