July 9, 2010

A triumph of function over form

I’ve been trying to borrow a prop pitch gauge for ages now but although I thought I had one lined up, it hasn’t materialised. Ginge over at Saxon Microlights has one but we don’t know if it will work with my prop. The same goes for Bob. His is for a pusher arrangement anyway so it may not work with my tractor prop, although I don’t know for sure if there’s a difference.

But anyway, I really want to put the prop on tomorrow as I’ve now bought a new battery and with the prop fitted, MYRO would be more or less ready for inspection. I was searching the Internet over lunch for topics to do with pitch gauges and came across a very interesting article on how to make your own. It was actually for model aircraft builders, but as they said in The Flight of The Phoenix, the principles are just the same. The article described a design that was relatively quite elaborate with an adjustable protractor so it could be used to adjust pitches over quite a range but I realised that what I need is something far simpler, just the ability to set my prop blades to one specific angle. It got me thinking.

My prop is a 3-blade Arplast of 165cm diameter. The ‘rules’ in the form of what is called a ‘MAAN’ to which I must strictly adhere in order for my set up to be approved, are quite specific. I must set my blades to a pitch of 14 degrees at a radius of 53.5cm. The more I thought about it the more I realised that using a similar method to the one I read about at lunch time, this is actually a lot easier than it sounds.

All I needed was a flat board about 70cm long with a raised block that the prop hub could be mounted on at one end with all three prop blades in place and a line drawn across the board at the 53.5cm position at which a 14 degree wedge could be positioned. Then for the first blade, with the pinch bolts loose enough so the prop blade could be rotated in the hub, all that I would need to do would be to position the wedge on the line and rotate the blades until they sat tightly on the sloping face of the wedge and then tighten the blade pinch bolts. With the process repeated for the other two blades, that would be ‘job done’. Piece of cake 😀

It isn’t often that you can conceive of a plan such as this, find the odd materials that you will need in your garage, set to with saw, drill, screwdriver etc and end up with a finished article that works as planned. But that’s what I did this evening and it only took an hour or two! The most accurate part was making the 14 degree wedge. For that I used an engineering adjustable protractor that used to be my father’s and I’ve had since I was small boy, to mark a line at the correct angle on a suitable piece of timber and then I carefully cut down the line using a hand saw. I completed the job by planing the sloping face to make it nice and smooth.

Initial tests indicate that it works like a charm – but that’s not surprising because it’s so simple, what can go wrong? It was getting dark by the time I finished but here’s a pic showing the gauge in all its glory with a blade and the wedge in place showing how the 14 degree setting is achieved.

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It may not be very elegant but who cares if it does the job. I think it will – but I’ll find out for sure tomorrow 😉