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Rotor Blades R/C Helicopter Rotor Blade Discussions

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Old 05-18-2018, 10:08 PM   #1 (permalink)
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Default An interesting tool for blade calculations

I found this online calculator today while trying to figure out how much effect an out-of-balance blade will have on your heli.

I'm sure everyone is aware of the immense forces put on the blades, but it is quite amazing when you do the actual maths.

A 600 size blade at 2400rpm has about 600kgs of energy behind it!! Yikes!
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Old 06-11-2018, 07:28 PM   #2 (permalink)
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You have to be careful using a tool like this- because isnt designed for a 3D object- and you have to know what the radius is. The radius for us is the blade CG, not the span/tip. A 130g 600 blade at 2400RPM sees about 280kgf at the root. But that isn’t energy, it is just a force that the grip/bolt has to offset to keep the blade attached (since the blade wants to continue in a straight line, not orbit the head).

Figuring out unbalance is not all that easy. If you figure blade one has 130g of mass at a 14” CG from the center of rotation (main shaft), and blade 2 has 131g mas at the same distance, you have 281 and 284Kgf on the blades respectively. So, there is a 3Kgf difference at that speed. But now what? Does this only operate on the head, or does the whole machine get pulled around? And that matters because that defines the mass in play here. If it is the whole machine, and that has a mass of 10lb, you have about .6 g acceleration. You have to convert that into some kind of ‘how bad is this’ metric, which for props and rotors is normally measured in inches per second (IPS). You can find the math for this only- it is just the acceleration over the RPM multiplied by some constant. But you will get a value of about 0.9IPS, which is ‘very rough’ by industrial standards. But this means that if you measured the motion of the machine, it is moving around by about .042” as a result of the balance error. That would be hard to see with the human eye, especially at that speed.

Thing is- the whole machine isn’t being pulled around. The main shaft bends a bit, the bearing supports likewise, etc. So this is really tough to solve, and much easier to just measure with a sensor. But, 1 gram unbalance at the blade CG is a lot for our hobby, and it shouldn’t surprise people that this much error in balance is going to causes a lot of vibration- probably too much for the FBL- and a lot of unnecessary stress on components (bearings, servos, motor, etc).

Hope this helps a bit- it gets confusing if you don’t break it out into steps IMO.
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