Dribbler

Here's my progress on designing and building the Dribbler.

The basic shape has already been sorted out for the Dabbler, although I have modified the profile shape significantly to flatten the leading edge to reduce drag. Here's a picture of the new profile shape...

This profile is much flatter at the nose than the Dabbler, which is almost vertical at the nose. This will hopefully reduce drag, and along with the inflatable battens, improve luff resistance. This profile shows a 20cm circumference leading edge. The kite is actually built with a 30cm circumference leading edge. This is because I managed to get hold of some bigger tubing, and a larger tube should add rigidity.

Other changes are as follows :-

	Angle of Attack changed from 3 degrees to 5 degrees. This should increase pull. This will also reduce depowering, but the Dabbler had too much depower anyway and tended to luff. I'm also hoping the Dribbler, being made from chicara, will have a better wind window, and therefore I think I can compensate for the wider window with a bigger angle of attack and not lose any depowering ability. This should also accentuate the difference between powerup and powerdown.
	Increased aspect ratio. It's a bigger kite, and therefore will be more sluggish in turning. Increasing the AR from 3.7 to 4.5 will compensate for this.
	Reduced taper on the wingtips. Increasing the AR results in smaller wingtips, and reduced control. So, I have made the wingtips 20% larger in proportion to the rest of the kite, so that controllability is not lost. This also gives increased power-up/down control.

1st-9th December, 2001 - I took a week of holidays to build the Dribbler. I also finished my buggy, and took it to Bribie for a test run with the Dabbler 2.2. Unfortunately conditions were not good, with a high tide, very little wind, and intermittent rain. We had fun anyway, and hopefully it will be better next time.

OK, here's the details...

The Dribbler is being made from chikara. This is a material specifically designed for kite building, and is a 3/4 ounce ripstop cloth that is water proof, and seems to be airproof also, where many other ripstops will absorb water, and also leak a bit of air.

Kite Magic only had yellow and blue in stock, so after much careful thought, I decided to make the Dribbler yellow and blue.

My kite program was enhanced to model the leading edge inflatable tube and add this to the plans. The plans were taken to a local plan printing shop, and for AUD$12, they printed them out full size in about 5 minutes. Way to go!

I marked, cut and sewed for a day, and the main skin was together. Then came the inflatable bits...

Inflatable bits

This is the real crux of the Dribbler. Last time I ordered some fibreglass poles from Kite Magic in Sydney, they came packaged in some plastic tube, and I realised that here was the answer to all my troubles. It is called HDPE (high density polyethylene) tubing, and comes flat on a roll, so can be cut to any length as required.

To make each bladder, I cut the tube to length, sealed one end of the tube with some really sticky silicon stuff. I tried contact cement, but it was no good. It kept coming apart. For the other end of the bladder, I made some flap-valves from short lengths of vinyl tube with a flap of HDPE on the end, and sealed them into the end. After inflation, the valves still leak slowly, so I bought lots of nozzles for the silicon tube, and filled them one by one with silicon, and let them dry. These fit really well into the end of the valves, and work really well as plugs to make the bladder air-tight. I don't know if they might fall out, but time will tell.

After a day of inflation, the bladders still had air in them, but the pressure had definitely gone down. They are all like this, so it's not holes. If it was, some would be more deflated than others. There are several possible causes

The plastic is not fully airtight. I have heard that polythene, or polyethylene is air-permiable. This is yet to be verified. I will try a different sort of plastic next time, and I may have to make my own heat-sealed tubes.
The silicon is not fully airtight. It may be that the silicon does not adhere well enough to the plastic, and is letting air through between the silicon and the plastic. I'm also not waiting several days or more for the silicon to dry completely, because I'm impatient. This may be causing problems.
The valve plugs are not air tight. It looks like they are, but maybe I should grease the plugs to make sure, but then they might fall out.
Somebody snuck in to the room and deflated the tubes while I wasn't looking. Our silky terrier was looking very guilty about something the other day.

Anyway, they seem to stay up well for a few hours, so I guess maybe I just have to blow them up again if I want to fly for more than a few hours.

Deflating the bladders is easily done by pushing a pencil through the valve to hold it open.

The plastic also seems to be prone to small rips, so I will keep an eye on this, and also keep an eye for alternative solutions in case this doesn't work long term.

Inserting the bladders

I had some difficulties fitting the leading edge bladder in with the valve at one end. I ended up inserting it from the middle outwards, so that the whole bladder doesn't need to be fed through the small opening at one end. This didn't work so well, because the valve didn't fit through the end. I have now cut the leading edge bladder into two, one for each half of the kite, and the two valves are in the middle of the kite, accessed through a velcro flap. The non-valved ends are fairly easy to push almost all the way to the wingtips. This will also make it easier to repair holes, as only half the bladder needs to be removed. For major problems, I also only have to replace half the bladder.

The other bladders are fairly easy to get into the kite by pulling through with some string.

17th December, 2001 - Finished the kite last night. Ready for first test flight. I don't want too much wind for the first test flight, because I don't want to double-stitch the seams until I've tried it out.

26th December, 2001 - I've had a few test runs now. Unfortunately, the wind is very variable near my house, so it's hard to get a good test, but I've launched it a few times, and had a look at the results...

It flies! Yay! It flys well, pulls hard, and is very stable - all of this when it's inflated well. The biggest problem is that it doesn't stay inflated. The bladders leak, and the valves aren't that good, so as far as the inflatable bits go, it's back to the drawing board. But the rest of the design is fantastic.

The kite has flown past vertical, so I'm not worried about wind window any more. It didn't collapse at all - it just floated back into the window and sat there. It pulls like a truck in fairly low winds. It is quite controllable, although four-line steering is no good. When it's pulling hard, it takes too much effort to twist the handles. I'll be adding the cross bridle at some stage, so that will help. Another alternative is to use a control bar instead of handles.

The wingtips need support again, just as they did with the Dabbler, so some rods have fixed that, although they are removable so I can still pack up the kite into my backpack. These are not 100% necessary, but make the kite more stable.

So far, the only reliable way to take off is lie the kite on its back, leading edge away, and vertical launch. The bladders don't hold the kite rigid enough to do anything else. The kite just folds flat over itself. Without struts, I'll need much more pressure in the bladders to hold the shape.

27th December, 2001 - Did some repairs on the kite in preparation for holidays at the beach. I repaired over 30 holes in the 7 bladders! Time to find a new sort of plastic. At the beach, the wind was howling the whole time, and I was too chicken to fly a 5.5m kite in 30 knot winds.

The next tests will be with LDPE garbage bags. If that doesn't work, I'm giving up on PE altogether, and I'll have to try to find some latex or rubber composites somewhere. I'm also looking at PVC, which might be good if I can get it thin enough so it doesn't add too much weight.