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

Maybe that was some sort of damage.  Not normal.

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

True.  But the blade lift *adds* to the axial tensile stress at the L.E.
and T.E. for a positively cambered airfoil.  Because the lifting forces
come and go with blanking issues and because near the root  torque bending
(pulsed) has the same effect, fatigue issues exist as I stated.  The
helicopter blades you cite either use a flapping hinge or a special bending
beam to eliminate bending in the blade.  As in all structural designs, if
you make it stiff it must be proportionally strong...sometimes making it
not stiff requires less strength.  I'm sure a G.A. prop could be designed
with flapping hinges but I doubt that you would like the coning angle.

Jeff Spitzer

 
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