Foil Kite Turbocharger

• I am a Peter Lynn ARC fan. IMHO, this kite design is so exceptional that it should be admitted to the Smithsonian Air and Space Museum, in it's hall of fame no less:
• It is the simplest possible expression of a foiled kite, a foiled "sled"
• There are no bridles, the two flying lines and two control line attach to the four corners
• Like with most sleds, the arc shape locks in the angle of attack of the center
• The arc benefits from tremendous "sled boosting effect"
• It is a high performance design, extremely efficient due to the tension in the upper skin that flattens out the billowing of the cells, as much on the upper skin, as the trailing edge

In 2002, Peter Lynn put out the F-ARC. It was considered an extreme kite, and it was, the 16m2 F-ARC 1600 having a scary aspect ratio of 7.0! Still today, it is still popular with some buggy and kite skiers. This said, I have quite a collection of F-ARCs that were produced in that era. I have cloned several, making a 1900 (AR 9.0) and a 1400 out of two 1600s and making a replica of Brian Holgate's modified F-ARC 1200 (cut down to 950) which he used to beak a world buggy speed record. It is an extremely fast kite!

Because the market shifted to LEI kites at that time, Peter Lynn unfortunately got stuck with a huge inventory of F-ARC kites. Over the years, they were liquidated, some given away,... I saw one for sale just the other day on e-bay going for a fair price. Gavin Mulvay who worked at Peter Lynn NZ may still have some F-ARC 1600s if you are lucky.

If you are able to pick one up, you should, but I warn you, it has it's quirks. This INSTRUCTABLE aims to eliminate these undesirable tendencies, making this kite all the more interesting. This image sums the benefits nicely. Warning no 2: They can be scary fast and powerful!

• A brush-less computer fan
  • With the appropriate size so that the foam body of your turbocharger keeps some structure
  • With a capacity suitable for your kite. I chose a 92 x 92 x 25mm model with 50 CFM
    • Test it before you start by putting it in the zippered slot
  • Note: Some are heavier than others. Shop around!
  • Sleeve or Hypro bearing type
    • Ball bearing models will rust!
    • Hypro bearings are MAGLEV, in other words, the impeller floats with no contact at all!
    • That's the cat's meow!
  • Consider the voltage you need to power it:
    • (4) NiMH @ 4.8V works fine with 5V models
    • (3) Li-Ion in series or 3S LiPo at 11.1V will work with 12V models
• A slab of foam
  • As thick or thicker than your fan. I used 1" foam for my 25mm fan.
  • Big enough for the air inlet you want to turbocharge
  • It can be extruded EPS, but polyethylene packing foam is better, more robust
  • Some double sided tape to bond in in the inlet duct
• Batteries
  • Capacity in function of the autonomy. Divide the capacity by the current to find out...
  • GOOD: (3) 18650 with 2600 mAH which will run 10 hours @ 0.2A, but they are too heavy
  • BETTER: (3) AA size Li-Ions (14500) with 1000 mAH, 4 hours and much lighter
  • BEST: (1) 1000 mAh LiPo 3S battery coming in at 70 grams.
• Wires and switch.
  
  • A power switch can be used, if not, connect the batteries when needed
  • Limit switch can be used to turn the fan off when internal pressure is adequate
  • If ever the pressure drops, the fan kicks in automatically.
  • Be creative. Hint: There are many ways to do this, some better than others.
• Odds & Sodds
  • Doubled side adhesive tape and adhesive backed hook & loop fastener (velcro)
  • A bit of aluminum rod (1/16" welding rod)
• If you are clever, add an ARDUINO to have some intelligent power management:
  • Variable speed control according to pressure demand
  • You can read RPM on the yellow lead (back pressure lowers RPM) or measure current drawn
  • With a Barometric pressure sensor...
• Have fun!

Computer fans are amazing. They use highly efficient PM Permanent Magnet motors. Some have RPM monitoring, some even have speed control, but the very basic two wire ones are all we need for this project. One single 50 CFM fan is enough to inflate a 16m2 kite with a 30 foot flat wingspan in just 3 minutes.

