Re: Re: Rubber testing: Chiming In
What kind of thrust does an indoor prop have? Those things don't move
enough to even pull a model out of your hand let alone fly by themselves.
Maybe that's why those .000001 gram models of Kang fly so well?
My biggest disappointment was when I let my first attempt at indoor go and
it made one complete circle into the floor. I was young, the model was
covered in silkspan and used pirelli..1/4" I used an AJ something or other
prop system...hand wound of course! That's all I could get.
Needless to say, I had no mentor and I was about 9.
Rick Pangell
Editor of "The Max-Out" Newsletter of
The Magnificent Mountain Men FF Club of Colorado
In a message dated 11/13/2012 3:28:42 P.M. Mountain Standard Time,
wdgowen_at_gmail.com writes:
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 - 14:45:42 CET
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