Building techniques

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eta150
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Re: Building techniques

Post by eta150 » February 28th, 2011, 5:44 pm

illusionist wrote:Should the lower rotor be set at a higher pitch than the upper rotor? I would think so, since it needs to move the air faster, that is already in motion due to the upper rotor.
Nah, the lower rotor should probably be lower pitch because you want most of the lift to come from the top. More top lift+less bottom lift=more stable. At the same time though, you do want maximum lift.
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Re: Building techniques

Post by TreeGirl_Yesteryear3 » February 28th, 2011, 7:21 pm

I have less than a week to get my third helicopter to fly. I've tried everything I can think of but I just can't seem to get it in the air. I think the rotors are probably turning too slow, but I changed out the attachments for the free rotor and it didn't help. I tested the coptor with only the free rotor attached. After I wind it (no matter how many winds I put in) the free rotor turns relatively slowly. However, when I hold it by the rotor and let the motor stick turn by itself, it turns very quickly, leading me to believe that my bearings etc. are not the problem. Obviously the rotors are going to turn slower than a stick would since they have more surface area and more resistance, but it just doesn't make sense! The only thing I haven't tried is making my rotors with curved ribs, but I have neither the equipment nor the time to do so now.

More on my design. I have 40 cm rotors made of 1/18 balsa, two long sticks connected by spars, the tips of the rotors are 4 inches apart (now that I think about it, i could change this measurement and haven't yet... would it help... it seems i remember seeing this measurement be less than 3 inches...?) My motor stick is 12 inches long and is the typical rectangle of balsa wood (I've tried other kinds too but they didn't help). The free rotor is attached to the motor "stick" via a tubular bead stuck through the short side of my flight stick/rectangle. Then there is another round bead separating that bead from the rotor itself. It's not exactly conventional, but its similar to the design found here: http://www.youtube.com/watch?v=mJPFSg1uLAQ

I'm DESPERATE. please help

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Re: Building techniques

Post by jander14indoor » February 28th, 2011, 7:55 pm

The data I collected last year clearly showed (consistent with theory) that making the second pitch rotor higher pitch than the first increases system lift. BTW, theory says you need to increase the lower rotor pitch to keep the same lift as the upper, not more than the upper.

BUT, that was with closely spaced rotors, further apart reduces the effect. IF. both your rotors are close together at the top, its probably worth it, and wouldn't decrease stability as eta 150 speculates.

On the other hand, if your rotors are at top and bottom, not as much benefit, and you do have more risk of the stability issue mentioned.

TreeGirl_Yesteryear3 wrote:<SNIP> you can read the note, quote only included because I'm changing topics
No, doesn't sound like bearings are the problem.
OK, need more info on your copter.
What is the vertical spacing between your spars (the long sticks along the diameter, the ones between those are actually ribs)? That changes the pitch for a given end spacing, very important to match rubber size to pitch. If you have too a large vertical spacing you WILL have slow turning props unless you get really big rubber. Vertical spacing should be VERY approximately in the 1 to 1.5 inch range
How much does your copter weigh? These things are VERY sensitive to weight.
What size rubber motor are you using? Is it thick enough to turn the rotors?
How hard are you winding your rubber band? If not near to breaking, you aren't winding hard enough, probably the most common problem with otherwise flyable designs.

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Livonia, MI

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Re: Building techniques

Post by TreeGirl_Yesteryear3 » February 28th, 2011, 8:10 pm

from the inside edges of the spars, the vertical spacing is one inch. As for the weight, I unfortunately don't know at the moment. I just built this coptor over the weekend and I haven't had a chance to take it to school and weigh it. The motor stick could probably stand to be lighter, but at this point I was trying to get the rotors turning. Like I said, the rotors are 1/18 LIGHT balsa, lightest strong pieces I could find. The covering is, unfortunately, plastic bag from the grocery store that you put fruit into. I ran out of mylar on copter 2. Other than using mylar, I could also try a lighter glue. I'm using super glue currently, but it's been said that it works.... so....

