How to build a 120 gram Acro Quad that can throw 16" props.
Sometime, weigh your quad frame (everything not electronic). It will probably weigh more than 500 grams. And, if you crash it hard, it will break ... into expensive pieces.
Wouldn't it be nice to have a quad frame that weighs 1/4th of that, flies more than 15+ hard minutes, costs the same or less, and you can ram it full speed into a brick wall and it just bounces off laughing (well OK, the motors shear off to save the props, but it's designed to do that).
My brother, Mar (yes, as you will see, mom, actually his wife Sin, named him right), is our durability test guy. He has crashing down to a rare art form. As impossible as it might seem, he holds the world record for crashes in one day of the same copter where each crash completely broke the copter apart. He has done this five times in one day! I was a witness. I was there! His daily average is two. He has crashed more times than I've flown. How does he accomplish this? He is Master Mar with a flashing grip on High-temp hot glue that moves faster that one can see. And he has pounds of the stuff.
This build was dedicated to him. In fact, I just put it in the mail to him. No need to package it. It's indestructible, even in the hands of our US postal ogres.
Note: Ignore the skewed APM. It’s just sitting there during a fit test.
The basic features of this quad pictured in the photo above:
- good for up to 16” props
- regular X
- 3300 mAh battery for 15+ minute acro high-energy flight
- sharp glass motor mounts to shear off the four nylon bolts holding each motor during a crash to save the carbon props
- low profile light foam legs to cushion landings.
- frame that can withstand a 250G crash
- battery (the heaviest part) mounted close to the plane created by the props for easier acro
- GPS mostly protected by the battery and motors from a direct hit
Marty (or Mar as his wife calls him) – If I was to do this again when my brain was working, I’d create a battery cradle that was taller where the APM would have a slot for it directly under the battery and closer to the CG (and as you suggested, out of the sunlight).
So basically, the ship is made of two phenomenally stiff carbon tubes bonded to an electronics platform, four motor mounts bonded to the tubes, two standoffs for the GPS/Mag, a battery cradle, and four darling little feet. That's it. Only 41 parts total counting every washer and nut. Now for the fun of it, count just the screws, washers, and nuts on your quad. Yup, more than four times as many part—just counting those. Getting the feel for why your quad can break? Every hole, every joint, is a stress point. The parts (total cost less is than $100 US) are:
- 1 Rockwest Fabric Tube 45525 for the two continuous motor masts
- 1 ACPSales Single Ply Carbon ¼” Nomex sandwich panel (12" square) for the electronics platform and battery cradle
- 1 CSTSales Fiberglass plate .06" for the four motor mounts
- 2 Nylon Hex Standoffs with nuts/screws for the GPS/Mag
- 1 APCSales Carbon Ribbons .31” x .032” for the two battery cradle slats
- 16 nylon bolts for the motor mount
- 1 Pipe foam insulation 1/2" for the four little feet
- 2 Velcro straps
- Scotch Weld EC-2216 adhesive (basic frame welding places)
- Hot glue (APM to the electronics platform; pipe insulation to the motor masts; stuff you don't want to be permanent)
- Some waxed dental floss (or zip ties)
In the above photo, the battery cradle was made of sandwich panel material and two carbon extruded slats. The edges were treated with the 2216 adhesive to increase crush strength. The battery straps are special as they have an additional layer where they go through the carbon sandwich panel to keep them from wearing on the carbon edge. If one sees the bottom side of the sandwich panel start to crush from the strap holding the battery, a small composite plate (.040 thick x .5” x .5”) should be added to spread the load over the lower skin. (Marty – after a crash check for this and add reinforcement if needed; if you see crushing, you’ve exceeded 15 Gs).
Marty – Note that because of the last minute layout change, the wires going from the APM to the GPS/Mag go under the cradle slats and over the motor mast where you will find two indents (center of the above photo). Once the wires are located, put tape over the wires where they cross over the motor mast to keep the battery from wearing the wires. APM 2.6 uses longer wires (about 5”) to allow you to put the GPS/Mag, in this case, about 2” away from the battery, which your magnetic compass tests showed should be adequate.
The receiver, not shown, goes on top fore/dead-center of the battery on its side between the elliptical hole and forward edge of the Electronics Platform with the wires pointing port.
