A New Twist on Old Rubber Power

A New Twist on Old Rubber Power

A New Twist on Old Rubber Power

Replacing FAI Tan rubber with latex surgical tubing

By Jesse Brumberger [email protected] | Photos by the author

As seen in the February 2023 issue of Model Aviation.

One calm evening last summer, I was enjoying launching a little glider using a hi-start setup comprised of kite string and some 1/8 × 1/16 internal diameter (ID; 3mm × 1.5mm ID) latex surgical tubing. The idea struck me that this small diameter, high-quality tubing might be an interesting alternative to the venerable flat rubber strip that has been utilized in rubber-powered model aircraft for more than 100 years. We might, in fact, need some alternatives if rubber strip goes the way of many other traditional Free Flight modeling materials.

For starters, a tubular cross-section is the most efficient geometry with respect to torsional stiffness, which indirectly implies that a tubular rubber motor might be able to store more energy than a flat-strip motor of the same mass—if its rubber could withstand higher stress and get enough turns wound in.

The author’s display of surgical tubing is shown next to FAI Model Supply Tan rubber.

The author’s display of surgical tubing is shown next to FAI Model Supply Tan rubber.

Secondly, traditional rubber strip commonly snaps when a tear initiates at a tiny nick along an edge, or an edge stretches unevenly against a wire hook—a stress-concentration point, in engineering terms. The tubing, of course, has a smoother surface finish and no edges. That smoother surface and seemingly better quality of the tubing’s latex suggests that it might indeed withstand higher stresses than the strip rubber.

I also envisioned pushing the ends of the tubing straight onto a propeller shaft and fixed tail spindle using a hose-barb and clamp-band type of arrangement: no hooks or knotting of the rubber necessary. For simplicity’s sake, I began with classical wire hooks and tied loops, but then I saw that none of the tube motors broke at the hooks during tests. Concerned about a hastily rigged clamp slipping and being launched at high velocity, I skipped trying that configuration this time around.

The 1/8-inch surgical tubing has a comparable cross-sectional area to two strands, or one loop, of 1/8 × 0.040 (3mm × 1mm) FAI Model Supply Tan rubber strip. I set up the high-tech Rube Goldberg with the materials at hand to get around having to procure a torque watch. I then measured torque versus turns for a straight piece, or "leg," of the tubing and a loop of the strip rubber. Results are shown in the graphs.

This is the author’s torque-test setup.

This is the author’s torque-test setup.

Torque versus turns over five successive windings is compared.

Three successive windings of strip showed very little change and are consolidated into one average line in the graph. The tubing markedly lost strength after the first winding, failing at 340 turns on the second winding.

The torque of the tube motor was closer to that of the strip motor than expected because the diameter of the tubing quickly necked down as it stretched. The tubing could store more energy than the strip per unit mass, but only for one winding, after which it greatly weakened. Close observation revealed faint strain rings spiraling all along the tubing’s length from the combined tension and torsion in the wall of the tubing. That’s where breakage would neatly take place when fatigued or over-wound. Surgical tubing evidently holds up better when it stores energy in straight tension, such as when catapulting a glider aloft.

The author twisted a loop of tubing into three sets of increasingly bulky knots.

The author twisted a loop of tubing into three sets of increasingly bulky knots.

I then tested a loop of the tubing against two loops of the strip. As seen in the photo, I twisted the loop of tubing into three sets of increasingly bulky knots. Had my data points been more precise and numerous, maybe I would have seen two inflection points in the slopes of the curves as winding progressed from knot set one to two, and then two to three.

The increased bulk of the knotted tubing provided noticeably higher torque this time; however, it was also apparent that loading was too high for the tubing to be stretched out during winding as far as strip rubber could be. I stretched the tubing approximately three times at the start of winding, compared with roughly four times for the strip.

Here’s the wrap: A few calculations revealed that the surgical tubing could absorb slightly more energy per gram than the FAI Model Supply Tan sport strip for the first winding. That then dropped toward a safe capacity of approximately 7.5 foot-pounds (10 newton-meters [Nm]) per gram for both forms of latex rubber. The tubing will deteriorate and break faster than the strip rubber based on subjective observations from additional repetitive winding tests.

This is a good place to put in a plug for winding safety and eye protection because the 10-gram motor in a P-30 contest model, for example, can hold approximately 75 foot-pounds (roughly 101 Nm) of energy, similar to a .22 caliber pistol cartridge!

Good old FAI Model Supply Tan rubber, while we can get it, has more favorable characteristics than surgical tubing. Perhaps tubing is worth a try in a rubber Speed model that needs an early burst of torque. A single leg of 1/8-inch tubing might be workable in a small model because it would not need any rubber lubrication, a bonus in a tissue-covered fuselage. Costs are comparable by weight, although the strip will last for more flights.

For reference, I was winding roughly 60 turns per inch (TPI) into a new, one-loop, 1/8-inch strip motor and approximately 50 TPI into a new, single-leg, 1/8-inch tube motor. Corresponding numbers for the double loop of strip and single loop of tube were roughly 50 TPI and 45 TPI, respectively.

I’ll wind up this piece by encouraging experimentation. A few years ago, I experimented with inflating surgical tubing to provide jet propulsion for models, after being inspired by a commercial toy I had as a kid that worked on that principle. (It doesn’t perform well and tends to burst; wear eye protection!)

It’s fun to see what novel arrangements can be made to work. The delight when a do-it-yourself project is successful can easily top the expected high-performance fun from a mainstream commercial product. Let your inner nerd fly!


FAI Model Supply

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Rubber motors are characterized by their length and cross section. The cross section of a multistrand strip motor is the product of the number of strands times the strip thickness times the strip width. The cross section of the two strand 1/8" motor is 0.01 square inch. (Normally I would take the thickness as 0.042", but we will go with the author's value.) The cross section of the tube is 0.0092 square inches. They are close. Rubber is characterized by three parameters; a turns coefficient, a torque coefficient and a specific energy. The turns coefficient is derived from testing sample motors, winding them to the breaking point. Every batch of rubber is different. The breaking number of turns for a motor is the turns coefficient times the length of the motor divided by the square root of the cross sectional area. The author does not give us the length of his test motors. A turns coefficient for some pretty good Tan II was 10.64. We can use that to estimate the breaking turns per inch of these two motors; the strip motor is 106.4 turns per inch, the tube motor gets 110.9 turns per inch. The lower cross section gets more turns. The author reports 60 tpi for the FAI motor and 50 for the tube. This is a significant discrepancy. Specific energy is the energy, usually expressed in foot pounds, that a pound of rubber can store. This is the energy that appears under the UNWINDING curve. The winding curve will be higher by about a third compared with the unwinding curve, but it is the unwinding torque that drives the propeller. The author does not explain how he derived the value of 7.5 foot pounds per gram, but that equates to 3,401 foot pounds per pound. That is in the realm of possibility for excellent Tan Super Sport, but it is somewhat unusual. The data provided here does not allow us to properly evaluate surgical tubing for use in powering our little airplanes. Perhaps the author can enlighten us further.

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