X-Mozilla-Keys:                                                                                 
Gary C. Buster LPT wrote:

OUCH!!!   Paul, wouldn't the prop be cleared a little better for wheel
landings (2point on the mains) with maybe a 7 foot or so???

Gary C. Buster LPT
gbuster@ballistic.com
Tyler, Texas   

Sure it would but if the gear box is 5:1 I think we would be 
in trouble with a seven foot prop. Best to design the airplane with
the worst case in mind and keep your options open. 

The Lancair IV does not have the large diameter option
for a couple of reasons. First is the limited height of the 
tri gear. Second is the direct drive engine.

I would also rather not use a three or
four blade prop because I think a two blade is more efficient
for top speed. Rare Bear had to go to four real wide blades
to absorb the HP and still takes off and lands in a 
three point attitude.

The tip velocity of the prop is the vector sum of the
forward speed and the tip tangential speed. I think it 
might be better to slow the tip tangential speed while still
absorbing the engine HP with a two blade prop. This means 
changing the PSRU ratio or decreasing the diameter of
the prop and making the blades wider. I think changing 
the ratio may be preferable. BTW The old Schneider Cup
racers had pretty efficient 9.17 foot diameter props 
in their day.

It is a very interesting problem. The Osprey V22 is an 
extreme example. A lot is going to depend on the ratio 
of the PT6 clone gear box. 

If I were building a Reno racer I think I would have
a long talk with Don Bates and run a few configurations
through his prop optimiser program.

Paul Lamar
 
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