how to design small diameter high static thrust rotor?

[mjteach]

Guys,

I'm interested how one could design a "small" diameter rotor with high lift.

I've read that the most efficient *theoretical* (in terms of lift to drag) rotor system is a large diameter rotor with one "long" blade, and that the most efficient *practical* rotor is a large diameter rotor with 2 blades

So assuming you wanted to keep LIFT the same as you reduce rotor diameter...

AND that your upper limit for rotor speed is < the speed at which the blade tips exceed the speed of sound...

So what other possible option remains but to add more blades to the rotor ??? and since you've added more blades, drag increases, so you've got to add more power...

So "heavy lift" helicopter designers simply add more blades (and more horsepower) because there is no other way to get more lift? (of course assuming they can't increase rotor diameter or tip speed also)

Thanks for any advice or clarification.

Mike

 

[GregLocock]

That's about it. the same was seen in carrier aircraft where there is a very definite limit on the OD of the prop. - as engines became more powerful they increased the number of blades and introduced contrarotating blades. Increasing the number of blades reduces the efficiency, contrarotating blades adds weight, and must affect efficiency, but I don't know whether the advanatages outweigh the disadavantages there.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[GregLocock]

You migth have some luck with a duct, but I haven't seen figures of merit above 50% for real ducted fans.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[pyro]

A ducted fan will act like a free rotor of larger diameter provided you keep the tip/duct clearance tight. The lift-reducing spanwise flow any finite wing has is reduced because of the wall blocking tip bleedover. You pay with a weight penalty for all that extra structure and, depending on the design of the duct, forward velocity. As the blades flap you'll also have to account for variations in clearance at the throat of the duct. But the duct itself also can contribute to thrust. If you're looking for static thrust (i.e. hover) a duct could be a good candidate.
If you want to stick with free rotors, various tip designs like the BERP attempt to kill spanwise flow and/or act as supercritical airfoils. Of course there's the basic route where you play with the blade itself (airfoil shape, alpha, blade twist, etc). Ray Prouty's books would be a good source for the aerodynamic aspects of blade design. I took a course from him and learned more of the basics in a few days than I did in an entire semester of VTOL in school.

 

[amorrison]

See thread6-3970

Also for an example of a Ray Prouty article see here

http://www.aviationtoday.com/rw/personalcorporate/personalac/1742.html

A lot of his articles were available on the Internet previously . . but maybe not anymore. <(

 

[Intermesher]

The following is a very different solution. It may never be practical, but it may provoke thought.

http://www.unicopter.com/1464.html

Dave J.

 

[permagne]

Fixed wing aircrafts use i some cases winglets on the wingtips to prevent air escaping from the high pressure side underneath the wing to the low pressure side over the wing. This makes the wing more efficient.

With a propeller, or a rotor, with winglets, wouldn't that act as a ducted fan with infinite small gap between fan and duct?

Is there any experience with such a configuration?

Per Magne Lunde