The above design is a side profile of the two end pieces of the battery cradle. It’s designed so the battery nicely fits into the top cradle to handle the side loads.
In the above photo, note that the battery needs to be face up so the battery wires run aft and port to keep the wires away from the GPS/Mag. The placement of the battery cradle is also configured with this uneven wire weight distribution in mind. When you are fully assembled, the cradle can be adjusted with a hot knife.
The bottom side holds the ESCs. Your ESCs will be covered. I love Jim’s idea of using waxed dental floss instead of zip ties. Saves eight grams. All four capacitors of the ESC can be put pointing aft so the power wires point towards the battery (use a wire nut to connect ESC to the battery or something else if you like). Going left to right (viewing upside down) the ESCs are 2 (starboard-aft), 4 (port-aft), 3 (starboard-fore), 1 (port-fore). This is done to minimize wire length. Note, however, that ESC 4 (the second one) can point either direction.
The four darling little legs are to protect the ESCs from hard landings.
Marty - It might also be advisable to hot glue a ½” long by ¼” wide x3/8” tall piece of foam at the tips of each boom on the bottom side to act as bumpers on uneven landings and crashes. Because you are using the small motors, I had to cut out screw access ports at the end. This doesn’t mess with the stiffness of the copter, but if an end of the motor mast perfectly his a rock, it might micro-fracture some of the composite layers at the tip. A small bumper will lower the G-force by a factor of about 10.
The above picture shows:
- the APM 2.6 outline; Marty –The location might seem a bit odd at first but it is the only place where the port props won’t touch it. Please mount it directly to the EP with your high-temp hot glue. Keep the layer thin. Vibrations will be almost non-existent on this copter it is so stiff. As an acro copter you want the APM to immediately feel everything. Please do a hover with the appropriate logs turned on so I can analyze the log file for vibration, stability, throttle, and watts.
- the white zip ties (you will use dental floss) that holds the four ESCs. Two of the zip ties go through the battery cradle. One goes directly through the electronics platform then takes a 90 degree bend through the cradle end-legs to the next hole. The other goes up at an angle through the electronics platform going under the battery cradle leg coming out on the correct side of the leg. I like the angled hole better.
The above photo shows the APM 2.6 location. Note that the aft starboard corner of the APM is close to the motor mast. There is just enough clearance to get your four pin control wires in and out of the box. On both ends, those wires will be clear of the props. Run all of the motor wires on the sides of the boom. On top wires are exposed to prop damage during motor ejection and on bottom they are exposed to ground impact.
Put a silver permanent mark on the port side of the battery where it should line up with the cradle support for quicker copter balancing the next time you insert the battery.
These are the universal motor mounts as they came off of the CNC router. The shape is critical. The adhesive needs about 1” of length total to obtain a half ton of holding load (hence the elongated tab at the ends). This means that the motor masts need to be .7” longer than the distance between motor centers (1.4” total for a continuous motor mast). If you don't have a CNC, just add a few grams of weight. Drill the center holes. Then cut these rectangular with a saw. Then rather than slits, just drill the exact holes that you need.
Note: Jim – When you bond these to your tri-copter, put a piece of tape over each side of the motor mounts to shield the screw locations from the adhesive only leaving exposed the center tabbed section and most of the hole. The tab at each end is .3” wide and that is all the width that the 2216 adhesive needs to give each motor mount the half ton of gripping power. Also, check the bottom of your motor to see if it extends farther than .060” from the back. If so, drill out the middle or add washers to the mount.
Also note that the motors need four nylon screws to center the motor when using universal slits (versus just two). But no worries. The glass edge of fiberglass is much sharper than the plastic and wood you have been using. The glass will shear four nylon screws when the G-loads exceed the ability of the blades to stay in one piece (at least that’s the idea).
Marty, good luck with the electronics. Jim, good luck with the tri-copter.
Marty – The forum would probably like to see the copter with all the electronics mounted, so please add a reply when you get there. They would also love to see video of flips, a crash, and what happens to the copter (where is it damaged if at all). You can link those videos and photos in replies. This is a test durability acro quad, so let’s see some test results.
Hi Forrest, this is a really nice article on a simple but very light and strong quadcopter.
