Small craft foam wheels and other bits

15 July 2016

Too much work for little bitty wheels! Anything round and has a hub would qualify as wheels.

Getting the CG also means getting the wing and tail incidences and the thrust line correct. They all work together and influences one another. If I view the tail as a stabilising force, the tail should be flat to the airflow, little or no lift, and the wing propped up slightly at a positive angle for stable flight. But the more it is propped up, the more 'one-speed' the plane becomes. So I could also set both the wing and tail at zero degrees to each other and use a bit of 'up' elevator for stability. From the slowflyer example, the wing has lots of positive angle to the wing, relative to the tail. I had to 'down-trim' the elevator for that CG position I want and the slowflyer flies with the fuselage nose pointing down. In other words, I have simply built in more downthrust. I have elevator control in a RC model, I could simply have set both wing and tail at zero angle to each other and used a bit of 'up' trim, and let the plane figure out itself what angle to hang the wing, probably 0.5-1.5 degrees, very minute anyway. By doing so, we can then approximate the CG placement base solely on the distance and relative size of the tail. The real CG depends on how the plane glides during test flying.

Thrustline angles are dependant on where the thrustline is relative to the centre of gravity and centre of drag of the plane, how long the propeller is, relative to them, diameter, pitch, area, shape and drag of propeller, and its spinning speed, and the flying speed of the aircraft. The wing is the biggest drag element. Just put in say 3 degrees downthurst and 1.5degrees right for general 'sport' flying unless doing aerobatic types (eh, maybe 1.5degrees eachway?). Whatever it was set to, the real thrustline depends on how the plane goes under full power during test flying. Adjust the thrust line to get similar power on and power off glide behaviour, quite impossible, but close to is enough. After CG is determined, more down thrust if plane zooms up under power and more side thrust to counter the torque. Thrustline acts on 1) CG, 2) centre of drag, and 3) the vector force which could cause a turning moment on the previous two points (vertically and horizontally), and 4) the convolution of propeller and spinning speed and is relative to the range of flying speed. Nothing much to work out, just try it in the air and observe the plane's behaviour.

For directional and roll stability, it doesn't really matter because we can manage even if it is without aileron and has only rudder control, but make sure the CG is centre to the fuselage and the fuselage is straight to start with. To improve rudder controlled planes, some form of dihedral: 1) the wing's dihedral or sweepback or 2) lateral area or CG above the CG/centre of drag, is required. Otherwise, the model will skid but doesn't translate to bank and the turning radius is large, therefore the turning response is slow. Another basic thing to watch out for is that rudder controlled planes has another quirk, if the fin and rudder is high up relative to the CG/drag centre (and nothing below it), it rolls the plane instead of yawing the plane. The rudder deflection will act like aileron and this rolling motion is not in the turn direction desired. Just lower the fin and rudder, probably don't need so much area anyway. This is borne out of my experience with my BD 5 where I got the area too big and they are way above the fuselage. So, it got to look right as well.

9 June 2016

The craft foam rimmed spoked wheels are installed on the big slow flyer and has lasted one flying session (a few landings). They seems to be working fine so the construction method used is ok.

Those wheels are for show but they are not attractive because their rims do not spin centrally. Obviously the acrylic setting tool is not good enough for such diameters.

Maybe a wheel jig like this will ensure that the cones and hub comes together correctly. I could also it to glue the silicon tubing tyres.

For the tyres, I don't think I have to slit the silicon tubing. The craft foam tyres have been strong enough, maybe gluing the tubing directly to the rim of the cones will work too. After the cones and hubs are glued, I use the rim of the cones to mark its circumference on a piece of paper. The girth (length) of the tyre will be longer, work out the maths by adding in one diameter in the formula and cut a tubing to length. Join the ends of the tubing to form a ring by heatshrink sleeves and hotglue, make sure it is not buckled. Then spray black.

Re-insert the cones and hub spoke wheels to the wheel jig's axle wire, supported by the lower ring. Slip the tubing ring to the spoke wheel, set the tube-ring to height, put some talcum filler, and then some superglue with the tip of a fine point tool.



