F15 SG

27 February 2017

F15 was completed, test flown, adjusted and handed to Mr. Wong. Which he subsequently lost sight of after a few minutes in the air. He didn't find it, so we formed a 3 men search party, panned out and Mr. Tan found it (Mr. Wong went away much further).

The F15 is now "F15 SG" because I applied a "I ♥ Singapore" temporary tattoo to the wing. Temporary tattoos work well on foam board.

Specifications:

• Wing Span 10" thereabout
• 8.5mm motor with Hubsan 55mm propeller
• 260 mah 1S cell
• AUW 32 gm
• All flying tailerons
• 3 Channels
• Special ability: 1) Flick roll. Enter high angle, torque and CG takes over and model performs a half flick roll. 2) I didn't get it to spin on the few flights I had. The twin fins acted like dihedral on the planform and the model is very stable.
• Time: 3-6 hours prototyping, I think 2 hours maybe sufficient if everything is gathered beforehand and I didn't have to set the transmitter mixes. If many identical models were made at once, it would be even faster especially with hotwire jig.
• Cost: At the field, a few of us figured that the cost of the expendable airframe, i.e. excluding re-usable parts such as receiver board, 1S cell, control horns, motor and propeller, is less than a dollar if all material costs were divided proportionally to the quantity used.

Special interest:

1. 8.5mm was pushed into a very light and flexible open cell foam block which acts as a motor mount. I didn't use any glue.
2. Open cell foam motor block was hotglued at four corners to the underside of the receiver board. Since the open cell foam is so flexible, I didn't have to make any holes or grooves.
3. I looked for but didn't find any problem with underslung motor or thrustline adjustment on this model.
4. My first use of temporary tattoo!

Both for Mr. Wong

That's my transmitter

32gm AUW

21 February 2017

Comments on 20 February 2017.

1.5hrs to come to this stage.
I pasted (glue stick) the modified plan to 2mm foam sheet, cut away and peeled off the plan which I am keeping to pass it to Mr. Wong.

The 10mm nose pod was similarly cut and was slotted to the planform with one fin temporary fixed to perform test glides. The purpose of the test glides was to note the CG position. The CG is approximately 1 cm back from the swept leading edge.

The weight of the full set of gear, including the new 1S, is 18gm.

I marked the centre line and the tailerons pivot line on the bottom side of the planform and taped top and bottom in a fore and aft manner with packing tape for some durability.
I made the pivot mechanism to exact length. The ends were simply very short tubing (1mm long) superglued in placed.
I cut away the tailerons, glued the twin fins and pushed the pivot mechanism through the fins. Then I glued the tailerons to the free revolving tube. Kicker on the tubes, CA on the tailerons. I have still to hot glue the bamboo axle to the planform.
Airframe weighed 10gm.

Currently all in is 28gm.















Todo:

1. Remove and replace battery connector and solder motor leads directly to board (or the connector to board, I am still undecided).
2. Cut planform to accept board (square for the two servos tops or only the horns? I'm undecided but am leaning towards square hole because some clearance is required under the servo horns to prevent binding/scraping.
3. Make and mount two horns to the tailerons.
4. Install two pushrods. They will be thin wire z-bend hotshrinked to 1.8mm bamboo dowel on the servos ends, either hotshrinked z-bend again or right angle bend with nylon keeper at the tailerons ends. Then I will hotglue the bamboo axle to the planform.
5. Run battery connector and decide mounting of battery location. Check it is of appropriate length and will not interfere with the prop.
6. Fix the motor to bottom of pod. If I am using the simple 10mm pod, I think I can superglue the motor to a mount (balsa, ply, foam, aluminium?) and have the mount pivot on the pod for thrustline corrections.
7. Option to cover bottom of intake.

Should I replace the pod with a better looking one? Maybe a plunge moulding of the canopy?

