Building a Better Quad Frame

Coby · April 25, 2018, 8:22pm

Yes, mud and water can be a driving factor:

https://twitter.com/BoxBotix/status/741616763922571266

In relation to the topic of this thread it is essentially a monocoque frame. Also a good lesson in not prioritizing the right requirements e.g. water/dustproof modularity over just about anything else, which lead to weight gain and increased complexity for no/little gain to the end user in real world use. Which is why I preach about design in context and why I think @rob215x should continue to update his requirements.

I don’t see mention of being resource constrained with no shop, which has already driven a huge choice of using wood. So, IMO, as this thing evolves, so should that statement.

rob215x · April 25, 2018, 10:50pm

I appreciate all of the feedback!

Actually, my blog already has that constraint. I might modify the original statement. I haven’t quoted myself before but here goes, lol…

So yes, I agree with you on that point!

Coby · April 26, 2018, 12:14am

@rob215x Really like what you are doing here and I don’t want to be pedantic, but designing a frame to be built without a shop has a huge influence on the design and ensuing discussion. I think it’s a big enough deal it should be mentioned from the start:

To design and build a better quadcopter/X8 frame that is open-source, light, strong, versatile, and is especially suited for DIY builders who have a very limited budget and only basic tools.

rob215x · April 26, 2018, 10:05pm

@Coby I just updated my initial blog post. The Purpose statement has been revised with your suggested text because it was always about that anyway. I added an initial statement to the Contents, and I added a final item to the Project Goals (essentially moved from the original Purpose statement.)

Thanks for your suggestions.

rob215x · April 27, 2018, 3:51am

09 - Nylon Bolts? Really??

Based on the responses to my previous post, I thought it would be a good idea to include the following disclaimer about using nylon bolts with your motors or motor mounts…

***DISCLAIMER***
If you are unsure about using nylon bolts to mount your motors, don't use them! If you are building anything heavier than a 2.5kg drone, don't use nylon bolts on your motors! If your payload is bigger than a GoPro, don't use nylon bolts on your motors! If you're not going to check your motor bolts before and after each flight, don't use nylon bolts on your motors! If you are unsure of the source and quality of the nylon bolts, don't use them on your motors!

Okay, now that I’ve warned you, nylon bolts DO work. This is NOT just theory. This is experience. I have over 100 flight hours using nylon bolts to secure the motors on four different frames, using motors from 1100kv to 380kv, with 8", 9", 10", 12", and 15" props. I have never had a single failure! (except during crashes, when I WANTED them to fail!! More on that later!)

That is a custom frame I built in 2015 out of basswood and carbon fiber to replace a heavy QAV400 frame. It has 1100kv motors and 9" props. The motors are all held on with nylon bolts! If you watch the video, I push it pretty hard in my tests. I did a dozen more tests before that video, in various stages of tuning. I also did speed runs at 65mph. Not one time did a single nylon bolt fail or come loose. I checked them before and after every flight.

Here’s the frame from that video: (If you want the plan for that frame, Imk and I’ll post it!)

Here’s a closeup of a motor mount:

This was the first version of my motor mount on a previous frame. The arms are 10mm OD braided carbon fiber tubing from Dragonplate. The motor mounts are made from carbon fiber / balsa sandwich panel. The bolts are 3mm nylon. One of my friends built something similar, and was also using nylon bolts. He had a motor mount fail because the mount peeled away from the tubing. The nylon bolts did not fail. Anyway, when I heard about that, I thought it would be a good idea to improve the design a bit.

Here’s the improved design. I simply added a piece of 1/32" (0.79mm) carbon fiber plate to the bottom of the tube. I had to use longer nylon bolts. You can see the bottom plate flexes a bit and that might have helped prevent the bolts from coming loose. However, I never had any nylon bolts come loose on the previous frame, or on my next two frames.

What Happens in a Crash??

Watch this short video and you’ll see a 140 foot free fall and crash.

Yes, I crashed into a recently mowed grassy field. But still, it was a crash after falling 140-145 feet. Nothing on the frame broke! None of those lightweight motor mounts broke. The receiver antenna mast and the GPS mast broke. The video transmitter antenna mount broke. Three of the motors were ejected from the mounts because the nylon bolts sheared on impact! This is why the frame arms didn’t snap. This is why none of the balsa / carbon fiber motor mounts broke.

The physics is pretty simple. You have a large, heavy mass (motor and propeller) on the end of a lightweight tube. When you hit the ground after falling at a rate of 9.8 m/s/s (acceleration of gravity), the momentum of each motor mass has to go somewhere. Something is likely to break. I would rather it be some nylon bolts than my frame or motor mounts.

