RC Aeroplane work log #1, Motors, mounts and miracles.

[Scythe]

So I recently embroiled myself in the fascinatingly expensive world of remote controlled aircraft. My workmates had been dabbling in the field for a while and I thought I might try it out. My first purchase was an Esky Honeybee King 2 helicopter. It's lots of fun to fly, but it's the same kind of fun you derive from balancing a priceless, delicate glass heirloom upside down over a most assuredly solid concrete slab. You can't really enjoy yourself whilst flying it, but it somehow results in fun. Go figure.

The HBK2 (as they're known in "the biz") came with a fairly average set of servos, and a crappy analog set of transmitter and reciever. Following advice from the internets, I picked up a Spektrum DX6i Tx/Rx set and a few HS-55 servos. The transmitter is a damn nifty bit of kit, able to control aircraft and helicopters with equal ease and includes an intimidating set of configuration options. I figure that it's worth the investment since I'll be able to use it for other projects later.

I decided to buy a cheapo airplane to fly during my helicopter's downtime (waiting for parts delivery from Hong Kong). I settled on the Cessna 182 from ElectricRC. I cut a deal with the owner, Tim, by which he'd remove the Tx/Rx, battery, charger and fifty bucks of the price. It comes with a decent brushless motor, a speed controller and it was compatible with the three-cell, 1300mAH Li-po batteries that I'd bought for my heli. I can recommend ElectricRC if you're looking to buy an electric aircraft in/around Brisbane.


My plane, photo credit to ElectricRC.

A long standing issue my workmates had encountered was the fact that once a plane gains a lot of speed in the negative Z direction, there an almost inevitably one result: A gut-wrenching thud and a spray of bits. Most of the planes can be easily enough reassembled, with the exception of the motor. This precision bit of kit is very intolerant of the merest deformation. And given its leading position on the front of most planes it is most often the component subject to the most ouch.

My workmates had experimented with various reinforcements and modification, to the point at which one plane almost entirely consists of carbon fibre strips and duct-tape. One helpful innovation is the prop-saver. It's essentially a bit of plastic or metal that bolts to the motor shaft, on to which the prop is attached with a rubber band. This provides a degree of energy absorption, hopefully saving both the prop and the motor. However this just wasn't enough in some of the more energetic ground intersections, motors were still ruined.

When I bought my plane I had all kinds of plans for making the motor a bit more robust. Of course all these plans went right out the window when I first laid my hands on it. I eagerly rushed to the nearest field and had a blast flying it about:

http://www.youtube.com/watch?v=3FcSfdSUTyQ

Of course, a few flights after this one resulted in my plane reenacting a scene from "World's most ill-advised flightpath corrections" and cratered quite badly, bending my motor shaft. Cursing my childish enthusiasm I went home, head hung low.

The next day I put some serious thought into a means of mounting the motor on the plane in a non-rigid manner. Something that would reliably detach in the event of an unplanned attempt at Vernsian subterranean exploration. The breakthrough came in the form of magnets. Three magnets, holding the motor mount onto the front of the plane, replacing the flimsy screws that previously did the job. Three magnets also meant that they could be used to transmit power to the motor. A pair of metal plates, epoxied on to the plastic parts, wires soldered on and a magnet between them. Vary the strength of the magnets until the motor is reliably held in place, but detaches in the event of a crash. Voilà!


Front of plane, plates attached.

I lapped down the surfaces of the plates with some wet'n'dry on some glass. For magnets I used a pair of button magnets I had laying around, and a magnet from a hard drive. I took the whole shebang for a test flight today. The result? Maximum success.


Motor mount.

The thing flew magnificantly. I crashed three or four times, each time the motor detached perfectly. A few moments spent clipping it back on and I was away again.


Some blur, and both together.

On my last crash one of the plates became unstuck from the motor mount. The epoxy was having trouble holding on to the plastic. Once I got home I cleaned the old epoxy off, scored the surface of the plastic deeply with a knife, and reapplied another layer of epoxy. Good as new.


Much more blur, and the two barely discernable parts held together with magnets.

The hole you see on the right there was the result of a midair dogfight with a pair of Plovers. They had developed a cunning tactic of splitting up; one swooping the distracted pilot on the ground and the other trying their best to peck the plane out of the sky. They're a ground-nesting bird and are thusly quite defensive. No other injuries were sustained on bird, plane or person.

Most of my crashes were the result of a perfectly executed landing onto bumpy grass. The tiny wheels on my wee Cessna couldn't cope and it just tumbled end-over-end. I might set up a skid arrangement.

Thanks for reading, hope you enjoyed it,

--Tjhowse--

[Scythe]

As an addendum to this post:

Some time ago I crashed the plane into a gum tree. The plane was hanging by the prop until the magnets detached. The plane fell to the ground but the motor disappeared. Whether it was flung away when the branch sprang back into position, or still stuck in the tree, we will never know. Our search for the wayward motor was curtailed by the setting sun.

Score one to nature.

My next project: A UAV. Stay tuned.

--Scythe--