Monday, 9 January 2017

DHC-1 Chipmunk - wing

After about a 18 month break, I have restarted work on the Chipmunk. The Fuselage has been parked for the time being and I have started work on the wings. Nothing too challenging at the moment, it is simply a case or reigniting my enthusiasm, getting a bit of progress and hopefully, I'll now see this through to completion.

As before, it will have to fit in with whatever else is happening in the Shed, but now I have got the irritating bits of the design out of my system (it's scale accuracy), I feel much happier and I'll now try and get this completed and with a moderate amount of scale detail. Like all large projects and eating elephants, its best to approach the task, one bite at a time.

The parts kit is almost complete (centre rib and 2 1/16 ply brace/doubler ribs not present - but that's not a problem) although the balsa wing rib wood is rather hard and prone to splitting. Still quite usable but I am not sure how it will survive a hard arrival.

Lots of clamps used to make sure the joints are nice and tight. The number of braces in the centre are in my opinion rather excessive but that is what we are stuck with. The wing certainly wont fold in flight. It was actually far more difficult to get the two panels to align correctly surprisingly. It took me 3 attempts to get it to where I am happy with it.

 Progress so far has been slow but the upper wing sheeting has been added. I can how start work on the aileron push-rods and the undercarriage. A bit of belt and braces while work progresses on the overly delicate Hornet Moth

I was never happy with the original method of retaining the torsion bar undercarriage legs which used tinplate straps binding the legs and then screwed to a hardwood block. The centre section is so over engineered - this looks like a potential weak point.

My approach has been to implement much larger beech blocks and cut a deep channel. Aluminium plate saddle clamps will hold the leg in place without risking damaging the wing skins  which sit below the torsion bar.

The anti twist stop will use a metal saddle clamp - bolted through the wing ribs

 Hobby King Oleo legs grafted onto the end of the torque rods. These are designed to be clamped. Its quite an easy task to drill out to 4mm to fit (interference) with the 8swg (4.1mm) diameter rod.

M3 hole drilled and tapped all the way through and cap head screws used to clamp the legs in place. A small flat will be ground on the legs to prevent them twisting

The sprint tension is too tight. The spring needs to be reduced in tension so that the leg compresses by about 6mm with (guessing) about 1.5kg load on them. I don't want to soften the spring, simply reduce the preload - the easiest approach is to incrementally reduce the spring length.