2014’s ASME (American Society of Mechanical Engineers) Student Design Competition involves making a vehicle ‘Lighter than Air’. The goal is making a multirotor craft limited to 28″ in diameter that is scored on total craft weight including payload. Having no experience in UAV design I decided to build a multirotor in order to gain valuable experience before we build our competition craft.
After many hours of research spanning a few weeks I came across rcexplorer.se, a website showcasing building a simple tricopter from easy to acquire components. From there I decided to build a tricopter… light, maneuverable, and portable.
I designed the frame in AutoCAD and cut it out in 3mm thick plywood using my home laser engraver. The goal was to have all the major components zipped tied securely to the frame. However, in doing so, the front booms are unable to fold backwards like David’s tricopter at rcexplorer.se, but they did fold forward.
The flight controller is a HKPilot Mega V2.5, a generic Ardupilot. The Ardupilot supports many design layouts, GPS, OSD, external power sensors, servo based camera gimble, and some other nice features like altitude hold. I choose the Ardupilot over other flight systems such as the MultiWii or the KK2, because those systems lack many of the aforementioned features. Oh, make sure to place open-cell foam around the onboard barometer as this prevents reading errors due to wind pressure and direct sunlight contact. The lack of foam caused several crashes when the control board went into land mode during a battery failsafe.
A future upgrade is switching out the stock TGY 9X receiver for a FrSky module with failsafe support. If the radio fails or goes beyond communication range, having the tricopter fly off never to be seen would be a sad day, but much worse would be damaging property or harming another.
The ESCs are HobbyKing 30A with an integrated 5v UBEC supply at 3A and are mounted via zip ties. The firmware has been updated to Simon K, firmware designed for multirotor applications.
The motors are Hextronic DT-750s (750KV) driving 11×4.7 APC props. They are cheap, yet effective. With these type motors you have to be careful not to over tighten the mounting screws or the screws will compress the base and shaft creating unwanted friction. I foresee an upgrade in the future.
The motor mounting plate is zip tied down to the booms which is great because in the event of a crash the motors will break away decreasing the likeness of motor damage. The motors leads are directly soldered to the ESC power leads which is not really a good thing. Ideally there should be barrel connectors allowing the motor become unattached without ripping the wires apart.
The landing feet is attached by, guess how…. zip ties! This is the only area where zip ties actually made for a frustrating situation. Being a new pilot, I tend to make many hard landings and those zip ties break away rather easy. Once I am able to control the craft better the zip ties will not be a detriment to my sanity. Until then, I may just screw the landing feet into the booms.
At first I used prop savers to attach the props to the motor shafts. They worked OK, but not much better than OK. Once I tried spinners the difference in flight characteristics was dramatic. The craft was more stable, used less power, and my confidence in the craft increased.
A point of consternation was the yaw mechanism. However, after seeing what others did, I came up with my own design. A few hours drawing up parts and making them several times on the laser engraver I had a working yaw mechanism! The servo is a TGY-390DMH, a metal gear, ball bearing, digital servo speced 5.4KG / .11sec weighing a modest 22.5 grams. The peace of mind knowing the servo will perform as required is well worth the $28.
The yaw mechanism is glued together using Loctite Gel Super Glue and is rather solid. Only after a hard crash the motor mounting plated popped off… a few dabs of super glue later the mechanism was completely fixed.
The booms are made of solid pine cut down to 0.75″ x 0.5″ and given a nice roundover. Because 8-32 bolts were used the mounting holes are little larger than they needed to be. Also, because I did not use a drill press the mounting holes are slightly off-center leaving one side of the boom with little material between the boom’s edge and the hole’s edge. Over the course of several crashes I broke both booms, and one of them I broke twice! As you can see in the picture I screwed a thin wood brace onto the side of the boom. This quick fix eliminated the booms breaking, that and just getting better at piloting and not crashing. =P
Lights! A few strips of 3528 120LED/m LEDs were added to underneath the booms. Intended to be used a navigation / orientation lights they turned out to be really cool. However, during low altitude flights, as navigation lights they did not work well – I could not see them! A future upgrade is placing them on a disk near the motors so the pilot can see the lights no matter the craft’s altitude and orientation.
Flying at dusk is a fun experience.
Ok, so there we have it, my first tricopter. Now that I have acquired some really good experience it’s time to make our competition UAV!