Rubber Band Length

[MustachiousPashos]

I noticed that a rubberband that is not stretched out, releases more of its turn faster. Would putting slack on a rubber band motor be benfecial to flight endurance?

[jander14indoor]

I assume you are talking an unwound motor when you say slack. Fully wound there should NEVER be any slack left.

That said, there is probably and ideal hook to hook length for maximum energy relief, but based on experience it is a VERY broad peak. In other words, you don't loose much for large variations from the ideal. As a starting point, if the motor is stretched before winding it is already storing energy you'll never get back. If the distance is TOO close you start gettin unwinding errors, but you are talking something like less than a third the un wound lengthe before it starts getting hard to manage. So you have a range of hook to hook distances of about the unstretched motor to about 1/3 its length to work in.

Now, as a practical matter, one of the keys to this event (as has been mentioned in other threads) is matching the rubber to the rotor. For that you'll need to constantly play with motor lenght. As its a little inconvenient to be constantly changing hook to hook, I'd just pick a reasonable length and focus on matching rubber and rotor.

Jeff Anderson
Livonia, MI

[sr243]

I have a helicopter that has a motor stick of 30 cm (maybe slightly less). I am using a 20cm(length when tied up, half of the length of rubber cut) .125'' rubber band. I had difficulty getting more than 600 winds on it. When I did 500 winds, there seems to still have about 150 or so winds left. I was wondering if I should change the length. It seems like both the winds and length are low, but there are still more than 100 winds left when it hits the ground. Any suggestions would be appreciated.

[jander14indoor]

First, what's the risk, try anything you think will help and see if it works. If not, you haven't wasted time, you've just found something that didn't work in THAT circumstance.

Second, lets talk details.
- How many winds does it take to break your rubber band? How close is 500 to breaking? As already said elsewhere on this list, turns are fuel, if you aren't winding to just short of max with any motor you are leaving FREE time on the floor.
- Do you lube your motor? If not, you are leaving a lot of winds behind.
- IF you are winding to max, try lots of motor changes. Longer, shorter, fatter, thinner, etc. To start, just do a little of each, say 10% of all combinations (that will be four by the way) Keep careful data and it will tell you which direction is right. Then go further in THAT direction. Look up something called Design of Experiments. Its a powerful statistical tool to allow you to vary multiple factors AT THE SAME TIME. Now this is sacrilege to some, but done correctly it is statistically AND scientifically sound, and much more powerful and efficient than one factor at a time changes.
- Finally, don't worry too much about winds left if you are winding to max. It can give you hints where to goe, but let the stop watch be your final measure. That's what the event is judged on, not winds left. Oh, and if you graph turns vs torque, you'll also see that if you have 20% of the winds left you have much less than 10% of the stored energy left.

Thanks,
Jeff Anderson
Livonia, MI

[sr243]

^ thanks. jander14indoor.
I did wind it above 500 a couple times (550 to 600), but ultimately the final break occurred at about 500.
I did lube the rubber band, however I am not sure if I am using the correct lube. I am using something like this http://www.amazon.com/ArmorAll-10228-Protectant-28-oz/dp/B0007898UI/ref=sr_1_2?ie=UTF8&qid=1328147190&sr=8-2
I guess I need to test out different rubber bands, but it seems like I would have to increase the length substantially to get to 750+ winds and make it spin for that long. Right now it last around one minute if I go just above 500 winds.

[jander14indoor]

That's a perfectly good lubricant for rubber motors. No long term degradation and good lubricity. There MAY be better stuff out there, but the difference is small enough that the data isn't strong.

Jeff Anderson
Livonia, MI