This discussion on "static" thrust goes on all the time on the E-flight forums. Yes propeller efficiency is better as it moves through the air, but even when the rig is standing still the propeller is exerting a force on the air which in turn moves. I've been trying to use a wind tunnel study from University of Illinois (
Urbana-Champaign to understand what is going on.
http://www.ae.illinois.edu/m-selig/props/propDB.html
On this site there are plots of the coefficient of thrust as a function of J, a dimensionless airspeed. As J decreases the thrust for a given rpm levels out. The tests are made in a small wind tunnel.
If the idea behind the tests Mike is proposing is to get at the power from the rubber, you could use anything, even a plastic P-30 prop, to provide the load. and as long as you kept track of rpm you'd have a consistent amount of air that the prop was moving.
It's true if you try to see what a big, slow turning prop will do when you're taking the energy out of the rubber over 60 minutes, you'll have a hard time seeing much of a force. If you find a prop that lets down the motor in a minute or so you have a much stronger force.
I fly a Dornier Falke no-cal that will reach a 40 foot ceiling in about 25 seconds with a Peck 6 inch prop out front while turning in a 50 foot circle. It will fly out of my hand from a standing start, but won't go vertical.
The efficiency of a prop on a static rig is undefined, mathematically, but the air still moves.
Joe
--- In Indoor_Construction_at_yahoogroups.com, William Gowen <wdgowen@...> wrote:
>
> 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 - 17:42:37 CET