Standard Libelle tie-down system

Initial Ideas

General arrangement

The rope attachment point is fixed to the top of the moulding with a single M5 bolt. The triangular thing behind it is a chunk of fibreglass to make sure the rope can't fall off if it goes slack. It would either be that or an alloy strap held down by the M5 bolt with its rear end bolted to the moulding behind the spool.

The moulding will have a wide piece of 1mm alloy moulded into the inside to spread the load. This will be 75mm wide at the TE, tapering to 25mm at the bolt, wrapping round the TE and remaining 75mm wide on the underside, where it extends at least 50mm forward.

A piece of 1.6mm epoxy plate will be bonded to the top surface under the rope attachment point. This would either be 25mm wide with a 25mm semicircular front and extending 50-75mm toward the TE or a 50mm disk. The M5 bolt fits through both reinforcements, probably with a countersunk head so it fits flush with the inside surface with a nylock on the outside.

Obviously there's foam rubber glued on inside the whole structure so there's only soft rubber touching the wing.

Rope attachment point

This drawing assumes the rope retainer is made from a slice of 25mm alloy rod, fitted with a single flange and attached to the moulding with a single M5 bolt.

Rope attachment point

Final arrangement and materials

This turned out to be pretty much as first sketched. The first thing to go was the internal alloy strength member when further thought made it obvious that that two layers of 235gsm carbon cloth would be at least as strong and a lot easier to make and fit. After all, the body of each tip holder was always going to be moulded, so slipping another layer or two of cloth into the layup was a no-brainer. As I had some rather nice 100 gsm twill-weave glass cloth I decided to use that for the outer surfaces of the mouldings and, because I needed a nice thick laminate if I was going to countersink it to take the M5 screw heads used to secure the rope attachment points, I added a layer of 100 gsm standard glass cloth on each side of the carbon. This resulted in a 1mm thick moulding which, added to the 1.5mm epoxy plate gave me 2.5mm to countersink for the screw heads. Perfect.

The rope attachment point was made exactly as I'd first drawn it: a piece of 25mm alloy rod turned in a lathe to give concave sides to suit the rope and topped with a 50mm disk made from hard 1.5mm alloy so the rope would have to be really slack to slip off. I decided to use a 10mm strip of 1mm alloy rather than the epoxy plate solution because it is equally strong and less likely to catch on anything. Each attachment point was held in place with two M5 screws with countersunk heads. These were fitted from the inside after countersinking the mouldings the leave the heads flush with the inner surface. I used Nylock nuts so the screws would be unlikely to work loose.

The reinforcement under the rope attachment point got redesigned so it was the same width as the disk (50mm), with a matching semicircular front and extends all the way to the trailing edge. I chamfered its edges and fitted it under the top glasscloth layer instead of gluing it on the outside because this made a neater as well as a stronger job.

Martin Dilly, a long-time model flying friend was kind enough to turn the rope attachment fittings for me. When it turned out that Claw tiedown anchors, which I'd always thought were really cool, were a similar price to traditional corkscrew anchors and came with rather nice soft 12mm rope, I bought a set and then found that a pair of Petzl Basic Ascenders would do nicely to lock the rope. I'm certain there are cheaper alternatives, but these were available locally.

I lined the mouldings with pieces cut from a 6mm foam sleeping bag underlayer and stuck these in place with double sided tape.

The rest of the materials were on shelves in my model building workshop or, in the case of the laminating epoxy, in my fridge.

Making the mouldings

I made an initial, abortive attempt to wrap a wingtip in 6mm rubber and make a mould over that. Bad idea. Didn't work at all well. So, I made drawings and measurements of a tip and used that to build a male mould at home. This was a framework made from 3mm balsa and planked with 1.5mm balsa sheet to form a solid shape with square trailing edge and tips, though it was a fairly good representation or the curves in a Libelle wingtip. Blocks of blue foam were glued along the trailing edge and the edges representing the wing tips before being carved and sanded to shape. Finally, the surface was toughened by finger-rubbing two coats of aliphatic wood glue into it.

After this, it was a simple matter to cover the structure with a thin release film and use laminating epoxy to lay up the mouldings, one on each end.

Photographs

Click on any of these photos to see a bigger version.

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Frame for the male mould, made from 3mm balsa and intended to represent the TE of two Libelle wingtips when wrapped with the rubber liner for the tip mouldings.

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The frame after planking with 1.5mm balsa.

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The completed male mould, showing blue foam carved and sanded to represent padding wrapped round the TE and tip shape.

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The completed male mould, top view.

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Turned alloy rope attachment points.

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Tiedown mouldings on the form, cured and ready to be removed.

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The completed tip mouldings put back on the male mould after separation and cleanup.

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Completed tip holders, rope attachment points installed on the mouldings with countersunk M5 screws fitted from the inside. The 10mm alloy strip, secured with a second M5 screw, is used to ensure that the tiedown ropes can't come off if the ropes should work loose. Holes in the lower surface give screwdriver access.

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Completed tip holders, soft foam liners in place.

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Rear view of the tiedowns in use.

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Top view of the tiedowns in use.

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Front view. By putting the centre of the Claw exactly under the wing LE and somewhat inboard any chance of the rope slipping off the tip is avoided while at the same time not putting any fore and aft load on the wings.

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Another view of the tiedown system in use.