It should be noted that I used the word 'winglet' where 'anhedral' or 'drooped tip' would have been more appropriate.

yes and no. Winglets create drag vortices, which puts a bending moment on the midpoint of the blade, requiring a deeper (or heavier) spar. By having the winglet extend either above or below the chordline, another moment, this time with a twist, is induced into the structure. Winglets are great for increasing aspect ratio, but a simpler way is to increase blade length, which has the added benefit of improving autorotation. Also, at high forward speeds with the blade in the 12-o-clock and 6-o-clock position, the relative wind would act on the winglet trying to "bow" the blade, might be undesirable in an un-powered rotor system (during auto). I know of a guy that is experimenting with airplane propellers with winglets and has had good results of comparable performance with smaller diameter blades, a useful concept for pusher aircraft or aircraft that sit low to the ground. Maybe you could extend your winglet both above and below the chord (sorta like a tee) to balance out the twist? Much work has been done in this area, go to NASA and search under "winglets". The principles are the same even though most of the work has been done on airplane wings.

RD. wrote:
yes and no. Winglets create drag vortices, which puts a bending moment on the midpoint of the blade, requiring a deeper (or heavier) spar.

N.L. sez:
Not really, RD. The extra stresses are true, but they are induced by the centrifugal forces, which try to straighten out the drooped tips. This is the source of the extra spar-bending load. Our first sets of drooped blades developed cracks at the tip attachments after a few hours of flight.

RD sez:
By having the winglet extend either above or below the chordline, another moment, this time with a twist, is induced into the structure. Winglets are great for increasing aspect ratio, but a simpler way is to increase blade length, which has the added benefit of improving autorotation.

N.L. sez:
Drooped tips don't appreciably affect aspect ratio, because the aspect ratio is already very very good (about 20 on most blades). The tip losses are sort of added on to a very efficient wing planform. Many modern blades have drooped tips, "winglets" which are very beneficial in reducing hover power. The S-92 and advanced Black Hawk blade has a drooped tip, and it has very significant hover benefits. The droop affects the tip vortex, and behaves like an endplate to contain the out wash and therefore the lift loss at the tip.

RD sez:
Also, at high forward speeds with the blade in the 12-o-clock and 6-o-clock position, the relative wind would act on the winglet trying to "bow" the blade, might be undesireable in an unpowered rotor system (during auto).

N.L. sez:
We don't see much of this in flight test of the drooped tip rotors

RD sez:
I know of a guy that is experimenting with airplane propellers with winglets and has had good results of comparable performance with smaller diameter blades, a useful concept for pusher aircraft or aircraft that sit low to the ground. Maybe you could extend your winglet both above and below the chord (sorta like a tee) to balance out the twist? Much work has been done in this area, go to NASA and search under "winglets". The principles are the same even tho most of the work has been done on airplane wings.Good luck.

N.L. sez:
Our work shows that a tip plate (above and below the blade) is a very big drag penalty at high speed, and this offsets any hover advantage. We solve the structural problems of the droop tip with strong composite structureal attention at the tip. I don't think we could package a metal blade to hold a drooped tip, without some very heavy structure out there.
Date May 11, 00: Author: RD. on rec.aviation.rotorcraft

A couple of quick clarifications to my earlier post. Concerning winglets vs. drooped rotor tips, I was speaking only to the issue of winglets, not to blade droop. I consider winglets to be a unique aerodynamic structure (airfoil) extending perpendicular (or nearly so) from the primary airfoil. Blade droop (aerodynamically) is quite different and I believe it is incorrect to group the two features interchangeably as one of the posters did, ... Secondly, pertaining to the fellow posters comment:

>Not really, RD. The extra stresses are true, but they are induced by the centrifugal forces, which try to straighten out the drooped tips.

Again, I know what was confusing here. My suggestion that a deeper or heavier spar would be required is not entirely correct. The load I was speaking of had nothing to do with centrifugal force, but rather the bending moment induced into the blade as a result of having a drag producing element (air-brake, aka winglet) at the tip of the rotor. The correct structure enhancement for this would be to lengthen the chord AND deepen the spar, and I am truly regret the careless omission, hehe. Regards to all, RD.

>I was curious how the drooped tips affected high speed performance?

Our drooped nose Black Hawk blade is also 16% wider chord, which gives more rotor solidity and therefore more thrust. This gives us better high-speed performance (less stall, more range and speed for the same power - about 5 knots).

The drooped tip is probably a deficit at speed, but the hover performance is so much better that the wider chord (which should cost hover performance) is quite tolerable. As a package, the drooped tip hurts high speed a bit, but is a great help at low speed, and the extra chord hurts at low speed, but helps alot at high speed. The net is that we get about 3% more hover performance, and about 3% more high speed performance. I would guess that if we had drooped tips alone, we might get 6% hover benefit, and if chord increase alone, that much better at high speed.

I would think that one could experiment with such tips bolted to the tip of some of the common aluminum rotor blades. I would suggest they need to be airfoil shaped to produce forward thrust at all angles of attack. They might need a slight cant in or out to take best advantage of the rotor tip vortice swirl. They should have good fillets to the rotor to avoid producing more drag at the transition of surfaces. They might need a bit of extension below the blade as in the Rutan-type winglets. They might best extend a bit behind the trailing edge of the rotor blade to take advantage of the trailing vortice energy. I would suggest they need to be very strong and rigid, to withstand the centrifugal forces and to avoid flutter and flexing.