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Motor Mount Alternative Solution

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Hi Guys,
I wanted to share my experience with you regarding the Inspire 2 motor mounts.
I first became aware of the potential for failure through this forum. Thanks to the community for the diligence to get the word out.
I originally had an Inspire 1 and then an Inspire 2. Once I heard of the glue connection and looked into the catastrophic failures, however rare they may be, I needed to create some peace of mind for my own expensive aircraft.
Full disclosure, I am a licensed professional structural engineer with a Masters Degree in Engineering.
I use my I2 in my structural engineering company for structural inspections as well as with my other photography/videography company for cinematography, photo and video needs. The last thing I wanted was to find out that my motor mount was spinning on the arm, or worse, completely detached from it!
I broke out my tools and disassembled one of the mounts to take a look. Of course, I confirmed that the connection is strictly adhesive without any mechanical attachment. And I also confirmed that the end of the carbon tube is cut in a scalloped configuration instead of square cut.
I wanted to lock the motor mount on at a low cost and with a positive connection. I saw the other solutions out there and each of them had their own merits. So, I am definitely not taking anything away from anyone trying to find cost-effective solutions founded on sound engineering principles with an easy installation. Kudos to everyone taking the plunge to provide a solution.

I, on the other hand, took a different approach, mainly to preserve the beauty of the Inspire 2, while providing strengthened connections of the mounts...
I first did a quick calculation of the approximate stresses on the mount and determined that my intended solution would be many times stronger than necessary.
I broke out my painter's tape, Sharpie, cordless drill, 1/8" diameter bit, rivet gun, 1/8" aluminum rivets and aluminum rivet washers and dove in.
In a nutshell, I drilled through the mount and tube and riveted them together. I used a temporary piece of wood to back up the tube to protect the wires while drilling and, after riveting, I added permanent protection tape inside the tube to avoid wear on the wires.
Done.
Incidentally, I did get the idea from an obscure Chinese YouTube video I found on the topic. It was in Chinese, but I watched and it was clearly understandable.
Once the mounts were reassembled, I dabbed the tops of the aluminum rivets with black paint and voila!

The irony is that aircraft have used rivets since the advent of aluminum in the airframe building process. Here we are again.
 

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Dear Moderators,
I chose to create a separate thread for this so as not to undermine the business efforts of those that are marketing motor mount products for sale.
 
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Hi Guys,
I wanted to share my experience with you regarding the Inspire 2 motor mounts.
I first became aware of the potential for failure through this forum. Thanks to the community for the diligence to get the word out.
I originally had an Inspire 1 and then an Inspire 2. Once I heard of the glue connection and looked into the catastrophic failures, however rare they may be, I needed to create some peace of mind for my own expensive aircraft.
Full disclosure, I am a licensed professional structural engineer with a Masters Degree in Engineering.
I use my I2 in my structural engineering company for structural inspections as well as with my other photography/videography company for cinematography, photo and video needs. The last thing I wanted was to find out that my motor mount was spinning on the arm, or worse, completely detached from it!
I broke out my tools and disassembled one of the mounts to take a look. Of course, I confirmed that the connection is strictly adhesive without any mechanical attachment. And I also confirmed that the end of the carbon tube is cut in a scalloped configuration instead of square cut.
I wanted to lock the motor mount on at a low cost and with a positive connection. I saw the other solutions out there and each of them had their own merits. So, I am definitely not taking anything away from anyone trying to find cost-effective solutions founded on sound engineering principles with an easy installation. Kudos to everyone taking the plunge to provide a solution.

I, on the other hand, took a different approach, mainly to preserve the beauty of the Inspire 2, while providing strengthened connections of the mounts...
I first did a quick calculation of the approximate stresses on the mount and determined that my intended solution would be many times stronger than necessary.
I broke out my painter's tape, Sharpie, cordless drill, 1/8" diameter bit, rivet gun, 1/8" aluminum rivets and aluminum rivet washers and dove in.
In a nutshell, I drilled through the mount and tube and riveted them together. I used a temporary piece of wood to back up the tube to protect the wires while drilling and, after riveting, I added permanent protection tape inside the tube to avoid wear on the wires.
Done.
Incidentally, I did get the idea from an obscure Chinese YouTube video I found on the topic. It was in Chinese, but I watched and it was clearly understandable.
Once the mounts were reassembled, I dabbed the tops of the aluminum rivets with black paint and voila!

The irony is that aircraft have used rivets since the advent of aluminum in the airframe building process. Here we are again.
I posted the YouTube video awhile back, language barrier of course in Chinese so I was stuck.
Did you make a vide to follow? It looks simple but **** a step by step guide in English would be awesome.
I couldn't decipher all the tools he used.
 
