Rubber

[StampingKid]

Finished build last night. For the first time in five tries,over two years, I finally came in under 7 gr. I''ve always been close--7.1 +/- . But this 6.35 grams should give me plenty of room to adjust cg. Excited to get this one to the gym. But first I need to finish my torque meter. I have everything for Harlan's plan--need only to assemble and solder. Am I correct that the two things I need to concentrate most on with the torque meter is the relative breaking point of each size of motor and the torque at which this plane flies level? Second question is will the same torque on different motors fly the plane the same?

[WrightStuffMonster]

. Am I correct that the two things I need to concentrate most on with the torque meter is the relative breaking point of each size of motor and the torque at which this plane flies level? Second question is will the same torque on different motors fly the plane the same?

There is a bit more too it than that, though you have the basic idea right. A good rule of thumb in choosing the correct size of rubber band is that you should have about as many turns left at the end of the flight as you backed off at the beginning (assuming you are flying in a low ceiling gym and backing off alot). This is the best place to fly because you are in the flattest part of the rubber bands torque vs. turns curve and will have and nice slow climb a good cruise and a long decent. The same torque on differently size rubber bands will fly your plane differently because each will have a different cruise torque. But it is a useful metric when you are comparing two motors of the same size. If they are wound to the same torque and turns they should fly similarly. I can dig up an old torque vs. turns graph if you think it would be helpful.

[leetx]

I want to share with everyone what we can wind on our rubber motors. These are what we have achieved and can serve as a guideline. But you MUST wind and experiment yourself. If you don't break rubber motors, you won't know how much they can be wound. From the discussions in this forum, I feel that most students do not fully wind their motors.

Our motors with 2 o-rings are between 1.48 and 1.50g. We use lightweight o-rings of approximately .015 g/pair.

0.093" (3/32"), motors are between 11 3/4 to 12 1/2 " long. We wind to approximately 1.0 in-oz (our torque meter is NOT calibrated -- this is my estimate):

First wind: 1410 to 1470 (94 to 98 on 15:1 winder)
Second wind: 1440 to 1500 (96 to 100)
Third wind: 1470 to 1545 (98 to 103)

0.083", motors are between 14.5 and 15". We wind to approximately .96 in-oz:

First wind: 1725 to 1800 (115 to 120)
second wind: 1770 to 1875 (118 to 125)
third wind: 1845 to 1950 (123 to 130)

When we aim for a 30' height, we will typically back off 150 to 180 (10 to 12) before launch.

We broke many many motors, some purposely, to arrive at our winding guidelines. Now, we almost never break motors near the peak winds. Likely, we can be even more aggressive, but during competition, it's good to know that the motor will not break.

When our motors break, they typically break well before the peak winding torque, and I attribute these to damage incurred on a previous wind. I don't believe we have broken a motor on its first wind winding to our peak torque. Always examine your motors to look tears -- takes only 5 seconds to examine it. If you see a tear, you shouldn't wind it to its peak. You might throw it away or use if for miscellaneous testing where full winding is not needed.

You must also track how many winds you have put on a particular motor. A motor will allow more winds at successive winds when wound to the same peak torque. If you know how many winds a motor has yielded in its first and second winds, you will have a good idea how many will be allowed on the third wind.

Hope this helps.

[smartkid222]

Out of curiosity, was the rubber you were using lubed? If so what kind of lube?

You must also track how many winds you have put on a particular motor. A motor will allow more winds at successive winds when wound to the same peak torque. If you know how many winds a motor has yielded in its first and second winds, you will have a good idea how many will be allowed on the third wind.

Hope this helps.

Yes, the more times you wind an individual motor= you can put in more winds than the time before,
but also realize that rubber has an energy curve. I've spend considerable time just know looking for a post that describes this but i can't find it. In summary, rubber that is wound close to it's breaking point becomes "tired." Therefore, after the motor becomes "tired" if you keep winding the same nuber of winds on it, you will not get enough the same amount of energy out of it.

[leetx]

Out of curiosity, was the rubber you were using lubed? If so what kind of lube?

Yes, always use lube. Armor All original formula.

but also realize that rubber has an energy curve. I've spend considerable time just know looking for a post that describes this but i can't find it. In summary, rubber that is wound close to it's breaking point becomes "tired." Therefore, after the motor becomes "tired" if you keep winding the same nuber of winds on it, you will not get enough the same amount of energy out of it.

I found the post from last year by jander14indoor and have pasted it below.

We use the standard tan super sport. At the winds I have posted previously, we have ALWAYS improved flight times on successive winds, up and through the third wind and mostly through the fourth wind. Although the numbers I have posted may seem a lot compared to the experience of many students, I believe we are not winding extremely hard. In experimenting with when the rubber breaks, we have noticed that .093" breaks at an average of 1.2 in-oz, and 0.083" at 1.1 in-oz. (Again, please remember that these numbers are estimates as our torquemeter is NOT calibrated.) For our discussion here, it is the relative torque readings that matter. For the 0.093" rubber, I have stated that we wind to a maximum of 1.0 in-oz, which is less than 85% of the breaking torque; and for 0.083" rubber, less than 90%. It is my experience that at these winding torques, the rubber's performance improves through the fourth wind. (I cannot say much for the fifth wind -- we did very little testing there.) Our best time of 3:40 (30' no touch) came on the fourth wind.

jander14indoor says that winding to 95% of breaking torque may tire the rubber sooner. Seems reasonable. I just don't have any personal experience.