I am especially fond of your material list and will be using some of your suppliers for sure.
I do have one question, On looking at the 3M data sheet for the Scotch Weld EC-2216 adhesive they do not show it as being used with Carbon Fiber products at all.
I wondered if you have separate published information covering the validity (and strength) of this application or if this is simply from your personal experiences with this product.
I have been very skeptical of gluing to carbon fiber because of the general known problems but would truly like to find a product I could rely on.
In reality, I really don't know the bond strength of 2216 to carbon (but would guess it is quite high given the shape of carbon chains). But I don't think that is the question you are asking, as one rarely bonds directly to the carbon in carbon fiber laminates and composites. I think what you are asking is what type of matrix material did we test?
Note that in the spec, 3M tested various matrix materials including the two substrates Fiber Reinforced Polyester to Reinforced Polyester. The fiber was probably glass, Kevlar, or carbon. The results was 1660 psi with the adhesive exceeding the substrate ability, which failed first.
I'm guessing that you work with either carbon reinforced thermoset or thermoplastic material. If thermoset, probably epoxy. If thermoplastic, probably PEI or PSS. Between all of those, I'll guess epoxy and discuss that.
While removing weight from the Boeing 777, one of my productive efforts (several hundred pounds) came from the floors--converting from fiberglass to carbon. Went through two years of corrosion testing and insert shear tests to prove the point that carbon was stronger, lighter, and more corrosion resistant than the mixed material of carbon/glass/aluminum (counter intuitive but true when done correctly).
Part of those tests included shear strength tests of the bond between the Aluminum inserts that house the screws that go into primary structure and the Reinforced Carbon Epoxy Laminate. An insert didn't pull until far over 1800 lbs (the b-basis allowable). The area on the insert flange was about .34 sq inches. So in shear we could with 95% confidence rely on the 90% of the inserts getting 5300 lbs/sq of shear. In tension, the panel core to carbon skin layer would have failed first.
For epoxy and thermoplastic materials, I haven't really found any adhesive that comes close to 2216 as long as the temperature range is OK. An example is Orange. The specs are similar to 2216, but when I actually tried it on the ships, there was no comparison. So your skepticism is warranted.
Most all of these really good adhesive weaken dramatically at higher temps. In fact, that is how we do repair work. Use a heat gun to soften up the bond that the parts pull apart.
It is also important to understand or design control the type of forces at work on the bond. If it is peel, while 2216 has three to eight times the peel strength of most other adhesives, it is still only about 22 lbs per inch. But this can usually be controlled by designing the bondline so peel is not an issue.
The documentation is open to the public, but you have to do through Boeing or Airbus to get it (each of which have their own test data).
Thank You Forrest,
I have been Googling a bit on the best adhesive / glue for carbon fiber pre-made products and the somewhat similar Locktite 9430 and 9340 Epoxy adhesives also seem to be popular, but I do understand what you are saying.
In fact the carbon fiber products we will generally be using are, in fact already epoxy - carbon fiber laminates and we are bonding to the laminate, not just the carbon fiber.
It is excellent support you offer for the 3M 2216 and I understand that surface preparation is also very important for CF products I found a good reference here for both cutting and bonding:
Clearly this is a field in which there is not much readily (publicly) available information and the adhesive manufacturers I think are purposely NOT being specific in relation to CF products.
Possibly this relates to the high degree of variance in the internal bond (and strength) quality of various premanufactured laminated CF products.
From what you have said however and what else I have found, I am certainly willing to consider that both the 3M 2216 and the LockTite 9340 and 9430 are likely to provide a sufficient bond for anything we are likely to use them for so long as adequate preparation and a satisfactorily large bond site is maintained.
What end mill tooling are you using to mill your fiberglass and carbon fiber, it seems a bit coarse, carbide I presume.
Thanks for the prep link. Let me know if you end up liking the LockTite. Also recommend not
I'm using cutters for glass and carbon. With my CNC, however, I have no control over speed, just feed. It would be nice to pull a part off the CNC and not have to sand it!
Also with the smaller parts, I'm haven't fount a 1/8" diameter burr bits that I prefer. So have been using the two fluke ones modified for composites. If you come across a better 1/8" bit with a 1/4" shank that doesn't mind regular router speeds, let me know. Would love a personal recommendation.