I was visiting a friend in Virginia so I didn’t have much with me for repairs. I got some wood dowels and popsicle sticks at a local craft store and made new GPS and receiver antenna masts. I put on my spare props, put in my spare nylon bolts, and I was flying the next day. I never did figure out why I crashed. I think the battery chemistry failed because it wouldn’t charge properly afterwards. But, the log doesn’t indicate the battery failed so its still an unsolved mystery.

I still have that frame. It still flies great. It still uses nylon bolts.

Load Testing and Vibrations with 15" Props

Here’s a 650 class frame with 380kv motors, 15" props, and 6S battery. Everything was working but the motors were getting really hot. I eventually traced it to a defective Pixhawk clone. But it took dozens of tests, both on a stand and outside. The motors are attached with nylon bolts and even with all of the vibrations and high torques on the stand, I never had one bolt come loose.

Once I eliminated the defective flight controller and got a Pixhawk CUBE, it flew well and I concentrated on the gimbal and a picture-in-picture video switcher. My flight times ranged from 30 to 40 minutes, and I flew many, many test flights. I never had a nylon bolt come loose. This frame uses lightweight 3D printed mounts that are 7g each. The nylon bolts seat directly against the mount. They do not have a flexible side like my previous mounts.

Finally, about a month ago, I was careless. I was in a parking lot, doing some PID fine tuning. The ship was in Loiter, about 15 feet up. I should have had a friend holding the controller but I didn’t. I was going back and forth between Mission Planner on my laptop and my controller. By accident, I brought it down suddenly and hit the pavement HARD! Two motors ejected and one was loose but still hanging by one of the nylon bolts. One of my landing gear legs, which was attached to the end of a rear arm, caused some damage to that arm. Three of the props were slightly damaged but enough not to use them for flight. Nothing else broke. None of the lightweight, 3D printed mounts broke.

Lesson learned- Let someone else be the pilot while you send PID parameters from Mission Planner!!

In conclusion, I like using nylon bolts. They have proven themselves continuously for almost 4 years and hundreds of hours combined test stand and flight time. Still, don’t forget my disclaimer! You don’t have to use them.

Hugues · April 27, 2018, 8:57am

The advantage I see for having a programmed break point at the motor/frame junction, thanks to nylon bolts, is that in case of crash or in case your hit an object, the motor/prop will not be instantly stopped which will safeguard the ESC and the rest of the electronics and wiring upstream. Why ? Because the autopilot will react to the non attitude correction response by throttling up to the max, driving amp draw to the max and eventually frying your ESC(s). Unless you have an emergency motor brake which is software activated, and fast enough when crashing.

Let’d do some maths,

The tensile strength of injected nylon parts is about 100 MPa (or Kgf/cm2).
Usually nylon bolts are M3 diameter or 3 mm in diameter.
So we can calculate the tensile strengh of one M3 nylon bolt, i.e.: 3.141516 x 0.15^2 = 0,07068411 cm2
Thus the max weight that a M3 nylon bolt can sustain is 100 x 0,07068411 = 7Kg

If you bolt your motor mount with four M3 bolts, that is a max supported weight, per arm of your drone of 28 Kg.
So four arms = 112 Kg
I think thus we are super safe using nylon bolt on a 650 class ship that weighs in the 5Kg range.

fnoop · April 27, 2018, 9:52am

This is a logical fallacy, you cannot infer your conclusion from the events. I’ve had many crashes onto both soft and hard ground from significant heights, and I’ve never had a frame arm snap or even damaged, nor have I ever had a frame broken/snapped/damaged beyond some surface damage. Following your style of reasoning I could infer that my frame and arms are a new magical design that is unbreakable under any circumstances, or that the colour black protects against any damage.

It does (momentum), it goes into the ground - particularly if you land on organic surface - anything softer than concrete. Yes, some will transfer across the arm but by the time the nylon bolts shear (and as @Hughes points out they are very strong) then a significant amount of energy will have been transferred down the arms anyway.

Excellent point. However, if you look towards motorsports as good example, they go to extreme lengths to contain rotating devices (wheels instead of props/motors) in the event of a crash. It could be incredibly dangerous to deliberately design a set of fast rotating motor/props to fly off in random directions in the event of a crash - if anyone or anything got in their way the injuries could be very serious. I would rather have the motor/prop stay firmly fixed to the crashed craft and risk damage to a few $$ worth of bits. Arducopter has crash detection which shuts the motors off in the event of a crash and has always protected my electronics from crashes - i’ve had a single DJI motor burn out in all my crashes.

