Round tubes offer the following pros:
- handle twist better than square tubes. Thus for the strength, are far lighter.
- are more readily available (pipes, tubes, arrows, kite frames, golf clubs and sticks of all materials).
- easier to cut (square tubing can twist and be damaged by cutting forces)
- easier to peg (fits into a drilled hole and easy to find extenders)
- less expensive
Round tubes are thought to have the following cons:
- difficult to mount motors
- difficult to join
None of the cons are real if you know how to work with tubes. Mounting round tubes to motors is easier and faster than with square tubing. Round tubes are also faster and easier to join together. The resulting joins are also far lighter and better.
In the H-frame forum, I was asked to share these build techniques so have decided to demonstrate the methods on the most complex multi-copter one can build, an Octa-V. I'll do this step by step. The result will be a multi-copter that reduces frame, screw, gusset, and motor mount weights by more than 40%.
The steps will be Design, Assembly, Charmin Test, and Flight Test
Installment 1: Design
First, both simple and complex multi-copters share something in common. When using round tubes for arms, there is no reason to cut a perfectly good tube in half for each arm. And then add a bunch of weight and fasteners to hold the halves together. How this is done will become evident in the third installment, the Charmin Test. For now, just know that each tube is continuous (no breaks, no joins). Opposite rotors share the same boom in a quad, hexa, or octa. All of those fasteners are gone. The cross beams on a V or H are also continuous. Assembly and disassembly is quick.
Most quads are so simple that one grabs two pieces of wood the same size and slap them together. Done. If the angle of view isn't good, just move the camera forward a bit. But, if you want to know the exact length of a quad boom based on prop diameter and platform size so you can minimize weight to get longer flights, then I've attached a worksheet that does the math.
An Octa V is a bit more complex. It is specifically used for camera work. So you need to optimize the motor boom angle and aspect ratio of the frame to achieve the desired Field-of-View for the camera (void of propellers), It also uses 8 motors so that if one dies, the copter can return to the ground with the $12K of camera/lens in tact. You also need to minimize platform vibration, so the platform needs to be large enough for the electronics, gimbal mount, and at least 1.2" (30mm) from the prop radius.
I've attached an Excel worksheet that does all of the calculations for optimizing weight. There is an instruction sheet if you want to ever build one and calculations for a Quad X, Quad +, Quad Spider, and Octa V.
The next installment will be Assembly.
P.S. I'm not experienced nor am I an expert. I'm just a tinkerer like many of you. There are builders out there with far more experience and wisdom. I'm hoping that this blog will allow us all to share ideas on building strong, fast, and light not only for initial build, but also for crash repair.
Replies
yes. it is more accurate for two reasons.
- the drain rate and voltages change in each phase impacting actual capacity of the battery in a manner that can't be accurately averaged.
- the other is that if the voltage of the phase is less than what can keep the ship afloat even at full throttle, the capacity in that phase is not counted.
Hey ... like minds ...
I'm slightly convinced that more motors can't fly as long. As you recall, i started with an Octa because of your logic.
- on the plus side, more motors can share the common weight of the frame and electronics).
- on the negative side, it takes longer masts and because of the additional length, stronger masts.
on the duration ships, the common mass (battery is not included because it's a function of mass) was never great enough to overcome the added structural weight.
but, i guess FPV/camera being added to the ship might tip the balance. thanks for the insight.
looking forward to checking out your worksheet.
quick answer on Po vs Io is yes, nearly. Po is 3.7 V/S and Io is 3.6 V/S so a 5S Po would at 1/2 volts more. figure about 1.4% per volt loss in efficiency.
If you think you find a good KDE, i'll get and test it. Took a quick look at the 330 KV motor. Heavy is right. On a net lift basis, not too impressive. But then again, the lowest test was at 50% power. So efficiency might be OK at 30% - 40% power.
