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

Jeff Spitzer wrote:
I'm not sure why you're asking me, Paul.  The Predator prop doesn't exhibit
a flapped nature near the root.  

I remember seeing a small groove on the underside near the trailing edge
when I was out at El Mirage. Looked like what one sees when they look at the bottom
of a flapped wing. PL

I'm no prop design expert but have toyed a
bit.  Usually the root trade-offs are driven more by structure than aero
due to the limited impact of bad aero.  The impact of bad structure can be
profound.  A "flapped" section will have thin material very far from the
neutral axis of (lift) bending thus causing excessively high stresses.
This is always a primary consideration in proper engineering analysis of a
propeller blade.  Typically a cambered airfoil looks somewhat like *the
opposite* of an I-beam in bending.  This is a major reason why non-cambered
airfoils are typically used.  The thin T.E. and L.E. are on the neutral
axis and therefore don't crack.  

I am not sure the cord wise bending loads are all that high back there. Most 
of the load is in axial tension. Blade axial bending loads are mitigated by the huge
tensile
loads due to centrifugal force. If this were not true a helicopter would not work at 
all. In fact these blade axial bending loads can be nearly canceled out by coneing 
the blades forward. Some advanced composite commuter plane props are now using 
this technique.  PL

If you run a standard prop program you
find that for "best effieciency" your prop has a huge root chord like the
original prop on the carter copter.  They took the approach to separate the
structural spar from the aero shell to allow use of such a goofy root
chord.  

In the end there is no benefit and only skin friction losses that
the simple prop optimizer program didn't mention.  So...what really happens
is a designer goes "yeah, so" and ignores the prescribed chord profile and
adjusts for structural reality and other trade-offs.  I think the same
applies to your flapped root, although I'm nowhere near sure.  I don't see
any major benefit.  I think the Lopresti thing had more to do with the
prop-pulse-phasing with the intake than prop thrust or efficiency.  I
really haven't been paying attention to the ACRE prop project so I really
don't know what your claims are, where you guys are coming from, or where
you're going.  Just my $.02 on the direct question about root flaps.

Jeff Spitzer

 
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