Rubber Physics
Mandrel dipped rubber tubing generally has superior mechanical properties such as abrasion resistance, and maximum elongation (percent stretch before breakage) when compared to extruded tubing. Mandrel dipped rubber tubing has a lower spring rate (tension per unit displacement) than extruded tubing of the same cross-sectional area. This means more energy can be stored in the dipped tubing than the extruded because it can be stretched further before reaching the same tension as the extruded type. This additional distance at equal tension provides two distinct advantages when applied to launching model sailplanes.
1. More potential energy is stored in the stretched rubber as given by the equation:
Potential Energy = 1/2 x Tension x Distance stretched This is an overall benefit affecting both altitude gaining launches using a high-start, and the bungee catapult type horizontal launch. You have more "fuel in the tank" for a more energetic launch, no matter what the trajectory. Have you ever seen a "zoom" launch from a high-start? No? Then you've never seen a Hollyday Designs high start rubber in action! Abundant stored energy makes this possible. The plane may have already reached the apex of the climb, but the high start still has a lot more to give... so ZOOOM! 2. Given two identical high-starts, one using dipped rubber and the other extruded rubber, with the same plane and conditions, the dipped rubber high-start will launch the plane higher than the extruded rubber high-start. The longer stretch distance of the dipped rubber from the fixed anchor point in the ground means the sailplane will travel along a larger radius arc than the extruded rubber while launching. The conversion of rubber tension to altitude will occur at the same rate regardless of highstart length (depends solely on aerodynamic efficiency of the sailplane, similar to L/D ratio) and the sailplane will fly upward at the same speed (same rubber tension as an extruded rubber histart). The total launch time will therefore be longer because there is a longer distance (arc circumference) to travel before rubber tension is expended. Traveling at the same airspeed for a longer time at a positive angle of attack means it will achieve a higher altitude at the apex of the launch.Tension / Displacement Curves
The numbers don't lie. Graphs of actual rubber samples of Hollyday Designs' High-Start Rubber, and other brands.
Coming Soon!
(Performance Statistics Table with drop down boxes for the curve gifs.)
High Start Web-Links...
Our rubber tubing is equal to the fine material once sold by WACO, or Weston Aerodesigns Co, and generally thought to be the best rubber for launching RC sailplanes. Many people have told me that it is as good as the now defunkt WACO rubber. See some of Frank Weston's comments about his highstarts.
For more information on high start physics and general discussion take a look at Dick Williamson's work at Charles River Radio Controllers' club website: Physics and Dimensions for High Starts. Nice Job, Dick!
A construction article about a foot operated trigger for a bungee launcher device appeared in the September, 1999 issue of Sailplane & Electric Magazine.