With a constant-speed prop the graph of propeller efficiency vs airspeed
(or vs advance ratio) is still curved, but not as much. Constant-speed
props do NOT settle on maximum efficiency, by the way, though they're
generally fairly close to that.

On your second question, I don't understand what curve your discussing.
What is plotted on the vertical and on the horizontal axes? and what are
the other parameters describing the curve?

John.

"xerj" wrote in message
news
Thanks to both of you.

I'm getting closer to understanding, but a couple of things are still
eluding me:-

1) What happens to the curve with constant speed props?

2) I don't exactly know how to put this, but if a curve labelled "65%"
showed, say, 100HP at 60 kts and135HP at 120 kts, that means that the
35HP
difference is due to prop efficiency? The reason I say this is because
if
the entire curve represents a 65% setting then the power shouldn't
have
changed, right?

Thanks in advance, and much appreciated for the answers so far.


"Todd Pattist" wrote in message
...
"xerj" wrote:

I understand the "power required" curve as plotted against
velocity, but
the
concept of "power available" plotted against velocity is escaping
me.

How is this curve derived? And why is it "curved"?

In the aircraft we fly, the Pa curvature comes about from
prop efficiency and engine limitations. A prop is most
efficient when the prop airfoil is operating at it's best
L/D angle of attack. Any RPM slower or faster than the RPM
needed at the current airspeed to produce that AOA reduces
prop efficiency. Of course, changing the engine RPM changes
the power available from the engine.

At the upper speed end of the scale, the engine is operating
at redline, and the prop AOA has dropped to zero producing
zero thrust and zero power. Any faster and the engine
either overspeeds or the prop begins producing negative
thrust.

Todd Pattist
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