Helicopter B/C [Trial]

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Re: Helicopter B/C [Trial]

Postby smartkid222 » November 2nd, 2009, 2:12 pm

For people who did this event at nationals last year what would be considered a "good" flight time?
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Re: Helicopter B/C [Trial]

Postby smartkid222 » November 13th, 2009, 7:37 pm

also for NY they have decided to use the revised rules instead of what they origionaly had.
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Re: Helicopter B/C [Trial]

Postby jander14indoor » November 14th, 2009, 4:35 pm

Helicopter wasn't flown at nationals last year. This coming will be the first.

30 seconds or so is fairly doable, I believe that in Ohio there were times above a minute with last years rules, heavier copter and smaller rotors. I'd expect 1:30 to be doable this year.

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Re: Helicopter B/C [Trial]

Postby shaneisgreenland » November 16th, 2009, 11:58 am

Alright, so I'm confused on the way to approach this. It says we can buy a kit, but would it be easier to just make it by hand and with wood? Also, I saw the earlier post on the different ways to make a helicopter, but I'm still confused about how I to go forward. I know it sounds vauge, but I would be very appreciative if I could get some ideas on what to do. Thanks!

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Re: Helicopter B/C [Trial]

Postby jander14indoor » November 16th, 2009, 1:55 pm

No problems, that's what this forum is for, questions and answers.

Kits vs plans vs own design.
- Kits are great for someone starting off as they have all the materials in one place. However, they are seldom the best possible design or materials as the compromises needed to keep prices reasonable and still make a profit necessitate short cuts. Plus, there are no purpose built kits for this event that I've seen yet. There are helicopter kits out there, not optimized for these rules. Some are wrong size or weight, some have rotor pre-made (illegal with these rules). Look for the Penni Copter, Wright Bat or Ceiling Walker. BUT, you may still be money ahead buying one, building it, and flying it if you have no experience. You get some building experience, and some flying to get an idea of the principles.
- Plans. Eventually there will be good plans available that fit these rules, not yet. With plans you can select the best materials and significantly improve your helicopter. Strength with lightness. And you'll know how to repair it, a real positive. With a good plan you can compete with the best.
- Own plans. If you know enough and have enough experience, this may give you a special edge. But without the knowledge or experience, its a path to disappointment. So start with kits or published plans. Learn what makes them work. Make small changes to see what improves them. Then expand your modifications, try those special ideas, etc.

Where to get started.
As I said above, to get started buy one of the available kits. Build it, fly it, understand how it works. At the same time research. Understand the principles, etc.
The conventional wisdom says a two rotor ceiling walker design will work well. Look for plans on the internet, or in some of the reference books. Indoor Flying Models by Lew Gitlow and Building and Flying Indoor Model Airplanes by Ron Williams both have excellent helicopter plans. They are a little hard core for this event (lighter and larger) but as a starting point give the idea of this type of copter and how to build the rotor. Pick a design, scale it to these rules, and beef it up to the rule minimum weight for strength and it should fly.

Hope that gives you a starting point, feel free to ask more questions. The more specific, the better, but ask anyway.

Jeff Anderson
Livonia, MI

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Re: Helicopter B/C [Trial]

Postby smartkid222 » November 27th, 2009, 4:34 pm

Jeff, any advice on making solid balsa props?
How do u come up with the design for the blades? Is there a program that generates the best shape for the blade (kinda like the one used to make a simplex airfoil)? Is each blade supposed to be symettrical (in addition to the blades on each rotor being symetrical to each other)? What material is used to make a templete after a design is decided upon?
Thanks in advance.

*edit (7:52pm) also, just to make you aware, NY has modified their policy and is now using the current 9-24-09 rules found here:
http://www.newyorkscioly.org/SOPages/HELICOPTERDURATION10.pdf
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Re: Helicopter B/C [Trial]

Postby jander14indoor » November 27th, 2009, 7:40 pm

Who said solid prop? Not required in the rules. Oh, and there is NO theoretical difference between a prop and a rotor, just differences in how used, and somewhat on what simplifying asusumptions work best.
That said, the method of making isn't what you are really asking I assume, but how do design a working prop. Lets go from simple to more complex.

