Taking force and distance readings during a pull test. The smooth tubing keeps the rubber from breaking during the test.
A pull-test comparison of various common batches of Tan rubber.
I have flown with batches of 10/97 and 3/02 Tan II rubber for many ears, and my reserves are running
thin. I wanted to do some comparison
tests between the old Tan II and the
current batches of TSS to see how rubber
has changed throughout the years.
Fred Pearce published two articles
in the March and April 1979 issues of
Model Aviation about his method of
pull testing rubber. I have followed his
method with one exception: I increased
the 430 factor to 480 in his second
equation, to adjust for the better quality
of modern rubber.
Fred’s method was rather easy. A loop
is made, stretched to a certain force, and
held for five minutes. It is then stretched
to a new force and the data recorded as it
is relaxed. Fred’s testing device was rather
large, so I decided to use shorter loops so
I could test it in my model room.
For these tests, you will need a digital
scale that will weigh up to 1 milligram.
Knowing the exact weight of the rubber
loop and knot is crucial for accurate
results (see my past columns for scale
suggestions). You will also need a pull
scale to measure the force. I used the
American Weigh AMW-SR- 20 digital
pull scale that reads up to .02 pounds.
Next you will have to make a measuring
I printed some rulers I drew up in
CAD with graduations every 1/10 inch
and glued them to a 6-foot x 1-inch x
F1 (pounds) = 45 x weight in grams/
initial loop length
My 5.8-inch loop weighs .475 grams
(minus .025 gram for the knot), so it has
a rubber weight of .450 grams, which
means the F1 force needed is 3. 49
pounds. Now pull the rubber until 3. 49
pounds is reached on the scale then
hold it at that length for five minutes
(using the spring clamp to hold the
scale), and record the stretched length
After five minutes, relax the motor
and let it rest for at least an hour to
recover before performing the next
129 Model Aviation JULY 2017 www.ModelAviation.com
Testing rubber for strength
by Don Slusarczyk
sponsored by HOW-TO issue