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
Interesting explanation thank's, it's better to paint the APM upside white?
if using the silicone product, it goes on so thin it probably doesn't matter. but yes, you are absolutely right, the lighter the color the more reflection, so why not. just be careful to tape off all external connectors, etc.
thank's Forrest
the flight time are coming out of eCalc estimates. It could be very wrong and only real trial will tell...
Update on battery pack : received the Panasonic 18650B 3400mah cells.
As previously posted, the optimum for my configuration is to build a 4S7P pack.
I find the easiest to solder the cells like this:
I took cells that come with soldering tabs which are spot weldered on the cell, which makes it much much easier to solder without risking to short them.
The colored lines will make my balance charging lead. The thick red and black will obviously be the main lead.
Just need to solder now....
@Hugues,
let me know the type of balancer board to work with your battery pack.
To my understanding you need 2 x 8 pins out smart balancer boards
since Li-Ion 2P cell can safely work for months if only high current charge/ discharge temperature limit is individually monitored.
Do you preselect individual Li-Ion cells against internal resistance ?
this will be great. never had the guts to try it aligned (always interlaced). if your battery pack survives a few crashes, we'll know that this config is OK.
i also track the added weight of wires, adhesive, carbon reinforcement, insulation, etc. my Li-Ion packs seem to average about 2.3ish% more than the raw batteries. let us know how you do.
weight ideas (for what they are worth):
- the location of the + & - leads. if you center the final lead at the center of the final parallel wire, then the amps in the parallel wire are less (rather than 10 amps there is only 5 amps coming from two directions to the middle) thus the parallel wire can be smaller and lighter. But this might not trade well with where the main leads need to go.
- given that you have chosen a single stack, you might not need cell 2 and cell 1 parallel wires. i might be wrong on why, so check my logic. when balance charging, each battery is a unique cell. so you have 28 cells. no charge can deal with that. what i've seen sometimes when i cause a short, is that the affected cell won't fully charge during the "total battery" charge. So I put leads on that cell alone and fully charge it. Once it is at full charged, the battery is back to normal. On the next discharge/charge cycle, the cell issue doesn't reoccur. The problem is fixed. My belief is that you can only charge the battery solo, not in a parallel of 2 or more?
Good luck soldering. I've done more stupid things causing bright flashes ... like forgetting that the solder wire is on a reel potentially connected to a ground.
Thx. I do not understand what you meant in the second bullet : do you mean I cannot charge a cell composed of 7 batteries in parallel ? It was my plan to charge each of the four cells of the pack (a cell of the pack is 7 batteries in parallel) by connecting the charger+ to the cellx wire and the chager- to the ground wire. X varies from 1 to 4 (4 cells). Am I wrong in this charging procedure ?
for a charge, you can actually just charge the entire thing at once. it will probably charge to 100% just fine. you can tell by recording the final voltage. if it starts to drop, then you have a cell issue (one or more).
the only problem comes with using Balanced Charge, which is the same as a regular charge with an extra step at the end. Balanced charge at the very end looks at individual cells and ensures the there isn't a slacker. If there is one, it individually charges that cell. problem solved.
In our LiPos, a 4S will have four cells. Each cell is the same as your tubular batteries, except that LiPo cells are normally thin, flat, and rectangular (a rectangular pancake). Increase a LiPo from 3S to 4S, one more pancake cell is slapped on the side making the battery thicker. Thus a 4S battery has five other little wires (besides the two massive ones) so that they can be also balanced charged.
- little black = ground for all four cells
- little red = cell 1
- little blue = cell 2
- little white = cell 3
- little yellow = cell 4 (color schemes and order varies)
so lets say after the battery charge, it's in balanced charge and then does it's final check. They all read 4.2 volts, but cell 2 reads 4.0 volts. In balanced charge, the battery then sends a small current down the little blue wire until cell 2 is at 4.2 volts. each individual cell is now balanced.
Battery voltage is driven by the weakest cell. so what balance charge does is to bring the weakest cell up to where it should be. With a 28 cell battery, like yours, this brings new challenges.
what i've seen is that with LiIon, i haven't had a strong need for balanced charge. regular battery charge works fine. if i short a cell and it starts working funny, individually charging that cell once seems to permanently fix it.
but, if you only believe in balance charging, then here is a solution. lets say your charger can balance charge a 6S battery (6 cells at once) and you have a 4S7P.
- add little + wires to every single cell (all 28)
- connect each set of the cell wires in the same series to a 5 pin connector
- with the first pin going to ground
- repeat that for all 7 parallel paths so you have seven 5-pin connectors (each S + gnd)
- then you can balance charge each parallel individually
- during each of the 7 charges, if in balance charge, at the end it will look at the four individual cells and pump more amps into it if its resisting during the main charge.
i've found it easier (and less weight), so far, to just charge the darn battery at the max rate and watch final voltage for any peculiarities. thus no little wires coming off of my Li-Ion packs. Just the two big ones.
This brings up charging. You can charge Li-Ion at 1.625 amps per parallel. So you will be able to charge at 11 amps. Most charges only go to 5 amps, which is probably a better charge rate.
If i've got this wrong, let me know.
For your LiIon batteries there is simply a two step charging profile as follows:
Constant current - The charger current is limiting set to about 1C or as per the battery specifications. The voltage increases to maintain a fixed current as the voltage of the battery and the voltage of the charger differential decreases (increasing internal resistance).
Once the voltage reaches a set voltage per cell (usually 4.2V per cell) the profile is switched to the Saturation charge.
Saturation charge - is where the voltage is held constant and the amps continue to be pumped into the battery although at an ever decreasing rate. Some others call this the absorption mode.
There are tricks to get more energy into the battery but all these methods shorten the LiIon battery life.
Once the battery capacity has been nearly reached, the charge is cancelled to prevent over charging.
As you add more energy to the battery, it's weight remains the same unlike other forms of powering aircraft using fuel tanks.
Jim
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