On 2 May 2004 17:02:05 -0700, (Paul Lee) wrote:

Yes. pushers can get good prop thrust efficiency easier since the
"thrusted" air behind is not impeded by any structure. Air molecules
move randomly at about 1000KTS and will easily fill in the void in
front of a pusher prop to be used for thrust. In a tractor the
thrusted air is deflected by the fuselage resulting in more thrust
loses. Thats why tractors generally have bigger props to "reach"
around the cowling and over wings.

Not sure that prop efficiencies are that much greater for the pusher
than for the tractor engined airplanes Paul. While it's true that the
pusher prop doesn't throw it's thrust against the fuselage, the
fuselage is nevertheless affecting things. You always know when a
pusher flies by because of the characteristic whapping rasping sound
the prop makes. It makes this sound because the airflow to the prop
is masked by the shape of the fuselage and wings at various places.
Around the bottom of the fuselage the prop sees relatively clean air,
but when it passes the wing, it hits a mass of downwash from the wing.
Then clean air, then turbulent air again. Plus, the mass of the
fuselage itself masks off some of the air the prop sees. The
turbulence is so severe that it's my understanding metal props are not
recommended for EZ's.

In addition, the diameter of the prop on tractor airplanes isn't
generally larger because it has to be to generate thrust around the
fuselage, it's larger because it can be. Props on pushers generally
have to be smaller in order to not grind it off on the ground in case
of inadvertant high AOA. Over rotating with an EZ risks a prop
strike.

Most tractor engined airplanes don't have that issue. Rotating for
takeoff moves the prop away from the ground, not closer to it.

My understanding is that Burt Rutan has said that front engine or
pusher, the efficiencies and design advantages pretty much cancel each
other out if you design around them, in other words neither design
offers a clear advantage over the other.

Corky Scott