Thursday, February 21, 2013

Making a 2m Rokkaku - Spars and bridle

Part 3 of 4

Carbon tube spars
Wrapped carbon tubes seem to be the spar of choice for many Rokkaku flyers, they are certainly the lightest and quite stiff but a little expensive locally.

Kites and fun things have a great range of SkyShark tubes way cheaper than I can find in Australia. The P4X camo are US$3.50 each, an absolute bargain even with $42 shipping. I ordered 12 spars plus internal solid carbon ferrules and vinyl caps to fit.

The P4X spars are 0.297"OD, 0.244"ID, 32.5" length, or in modern units 7.5mm OD, 6.2mm ID, 82.6cm length. It's pretty much impossible to find full length spars so joining is essential. Most of the wrapped carbon tubes are made as arrow shafts, hence the standard 32.5" length.

It's best to avoid joins where spar and spine cross and at bridle attachment points so some nutting out is required to determine what lengths to cut.

The cross spars are roughly 160cm requiring a 3 piece design. 2 x 3/4 lengths with a 1/2 length in the middle. That left 4 x 1/4 cutoffs.

The spine is 200cm requiring a 4 piece design. To reuse some of the cutoffs  I used 2 x (1+1/4) lengths. The 1+1/4 being permanently glued with an internal ferrule so it's actually a 2 piece spine.

Ferrules are glued in using epoxy.

The next job is to carefully fit them to the kite, trimming the ends evenly until they are the right length. Take it slowly, you don't want to cut them too short. Then you can pop the vinyl caps on to cover the ragged ends.

Bridle
The final task is to make up the bridle. It consists of top and bottom Vs (black in the photo) joined by another V (red) with a short loop at it's apex (green).

For the top and bottom Vs take 3.6m of 200lb dacron line (making a 1.8m V), pass the ends through the kite bridle points and tie to the spars using a larks head hitch. Carefully find the centre of the V and tie a small loop. The line for the third V is about 2m long (making a 1m V) and is tied between the top and bottom bridle loops using a bowline knot.

The final short loop is made from about 30cm of line and attached by a prussic hitch to allow angle of flight adjustments. This hitch grips under tension but can be moved under no tension.

Initial placement should be about 8" higher than the middle. You might need to slide it higher or lower to tune the flight angle.

Here is my first flight of this wonderful kite.

and another compared to the Skydogs Pirate Rok


Next post will cover spar bow lines and kite angle of attack.

8 comments:

  1. Hello Andrew,
    Can you suggest me some best replacement on P4X Camo tubes.

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    Replies
    1. Hello Soumya, You could try fibreglass tubes or even bamboo. What can you get locally? The Skyshark P4X spars are the cheapest and best option for me.

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    2. Thanks Andrew. By the way, can i use any type of Carbon Tubes. Bcoz, It's harder to get in India. But one RC Quadcopter retailer has Carbon Fiber Tubes with 8mm OD, 5mm ID and 1000mm Length, which is out of stock now.
      Regarding Sky Shark, what is difference between P4X Camo and P Series Tubes.
      Did you got the ferrules from the same site? And what is the purpose of the cap.
      Hope you dont mind with my questions.

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    3. That 8mm CF tube might be too stiff and heavy with a 1.5mm thick wall. There are "pultruded" and "woven" CF tubes. Skysharks are woven, which means they can be lighter and more resistant to splitting. Pulltruded tubes can certainly be used and may be cheaper.

      According to the website the P4X is very similar to the P400 Skyshark, I only use the camo spars because they are cheaper. Some KAP folks use P200 and P300 also.

      Yes I bought the ferrules from the same place and the caps are just to prevent the spars wearing through the webbing kite pockets.

      Please ask whatever questions you want. I'm happy to share what I know.

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    4. Thanks Andrew. Since I'm in India, I'm going to get all these thing shipped from US (have to Pay a lot). And so, I'm asking everything at once. Does the Fabric from emma kites works fine? I think, 5 yards will work fine to make a 2m Rokkaku.

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    5. That's what I have to do also. Postage was about A$40 so I buy lots of spars for future projects. Emma Kites ripstop is excellent for kites. It's very light and strong, not first quality but it works well.

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    6. Andrew, where can i get those fine Dacron Lines with the suggested lbs (strength). Why is the fabric from Emma kites are not First Quality and what does it concerned about.

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    7. Do a search for Ashaway Dacron kite line, I bought mine on Fishpond.com.au but it came from US somewhere. The cross threads (weft) on my Emma kite cloth was curved rather than running straight. Doesn't seem to make any difference but with top quality ripstop all threads would run straight.

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