Re: Rubber testing: Chiming In

From: Leo Pilachowski <leop_at_lyradev.com>
Date: Fri, 09 Nov 2012 23:00:40 -0000

The measured specific energy of rubber is measured on the energy released during relaxation (that is, on the unwind or the return from a stretch). So the hysteresis is not ignored. Also, a torque/force curve can be ploted against unwinds/distance to get an idea of how the rubber releases the energy on relaxation. However, the most important number for indoor fliers is the specific energy. For a given number of winds, more specific energy means more average torque. Or, for a given average torque, more specific energy means more winds. More winds means longer flight times and a longer time wins a contest.

LP

--- In Indoor_Construction_at_yahoogroups.com, themaxout@... wrote:
>
> 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_... 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_..._ (mailto:themaxout@...) 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_..._ (mailto:j.bartek@...) 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@"_ (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 - 15:00:46 CET