First level prop plade uses flat plate blades. I strongly suggest you start hear and get a working helicopter. Lets think of just two blades to start. They stick out of the axis of your system at right angles. Each blade is a simple plate, matching the other symetrically. Start with them flat to the plane of the blade. Spin things and nothing happens. Angle the leading edge forward and you get thrust backwards. Angle them back and you get thrust the opposite way. Problem is, not very efficient. The blades are only working at the correct angle of attack at one diameter. But they work. My trial copter last year used just such blades and flew OK. Needed more work, and it could have flown better.

Next level is to realize that even for a hovering copter the rotor is working in a stream of moving air. Ideally they air is a uniform stream. Now think of the rotor as a screw that moves screws through the air. If you think about it a while, the different radii of the blades move at different speeds through the air and need to move at different angles to work correctly. Lets make some simple assumptions and see what happens.
Assumptions, you want your rotor to move through the air 10 inches everyturn (alternatively move the air ten inches for every turn while the rotor hovers). You have a 10 inch diameter rotor.
At the tips, the blade has to move along an angled path for no 'slippage' (more about that some other day, this is going to be LONG as it is). So think of the tip moving along the surface of a cylinder 10 inches diameter by 10 inches long in one turn. Take that surface, unroll it and lay it out flat. You have a rectangle 10 inches high by pi*d or 31.41529.... inches. The tip of the blade runs along the diagonal. Now we need some trig and geometry. The move (by pythagorean theorum) c**2 = a**2 +b**2 or c=sqrt(a**2+b**2)=sqrt(10**2+31.42**2)=32.9 inches at an angle of invtan(10/31.4)=17.6 degrees. If the blade is angled at the tip to that amount, it will ideally screw through the air 10 inches.
But what happens away from the tip, say at the 5 inch diameter point. At this point the blade still moves 10 inches forward, but along a smaller diameter cylinder. Again unroll this cylinder and the cylinder is now 10 inches high by 15.71 inches. Again moving along the diagonal we now need an angle of 57.5 degrees.
If you figure this out along the length of the blade you will find its flattest at the tip, and steepest near the center. Its a shape called a helical blade. Its more efficient than a flat blade as the prop is working equally along its length. Turns out its ridiculously easy to shape such blades. Go back to that cylinder, cut out a pie shaped wedge. Cut along diagonal edges and the surface is that helical blade. Its pitch (distance moved each rotation) vs its diameter is controlled by the width of the pie vs the thickness. You can figure it out by geometry and trig again.
Look around the web and I think you'll find equations and solutions to any given rotor desired. I know you can find it in some of the reference books on rubber powered airplanes. Better explanations.

But even that's a simplification. A prop isn't a screw, its a complicated wing. You can treat each diameter as a wing in a flow field that matches those cylinder surfaces we used for the helical blades. But to lift efficiently, they have to make an angle of attack, thus slippage. In other word, a real prop with a 10 inch pitch moves less than 10 inches if you want any useful thrust. But you can get a pretty good starting point with the helical approximation.

Even worse, for a hovering helicopter you can't really treat the flow field through the prop as uniform (while it is a reasonable assumption for an aircraft prop). The center will be very different than the edges. But treating this is at the edge of MY theoretical understanding. What it means in practice is the most efficient blades deviate from helical form slightly. But I think at that point we are getting way ayead of ourselves for a working helicopter. Its at the range of gilding the lily. IE fighting for the last few seconds of optimum flight when you may not have the basics yet.

OK, hope that helps, research the net, ask questions. Props can be complicated, but very simple ones can work for this event.

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Livonia, MI

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Re: Helicopter B/C [Trial]

Postby cypressfalls Robert » December 14th, 2009, 4:18 pm

I just learned today that my state also has helicopter, so my question is that this is basically wright stuff except the fact that you want your plane(helicopter) to go up and down?