Hi Guys,
I wanted to share my experience with you regarding the Inspire 2 motor mounts.
I first became aware of the potential for failure through this forum. Thanks to the community for the diligence to get the word out.
I originally had an Inspire 1 and then an Inspire 2. Once I heard of the glue connection and looked into the catastrophic failures, however rare they may be, I needed to create some peace of mind for my own expensive aircraft.
Full disclosure, I am a licensed professional structural engineer with a Masters Degree in Engineering.
I use my I2 in my structural engineering company for structural inspections as well as with my other photography/videography company for cinematography, photo and video needs. The last thing I wanted was to find out that my motor mount was spinning on the arm, or worse, completely detached from it!
I broke out my tools and disassembled one of the mounts to take a look. Of course, I confirmed that the connection is strictly adhesive without any mechanical attachment. And I also confirmed that the end of the carbon tube is cut in a scalloped configuration instead of square cut.
I wanted to lock the motor mount on at a low cost and with a positive connection. I saw the other solutions out there and each of them had their own merits. So, I am definitely not taking anything away from anyone trying to find cost-effective solutions founded on sound engineering principles with an easy installation. Kudos to everyone taking the plunge to provide a solution.

I, on the other hand, took a different approach, mainly to preserve the beauty of the Inspire 2, while providing strengthened connections of the mounts...
I first did a quick calculation of the approximate stresses on the mount and determined that my intended solution would be many times stronger than necessary.
I broke out my painter's tape, Sharpie, cordless drill, 1/8" diameter bit, rivet gun, 1/8" aluminum rivets and aluminum rivet washers and dove in.
In a nutshell, I drilled through the mount and tube and riveted them together. I used a temporary piece of wood to back up the tube to protect the wires while drilling and, after riveting, I added permanent protection tape inside the tube to avoid wear on the wires.
Done.
Incidentally, I did get the idea from an obscure Chinese YouTube video I found on the topic. It was in Chinese, but I watched and it was clearly understandable.
Once the mounts were reassembled, I dabbed the tops of the aluminum rivets with black paint and voila!

The irony is that aircraft have used rivets since the advent of aluminum in the airframe building process. Here we are again.
Upon releasing the Chinese video there were many heated discussions about drilling through the CF tube. Some experts on CF materials have concluded that structural integrity of the CF tube could be significantly compromised when drilled through so close to the end and especially when the CF tube has already been cut in half. So, it may look like the most straight forward and logical solution but it would not only void the warranty of the craft but maybe even do more harm than good at the end.
 
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@Ftolino aggressive on occasions but I’ve learned to admire your opinion none the less;)
What’s your thoughts on this solution?

@mmarian I appreciate you being straight up and to point also:)
We’re all grown men, if some can’t handle legit questions I can shop elsewhere.

Warranty wise I’m not much concerned. I know others are but I’m only concerned with falling out the sky.

I wonder if the arms are 100% carbon or just coated on the outside. Just my 2 cents after inspection.
 
I wonder if the arms are 100% carbon or just coated on the outside. Just my 2 cents after inspection.
Believe that you are correct in your suspicion. After seeing crashed I2 I tend to believe that tubular structure of craft is made of some type of plastic (ABS, PVC maybe) with single layer of carbon fiber. While this certainly not a bad solution to increase rigidity versus single polymer structure & save costs, DJI advertising of carbon fiber frame is misleading.
That is besides the fact that full carbon fiber components with integrated aluminum tips for connectivity would be more solid engineering solution, especially on the long run.
 
I am much more concerned about the pressure on the tube thru clamps than thru one small hole.
As mentioned from n5101 it is probably not full carbon fiber, it`s just coated and I doubt that simply one hole risks the stability.
I have seen many copters and planes where srcews holding carbon fiber parts together and no one was concerned that the
huge amount of holes is an disadvantage to the system.
Drilled and done correctly this Rivet solution is doing less stress to the tubes and mounts and supports also the existing glue
to "not give up".
I learned from all my engineering lessons that it is better to connect 2 points directly (glue plus rivet (or screws)) instead of try to
hold them together externally.
Personally if I find fitting washers to the outer diameter of the mounts*, I would do it with torx screws and self-locking screw nuts
* So they are straight on one side and fitting to the mount on the other side to gently distribute the pressure.
At the end you need only a minimum of pressure for srews or the riviets, because simply the metal bolt of the rivet prevent any mount
rotation.
The rivet itself or screw-nut is just there to avoid that the bolt is falling out the mount.

If you are concerned about the mounting hole, there are much more modified areas: on each side the cut out and the cut out in the
middle of the tube.

Please don`t get me wrong, I appreciate any opion here.
But as soon someone comes with a simple, very effective and extremely cheap solution, someone comes and complains.
Just the bolt of the rivet is the proof that it can not rotate anymore, no assumptions here, just mechanical impossible.
So far I have seen no proof from the other systems that they are doing their job in case the glue fails, may be I missed some positive
report here, but currently they all assume they can prevent a motor rotation good enough to not fall out the sky?!
On the other hand: If the holding mounts create to much pressure and therefore stress on the tube they can create another more critical
point of failure than a small 3mm/ 1/8" hole. Keeping asside that the massive cut outs at the ends and in the middle are not even more critical.
As I said before, even then it seems the tubes are not raw 100% CF, instead of a mixture of ABS(?) and CF, which is not that sensible to cut
outs and holes.

