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Tri copter boom lengths

cranialrectosis

Faster than a speeding faceplant!
Mentor
#1
I seem to recall a discussion where it was suggested that on a tri copter you want the front booms shorter than the rear boom. I think the idea was to offset the lesser amount of thrust from the rear motor by reducing the amount of leverage the front motors have.

I don't seem to be able to find the thread.

Is this theory sound? Is there a benefit to making the front motors work harder?

If so, how much shorter than the rear boom should the front booms be?
 

Craftydan

Hostage Taker of Quads
Moderator
Mentor
#2
dunno about that discussion, but from the physics, shorter front booms would make it less-stable-more-agile in roll. shorter tail will make it less-stable-more-agile in pitch

As for motor loading, it depends on how close the CG is to the "motor centroid" -- the weighted average of motor position weighted by thrust, not by, well, weight.

If all motors were equal, and the booms were positioned such that the motors were on the corners of an equilateral triangle, putting CG in the middle of that triangle would even out the loading -- the neutral point. So what happens to the neutral point when we move things around?

pulling front motors together, or moving the back moter farther back, would move that neutral point backwards (going to the extreme, pulling the front all the way together would make neutral 2/3 back from the front motors).

making the tail stronger (more thrust for the same throttle signal, presumably from a bigger motor) would push neutral forward.

Of course, like fixed wings, you don't have to put the CG on neutral, but unlike fixed wings, tail/nose heavy only affects motor loading. If it's within the motor's power curve to keep up with the heavier load, it'll be fine in hover. A tri might drop or raise the tail in a full throttle burst if CG is off neutral, unless mix limits are set to compensate.
 

Cyberdactyl

Misfit Multirotor Monkey
#4
Having the principal axis of inertia of a tri or quad offset from the MR's optimum physical center of lift, can play all sorts of wicked mischief on flight physics. The asymmetric dead cat design is a classic example.

Also for acro it can help greatly to keep as much mass as possible in the same plane as the lift points, keeping the principal axis of inertia in line with the angular momentum vector so the craft does not expend energy on torsional flex of the structure.

There's a great analogy to a quad or tri yaw rotating. . . in a tumbling asteroid and its slowing of rotation over time. The equations have been fairly well known for 60 years. Here's a fellow explaining the math in detail.

The video which explains the study graphically, primarily looks at energy loss from non-principal-axis rotation and how that slowly eats away at the rotational wobble, forcing the asteroid into a more perfect principal axis of rotation spin over time. The physics and energy loss from COG being a distance from the principal axis is very analogous to the movements of a multirotor. :)
part 1
part 2
 
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cranialrectosis

Faster than a speeding faceplant!
Mentor
#5
Here is why I like FliteTest.

You can go down the rabbit hole here just as far as you want. Well, until our wives call us for dinner...

Thanks, Cyber. I have seen your racing quad design and am eager to see it built. It employs some of the physics you are describing here in a beautiful graphic representation.

For now I think I will keep my first tri copter as per the original recipe and tweak it later like I have with the AnyCopter which is on its 8th build for all the tweaking.
 

Deeleins

Junior Member
#6
My tri has slightly shorter front arms to compensate for the increased angle due to using 12mm instead of 10mm booms- (and keeping the centre boards and drill holes the same.) as suggested by the great man rcexplorer himself. Well - actually I kept the booms the same but moved the front motors in a bit - never got round to cutting the extra off.
Good luck with the build- lets see a photo when you're done!