Rubber

[blue cobra]

I suspect that was a joke.

But it's strange- I just got a 45:3 winder, but the rubber takes just as many winds as with my 15:1.

[StampingKid]

If it was meant as a joke a true nerd would have related to pi.

[Taran]

I just ordered a 60:4 winder for my airplane.

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Wright Stuff
Meteorology
Can't Judge A Powder

[StampingKid]

Looking at my data, it seems that with all other things being equal that I am getting my better times on the second wind and second best on just abusing the the rubber on the fifth and sixth when you pack way too many winds in and don't detorque at all. I know that the latter is not a good competition practice as is not consistent but is there a reason why I am getting better times on that second wind? Am I not stretching the rubber out enough on that first wind?

[jander14indoor]

Not sure I can give you a WHY with any authority, but your experience is consistent with the history of using rubber to power airplanes.

The recommended practice is to 'break-in' rubber before using it in competition. Details of recommendation vary, but all involve signficant (80 or 90 percent of breaking) stretch of the rubber, holding for a short period of time, relax rubber, let it rest for a time and inspect for flaws. Multiple flyers have done this and collected data, showing significant improvement in terms of energy storage and return (which results in longer flights).

Your first wind is serving as the 'break-in' for your motors. They then perform fairly well for a while, depending on how hard you wind. If you wind to only 70 or 80% of max, it may last MANY flights. If you wind to 95% plus (and you should for competitions), don't expect more than a few good flights from a motor.

Now, while I can't say with authority why this is so, I can propose a theory. Rubber as manufactured has a fairly random structure to the long chains. When you stretch it HARD I suspect the long chains line up in some fashion, changing structure permanently. In the process they absorb a lot of the energy input by that first stretch and don't return it when relaxed. On the next wind (or stretch) the chains are already oriented, little furthur change occurs, and most of the energy input is returned to power the plane. I'm fairly confident that's what's happening mechanically (my degree in metallurgy isn't totally irrelevant to rubber), but can't explain any better on a micro level.

Hope that all makes sense.

Jeff Anderson
Livonia, MI

[StampingKid]

Sort of like tempering steel to just below its critical temperature to improve its ductility. And if not, I understand that there is definitely a correlation between the stretch and optimizing the energy output of the rubber.

[Littleboy]

Does anyone use a π:1 winder? I googled it and on one of the pages they had one.

[jander14indoor]

Sort of like tempering steel to just below its critical temperature to improve its ductility. And if not, I understand that there is definitely a correlation between the stretch and optimizing the energy output of the rubber.

Actually I suspect the rubber as it comes from the manufacturer is more like the tempered condition for steel. I was actually thinking of a different model, cold rolling steel to increase its tensile strength while its brittleness also goes up. Problem is, I strongly suspect at a molecular level the mechanisms aren't the same, even if similar, so I can't touch on the how question. I know how steel cold hardens, I don't know how rubber does it. Wrong courses of study. But the macro affect is certainly verified by many sources.

Jeff Anderson
Livonia, MI

[StampingKid]

How can you adequately estimate what is the usable portion of the power curve? Generally, I think I can launch without torque problems at just below .4lbs/ft. And it looks like I can still ascend or cruise at .2 lbs/ft. The latter though is an estimate. Is there a more definitive way?

[jander14indoor]

Have you plotted the torque curves of your rubber? Both wind and unwind? You can use that with the data you've gathered about how much torque you need for level flight to start estimating flight times by plotting against the curve. Can also start estimating the effects of backoff, different widths against ceiling height, etc. Note, the area under the unwind curve is the energy you fly with. Once you know how much energy you need to get to an altitude, you know how much you have left for the rest of the flight. You can start playing games with where you launch at, etc.

You can also start looking for improvements in your plane trim. Example, a plane that cruises level at .2 is better than one that cruises level at .3. Don't need a whole flight to look for improved cruise trim.

And so on. Here's where keeping data is critical, and where the consistent winners really work hard.

Jeff Anderson
Livonia, MI