Helicopters B

[Olympian_Cau2134]

Does anybody know where you can get the rubber bands by the foot for the helicopters

[jander14indoor]

www.faimodelsupply.com is the source for competitive rubber, you can buy it there direct by the 1/4, 1/2, 1, 5 & 10 lb quantities, or by 16 or 32 ft lengths.

Frankly, I would suggest at least 1/4 lb of 1/8 inch unless you already know you need another size.

To be successful with this event you need to fly lots and gather much data. One of the big problems I see every year, is competitors afraid to wind their motors and launching with a half full tank of gas. To be successful you MUST wind these motors to near breaking, meaning motors will only last a few flights. Limiting yourself on rubber is not smart.

Also, try www.f1d.biz which resells FAI rubber.

Jeff Anderson
Livonia, MI

[Olympian_Cau2134]

What size piano wire should you use for the shaft

[jander14indoor]

Commonly available sizes at your local hobby shop.
0.015 inch diameter will bend with most helicopter motors.
0.020 inch diameter works, probably about as small as you can go. MIGHT be a little small for really fat motors/inexperienced teams.
0.025 inch diameter will solve any problems with 0.020 but IS heavier.
0.032 inch diameter is overkill.

my experience.

Jeff Anderson
Livonia, MI

[chalker7]

I generally prefer .020. Thicker wire is pretty hard to bend since it is so strong, resulting in imprecise shapes and potentially unbalanced shafts.

Commonly available sizes at your local hobby shop.
0.015 inch diameter will bend with most helicopter motors.
0.020 inch diameter works, probably about as small as you can go. MIGHT be a little small for really fat motors/inexperienced teams.
0.025 inch diameter will solve any problems with 0.020 but IS heavier.
0.032 inch diameter is overkill.

my experience.

Jeff Anderson
Livonia, MI

[baker]

I generally prefer .020. Thicker wire is pretty hard to bend since it is so strong, resulting in imprecise shapes and potentially unbalanced shafts.

Commonly available sizes at your local hobby shop.
0.015 inch diameter will bend with most helicopter motors.
0.020 inch diameter works, probably about as small as you can go. MIGHT be a little small for really fat motors/inexperienced teams.
0.025 inch diameter will solve any problems with 0.020 but IS heavier.
0.032 inch diameter is overkill.

my experience.

Jeff Anderson
Livonia, MI

Agreed.. .020 bends easy with pliers, cuts easy with side cutters and keeps it's shape very well even with a ton of knots on a motor. You'll break the motor stick before deforming the shaft hook.

[jit123]

Are we allowed to help give the Helicopter a boost?

[jander14indoor]

If you build it right and get the rubber and rotor match right, you won't need to or want to.
Properly built, a helicopter will fly out of your hand at ground level and fly to the ceiling in almost any site available, including the fieldhouse at Univ of Illinois with its 90 ft plus ceiling.
Boosting it is more likely to break it.

Jeff Anderson
Livonia, MI

[aznman908]

I have done helicopters in the past, but i have made a rotor on top and bottom. They were traditionally only two blades for each rotor. I was thinking of building a rotor with four blades on top and two on the bottom. I have seen first and second place team do this design. The problem is that my old partner said that using four blades is not as efficient. I was wondering if that was true or not.

[jander14indoor]

I have done helicopters in the past, but i have made a rotor on top and bottom. They were traditionally only two blades for each rotor. I was thinking of building a rotor with four blades on top and two on the bottom. I have seen first and second place team do this design. The problem is that my old partner said that using four blades is not as efficient. I was wondering if that was true or not.

Well, this is one of those questions with a yes, but... answer.

First, a wing (and a propeller(rotor, same thing) blade is just a twisted wing flying in a very small circle) works best if the air going over it is nice and uniform.
- If you have two wings following the same path, the second is in the disturbed air of the first, so not working as well.
- Theoretically, the most efficient rotor is one with a single blade as the disturbance has moved well past by the time the blade comes around again. Now this has other challenges, thus the bonus to get you to try it this year.
- For a rotor with two blades, each is in the wake of the other, and they have half the time for the air to move past so they disturb the air for each other a little more than a single blade.
- Three blades, are even close, even more interference, loss of efficiency.
- Four blades, even closer, even more interference, even more loss of efficiency.

But, the lift off a wing, is proportional to its area. Assuming all things equal, twice the area, twice the lift. Two blades have twice the lift of one, three, three times, and so on.

So, you have two competing things going on.
- Four blades will have twice the area, and should have twice the lift.
- But, they interfere with each other more so you don't actually realize twice the lift.
- Whether its worth it depends on which increases faster, area or interference.

There are messy theoretical calculations that tell you how fast each changes, but they depend on a lot of data we typically don't have for models of this size.

There is another way to increase area and that's fatter blades. That brings its own set of efficiency tradeoffs. Long skinny wings are more efficient than short fat ones.

How do you figure this out, Build all these variations and see. Remember, keep other things equal. Same weight, same pitch, same rubber, same winds, SAME WEIGHT.

So, direct answer, a four bladed rotor is less efficient than a two bladed one. BUT, it may be worth it anyway for the increase in lifting area. Try it and see.

Jeff Anderson
Livonia, MI