Rubber Motor Testing
August 1968 American Aircraft Modeler
Rubber-powered free flight, to the uninitiated, might appear to be a simple sport. Maybe even to the casual free flighter is simple. Wind the rubber band, launch the model, recover the model, repeat. Truthfully, that's about the way it has always been for me. However, many free flight enthusiasts are more competitive and like to know how to get the most performance out of every inch or ounce of rubber. As with most aspects of every hobby, the science of rubber motors has become quite precise. Even as far back as 1968, when this article appeared in American Aircraft Modeler, hobbyists were experimenting with rubber dimensions, chemical composition, elasticity, weight, tension, turns-per-inch, and every other conceivable parameter available for measurement. Typically, experimentation begins with observing and recording behavior of physical actions and reactions, then a theory is derived from empirical testing. Equations are developed that match measurements, and then more measurements are made under new conditions to determine whether the derived formulas pertain to all scenarios. If not, then new equations are derived to account for idiosyncrasies. |
One of the most interesting findings, in my opinion, is that it takes about 24 hours for a rubber motor to restore itself to full strength after being wound. The main lesson from the story, however, is that breaking in the rubber by stretching it rather than winding it accomplishes the same result, but saves wear and tear on the rubber by not needing to be twisted to break it in.
Rubber Motor TestingJim Horton
AFTER getting poor and varying results with several pounds of Pirelli rubber, we decided something was lacking in our technique. A simple straight forward test was needed to determine the physical state of a rubber motor. Such a procedure, useful while breaking in the motor, would also indicate its contest-life capability and serve, too, as an indicator of quality from one batch of rubber to the next.
Unlimited design by Horton has enviable contest record of 11 trophies. Motor test procedures described in articles were used on this model.
Rubber motors were stretched four times original length for a period of 72 hours. Cool, dark basement was convenient. Light has an adverse effect on rubber.
Motors were made up to required length around nails in workbench. Additional nails at one inch intervals indicate permanent break-in stretch.
We decided to measure the force, in pounds, developed by a motor stretched to a fixed length. A simple spring scale measuring from zero to 50 pounds was purchased at a hardware store. Our Unlimited design motor, 18 strands of 1/4" x 1/24" Pirelli 32 in. long, was stretched to three times its original length, and readings were taken on the spring scale.
Throughout the contest season readings were taken not only after motor break-in but also after each contest flown with four of the motors. Chart 1 gives the force profile of these motors. Standard, conventional break-in procedure was used -- motors were stretch wound, increasing the turn count by 10% each time until the maximum turns-per-inch (18 in our case) was reached. Our results, as indicated on the chart, show that after a motor is completely broken in, the force line becomes constant.
Armed with these readings we decided that, if motors could be broken in without winding, their useful life could be extended. To reduce erratic results a controlled stretch method was used. Motors were stretched four times their original length and readings taken with the scale at 24-hour intervals. We were amazed to find that it took three days to break in a motor. At the end of this period, "controlled-stretch" motors were giving the same tension force readings as those obtained from broken-in motors of the previous contest season. Permanent stretch was also the same -- motors increased their length three inches, from 32 to 35, after break-in.
These motors have been used in two contests this past season and wound to 18 turns per inch on each flight. So far we have not broken a single strand and the force readings have remained constant. Chart 2 gives the force profile of the motors used.
Some interesting byproducts resulted from this testing. At one meet last season, we had a real dud of a flight after making two. maxes. Naturally, we were ready to place the blame on a dead motor. A quick check with the scale indicated, however, that the motor was in perfect shape, shooting down another good excuse. Another interesting point was noted: it takes the motors 24 hours to fully recover from a winding to maximum turns. Readings taken immediately after full contest turns will not be the same as 24 hours later. All chart readings given here were made after this rest period in order to keep results consistent. It's amazing how strong the motors were; even after a season's flying, they registered the same tension force readings. In fact, motors were retired due to age rather than weakness. Strands popped, causing an excess of knots.
One strong message was clearly given on the force charts. Motors cannot be wound to a contest turns maximum without proper break-in. Fresh Pirelli will develop high stresses and has been known to blow at 50% of normal turn capacity.
Remember, this article is specifically for the motors we used. However, this procedure can be used to evaluate any size or length of motor. We feel these tests useful for the contest flyer.
Charts compare both conventional and controlled-stretch break-in procedures over two seasons
of competition. Force readings and amount of stretch were the same for each.
Note: I'm not sure what the horizontal axis represents, other than events in time, but then
why is the Johnsonville Meet (chart II) offset from the center of the grid?
Posted July 10, 2011