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.