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95. Blip Switch

15/02/2014

We’ve not been totally idle over the last couple of weeks, but there’s been nothing major to report or photograph.

The ends of the cabane struts have been fitted with steel caps to protect them, since we envisage rigging and derigging them regularly. They are now awaiting final varnishing, along with the wing struts.

I went through the whole fuselage, lockwiring the turnbuckles so that they don’t come undone with the vibration.

Oh – and I’ve fitted the bungee to the tailskid so that it now has suspension.

The bungee fitted to the top of the tailskid to provide suspension. Also you can see the lockwiring of the fuselage bracing turnbuckles. There are

The bungee fitted to the top of the tailskid to provide suspension. Also you can see the lockwiring of one of the fuselage bracing turnbuckles. There are are another 77 inside the fuselage, and loads more outside…

I’ve screwed the rear decking in place, with the aluminium strip over the front end to which the lacing hooks (for the fabric attachment) and the leather cockpit padding roll are fitted.

And I’ve removed the front carrythrough cable which we’ve established was NOT fitted originally, replaced the eyebolts with plain bolts, and reset the heel troughs in their proper position.

You can see most of this on this video, which also demonstrates that I will need to lose even more weight before being able to fly it comfortably!

And I’ve deciphered the parts list and worked out how the original blip switch was made. You may remember that the very primitive throttle arrangement on a rotary engine won’t run slow enough to allow you to land the aircraft, so a blip switch is fitted to the top of the stick that cuts out the ignition, allowing you to cut the engine intermittently in order to get back on the ground under control. We don’t have an original drawing for the switch, but the parts list identifies all the constituent parts, so using that I’ve made a reasonable reconstruction of how it must have been.

Blip switch - Original

Looking at it, I wasn’t entirely comfortable with remanufacturing it exactly, for various reasons. Firstly, it earths to the stick, which isn’t itself earthed particularly well. One could imagine it preferring to earth through your hand, which might be a bit distracting!

Secondly, I wasn’t very comfortable about the strength and longevity of it – this is must be regarded as a primary flight control, and we wouldn’t want it falling apart or jamming in flight.

So I decided to replace it with a modern switch, and amazingly there’s a specialist shop just down the road in Tenbury Wells, AES, which specialises in vintage car electrical components, and they have a switch that’s a perfect fit. It’s even got a big red button, just like the original.

Blip switch bottomBlip switch top

Getting it secured in the top of the stick took a little more headscratching, particularly since we were determined not to alter any of the original parts, but we think we’ve got there now.

Blip switch - New

We trimmed the thread off the outside of the switch body with a hand file, and found some thin-walled aluminium tube 7/8in od that would fit over it with a good deal of persuasion. It just fits inside the top of the brass tube – again, with a bit of filing and persuasion – and we’ll secure it with a grubscrew that fits flush to the outside of the brass. Then we’ll find a bit of 1in od tube – probably aluminium also – to glue into the top of the grip and help to reinforce it, and this should be a sliding fit over the 7/8in tube.

Then we can thread the wires up through the original slanting hole in the brass tube, attach them to the underside of the switch (two-core, so that the earthing connection can be led back to a proper earth) and locate the switch in the top of the brass tube, fixing it with the grub screw. Then we slip the grip over the switch and the top of the stick, using the original nut and bolt to fasten it in place.

And here it is, done…

Bristol Scout Blip Switch 001

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One Comment
  1. Robert Hughes permalink

    Have a look who built the AES website :O)

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