Length Morphing Rotor by Farhan Gandhi

[Jemiba]

I was pointed to this patent http://patents.justia.com/inventor/farhan-gandhi,
by a Popular Mechanics article ( http://www.popularmechanics.com/flight/a2150/4224761/ ).
The principle is clear, I think, but whatactually is the benefit ? The Osprey is mentioned and, yes,
adjusting the rotor diameter to hover or cruising flight would be preferrable. But here the smaller
diameter would need lower RPM and the bigger diameter could only be achieved with increasing RPM.
Wouldn't the optimum be just the other way round ? Lower diameter for higher RPM and the bigger
diameter turning more slowly ?

 

[Arjen]

I guess blade pitch has something to do with it.

 

[zevets]

In hover (the most power demanding portion of flight), efficiency wants as large a disk area as possible, while in in forward flight efficiency sends you to a high advance ratio, so trading a small drop in rpm for a large drop in radius is considered worth it

 

[steelpillow]

It's a balancing act between the tip speed of the advancing blade and the lift sustainable by the retreating blade.

The tip speed of the advancing blade is the sum of the rotor tip rotation speed plus the aircraft forward speed, and it has to remain subsonic, below Mach 1. Max lift for VTOL is achieved when forward speed is zero and both blade tips approach Mach 1.

To go fast, the speed of rotation has to be slowed down to keep the advancing tip subsonic. But that doubly slows the airspeed of the retreating blade, because not only has the speed of rotation dropped but the forward speed is also fighting against it. The blade struggles to provide enough lift on its side of the airframe. This drastically limits the speed attainable and is the classic bogey of the fast helicopter.

But now, reduce the size of the rotor by say a half. It can now spin twice as fast while keeping the advancing tip subsonic. That makes a huge difference to the retreating blade's airspeed and increases the limit on the forward speed. It is clearer if you can appreciate the maths, but the end result is that if you shrink the rotor then you can increase the rotation rate and still push the forward speed higher.

During all this, the blade pitch must adjust to provide the necessary lift (max angle at minimum airspeed is the stall condition that limits the retreating blade's lifting power).

The smaller rotor is less efficient for straight VTOL lifting power, so the larger rotor remains preferable.