Re: 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 - 07:13:40 CEST
This archive was generated by Yannick on Sat Dec 14 2019 - 19:13:46 CET