My thought is to use the GoPro as both FPV and as the camera. I'm completely up in the air on the gimbal. Looking for something that is good, but can be lightened using composite sandwich plate. Any suggestions?
I just found this 3D gimbal that weighs 188g. I know nothing about it so I don't know if its any good: http://www.amain.com/walkera-g-3d-3-axis-brushless-gimbal-ilook-gop...
The Zenmuse 3D gimbal costs twice as much and is listed at 168g.
A GoPro HERO3 weighs 75g.
so i should be looking at a front mounted 2-axis (or modify one to be that way) and one that does allow for more than 90 degrees down (like 130 down/back).
do you think your friend with 3-axis is getting better footage than you can with only 2-axis? and if so, is it because the video is more stable in forward flight? or is it just because he can do more tricks with the camera?
One of my deciding factors for choosing the QAV400 was the fact that its 2-axis gimbal mounts to the front of the ship and the camera can tilt up or down 90 degrees. I can adjust the tilt remotely with a pot on my radio. I have filmed straight down but not up... yet.
If you use a 3-axis gimbal, you will lose the ability to tilt all the way up, but you gain the ability to rotate the camera, independent of your ship orientation. If your gimbal is hanging underneath, you could even film behind you. If you mount it up front like mine, the props/ship will be in view if you rotate the camera.
The only reason I might go to a 3-axis gimbal is to stabilize the camera yaw. I know a guy who has a 3-axis gimbal hanging off the front of his QAV400 and he shoots beautiful footage. He has not done any weight reduction or other optimization so he only gets 7 minute flights.
also been looking at folks that modify the gimbals so they stick out front. they tap into the arm going to the damper mounts. get rid of or move the damper section behind the camera.
as a photographer do you ever shoot up?
i'd like the ability to shoot straight up when climbing between trees. i've noticed that many gimbals will rotate that direction but you end up shooting the mounting bracket. is there a cure for this?
meets/exceeds my base requirements of +90 and +45. and light. looked at a youtube shot with it.
so should i be using the Pixhawk to stabilize it or rely on the gimbal?
An issue using round tubes (my preferred choice) is mounting the motors securely at the end of the tubes. Typically the BLDC motor's shaft sticks below the thin firewall plate mount and the obvious tempting solution is to drill a hole through the tube so the shaft spins freely without rubbing against the tube. Bad idea though!
In crash studies, this hole at the end of the tube weakens the tube and it splits from the end in hard landings with future torsional rigidity compromised. Better method is to make the motor firewall thicker. The lightest way to do this is by using balsa core CF panel available from a number of sources. For most motors, 1/8" panel is thick enough and works well. When this firewall is bonded onto the tube's end, the motor firewall is on for good. And provided the motor is secured to the balsa firewall using nylon screws, in a crash the motors separate without damage to the firewall or the tube. That is unless you fly like my other brother Marty, who can break anything and was the inspiration for the "Don't Fly Like My Brother" instructional videos.
https://www.youtube.com/watch?v=nBq8YHShkVU&list=UU82flMBYfAPw-...
See the two pictures attached. Note the 3M Structural Adhesive application which was applied using a wood tongue depressor modified to the proper fillet radius. And as my brother Forrest and others have mentioned, be sure to use spiral wound tubes because the less expensive pultruded tubes just do not have the torsional stiffness we all require for a durable build.
Jim
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Your bonding looks really great (much better than my jobs). I put tape over the holes; put adhesive on the motor mount and press it onto the motor mast; align the two by taping them onto a full-scale drawing; let it cure; peel off what I can of the tape, then carve out where the head of the screw goes when adhesive ran too thick. not always pretty and the last part is a pain.
Jim - Could you talk a bit more about your tongue depressor technique? Do
you put adhesive on the motor mount first, press it onto the motor mast,
and then wipe away excess with the radius fillet tongue depressor? Or do
you press the two parts together first and then apply the adhesive with the
tongue depressor?
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