So even though the numbers I have posted seem A LOT -- I think there is actually more to be had, and Tan SS can hold even more winds. I have provided these as guidelines and especially to encourage most of you to winder harder! But you must experiment yourself.

Hope this helps.

OK, thanks. On replacing rubber after every 2 flights. That doesn't seem quite right. It couldn't possibly be that FAI wants to sell more rubber, or could it? However, there must be a point where the rubber is stretched past the optimal elasticity, I think. <SNIP>

Weeeellll, we're back into true but...
FAI is talking about winding motors VERY close to breaking when they say 'tired' after two flights. From my and many others experience this is true. You can even see the effect as such motors will show little edge knicks and other signs of stress. If you are winding well short of max you don't do much damage and you will get many more flights in. Resting motors helps with this, lets those long stretched out chain molecules in rubber reset.

Of course the question arises why is FAI warning about this? Turns out they are trying to help the customer with knowledge, not just sell more rubber. A competitive free flighter (even indoors) WILL wind motors very near max in competition, every time. Why you ask? Because you can get more winds in the motor by winding to near max and unwinding to launch torque than you can by winding to launch torque to start with. More winds is more fuel is longer flights. That's how you win.

One thing I've observed is that many SO flyers have only a few motors from a kit or small bag of rubber so they tend to underwind to avoid breaking the motor. Understandable behavior, but very limiting to flight time. Though conducive to longer motor life.

So, what to do.
First, realize rubber is fairly cheap. At 1.5 gm per motor, you can make 35 +motors from a $13.20 with shipping 1/4 lb (113 gm) box, so get extra rubber. A half pound box for 70 motors is only $19 or so.
Next, learn where you motors break! This is where a torque meter comes in handy as motors break more on torque than winds and you can see how close you are getting. You'll find the max torque is way beyond reasonable launch torques in all but the very tall sites. You can do this with winds too, just not as consistently.
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.
Now, wind past your flying torque, but well short of breaking for practice. Learn plane settings, etc. Reuse motor as long as performance doesn't degrade or rubber breaks. Fly an occasional practice flight by winding to 95% or so of max and backing off to flying torque. You'll find you add a significant number of turns.
In competition, do just enough practice flights to get dialed into the site. Do one practice full winds and backoff. For the flights that count, wind as close to breaking as you dare, backoff to launch torque and go. Everytime.

[smartkid222]

Yes, always use lube. Armor All original formula.

Everyone says how lube allows you to put more winds into the rubber. This is its purpose: to allow you to wind the motor more before it breaks. Has anyone actually tested this though? or has everyone just accepted this as fact without verifing it?

Im saying this cause i tried to test this and my results showed that lube didnt' make much of a differnece. I would say, however, that my tests had a very large margin of error, so they dont hold much value.

[smartkid222]

also while on this topic, have you ever tested other lubricants besides armor all? At a hobby store i go to they sell lube. The owner claims that the it works better than armor all and the properties of the two are very different. Again i didnt' find much of a difference between not using it and using it.

[jander14indoor]

There is extensive data on the value of lube to energy storage and return for wound rubber motors. Fair amount on what type of lube. Reported in the US National Free Flight Society's annual Free Flight Symposium or the British Free Flight Technical Committee's annual Free Flight Forum. With proper repetition and statistics applied.

Lube is critical to winding hard without breaking. Makes a BIG difference.
Which lube is important to energy storage and return, less so to life.
Bottom line is silicone based lubes (of which Armor All is one variant) were all much better than no lube, better than the old dish soap and glycerin of our youth, but only small differences between the various silicone lubes. Armor All can be improved to near the performance of the fancy lubes by drying off much of the solvents to thicken it.
There's also data on which lubes degrade rubber over time, hint avoid petroleum based lubes.

Now, this is Science Olympiad, not Trust Jeff Olympiad. Feel free to look up my sources or replicate the experiments to confirm or challenge. Might be an interesting discussion on testing methods, results, etc.

Comment on hobby store (and salesmen in general) who's trying to SELL you something. Where's HIS data, and do you trust the method and source? Not saying he's wrong, just has other motives (profit).

Thanks,

Jeff Anderson
Livonia, MI

[smartkid222]

alright thanks.
BTW i didnt' trust the owner but i wasn't paying for it so it didn't hurt to get it.

Now my next question is kinda stupid/simple but there's reason behind it.
It is advised that during testing the rubber motor should be wound OFF the plane. This is because it could break and potentially damage the plane. When winding off the plane one side of the rubber is attached to the winder. What is the other side attached to?

[blue cobra]

alright thanks.
BTW i didnt' trust the owner but i wasn't paying for it so it didn't hurt to get it.

Now my next question is kinda stupid/simple but there's reason behind it.
It is advised that during testing the rubber motor should be wound OFF the plane. This is because it could break and potentially damage the plane. When winding off the plane one side of the rubber is attached to the winder. What is the other side attached to?

Preferably a torque meter, but if not you can make a hook out of some stiff wire and attach it to something like a 3-inch C clamp and clamp that onto a table or the bleachers.

I have a feeling I might be winding wrong. I can only really get 90 winds into .090 TSS. Maybe I'm winding too quickly or moving in too quickly.

Also, with regards to dewinds, how many are typical? And if I had to dewind, say, 5 times, and I wanted 90 winds, would I have to wind 95 times, then dewind 5?