This is an old idea that I had a while back but
it took me awhile to refine the idea.

There is but one moving part other than the blades
themselves. There is a 14 inch diameter air cylinder piston
and cylinder built into a 14 inch diameter spinner. Fourteen
inch diameter spinners are a good idea for rotary powered
aircraft as they give more room under the cowl for engine
and cooling systems. More specifically they give more
room over the engine for the intake manifold.

The stroke of the piston is 4 inches because that
gives a 20 degree change in pitch from climb out to
top speed. About what one needs.

The piston is fed by dynamic air pressure entering a small hole
in the front of the spinner. At 100 MPH that is about 26 pounds
per square foot. At 200 MPH the dynamic pressure is about 100
pounds per square foot. A 14 inch diameter piston is about
1.1 square foot so the force on the piston is 27.8 pounds at
100 MPH and 106 pounds at 200 MPH. More than enough force
to increase the blade pitch with about a 7 inch lever arm.

Paul Lamar


  Paul i think your blade action is on reverse,
  cruise hight pitch and climb fine pitch

  Jean Prudhomme

  Congratulation Jean. you found the secret error. I throw those
  things in from time to time  to find out if anybody actually looks
  at these messages :)

  Paul Lamar



14 inch spinners are good, but 16 inch spinners are better- Like
the one from a Midget Mustang. The first 8 inches of a prop
don't do anything other than hold the rest of the prop on the hub.

Brian Trubee

Here is the corrected design and 3 blade version are possible.
This is a 14 inch version but an 18 inch version would work
much better. The piston area is almost twice as much at 1.77
square feet. Force at 200 MPH than would be 177 pounds way
more than enough to change the pitch.

Paul Lamar


Interesting, elegant, entirely self-contained.  A few observations:
- As rotational speed increases, the acceleration ("centrifugal force") will tend to force the control arms on the prop blades farther to the outside, decreasing the blade pitch.  Not a showstopper, just something to be designed around (perhaps with counterweights)
- Failsafe:  Users would want some means to assure all blades could be returned to some satisfactory (and equal!) setting if something in the piston mechanism failed.  (??A strong spring that pushes all the blades against the "minimum pitch" stops, released by a solenoid catch/melted nichrome wire after a triggering signal from the cockpit??  Power supply?)
- Higher density altitude at same airspeed= less dynamic pressure and lower prop pitch, when what we'd want is greater prop pitch (right?).  So, the design would have to be optimized for a particualr density altitude and conditions outside this would be somewhat non-optimum.  Just a compromise, like so much of engineering.
- As everyone realizes, the devil is in the implementation--bearings, seals, vibration tolerance, flutter control,  etc

Mark W.

I figure we would set it up for cruise at 12,000 feet.

Another trick is to dive the airplane briefly and force the
pitch to increase then slowly level out. Pitch is proportional
to speed squared.

This technique is kind of an urban flying legion but in this case
it could work.
       

I also think the piston and cylinder are best done in carbon
fiber composites. A small diameter center cylinder can guide
the large piston.

The pivot axis of the blade is best done at the 1/4 cord point
to minimize the torque required to change pitch. If the pivot
axis is ahead of the 1/4 cord point the pitch will tend
to flatten automatically.               

If the pivot axis is behind the 1/4 cord point the pitch will tend
to increase counter acting the acceleration tendency to force the
control arms on the prop blades farther to the outside, decreasing
the blade pitch.  Small adjustable trim tabs on the trailing
edge of the blades can be used for fine tuning.

Paul Lamar