The silicon tubing has arrived but before I try it out, one of the previous cones made snap at the joint line, so I will rejoin it and clean up the cones before making the wheel jig.

Ha! Here's an idea for the wheel jig: 1) Mark the exact centre of two identical plastic screw caps, 2) drill the centres with drill bit the diameter of the hub tubing and we are done.

If plastic cones were pre-drilled, the two plastic cones are sandwiched between the two screw caps with the hub tubing running through the caps (hot glue, so it can be removed later). Spots of superglue will secure the cones at the rim. Glue the silicon tubing around the rim. Remove hotglue from one end of the cap and remove the cap. Dab some superglue to secure the exposed hub to one side of the wheel. Then remove the other cap by breaking the hotglue bond and apply some superglue to the otherside of the wheel's hub. Trim off and we have one spoked wheel.

If the plastic cones were not pre-drilled for the hub, then the method is: sandwich the two plastic cones as before, eyeball that the rim and the screw caps are concentric, spot glue the rim with superglue, drill the cones through the screw caps. Remove screw caps and glue the plastic hub tubing, trim. Glue the silicon tubing around the rim, eyeballing that it remains central. Obviously, with this procedure, you have to hold the caps against the cones gently and consistently.

27 May 2016

The template for the spoked wheels was printed out and squares of pvc plastic taped to the paper template in turn. A pin mark was made to a plastic squares before taping to the template. This marks the centre of the cone where all spoke lines radiate from. I scribed the 17 spoke lines on the plastic using the same plastic headed dressmaker pin against a straight edge, starting from the centre pin mark radiating outwards, sighting the printed spoke lines of the underlying paper template.
When the 4 plastic squares were all scribed, I ran my Pilot G-2 0.7mm black pen over the scribed lines. The plastic squares were left to dry when I went for lunch, then a soft tissue was held against each square and the ink buffed away in circular motion. This left the scribed lines with the black ink.

The four squares of plastic have their holes pierced through and enlarged with a rotary diamond bit (the long tapered one that looks like a spike with glittery surface). Pushing the tool while rotating with the fingers had the holes enlarged and the edges protruded which ought to be useful when gluing the bushing tubing for strength. The rotary mandrel (the one that has an enlarged head holding a bolt) was assembled this way: the plastic washer was fed to the bolt, then two square plastic pieces, the paper template (this time, the paper template has the diameter of the wheel marked on the 17 spoke lines), the final two square plastic pieces and finally the mandrel. The assembly was gripped in a dremel rotary tool, held secured against a table surface with the left hand, and spun at the lowest setting. The small dots marked on the paper template can be seen through the square pieces and a cutter was introduced with the sharp edge pointing outwards. The first two pieces were cut accurately but after that, I could cut the remaining two pieces. A lot of plastic bits and dust were flung around and it was messy and it hurts when a scrap stung my forearm. I decided I don't want to cut the remaining two pieces with the cutter. I saw that the last two pieces on the mandrel were bigger than the first two pieces and were not round. I used a 60 grid sandpaper block and sanded them down. It took about the time to write this paragraph.

The segments of the discs were cut out and clear transparent tape was applied to the unscribed surfaces on one edge of the discs. The tape should be slightly longer, so that the corners of the tape will go past the spoked discs. The other edge of the discs were brought together with the scribed lines inside of the formed cones going past the first edge that had the tape on the other surface. The edge was released until it matches the first and the tape brought in contact, thus holding the coiled discs in a cone position. A little superglue was applied with a small scrap of the segment and the cone placed on a flat surface for curing. The exposed tape applied in the previous step held the cones in position. The cones were weighted down to ensure that the cones do not cure twisted. It only took a few minutes for this operation.


These were the four cones after peeling away the tape pieces. The butt joints held the cones but they were slightly raised off a flat surface at the rim. It could have been better if I had weeighted the cones better, on the joint lines.