These are really interesting points and facts however, I’ll certainly look more closely at nylon bolts in the future. If they are a significant weight saving then worth looking at, if they’re not overall then for me it’s not worth the risk.

rob215x · April 27, 2018, 11:02am

I’m really not fond of logical fallacies and I try to avoid them whenever possible. I really don’t want to argue back and forth on this blog but I’m going to defend my position. First, I’m going to present an important definition for the benefit of young people who may read this blog at a later time:

hy·poth·e·sis

noun
a supposition or proposed explanation made on the basis of limited evidence as a starting point for further investigation.

My hypothesis is that one or more of the arms or motor mounts on that specific frame would have broken in that crash if I had not used nylon bolts. This is based on the following facts:

The arms were made of very lightweight, unidirectional carbon fiber tubing which is 7mm in outside diameter. See my above photo of the frame that weighs 94g. This tubing fits inside sections of 10mm braided tubing that I used on the previous frame.
The motor mounts were made from very lightweight, balsa/carbon fiber composite that I have managed to break in previous experiments.
There was enough force present in my crash to shear 12 nylon bolts, so it didn’t all disappear into the ground.

I hope this makes it clear that my previous statements were not logical fallacy at all, but were suppositions clearly based on factual evidence. I am very happy to be corrected when I’m wrong. I ask everyone to check my math because I make mistakes, but please be constructive when commenting on this blog. I am here trying to help people.

Thank you.

Hugues · April 27, 2018, 3:48pm

Indeed but you can also hit an obstacle which blocks or slows down your prop, without a crash detection happening, which happens the most in my experience.

Coby · April 27, 2018, 9:55pm

Hope my blabbering helps. I don’t get out of the shop much, which can lead to long comments at times.

I think your big warning about nylon bolts is spot on. They can be used, and as hugues points out are strong enough, but they do add different risks compared to steel or aluminum. I think the biggest risk you will find for people new to righty tighty lefty loosey is that they are very easy to either cross thread or over torque. If they do not break in this process they can be left in a less than great state, which is very hard to visually inspect. Your big bold warning covers that I think, so as long as people are educated on the risk/benefit I say have at it.

CLabeck · April 27, 2018, 9:58pm

I had experimented with lighter than steel screws/nuts at one point in time in an attempt to save weight by all means…

…I did not go for Nylon but instead for aluminium screws/bolts.

i tested 7075 (aircraft aluminium) and got decent results but the screws were prohibitively expensive. i then tried custom made screws from 6061 aluminium made in china - with discouraging results

The result was that a) I could only tighten screws to <0.4Nm max ( or the screws threads would give in) and with that mini tension I got very significant vibration issues. the ship at that time was 4kg TOW with 18" props, all CF frame.

i switched back to steel screws,and all vibration issues were immediately resolved…

I can’t speak about small/light birds - and my setup was 100% CF parts - so a different setup - but for the bigger birds 5-15kg my personal experience is you get best stiffness (=least vibrations) with steel screws/nuts (at,least,at M3 or M4).

fnoop · April 29, 2018, 6:29am

Hi @rob215x , sorry if you feel my comments are unfair on you (and @Hugues). My intent is not to belittle or be negative to try and make myself sound clever - in fact one of my pet hates is when people troll forums beating on others who are trying to be constructive and contributing knowledge. I’m a very literal person, so I’m simply discussing the interesting points brought up here but I can be a bit Sheldon-like sometimes. Apologies for any offence caused (unintentionally).

I do stand by what I said though, from my point of view, and I don’t think there’s anything wrong with constructively discussing and disagreeing with what others post in public - it’s important if there are potential problems or errors for others to see that. I also know next to nothing about hardware, so happy to accept I could be completely wrong. I do know however that designing heavy dangerous objects to deliberately ‘rapidly disassemble’ in a crash is a terrible idea from a safety point of view. There are many documented incidences of injury and death from this kind of issue - if you look at almost all vehicle engineering they are designed to deliberately contain and self-damage in the pursuit of safety.

I am interested in what you are doing because I think there is a huge disconnect between the great software we have at our disposal and the lack of similarly great hardware for the end-user to get flying with. I have also followed @fffrantz and @Hugues projects with great interest over the years. They have done some quite brilliant work and produced really great results, but with limited practicality/reproducibility. I am still searching for practical, easily reproducible recipes for great hardware, which is what brought me here. My points/questions here is in pursuit of that, trying to point out or find out more about the areas where I see others have gone wrong in the past.

