Re: Re: Rubber testing: Chiming In

From: William Gowen <wdgowen_at_gmail.com>
Date: Tue, 13 Nov 2012 17:28:38 -0500

Awhile back I was thinking about a similar problem with my beginner
class models and their flaring props. I wanted to measure prop flare at
various torque levels. My plan was to use an electric motor to drive the
prop. The torque could be measured and the prop photographed to give
some info about what was happening in the air. But, as John describes, I
realized that a stalled prop was not going to behave like a prop in the
air. I had an email exchange with Dean Pappas about what kind of motor
to use. He came up with the idea of mounting the whole rig on a cart
that could be driven or pulled across the floor at a controlled speed.
This would be the ultimate poor man's, low RE number wind tunnel. Of
course the idea died at that point but maybe someday......

On 11/13/2012 12:05 PM, John Barker wrote:
>
> Rick
> I don't want to pour cold water on your idea for using a test rig to
> get continuous data on how the rubber motor performs throughout the
> flight because good, recorded data is usually the way of making
> performance increases. However I believe that the sketch of your
> proposed set up has a fundamental flaw that will make it unsuitable.
> The propeller in your test rig is static, by which I mean that,
> although it is rotating, it is not moving forward and static thrust is
> not really of much interest. Unless the propeller is moving forwards,
> as in normal flight, the angle of attack of the blades will be much
> higher than the 'in-flight' angle of attack and any thrust and torque
> figures will be meaningless.
> However there is a well known solution to this problem which dates
> back to the pioneer days of powered flight - the Whirling Arm. The
> easiest way for me to describe this is with a few crude assumed
> dimensions. Assume a room is available for experiments that is 12
> feet square. In the centre of the room erect a vertical pivot. Take
> a piece of wood say 1 inch square and 10 feet long and drill a hole in
> the middle. Put the hole on the pivot and then the piece of wood can
> be spun round, clearing the walls by about a foot. In practice the
> beam would probably need to be built up to provide stiffness with
> lightness and would need balancing about the pivot. The pivot would
> need to be a low friction bearing. It would then be possible to mount
> a motor stick, with motor and propeller on the end of the arm, wind
> the rubber, release the propeller and watch the arm spin round.
> Obviously if you timed and counted a number of circuits you could
> calculate the 'flying speed' of the propeller.
> I believe Bernard Hunt had a whirling arm at one time and I know Dr
> Bob Bailey had one because he described it in the 25th Nffs Sympo. I
> recommend that you read his paper because it will almost certainly
> help you to fabricate a better test rig.
> John Barker - England
>
Received on Tue Nov 13 2012 - 14:28:41 CET