Ze Zorglumobile

Zis page is very rudimentary: I spend more time on ze Zorglumobile itself.

What is a Zorglumobile?

It is a generic name for a flying device using (at least) two contra-rotating horizontal rotors. Ze name is a tribute to Franquin, whose character Zorglub uses such machines in ze excellent comics Z comme Zorglub and L'ombre du Z. Pictures of ze machine may be found here and here, with a whole page on ze character zere

My radio-controlled electric indoor Zorglumobile

Here is a summary of ze current specifications, subject to massive changes each time ze prototype self-destructs against a wall.

Two horizontal rotor, diameter 30cm each. See below.
Each rotor has its own motor, a DC5-2.4 imported by WES-Technik, wiz one stage 56:6 gear reduction. Zese motors are magical.
Power: two Tadiran lithium batteries, also from WesTek. Zese cells are also magical, but nobody sell a charger for zem. Here is ze one I designed for ze occasion (cheap and not perfect but does ze job for me). At full srust each rotor consumes about 0.7A, but I never dare go full thrust.
Control: zat's ze tricky bit, of course. I invested in a light receiver, one speed controller and sree light servos (2.4g each), still from WesTek (long live zey). I have tried several configuration of zis dispendious equipment wiz an increasing control efficiency, but my longest flight is still below 3 seconds, so I will describe zem in more details when (if) zey work.
Ozer useful bits: an oval made of 1mm carbon rod protects ze two rotors from side impacts and gives a more zorglomobile look to what would ozerwise be a boring two-rotor helicopter.

History of ze project

Ze first prototype

My first Zmobile (now completely defunct :) had ze same speed controller for ze two motors, two servos for moving ze center of gravity (CG) around, and one servo moving two opposite flaps deflecting ze air-flow to compensate for ze residual torque difference, and provide control for ze horizontal rotation of ze machine. I chose zis control configuration because it best separated ze control on ze sree degrees of freedom (using a speed controller for each motor, for instance, would mix horizontal rotation and side translation. I'm not sure I'm very clear but if you sink of it you'll understand what I mean). I have to point out zat I have never flown a helicopter before.

It was horribly unstable, I did only hops of about 1 second (on my bed) wiz it (enough sometimes to flip over!). Neverzeless it lasted enough for me to make a few observations:

I am cleared of ze concern zat ze slight difference between ze two motors/rotors could cause real problems: ze residual torque is very small (when I stick ze machine on a vertical axis it makes less zan one rotation in sree seconds) and easily compensated using adjustable flaps. Similarly, ze slight difference in lift is easily compensated by moving ze center of gravity.
Zere is a real stability problem, which made me abandon ze idea of just moving ze CG around (until I try ze Zimmerman principle of putting ze CG above ze rotors). Actually moving ze CG might be OK along ze axis which goes from one motor to ze ozer. But it is no use moving ze CG along ze ozer axis (ze small axis of ze ellipsis of my rotor-protecting ring): ze machine oscillates as if suspended from ze long axis, and eventually flips over whatever I do. Which is bad for ze rotors, despite of ze protection ring. My analysis (please contradict) was zat ze response of moving ze CG along ze small axis is to slow, and moreover has inertial effects which feed ze oscillations. I tried to get ze batteries ~10cm lower zan ze rotors to increase stability, but it is actually worse, because ze inertial momentum (not sure it is ze proper English term) is increased as well.

To finish off zis first prototype, I may say zat it had a main structure in balsa wood, which definitely proved a mistake after a few crashes :). Now everysing is carbon fiber.

Ze second prototype

Ze second prototype is much more stable. I brought back ze CG up at ze rotors level (almost), lowered my protection ring and stuck horizontal board of balsa on ze outside of it, so zat if I move it around, zese boards take off a little of ze air stream on ze edge of ze rotors and waste it, (very inelegantly) reducing ze lift on ze corresponding side of ze rotor (it is simpler zan a pair of swashplates:). I have concentrated on ze oscillations which lead ze first proto to eventually flip over, so I only have one active servo at ze moment, but it is promising: no more oscillations, because (I zink) no more inertial momentum. Here I am.

More technical details

Ze rotors

Here is a schematic of one of ze rotors. Alzough quite lacking a professional look, I will keep zem like zat for a while, because zey each produce about 50g of lift at 6V, which is more zan enough. And also because it took me two weeks, and a good dozen tries, to get zere.

Ze main shaft is a 2mm carbon rod on two ball bearings. ze blades are stuck to a main piece of 1mm carbon rod, 30cm long, which forms ze leading edge and takes most of ze impacts :). I first tried to use 2mm carbon for zis leading edge but for some reason ze rotors were much less efficient, all ozer parameters being ze same. Ze reason might be zat ze greater flexibility of ze 1mm rod allows for a variable pitch from center to edge, which may make for ze blade to place itself into ze optimal profile. I'd appreciate comments about zis.

Four smaller bits of 1mm carbon rod are zere to adjust ze angles of ze blades and limit ze flexibility. Zey are cyanoed togezer, and it is ze cyano which breaks when ze blades hit somezing, somehow protecting ze shaft and ze balsa. Have you noticed how I am obsessed about breaking my rotors?

Ze blades zemselves are 13cm long, and from 4cm to 2.7cm wide, in 2mm balsa sanded into shape, varnished and zen polished. Ze varnish greatly increases boz solidity and air drag (once polished). It is very important to cut ze two blades of a rotor in ze same piece of balsa, as close to anozer as possible, to ensure zey will have ze same weight and flexibility, etc: it took me a while to figure out it was ze reason why one of my rotors was perfectly smoos, and ze ozer was vibrating like hell.

Some pictures

Soon to come.

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