Winding Rubber Motors

An article by John Kagan from Model Aviation Oct 2008


WINDING RUBBER "motors" is another one of the Indoor FF art forms. Peak performance is heavily dependent on winding the most energy into a motor, but go too far, and snap! You have two smaller, useless bits.

After breaking a motor, it is common to hear the oh-so-helpful advice: "back off one"—a sarcastic suggestion to unwind to the point just before it broke. Getting the maximum amount of energy into, and then back out of, a motor is a tricky feat, but here are tips that can actually help before it' s too late.

Use rubber lube. Lubricating the motor before winding prevents chafing that will cause it to break prematurely, and it lets more energy in and back out.

Rubber lube has traditionally involved special concoctions and guarded secrets. In times past, the preferred formula involved a base of glycerin and green soap. For many who grew up with it, that distinctive scent means "model airplanes" as nothing else does. It is still a decent choice, and it has the advantage that it can be washed off the motor if required.

These days, though, most people use some form of silicone emulsion/solution. It lasts a long time on the motor, doesn't weigh much, and is extremely slick. It is so slippery that care must be taken to find a knot that won't come undone. Some knots that work with glycerin and green soap will untie themselves before your eyes when silicone lube is used. Silicone is available in many products, from hair spray to vinyl conditioner. My top-secret lube involves pouring Armor All (original formula) in a shallow pan and leaving it for several days until the liquid changes from milky white to a viscous clear. Put it in a dropper bottle and go fly.

Another surefire tip is to stretch the motor as you wind. It helps the knots stack evenly and allows more turns to be packed in.

How far to stretch and how fast to come in is a matter of opinion. For volatile rubber batches (such as 7/97), it helps to use a moderate stretch and come in well before the motor gets tight. This reduces the chance of overtaxing the motor and starting any notches that might cause it to break in flight. Then bring it up to peak torque once you've reached the desired final length.

For sturdier batches, I get better times when I keep the motor taut. It is easiest to watch the motor rather than the torque meter during most of the wind. Start coming in when the knots stack up, which happens at intervals. If done right, you end up with a rubber motor that has a neat row of knots and minimal "grapevining" (scraggly little sections sticking out on the sides).

The speed of the wind is also a matter of opinion. Indoor legend Jim Richmond takes forever, using roughly a 10:1 ratio winder. I vary but often find that winding more quickly gives me a better feel.

Either way, you want to go slower near the end. The beginning is less sensitive, but packing in the last few turns can be delicate. Slowing down lets the torque even out across the motor and may even dissipate built-up heat.

You can actually watch the torque drop as you let it sit after the first few times you bring it to peak torque. Repeat until the torque stays at the peak.

This brings us to using a torque meter, which is not much more than a wire, an indicator needle, and a dial face. You can build one fairly easily by referring to the article on the Indoor Model Specialties Web site (see the source list for the address) or buying a kit for not too much loot from Peck-Polymers. (Contact information is in the source list.)

A torque meter will help you approach a motor's maximum energy storage without going over. Test a piece of similar-size rubber to failure, and you will have an upper limit. Just remember that a motor's maximum torque will go down as it stretches and gets thinner after successive uses.

A torque meter will also give you a good way to back off turns to avoid outflying a given site with fixed-pitch classes. For maximum-duration flights, wind to max torque and then back off the peak portion of the torque curve. Backing down to a given torque, rather than just removing a certain number of winds, is an accurate way to hit your height target.

Finally, use 0-rings. The ones we use are cut from hard plastic such as high-pressure tubing, nylon pushrods, or even plastic ice cream push-up sticks.

The rings are threaded onto the motor before tying and allow a fully wound motor to be transferred to the model without losing turns. It also keeps tight, slippery motors from zipping between your fingers and thrashing your model. (Guess how I know.)

If any of these tips are new to you, try 'em the next time you fly and see if they don't help improve your times!

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