Helicopter B/C [Trial]

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illusionist
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Re: Helicopter B/C [Trial]

Postby illusionist » July 24th, 2010, 8:40 am

Alright, thanks guys
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Draylon Fogg
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Re: Helicopter B/C [Trial]

Postby Draylon Fogg » July 27th, 2010, 11:44 pm

what is the best way to make a rotor??
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Re: Helicopter B/C [Trial]

Postby jander14indoor » July 30th, 2010, 6:06 pm

'Best' is of course a very subjective term.

Here's one method that's fairly conventional among the indoor crowd. I'm going to give this in words, if I get the right energy some day I'll post pictures. Or links to existing examples.

The conventional solution is a dual rotor copter, both rotors mounted coaxially and counter rotating. One is attached firmly to the motor stick, the other rotates free on a wire shaft. In flight, BOTH turn in opposite direction as one end of the motor torques on the 'free' rotor, and the other end torques on the 'fixed' rotor attached to the motor shaft.

Lets start with the 'fixed' rotor.
• We'll start with the axis of the rotor which is the motor stick itself. It should be about 12 inches long and weigh about 1.5 gm.
• Select two spar sticks for the rotor. These are long straight stiff pieces of wood. Just under 40 cm long, probably 1/16 by 1/16 and weighing 0.3 to 0.4 gm. These pieces are key, select them carefully.
• Now, glue the first spar to the rotor, oh say 2 inches down from what will be the top of the motor stick. Glue it so it is centered on the motor stick, opposite side from the rubber band. Make sure it is SQUARE to the motor stick. A jig is handy here. Don't try to do this free hand. You won't be accurate enough.
• Now, measure up the motor stick, say 1 1/4 inches. You are going to attach the second spar here, again centered on the spar.
o BUT, you have to rotate this spar around the axis so the tips of the spars are separated about say 4 inches when looking down along the length of the motor stick. A little trig gives me about 28 degrees of angle rotation. And that's clockwise looking from the top of the stick, by convention. It only matters because the free rotor you build next MUST rotate the opposite way. As long as you wind correct, either can work.
o Now, to attach the spar at an angle with a good glue joint, you'll need to sand a shallow angled notch into the motor stick on the same side as the first spar at the marked location. The angle is of course 28 degrees in the direction you want. Glue the second spar into this notch, again SQUARE to the motor stick, but angled to the first spar when looking from the top. A jig of some kind is handy.
• The next step is to put some ribs between the spars to define a twisted surface to put tissue or mylar covering on. Measure and mark along each spar 5, 10, and 15 cm from the motor stick. Using light balsa, cut and fit a rib between the spars at each of these location and at the spar tips. Make sure you don't cause the spars to bend!! These ribs can be straight, or have a slight curve like a wing. This is finicky hand work, cut, fit and reject if not perfect. Once you are sure the rib is right, glue it in place. Of course a jig is handy, be creative.
• Now, cover the rotor from the 5 cm rib out to the tip. The inner 5 cm theoretically adds thrust, but as a practical matter, most of the work is done at the tip and the inner area just adds drag and weight. Covering is just like covering a Wright Stuff wing, except the surface is curved in three dimensions. It can be done in one piece, but treating each section on its own might work better.
• Voila' you have a rotor.

The 'free' rotor is almost exactly the same except for the axis. For the free rotor the axis is the shaft (think prop shaft wire on a Wright Stuff plane). The spars are selected the same and are just as critical here. You glue them to a very long prop shaft (rubber hook at one end, locking hook at the other). Same spacing, same rotation difference, but in the OPPOSITE direction as the fixed rotor. It’s a little tough gluing to the wire securely, so you may want to cut two 1/16 square spacers about 1 ¼ inches long to slab alongside the wire and glue between the two spars. You also want to glue the locking hook to the top spar VERY securely. Everything else is the same.

Hope that helps there are other methods, but I've already taken a lot of space.

Note, the dimensions I gave are approximate!! They should give you a good flying copter if you keep the overall weight to 4.0 gm MAX. The ideal is for YOU to determine. But don't be too surprised if you find the best thrust is with the lower rotor at a slight higher pitch then the upper! And maybe a three or four bladed rotor is better. Only testing will tell.

Jeff Anderson
Livonia, MI

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Re: Helicopter B/C [Trial]

Postby Draylon Fogg » August 6th, 2010, 6:01 pm

ok thank you, i cant wait till school starts so i can get building :D
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Re: Helicopter B/C [Trial]

Postby calgoddard » August 11th, 2010, 4:47 pm

Jeff:

I wanted to thank you for your very informatve post above.

I am looking forward to this event next year. While we will not have Wright Stuff, this new helicopter event looks challenging and exciting as a replacement flying event.

There is little available in terms of published indoor rubber powered helicopter designs intended for max duration. I like this because it provides an incentive for students to design and test next year.

Running Helicopter Duration as a trial event for a couple of years has proven that the basic rules used in those years are viable and will result in a nice competition. I hope that the 2011 rules are similar. Past experience indicates there will be minimal changes, if any. I understand that the Helicopter Duration trial event went very smoothly last year where it was held.

We will get the final rules for Helicopter Duration in September. In the meantime, I plan to build and fly a helicopter that meets last year's trial rules and I am confident that the knowledge gained will be invaluable even if the rules are tweaked a bit for 2011.

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Re: Helicopter B/C [Trial]

Postby jander14indoor » August 13th, 2010, 7:07 am

Changes should in fact be small. Mostly tweaks to address the minor issues at the trials or hypothesized problems in interpretation. Overall helicopter duration ran well at the regional, state and national trials last year.

As far as getting a jump on next year, last years national trial rules are a VERY good place to get started.

Jeff Anderson
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Re: Helicopter B/C [Trial]

Postby illusionist » August 25th, 2010, 7:58 am

What are the advantages of having a longer/shorter motorstick (greate/less distance between the two rotors)? Stability?
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Re: Helicopter B/C [Trial]

Postby illusionist » August 25th, 2010, 8:15 am

Also, In a dual counter-rotating rotor design, do the two rotors have to be aligned directly on top of each other, or is it okay if the top one is further away from the center of the motorstick than the bottom fixed one?
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Re: Helicopter B/C [Trial]

Postby robotman » August 25th, 2010, 4:09 pm

Also, In a dual counter-rotating rotor design, do the two rotors have to be aligned directly on top of each other, or is it okay if the top one is further away from the center of the motorstick than the bottom fixed one?
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Re: Helicopter B/C [Trial]

Postby paleonaps » August 25th, 2010, 4:12 pm

Yeah plus this thread will probably be locked soon.
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