Re: Re: Rubber testing: Chiming In

From: John Barker <john.barker783_at_ntlworld.com>
Date: Wed, 14 Nov 2012 01:56:01 -0000

Rick,
You asked what sort of thrust an indoor propeller provides. The answer is, 'not very much! A Penny Plane airframe weighs about 3g and carries about 2g of rubber so about 5g flying weight requiring a lift of 5g. I reckon the L/D ratio will be about 4 to 5 so the thrust (to equal the drag) must be only just over a gram. An F1D would be even less of course so I am not surprised that you wonder if an indoor propeller could turn a whirling arm. Read on!

In my previous post I assumed that most people would be familiar with a whirling arm rig and did not spend much time describing one. That was a mistake and did not give a proper idea of Bob Bailey's light weight rig. His arm was a 1/2" square box of 1/16" balsa sheet and was about 3 feet from the pivot to the motor stick. The pivot was just 16 swg wire running in aluminum tubes with a couple of 'Teflon' washers. The rig was statically balanced to eliminate errors if the pivot was not vertical. It is interesting that when testing F1D propellers a 1/32" balsa drag plate,9" x 3.5" had to be added near to the end of the arm to slow the rotational speed so that the forward speed of the propeller matched that o an F1D. I was amazed to read that a 1 degree change of propeller blade angle gave detectable changes of torque and rpm on the rig.

John Barker - England
Received on Tue Nov 13 2012 - 18:05:42 CET