Tuesday, February 26, 2013

Rokkaku pushed to the limit

Continuing on with rokkaku testing, here's a video in Bft 5 or up to 20kn. That's beyond the recommended wind range for this kite but I just had to see how it would behave.

Line pull was averaging 20lb and peaked at 25lb, which is unpleasant.

The Skyshark P4x spars flexed alarmingly but survived.

Sunday, February 24, 2013

Rokkaku bridle and bow adjustments

Here are two videos showing the tuning adjustments used on a rokkaku kite.

If either of these adjustments are a long way off the kite will either pull like a donkey and not fly high, or swing around wildly from side to side, or fly forward then drop back repeatedly.

But within a small range of adjustment you can choose how high it flies or how hard it pulls.

Angle of attack or bridle height adjustment.

Spar bow adjustment

Friday, February 22, 2013

Making a 2m Rokkaku - bow tension and flight

Part 4 of 4

Spar bow lines
A rokkaku is a flat kite which means it has no inherent stability. Some bend in the spars is required. Adjustable tensioning lines bend the spars like an archer's bow.

In the sewing instructions I mentioned how to create an end loop in the pockets for the bow lines. This photo shows how the end loop works.

The lines are about 2.2m long, tied at one pocket, looped through the other pocket then back to a sliding clip.

I made the sliding clip from UPVC plastic which can be heated and bent into shape. A 30mm x 15mm piece is drilled with two holes, rounded off and heat bent with a slight curve.

The clip slides along the line and holds the bow under tension.

The amount of bow can be adjusted to change the flight characteristics of the kite.
More bow gives less pull, more stability and a lower line angle, better for stronger winds.
Less bow gives more pull and a higher line angle but will reduce stability. If the kite is wiggling side to side you need more bow.

Some folks say you must have more bow in the bottom spar than the top, others say they can be the same. Mine seems to fly well either way.

I tend to set up with 6 to 8" of bow in the bottom and a little less in the top.

Angle of attack
The final small loop in the bridle is used to adjust the flight angle. The prussic (prusik) hitch, or double larkshead hitch holds it in place under tension but can be loosened and moved under no tension.

With the loop at the centre point of the bridle the kite will pull like crazy and sit on the ground. Moving the loop up a long way, say 12", will make the kite go up a bit but tip over towards you.

This ideal bridle position is somewhere between these two extremes. Try placing the loop about 8" above centre and see how it flies.

If the kite flies but with not much pull and is luffing or tipping over at the top move it a bit lower.

If the kite is pulling hard and not rising far move the loop a little higher.

When it is in the correct spot the kite will climb up high and sit there with moderate pull on the line. You can now dictate exactly how much line pull you have by moving the loop up or down in small amounts. If you need more pull for lifting an SLR move it down a bit, and if you want less pull for easier flying move it up a bit.

The rest is up to you, go fly the kite and have fun.
I'll make a video showing different bridle positions and the resulting flight characteristics soon, maybe different spar bowing too.

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.

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.

Making a 2m Rokkaku - Sewing

Part 2 of 4
Ripstop is very light and slippery which makes accurate sewing a little tricky. I pinned the panels together to sew the seams. Other kite makers use tape, glue or melting tools but pins worked for me.

Common sewing stitches used are straight and zigzag. Straight stitch has less stretch and is good for seams.  Adjust the stitch length to about 8 stitches per inch. It's essential to practice first on scrap cloth to ensure the tension is correct. Consult your manual or ask an expert to adjust the tension. I used zigzag on the corner patches because that seems to be what the experts do.

Panel seams
Pin the panels together face to face and sew along the 15mm seam line pulling the pins out before you run over them. Open out the panels, fold the seam over, and sew it down flat. The raw edges will be exposed but coated ripstop won't fray. For a really professional finish you can fold the top edge under before sewing down. Might try that next time.

Corner reinforcing patches
Corner reinforcing patches are cut like pieces of pie from a 150mm diameter circle of stiffer cloth. Once the panels are sewn together position the circle over the top corner, mark the angle and cut out the wedge. For the side corners you only need to fold the remaining part of the circle in half and cut along the fold.

Once all the panels are sewn together you can sew on the corner patches but first fold and make a crease all the way around the kite along the hem line. Now position the corner patches a few mm in from the hem line with pins and just sew around the curved edge.

Now for my favourite bit, the double folded hem. I like this bit because it's easier than seams and it finishes off the kite, apart from a few more patches. Fold the fabric edge over to the crease then fold again to form a 10mm hem. Sew down using straight stitch all the way around the kite. You can fold as you go.

