Rubber

[avzq]

I have a few questions about rubber:

- Are o-rings worth the extra weight (even the ones made from nylon tubing)? Could it be better to allow just the tips of the rubber to slightly unwind before hooking them to the plane? Or can I use just one?

- Is motor torque and altitude related linearly? (I'll test this but I'd appreciate more experienced opinions.)

- Without a torque meter, what might be an effective way to find/collect data for the number of backed off winds necessary to reach roughly the same torque as a set number of raw winds? (Say 800 raw turns consistently got me close to a desired altitude. The next flight, I wind to near breaking and back off a certain number of winds to reach the same torque/altitude as 800 turns. How could I find this number/range experimentally or using other data? Say I also wanted to do this for 900 turns, 1000, etc.). This makes me realize the importance of a torque meter; unfortunately, the one I've ordered has not arrived yet.

Sorry about the lengthy post! Thanks in advance for your responses.

[Crtomir]

I have a few questions about rubber:

- Are o-rings worth the extra weight (even the ones made from nylon tubing)? Could it be better to allow just the tips of the rubber to slightly unwind before hooking them to the plane? Or can I use just one?

Wright Stuff is pretty generous with the weight. Unlike gliders, nearly everyone can build a plane that is 7.5g (last year's rules). The rubber motor max weight limit was 1.5g. Two (large) o-rings weigh about 0.1g. That's about 7% of the total motor weight. So, yes, technically, you could maybe get a little longer rubber motor without o-rings than with them, but you would be doing that with two problems. One is that o-rings make it much easier to slip the motor on and off the hooks. Without o-rings, you have to undwind the end a little (losing a little of what you gained by not having o-rings to begin with) and risk damaging your rubber when you slide it onto the metal hook. The other problem is that the o-rings are fatter than the metal hooks so the tension force exerted on the rubber is spread out a little more meaning the pressure is less on the rubber ends than if you didn't have o-rings. Without o-rings, you are more likely to snap your rubber at the hook. That being said, you can experiment and see what works for you.

- Is motor torque and altitude related linearly? (I'll test this but I'd appreciate more experienced opinions.)

I don't think so. That's why you need to take data. Lots and lots of data.
Torque affects the speed the propeller rotates which affects the thrust. As the thrust goes up, the speed goes up until the thrust equals the drag. Then the plane continues at constant speed. Lift also increases with increasing speed. I believe both drag and lift increase according to the square of the speed. So that's not linear. As the rubber motor unwinds, the torque drops off in a nonlinear way also. Thus, the speed at launch is highest, but drops off nonlinearly. The lift, then, drops off nonlinearly as well. It would be an interesting exercise to plot altitude vs. motor torque and see if you could fit that to a theoretical curve based on what you know about the torque vs. time unwinding of the motor and the fact that lift is proportional to the velocity squared. But, it is probably best just to take a bunch of data.

- Without a torque meter, what might be an effective way to find/collect data for the number of backed off winds necessary to reach roughly the same torque as a set number of raw winds? (Say 800 raw turns consistently got me close to a desired altitude. The next flight, I wind to near breaking and back off a certain number of winds to reach the same torque/altitude as 800 turns. How could I find this number/range experimentally or using other data? Say I also wanted to do this for 900 turns, 1000, etc.). This makes me realize the importance of a torque meter; unfortunately, the one I've ordered has not arrived yet.

First of all, you can make your own torque meter fairly easily. Go to http://indoorspecialties.com/index1.html and click on the "Articles" menu item at the left. Then click on the "Build a Simple Torque Meter" item. There is a nice, well-written document to guide you. You really should have a torque meter. I know some teams that do everything by number of winds, and that could get you pretty far, but not far enough. Since it sounds like you will have a torque meter soon (hopefully), you can still make some progress just by testing off the number of winds. For one thing, for a given rubber motor length and thickness (or weight), you will want to know the maximum number of winds you can get before breaking for the first, second, and subsequent winds. The rubber motor stretches out pretty significantly after the first 2-3 winds. For example, we used 1/16" TAN SS rubber. With about a 41-42cm loop that was just under 1.5g, we would typically not wind it more than 1500 winds for the first wind, 1600 winds for the second wind, 1700 for the third wind, and 1800 for all subsequent winds. Sometimes, if the students were brave enough, they could get up to 2000 winds, but that was risky.

[avzq]

I have a few questions about rubber:

- Are o-rings worth the extra weight (even the ones made from nylon tubing)? Could it be better to allow just the tips of the rubber to slightly unwind before hooking them to the plane? Or can I use just one?

Wright Stuff is pretty generous with the weight. Unlike gliders, nearly everyone can build a plane that is 7.5g (last year's rules). The rubber motor max weight limit was 1.5g. Two (large) o-rings weigh about 0.1g. That's about 7% of the total motor weight. So, yes, technically, you could maybe get a little longer rubber motor without o-rings than with them, but you would be doing that with two problems. One is that o-rings make it much easier to slip the motor on and off the hooks. Without o-rings, you have to undwind the end a little (losing a little of what you gained by not having o-rings to begin with) and risk damaging your rubber when you slide it onto the metal hook. The other problem is that the o-rings are fatter than the metal hooks so the tension force exerted on the rubber is spread out a little more meaning the pressure is less on the rubber ends than if you didn't have o-rings. Without o-rings, you are more likely to snap your rubber at the hook. That being said, you can experiment and see what works for you.

