"Mxsmanic" wrote in message
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I'm surprised that propwash would matter, since the airflow from the
prop should stay in roughly the same place no matter what the attitude
of the aircraft.

In addition to what's already been written, it's not true that "the airflow
from the prop should stay in roughly the same place". Aircraft attitude
affects relative wind, which in turn affects where the airflow from the prop
goes. This is a significant effect. As well, aircraft attitude affects up-
and down-wash from aerodynamic surfaces which also affects where the airflow
from the prop goes.

Air moving back from the prop doesn't do so independently of all the other
air around the airplane. It mixes with, interacts with, is affected by, and
otherwise reacts to all the other air in response to everything else that is
going on.

That is, it would be like putting a big fan on a
sailboat to drive it forward.

Depends. If you put the big fan at the front of the sailboat and direct it
rearward, you can not only propel the sailboat, if you put a sail in the
flow you can redirect the flow to accomplish different things (though why
you'd bother on a sailboat, I don't know).

Likewise, in an airplane you can redirect thrust to provide aerodynamic
control. There is a very minimal reduction in the thrust (as the
redirection creates some drag, offsetting the thrust) while the control
surfaces get more air to work with. You are right to think that you don't
get something for nothing, but in this particular situation, the cost (in
drag) is insignificant compared to the improvement in control effectiveness
(from redirecting the prop thrust).

As an extreme example, consider the airshow pilot who can change the pitch
attitude of his tailwheel airplane on the ground by locking the brakes and
using engine power and elevator control to raise or lower the tail as he
desires. The fact that the engine and prop are attached to the airframe in
no way prevents the thrust from the prop from being used by the elevator to
effect a pitch change.

Pete