Rubber

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Lily Essence
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Re: Rubber

Post by Lily Essence » April 20th, 2011, 10:38 pm

illusionist wrote:
smartkid222 wrote:
illusionist wrote:Okay, so I tried a really low pitch (.8 vertical spacing and 3.5 tip to tip) and with 1,000 winds on 1/8, it rises about 14 feet, and falls back. So clearly lift is the issue. From what I know, if the pitch is higher, it should get more lift. But, in order for flight with 1/8, I needed to use a pretty low pitch. When I tried 1 inch with 2.75, it is too high for the rubber to provide the sufficient amount of lift. So is my reasoning right? I need to use a low enough pitch so that the rotors will actually spin with the power of 1/8, yet provide lift. Does it all come down to finding the right pitch, or is there anything else?
Btw, I'm using 1/8 FAI Tan Super Sport rubber. My helicopter is 4.05 grams.
Thanks in advance. I know I've asked similar questions a few times before.
So you're saying you are getting really low times? How many winds were left out of 1,000?
Yeah, with the .8 pitch, I got all of 17 seconds :cry:
There are at least 700 winds remaining when it lands.
So if a lot of winds are left, it generally means that I should lower the pitch right? But I'm already at a ridiculously low pitch...
In my experience, that means that you should use thicker rubber or less of the current rubber. Your pitch is really low already...
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Re: Rubber

Post by mrsteven » April 21st, 2011, 2:51 pm

Yeah I'd change that next one you build
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Re: Rubber

Post by Lily Essence » April 24th, 2011, 3:27 pm

I was reading through some of the old Wright Stuff boards, looking for information over topics that overlapped with Helicopters.

I'm curious to know what "launching torque" and "max torque" is. I understand torque to be the tendency of a force to rotate an object about an axis (like our rotors around the rubber). What exactly is "launching torque" in regards to flight times? Is there a way to find the perfect "launching torque" for helicopter? Or does a helicopter not have one at all?

How does winding and then /unwinding/ help "control launching torque"? I don't understand the point of unwinding... Aren't you leaving off winds and power that will lead to a greater flight time?

And a side questions that still has to do with rubber: How thick isthe rubber you guys are using? I have a notion that thinner is better, but only if your heli can generate enough lift on thinner rubber...

I feel like I have stalemated at a 2 minute time and I want to see what I could improve on without having to build a whole nother helicopter.
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Re: Rubber

Post by chalker7 » April 25th, 2011, 12:49 pm

Lily Essence wrote:I was reading through some of the old Wright Stuff boards, looking for information over topics that overlapped with Helicopters.

I'm curious to know what "launching torque" and "max torque" is. I understand torque to be the tendency of a force to rotate an object about an axis (like our rotors around the rubber). What exactly is "launching torque" in regards to flight times? Is there a way to find the perfect "launching torque" for helicopter? Or does a helicopter not have one at all?

How does winding and then /unwinding/ help "control launching torque"? I don't understand the point of unwinding... Aren't you leaving off winds and power that will lead to a greater flight time?

And a side questions that still has to do with rubber: How thick isthe rubber you guys are using? I have a notion that thinner is better, but only if your heli can generate enough lift on thinner rubber...

I feel like I have stalemated at a 2 minute time and I want to see what I could improve on without having to build a whole nother helicopter.

These are wonderful questions, I'll try to answer them as best I can (and I apologize if any of this is confusing, I'll clarify if you point out difficult parts). First, a huge caveat. A lot of this terminology comes from its use in Indoor Free Flight airplanes, of which Wright Stuff was a part. One of the reasons we developed helicopters was because there isn't a whole lot of expertise out there for them. While a lot of the same techniques and practices might overlap, a few may not. Hopefully, with it being an official event a lot of you are figuring out these differences and coming up with answers.

Ok, so, the fundamental relationship here is between your rubber band, the number of turns on the rubber band and torque. As you describe, torque is essentially the rotational version of force. The more winds you put on the rubber band, the more "twisting force" or torque it has on it. You can, and should, measure the torque on your rubber motor while winding with a torque meter. There is no tool that is more valuable for increasing flight time. I cannot emphasize enough how important it is to fly with a torque meter. It doesn't have to be fancy, it can just be a simple wire that twists torsionally with a hook on the end and a pointer. You can find both torque meter plans and torque meters to purchase on various indoor freeflight sites.

As you wind, the torque increases, but it is not a linear relationship. At first, it increases fairly rapidly, then through the middle portion of the winding the torque stays more or less constant and then when you get close to breaking it increases very rapidly. During unwinding, this relationship is similar (although inverse and the torque is generally higher at an equivalent number of winds during the unwinding than the winding). If you graph the relationship with winds on the x axis and torque on the x you get something (very roughly) like this (you can do a much better job making your own with a real motor and torque meter):
/
/
______________/
/
/
/

With helicopters, torque pretty accurately converts into overall thrust. The more torque, the faster the rotors turn, the more overall lift (I'm simplifying here, but that's the general idea). In order for a helicopter to stay aloft, total lift must be equal to or greater than the overall weight. If the lift is more than the weight, the helicopter will climb, if it is equal to, the helicopter will hover and if it is less, the helicopter will descend. The problem is the margin for error is extremely narrow on helicopters. As a result, nearly all rubber powered helicopters skip this hover/cruise phase. Most teams have helicopters tuned such that the torque during the middle portion (the flat one) of their flight provides enough lift to keep the helicopter climbing (this is very different from Wright Stuff). I suspect that teams might be able to get more time if they tuned their helicopter/motor combination such that they can take advantage of the cruise/hover portion of the unwinding, but I have no hard data on that.