I have always liked minimalist designs and this is a good example how you can do more with less (I should have been a politician with such a slogan).
I was wondering if to gain room to place all of the electronic components you could not use perpendicular vertical panels, like computers daughter boards?
No doubt. Every square inch of electronics platform (EP) weighs half a gram on this ship and probably 4 grams or more on most ships.
Brother Jim showed me one of his tricks of simply putting the receiver on its side. It's even easier to access the connectors.
If i did this design again, I'd make the battery cradle 1" taller to put the batteries almost at the prop plane (nice for an acro ship). Then cut a slot in the cradle where the APM or Pixhalk would go under the battery so sunlight no longer impacts the barometer.
You want to put the GPS/Mag as far from the battery as you can and don't know if it can go under the prop to eliminate more EP real estate (never tried this and have no idea what would happen with an intermittent signal cause by the prop passing over it). so that means some sort of appendage going out from the electronics close to the prop tips.
The ESCs can be tied to the side of the booms versus on the EP. This is actually a good place for two reasons:
- the motor masts provide some protection from ground strikes (complete protection if the ESCs are not hung lower than the mast
- it puts the ESC in the prop wash so you can strip off the heat sink and just coat them with silicone.
Prior to the trip to my brothers in Phoenix, I'd been reluctant to do this. After all, doesn't interference with the props wash means that there is a force pushing down on the ship. So Jim and I tested this. We found that this concern, while real, it is highly overstated. Each ESC hung on the side under the props will cause about 1 gram of thrust resistance (the heat sink, no longer needed in this config, weighs 2.5 grams).
So yes ... we just got rid of half of the EP ... maybe 6 grams on the above ship and about 400 grams on most other ships. The EP still needs to provide the X structure of the quad, but floor panels are increadible in shear, so with the 2216 adhesive, it doesn't take much (maybe just four 2" gently tapered spokes) with appendages for the GPS/Mag and APM/Battery cradle.
I like it.
As long as going vertical means staying between the lowest motor mast and prop tips, then the electronics are somewhat protected.
So on the octa duration quad that set the record, I bonded vertical fins on the motor masts just like you suggested and mounted two ESCs on each side of each fin.
I have the same problem for small diameters, my 24" by 24" homemade CNC does use full sized routers among others.
And I am using a Bosch manually adjustable speed one (electronic forcing) but it still doesn't go any faster than normal routers, just slower, so not exactly what we are looking for.
I'd like to mount a Kress spindle, but they are just to Damn expensive.
Fiberglass and CF are hard to cut by any means even the water jet guys don't like CF.
I'm researching mills a bit, I'll let you know what I find.
FYI I have included several of the suppliers you have listed on this page that I am currently working on:
If you have any thoughts for some brief specific information you might think would be worthwhile to add to this page, please let me know.
I am planning on linking to this discussion but haven't put it in yet <actually I just did!>
Apparently the wiki is running low on disk space, but I think that is probably due to WP currently saving way too many short term revisions (66 of this page currently).
Once we get that straightened out I am planning on getting into greater depth on particular techniques and important considerations.
Jeez Forrest - that is the ugliest frame I have ever seen but hey if Marty can't break it then nobody can and that's worth lots. We'll work on the frame's sex appeal later. What's wrong with your carbon mini frame design that was kind of cute and flew so good?
FYI - the Don't Fly Like My Brother Videos in the APM2.x Wiki on how to use the Mission Planner feature Marty's crashes exclusively and there is no shortage of them if anyone is interested in watching. LOL
Nice work Bro. Hope to break your endurance record before you return from vacation now that I've got SimonK ESCs that don't brown out my motors before the Li Ion batteries are used up.
Some pilots are attracted to men ... some to women ... :-)
Hey Mar - nutin wrong with flyin a manly-man copter! Think i'll make a pink dress for Jim's copter.
Wow Gary! Great body of work. Fascinating insights too. Great read.
If you want an extra set of eyes to help, let me know. Be glad to help. Also do you plan to talk about ship (frame) efficiency--measurement and best industry practice? Or is that outside the topic?
Possible frame build links are as follows (Frantz versus Franz):
Ive included links to all your various and interesting copter build pages in the advanced multicopter wiki page here: