# How to go about designing a heavy lift (3kg payload) multirotor frame?

Hi

I'm working on designing and building a multirotor drone to lift a ~3kg payload with a group at my university. My role is to help design the frame, and while I have a few ideas, I need some advice on the best way to get to a finished design.

Obviously the key challenge here is to minimise the weight of the airframe without sacrificing too much in terms of strength/stability. I've played around with Fusion 360's generative design to try to come up with some 3D-printable parts, but I'm now wondering if this is overthinking the problem.

If anyone has experience building heavy-lift drones, I would greatly appreciate your advice.

Some specific questions:

• Is it feasible to 3D print part or all of the frame? If not, what is my best option?
• Would an octocopter be better/worse than a hexacopter configuration?

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### Replies to This Discussion

Greetings Jack

I assume this an Engineering term project, the best way to do this is

to use the method of material selection in regards to strength density modulus of  Elasticity etc

one can not suggest the best material without seeing your designs are you

using tubes for the arms or square or rectangular arms. There are many variables to cut down on weight I will recommend

On the question of configuration(Hexa Vs Octo)

All goes to the basic laws of physics,

Given the same size motors as regards to the output and efficency of the motors, the octocopter will do a better job because here the variable is the number of motors, more motors will mean that each of its motors will carry less load compared to the hexa motors

divide the total load by the number of motors,  the less a load a motor will carry the better it will perform.

Now if the hexa motors are bigger motors than the octo motors then its a different story, there are variables you have to look into in sizing your motors for your design look at the torque the output of the motors the stall speed and the efficiency of the motors.

Hey Jack, As a college team project, you will probably not get a from-scratch design done in one semester. Just go get a Tarot copter, scratch the name off, and say Lori L. said to do it.(just kidding)

You cannot ignore the fact that this design has all been done before.

Seriously though; the state of the art is now high-current motors on a simple frame of carbon fiber and BetaFlight  here. open-source flight controller firmware. For example this by Catalyst at \$340 and this by iFlight at \$160. On the iFlight (850mm), chop the arms down to get 550mm between the near motors. Note the structural elements where the square CF tube comes to the base plate. Iflight uses added vertical CF members locked in and Catalyst uses NOTHING but standoffs. IFlight saves money by building up the tube length with parts and pieces of flat CF for the motor mount while Catalyst just clamps the motor to the frame. In a crash, the iFilght is repairable with small parts while the Calalyst tube is smashed up! One note. Square CF tube is VERY expensive. I have used anodized alum tube in the past; 0.625 x 1.125; forgot the source. Now going to Rock West Composites for 20mm x 35mm rectangular tube here.;  \$39/foot.

The 850mm is amazingly called "X- class" and the 550mm the "beast" class. Their genesis is the racing drone folks who want something bigger. Funny thing is they didn't realize these sizes existed when drones started out TEN YEARS AGO. See my old crate on YouTube at "Rockford Quadcopter" here  , and some of the old 3DR designs on the web.

These new designs are using the iFlight F7 TwinG controller here. (supports 8 motors) or similar, and the phenomenal Succex 80A ESCs (I have the 4-in-1 60A)(which use highly efficient DirectFETs). With the separate ESCs,  use the Advanced Power Devices (APD) power dist board APD360 or APD500 from  here, and then an 8S battery for juice. In addition, normally these racing machines would use the MasterScrew 12inch tri-blades, but you might want to change that. I''m getting some of these props to try myself for low speed application; currently I use 12inch SlowFly props. Their pitch is for highest air speed.

The drone racing up-size wannabes use low 400KV motors cranked up to 8S, but I am using 770KV motors. My goal like yours is low aircraft speed, but high lift.

The problem with an octo-copter is just TOO many parts n pieces. That PDB I mentioned is only for a QUAD, so with a hex, you could just connect a motor to each of the two sets of battery pads. Since the motor connection is right there at the battery lead, no added power is overloading the four port PDB. Advantage hex over the quad is it can still fly if one motor fails, but today the motors and electronics are so much better than five years ago that is doubtful to happen. Maybe just go with a quad!

Additionally, if you or some members of your team are doing the electronics too, take a look at my last and latest flight controller design here. It's not in production, probably never will be, and I just released the design into the public domain for anyone to study; especially academics and students (it has extra notation ot normally found on production schematic drawings). All of the design files, BOM, pin allocation, gerbers for final design, are there.  Coincidentally I posted a write-up on it yesterday and it is currently on the front page of DIYDrones here. And at DIYDrones SAM E70 Flight Controller.

You might want to pass the design resource onto some of your academic friends involved in drone/robotics electronics. There is a lot of sensor related how-to. No doubt the Chinese will get wind of the completed flight controller design and pump clones out like cloned smartphones.

I've been in your situation myself and most likely the project will not finish for this or that "unresolved issue"; it's only an academic exercise. Like this last un-built three-year-effort flight controller design I did which turned into an academic exercise;  I'm retiring.

That's all. Good luck!

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