Ideas for plunge moulding canopy:

1. Carve mould from blue foam.
2. Mount mould in vice.
3. Make two rectangle frames using Ice cream sticks hotglued together.
4. Hold the thermoset plastic sheet between the two frames with binder clips.
5. Heat plastic with hot air gun and plunge it down on the mould.

Ideas for pod:

1. Heat bend single sheet of 2mm foam over the rear end of current pod. The foam must be long enough at the front to reach the back of the nose cone. Trim to shape.
2. The top front portion from the back of the nose cone to the cockpit will be a single thicker piece of 10mm foam and glued to the previous piece.
3. Glue the canopy in placed.
4. Glue the bottom piece and the nosecone (2 pieces of 10mm foam?)
5. Sand to shape.

17 February 2017

Detailing the idea.

The planform can be drawn on tracing paper over the printout, allow that the two sides (and fins) extend to the nose intake by widening the intake and drawing in the two side strakes.

There won't be a happy coincidence of finding the correct sized tubing and wire/rod axle. Here are two alternatives to making a good fitting tubing over the selected wire/rod axle:

1. tape the wire/rod, making it slightly bigger for rotational clearance. Roll the tubing from paper with white glue (messy but definitely no superglue), extract wire/rod and remove tape.
2. tape the wire/rod for clearance as before, slip a heat shrink sleeve and shrink it down, remove tape from wire/rod.

Obviously if I have another wire/rod just slightly thicker I wouldn't need an extra process of taping and removing.

I think method 2 using heat shrink sleeve is simple and can work on this model. I might have a problem extracting the heat shrank sleeve, if it is pulled, the clearance gets lesser and binds to the wire/rod, so I will have to be extra careful when heat shrinking (or if I use different sizes of heatshrink?). If I need a harder tubing, I can substitute with ear bud plastic tube, but this time the tube is heated and stretched/rolled. By stretching, the tubing reduces in diameter and I probably don't need to prepare the wire/rod with tape. Either way, it is not fool-proof. I will give heat shrink a go unless I can find a match for the axle and pivot tubes. (20 Feb 2017: well, I made them match by scraping and sanding bamboo dowels to rotate freely in the cotton bud's tubing. Also the white tubing is smaller than the blue one. And I realized that the previous idea of taping the axle is unpractical.)

Since the motor is below the wing, I have wing drag at the top causing a pitching up moment. The CG will also be above the propeller, another pitching up moment. But if I deflect the exhausts at the rear, I might be able to introduce pitch down moment which is also proportional to the thrust. By having the motor glued on so that it blows on the bottom of the wing, I can amplify that effect. It is not all bad, considering that the CG is below the wing, I ought to have some static 'right-side-up' stability, just like a high wing model. To lower the CG and have a clear path for the pushrods, I can mount the board by cutting a rectangle on the planform and expose the servo horns topside so the pushrods can ran above the wing. Maybe the board is so deep that at the same time, it can also be glued to the bottom piece of the duct, making the duct structure strong, which is also good as a motor mount. (20 Feb 2017: No, the receiver is not that tall.)

I laid out the printouts, drew cheatlines and pivot points, looking out for the line of pushrods and thought where can the 1S be located. I thought about building a box fuselage with hinged canopy top for 1S replacement. I was not able to lay out all the visualized shapes, if I was to include the fuselage pod onto one piece of 300x450mm.

I thought of shaping the pod from 1" blue foam and use it as a mould. The mould will be wrapped in cling film, then wet paper strips applied to create a shell.
But it is so much simpler, lighter and probably stronger, to cut a profile from the white 10mm foam. And a slot to slip in a Velcro to hold the battery directly underneath the pod. (20 Feb 2017: Thang bought on my behalf, the 1s 240 mah 45C from HK, 5 cells with the appropriate white spade connector for SGD 20.00, good buy and quick delivery. The cells are compact and light too, the idea of a build-up-pod or slip-in-pod gets very attractive.)

