Helicopters B

[retired1]

Because of decreasing torque, with current methods, it is impossible to have the chopper go to just short of the ceiling and hang there for an extended period of time. It might go up slowly and come down slowly, but it will not just hang there for any extended period of time.
The following is my opinion based on some experience but a lot less than jander's. Start with a 12" stick. Have the rubber just taunt And then vary the width and record the times. One way to work the length of the rubber is to start with a slightly loose one and then tie another knot and remove the first knot. It is now tighter and weighs less.
Use lubricant! after each flight or no more than two flights. Pre stretch your bands , possibly the day before. The band will get progressively weaker with each use. Letting it set for a day will recover most of its power. Keep a log on length, width and weight of each one.
Max winds on a 12" boom will require a heavy boom or a truss. I think that the truss is the way to go. Just hope that the band does not break and tangle in the truss.
A winder is mandatory! I prefer the 10 to 1 ratio as it makes knowing band turns a lot easier than 15:1 5:1 is too slow.

The number of turns that a band will take before breaking depends on the length and then the width. Last year, we had several premature breaks. On checking the bands with a slight stretch, we found that some of the sides were scalloped, normally within a couple of inches of the knot. Never figured out why.

Build a stock chopper and fly it several times before thinking about changes. Loading the band is a lot more difficult than it looks. It takes coorination of both of you.

[jander14indoor]

Stop worrying about motor stick length, there is NO reason to match it to rubber length!!!! It's very common for top level indoor competitors to fly very successfully with motors 2 to 4 times the length between the hooks. It just dosn't matter.

Reason, as soon as you wind your motor a few turns it will shorten drastically and be in tension on any motor stick. And these are the weakest turns contributing least to length of flight.

About the only thing you don't want, and even this isn't very important, is to have a lot of stretch on the motor before winding. There you've already stored energy into the rubber that you can't get back, no matter what. And any given band only stores so much energy.

About the only thing the motor stick length affects is stability. Too short and the copter tends to wobble more. But any reasonable length works, unlike Wright Stuff or Gliders where fuselage length plays a key role in balance and stability.

So, just pick a length build your copter and start testing.

Oh, an alternative to the test plan above. Instead of motor length you can use motor mass. This will be slightly more independant than length, but either can work.

Hmm, you ask how to vary the motor length. You are using competition rubber like TAN Sport or TAN Super Sport, right? The stuff that comes in continuous strips that you tie in a loop? You just tie the loop however long you need it and cut off the excess tails from the knot.

If you are using office rubber bands, you are WAY behind the curve, they simply do not store enough energy for this event.

Quick comment on breaking your bands. If you intend to win you will break a lot of motors. Get enough rubber to not be worried about winding and breaking. Turns are fuel in the tank, if you aren't winding to close to breaking, your are entering an endurance race with half the fuel in your tank.

Another comment on breaking motors. If you wind to torque you will find that a given width WILL break consistently at a fairly consistent torque. Turns are a very poor predictor of breaking by comparison.

Finally, for now, look back on this string, or last years helicopter discussion for discussion on the importance of winding to torque instead of turns and about the importance of taking data on your motors so you understand how they store and release energy.

Jeff Anderson
Livonia, MI

[chalker7]

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About the only thing you don't want, and even this isn't very important, is to have a lot of stretch on the motor before winding. There you've already stored energy into the rubber that you can't get back, no matter what. And any given band only stores so much energy.

Jeff is, as usual, entirely correct with his above post. Just a clarification on the section I'm quoting above. What's being said is you don't want the rubber band to be stretched on your motorstick without any winds. That is to say, you don't want a motorstick that is longer than any rubber band you might use (also, longer motorsticks weigh more and are less stiff...)
You do, however, want to stretch your motor out quite far before starting to wind. The best winding technique is to stretch your motor out about 3-4 times its resting length and then start winding the motor. Put approximately half of the max winds on the rubber band while it's stretched out like that, then start slowly walking in. You want to put your last turn on your motor when you reach the hook to hook length of your motorstick. This method lets you get a couple more turns on your motor and evenly distributes knots along the length of the motor while helping to avoid some of the notching you will inevitably end up with after every use (provided you are correctly using lube.)

[JimBob69]

Hey guys,
Just a quick question regarding design specs...what density of balsa would you guys recommend? Considering that I don't want it to bend whie the rotor spins although it should still be bendable enough to actually build an efficient rotor.
Thanks,
Jim

P.S. I'm considering using/buying the Freedom Flight kit and then tweak it based on what I can conclude from testing but for now I'm simply constructing one for myself.

[jander14indoor]

Use whatever density it takes to hit the minimum weight! Seriously. Except the motor stick, bending isn't much of a problem in this event. Not "no problem" just not much.

To figure it out, make a bill of materials for your design. List each part, its dimensions. Figure out the volume of wood, metal,covering, glue etc. Guess at a density, figure out total weight. If more than minimum, use lower density balsa/smaller pieces. Iterate until your BOM is right at or just below the minimum. Build, measure and adjust.

From experience, but must be adusted for your design, 7 lb/ft3 for the spars, 4-5 for ribs, maybe 6-7 for the motor stick, but it has to be a STIFF piece of wood.

Jeff Anderson
Livonia, MI

[2017Kortman]

Hey, could someone tell me what rotor pitch produces the best flight time? Thx

[mrsteven]

Hey, could someone tell me what rotor pitch produces the best flight time? Thx

There isnt one, there is a relationship between the pitch and width of rubber used. You want to match the rubber to the pitch, by making a helicopter and testing a variety of rubber strips with it to find the optimal width.
Or likewise, you could start using a set width and rebuild blades of different pitches to match, if you know how you can also make varible pitch rotors to test.

Also a huge dependant situation is your ability to build accurately and consistently. Or even rotor design (helliptical vs elliptial) will have a large change in performance

[2017Kortman]

Thanks!

[2017Kortman]

Okay, my coach gave us some homework, and I need to know how rotor pitch affects flight time. Any replies would be greatly appreciated.

P.s., I can't seem to upload a profile picture. It always gives me a message saying that it's to big or something. Any advice?

[2017Kortman]

Figured out the picture! (still need answer for my other question)