Good luck for your build/project, it has some great ideas.

rob215x · May 1, 2018, 5:50am

10 - Packing Material and Drone Design

As I’ve mentioned, this blog is for people who want to build their own quadcopter frame with inexpensive, easily sourced materials and minimal tools. I also want that frame to perform really well because its the same frame I’ll be using for my photography business. I’m trying to bring new people to the Ardupilot community and hopefully this blog will become a good resource.

For any of you building your own frame for the first time, I’ve been experimenting with many different materials in order to save you some time and money. Physics theory is great but my posts are based on results from trying things in real life. I like using math to explain why something works but there are times when you just need to build it and see what it does.

The other day, I bought some outdoor patio furniture from Home Depot. The box had some packing material made completely out of brown paper, like they use for grocery bags. I was amazed by how strong and light it was. One piece seemed like a good size for a 680mm quad frame (like the one I’m currently building in this blog). So, I weighed it…

The overall dimensions of this piece are 94mm wide x 60mm tall x 52cm long. There’s a single layer of brown paper on the top and bottom, with strips of paper arranged in a honeycomb pattern in the middle. Next, I clamped it into my torsion test stand… (the video says 38cm but that’s the distance from clamp to clamp- it was too long for my test stand)

Amazing results!! Its almost as stiff as my “X” frame and certainly better in torsion than any other frame I’ve tested so far, either commercial or DIY. But how can I use it? I can’t just stick tubes in it. Its made out of paper.

I think the design principle is what’s important here. How can we use this design to make a stronger, lighter frame with other materials?

Today, I went to my local arts and crafts store. I wanted to see if I could find some lightweight material I could put between two pieces of birch plywood to make something like the amazing packing material. I returned home with some blocks of craft foam. Its the stiff, hard foam that’s usually green or white and is used for flower arrangements. It comes in solid round balls, cones, and blocks.

I cut 2 pieces of birch plywood 100mm x 300mm x 1.5mm (1/16"). I had 2 different sizes of foam blocks, 30mm thick and 48mm thick. The 30mm didn’t feel stiff enough so I used the 48mm block. I weighed the pieces…

Its slightly heavier than my Drogon v2.4.6 frame body…

Note: I use two different scales. I compare the readings from both to ensure my measurements are accurate. The silver scale is more accurate for very small things because it measures to 0.1g. At anything over 20g, both scales show the same integer value.

Next, I glued the pieces together with 3M 6070, an adhesive made for foam. Once the glue was dry, I weighed it again and it was 120g. Finally, I mounted it in my torsion test stand…

Well that was interesting! Although it isn’t nearly as stiff as the paper packing material, it is slightly stiffer than my Drogon 2.4.6 design. I’m going to stick with my current design for now, but I think this experiment was definitely worth the effort. Here’s what I learned:

A honeycomb center structure glued between two sheets of material, make a very stiff structure. The material on the top and bottom doesn’t even need to be stiff by itself. It could be a thin layer of veneer or carbon fiber. This is essentially how Nomex is constructed.
Craft foam isn’t as good as the paper honeycomb for a center structure. Maybe there are better types of foam out there for this purpose. But for now, I’m trying to stay away from exotic materials that are expensive and hard to find. Nomex is expensive!
Looking at the math/physics of this experiment, increasing the height (or more accurately, the area) of a frame’s cross section, greatly increases its stiffness. Switching from the 48mm thick foam down to the 30mm thick foam, I can feel the lack of stiffness just twisting it by hand.
Is anyone wondering how much it would weigh if I used carbon fiber instead of birch plywood? For the following weights, I’m adding 6g for glue since I know that’s what I used to make the one I tested. If I used 1.0mm carbon fiber, it would actually be heavier at 144g. If I used 1/32" (0.79mm) carbon fiber from Dragonplate, it would be 102g. I also checked a piece of 0.5mm carbon fiber plate I got from eBay and the total came to 103g. If I used 1/32" birch plywood, it would be 92g. Given the cost of carbon fiber, I’ll keep using birch plywood for my prototypes.

Stay tuned for more experiments! I have many things planned. I will add motors and props to something soon!

james_pattison · May 1, 2018, 10:16am

My inner engineer is having a little happy dance. I read your first post in this blog AGES ago, and hadn’t come back. I’ve been missing out. Love this thread!

MantaRay · May 14, 2018, 7:52am

@Hugues I think your calculation is off by a factor of 10. 1 MPa to kgf/cm2 is 10.2

rob215x · June 24, 2018, 12:56am

11 - Drone Frame Calculator

Its been a while since my last post but I have a LOT of stuff planned so I wanted to give everyone an update. Building frames from scratch, using a variety of materials, can be time consuming. You’ll buy some materials that seem like they could be lighter but when you cut the pieces and glue them all together, you may have only saved a few grams overall. Worse, your frame might even be heavier. Some people fill notebooks with measurements and weights of all the different parts, while others put everything into an Excel spreadsheet.

