Re: Re: Rubber testing: Chiming In
Bill,
Everybody seems to write down the motor pedigree, winds, backoff turns and
all that, so there is plenty of parametric data out there. What I suggest
is trying to get halfway to the goal line in the shop when testing motors.
Flying in Lakehurst is a long term testing program for most.
Testing the rubber gets you "energy" in foot-lbs/lb...that always seemed
kind of odd to me. To me it implies if you took a pound of rubber you could
stretch it and chuck it up in the air that many feet. Odd way of
expressing it. But it does compare all of the rubber batches. It ort of ignores
the hysteresis and loss of energy ability of the rubber batch.
Wouldn't it be cool if the profile for Brett's model could be extrapolated
for his model and used parametrically for the next time he, or anyone,
would be flying there?
Think of the possibilities of prop testing in your own shop instead of
having to wait until you could fly at a decent site.
For the instrumentation..i.e. load cell selection and al that
(Kirda)...find a commercial one that will measure in the parameters of torque, tension
and whatever you would expect to see. I didn't say it would be cheap, but
the Gowen torque meter with a stopwatch is a great solution. And for
thrust, a spring with a ruler and a stopwatch can work too.
The secret is "measured value" versus time. The torque profile I would
think would be of great value...especially with measured against the data
that is written down.
* Launch torque vs. climb rate to get to the cruise torque
* What's the cruise torque versus time of a given model / prop /
rubber, etc.
* all those things
This would make a great Sympo article...for someone of tech ability that
has some time on their hands. Plus folks willing to part with their data
sheets(!).
Rick Pangell
Editor of "The Max-Out" Newsletter of
The Magnificent Mountain Men FF Club of Colorado
In a message dated 11/9/2012 2:13:20 P.M. Mountain Standard Time,
wdgowen_at_gmail.com writes:
Hey Rick
Weren't you the one asking awhile back whether it really mattered what
size rubber motor you used on an indoor model? (I'm still a little ashamed of
my answer)
Seriously, I do a lot of testing but I call it motor testing rather than
rubber testing. I make my best guess as to what's going to be the right
motor for a particular site and then test all the motors I plan to use. I
haven't found it necessary or advisable to wind them hard for the testing. My
usual method is to do a wind to about 60% of max torque, back off turns to an
estimation of the launch torque I'll need, and then take torque readings
for every 200 turns (10 winder turns for me) down to zero. The summation of
the torques at each step gives me a "score" for a particular motor. A higher
score is better in nearly any situation.
A fly in the ointment is that variation in the length and weight of
possibly "right" motors will skew the results toward heavier motors. I use a
correction factor in the final "score" that tries to balance the effects of a
higher wing loading vs. higher available energy.
This may not be exactly what you're talking about but it does tell me
which motors are going to perform better in the air. That's what's important in
my view. Another really important aspect of this kind of test is that I
can avoid wasting contest time flying on a bad motor - or maybe a better way
to put it is a motor that's not as good as a "good" motor.
On 11/9/2012 3:49 PM, _themaxout_at_aol.com_ (mailto:themaxout_at_aol.com) wrote:
To all....imitation is the best form of flattery. Sotme time back I put
out the attached little dissertation. It seems indoor model power systems
are the key component for a given model. If one had sufficient test dta for
a great performer, say Brett's or John's models at peak performance,a nd I
mean the rubber/prop system "as flown."
In my former life, the mantra was test like you fly. My thinking is you
need the whole flight profile for a give rubber/prop configuration. The
torque, prop dia, etc.
If this does not come through, email me directly and I will send it
directly to you.
This begs for somebody willing to get out the stopwatch and the meters and
make up the test fixture.
Rick Pangell
Editor of "The Max-Out" Newsletter of
The Magnificent Mountain Men FF Club of Colorado
In a message dated 11/9/2012 1:18:28 P.M. Mountain Standard Time,
_j.bartek_at_sbcglobal.net_ (mailto:j.bartek_at_sbcglobal.net) writes:
Mike
Looking at your initial post, I see that you'll be using 5 X extension on
0.053 inch rubber for the start of your "wind to failure" test. Not sure
how long a loop would last stretched that far. I haven't used that much
stretch for Bostonians and such with wider rubber. Do you measure force to
stretch the loop with a fish scale or similar? Perhaps adding weights to a
suspended loop, like a high school physics Hooke's Law experiment would give
you that.
A starting point would perhaps use a 6 inch loop of 0.053 inch rubber
stretched between 2 No. 10 wood screws 18 inches (3X) apart in a board. The
larger wood screws might be smooth enough to keep the strain from
concentrating there.
I've got a box of FAI tan in 3/32 inch (0.094") and should generate some
preliminary data before we talk "exact" though.
Joe
--- In _Indoor_Construction_at_yahoogroups.com_
(mailto:Indoor_Construction_at_yahoogroups.com) , _"mkirda_at_..."_ (mailto:mkirda@...) _<mkirda@...>_
(mailto:mkirda_at_...) wrote:
>
> Be happy to if you could write up exactly what you want me to try.
>
> --- In _Indoor_Construction_at_yahoogroups.com_
(mailto:Indoor_Construction_at_yahoogroups.com) ,
>
> Maybe
> > > Mike Kirda could take some loops of his rubber and stretch them over
> > > some nails in a board, with and without lube, to give a "days to
break"
> > > number.
>
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Received on Fri Nov 09 2012 - 13:50:54 CET
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