Dynamic spar bodge

Mike LeDuc came up with a brilliant mod for the ITW Levitation delta. He spotted the tendency for the spreader spar to snap under excess load and instead of the obvious ‘replace with a stronger spar’ solution developed an elastic version which would shorten under load. His design is elegant and has proved successful. I want one.

I began to collect the parts I’d need to make it with help from Manfred Kluge at MK kites/Kites 4U and came up with 2 x 825mm pieces of ‘Excel Cruise -6’ carbon with nocks.

The hard bit was the captive nylon bushes Mike used: they require bonding and precise pinning. I didn’t like the idea of pinning anything to the precious carbon in case I over did it and split the spar- carbon is funny stuff and I treat a bit like glass, it’s likely to shatter unexpectedly if you abuse it.

So what to do? I put the bits I had in a corner and got on with other things. Once in a while I’d lay out the pieces out and ponder: how am I going to get a parallel motion constrained between the 2 pieces?

The tried and tested method of beefing up the Levitation is to replace the spar with 8mm carbon. I have done this and it works well. I decided to do the same for the PFK Nighthawk in an attempt to smooth its jerky behaviour in Bft 5. If it works for the one delta surely it should work for another? It does: I ordered the required Skyshark 8P, cut it to length and the tawa spar was replaced: easy!

The Nighthawk is softened a little by using the carbon and its weight is reduced a fraction. I was pleased with the result….and I had a 2 short pieces of 8P tube left over…and that got me thinking:

Tube ends2 pieces of 8P 160mm long provide a rigid constraint for the 6mm carbon spars..

Wrapped tubes..the ‘rigid’ constraint is duct tape ( the engineering ‘solution’ for things that shouldn’t move) I wound the tape carefully, starting at the centre, to avoid twist and to keep the weight balanced- if nothing else this would be ‘proof of concept’ while I thought of something better…the old ‘temporary permanent’ solution.

Parts 2MK kites supplied lovely ally nock ends, plastic ones are available but I like the purple anodising. The 6mm Cruise spar slides easily in the 8P guide.

Assy 1I have a good stock of ‘size 84’ rubber bands for powering my silly putty rig. 2 looped together proved to be just the right length. The replacement spar fits the kite easily and it flew well, at first the kite heeled to the left so I slid the tubes to the right and this centred it. As I wondered if I needed some method of centring the kite flew steadier and steadier. On returning it to the ground I found the tubes had begun to twist apart forming a dihedral, I began to think, provided the movement didn’t continue, that this is a self centring spar!

Assy 2The dihedral is very slight and has not shifted so far. There is a tendency for the opposite ends of the spars to touch the kite sail as they push inwards under extreme load.

ITW Mod 1I have put about 2 hours flying time into it in various winds and it’s coped well. I check the orientation and centring of the tubes before flight to make sure they don’t present an anhedral aspect.

I think the thing would naturally slip round to the dihedral position but I don’t want to find out in mid flight. An exaggerated view of the geometry shows why, once the 2 tubes have shifted to form an ‘x’ they become self centring:

Assy 3The stretch of the duct tape has found its own centre between the 2 tubes and this balances the elastic movement: a happy outcome provided the tape doesn’t stretch further and un-stick altogether.

For a parts bin lash up this has worked well and I have avoided epoxy bonding and introducing pins to the carbon. It may be a bit on the heavy side but it definitely improves the wind range of the kite.

Test shot at 16mph wind speed. Things didn’t go so well on recovery, the kite slewed into a tree and, in tugging it out I knackered the spar, the duct tape stretched to the point where it no longer locks the ‘x’…a stiffer bond is needed!

B

About billboyheritagesurvey

Heritage worker
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5 Responses to Dynamic spar bodge

  1. Ramon says:

    My trusty old Levi Light started to fly off centre since end of December 2013 in somewhat stiffer winds. While my Levi Light flew like this, Tom’s (de kist) flew straight as straight can be. Then this daily digest came through the electro-post and this solution made me think… I want one too! With a bit of luck I have all the materials in stock (lots of duct tape for sure!) so I might have my version ready tomorrow. For a reason unknown I never was triggered to look for such a simple solution, not even by the stuff Mike exposed on the forum, but your blog did, Bill!

    Ramon

    • Thanks for looking in Ramon,

      So far the bodge version has worked well, I worry about it as it’s possible it has introduced new failure modes to the kite ( snapped elastic, ungluing tape etc etc).

      I have noticed a reduction in the lifting power of the kite. I have enjoyed flying 2 rigs before the mod but can’t repeat the trick after.

      Mike’s approach to this is very thorough and his work gave me the confidence to try out a ‘back yard’ version.

      Let us know how it goes,

      B

  2. Ramon says:

    Instead of using duct tape (I’ve been thinking about this today) I’d like to use a technique used in angling: http://www.abc-sportvissen.be/images/images%20vissersknopen/oogwikkeling1.jpg . With monofil 40/100 it should be a strong solution to attach the two 16 cm 8mm carbon pieces together. I am also thinking about a slick solution for the dynamic part of the contraption.

    I will put something online as soon as I have something usable.

    Ramon

    • Excellent idea Ramon, it needed a stiffer bond.

      Yesterday I was testing this out and crashed the kite on landing- no big deal but it snagged a tree. I tugged the line and down she came. I decided to check the kite over and flew it in a blocky 8-13mph and it heeled badly. I checked the spar and the tape had stretched to the point where the tubes could move off centre. I centred it and the next gust repeated the effect.,

      The centring effect is lost unless the angle is rigid. I’m thinking of binding the centre to a hanger like Mike did so that it can’t slip.

      B

  3. Ramon says:

    Bummer, Bill! Anyway, I checked out Mike’s way of doing it and I must say, it’s a great way of doing it. I checked my stock and most of the stuff I need is available. I only need those nifty pieces he is using.

    Today I finished the work on a new Kite antenna. I need to modify one little thing – I now employed a single Brooxes Hangup, but I’m sure I get a better result with two (stuff on Flickr). With a bit of luck we’ll be on the air from a meadow somewhere next weekend.

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