Re: Indoor Props - Pitch versus Ceiling Height

From: Peter Hess <hesspl_at_comcast.net>
Date: Mon, 16 Apr 2007 10:20:29 -0400

John:

Thank you for your response. I am afraid that I know very little about science of propeller behavior, but you discussion was very informative and, when taken together with Marty Sasaki's observations, helpful. As I said in my response to Marty's post, my conclusion, inexpert as it is, is that I should revert to lower pitch props and rely on increased flare to produce the higher pitch that will help to control launch torque.

Thanks, again.

Peter Hess
Canton, CT

  ----- Original Message -----
  From: John Barker
  To: Indoor_Construction_at_yahoogroups.com
  Sent: Monday, April 16, 2007 5:45 AM
  Subject: RE: [Indoor_Construction] Indoor Props - Pitch versus Ceiling Height


  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 - 07:39:25 CEST