As for the rubber motor. Again, I'm not sure. I was using leftover rubber from wright stuff, and the bag was labeled, so i can find out, but i didn't pay attention at the time. If I measure it, it's 2.5mm by 1.5mm. Again, i am unable to weigh it until tomorrow.
I hadn't realized that the size of rubber would have that big of an impact. I'm fairly certain that we have a good variety, but are there some guidelines so i'm not just shooting in the dark? obviously I'll have to experiment to find which works best for my coptor, but I never thought the rubber would be the difference between turning lamely in my hand and flying.

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Re: Building techniques

Post by jander14indoor » March 1st, 2011, 5:46 am

OK, vertical spacing of about an inch should work.

Super glue for the wood joints is OK if not used to excess. If you apply direct from the bottle, that is excess. One drop should just about glue all the joints in a rotor. Big difference in weight there. If you are using superglue for the coverings, STOP!! That's a VERY heavy way to go. Instead use spray glue, and only a very little of that. Sort of mist the glue lightly over the upper surface, don't spray directly.

Grocery store bag covering can work if you get the really crummy light stuff, not the strong, heavy good stuff. Best to collect samples and weigh consistent size pieces, select the lightest.

OK, said it before and I'll say it again, not building with a scale next to you is a VERY hard way to hit 4.0 gm consistently. These things are VERY weight sensitive, as little as an extra half gram dramatically affects flight. Weighing after its all built is too late to change things. Either build a scale (surprisingly easy to make a good scale to 0.01 gm accuracy) or buy a cheap electronic one on line, available for less than $30 accurate to 0.01 gm or so. Search previous discussions for recommendations.

Rubber, ahhh, here we may have something. With the rotor you 've described you should be able to turn it fast enough to fly (if the weight is low enough) though these things never really spin fast like a WS prop. Rubber is both your fuel (total mass of rubber) and horsepower/torque (cross section or width of rubber). Using more rubber means more fuel (and of course more weight to carry so there is a limit). So you need to know how much you are using. Using wider rubber means more power/torque to turn the prop faster creating more lift. If you are using old WS rubber, sound like its 3/32 inch width, that's probably too thin. Wound to the max it may not turn the rotor hard enough to fly. Try wider rubber, say 1/8 inch wide. You won't get as many turns, but you'll get enough power to fly. If you don't have any different sizes, try making up a three strand motor of what you have, its possible, though not common. Tie a loop in each end. Hook one loop on the prop, run it to the hook, back to the prop and then the other loop back on the hook. Or just double it up for 4 strands. Probably over kill, but is should fly.

Hope that helps, feel free to ask more,

Jeff Anderson
Livonia, MI

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Re: Building techniques

Post by Frogger4907 » March 1st, 2011, 7:39 am

If you are doubling it over you might consider, making your hook higher up on the motor stick because you wont be able to stretch it out as much
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Re: Building techniques

Post by smartkid222 » March 1st, 2011, 12:24 pm

jander14indoor wrote:The data I collected last year clearly showed (consistent with theory) that making the second pitch rotor higher pitch than the first increases system lift. BTW, theory says you need to increase the lower rotor pitch to keep the same lift as the upper, not more than the upper.
Jeff Anderson
Livonia, MI
Here the conventions get tricky.
I assume "second" means bottom rotor?
You said "increase the lower rotor pitch to keep the same lift as the upper." I assume that the lower rotor is fixed and the top rotor is free. Would you do the opposite (decrease the lower rotor pitch) if the orientation was reversed (lower is free and top is fixed)?
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Re: Building techniques

Post by sj » March 2nd, 2011, 4:20 pm

Hey. How are you guys storing your helicopters between competitions? Our amazing helicopter was broken some how even though it was in a good box on top of a high cabinet.
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Re: Building techniques

Post by Frogger4907 » March 3rd, 2011, 7:08 am

sj wrote:Hey. How are you guys storing your helicopters between competitions? Our amazing helicopter was broken some how even though it was in a good box on top of a high cabinet.
I have a bin with two 6 in pieces of 2x4 stacked on top of each other and bolted to the bottom. There is a slot the size of my motor stick in the top so I can insert the helicopter in it and two small bungees strap around and hold it tight in it's slot so it's hovering off the bottom of the bin... Some rice is in my bin as well to reduce humidity.
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Re: Building techniques

Post by kjhsscioly » March 15th, 2011, 5:17 am

Frogger4907 wrote: Some rice is in my bin as well to reduce humidity.
Does the rice work? we should try that... IL has hugely fluctuating humidity

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