Helicopter B/C [Trial]

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Re: Helicopter B/C [Trial]

Postby SOCoach » April 19th, 2010, 7:13 pm

Oops . . . sorry Jeff, should have read the post above mine . . . so flat blades with an approximate 20 degree tilt.

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Re: Helicopter B/C [Trial]

Postby jander14indoor » April 20th, 2010, 3:33 am

Do the wings on the helicopter need to have airfoils like the wing of a wright stuff plane, or simply flat with a twist? I guess I am not sure how it generates its lift . . . . simply by moving the air?
No problem, and actually a good question.
Ideally, the blades (or wings) of your rotors should be airfoil shaped and have a twist as they do create lift, just like a wing. Problem is, they aren't operating in a uniform velocity field like a wing, even if the air is flowing as a body uniformly past or through the rotor. The tips are moving much faster through that field than the hub so need different angles to have an appropriate angle of attack.
But, just like Wright Stuff, they will work even if flat, if angled properly. And even if not twisted. Just not as efficiently. The poster I was answering with the flat blade suggestion wanted a quick and dirty solution to a working copter in four days. Flat is quick, dirty and inefficient aerodynamically, but quick. And it will fly better than you expect. I was getting 30 seconds with last years trial rules and flat blades. No where near the capability of these things, but it flew. Thirty seconds would have one a lot of regional trials I saw this year. Probably not so much next or the year after when the students get things figured out.

Oh, side comment. ALL wings generate lift by moving the air. For the plane to go up, it HAS to push the air down, equal and opposite reactions and all. The shape of the airfoil just helps some do it more efficiently than the others. The trick is to keep the wing (or rotor) moving in a stable, consistent fashion.

Thanks,

Jeff Anderson
Livonia, MI

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Re: Helicopter B/C [Trial]

Postby jander14indoor » April 20th, 2010, 5:18 pm

I love the internet. For all those who've been asking for some idea of what a good helicopter to these rules looks and flies like here's one from this year on youtube: http://www.youtube.com/watch?v=5RwygrzF ... re=related

As you can see, it does fly. Needed more power/winds, don't know if the pitch was right on the rotors, or what the overall weight was, but it LOOKS like what at lease one successful design looks like. Probably capable of MUCH longer flight times if tweaked correctly.

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Re: Helicopter B/C [Trial]

Postby brobo » April 20th, 2010, 6:55 pm

Crunchtime! I'm really worried right now about the design of my helicopter... most of my doubt is probably just nerves, but can anyone give me some pointers as to if my design is good? I'll do my best to describe it.

Not including the propeller/rotter, our helicopter is about 15 cm tall. It is a triangular prism in shape, with each side of the triangle measuring 7 cm. I built some supports in the shape of X's on each side to stabilize it and keep it from breaking. We have a piece of paper covering the upper end, and that is where our rotter is. It has two blades, each 15 cm long, completely flat, and tilted to about 20 degrees. I'm not sure what my partner is planning, but we will attach the rubber band to the bottom of the helicopter, and the other end to the bottom of the rotter, and spin the blades to wind it up. All together, our helicopter weighs about 8 grams (a little heavy, I know).

Any thoughts? Too heavy? More blades? Any input is greatly appreciated. Thanks!
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Re: Helicopter B/C [Trial]

Postby smartkid222 » April 20th, 2010, 7:18 pm

WTH! Thats my partner at regionals! i dont remember anyone taking video. Gah. oh well lol

I would say that it needs to be lighter. What materials is everything made out of? do you mean actual lined paper?
but more importantly i guess... does it fly?
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Re: Helicopter B/C [Trial]

Postby brobo » April 20th, 2010, 7:38 pm

Well, thats the problem... we haven't tested it yet. Its made out of just small light wooded sticks... balsa wood I think. The paper is pretty heavy, but there isn't that much- less than a quarter of a square inch, I would guess in total...
thanks, I will look and see if I can make it lighter :)
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Re: Helicopter B/C [Trial]

Postby jander14indoor » April 21st, 2010, 5:39 am

<SNIP>
Not including the propeller/rotter, our helicopter is about 15 cm tall. It is a triangular prism in shape, with each side of the triangle measuring 7 cm. I built some supports in the shape of X's on each side to stabilize it and keep it from breaking. We have a piece of paper covering the upper end, and that is where our rotter is. It has two blades, each 15 cm long, completely flat, and tilted to about 20 degrees. I'm not sure what my partner is planning, but we will attach the rubber band to the bottom of the helicopter, and the other end to the bottom of the rotter, and spin the blades to wind it up. All together, our helicopter weighs about 8 grams (a little heavy, I know).

Any thoughts? Too heavy? More blades? Any input is greatly appreciated. Thanks!
Yeah!! description and specific questions. That we can help.

First, LOSE WEIGHT!!! Weight is your enemy, it kills flight time.
Second, your structure sounds much too complicated and overbuilt. All you really need is a stick. What do you think all that sturcture is doing, besides adding weight? And why the paper on top? It could be interpreted as a horizontal lifting surface and get you second tiered.
Third, what's going to react your rotor torque? While that rubber band is trying to spin the rotor, it is equally trying to spin the sturcture it is attached to. Which do you think will get all the spin? The rotor fighting the air to create lift, or the body with little resistance to spinning? You need either a VERTICAL flat surface to reduce the body from spinning, or attach a second rotor to the body to spin the opposite of the top rotor and add to the lift. Make sure all the motor spin effort goes into SOME rotor for maximum lift.

Suggestion, read back up this chain a page or two. You'll find pointers to several designs that will work and illustrate some of the points made above. You'll also find a youtube video of a working helicopter to these rules to show you ONE solution. There are others.

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Re: Helicopter B/C [Trial]

Postby SOCoach » April 21st, 2010, 10:45 am

My kids are going to start working on copter designs today and start building Thursday. I have stressed the importance of keeping the weight down. Along those lines . . . what seems to be more efficient . . a second rotor at the bottom of the motor stick or flat surface perpendicular to the top rotor to prevent spin. It seems the first rotor idea would be hard to keep weight down . . . . what have others had success with?

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Re: Helicopter B/C [Trial]

Postby jander14indoor » April 21st, 2010, 4:00 pm

Either design can be made to work at the target weight. See my discussion on bill of material importance to building to weight in the Wright Stuff thread.

As to efficiency, have them consider this. The Wright Bat style will fly, but, is the lower surface contributing any lift? Is it doing ANYTHING aerodynamically (think drag)? Is drag that isn't the result of doing any useful work EVER good?

Now, consider the two rotor design. Is the second rotor doing anything useful? If so, what about drag again?

What's the better balance? I know what I think, but I want you out there to think about it. Once you understand this, and HOW YOU ARRIVED AT THE ANSWER, you'll be a long way on the path to understanding engineering design!

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Re: Helicopter B/C [Trial]

Postby SOCoach » April 21st, 2010, 5:23 pm

Thanks for the tips Jeff, I think I have enough to get the kids started.


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