Helicopters B

[Bazinga+]

I was just wondering what do people think is the best rubber length? should it be longer than the body, and if so, by how much? Because i know that there used to be a limit on rubber motor weight, so does that mean it is advantageous to use longer rubber? Also, is it true that the optimal angle of attack is 15 degrees? Because its been working quite well and theres also this graph;
And there are many more which say the same thing.

[jander14indoor]

Weeeellll...
Not meant to be a deterrent, but definitely a challenge. Before attempting you should be sure to achieve a high degree of success with conventional designs.

But frankly, that's been true for each of the bonuses we've devised for the SO flying events. You have so much to learn overall that its best to master the 'conventional' solutions before trying out of the box stuff.

Note, single bladed propellors are fairly common in some model aviation events due to their advantages. So they can work.

Unless chalker7 has been busy and not told me, neither he, the other helicopter rules writers nor I can give you optimum design parameters to maximize performance in a helicopter role. Nor whether the bonus in size or time is enough to make the trip worthwhile. We think it is, but ultimately YOU will demonstrate whether it was or not.

But that's no different than two years ago when there was a bonus for Chinook configurations. We knew it was theoretically possible, weren't sure how to do it as a practical matter, weren't sure if the bonus made it worth it. There might have been a side bet or two about how it would turn out. By the national tournament it was VERY clear the students had solved the practical problems and clearly demonstrated it was doable and worth it.

Jeff Anderson
Livonia, MI

[jander14indoor]

Again, optimum rubber length has NOTHING to do with motor stick length. If you land with no turns, your motor is too short or you didn't wind hard enough. If you land with a lot of turns your motor is too long or could be fatter, probably too long.

Yes there was a motor weight limit, it was used to limit flight time for Wright Stuff as the rules allowed flights too long to contain in the competition time you have at an SO tournament. We felt it was better to have light, elegant, slow flying planes and limit motor size than to increase the mass of the planes, meaning they had to fly fast and crash hard. Turned out reasonable rules for helicopters gave us reasonable flight times without limiting motors, so we dropped it. And it added an additional variable as helicopters seem to have less variables than Wright Stuff for you to manage.

So, longer, really heavier, motors are good, to a point. At some point they add to much mass and degrade time.

Carefule, angle of the blade to the body (I assume that is what you are talking about with the 15 degrees) has little to do with angle of attack. Angle of attack is ALWAYS angle of lift surface to the air stream. When your rotor is turning, it is moving the air downwards so that the physical angle of the rotor blade is not the same as the angle of attack. The only way to figure out true angle of attack is to measure air stream velocity, rotor rpm, and then do some fancy trig work.

Also, the angle of attack will change across the radius since the air stream is fairly uniform across the radius (if you designed your rotor right) but the speed of the rotor blade through that air stream changes as the radius changes.

Note, that's not to say there is anything wrong with your design, just to correct use of technical terms.

Regards,
Jeff Anderson
Livonia, MI

[chalker7]

I assure you this is not the reason the single-bladed bonus was implemented. In general, bonuses are offered as an extra challenge for teams who feel they have already mastered the "original" set of rules.

And nope, Jeff, I have not been busy and can't offer any advice. Theoretically, single-bladed rotors should be more efficient since the blades are not going through disturbed air from the other blade. That said, the obvious challenge is balancing the rotors...I'm really looking forward to some awesome solutions just like we saw at UCF in 2012!

I gave up on single bladed rotors. We spent many weeks until we collectively dismissed it as a deterrent, meant to slow people down from making their helicopters.

[chalker7]

And nope, Jeff, I have not been busy and can't offer any advice.

Just a quick note, I meant to say I haven't been busy with optimizing single-bladed Science Olympiad helicopters. I've been absurdly busy in my other lives, otherwise this would be a very fun project with a high priority.

[helicpters_rule]

just letting people know that single bladed is posable by accident as my partner released the helicopter he hit off on of the bottom blades on a 2 bladed bottom and it flew for 1:25

[1nxtmonster]

I built a homemade that gets a time of 0:50. It has about a 11 inch long main body, and a 23 cm rubber band wound to about 750 turns. Would there be any sense in doing a significantly longer band (e.g. 35 cm) and winding it to 1200 winds? Or would I just add unnecesary weight? i don't have much rubber left to waste, so I figured i'd ask first...

[jander14indoor]

Well I hope EVERYBODY'S helicopter is home made. If you mean scratchbuilt vs kit, more power to you.

Need more information to answer accurately, but here's some questions on what we'd need and generic recommendations that depend on your answers..

What kind of rubber are you using, what's the thickness?
What's the weight of your helicopter without rubber? If its not very close to 3 gm you might gain time faster there than messing with the rubber.
How close to breaking is your motor at 750 turns? This sounds low unless its a very fat motor. Hm, though, 23 cm, 9 inch loop, yeah sounds kind of short. I know students worry about breaking motors, but if you aren't breaking them occasionally you are flying on less than a full tank of gas. Turns are fuel! If you are so short you are worried about it try to get more rubber. Hard. It takes lots of experimentaiton

Assuming you are doing all the above right (min weight, max turns), how many turns does your helicopter have when it lands?
If lots of turns left on landing, you can shorten the motor slowly and gain some time because you carry less weight.
If few turns left on landing (if you have a 3.0gm heli I'm guessing not that motor sounds short), you can safely lengthen the motor. I was going to say go larger in small steps, but if you are limited on rubber, you might want to jump a long ways. That way if you have too much you can shorten.

Hope that helps,

Jeff Anderson
Livonia, MI

[Bazinga+]

I recently got some 3/32 inch rubber (.1 inch or .25 cm), and is there any way this could work better than 1.8 (.125) rubber? its tan super sport in case ur wondering. Our helicopter seems to be generating jusat enough lift to fly with 1.25....

[1nxtmonster]

Well I hope EVERYBODY'S helicopter is home made. If you mean scratchbuilt vs kit, more power to you.

Need more information to answer accurately, but here's some questions on what we'd need and generic recommendations that depend on your answers..

What kind of rubber are you using, what's the thickness?
What's the weight of your helicopter without rubber? If its not very close to 3 gm you might gain time faster there than messing with the rubber.
How close to breaking is your motor at 750 turns? This sounds low unless its a very fat motor. Hm, though, 23 cm, 9 inch loop, yeah sounds kind of short. I know students worry about breaking motors, but if you aren't breaking them occasionally you are flying on less than a full tank of gas. Turns are fuel! If you are so short you are worried about it try to get more rubber. Hard. It takes lots of experimentaiton

Assuming you are doing all the above right (min weight, max turns), how many turns does your helicopter have when it lands?
If lots of turns left on landing, you can shorten the motor slowly and gain some time because you carry less weight.
If few turns left on landing (if you have a 3.0gm heli I'm guessing not that motor sounds short), you can safely lengthen the motor. I was going to say go larger in small steps, but if you are limited on rubber, you might want to jump a long ways. That way if you have too much you can shorten.

Hope that helps,

Jeff Anderson
Livonia, MI

Some clarifications:

By homemade, yes, I mean not from a kit. Rubber is 3/32 peck polymer rubber band lubed with rubber band lube. Copter weighs in at 3.2 grams. It will break at just past 750 (e.g. 800) consistently. At regionals I broke a rubber band trying to get it up to 790 winds. Helicopter has about one-third to one-half turns left when it lands. So I should try making longer and shorter bands? Is any brand of rubber band better than another? Or is tower-hobbies' peck polymer brand good enough?