|04-14-2009, 07:14 PM||#1|
I am: mael i
Join Date: Mar 2009
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2 or 4 rotor?
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it would make sense to me that a 3 or 4 rotor head would provide more lift then a 2 rotor head but i figured id ask
i plan to run the Spartan RC AP2000i so i think it will provide the stability
anyway please let me know what you think
also an one stretch a jr voyager e if so what parts did you use?
thank you Mael
|04-15-2009, 09:26 AM||#2|
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I am: Gary L
Join Date: Aug 2008
Location: Tecumseh, Ontario
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Re: 2 or 4 rotor?
There is a lot of nice theory involved, but you will SHARE the lift, (carrying the weight) between any number of blades. Disc loading would be identical if the aircraft weight is the same, but the blade loading goes down.
Unfortunately, in our case that does not mean we also increase stability.
Our original 2 bladed designs use a Bell/Hiller setup with a flybar for two reasons: First is to reduce the load on the servos. You basically "Fly the Flybar", ie. your cyclic control inputs go there. The paddles out on the end of the flybar act as aerodynamic levers to increase your inputs up to the main rotor blades. This is an internal input.
Second, the flybar acts as a damper against short-duration inputs, such as wind gusts, coming from outside the heli. It will try and return the heli to whatever state it was in before that force acted on it.
Now, let's say you go with 3 or more main blades. Naturally, you cannot use a flybar for this type of rotor system, so you have not only removed the device that gives you that natural stability, but since the servos now have to control the pitch of the main blades directly, their loads have increased dramatically. In your case you would seriously need to look at the capability of the servos.
Sure, any spinning mass will have inertia and some form of natural resistance to change, but in this case that also means you have to add your own control inputs to counter excursions from your desired state.
p.s., here's the really fun part:
Due to the fact each blade does not carry so much of the weight its Angle of Attack can go down. Therefore the Induced Drag from producing lift decreases, partially offsetting the increase in profile drag from the extra blades.
However, this system is what is called a "Rigid Rotor" and other means to adapt for Lead/Lag and other forces have to be made. In our little ones this is done with the O-Ring dampers in the center holder. Simple when the one feathering shaft attaches both blades, but this can get interesting when you try and adapt that capability to multiple rotor designs.
In earlier designs there were hinges and oil-filled damper systems, (go look at a 50's era Sikorsky. There were more grease fittings than I can remember). Now, along with rubber isolators at the blade root, composite blades have the rigidity AND flexibility to compensate for those forces.
Hope I didn't get too complex. My mother told me there's be no more math when I got out of school!
Last edited by Cougar429; 04-15-2009 at 09:30 AM.
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