When I started build model aeroplanes back in the early 1950's we used 'clothes horse' hinges for our control surfaces. As shown in the first diagram they were short strips of half inch wide ribbon stuck to the balsa wood surfaces with balsa cement. They worked well and did not fail, but had two disadvantages. It was difficult (impossible) to get them really tight so that there was always some slop and the gap would widen or close as the pushrod moved the surface, which was not desirable with high performance models. The other disadvantage was that my big sister would not always give me her hair ribbons nor the shoulder straps from her underwear.
We changed to using what we called 'sewn' hinges. Linen thread was so strong that it could not be broken by hand and it did not stretch. This was literally sewn loosely through the soft balsa wood with a needle as shown in the second diagram, and then with the surfaces clamped in alignment starting from one end the figures of 8 were tightened one by one with a pair of pointy nosed pliers until knotted off at the other end. As each loop was tightened the thread locally compressed the balsa wood which removed all slop. This was then secured with balsa cement smeared with a finger over the thread on the flat wood surface. These hinges were always very free of friction. See drawing 2.
These were excellent hinges. Most modellers drew straight lines so that the stitches were even and looked good, but in my rough and ready way I did it by eye, but always varied the length of each figure of eight so that no two adjacent holes were through the same line of grain of the wood. Wood will want to split along its grain line, and if all the holes are in the same line then that line would be a potentially weakness if the control surface was over stressed. By spreading the holes to different lines of grain I felt I was spreading the stress points and resulting in a stronger structure. Not as pretty, perhaps, but performance always came first. See drawing 3.
When I first saw James Wharram's sewn hinges I was surprised, not by the fact that he was using rope hinges, but that he was A) Countersinking the hinges into the wood, and B) placing the holes so close together in a straight line. By cutting away the wood to make the finished hinge flush with the surface of the wood it may look prettier, but it means that the hinge is in weaker (lesser) wood than the rudder itself, and as Arne so rightly suggests, a line of holed close together usually has a label saying, 'Tear along here'. I suppose I wasn't surprised that the hinges were in tight little groups and placed where pindles would usually be placed, but our modelling method of spreading the hinge right along the surface does help to spread the load.
When I decided to try and prove that a precision wind vane steering gear could be built with rough workmanship for less than £25 instead of £2-3K I learned a lot. The rough and ready workmanship is something I have perfected in my lifetime, but when the tube of glue I bought to try out cost more than a third of the planned budget I realised that even with sewn hinges and scrap wood the budget was going to be broken.
Using my irregular and spaced sewn hinges on thicker and harder wood than the balsa used on the models did show new problems to me, but also new solutions.
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I did not countersink the stitching into the wood, but left it on top of the surface. Not pretty, but not weaker.
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I left a gap of at least a cord thickness between each hole for the figures of 8 to spread the load.
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I varied the length of each figure of 8 to spread the load over various lines of the wood grain.
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When I tried to pull each figure of 8 tight in turn I found that the cord (in my case 3 mm pre-stretch polyester) it would not pull over the sharp edges. The solution is to round off the holes drilled through the wood so that the cordage can slide and tighten. See drawing 4.
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Once tightened as much as possible, and knotted off, it is then possible to drive long thin wedges down between the cords and the flat wood surface to tighten the stitching further. If deemed necessary this can be done on both sides of the wood and on both the fixed and the moving surface, making 4 places in total. I did not do this in practice on the wind vane, but may when the cord stretches as it beds down. See drawing 5.
One problem when using sewn hinges for a pendulum rudder is trying to decide the foil balance, as the moving surface does not turn around a fixed point. Hinges sewn as discussed here are actually roller bearings, where the moving surface is rolling around the the fixed surface, and the line joining the centre of the circular faces swings through half the angle that the moving surface makes. On top of that you have to include the thickness of the cord between the two rounded surfaces. Great fun if you are trying to get a 22% balance, but here the rough workmanship helps as a badly carved foil moves the balance point further aft than the best 25%.
One extreme thought for the purist is that if the hinge line between a rudder and skeg is sewn as in drawing 2 or 3 all the way down and spreading the load along the hinge line it may have another interesting advantage. Those with an aeronautical bent will know that vortex generators placed where there is an abrupt bend on a foil can keep the flow attached and delay the stall. This would suggest that the zigzag stitching proud of the surface at the hinge line of the skeg might delay the stall of the rudder if it is turned too fast or too far. Just a thought?????
I hope someone will have the patience to read this all the way through.
Cheers, Slieve.
