The graph you referenced involves three different power (throttle)
settings, all at sea level. Keep in mind the airplane will be in level
flight ONLY at the two intersections of the Pr and Pa curves; in between
(Pa Pr), it will be climbing. (Outboard of those intersections, it
will be descending, unless stall conditions intervene, in which case it
will REALLY be descending!) Keep at it; this stuff will eventually make
common sense.
John Lowry
Flight Physics
"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
(Remove DONTSPAMME from address to email reply.)
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