# Symmetrical Tricopter and T-Copter Design Theory

by cpo | December 18, 2013 | (26) Posted in Tips

In this video, I discuss what I have learned with regard to basic symmetrical tricopter and t-copter design. I show how the tricopter and the t-copter are the same from the flight controller's perspective, and from the perspective of a symmetrical build design. I also show how to identify the proper rear motor position (via rear boom length) for given front motor positions. Oh..and I show how how this T-Copter calculator works just a well for tricopters...and how it validates my "circular" logic of tricopter symmetry.

http://lloydhassell.brinkster.net/rcgroups/t-copter%20dimensions%20calculator.html

I am still designing my tricopter, and you can find my design log at:

JuggNugg on December 21, 2013
Interesting Video! Does the perpendicular bisectors apply to quads too? I think they do, but I'm not 100% sure. Awesome video, I never considered the effects of how a 1/2" boom would interfere with the center of gravity.
cpo on December 22, 2013
Yes, The "line" solution should work just fine on a symmetrical quad. I haven't been able to uncover a pattern using motor-to-motor circles on a quad design yet. It looks really pretty when I am done, but doesn't show me the optimum cog point. : ) I only played with it a short time based on your question, however.
JuggNugg on December 23, 2013
Thanks! :)
Milk Man 1954 on December 22, 2013
great video. gives me a lot to look at on my tricopter that i'm building now.
cpo on December 22, 2013
Thanks! Glad it's helpful. Of course, it's much easier to play with a model on the computer to sort this stuff out...a little more challenging to work out with the actual parts, but I think I am going to try and do a video of how to do it on a built tricopter so folks that make small design changes during the build can find our where the optimum COG point is of their completed project. It will likely be the "string method" :-) Now I have to build my tri..
cpo on December 22, 2013
Oh...and just to be clear...the optimum center of gravity point is a design element. It defines the location that you should balance everything around in the build. I also want to make sure it is understood that changing the boom lengths can change the optimum COG location, but doesn't automatically balance the copter to that location. You'll have to adjust COG of the build to match the COG of the design (if you want). Of course, the flight controller can and will easily compensate if your actual COG is slightly off from the designed "optimum" COG. I just believe that the closer you get to that point, the more stable everything will be in general.
Exhodus on December 22, 2013
Excellent video! Thanks a lot!
It made me realize that the tail boom on my tri is way to long, and thus the CG is completely wrong.
You saved me a lot of headaches man,thanks again!
cpo on December 22, 2013
There may be legit reasons to have a tail boom longer or shorter..but it just wouldnt fit what I defined as a symmetrical tricopter. I am sure experienced tri builders can chime in on what circumstances it may make sense to deviate.
FKreider on December 22, 2013
What program are you using in this video? It looks pretty user friendly
cpo on December 22, 2013
Yeah....its nice.....but at a cost. Not everyone will have access to Solidworks, though. I was planning on trying to recreate that capability in Sketchup using SketchyPhysics plugin...but it won't be until thr nect thing I need to model up. I also really like Inventor Fusion [still available as a free download] and I think I can do some of the moveable assembly in that.
FKreider on December 22, 2013
Looks like inventor fusion has been upgraded to fusion 360, looks to be a free download, it's worth a try! Thanks for the tip
cpo on December 22, 2013
I was beta testing Inventor Fusion for a while, and I am pretty sure it's subscription based. The download is free, but you need to log in to an account with a subscription to use it. I am pretty sure... but I haven't looked at it in a while. Inventor Fusion, however, is just free. Check it out though, because I could be wrong.
galaxy engineer on December 22, 2013
Hey......great video! A bit on the long-ish side but through nonetheless.
I am currently building one of Davids tricopters from scratch and this video did answer some questions I had about Davids design as well as general designs. I did wonder what the big deal was to the further spread of the front motors due to using the 1/2 inch square booms.
This is my first tri copter build and I chose Davids because of its simplicity and it just plain works very well. I am not straying to far from the inted plans as I intend to write an article about it. I think I am going to go ahead and notch the front arms where the ''stopper'' bolts engage to bring the arms back to 120 degrees and restore asymmetry and the ''spirit'' of Davids design. I dont think it is necessary to do this and still have a great flying Tri just trying to keep it simple and to specs.
Anyway......great video and thanks for your time in making it and providing the information to the masses!

Brett Hays
cpo on December 23, 2013
Well...I did title it as a "Not-so-quick-look" : )

You can't go wrong with David's design. Too many people are flying it to argue with! : )

Are you cutting your own body, or buying the pre-cuts? My first thought on restoring the build to true 120° front arms would be to simply move the location of the "stopper" bolt slightly, but notching the arms should work as well. If you were cutting your own, it would be simple to just move those two holes... if you are buying the pre-cut, it could still be done, but you'd just have to drill new holes near (but not too close) to the original ones -- in which case notching may still be easier. Good luck with your build!
galaxy engineer on December 23, 2013
Yes I think your right about Davids design. I decided to go with a total scratch build and ran off the template from his article and scrolled out my own frames. It was sooooo simple to do. I think even doing it by hand with a coping saw would be a no brainer. I had already built it up when I saw your article so simply notching the arms will bring it all into asymmetry. Like I said I dont think flying it as is would be the end of the world just wanting to keep the spirit of Davids build and offer it up as an option in the build article. I thought about moving the holes but then I get into having to break out a ruler and make some calculations.....lol.....notches=easy......haha!
Red20RC on December 23, 2013
Great work. That was 20 minutes of my life I was happy to invest - I learnt a lot that I can take with me into future builds.
Also have a desire to try it some of this 3D design software now!
cpo on December 23, 2013
Thank you! 3D software is really fun and lets me sort things out that are in my head before I try them in real life. I have a 3D printer coming soon (got in on the Rigidbot Kickstarter this summer) and when it arrives, I'll be tinkering a lot in 3D. The hardest part for me was finding the one program that intuitively does what I need it to do. And they are all just different enough to drive you crazy!
jesses skyland on December 23, 2013
Hi
So cool that you did that presentation. I am designing a tri in my machine shop and will be using aluminum or magnesium to build the foldable frame connection. I do not yet have the Gopro so trying to see the sight lines is hard, so this Demo you made makes me more confident. Simplicity, crashability / survivability / vital parts protection, quick fix swappable parts These are the things I am thinking about for the final design.
cpo on December 23, 2013
Nice! Sounds like fun. I wish I had a machine shop... hmmm... Maybe I can ask for one for Christmas. LOL!
jesses skyland on December 23, 2013
If you live in or near Petaluma CA you are welcome to use my equipment! And that stands for any of you guys at Flite Test. Theo you guys have a lazer cutter.
cpo on December 27, 2013
Wow! Quite an offer. I'm on the other coast...but thanks!
liveyourdreamsRC on December 26, 2013
This should help a LOT in trying out designing in the future. Thanks for all the info! Very cool stuff, and I really like how you explain everything to a good extent but kept it interactive and fun to learn- keep that up and again, great vid!!
cpo on December 27, 2013
Thanks for the feedback!