by Herrbeta
 P: 3 Hi all! All the way from Sweden comes a small question... Planning to build a simple RC helicopter which, in the first stage, only should be able to take off. My idea was to use either a small lawnmover engine, or one from a chainsaw and build the helicopter with two props, turning opposite directions. I´ve calculated a bit on this but having difficulties with how much power that is neccesary for let´s say a craft that weighs about 20-30kg (44-66Lbs). I guess this is highly depending on the blades efficiency though. Do you think a craft like this is possible to build with two moderate sized homebuilt propellerblades, and in that case, if you should assume their efficiency (make it low...) How much power must the engine approx. produce? I realise this is kinda vague, but I really would appreciate an answer. Maybe someone built something similar. Regards /Johan, Sweden
 P: 65 Okay... What you're proposing is a little ambitious, but from a very basic perspective, let's analyze hovering flight... You are going to want a relatively low disk loading... let's say 10 psf (lb*ft^-2). So in hover, thrust is equal to weight, so that makes the rotor area... DL = 10 psf = (T/2)/A = W/(2*A) => A = W/(2*DL) = 66 lb / 20 psf = 3.3 ft^2 So, EACH rotor needs to have an area of 3.3 square feet which means a rotor radius of... r = 1.02 ft (on EACH blade) So, the ideal power needed would be... P_ideal = T^(3/2)/sqrt(2*rho*A)... where rho is the mean local air density... for Earth sea level... 0.0023769 slug-ft*ft^-3... so... P_ideal = 2*(W/2)^(3/2)/sqrt(2*rho*A) = 2*(33*lb)^(3/2)/sqrt(2*0.0023769 slug-ft*ft^-3*3.3 ft^2) = 5.5 hp So, let's assume a figure of merit (rotor efficiency) of 0.6 which is pretty accurate for a home-built contraption... so the actual power needed is... P_actual = P_ideal/FM = 9.2 hp This is the amount of power that needs to be delivered TO THE ROTOR. Any losses from your transmission system from the motor to the rotors will necessitate additional power. Now, all this has been just basic, analytical work. In practice helicopters are evil, evil contraptions. You may be able to get it to lift off the ground, but once there, you have to have some way of controlling the flight. The control aspect is one of the most difficult in helicopter design, and I'm not trying to dissuade you from trying, but know that this will not be a trivial task. Anyhow, good luck and godspeed. Cheers...
P: 65

 I appreciate you are taking the time to answer this, in fact, useless project.
Never say that... no project is ever useless. The most informative learning in engineering comes from actually DOing something.

Anyhow...

psf is "pounds per square foot" [lb/ft^2]

Disk loading is a measure of the amount of thrust per unit rotor disk area, thus...

DL = T/A

It is a metric for how much of the aircraft's weight each square foot (or meter) of the rotor disk supports. A lower disk loading means less weight per unit rotor disk area, but it also means longer rotors (and thus more weight). Typical helos have disk loadings of 5-15 psf, so I picked 10 as an estimate.

I don't know how far along you are in aerodynamic knowledge, but if you can, pick your airfoil so that the lift imparted by the rotor (remember lift is thrust in a helo) gives a disk loading in this 5-15 psf range. Too low and your rotors are too big. Too high and you risk aerodynamic stall on the rotors, not to mention the added weight.

Also, remember belt driven transmissions have huge losses associated with them (including slip), so if your rotor figure of merit is 0.6 (as I guesstimated) you need 9.2 hp at the rotor. If you have a 12 hp engine, that means your transmission can be no less efficient than... 0.76. 76 % is a fairly high efficiency for a belt driven system. I would consider going with a chain (to prevent slipping) or a drive shaft. Although the drive shaft will be more complicated, it should give you better efficiency. Also, your 12 hp motor may not ACTUALLY be putting out 12 hp.

So, again, good luck.

Cheers...
 PF Gold P: 8,964 As a side note, I saw that you plan to build the rotors yourself. I don't know what material or manufacturing technique you plan to employ, but be aware that they must be extremely well-balanced and of equal aerodynamic properties. Having one blade heavier or with more pitch than the other would set up oscillations that you can't control.
 P: 3 Thanks again. I will think a bit on this and hopefully soon begin working on the project. When it comes to rotors, I understand that homebuilt probably will be too much of a difficulty. I´m thinking of aluminum drop shaped (not sure about the spell)blades that is used on small wind power stations, but since I´m not that much of an aerodynamic expert I guess I just have to try it out. I will post the results when done, succesful or not.
 PF Gold P: 8,964 It might be worth your while to check into pre-built helicopter rotors sold for RC models. Wolram is something of an expert in the field. Try firing a PM to him for advice. I'm sure that he'd be more than happy to help. Good luck!
P: 5,095
 Quote by Danger As a side note, I saw that you plan to build the rotors yourself. I don't know what material or manufacturing technique you plan to employ, but be aware that they must be extremely well-balanced and of equal aerodynamic properties. Having one blade heavier or with more pitch than the other would set up oscillations that you can't control.
The balance of a rotor system is done on the aircraft. You have two separate tasks; tracking and balance. They are both done with manipulating weights on the blade tips to get the blades rotating in the same plane as well as reducing imbalance. That way that allows a bit wider allowance for rotor blade manufacturers.
 PF Gold P: 8,964 Well I'll be dipped... That's the first I heard of that. Can you get into a bit more detail?
 Sci Advisor P: 5,095 Sure...For a track and balance we would put a small metal tab that extended below the bottom edge of the tip of each rotor. On the inside face of the metal tab was a piece of reflective tape that had a line. Since we had three rotors per hub, they were oriented like this "|, / and -", one for each blade. Once the rotors were turning, from the cockpit for the front hub and outside for the aft hub, you can use, well basically a powerful timing light that reflected off of the reflective tape on the tabs. If your tracking was correct, you would have all three lines overlap and it looked like a wierd X type of symbol floating in front of you. If it was off, one or all of them would not intersect. From there, you estimated how much weight to add to the tips to get your tracking in order. Here's a link: http://www.chinook-helicopter.com/st...eas/blade.html
 PF Gold P: 8,964 Bloody awesome site, Fred! Thanks millions.
 Sci Advisor P: 5,095 No problem-o. If you look at the tail number history on that site, look at D model aircraft 86-01676 (Ditch Witch) and 87-0073 and 86-01671....Ahhhh memories.
 P: 1 You would be surprised by how cheap, and simple a helicopter, airplane, or hovercraft can be made. Look up one man helicopter on U-tube. It's a little scarey, but with some shielding, and other mods it might be made safer. It's place to start. I've seen good, though small airplanes at shows that use one, or two chain saw motors. Don't let these guys tell you it's impossible. You can find all kinds of plans that have already been worked out if you look hard enough on line. Look up Oskosh, or any air show also.
P: 43
 Quote by LunchBox Now, all this has been just basic, analytical work. In practice helicopters are evil, evil contraptions. You may be able to get it to lift off the ground, but once there, you have to have some way of controlling the flight. The control aspect is one of the most difficult in helicopter design, and I'm not trying to dissuade you from trying, but know that this will not be a trivial task.
LOL! Exactly what LunchBox says! Control and stability of helicopters can be a really tricky animal!
 P: 4,777 While stability and control is an issue, you can overcome it by using gyro rate feedback. Almost all RC helicopters have this, and you can buy said Gyros from Futaba or JR for about 200 bucks.

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