Rubber Motor Length

[RaspberryPhi]

My Team is stuck on the design for our rubber motor length because Guru Engineering's Kit doesn't specify how long (They say keep under the weight limit, but there isn't one this year). Has anyone tested and found a good length for their rubber motor?

[coachchuckaahs]

I do not know directly anyone who has tried the Guru kit. However, the whole crux of the Heli event after building is matching the heli to the rubber and vice versa.

In general other kits a good starting point is about 2g of rubber. But this is only a starting point.

Once you fly it on that, adjust the mass (loop length) and try again. Keep careful notes to see if you are moving in the right direction.

You will also find that it is not likely sufficient to characterize the rubber by width, such as "1/8"". The rubber as manufactured varies in thickness perhaps 15%, while the width is cut fairly accurately. So you will want to rank your pieces of rubber by "linear density", and record that parameter as well. Then, if you have a range of densities, treat that as a second variable.

Good luck in your testing
Coach Chuck

[jander14indoor]

To be explicit in what is implicit in Coach Chuck's response. Length is NOT a critical or even useful parameter. Critical is rubber cross section (however measured, and yes mass per length (linear density) is most accurate) and total mass.

Length is only handy to quickly cut a motor of known cross section linear density to a target mass.

Jeff Anderson
Livonia, MI

[sonicboom]

How should you go about changing variables in your rubber motor after a test flight? When would it be appropriate to decrease the linear density and mass and when would it be more appropriate to increase it? Also, would it be better to use a lower linear density in a higher-ceiling flying environment and use a higher density in a lower ceiling?

[coachchuckaahs]

If you have a range of densities, or you strip your own, then yes, you can vary the density. Heli appears fairly sensitive to density, so even the variation you get in most bags of rubber will be enough to be a variable.

If you cannot change density, then you can only change mass (length) of the loop.

You can judge which way to go, roughly, by observing turns remaining. This is a ROUGH guide, and should be refined with stopwatch observations.

If you have too many winds remaining, then it is coming off the ceiling too early, and this is due to too little torque to hold up your mass. So either increase your density, or decrease your mass. If you are leaving very few winds, then the opposite, go to lower density or increase the mass of rubber.

So, if you have at least some control of density, then it is a two-variable parameter space. Actually it is whether you control the second variable (density) or not, so do track it.

What is the correct number of turns to remain? It really depends on your heli. But something in the 200 turns range may be a good starting point.

All of this assumes you are already winding correctly and near max turns, and winding in a very consistent manner.

Note also that later uses of the rubber motor will act like slightly lower density. This drops off very fast in the first 2-3 uses, then a lesser dropoff each use until failure. So keep track of how many times you use a motor. It is important to use a broken in motor (2-3 full winds) in competition as it will store more total energy.

You are correct, A higher ceiling will usually benefit from a lower density, assuming your heli is stable during the whole letdown. Some helis, as the torque goes down, may stall a rotor and fall on its side, negating the benefit of a high ceiling. So for a given ceiling height you will want to do some density and mass variations to find optimum.

Does this sound like it will take a lot of flights to optimize? Yup! A heli can fly quite well with basic sizes and winding, but optimized flight takes significant testing.

Coach Chuck