X8 Octo Quad build log & maiden flight

Here is a short build log of how I converted my APM2.5 Quad to a X8 Octo quad.

First I dismounted everything which is always a pain, especially when you have to disconnect these little bastard DF13 connectors. By doing so a few times already (it is my fourth build with same APM...), I had finally to replace the telemetry cable by a new one. Oh well not too bad...

I could scrap the base 3DR power distribution board which was not meant for 8 ESCs and I got instead a vulcanUAV PDB which allows for (way too) many ESCs and 250 amps as seen on this picture.

I soldered also a 3DR power module and connected a Y battery splitter to be able to connect two batteries in parallel.

I prepared the motors. These are 3110-17 (700Kv) tiger motors. I love 'em. As you notice on the picture, I took my lessons from the first builds : every part is now tagged, either with a number, either with a color sticker, in order to know what motors goes on which arm and which cable connects to which ESCs (and so to keep the correct spin direction).

These motor cables are quite long so I decided to braid them, giving an extra EMI improvement and shortening them a bit:

By dismounting the quad, I noticed that the four screws holding the motors to their motor plate came a little bit loose, due to vibrations. I do not want on these parts to use blue loctite because it is a a nightmare to unscrew (for my next fifth build...). I used instead some hot glue as seen there:

(yeah, I know, I had to mix different screws, not pretty but who cares, nobody sees the bottom motor plate anyway...)

Then I laid on the floor the bottom plate, passed the braided motor cables in the arms, connected all the motor cables to the ESCs (thanks to the color stickers), placed the cables as neatly as possible. I shortened the ESCs power cables to the minimum in order to gain a clean placement. I soldered directly the ESCs to the PDB (no risk of bad connectors). By the way, a lesson I've learnt when you solder so much on a board : get a very high power solder iron otherwise the heat latency of the board gets unmanageable (and gets worse for every ESCs that is added).

I decided to keep only one BEC power cable from one of the ESCs. The other are snugged under the PDB (No, I did not cut them, you never know what will be needed in a next build)

Then it was time to work on the APM itself and all of its accesories : receiver, beeper, telemetry, GPS, etc. In order to get vibrations as low as possible, I tried what you see on the picture : APM is mounted on a small plate about 3cm above the center plate, with rubber dampers-white nylon spacer-silicon damper (what you see between the two metal washers).

This carbon fiber plate is thus never in direct contact with the rest of the frame. APM itself is placed on four corners of moongel (I put a balse plate between the carbon plate and APM to raise it a bit so that APM does not touch the metallic washers).

APM is pressed gently on the moongel with two rubber bands. Later on, after I attached the GPS, I also improved a bit the vibrations levels by adding moongel between the GPS on top of APM and the APM case, so that the APM is sandwiched between two layers of moongel.

This is how the Electronics look once finalized and all cabled.

Then I took the beast out for a short maiden flight as seen in the video herebelow:

To some extent, but you are ignoring the pitfall of an x8 system, which is that in practise the system loses efficiency at a rate greater than 30% - actually in all practicality my experience shows that with larger props (14 ish) efficiency loss is close to 50% as a result of inherent aerodynamic disturbances (a fact I am sure you are aware of). What's worse is this actually scales with rpm in a similar manner as logic would suggest.

In terms of increasing the weight proportionally we must also consider the general loss in efficiency is two fold - we are consuming more current, and in doing so lowering efficiency in terms of general turbulence and high pressure air being "regurgitated" in opposition to the rest or low throttle state. What this means in terms of our global work output is that we must factor for larger more heavy, by virtue of their nature, batteries - and to a lesser extent the motors themselves.

In terms of advantages you mention a theoretical hover at 70% throttle, in an x8 configuration this is completely pheasable, as a result of coaxial stability and a lower individual rpm contribution on a per motor basis to attitude correction. I have learned the hard way that this is not the case with x4 configuration, which even at 50% throttle, admitting characteristically short arms are present, a 10 lb ish lift can cause vortex destabilization in decent resulting in some not awesome stuff.

In short you are more or less correct - weight by nature of efficiency in a setup like this increases at such a high rate that if losses like I have experienced in reality propagate - will result in a "proportional" or marginally better set of performance characteristics (in my experience 15-20%).

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Comments

The Sun July 15, 2013 at 2:50pm

@Chris

No it could not.

If you had both props rotating the same direction it would work, but with CR setups it would not.

This is because that the inertia generated by swinging the props in a specific direction generates the force to yaw the copter, in a quad accelerating CCW motors yaws CCW and vice versa.

In an X8 the principle is identical, but you have props swing two directions on each arm, which means in order to yaw you need to decelerate/accelerate 4 motors on alternating sides of the arms, but one on each of them (the main reason for inherent increased stability in CR systems).

Thus if you had both on one esc decelerating both would just make it drop the arm, and you would have effectively no way to yaw.

