Re: Re: Rubber testing: Chiming In
John,
Works for me, but if all of the tests were performed the same the
comparison would be apples and apples. Prop pitch being a variable. If a series
of tests were run with the same prop and only changing the rubber, it would
have meaning. Using John or Brett's actual rubber / prop combo in the
first test would be revealing. Again, actual flying is the validation.
When the rocket engines, etc were being tested they were strapped down
too...etc, etc. You just have to accommodate and do the parametric
comparisons. My intention wasn't to make this a difficult problem, but I bet it
would be close.
And, when I used to run a design/test team, my standard answer was "There
is nothing you can do to ruin my day." With that in mind, and improvement
that someone is willing to suggest is more than helpful.
I used to get all of the new young engineers for some odd reason in my
design group. The first day I would sit them down and show them a calculator
and ask what it was. The answers would range form a "TI" or "HP" and the
like...then I would say that it was more computing power than what we sent
to the moon. We used slide rules and 3 significant figures.
My next comment was "Engineering is not an exact science, but math was."
In engineering we would take the worst result found in test, multiply it by
.7, then multiply by 1.2 to get an allowable and then calculate to get a
"positive" in the result. What are we doing analyzing to 12 decimal places?
(note, 2 times 2 should equal 4, but what about large values of 2?)
Being an engineer is like being a football coach trying to call plays (NFL
type, not that other thing). The rules are simple...run any palyyou want,
but the best you can do on any given play is get half way to the goal
line....
So, who makes the best engineer? It's the one who knows when to kick the
field goal.
Just my tongue in cheek answer...
It's a start. So, who is that person who thinks one could go all out on
this? Makes for a GREAT Sympo article.
Rick Pangell
Editor of "The Max-Out" Newsletter of
The Magnificent Mountain Men FF Club of Colorado
In a message dated 11/13/2012 2:19:25 P.M. Mountain Standard Time,
john.barker783_at_ntlworld.com writes:
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 - 14:00:56 CET
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