BTW the latest hot tip is to put a fixed flap on the
first third of the
prop blade where it is going the slowest. A plain
fixed flap should
give a C sub L of about two. I notice Jeff has these
on the Predator
props. Also if you look closely at the prop
PowerSport is pushing
(some sort of pseudo constant speed flex prop) it
too has a flap.
Picture in a minute relative to Mike's muffler
mount.

Paul Lamar

Vance Jaqua wrote:

About 3 years ago at Sun N Fun the Lo Presti guys had
some blades like that but I thought they came to their
senses since they seemed to disappear. The flapped
area will stall quicker at low speed and drag more at
high speed - looks like a lose lose proposition to me.
High Cl usually has a poorer lift/drag ratio which is
the
operative function- if you need more lift there, make
it wider. Vance

You are thinking inside the box Vance. Yes Lopresti did that
but the flap was short, extended out from the main portion of the blade
and tacked on. Each Lopresti flap had two additional end vortices just 
like Fowler flaps on wings. Tip (end) vortices are well known to cause 
high drag. After all that is where induced drag comes from Cl^2/pi*A.
  A of course being the blade aspect ratio.

The blades with these blended fixed flaps have no such vortices as the
flap and hence the high C sub L 's is smoothly blended and integrated
with the rest of the blade. 

Furthermore the range of angle of attack of a 66(215) airfoil is 
25 degrees with a 20% fixed flap. Minus 16 degrees to plus 9 degrees. 
With a 66(215) without a flap the angle of attack range is from minus 10 
to plus 10 or a range of about 20 degrees. (BTW don't be fooled
by the diagram on page 666 of Von Doenhoff. That is for a SPLIT
flap and not a PLAIN flap. See page 195 for data on a plain flap.)

This means it could still be generating lift when the inner third of 
the prop during takeoff and climb would normally 
be stalled due to unfavorable angle of attack and twist. 
The twist distribution of the blade would have to be adjusted of course.
In fact the blade may not have to be really twisted at all. All effective 
twist could be added by adjusting the airfoil on the flapped portion of the blade.
Did I make this clear?

Furthermore the inner third of the blade is going relatively slowly compared to the
outer 2/3rd so the drag is why less. Drag is a square law function of speed
of course. 

Fourth or perhaps fifth the torque restraint is the mechanism
the prop exerts on the engine to load it down. The drag from the inner third of the 
blade exerts less torque on the engine as the moment arm is reduced.

IMHO it is worth a try. Obviously some pretty smart people also think
it is worth a try.  

I would like to here from Jeff on this. Perhaps it is a military secret :-)

BTW one Peter Kumpel was doing some very interesting propellors along these
lines prior to WW II for the Horten brothers of flying wing fame.

Paul Lamar

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