Building Copters with Round Tubes - Stronger, Lighter, Easier to Mount Motors than Square Tubes - ArduCopter User Group

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.

Multi-Copter Design.xlsx

I've found the interlocking arrangement (A or C) to be the best. The batteries have a protective thin shell for insulation, that does not handle high shear well. Thus your interlocking method helps to remove forces from those bonded joints (the forces at the battery pack are high during a crash). The batteries are spot welded with Epoxy 2216 at every battery contact to ensure that the pack stays together and that the batteries maintain their distance from each other during a crash (don't want to create a short on impact).

To enhance stability:

With A (2 stacks of four rows of alternating 3-4 batteries), i might add a stiff carbon/nomex end plate to protect from shear in the direction of the axis of each battery.

With C (1 stack of eight rows of alternating 3-4 batteries), i add four right sized carbon tubes between the outermost triads of batteries (the outermost holes). This gives stability during a crash in the serial battery linkage and creates a fairly strong structure.

With D [B interlocked] (1 stack of seven interlocking rows of four batteries), ditto the comments of C.

The choice comes down to volume requirements and wiring simplicity.

Building in stacks present a unique problem. When building battery packs with more than 1 stack, one needs to weld a wire across two batteries and then fold the wire in half as the battery ends come together, end to end. To be able to selectively charge one cell, it is important to be able to access all of these joins. Thus stacked batteries should always have two layers. For example, a 20 cell battery can be 2 layers (in series) of 2 batteries end to end (in series) x 5 of those in parallel to create a 4S5P. Thus there is access to every cell. Another tip on those access points is that they should protrude:

- only enough for access and not enough to cause a short with the battery or ground

- in a direction that will not come in contact with the ground during a crash

- in a direction that will not poke through the protective covering during a crash

- in a direction that is accessible to a charger if the cells become imbalanced.

Also, with a photography ship, there is another issue. If you are of the ilk that the CMOS or CCD needs to be located at the center of the ship CG to minimize CCD translocation issues during corrective roll/pitch/yaw motion, then the battery pack needs to go around the gimbal. There are a lot of ways to do this. The simplest, if doable, is to build two battery packs and put them on either side of the gimbal. But, if you have an odd number of cells, then this might not work. See drawing below for one solution to such a design where it is necessary to wrap the batteries around the gimbal to put the CG at ship x/y center and at the plane of the props.

Replies

- cj > Hugues January 18, 2015 at 6:16am
  
  another mobius gimbal using the 1806 motors:
  
  http://www.rcgroups.com/forums/showthread.php?t=2325358
MR60

Hugues January 12, 2015 at 10:03am

30 Panasonic 3400 man 18650 on order
MR60

Hugues January 12, 2015 at 12:11am

About Panasonic 18650 pack design:

So in my case I would like to build a 4S7P pack. What would be the best "pack" design in your experience ?

I have come up with the following packaging arrangement. Are there other better arrangements and/or advices/pitfalls ?
- MR60
  
  Forrest Frantz > Hugues January 12, 2015 at 9:03am
  
  I've found the interlocking arrangement (A or C) to be the best. The batteries have a protective thin shell for insulation, that does not handle high shear well. Thus your interlocking method helps to remove forces from those bonded joints (the forces at the battery pack are high during a crash). The batteries are spot welded with Epoxy 2216 at every battery contact to ensure that the pack stays together and that the batteries maintain their distance from each other during a crash (don't want to create a short on impact).
  
  To enhance stability:
  
  With A (2 stacks of four rows of alternating 3-4 batteries), i might add a stiff carbon/nomex end plate to protect from shear in the direction of the axis of each battery.
  
  With C (1 stack of eight rows of alternating 3-4 batteries), i add four right sized carbon tubes between the outermost triads of batteries (the outermost holes). This gives stability during a crash in the serial battery linkage and creates a fairly strong structure.
  
  With D [B interlocked] (1 stack of seven interlocking rows of four batteries), ditto the comments of C.
  
