Fighting Kites

22 January 2016

If instructions were to be written.

1. Cut a 13" long piece of 9mm x 3mm wooden stick for the fuselage spine.
2. Mark across the top of a 9mm x 3mm wooden stick at the 1" and 3.5" marks. The 1" mark is where the nose of the wing will be located. The 3.5" mark is where the wing spar crosses.
3. Cut a 10" length of 0.8mm music wire. Fold in half, bend once at the folded end 1" from the folded point, splay open the ends until they are 5" apart. Then at each end, bend at the 1/2" point from the tip so they form a level surface on which the wheels may rotate. Install a pair of wheels and bind the undercarriage assembly to the bottom of the fuselage spine with the wire splaying out at the 1" mark of the spine.
4. Hot glue the receiver and servo block to the top of the fuselage spine, half an inch behind the 1" mark.
5. Draw the wing shape on a 2mm foam sheet. For the fighting kite's shape, the span is 15"; mark two lines 9mm apart at the middle of the span, that is where the spine will be glued to. The length is 12"; draw the leading edges and the trailing edges joining them at the middle points at the tips. Draw the elevons' hingelines with a right angle triangle, pointing to the nose of the wing, so that the hingelines will be approximately at right angle to the servos. The elevons measures 1" at their narrow ends.
6. Cut the wing shape and bevel the hingelines and glue it to the bottom of the fuselage spine.
7. Cut a small triangular piece of 2mm foam, 3" long, and 2" deep, and glue it to the bottom of the wing against the trailing edge. This is the tail skid.
8. Cut a 1" long piece of 20mm x 10mm foam for the motor mount and make a groove, 10mm diameter, in the centre to seat the motor. Hot glue the geared motor assembly to the motor mount and then hot glue them to the bottom of the fuselage spine over the undercarriage assembly, against the leading edge of the wing.
9. Glue a 2mm wooden rod of 17" length to the 3.5" mark on the fuselage spine, bend it at both ends and glue the ends to the tips of the wing. Snip off excess length of the 2mm wooden rod
10. Cut 2mm foam sheet measuring 5" long at the base, 4" high, 2" long at the tip and glue it centrally to the fuselage spine to form the fin.
11. Decorate, connect the battery and motor and go fly.

Oops, it is lengthy and boring.

13" length seems versatile enough. Maybe a simple RET biplane. But it would need to be longer if I want a monoplane.

12 January 2016

Photo on left was the first entry trawled from the internet. It shows the fighting kite that was popular in the early 70's. I was a young boy back then and I was never successful with this kite. I couldn't get it to fly.

It was a magical moment piercing 4 holes with a match stick. A pair is made across the bendy bamboo piece and the straight bamboo spine (a stout piece compared to the spar) and the other, somewhere near the tail across the straight spine.

Almost every household back then has a ready supply of needle and thread. I didn't even think of using needles, which is a good thing, because the holes would be too small for me to pass the thread through.

There was a procedure I followed to determine the length of thread to be used. The length has to go from the head, to the tip, to the tail and back to the head via the tip. This single thread was fed through the pair of holes at the crossed bamboo. It was evened out and tied so that the two strands are at the bottom of the kite, the side where the bamboo is separated by the tissue. The two ends were fed and tied to the bottom pair of holes. The thread was then picked up to suspend the kite, moving along the thread, the thread was pinched when the kite is hanging at a certain angle (I now suspect it is 15 degrees), and then while pinching the loop, a knot was tied which formed the point to attach the kite line. This wasn't easy, and help was usually solicited from my mother or my older siblings. Other magical moment were laying the kite over one's head and inducing a curve to the kite by rubbing it over one's hair for steady flight. The explanation was rubbing with hair makes it fly better. I don't remember correctly but I think I might have rubbed my kites the wrong way, on the bottom, that is, introducing anhedral instead of dihedral and pitching down instead of pitching upwards!

I got out mom's coil of sewing thread, tied it to the pinched loop and tried to fly the kite. The kite didn't last long and I messed up a lot of thread.

Everything seems bigger as a child, but from the photo trawled, it is very small. For a one-to-one scale model of the fighting kite, the wing area would be about 86 square inches for a 15" span. This is a good size for the WLToys' components.

RC version of the Fighting Kite

Power and Control

WLToys' motor and receiver to power and control because they are light, powerful, easy to install and mostly because they are cheap and available.

Size and Design Considerations

The planform shape of the fighting kite is almost a square, the balance point for this short-coupled model would be something like 20% or even shorter for sufficient inherent stability. For current purpose, I would guess that the 25% mark is 1/3 of the root chord (an equilateral diamond shape), but 20% CG mark is simply 20% of the root chord (a square wing), and this planform is somewhere in between.

I worked this out because I prefer to have all the gears placed infront of the 20% of root chord. If the nose tip to tail tip of the wing is 12", this means a working length of 2.5". Consequently, I shall mount the motor forward of the nose tip, by extending the spine forward an inch. Afterall, if weight is required to be added later after flight testing to bring the balance point to a better location, tailweight will only be a quarter of noseweight.