I don’t really like either approach, plus I wanted something to do the math for me…

I wanted to make a tool to help me calculate and keep track of all the weights of my frame parts.

What I thought would be a simple project, has turned into something much more complicated. Sound familiar, LOL? Anyway, I’m pretty far along with this new tool, and I’d like to share a preview of it…

Introducing the Drone Frame Calculator

I’m sure many of you have used eCalc and if you haven’t, its a great tool for calculating which motors and propellers to use for your drone. My tool serves a completely different purpose. The Drone Frame Calculator or “dCal” helps you compare different materials for building drone frames. It lets you store multiple lists of parts and weights online, so you can compare them against each other, or against your friends’ designs. And, it lets you calculate the weight of any size piece of material, if that material is listed in my database. I’m currently filling the database with hundreds of materials, from carbon fiber plate and tubing, to steel, aluminum, and nylon bolts, to plywood, balsa, foam, and carbon fiber composite sandwich materials.

An Example of dCal’s “Irregular Shape” Mode

Here’s one example of how you can use the Drone Frame Calculator:

Here’s the top plate from my H4 Alien 680mm quadcopter frame. It’s a fairly complicated shape. It would be difficult to estimate what that exact piece would weigh if it was cut from a different material. This is where dCal comes in handy.

As you can see, the original piece weighs 65 grams. Its made from 1.5mm carbon fiber plate with a matte finish on both sides. The material looks exactly like the 1.5mm carbon fiber plate I bought from GetFPV. The thickness measures exactly the same.

In dCal, I clicked the “NEW PART” button and selected “Irregular Shape” mode. I selected 1.5mm Carbon Fiber plate from the Materials drop-down menu. I entered 65g in the weight field, gave my part a name, and clicked “Save”.

Next, I made 2 copies of the part. On the first copy, I selected “1.0mm carbon fiber plate” from the materials drop-down menu and dCal showed me what the new weight would be, if I cut out the same shape from the new material.

The third example shows the weight if I cut the same piece out of 1/16" birch plywood. It would weigh less than half of the original!

More to Come…

I’m still building and testing this thing. It’s not quite ready for the public but its getting close and I wanted to share what I’ve done so far. This will be a FREE utility. I might run some Google ads to help with hosting costs but its running on my company’s server for now. I have many features planned, and I hope some will be influenced by this community, so feel free to pm me.

You can also follow my dCal blog

rob215x · June 30, 2018, 12:51am

Another Example

Here’s another example from my dCal app (Drone Frame Calculator). Earlier in this blog, we discussed steel bolts vs nylon and aluminum bolts. A common size bolt in many small to mid-size drone kits is M3 x 6mm. An H4 Alien quadcopter kit uses up to 96 of them, depending on the version and if you install all of the parts.

In dCal, I can just select different M3 fasteners from a drop-down list and enter the quantity. dCal instantly gives me the weight of my bolts:

Quadzilla · June 30, 2018, 4:41am

There is one more option milling 3D frames subtractively. if you can cut 2d on a cnc you can cut 3D body’s breaking a arm requires you to mill another part for a few bucks and mill time once the patterns are developed. I don’t think its not in the realm of the DIY hobbyist. Some of the advantages are also low GPS interference low vibration good protection of FC and esc’s. just my 2 cents.

rob215x · July 1, 2018, 3:15am

I agree. You can also get 3D parts printed locally now in many locations. I’ll be exploring both of these options in future posts.

rob215x · August 6, 2018, 11:48pm

More dCal Stuff to Help Build a Lighter Quad

As you might have noticed, I haven’t had a lot of time to work on this blog but I’ve been busy in the background, making tools to help accomplish the original goal - to design a better quadcopter frame, suited for DIY builders who have a limited budget and basic tools.

I’ve been adding materials and components to the database in dCal, my Drone Frame Calculator project. For example, I’ve added a fairly good selection of FPV cameras, by weight:

Visit this page here: https://design215.com/dcal/u/examples/camera-weights

I’ve also added a very simple flight time calculator when you know how many amps your drone uses to hover. The page has additional information for novice builders:

Visit this page here: https://design215.com/dcal/toolbox/flight-time-amps

I still have a lot of work to do on this project and I welcome any suggestions or requests. I’ve drawn up my next frame design and I’ll be using dCal to estimate its weight before I start building it. I’m going to be making a series of short videos demonstrating the build of this frame and hopefully it will accomplish the goal of this blog.