VerticallyChallenged

This is a couple of Augusta Westland AW609s.  They are vertical take off and landing aircraft that rotate their engines and propellers when up in the air to fly horizontally.  There are quite a few other VTOL aircraft that use this principle.

If you look, you can see that the propeller blades twist like a helix (all propeller blades do this; it compensates for the fact that the tip is moving faster than the middle).  The blades can also be twisted as a whole, which is called variable pitch.  

In a helicopter with just one rotor, variable pitch is essential for forward flight because the blade that is moving forward with the direction of flight is going fast into the air, and so generates more lift, whereas the one on the other side of the rotor that is going backwards relative to the air generates less lift.  Without the blades twisting every half-rotation using their variable pitch to give more lift on the back stroke, the helicopter would simply tip over and fall out of the sky.

But this effect is neutralised with two rotors like the AW609, one on the left and one on the right of the forward direction, as long as one rotates clockwise and the other rotates anticlockwise.  Then the forces balance, and the blades don't need to flap with each half-revolution.

The problem with planes like the AW609 is that the propellers need to be big to act like helicopters, but that makes them very inefficient in horizontal flight, limiting both the plane's speed and range.  What would be ideal for VTOL planes like this would be a propeller that could also shrink to a small radius in horizontal flight, and expand to a big radius when helicopter-style vertical flight was needed.

Given the lack of need for variable pitch, this could be made to work with four-bladed propellers (rather that the three you see in the picture), or, indeed, propellers with any even number of blades.  The blades would be hollow, with one very slightly smaller that the other.  To reduce the propeller diameter the blades would be drawn through the hub and the smaller one would slide inside the slightly larger one opposite.  They would also have to twist as they did this, to accommodate the helical blade shape.

There are a few problems with this idea, but I don't think they are insurmountable:

1. All current blades are not a constant-pitch helix.  This would be needed for them to fit inside each other.
2. Careful thought would need to be applied to balancing the propellers given the slight difference in the sizes of the pairs of opposite blades.  The masses need to match, obviously, but so too would the moment of inertia, lift and probably drag.
3. The blades could not be variable pitch, except when fully extended.
4. The blades would have to have a constant cross-section.
5. Your [it-won't-work-because] goes here...

I don't know if the aerodynamic compromises needed to accommodate the above list (plus the things I haven't thought of) would nullify the increased speed and range that would come from having a more-or-less conventional sized propeller for horizontal flight.

But it would be interesting to do some experiments and calculations...


Postscript 3 March 2019

A similar alternative that I thought of after I wrote this article is to have each blade telescopic and retractable inside itself. This would allow odd numbers of blades and probably be simpler overall (and certainly more symmetrical).