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Jul 05, 2019, 06:49 PM
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Thoughts on a really big tricopter?


I have been staring at a couple of T-Motor U8 170kv motors sitting on my shelf... or rather they have been staring at me! Has anybody here built, or know of, a really big tricopter (or maybe a v-tail quad) using two heavy lift motors/props? I would want almost all of the lift to be provided by the two U8 motors and 28" props up front. I was thinking about having the third motor (or v-tail motors) and prop be much smaller and there just to provide pitch and yaw. What do you guys think? Please provide some real life experiences with big tri copters or v-tail quads. Thanks!
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Jul 05, 2019, 08:36 PM
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I believe the motors should all be the same size if you are going to use props this large. The largest props I have ever used on a tricopter is 14" with 700kv motors.
Jul 06, 2019, 05:14 PM
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Sounds like a serious freaking machine! As far as the smaller motor in back- you'd get less adverse yaw from the smaller prop than one of the same size so the tail motor and fc would work less in that aspect at least. Finding a good size combination that functions well with the battery may be the hardest part of that equation.
IE: will you be able to find a smaller motor that you can run on the same cell count pack?

I ran a scratchbuilt mwc tricopter that ran 13 inch props for a bit it didn't end up being as fun as I had thought it would but I had made a crappy motor choice and wasn't happy with the build in the end..
Jul 08, 2019, 02:48 PM
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Quote:
Originally Posted by otterdive
I have been staring at a couple of T-Motor U8 170kv motors sitting on my shelf... or rather they have been staring at me! Has anybody here built, or know of, a really big tricopter (or maybe a v-tail quad) using two heavy lift motors/props? I would want almost all of the lift to be provided by the two U8 motors and 28" props up front. I was thinking about having the third motor (or v-tail motors) and prop be much smaller and there just to provide pitch and yaw. What do you guys think? Please provide some real life experiences with big tri copters or v-tail quads. Thanks!
just do it and tell us how it came out.
My largest TRI is 100cm motor-2--motor, 3 equal 475kv motors on 4S or 6S, swinging 15" or 17" props
https://www.rcgroups.com/forums/show...0#post37315865

You might consider bulding a bi-copter, something very rare these days
Jul 10, 2019, 10:12 AM
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Quote:
Originally Posted by mmelvis
I believe the motors should all be the same size if you are going to use props this large. The largest props I have ever used on a tricopter is 14" with 700kv motors.
Nope, not needed to be the same, I did it in the past some variation of a T frame, with front motors about 14-15" and a small higher kv tail motor of about 8".
The trick is to balance it well, by sliding the battery on the T "fuselage" arm. When balanced the tail motor does its job well for pitch.
Jul 13, 2019, 11:33 PM
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How did you determine the correct CG position? I think you have to find the sweet spot so you donít over or under power the small rear motor.


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Jul 14, 2019, 06:38 AM
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Exactly... geometry and physics.
Compute a balance having the arms length as ratio of props size squares.
If tail prop is half of front prop then a good start for CG fine tune is at 1/4 from the front boom of T frame.
Jul 14, 2019, 03:34 PM
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Center the weight as far forward and inline inline with the front props so the tail is only acting as a rudder and elevator not a load carrier just like a regular helicopter which is what this is just with 2 load props instead of a single.
Jul 15, 2019, 05:01 AM
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Nope, is not similar with a heli.
Tail must be under permanent load, and same disk loading as the front props, because the logic of current flight controllers thinks all the motors should have the throttle pwm in the same ballpark.
Having 50% on front props and only... say 10% on tail prop, will make pitch very sensitive and hard to control.
Jul 15, 2019, 07:41 AM
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Yup- similar to a heli- the pitch sensitivity needs to be tuned in rate settings. The current flight controllers are 10x better than what we had 6 years ago. To suggest they are less capable is just plain backward. I'm sure you have some explanation how it won't work but in the end you're wrong. Your theory doesn't leave any room for kv variation between motors either. The mixer in the fc DOES SUPPORT "custom tricopter" and extra "ppm to servo" can be mapped in smix.
https://github.com/betaflight/betafl...1.7-(and-later)
You should know better than to say "X" cannot be done considering you've been here since the forum went full retard and added the name "drone" to every single multirotor forum name.
Jul 15, 2019, 08:57 AM
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Is a matter of control resolution.
When tail motor pwm is 10% for level, then changing +/- unit will led to much bigger pitch variation then for the case when tail motor pwm is 50%, when the disk load is equally amongst all props.
btw, disk loading could have different meaning for you and me is the thrust divided by disk surface, similar to wing loading for a fixed weight platform.

No idea why you compare with the heli tail prop, which has rudder/heading role only, not being involved in pitch at all...

Not the last... if you place CG exactly on the front motors boom, how do you perform backward movement, i.e. drop tail ?
As advanced as they are, the current crop of FC, I am not aware any of them to be able to control reversing rotation of a motor...
Last edited by renatoa; Jul 15, 2019 at 09:17 AM.
Jul 15, 2019, 09:47 AM
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Quote:
Originally Posted by renatoa
Is a matter of control resolution.
When tail motor pwm is 10% for level, then changing +/- unit will led to much bigger pitch variation then for the case when tail motor pwm is 50%, when the disk load is equally amongst all props.
btw, disk loading could have different meaning for you and me is the thrust divided by disk surface, similar to wing loading for a fixed weight platform.

No idea why you compare with the heli tail prop, which has rudder/heading role only, not being involved in pitch at all...

Not the last... if you place CG exactly on the front motors boom, how do you perform backward movement, i.e. drop tail ?
As advanced as they are, the current crop of FC, I am not aware any of them to be able to control reversing rotation of a motor...
I'll ignore the first part of your post because it appears you didn't look at the link.
I compared with heli tail because that's what he suggested- big props on front smaller in rear. A heli motor only operates in yaw, where a tricopter tail can only operate in yaw and pitch due to motor orientation. Which answers both of your points there. The FC wouldn't be in charge of reversing the motor (which it most definitely could if it were AIO board)- the esc's do the motor reversing- have you never seen a 3d quad? They've been out for 4~ years or better.
I'm not sure exactly how the remapping/programming would be done but I know it CAN be done- and the basic info is on the link page.
This would fall under the "custom tricopter" category.
Jul 16, 2019, 08:18 AM
Hamburger
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renatoa got it right.
Reversing motor+props only works well for small props due to mass, that is where variable pitch shines but it is another world to enter.

In effect you want some mass forcing the tail down - which is easy by moving the CG (battery) from the T-line between front motors in the direction of the tail.

Whether you accept the different flight behaviour from very different rotor sizes is your personal preference.
Jul 16, 2019, 09:26 AM
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Not building a 3d quad- that was only an example of esc performance, not a suggestion for setup. He said a heli only moves yaw with the tail which is fine- but a tricopter has a rotating prop on the back that's always facing in an upward direction UNLIKE a heli. Therefore it has the ability to direct pitch. slowing or speeding up the prop for more or less lift to handle the pitch is no different than a quad slowing motors for yaw. It doesn't need to reverse.
Jul 18, 2019, 04:19 PM
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Quote:
Originally Posted by renatoa
Exactly... geometry and physics.
Compute a balance having the arms length as ratio of props size squares.
If tail prop is half of front prop then a good start for CG fine tune is at 1/4 from the front boom of T frame.


Thanks that makes sense but I donít understand what dimension to take ď1/4 from the front boom". I understand that is the CG location but how far from the front boom should the motor be placed?


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