Cheers!
 
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HI guys, I am glad to stumble upon this thread. A lot of information can be carried over from my original thread.

To credit myself, I am also a Mechanical Engineer (not Masters though, thats true dedication XD) and have been studying/reviewing and designing this motor mount issue.

I believe there has been a lot of misconceptions of this issue with the Inspire 2, from my observations from the community.

First, the motor mount failure rate is likely in large part of the poorly designed props and attachment features from DJI. They simply are not well thought through (apparently polymer shrinkage from the molds and interference fits were not given the correct tolerances in design) which leads to unwanted vibrations. The more the motor vibrates the more the epoxy bonding the motor mount to the arm has to try to translate that vibration to the carbon fiber tube. One fracture that forms in the epoxy WILL ultimately lead to failure in the jointing of the two. So first recommendation is to ditch the stock props. Buy either QUALITY folding props or my preference - T-Motor 15x5 standard props. (I prefer these as you can be sure to get a 100 percent perfect balance, vs folding props)

Second, where did the assumption of the carbon fiber arms to be a partial composite carbon fiber tube? I will dig into this more to confirm, but from my previous large Ardupilot building experience, these seem just as strong and rigid as full carbon fiber tubes, actually of higher quality (very tight and consistent plain weave). I doubt DJI would do this minor cost savings too, as only exterior lined carbon fiber tubes would weigh significantly more than standard full carbon fiber tubes. Like I said this is all assumption though, I will try to confirm this.

As for the Op's solution, I fear this is not a good method for a few reasons (not to say I am right or wrong, just from my observations and analysis);

  • You are introducing a potential of creating a fracture in the epoxy while drilling into it. Like I mentioned the vibrations will case that fracture to get much larger.
  • A fastener is meant to fasten to a flat surface. You are making it fasten to the bent surface. I personally do not think this a good approach, as you are going to create to high-pressure points onto the carbon fiber tube. Carbon fiber is very strong and stiff, but can fracture easily to have those traits. Even if you didn't while assembling it, those constant vibrations can easily "dig" at the carbon fiber tube and begin to fracture the material.
  • Lastly, you mentioned the worry of twisting off. In my thread I also mention how I feel twisting is not the motion of concern. More of a hinging motion in the direction of the force from the propeller is a larger concern. Which this remedy helps a little bit, but introduces a high percentage of risk in terms of fracturing the epoxy or carbon fiber tube.
I must say I do really enjoy everyone getting on board with this issue and looking at it from all angles, and am glad to join the convo :)

Regards,
Dustin
 
Interesting thread, but all quiet theoretical engineer assumptions...
WIth some craftmanship and hands-on material experience this is a good solution.

Though you should also solve the vibration issue itself.

Some tips:
Drill from the inside if possible and pre-drill with a 1mm. and support to avoid shooting through.
Choice of rivet/riveting tool is also important, add a washer (preformed) on the inside (to avoid puncture/crunching the inside of the tube)
Clamp the rivet gently as the tube wall is thin and Sandwiched material, you might add a little epoxy glue on the rivet (inside) to increase the clamp surface.

Good luck diy's
 
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Hi all.
Just wanted to share some toughts on this matter.
We are an operator since 2012 flying mainly on the power grid in norhern Norway. Rough conditions during all year operations.
Until now we only operate custom built Vulcan UAV, and these machines have an almost perfect reliability record with us.
Certified for building and testing our own machines.

We got an Inspire 2 to check out the possibilities especially in video making, and also just to get some experience with the ready made drones.
I must say that the Inspire has some serious capabilities especially for video, not so much for stills compared to the vulcans.
The Vulcans can also do great video, but the gimbals are more of a hassle (Gremsy?s mostly)

However, the build quality on the Inspire, and especially the motor and propeller mounts are nothing but "toy quality" compared to the Vulcans.
I would never use a stock Inspire for operationall purposes, period.

So currently I have upgraded to KDE direct folding props, witch we have lots of experience with, they are great.In the process of reinforcng the motor mounts, where "part 1" is to lock the alu mount to the CFK tube, the next part will be adding braces to the mount/ tube.The way DJI has mounted the motor to the arm is horrendous from an engineering standpoint, especially removing half the tube diameter where the alu mount is glued on.
From my engineering background, I feel that using 3mm bolts as shown is a good solution, but addistional bracing is required to finish the process.
Loosing some flight time is no concern to me, neither is the estethics, this is to be a working tool.
We are used to working with machines without warranty also, we fix ourself, but for others this might not be a good soulution as you surely loose the warranty over this.
The modifications also needs thorough testing from our side before entered into service, but so far soo good!

For comparison, check out the picture of a Vulcan motor mount, witch is structurally almost perfectly designed.
20180925_192155.jpg20180926_155346.jpg20180926_160916.jpgPresentasjon1.jpg
 
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