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Re: Helicopter B/C [Trial]

Postby andrewwski » December 14th, 2009, 7:16 pm

Not at all. It's not a plane in any way, shape, or form.

If by you mean the objective is the same, then yes, it's to stay aloft the longest.
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Re: Helicopter B/C [Trial]

Postby cypressfalls Robert » December 14th, 2009, 7:56 pm

andrewwski wrote:If by you mean the objective is the same, then yes, it's to stay aloft the longest.


yes thanks for the clarification

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Re: Helicopter B/C [Trial]

Postby shaneisgreenland » January 4th, 2010, 11:45 am

For the dual rotor design, how would you go about to create one from scratch? Would you suggest ordering balsa wood and going about it from there? Also, are there any plans/designs available to build a simple one? I want to start with an easy model, and then build from there, so that seems like the simplest, but I need to know how I should start building it.

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Re: Helicopter B/C [Trial]

Postby jander14indoor » January 4th, 2010, 12:33 pm

There are plans in books, look back up this string a ways to see my recommendation there.

I'm not aware of any plans on line yet.

Here's how I started my scratch copter when playing with the rules last year.
As usual with flying things, minimize weight and maximize lifting surfaces. The rotors are your lifting surface, so like planes built to max span, you'll want your rotors to max diameter.
Next, keep things simple to start.
Start with two bladed rotors, keep the max blade chord 'reasonable' (say 10% of diameter).
I made a conventional dual rotor design. One rotor locked to the motor stick, the other turning free in a bearing. When flown free the 'fixed' rotor and motor stick spins one way, the 'free' rotor spins the opposite (Newton's law of equal and opposite reactions works!).
Since I didn't know what pitch was needed to provide enough lift to pick up the minimum mass, I didn't worry about weight to start and made the blade pitch adjustable. I actually used flat paddle blades instead of helical pitch.
I then rigged up a test stand to test rotor lift. It had a heavy base to keep the copter from flying, and a bearing to lock onto the motor stick allowing it to spin, but preventing flight for now.
Set the whole thing on a digital scale, tare it out to zero. Wind the rubber and spin the thing up. The negative weight on the scale is the lift.
Now, play with pitch on the rotors until the lift matches min copter weight about halfway through the wind down. Note, the best pitch settings may NOT be with both rotors equal! Also time the process to get estimates of flight time. For all time the lift exceeds copter weight you are climbing, below is descending.
Now, go back and build a copter to min weight using the pitch info you gained from testing and fly it.
Once that's going well, consider different shape rotor blades. Try different width motors. Use the test stand to evaluate. Consider 3 or 4 bladed rotors.

And so on.

Hope that helps,

Jeff Anderson
Livonia, MI

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Re: Helicopter B/C [Trial]

Postby baker » January 9th, 2010, 1:30 pm

First try... 10 grams, 12 dia prop, 1/8 inch motor, flew like a rock. Oh well, back to the drawing board...

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Re: Helicopter B/C [Trial]

Postby cypressfalls Robert » January 9th, 2010, 9:03 pm

I have no idea where to start and (sadly) I don't know what a prop, motor stick, etc is so can someone define all these with a picture or something please.

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Re: Helicopter B/C [Trial]

Postby jander14indoor » January 10th, 2010, 11:10 am

baker, 10 grams is way too heavy, you need to cut that in half at least to get it to fly. Did you have one or two 12 inch diameter props? If one, how did you control the torque? What pitch(s)?

cypressfalls_Robert, a prop is the propellor or rotor which spins to create the lift in a helicopter (thrust for a plane). In a full size copter its the blades on the top of the copter which spin around, disk parallel to the ground. The motor stick is just the main structural element, usually a simple stick for these competitions, which holds the rubber motor. There's a hook on one end, and a bearing on the other to hold one of the rotors. The wound motor goes between the hook and the shaft of the rotor.

Hope that helps, if I can find more time, I'll try a drawing, but that's definitely NOT my strength.

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