Hope this helps.
Chris July 15, 2013 at 12:58pm

Thanks Hugues. Totally understand, I just wonder with the configuration of two motors mounted vertically, whether having identical speeds would cause a stabilization issue. For all intent and purposes the X8 configuration appears to be just a regular quad, so could the micro controller do it's job ok if it thinks it is dealing with just 4 motors?
MR60

Hugues July 15, 2013 at 12:23pm

@Chris, The purpose of an ESC is to control the rotation speed of a motor (propeller on that motor). The propellers's speed is managed individually by the microcontroller (APM firmware). So if you only have one ESC for two motors, assuming the ESC can deliver the amps for the two motors, the speed of these two motors will always be identical, which is not necessary what the microcontroller wants to stabilize the quad.
Chris July 15, 2013 at 9:08am

OK, I am a little new to all of this, but I am trying to learn and have many questions and ideas. I have seen someone connect two motors with one ESC just for demo purposes. My question is would this be possible and advantageous for an X8 set up? Apologies in advance if this is a dumb question or already answered.
MR60

Hugues June 25, 2013 at 1:41pm

sure I'll post some results as soon as I can
The Sun June 25, 2013 at 11:54am

That's interesting about the prop sizes, and yeah the coaxial setups are pretty damn neet in that sense. As for flying with the gimbal I would assume it will fly very well, x8s are well suited for videography setups.

You'll have to post some videos of your results at some point, I look forward to seeing them.
MR60

Hugues June 25, 2013 at 10:25am

Yes I am aware of the efficiency loss versus a true Octo. However based on a recent post of tests done by 3D Robotics (that they themselves read from a university paper, don't remember which) you can optimize the efficiency loss by placing 10% shorter diameter props on top versus bottom props. Thus in my example : 11" on top and 12" on bottom. Doing this has apparently proven a drastic current consumption reduction and a minorization of the efficiency loss. On that last point I've seen numbers from 10% to 25% so I can't really say.

Anyway I fully agree with you one the stability advantage of the X8 versus the only other config I've known so far the X4. It is immediately perceptible and it resisted quite strong gust of winds way better than my quad did (the quad was the same only with 4 motors instaed of 8 so the comparison is valid)

I still have to order and install my compact camera and brushless gimbal; I hupe this will fly ok...
The Sun June 24, 2013 at 1:21pm

To some extent, but you are ignoring the pitfall of an x8 system, which is that in practise the system loses efficiency at a rate greater than 30% - actually in all practicality my experience shows that with larger props (14 ish) efficiency loss is close to 50% as a result of inherent aerodynamic disturbances (a fact I am sure you are aware of). What's worse is this actually scales with rpm in a similar manner as logic would suggest.

In terms of increasing the weight proportionally we must also consider the general loss in efficiency is two fold - we are consuming more current, and in doing so lowering efficiency in terms of general turbulence and high pressure air being "regurgitated" in opposition to the rest or low throttle state. What this means in terms of our global work output is that we must factor for larger more heavy, by virtue of their nature, batteries - and to a lesser extent the motors themselves.

In terms of advantages you mention a theoretical hover at 70% throttle, in an x8 configuration this is completely pheasable, as a result of coaxial stability and a lower individual rpm contribution on a per motor basis to attitude correction. I have learned the hard way that this is not the case with x4 configuration, which even at 50% throttle, admitting characteristically short arms are present, a 10 lb ish lift can cause vortex destabilization in decent resulting in some not awesome stuff.

In short you are more or less correct - weight by nature of efficiency in a setup like this increases at such a high rate that if losses like I have experienced in reality propagate - will result in a "proportional" or marginally better set of performance characteristics (in my experience 15-20%).

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MR60

Hugues June 24, 2013 at 10:35am

Based on the calculation/simulation results of http://www.ecalc.ch/xcoptercalc.htm?ecalc&lang=fr , with my config:

-20 amps ESC

-Tiger motors 3110-700Kv

-12x4,5 inch props

-Weight of craft with 2x5000mah batteries =3692 g

-large camera and gimbal payload=1000 g

Total AUW = 4692 g

I would hover at : 51% of throttle

Thus this give me plenty of reserve for even theoritical even higher payload (although I counted already 1000g of camera and gimbal paylaod which is already way above reality).

For example with a camera+gimbal of 1500g instead of 1000g (50% heavuer thus), I would hover at 58%.

If theoritically we say we can fly with maximum throttle at hover of 70% (to keep some flight maneouverability), then I could embark a net payload of maximum about 3000g (or 3Kg, or 6 pounds).

If you do not see an increase of lift capacity between your quad and X8, it means you increased the weight proportionnally ?
The Sun June 24, 2013 at 12:37am

Braiding may help a little, but the actual em fields are generally so small that its relatively irrelevant.

I will be interested to see if the x8 really helps your lifting power that much, personally x8 has always been my favorite (non x4 or y6) configuration - as a result of coaxial stability and redundancy - but honestly I never really saw much of a carrying weight increase, and the increased current requirements never sat well with me.

For heavy lift stuff I always end up going back to the traditional quad - with huge motors and props and massive arm lengths. For filming my favorite has always been y6. It has all of the advantages that an x8 has in terms of stability and redundancy, despite being mildly more efficient.

Anyway keep up the good work.

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