Control options

There are two control options:

• rudder-elevator-throttle. Tips are set with dihedral if the wing is rigid but perhaps the washout effect is sufficient if it is flexibly covered, and a set of fin and rudder is required. Planned correctly, the V911 transmitter should work, except that the rudder is on the left stick (mode 1).
• elevons-throttle. No dihedral is required but a fin is still necessary. The Flysky transmitter with module is required for the elevon mixing.

Choice of Construction

I am proposing two methods of construction for the wing, a tissue(or plastic)-covered wing, and a 2mm foam sheet wing. A covered wing is more reminiscent of the past, while a sheet wing is simpler.

Motor Mount

The motor mount shall be a chunk of foam (any foam would do) with a semi-circular groove side to accept the motor. The foam block will be about 1" long and perhaps 10mm thick, but the size is not critical. After a WLToys F929 F939 geared motor is glued to the foam block, the block is glued to the spine. I think hotglue is suitable for gluing the block to the motor and the spine.

Foam is chosen because it can be sliced in the middle easily to set downthrust or right thrust after flight testing. This adaptability is desired, considering that a 5-6" prop is spinning on a 15" span wing. And if it breaks, there will be sufficient area exposed to just glue it back on.

The motor is to be mounted infront of the wing for 2 reasons, to give it a balancing advantage and also to have the thrustline above and foremost of the CG. The latter reason will minimise the angles required for trimming.

Spine

Daiso's 9mm x 3mm wooden strips is sold so cheaply and they are stiff. A length of 13" is cut, I think they come in 1m lengths, so 1 piece can make 3 spines. With the motor mounted, this spine will be glued to the wing's topside.

Curved Spar

Daiso's 2mm diameter wooden rod is sold so cheaply and they are flexible.
The spar will cross the spine at the 20% mark, or 2.5" from the wing's nose, i.e. 3.5" from the nose tip of the spine and the tips will be bent until it is at the middle of the wing, i.e. crosses the mark 7" from the nose tip of the spine. The length of this spar if it is a smooth arc is 17" (using a calculator found on the internet), but I think I should add an inch to it, that is, 18", so that the tips can be straightened slightly to follow the rhomboid shape of the wing.
Instead of doing it like the kitemakers did, using the tissue to hold the spar in position:

1. Tie the 2mm diameter wooden rod to the top of the spine, 3.5" from the nose tip of the spine. Maybe a dab of hotglue? I am never quite good with strings.
2. Tie the middle of a string to the tail tip of the spine. Reinforce with hotglue.
3. Bring each end of the string to go around each spar's tip, pull and when the tip is at the location (7" from nose tip of spine), tie off.

Wing

Place the framework of spine and spar and glue the bottom of the spine and the tips to the upperside of the covering material (tissue, plastic bag, 2mm foam sheet).

UHU Por can be used for tissue/plastic bag wing and hotglue would be convenient for 2mm foam sheet wing.

Radio Installation and Control Surfaces

The WLToys' receiver will be hotglued to the top of the spine, behind the motor block. Thin wires running in tubes are the pushrods.

• Elevons are cut and hinged from the 2 mm foam wing before the frame is glued on.
• For tissue/plastic bag sheet covered wing, strip elevons are taped to the strung trailing edges of the wing.
• Stabiliser and elevators are cut from 2mm foam sheet and glued to the top of the spine before gluing the Fin.
• Fin, and hinged rudder if required, are of 2mm foam sheet and glued to the top of the spine. They may extend beyond the spine's tail.  

Main Material and Equipment

Construction material bought from Daiso:

• 9mm x 3mm wood strip (for the spine)
• 2mm dia. wooden rod
• 2mm foam sheet (for the fixed or controlled surfaces such as fin, rudder, elevons and elevator)
• 10mm foam sheet (glued to spine to mount the motor), or any scrap foam block

Equipment from Banggood:

• WLToys' F929 F939 receiver board
• WLToys' F929 F939 geared 8.5mm motor and propeller assembly
• FlySky FS-TH9X 2.4G 9CH Transmitter with matching module, mine is a Flysky FS-GT3 Module FS-RM002
• single cell batteries of 100-250mah.

Other sources:

• tissue, lacquer
• plastic bag
• Hot glue or UHU POR
• strings
• tapes
• control horns
• wires, tubes
• Optional wheels, permanent marker pens, paint and such.

Options

A profile fuselage from 10mm foam sheet glued to the top of the spine. Cut opening for WLToys' receiver board, extending to the middle of the wing. This strengthens the spine, and gives some area above the CG which will help as a dihedral device.

The stabiliser can also be mounted on top of a 10mm thick foam piece. This piece can be part of the fuselage or be made separately. This brings the centre of lift higher and keeps the elevator from trawling the ground.

Install a pair of wheels. Bend the wire legs and heatshrink (or binding with thread) over the nose of the spine. Insert two wheels. This will bring the CG lower and enable rise-off-ground flights. If the battery is mounted to the bottom, this will also protect the battery and connection. While at it, might as well make a foam skid to protect the tail ends. If the model is to be equipped with undercarriage, the motor mounting block can be glued to the lower side of the spine. This will require slightly longer undercarriage but clears the top and the receiver board can either be brought further forward or for mounting the battery topside.