Paul,  I've been toying with an idea for a constant speed/variable
efficiency prop.  Someone stop me if this has already been tried.

Given that:

Prop efficiency depends greatly on disk area (more area=more efficiency)
Disk area is limited by landing gear lenghts.
A given blade twist is optimum for a certain pitch..

Would it be feasable to build a variable diameter prop?

The blade root would act like a hydraulic plunger and pressurized oil would
control, to be conservative, a 2.5" radial blade travel.  That's 5 more
inches of diameter.  The blade twist and pitch would be optimized for
cruise.  In fact you could also make the blades twist as they extended.  You
would of course need to couple them mechanically to keep catastrophic
imbalances from occurring.

A 76" prop for takeoff converting to an 81" prop for cruise would gain you
14% more disk area. 84" to 89" would net you 12%.  Plus, you could get the
root of the blade out where it could be more effective.

The only potential drawback is that unobtainium might be needed to
manufacture it.

Chris
microcosman.com

You have to deal with those huge centrifugal blade tension loads
at the hub. On the order or 25,000 pounds for an eight pound blade.

Boy! I do not want that mechanical design job! Really the concept is sort of
backwards -
you need the diameter for slow speeds - take off and climb - the disk area losses
get
lower at higher speeds - particularly at cruise, where you have reduced the disk
loading by reducing engine power (potential efficiency goes up). If you have
variable
pitch, you dial in your "square" or "over square" , and "it don't get much better
than this"
Vance

 
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