As for max torque/launching torque/backing off winds, there are several things to think about here. Max torque is whatever the max you wound to was, this should be very close to the breaking torque of the motor. Launching torque is whatever you launch at, for most teams this is probably equal to max torque. In helicopters, I suspect this is the proper strategy (as long as you are committed to hitting the ceiling). If you are very good and know your helicopter inside and out, you could get your helicopter to hover during the middle portion and control height by limiting your climb through backing off to a lower launching torque. This is what we mean when we discuss "no-touch" flying in free flight, which is more important (and easier) with airplanes than helicopters. You might be thinking that you should just wind to the launching torque, but actually by winding beyond it (to near breaking torque) and then taking winds off to get back to the launching you end up with more winds (due to the different shapes of the torque/winds graphs for winding and unwinding).

Next, rubber band width vs. torque and winds. Thicker motors will end up with more torque, fewer turns and steeper torque curves. Thinner motors have less torque, more winds and much flatter torque curves. More winds are definitely better, but only if you can actually use them all. You want the thinnest motor that will usefully provide torque/energy throughout its unwinding. This size will be different for every team, depending on the specific dimensions of their helicopter.

So, getting to the final point on how to increase time on your helicopters. Once you have well-built, flying helicopters (and you sound like you do if you are hitting 2 minutes), you HAVE to fly with a torque meter. Wind off of one every single flight. Take copious notes (much more than the 6 data points required in the rules), especially about winding, torque and winds left at the end. If you're landing with lots of winds, you're probably using too thin of a motor. If you're landing with no turns at all, you're almost certainly using too thick of a motor.

In general, just fly a lot and see what different parameters do to flight time. Your notes should be very informative if you are keeping track of everything and comparing flights. The best teams spend far more time flying and practicing than they do with construction. This is (in my opinion), the truly challenging part of the event.
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Re: Rubber

Post by chalker7 » April 25th, 2011, 12:50 pm

It looks like the boards don't like my ASCII art. I'll try to figure out a different solution.
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Re: Rubber

Post by sj » April 25th, 2011, 1:42 pm

chalker7 wrote: In general, just fly a lot and see what different parameters do to flight time. Your notes should be very informative if you are keeping track of everything and comparing flights. The best teams spend far more time flying and practicing than they do with construction. This is (in my opinion), the truly challenging part of the event.
I strongly agree. We spend a ton of time flying almost everyday using a torque meter which is the most useful tool we have. However we have found that winding back is not as useful if your body can handle the max torque of the rubber. I suspect this is because this event requires much more torque than wright stuff and hence every wind matters because you can only wind so many times. Our goal right now is to build a heli that is able to fly on rubber thinner than 1/8th because we have reached a point that 1/8 is almost fully efficient but we have too low efficiency on 3/32. We purchased a rubber stripper so hopefully we can make slightly thinner rubber which may increase our time more.

Also how fast are peoples helicopters descending? Ours drops at about a foot per second but this is on a 25~ foot ceiling and i suspect that the rate would be more like 3-4 feet per sec if it was falling from a higher ceiling and it un-wound all the way.
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Re: Rubber

Post by mrsteven » April 25th, 2011, 2:37 pm

So the torque meter is just so you can measure how much torque gives you the best flight and decrease/increase winds to get that torque number? So youre not really measuring the number of winds, but how much torque is on it before flying.

Like instead of saying 20 winds is the best flight (for example purposes) you say X torque is ideal?
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Re: Rubber

Post by sj » April 25th, 2011, 2:48 pm

Well no its more complex than that because different thicknesses of rubber can reach x torque at different winds. For instance say X torque is your max torque that Y torque is your minimum torque to climb. You want the rubber with the most number of winds between Y to X. For instance one thickness may not reach Y and another may not have a high number of winds between X and Y another may have too high an X where your body cannot handle it. Thus the importance of finding the perfect rubber is clear and you cannot do this without a torque meter.
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Re: Rubber

Post by mrsteven » April 25th, 2011, 2:51 pm

true true, but im saying given you have the perfect rubber, when winding youre not counting winds, but looking for the optimal torque on the meter?

I understand previous testing must first conclude the optimal rubber
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Re: Rubber

Post by sj » April 25th, 2011, 2:55 pm

Right i suppose, although in my case we look for anything past a certain torque as there is no reason not to wind to the max the rubber can take. I say this because hitting the ceiling doesn't matter in this event and there is no glide phase; it lands when it is out of winds and therefore the more winds the better unless the body of the copter is going to snap.
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