  The choice comes down to volume requirements and wiring simplicity.
  
  Building in stacks present a unique problem. When building battery packs with more than 1 stack, one needs to weld a wire across two batteries and then fold the wire in half as the battery ends come together, end to end. To be able to selectively charge one cell, it is important to be able to access all of these joins. Thus stacked batteries should always have two layers. For example, a 20 cell battery can be 2 layers (in series) of 2 batteries end to end (in series) x 5 of those in parallel to create a 4S5P. Thus there is access to every cell. Another tip on those access points is that they should protrude:
  
  - only enough for access and not enough to cause a short with the battery or ground
  
  - in a direction that will not come in contact with the ground during a crash
  
  - in a direction that will not poke through the protective covering during a crash
  
  - in a direction that is accessible to a charger if the cells become imbalanced.
  
  Also, with a photography ship, there is another issue. If you are of the ilk that the CMOS or CCD needs to be located at the center of the ship CG to minimize CCD translocation issues during corrective roll/pitch/yaw motion, then the battery pack needs to go around the gimbal. There are a lot of ways to do this. The simplest, if doable, is to build two battery packs and put them on either side of the gimbal. But, if you have an odd number of cells, then this might not work. See drawing below for one solution to such a design where it is necessary to wrap the batteries around the gimbal to put the CG at ship x/y center and at the plane of the props.
- MR60
  
  Hugues > Forrest Frantz January 12, 2015 at 9:59am
  
  Thx for these useful insights. About stacking, would small magnets be a solution? Like this:http://www.ebay.co.uk/itm/5-x-Battery-Magnet-Spacer-Convert-from-Fl...
- MR60
  
  Forrest Frantz > Hugues January 12, 2015 at 10:18am
  
  let me know what they weigh
MR60

Hugues January 11, 2015 at 10:07am

Here some simulations of future fight times when my CF build will be finished. Obviously I did not go for the lightest that could be because I still want this ship to be realistic for real flights (able to embark some payload like the mini mobius gimbal I'm working on, plus having telemetry and GPS). So my fight times will not be that great.

I wanted to see what would be the optimum Li-Ion pack (how many cells in series and in parallel) to get the most flight time. I will be limited to a maximum of 5S. I'm not limited in prop size by the frame (I made it 1 meter motor to motor). I am however limited in prop size and maximum voltage by the chosen motors, MN3508-380Kv (max 5S for props 16" or larger).

The ship config weight will thus be:

The various battery pack combinations are in 4S and 5S voltages for the following capacities and estimated flight times (from ecalc):

I then traced the graphs for the 4S pack combinations and separately for the 5S packs as follows:

The conclusions :

-for my ship config 4S is better than 5S, in terms of flight time (and less cells to assemble in a pack)

-There is an optimum pack for both voltages in a 9P mode (9 in parallel), or in other words for a battery capacity of 30.000 mah. In 4S, that is a battery weight of 2.1Kg (two times ship's weight without battery).

I think I will limit myself to a 4S7P battery pack because that is going to be expensive...(4x7=28 panasonic 18650 cells to buy)

-Flight times are not that great but I have seen worse

https://storage.ning.com/topology/rest/1.0/file/get/3701910458?profile=original
- Jon > Hugues January 11, 2015 at 10:18am
  
  Pretty impressive.
  
  With going for 4S are you able to predict flight times with 18 inch props?
- MR60
  
  Hugues > Jon January 11, 2015 at 1:20pm
  
  Here 4S with 18x6 props if the motors support it (according to ecalc I see a variation of up to 3 minutes in flight time results according to the brand of props I select.)
  
  https://storage.ning.com/topology/rest/1.0/file/get/3702682405?profile=original
- Jon > Hugues January 11, 2015 at 1:42pm
  
  That looks good. The hover percentage is probably more reasonable.
  
  According to the T-Motor spec 15 inch on 6S the operating temperature is 70C so with 4S I'm guessing it should be less, especially if you are flying it gently with 2C batteries.

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