Re: Re: SS 2009 good batch?
Several years ago there was a discussion of rubber winding on the FFML. I posted a comment about some F1D flyers taking 10 minutes to wind a motor to let the heat dissipate during the process. A very well known F1B flier told me that was ridiculous.
----- Original Message -----
From: themaxout_at_aol.com
To: Indoor_Construction_at_yahoogroups.com
Sent: Sunday, March 27, 2011 10:13 AM
Subject: Re: [Indoor_Construction] Re: SS 2009 good batch?
In a message dated 3/26/2011 11:36:51 P.M. Mountain Daylight Time, tapio.linkosalo_at_iki.fi writes:
In F1B, several guys have reported that the Tan rubber starts getting
brittle. It still has the good energy return, but it tends to break
abruptly, which is not so nice if it takes place e.g. during the flyoff
(where you only have a 10 minute window to wind the motor, pick the air
and start your flight). Many people say that they prefer to use SS, as
they can rely on the rubber holding together for the flight.
An article covering this phenomenon of breaking in the NFFS '09 Sympo by Fred Pierce. Rubber can be pretty complex and Fred explains some of the interesting features to consider.
Excerpting some of the article....
"Rubber undergoes two types of change-of-state when winding and then relaxing a rubber motor.
1) Stretching out and winding a rubber motor changes its molecular order from a mass of randomly oriented tangled polymer chains to a stretched out untangled more orderly state. It gives up heat, it reduces its entropy (a thermodynamic measure of the amount of energy available for useful work), and hence, the energy available is reduced.
2) Another, and more difficult to comprehend change-of-state energy loss (gives up heat) is the “crystallization" of the polymer chains. This is where a molecular crystal is formed following a well-defined pattern, or structure, dictated by forces acting at the molecular level (see phase change diagram). The natural rubber component in a blended rubber as the motor is stretched (strained) by winding, “crystallizes.” The technical term is “Strain Induced Crystallization.”
...Heat is a critical issue. One must understand the temperature behavior of the rubber motor as it is being wound and then how to cope with it. Most of the “good” rubber produced, say FAI Model Supply May 1999, had a practical upper limit of 80-85 degrees F, where it could rupture. If in winding, the rubber heated up due to the hysteresis, and if the starting temperature of the rubber was at ambient and the ambient temperature was high, the rubber could be at a temperature above well above 80 degrees F and subject to breakage. The example is indoor flying where motors would break well into the flight. The temperature at the floor was near 70 degrees F and the temperature at ceiling height could be near 100 degrees F. The motor would heat up due to the long unwinding time and still be under high stress. The long motor runs were not sufficient to keep the motor cool enough as it would absorb heat from the ambient surroundings.
...A significant feature is that only natural rubber can crystallize. The synthetic rubber in a blend like FAI Model Supply Super Sport does not crystallize when being wound, but does slightly when being held after winding.
A special note: Experiments have also shown that the speed at which one performs a stretch or winding can have a significant affect. The resistance to notch propagation from small nicks in the rubber is diminished the faster you stretch wind; speed can blow motors.
...It is possible to lower the motor temperature rise from winding. It is done as part of the common pre-winding stretch to increase potential turns. You hold the stretch, say less than a minute, relax it, and pull it out a second time to wind. The maximum temperature rise encountered by the motor will be reduced."
Rick Pangell
Editor of "The Max-Out" Newsletter of
The Magnificent Mountain Men FF Club of Colorado
Received on Sun Mar 27 2011 - 10:15:42 CEST
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