Total energy test results of various Tan II and
TSS rubber batches, as tested by the author.
This is the collected data for June 2016 TSS
rubber. Remember that the first force value
must be divided in half before adding to the
rest of the force values.
test. You will see the loop is now longer
The next step is to pull the rubber to
a slightly higher force based on the data
collected during the break-in stretch.
This new force is based on this formula:
F2 (pounds) = 480 x weight in grams/L1
During the break-in process, the
rubber was stretched to a length of
51. 6 inches to achieve the 3. 49 pounds,
so using 51. 6 as the L1 value in the
formula, the F2 force is 4. 19 pounds.
Stretching to this final value puts a lot
of stress on the rubber and knot, so I
suggest that you pull slowly during the
last 10% or so when you are getting
close to the force value.
After you stretch to this force value,
the data collecting begins. First, record
the stretched length of the rubber
at this force; the L2 length is used
to calculate the rubber stretch ratio.
Now move the rubber in 3 inches and
measure and record the force at this
new location. Move in another 3 inches
and record the force, then another 3
inches and record, and so on until the
motor is relaxed.
The data you have collected can then
be used to calculate the stored energy
of the rubber. The energy is calculated
by first taking half of the F2 force and
adding that to all of the other force
values measured at the 3-inch intervals.
For my motor example, the force values
were summed up to 16.295 pounds.
Because I used 3-inch intervals, the energy
formula can be simplified as follows:
Energy = 113. 4 x summed forces
(pounds)/weight in grams.
Using the sample motor, energy
= 113. 4 x 16.295/. 45 = 4, 106 feet-pounds/pound.
Stretch ratio = L2/initial loop length.
Using our data, that equates to 54. 6
inches/5.8 inches = 9. 41.
The stretch ratio can also be used
to approximate how many turns the
rubber loop will take.
Maximum turns: 4. 7 x stretch ratio
x loop length x square root of (loop
length/rubber weight in grams).
Maximum turns = 4. 7 x 9. 41 x 5. 8 x
square root of ( 5.8/. 45) = 921 turns.
I have included a graph showing
various batches of Tan II compared with
some of the latest TSS rubber. You can
see some batches of older Tan II rubber
had stretch ratios of more than 10
compared with TSS, which was slightly
more than 9.
The June 2016 TSS rubber, however,
has an energy total similar to some of
the better Tan II batches. Because the
total energy is similar, but the stretch
ratio is 10% to 12% less, to get similar
turns as Tan II, the TSS needs to be
approximately 10% to 12% longer for
loops of the same weight.
Until next time, keep the weights down
and the times up!
National Free Flight Society (NFFS)
FAI Model Supply
Model Aviation Digital Library
130 Model Aviation JULY 2017 www.ModelAviation.com