Another problem was the blooming of the superglue on the scribed inner side and some seepage on the smooth outersides. A cotton bud and some alcohol will clean up the cones.

Now I have to put them aside until I receive the 3mm x 2mm silicone tubing from Banggood to do the tyres. They are neater than the previous pair.

 

Here are ideas on the use of and the working of the 3mm outer diameter x 2mm inner diameter silicone tubing:

• Short rings may be used to reinforce the cones and provide greater bearing surface for the 2mm outer diameter plastic tubing.
• Short tubes may be used for the same purpose as the short rings, but instead of two rings per wheel, only one tube is required.
• I have yet to figure out a way to make a tool that can slice the tubing perpendicularly with a razor blade.
• Make a tool to slit the tubing. This comprises a 1mm packing piece to which is glued a #11 blade, the tip of the blade is exposed about 1.5mm from the packing piece. The packing piece with blade is then glued to a flexible strip of pvc. The tool is used by rolling the pvc strip around the tubing and inserting a second 1mm high packing piece, pinching with the left hand. One end of the tubing is tied stationary, the other end is fed through the tool and pinched with the right hand and held at a slight tension. The tool is moved forwards the stationary end and the tubing is slit.

 

25 May 2016 (attempts at a pair of 1.5 inches spoked wheels)

There is no spoke in this spoked wheel construction. Instead, it relies on drawn lines on clear plastic cones to simulate the spoked wheels. Why not? It is only a hobby.

These 1.5-inches wheels were intended for a Roger Sommer's Monoplane, approximately 20" wing span, 3-channel, powered and controlled by WLToys F929 F939 components.

I used a dremel mandrel piece to screw on a craft foam and spun it in my electric drill to cut the disc. Previous experiments was gluing a bushing hub before spinning and cutting; this experiment with a mandrel piece, the one with a wood screw, that is used for felt disc (for polishing) works.
In this case, it was for the crankcase of the 7 cylinder rotary engine, but I put it here because it was a more convenient tooling method.

I drew a circle of my wheel diameter and divided it to 16 segments.

 I traced the image on PVC cover sheet by scribing it with the back of a cutter then going over the scribed line with a black pen. The pen markings would be wiped off if there is no scribed lines. I cut with scissors, poked the centre hole, cut a single slit and coil the disc into a cone. The first half is glued to a bushing perpendicularly and then the other half introduced with the spoke lines staggered.

The result however showed a flaw in the concept. At one side of the spoked rim, the spoked lines are staggered but at the other side, they matches. The pattern I used did not consider the overlap.

I didn't use a compass so I used my protractor to draw a circle and marked out the segments. The overlapping portion is 8 degrees and the other 16 segments are at 22 degrees.

I used a compass cutter to cut the plastic disc and it was inaccurate; I have had better accuracy using a pair of scissors. Anyway, the disc was taped temporary to the template and sribed/penned. Then a single slit was cut and the disc overlapped by the two spoke lines of the 8 degrees segment. The centre holes were widened with a round file and the bushing added to one cone and then closed off with the other cone to form a spoked rim.

I have ordered 3mm (OD) x 2mm (ID) silicone tubing from Banggood after I misplaced my previous purchase but I wanted to try completing the wheels with 2mm craft foam as tyres.

I cut strips of the 2mm craft foam, and glued them around the spoked rims. I started at one end of the strip by applying a bit of superglue. The spoked rim was brought in contact to this spot and held until there was sufficient grip. More CA was applied to the spoked rim in small sections and the rim rolled onto the foam strip. When the foam strip made a round, the excess was cut off with a NT cutter and the free end glued in placed, butting against the fixed end.

 

 

I observed:

1. The pair of wheels are inaccurate and not the same sized because the compass cutter had to pass more than once around the plastic sheet.
2. The lines were not radiating from centre and the hole is probably not centred.
3. There is a twist to the cone because of the overlap.

