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.
Jeff Anderson
Livonia, MI