Rubber Motor Maximum Turns Calculator

Free Flight Rubber Motor Maximum Turns Calculator

Here are two quick and dirty calculators to determine how much you can safely wind up your free flight rubber motors. The first is loosely based on information in the book Making Scale Model Airplanes Fly by William F. McCombs when using Pirelli rubber (which is no longer available) that has been lubricated, broke-in and stretch-wound. I am not a math magician or a programming wizard so this isn’t completely perfect. I think the calculated values are a little low compared to those in the book.

The second is a calculator I created using data I collected with lubed and stretched 1/8″ SIG Sport Rubber.

Please don’t blame me if you use these calculators and your motor snaps and your plane gets destroyed (I’m too busy doing this to my own models). It is recommended to only use 90% of this value in repeated contest flying. Also, these values were determined for an outdoor temperature of 80° F. Fewer turns should be used in cold weather, more can be used in warm. However, motors are more likely to unexpectedly break in temperatures above 95° F.

Some info for newbies to the hobby. A “loop” is a strand of rubber tied in a loop. A loop is measured after it’s tied. In other words, a 20″ strand of rubber is tied into a 10″ loop.