To improve accuracy/neatness, I think:

1. Use better template. Scribe and draw spoke lines as before, against the better template.
2. Either hold 4 pieces of plastic together in a dremel mandrel (original use of mandrel is for cutting disc) or a bolt/washers/nut assembly, or scissors cut individually.
3. Cut the 8 degrees segment away, glue half of a short strip to the middle of one side of the disc (i.e., midway between hole and rim), bring the other side and butt each other. Glue the short strip to secure. Glue will flow by capillary action to the butt joint.
4. Try using the silicone tubing as tyres when it arrives.

So I get a friend to draw this template. The concentric circles are for visual guidance of round discs.

And although primarily for guiding the cylinders in rotary engines, he drew circles with 7 and 9 radiating lines. Who knows, maybe one day I need to simulate spoked wheels that are too small or thin to do them in two half cones and I have to use flat discs instead.

15 September 2015

The monowheel prepared the night before was sanded with the bushing in a portable electric screwdriver/drill. The edge was smoothened but it is still eccentric.

The bushing was inserted to the setting block and cut.





The remaining bushing was inserted to a square piece of craft foam.

The foam was held against the side of the tool so that the bushing may be perpendicular.





The bushing was held in the electric screwdriver.

The centre pivot point of the compass cutter was held in the bushing and the cutting blade brought to the spinning foam gradually.

The wheel was cut off with a NT cutter rotating against the bushing along the side of the tool.

The tools and equipment to make this small pair of wheels are: electric screwdriver, setting tool, NT cutter, compass cutter, and sandpaper block to sand the bushing. The materials are: Superglue and Kicker, 2mm craft foam, plastic tubing.



The pair of wheels is 15mm in diameter, the monowheel is approximately 8mm diameter.
The scale is accurate to 0.1gm, and the three wheels registered 0.0gm!

The craft foam and plastic tubing are easy to cut. They are also very light.The procedure of gluing the bushing before cutting the disc results in a better wheel with less eccentricity.
It is suitable for making small wheels because eccentricity becomes more obvious with small wheels.

14 September 2015

Tape the craft foam to the cutting mat.
Set the compass cutter.
Rotate the cutting mat, not the compass cutter.
Trim if edges were still joined.
A later experiement showed that having the tape at the bottom of the craft foam will help ensure a thorough cut.

Cut the short lengths of bushings from pastic tubing. They are easy to snap clean.


Find a hole which is big enough for the plastic tubing to slide through the arcylic setting up tool.

The acrylic setting up tool was made by Tan Kee Chiao at his office/workshop with his milling machine and pillar drill. It is a series of perpendicular holes on a half inch (or was it 10mm?)  thick of acrylic.




Use the pointed end of a bamboo skewer to position the wheel disc on the drilling tool. The hole will stop the bamboo. Then the disc is held against the setting up tool and the plastic tubing is inserted from the other side of the too.







Rotating the plastic tubing while applying force has the bamboo skewer replaced by the plastic tubing.

The plastic tubing is retracted but ensure that the craft foam disc is still in position.





The short bushing is inserted, then some UHU Por glue is applied to the exposed bushing.










A wire which fits the plastic tubing is inserted from the rear, through the bushing and left aside until the glue is set.

The other wheel is similarly prepared.







Short lengths of plastic tubing were glued and crimped to retain the wheel.

The wheels are not true. The bushing and the centres do not coincide and I suspect that the disc may not be at perpendicular to the bushing. They are however, good for current purpose.




Craft foam doesn't sand well and I did not have to use a drill and a sanding block.
If I shall require a better but equally small or smaller wheel, I would have both sides of the craft foam taped before cutting oversized with circle cutter.
Then slide the axle wire through the plastic tubing, feed it through the setting up acrylic tool, wire through the taped foam disc and then the plastic tubing, super-glue one side, remove tool, wire and snap the pastic tubing to 1" length.
Spin the wheel by chucking the tubing to a drill, sand true, snap the plastic tubing again and the remaining length of plastic tubing ought to be more than enough for a few more wheels. When the stub gets too short, then they can be used as spacers.