Google for "Custer channel wing" to see what extreme effect
prop wash can have.



"Peter Duniho" wrote in
message ...
| "Mxsmanic" wrote in message
| ...
| [...]
| 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
|
|