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PowerHelis . JR-Spektrum . Gyro Hobbies

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Main Discussion > elevated tail rotor design
 
 
AirWolfRC
Elite Veteran
Location: 42½ N, 83½ W

Giving it a bit more thought, side thrustof the t/r will not induce any torque on the fuselage or the main rotor. It will only make the heli rotate on its swash plate but on an axis down through the rotor head and perpendicular to a line from the tail rotor hub to the main rotor hub.

If anything, the only roll torque would be equal to the sin of the angle between a line through the tail and main rotor hubs and the rotor disk times the side thrust.
07-21-2008 04:13 PM
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Topher
Veteran
Location: Grosse Ile, Michigan

Quote 
perpendicular to a line from the tail rotor hub to the main rotor hub

Huh? It seems like your describing a plain, not a vector. The tail rotor will induce a roll on the helicopter about the roll axis. In order to compensate the main rotor needs to input roll in the opposite direction. The TR also induces pitch(the rotation) which also needs to be corrected by the main rotor.

An elevated TR in the same axis as the tip path plain will eliminate any roll that would be created by a TR not in the TPP however a pitching moment will still be created.

Get a life? How the hell do you fly a life?
07-21-2008 05:28 PM
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AirWolfRC
Elite Veteran
Location: 42½ N, 83½ W

You're not thinking about how the supposed torque is being induced. To induce a torque, you have to have a fulcrum about which a force is acted.

Ask yourself where is the fulcrum for the tail rotor force ?

A bit of vector geometry should be helpful.
07-21-2008 05:51 PM
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Topher
Veteran
Location: Grosse Ile, Michigan

At the center of gravity.

Get a life? How the hell do you fly a life?
07-21-2008 06:02 PM
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AirWolfRC
Elite Veteran
Location: 42½ N, 83½ W

At the center of mass, which is some distance below the rotor head.
And a tail force will want to rotate the heli about that center of mass point, which is NOT in the plane of the rotor disk, which is where the the applied force needs to be.

So, the applied force needs to be broken up into multiple component vectors.
07-21-2008 06:18 PM
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Topher
Veteran
Location: Grosse Ile, Michigan

How complicated to you want to make this? If you apply the TR force on the same plane as the CG, you wont have any induced roll, just a sideways force that is counteracted by a little aileron input and thrust. If its off from the CG that makes life a little more difficult and you will need the inertial tensor if you want to do a dynamic model.

Get a life? How the hell do you fly a life?
07-21-2008 06:41 PM
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Roscoe
Heliman
Location: northfield,ohio - USA

Guys, I think we ALL need a beer or 4 to digest all this; we are getting pretty deep here(not a bad thing,very good thread); for me, I think I'll go fly, it's a great day out there,calm and sunny.

Thanks for all the great info!

"Knowledge speaks, but wisdom listens. "~ Jimi Hendrix
07-21-2008 06:45 PM
 
 
Topher
Veteran
Location: Grosse Ile, Michigan

Nothing here is that complex, except maybe finding the inertial tensor of the helicopter because of the precession created by the rotors and moving parts. That would probably have to be measured. The rest is all linear algebra.

Get a life? How the hell do you fly a life?
07-21-2008 07:05 PM
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AirWolfRC
Elite Veteran
Location: 42½ N, 83½ W

I wouldn't call it linear algebra, more like vector geometry in 3D.

You still talk about CG. It's a center of mass in 3D.

And precession is not part of the picture when you consider we're primarily talking about a STATIC hover condition . . . . without getting into lateral flight.
07-21-2008 07:14 PM
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Topher
Veteran
Location: Grosse Ile, Michigan

Eh ok, vector geometry, but it all comes down to matrix algebra when you plug it into Matlab.

Isn't CG the same thing as CM? As far as we are concerned earths gravitational field is relatively constant. And yes if we are just talking about static flight then we dont need the inertial tensor either. The entire thing can be figured out with a FBD and matrix solver.

Get a life? How the hell do you fly a life?
07-21-2008 07:19 PM
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AirWolfRC
Elite Veteran
Location: 42½ N, 83½ W

Boy, you kids are spoiled these days. Matlab, eh ? These kind of problems aren't that hard anyway. I do that kind of stuff with a blank sheet of paper, a pen and a 4-function calculator. (ok, it also does square roots)

Matrix math is actually easy. Transpose two numbers and flip one of the signs of a vector and you're rotated 90º. I still remember when I wrote a trig package for machinists about 25+ years ago on a Radio Shack Model 4. Let the operator talk in terms of points, lines and circles.
07-21-2008 07:28 PM
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Topher
Veteran
Location: Grosse Ile, Michigan

"And I had to walk through 5 feet of snow uphill both ways......."

Whats wrong with using Matlab? I'm using Matlab/Simulink right now at work and without it I would never be able to get the work done that I do. Heck, even my TI-89 will do most matrix operations. Why work harder than you have too?

Get a life? How the hell do you fly a life?
07-21-2008 07:35 PM
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AirWolfRC
Elite Veteran
Location: 42½ N, 83½ W

I had a TI-59, with magnetic cards and a printer

The nice thing about doing it without things like Matlab is it helps you cut to the meat of the problems. Most times, it's easier. KISS principal
07-21-2008 08:00 PM
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Main Discussion > elevated tail rotor design
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