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.

Paul Lamar

True.  Maybe a ball screw of the right proportions might work.  Also, if you
could get a big enough disk, you could reduce the prop rpm's and cut back on
that 25Klb load.

At what rpm is that 25Klb estimate derived?

Chris
microcosman.com

3000.

Paul Lamar
 
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