- Is motor torque and altitude related linearly? (I'll test this but I'd appreciate more experienced opinions.)

I don't think so. That's why you need to take data. Lots and lots of data.
Torque affects the speed the propeller rotates which affects the thrust. As the thrust goes up, the speed goes up until the thrust equals the drag. Then the plane continues at constant speed. Lift also increases with increasing speed. I believe both drag and lift increase according to the square of the speed. So that's not linear. As the rubber motor unwinds, the torque drops off in a nonlinear way also. Thus, the speed at launch is highest, but drops off nonlinearly. The lift, then, drops off nonlinearly as well. It would be an interesting exercise to plot altitude vs. motor torque and see if you could fit that to a theoretical curve based on what you know about the torque vs. time unwinding of the motor and the fact that lift is proportional to the velocity squared. But, it is probably best just to take a bunch of data.

- Without a torque meter, what might be an effective way to find/collect data for the number of backed off winds necessary to reach roughly the same torque as a set number of raw winds? (Say 800 raw turns consistently got me close to a desired altitude. The next flight, I wind to near breaking and back off a certain number of winds to reach the same torque/altitude as 800 turns. How could I find this number/range experimentally or using other data? Say I also wanted to do this for 900 turns, 1000, etc.). This makes me realize the importance of a torque meter; unfortunately, the one I've ordered has not arrived yet.

First of all, you can make your own torque meter fairly easily. Go to http://indoorspecialties.com/index1.html and click on the "Articles" menu item at the left. Then click on the "Build a Simple Torque Meter" item. There is a nice, well-written document to guide you. You really should have a torque meter. I know some teams that do everything by number of winds, and that could get you pretty far, but not far enough. Since it sounds like you will have a torque meter soon (hopefully), you can still make some progress just by testing off the number of winds. For one thing, for a given rubber motor length and thickness (or weight), you will want to know the maximum number of winds you can get before breaking for the first, second, and subsequent winds. The rubber motor stretches out pretty significantly after the first 2-3 winds. For example, we used 1/16" TAN SS rubber. With about a 41-42cm loop that was just under 1.5g, we would typically not wind it more than 1500 winds for the first wind, 1600 winds for the second wind, 1700 for the third wind, and 1800 for all subsequent winds. Sometimes, if the students were brave enough, they could get up to 2000 winds, but that was risky.

Thanks a lot for your in-depth answers!
I've read that it's better to break in motors for competition; would this be stretch-winding them and using them on a plane before competition use? If so, how many times should I do this to competition motors?

[Crtomir]

I've read that it's better to break in motors for competition; would this be stretch-winding them and using them on a plane before competition use? If so, how many times should I do this to competition motors?

There are different methods. You want to stretch them out so you can get more winds, but you don't want them too stretched out so that you start loosing valuable torque (especially with really high ceilings). You want to stretch wind the motors at least twice before competition. The third and fourth time are probably going to give you the most total energy (area under torque vs. #winds curve). After that, they get weaker, but we still used them anyways.

Some people like to just stretch their motors by pulling them out 7 to 10 times the loop length and letting it sit for 10-20 minutes. There are different methods.

[jander14indoor]

Torque vs altitude will also change with motor width and mass because it changes how long you are at a particular torque.

On break-in, do some searching through these forums, especially the archives for advice. Winding motors is one way, but has to be done in a controlled way to be effective.
Also search the internet forums on freeflight. Indoor Freeflight on Yahoo Groups. Indoor News and Views newsletter. etc.

Jeff Anderson
Livonia, MI

[avzq]

Torque vs altitude will also change with motor width and mass because it changes how long you are at a particular torque.

On break-in, do some searching through these forums, especially the archives for advice. Winding motors is one way, but has to be done in a controlled way to be effective.
Also search the internet forums on freeflight. Indoor Freeflight on Yahoo Groups. Indoor News and Views newsletter. etc.

Jeff Anderson
Livonia, MI

I found a bit of your previous advice on breaking in motors:

Break in your motors because rubber motors change a LOT on that first wind, even if well below max. Breaking it in allows more repeatable flights for the rest of the motors life. It should be done by winding to 80% or so of breaking torque (some like higher, some just stretch and hold) and held for a while.

So to break in motors, should I stretch-wind to around 80% of breaking torque and hold for two cycles? And if I had two flight attempts during competition, is it better to use a different broken-in motor each time or the same one (I'm currently in a different free flight competition)?

[JasperKota]

is it better to use a different broken-in motor each time or the same one (I'm currently in a different free flight competition)?

I don't think it makes a significant difference, as long as the width is the same and weight gets pretty darn close to the same, but I personally used the same motor for each of my flights. Unless the motor breaks, of course, but if you're using a new broken-in motor (haven't done any hard winding with it), lubricated it well, the motor shouldn't break after just two flights.