Side and top spar pockets are made from 100mm lengths of 25mm wide webbing with 35mm folded over folded over to form the pocket. When sewing down the side pockets you can form an end loop for the bridle lines by sewing across 10mm back from the fold.

The bottom spine pocket needs to be adjustable to tension the spine. Velcro and webbing are used to form a pocket and adjustable flap.

Bridle holes and spar crossing points and ties
Four 40mm square patches are sewn on 400mm out from the centre of the kite and in line with the upper and lower spars, and another in the centre of the spine. Beware, the centre of the kite is on the folded edge rather than the middle of the centre seam. 

To make holes for the bridle to pass through I melted holes in the patch using a heated piece of coat hanger wire. Other kite makers fit brass grommets as well.

At the spine and spar crossing points I used a 80mm x 40mm patch. Onto the patch goes an 80mm piece of webbing which has velcro pieces sewn on to hold the cross spar. A 15mm stitching gap is left in the middle of the webbing for the spine to slide through. That way the spine always sits between the spar and the cloth.

Velcro is sewn on to the centre spine patch to hold the spine.

Here is a video of the process which will help make sense of these instructions.

Next post will cover spars and bridle

Tuesday, February 19, 2013

Making a 2m Rokkaku - Preparation

Part 1 of 4 
I have made kites all my life, but nothing as big or serious as this one, and have a fair bit of sewing experience. Between high school and uni I spent some time as a sailmaker's assistant and made my own sailboard sails. Wife Virginia encouraged me to use her precious Pfaff Tiptronic 2030 so I was almost set to go.

Great advice given by expert sewers is "new project, new needle". Respected kite maker  Gary Engvall recommends that a size 90 universal point needle is right for ripstop. So I bought a packet of size 90 universal points and 2 rolls of Gutermann polyester thread.

A Rokkaku kite is very simple being basically a flat hexagon with patches and pockets added and there are plenty of plans and How-to websites available.

I closely examined Rokkaku photos on Flickr. especially those by Cris Benton, Blue Kite Team, KAPPIX and Jones Airfoils and read through Gary Engvall's instructions many times.

I decided on a 5:4:3 size ratio and 2m high design. This means that the height is 200cm, width 160cm and main body 120cm high. This is a good size for a wide wind range similar to the Skydogs and Premier Rokkaku kites. It should have roughly twice the line pull of a ITW Levitation Delta making it perfect for lifting an SLR in Bft 3 and 4 and lighter rigs in almost any wind.

Now for sourcing the materials. Ripstop nylon varies greatly. Don't go to a normal material shop and buy their cheap ripstop, it's heavy, stretchy and only good for shower curtains, kite bags and tails.
For kites you need the stiff crackly stuff used for spinnakers. The most common weight seems to be 3/4oz. I used Emma Kites $6.00 ripstop. Although it's very light weight it worked really well. Might try ITW, Kitebuilder or a local sailmaker's spinnaker cloth next time.

You also need some stiffer reinforcing cloth for corner patches and other stress points. I had some cordura for my kite but dacron sailcloth is more often used. 1" wide webbing is also needed for corner pockets and spar loops.

Here is an A4 sized gif of my kite plan. Feel free to download and print it. Red seam allowances are not to scale.

It's important to cut out the panels with the long edges aligned along the warp or down the length of the piece of cloth. That way the finished kite will hold its shape better with less ugly stretch wrinkles. The warp has less stretch than the weft.

I made a template for the triangle panel which made marking out
much quicker.

Mark out the finished size of the panels on your ripstop in clear lines (to sew along later) then add the seam and hem allowances (for cutting out)

Next post will discuss sewing together the panels and adding reinforcing patches

Sunday, February 3, 2013

Camera settings for KAP

The biggest problem with Kite Aerial Photography is that the camera is moving all the time, sometimes to a scary degree. More than once I have seen the camera do the full 360 over the kite line.

So you must use a fast shutter speed. I use shutter priority and routinely set it on 1/2000th. That's Tv on Canon and S mode on Nikon. In light and steady winds you could use a slower shutter speed.

With the Canon S100 I also choose Auto ISO to prevent underexposure on dull days. With the Nikon I use ISO200 because Auto ISO is not available in S mode.

In shutter priority mode the camera chooses the aperture for correct exposure. With the shutter set at 1/2000th the aperture will most likely be wide open. This is OK because the subject is usually far away so depth of field is not an issue. Even with wide apertures, like f/2, everything will be in focus.

I'm undecided about Image stabilisation at this stage having tried with and without but not seen any difference.

I have the intervalometers set at one shot every 5 seconds on all three cameras.