The idea is to actively inflate the kite, rather than waiting for air pressure from forward movement coming into the air inlets to do that for us. There are any reasons why this is an interesting enhancement:

• It is so much easier to pre-inflate the kite prior to launch.
  • One of the flaws of the arc kites is that the kite may bowtie if not fully inflated.
  • This is tendency increases with AR, the wider the kite is.
  • The wingtips will often fold inwards, or the center may even dive in.
  • The kite can literally turn into a propeller and spin under great power and speed.
  • This is an experience you want to avoid, believe me!
• The kite can loose pressure in the lulls.
  • Oceanside, with steady winds, this never happens
  • But it happens often in gusty conditions, in the mountains and inland
  • After just few seconds of dead air, it can be game over!
• The previous events do not happen when the kite if fully inflated.
  • The pressure in the cells maintain the shape of the kite and if anything, wants to open it up.
  • The shoulders and the center no longer want to collapse inwards.
  • You know it is pressured up, it will lunge forward to the edge of the power window, very often lifting you off you feet.
• Some kites do not have sufficient inlets, or have become porous with age, lose pressure
  • Turbocharging them can fix that!

• Moreover, kites become particularly fast responsive when well pressured up, or over-pressured

The housing of the computer housing is heavy glass filled plastic, so we can cut that away, keeping only the core and the impeller. Mine was over 100 grams and trimmed only weighed only 60 grams.

You can then remove the label at the back to access the split washer that secures the impeller. You wont need to keep it since the impeller snaps into place by magnetism and will be captive in our new assembly in any event.

WATERPROOFING THE MOTOR

For use outdoors and in the water, you will need to waterproof the stator (static part) of the motor. You can do this with clear epoxy or even paraffin wax. IMPORTANT NOTES:

• Dip the entire stator, PCB and wires to wet it from all sides. Corrosion can be insidious.
• The coating must be thin, particularly with respect to the clearances with the magnet on the inside of the rotor.
• Be sure to mask the bushing as not to affect it's free spinning in any way.

You will need to design a new housing (duct) for your turbocharger to be 3D printed. Any CAD software or freeware capable of saving to an STL file will do.

Use the dimensions of the existing housing to get the proper dimensions for the duct.

Make the arms that hold the center deep and strong. You can come in closer to the impeller, around the motor. The plate under the motor disk will be thinner and the thicker arms will help with centering your rotor in the duct with precision.

Weight is critical so you can use minimal outside layers and light infill. I used 1.2mm for the skins and 25% infill for the core. Regular PLA was fine and in the case of an impact, I have kept the files to print another one.

I also left the print table adhesion apron around the part which helps in the assembly with the foam body (next).

Cut out the foam body to fit tightly in the center air inlet of your kite (see previous page showing the inflated kite).

Cut the center hole a little smaller so that your duct fits tightly. The print apron acts as a nice stopper.

Push in a small aluminum rods to keep the inlet mesh away from the impeller. In fact, since then, I have replaced this with a series of hoops with the leading edge profile using nylon weed wacker filament. This keeps the entire inlet mesh off the foam, maximizing surface area.

Use double sided tape (sailmaker's seamstick works great) around the perimeter to bond your turbocharger in place, tightly up against the inlet mesh.

If you design your turbocharger well, you can weigh in less than 100 grams!

Tape your power pack at the back of the kite inside the belly zip used to empty the kite and you should be good to go.

Weight

Weight and it's position is critical in a kite, particularly in light air. This is why so much attention was directed to weight reduction of the fan, located on the leading edge. But batteries can be more significant.

Perhaps you may want to run wires to locate your batteries elsewhere, or at least spread them out. Three (3) 1000 mAh LiPo cells, at 24 grams each, in series along the kite's webbing strap (on the center of lift) would be insignificant. Four (4S @ 14.8V) would give added punch but reduce the life span of the motor (10,000 hours according to the specs).

Fan types

EDFs could fit the bill but they are power hungry. Outrunners could easily be waterproofed with casting epoxy.

I would be curious to hack one of my many damaged mini drones. Four fans, each in their little 3DP duct could induce quite an airflow. It would be a matter of disabling the accelerometers to get all four fans running full throttle that could be controlled at the bar! Humm...

Notes:

This INSTRUCTABLE shares the principles and is not a specific recipe per se. By empowering you with knowledge, I hope to spark your curiosity and give you the tools to work with your components and adapt your design specifically for your kite, perhaps push the idea even further. This device should work on any Peter Lynn Arc, but perhaps you can turbocharge a closed cell bridled foil as well.

The main thing is that your curiosity gets sparked, you get embroiled in research, you make flamboyant discoveries, and mostly, THAT YOU HAVE FUN !

Cheers, Paul.