Helicopter B/C [Trial]

[smartkid222]

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/HE ... TION10.pdf

[jander14indoor]

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.

Jeff Anderson
Livonia, MI

[cypressfalls Robert]

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?

[andrewwski]

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.

[cypressfalls Robert]

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

yes thanks for the clarification

[shaneisgreenland]

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.

[jander14indoor]

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

[baker]

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

[cypressfalls Robert]

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.

[jander14indoor]

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.

Jeff Anderson
Livonia, MI