I found the right match of axle and tubes for the pivoting tailerons. A short length (just about 10mm longer than needed) of white fiberglass rod left over from a piece I picked up at the field and which I had used elsewhere as a pushrod (black paint and superglue remants), and the blue plastic cotton bud tubing. A very good match with minimal free-play. The 1.8mm bamboo rod is too large for the tubing. Then I held it in my hand and decided that the model can be made lighter if I use tape hinges. I didn't weigh the axle and tubing, but I think it is more than 1 gm. I have to think about more cheatlines, then I discovered that that would involves notching the fins for elevator movements and I don't want that. Then it dawn on me, I could chuck the 1.8mm bamboo and turn it slowly while sanding down the diameter with a hand held piece of sandpaper (20 Feb 2017: no, this idea didn't work out well, far more effective by scraping it with cutter and then sanding by hand). Quite simple and it will be lighter than the fiberglass rod (20 Feb 2017: fiberglass rod weighed 0.3-0.5gm, the thinned bamboo and the matching tubes didn't register on the weighing scale). For strength and rigidity, I will glue the inner portion of the axle to the planform, rather than having it floating in tubes. For a neater appearance, instead of gluing on very short tubes to the ends, I could pierce the ends through pieces of tape, sticky side out and non-sticky side towards the pivot tubes, and apply super glue to the 1-2mm exposed ends. This is to be done before the tailerons are glued to the pivoting tubes.

After marking the bottom of the planform for the servos cutout, tape it first and cut later. Plug in the motor before gluing the servos to the taped cutout with hot glue.

(20 Feb 2017: Wong asked for help. I will just do one for Wong and pass him whatever mould, plan etc so he can do another one himself. Sounds like a bad deal, but it is ok, because although I don't get to keep it, but why would I want to keep another plane? I will get to test fly it for sure anyway, it's the process that's fun, not the final product or the notion of possession.)

8 February 2017

WLToys F929 F939 receiver board, 8520 motor with hubsan propeller. Motor mounted to base of the plane, half arc of propeller protrudes beyond the bottom of the fuselage but there is no surface behind the propeller to decrease the thrust. Throttle and tailerons controlled.

Enlarge this jpg which I downloaded from the internet. The wingspan is approximately 10". I made a pdf of the image, auto-fit to A3 (150%), then enlarge the plan view and side view from A4 to A3 (approx. 141%).

Components

• 2mm foam: 1 x plan shape and 1 x pair of tailerons.
• 2mm foam: 2 x jet engine and fin side (optional: 1 x pair of air-inlet against fuselage side)
• 10mm foam: 1 x side shape of the fuselage pod with slot for wing (note: if the planform includes the nose, then this pod will not be scale, so if a scale side-view is desired, then it is better to build up the pod, or better yet, exclude the nose area from the planform. Regardless, it might be a good idea to magnetically attach the front portion of the pod to the rear portion of the pod or the wing. The F15 will crash many times and having a 'crash-proof' nose pod is beneficial and the additional portability is desirable.)
• 2mm/5mm/10mm foam: 1 rectangle piece to mount motor.
• 1 x carbon fibre rod and some plastic tubes for pivot mounting of tailerons
• 2 x pushrods and horns
• 1 x WLToys receiver board
• 1 x Velcro strap
• some 1S cells for flight

Assembly

1. Place wing upside down and glue carbon fibre pivot mount axle (can use axle mount to displace the axle to scale position).
2. Slip and glue the 2 side profiles (engine and fins) to the inverted planform.
3. Slide the plastic tubings to the pivot axle and glue the retaining tubes to the axle. The tailerons are then glued to the free rotating tubings (options are to make slots to receive and to add clear packing tape). Finally, install the tailerons' horns. The tailerons are not in the scale position, they are just under the wing/fuselage, although they can be lowered to scale position, but why make the effort?
4. Glue receiver board and make and assemble the 2 pushrods connecting the receiver and the tailerons, passing through the side profiles.
5. Glue the fuselage pod and install the Velcro.
6. Glue motor to the motor mount, connect to receiver board and glue the mount to the bottom of the air-inlets.