RE: Indoor Props - Pitch versus Ceiling Height

From: John Barker <john.barker783_at_ntlworld.com>
Date: Mon, 16 Apr 2007 10:45:34 +0100

The following does not address your exact query about propeller pitch in
respect to ceiling height but it does relate to your mention of the
difficulty of matching the rubber to the propeller.

If you draw the usual propeller velocity triangle with rotational speed
along the base and forward speed as the vertical then the relative wind is
along the hypotenuse. Assume the hypotenuse is at 20 degrees to the base,
so we are talking of a blade angle of about 20 degrees, in other words a low
pitch propeller. Now assume an increase in torque which doubles the
rotational speed, the new relative airflow (the new hypotenuse angle) is at
10.31 degrees so the angle of the airflow relative to the propeller blade
(the angle of attack) has increased by 20-10.31= 9.69 degrees. It is this
extra angle of attack that absorbs the increased torque.

If you go through the same exercise with a high pitch propeller with a blade
angle of 40 degrees then the change in angle of attack from doubling the
rotational speed is 17.3 degrees. This just confirms what is well known,
that a high pitch propeller is much better at coping with the initial burst.
However what may not be so widely appreciated is that the higher rate of
change of angle of attack with torque will make the propeller more sensitive
to rubber size and launch torque and will more quickly approach the stalling
point of the propeller.

Please don’t think that this is theory, it is just putting some numbers to
well known facts.

John Barker - England



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Received on Mon Apr 16 2007 - 02:48:33 CEST