About Turn Canard

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Location: Ravenshead Notts

About Turn Canard

Postby uplink » Thu Mar 19, 2009 12:42 am

ABOUT TURN

Own design canard electric glider

Designed, plans drawn and model built at the end of 2006 / start of 2007 - my intention was to make an electric powered canard which had, to some degree, the look of a real canard glider, - as opposed to a proof of concept, functional, box shaped one I’d built some 30 odd years ago.

Work began with the construction of several balsa and foam chuck gliders - experimenting with relationships of areas of wings, foreplane and fin and of dihedral and centre of gravity - aiming for something which was stable and wanted to recover no matter what obscure angle, or with however much sideways thrust I launched it.
Here you can see About Turn with some of her diminutive 'parents'!

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Having built an own design, bungie launched canard glider over 30years ago, and the fact that it had almost flown straight off the building board, without problems - I was hardly expecting the climb to success for About Turn, to be quite so steep, or so long - or so littered with crashes!

My problems started on the first flight back in April of 2007 - it revealed within a couple of seconds of launch that I had no control - or so I thought - I later found I had only lost aileron control and could have steered it with the rudder!
The reason hinged on the fact that this was the first time I had decided to use ‘industrial standard' 9 pin D connectors (at the join point between wing and fuselage) to connect through the aileron and flap servo wires. The two wing halves join the fuselage using the ‘rod in tube’ fixing method and are held laterally in position by a tight elastic band - goes from the hook at the root of one wing, through a tube across the fuselage to a hook on the other wing root - so the two wings are pulled tightly together, gripping the fuselage.
Thus the D connectors were held firmly connected… except, that is, at the moment of launch - when they weren’t !
The inertia of the long wings as the plane was thrown produced a lateral force greater than the elastic bands restraining force - this meant they slid away from the fuselage just enough for the connectors to disconnect (about 2 millimeters) and the elastic band was not strong enough to overcome the friction to force the connectors to connect again!
Result - a crash into the only brickwork within the visual range of the Goosedale flying field - luckily the damage was very light.

Carbon fiber spar seen here sliding into fus ali tube - to rear of that the elastic band - then the ‘D’ connector - then the piano wire locator rod:-

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This problem of the wings pulling out, was overcome by fitting a couple of brass tubes into which a ‘staple’ fashioned from piano wire, was pushed - so meaning the wings could not slide away from the fuselage - not even half a millimeter! …seen here not yet pushed down into the tubes…

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…a tad crude, but easy and effective.

Subsequent attempts at flight proved -

1. The motor needed more downthrust. (resulted in crash damage as it tried to stand on it’s tail!)

2. The ailerons were ineffective as the down going one produced bags of drag and so adverse yaw (resulted in bad crash damage to fuselage) …. corrected by going to 100% aileron differential ie. only up-going aileron movements.

3. Applying up tended to turn it left - found the piano wire coupling the two halves of the elevator on the foreplane had dug itself a bigger and bigger slot in the balsa - so when up was applied, the servo moved the right half of the elevator down (down elevator on the canard pushes the nose up) but if you simulated flight loading, the cross wire link to the left half of the elevator was hardly moving it at all - so the elevator was acting as a mini aileron system - cured by dismantling the elevator and epoxying the link wire really well to the elevator halves.

4. The chuck gliders had given me a position for the c of g and this appeared to be ok on the first few flights - but then on a flight disturbance I had the case where the main wing stalled before the foreplane (a real no, no on a canard) and she went into a very, very flat sycamore seed type rotation - just like she was an autogiro - descending very slowly - quite a sight ! (first time this occurred I almost managed to pull out of it - but needed about 20 foot more height - Fus snapped in two ..… a second time it happened, I just left it to spin slowly all the way to the ground - gentle touch down and no damage !)
So, I moved the balance point forwards a little with more nose lead, and then she was gliding nose heavy and needed full up all the time - I re-rigged the foreplane to a higher incidence angle, but that pushed the foreplane into a stalled angle, so the nose didn’t lift as I’d wanted - only thing was to get more lift from the foreplane by increasing it’s area a little - so, made it slightly bigger - put incidence back to what it had been - and that cured it !

5. The motor cut after about 3 mins flight due to the ESC overheating - so I added a couple of cooling holes and air scoops - cured.

6. The very thin, 30 something year-old, undercambered wings - which I had modified to use on this plane.
I recall, on a previous plane, whenever the normal loading on them was reduced by applying any amount of down, they had always been prone to flutter - and on this plane it was no different - they just lacked twist rigidity.
So, I opened them up and added some diagonal sub-ribs - that appears to have cured the flutter problem.

And so now - now she appears to fly pretty much ok.

Fitted with 8 cell Nimh 4/5th size 2200mAh flight battery that weighs in a 15oz
The total AUW is exactly 5 pounds (2.27kg). - due in part to those wings -I recall, 30 odd years ago a place was selling furniture type veneer very cheap and, at the time, I obviously was more than tempted to cover the wings in it - it has a density similar to concrete !

Motor is up front - unusual for a canard, but I've never been happy with handlaunching a plane with the prop at the back - too much whizzing around stuff, too close to your ear - and you have to remember to keep your hand moving downwards as you release the model - so that the prop that's coming through, doesn't find your hand!

The motor's a brushless AXI 2814-12 turning a folding 10 by 6 prop.

Wing span is 112inch (2845mm) with average of 8.93inch cord (226mm) giving an area of 1000sq inch (6.9sq ft)
Giving a wing loading of 11.6oz/sq ft - a bit high for a thermal glider (...those concrete covered wings !)

But on a canard - unlike a conventional plane, where the tailplane will only produce lift if a very rearward c of g is chosen - and indeed, on most stable c of g locations a tailplane actually produces a slight downforce, ie negative lift - on a canard the forplane is actually producing lift for all it's worth ! - so that can be added into the total lifting area equation.

Canard span is 26inch with area of 159sq inch

So total lifting area now = 8 sq ft giving a loading of 10 oz/sq ft... not quite so bad ! (thermal gliders aim to be between 4.5 and 9 oz/sq ft)

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Next you can see how the batt is slid into the fin - also see the position eventually arrived at for the c of g : 130mm forwards of the main wing leading edge.

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Below was the result of that lack of control which resulted from insufficient aileron differential.
Caused a slight pause in the development program!

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…and nose repaired - reinforced with a couple of carbon fiber tubes.

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Adding those stiffening diagonal sub-ribs:-

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Split flaps to act as air-brakes … on a canard a down going flap would be read as down elevator and an up-going flap as up elevator, so the braking achieved on a conventional plane, by having a flap go down to about 80degrees, is not possible on a canard - hence getting around this by half flap up and half down !

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Don’t know quite what it is - I just find something appealing about the canard layout.... though the crashes - not so appealing!

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Now THAT'S a thermal.

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