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Old Jul 01, 2010, 05:21 AM
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Tri-Copter and Quad-copter Rotations

Hi all,

I'm new to this forum and to RC'ing in general.

I've been looking into building a tri-copter and have seen many of the wonderful builds some people have put together.

I understand that in tri-copter builds, there seems to be a convention of adding a servo motor arm to the tail propeller to control the rotation of the tri-copter.

My question is; Is there any way to rotate the tri-copter without the need for an angled prop?

Also, some of the quad-copter builds i've seen use perfectly level props, how do these control rotations?

By rotation, i'm talking about absolute rotation around the Z axis (vertical axis). To simply rotate the copter with a tri-copter build using a servo arm I can understand. How do quad-copters rotate? Is this a combination of the counter rotating props?

I'd very much like to build my own and have started sourcing out parts for a tri-copter but I'd like to know if rotations can be handled without making the 3rd prop redundant with it's sole purpose being to spin the copter around.

Forgive me if this sounds a daft question, i'm just starting out with this so am trying to gather as much info as possible before splashing out any cash on parts.

Thanks for reading, any info you can provide would be much appreciated
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Old Jul 01, 2010, 05:56 AM
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Kiryat Yam, Israel
Joined Apr 2009
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Quads use counter-rotating props: 2 rotate in one direction, 2 in the other direction. Yaw (rotation around Z) is done by reducing the speed of one pair and increasing the speed of the other.

You can't use this trick in Tris, thus the need for rotating at least one prop around the X or Y axis.
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Old Jul 01, 2010, 06:58 AM
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So there is no way to rotate a tri-copter around the Z axis without an angled prop?

I guess I kind of understand how it works for quads and why you can't do the same with a tri-copter.

Wouldn't varying the speeds of each prop on a tri-copter induce some form of rotation at all?

I'd like to build a tri-copter but would prefer one without a dedicated prop solely for rotation. Perhaps I should just build a quad instead and be done with the confusion =P
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Old Jul 01, 2010, 07:24 AM
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Kiryat Yam, Israel
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The props on a quad are arranged such that for yaw rotation, prop speed variations cancel out all other effects (at least theoretically). To do this you must have symmetry. The same, BTW, is true for rotations around the other axes. That's the beauty of the quad design.
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Old Jul 01, 2010, 07:46 AM
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Does this mean that tri-copters are incapable of aerobatics due to the lack of props that provide thrust?

Ie: is a tri-copter still capable of performing a front-/back-flip?
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Old Jul 01, 2010, 10:31 AM
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Joined Jan 2010
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The tail motor has 3 functions, it provides Elevator for front and back flips by slowing down while the front accelerate and vice versa for down eleveator.

It also provides pitch control together with the other 2 motors, accelerating and decelerating together with the front ones, and then it provides yaw (rotation) to the tricopter.

That motor does not have the sole purpose of yawing the tricopter as it happens for the tail rotor of conventional helis.

The tri builds that you see without a servo, actually don't have a servo at the tail end, but they do have a servo in the middle of the frame, driving a torque rod that goes throught the tail boom.
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Old Jul 01, 2010, 10:52 AM
Tri-Quad-Hexa-Octo-copters!!
United States, TX, San Antonio
Joined Feb 2007
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Hi,
If you are after the orientation and look of a tricopter but do not want to use a tradtional tail servo, then consider building a coax hexa which does not require a tilt servo. You would need either a special board and corrrect firware, such as the KapteinKuk, Mikrokopter, Quadpowered, or UAVX, or a computer Tx and V-tail mixers, or the mini-mixer by ClarkST. More info here: Quadrocopter and Tricopter Mega Link Index
Cheers,
Jim
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