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Message
No, I think you are correct, Al
The "corkscrew" calculation will
almost always under-estimate the speed capability of a decent propeller as it
only considers the "Screw Pitch" (which is a notoriously flaky and difficult
parameter for an amateur to determine accurately in the first place).
What that approach fails to consider is that the propeller, of course,
is an airfoil and the blades produce lift similar to a wing. This lift
vector, of course, is complex due to the prop rotation, but it is
generally perpendicular (more or less) to the surface of the blade. This
results in more "pull" on the blade of the propeller and results in more thrust
and forward movement per revolution than just calculating the distance the
"screw effect" would pull your aircraft along. Therefore, you airspeed
will generally be higher than the "corkscrew" calculations alone would
indicate. That, at least is my understanding of the why you go faster
than the "corkscrew" calculation would indicate.
Ed
Ed Anderson
RV-6A N494BW Rotary
Powered
Matthews, NC
----- Original Message -----
Sent: Tuesday, July 27, 2004 9:30
AM
Subject: [FlyRotary] Re: Prop
Speeds
Subject:
[FlyRotary] Prop Speeds
Wow, that is 2935 RPM at the prop
with a prop tip speed of just under 900 FPS. I am still too shy to run
mine that fast, although I could easily get there. Interestingly, the
calculations say the max corkscrew speed of that prop at that RPM is only
197mph. He could pick up a little for being really clean, and maybe some
more depending on how they calculate prop pitch.
The pitch is a
measure of the angle of the chord of the blade; or sometimes the flat on the
RAF airfoil; it doesn’t measure the “lift” of the airfoil. So it seems
to me quite possible to go faster than the “corkscrew” speed of the
prop. But then I could be wrong . . .
Al
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