A 28cc model petrol engine for £30?

John Wheater

It’s true. I bought this new strimmer, 'weed whacker' for our American friends, on eBay for £22.00, although the vendor charged an exorbitant £12.50 carriage. Whatever, at a total of £34.50 it turned out to be an excellent purchase. The manufacturer’s name is Sovereign, model M28, and the labels on it indicate it is of German origin, not, as expected, Chinese. Checking on the World Wide Web it looks as though it really is German.

Rather than go into a lengthy discourse on the way I tackled the conversion, I am simply going to caption some photographs.

1. This is how the unit was received.

2. It is easy to strip, there were no odd torque heads on the screws or anything like that.

3. To remove the clutch unit, I held it in a Stilson wrench (Mole Grips if big enough would do) and gave it a whack with a hammer, it is only a push fit on the tapered shaft.

4. To remove the flywheel, I held it in my hand and give the end of the shaft a tap with a hammer. I used a copper mallet to make sure I didn’t damage the threads. I was ready for the Woodruff key, I didn’t want to lose it, and I put it somewhere safe until reassembly.

5. I cut off the flywheel housing using a cutting wheel in a Dremel.

6. Filed and polished all those nasty jagged pieces that I created in 5.

7. A prop driver was easy to machine. The thread on the shaft of this one was 8mm x 1.25, the driver is threaded internally so that it threads over the shaft.

8. A prop washer is also needed to go on the flywheel spacer to provide a good base for the prop. Now where did I put that Woodruff key?

9. Reassembled.

10. Prop on and contact! This one had magneto ignition which is why the flywheel needs to be retained, it carries the magnets. I couldn’t get mine to start with a flick of the wrist, it was a little stiff, so I used an electric starter. It fired instantly and settled into an easy beat once it had warmed and the choke was set to run. There was no need to adjust either the fast or slow running screws and I got 7,000 rpm on an 18 x 8 Master Airscrew prop. Transition from idle to full throttle was instantaneous. It promises to be a good motor and I am pretty sure it will hand start once it has run in.

Now I must build an airframe to take it! I think it will be going in a Jiant(sic) Jabberwock.

See how she runs! Click the monitor icon and stand back!

Detailing a scale Druine Turbulent

Garry Henderson-Smith

A couple of years back I obtained a set of free lift-out magazine plans for a scale Druine Turbulent, that sweet little VW-powered French homebuilt. The plan built up into a small but very tidy model suitable something for around a .20 fuel motor or electric conversion.

I passed the plans on to a friend of mine, Alan Trinder, who is a long time modeller and he duly put together an attractive plane powered with a G-Mark twin 2 stroke that performed quite well at a couple of scale rallies around Sydney.

Recently he decided to convert the Turbulent to electric power and so required a dummy VW engine to replace the formerly exposed heads of the G-Mark and having seen the finished product, I think the workmanship in the tiny VW is worth showing off.

The first step was to recreate the finned barrels and head of the flat four and this was achieved by cutting alternate fins from two thicknesses of ply, (·4mm and ·8mm) and stacking them to build up the appropriate depth of motor. The addition of tiny shaped pieces as casting webs and counter sunk plugholes, adds to the realistic look of this section. Pipes of the correct scale size were then bent to the exhaust and inlet profiles and spark plugs manufactured from bolts, wire and shrink tube.

As the pieces come together the distinctive style of the boxer motor forms and the final assembly and painting highlight the attention to detail, including a cast resin rocker cover with minute gasket and retaining wire.

The air scoops sitting atop the heads are scale as well as functional and direct cooling air into the speed controller, fitted inside the cowl. The final touch of "wrinkle finish" paint on the rocker cover, semi gloss black on the inlet tube and gloss black on the air scoop add to the realism of the build.

For a motor just 55 millimetres across, the detail is excellent and only awaits the final weathering and exhaust staining on the cowl.

Making a vacuum box

Malcolm Ferguson

It occurs to me that readers may be interested in how to make a vacuum box which seems to be needed when making lightweight bits for electric jobies.

Complete vacuum box and frame

The pictures should give a clue as to what's needed. A piece of aluminium tube to connect to the vacuum cleaner is glued in but is obscured by the finished nacelle (but see detailed picture below); the little blocks are needed to give the excess plastic somewhere to go, otherwise a crease ruins the corner, the uprights are only guides when placing the frame - the plastic cools rapidly and one does not have time to muck about - the top of the box is MDF (medium density fibre board) and the holes help the air to evacuate, the inside sides and bottom are best sealed, say, paint/varnish.

Vacuum box construction detail

A sheet plastic called Vivac is good and tough but PVC will do but is fairly easily breakable, styrene goes brittle with time, 0.5mm thick is good for most things but 0.2mm can be used for things which carry no load; different materials require different temperatures - I use about 130°C on fan grill - an expert has told me to heat the plastic (in the oven and clamped in the frame, the screws go through the plastic also) till there is 3inches of sag, with the vacuum cleaner running push the frame down onto the seal and it's done. You have to be quick, though, it all happens in 2-5 seconds, in fact, the first time I tried I used 0.2mm styrene sheet and as the frame contacted the seal there was an almighty bang as the vacuum took charge, I got a hell of a fright.

Set-up

Plastic sheet is commonly used by commercial molders and round about 3/4 pounds a sq/mtr. It would pay to ask about the material as names change internationally.

Plug and finished molding

I'm told the best stuff for the plug is also MDF laminated to required thickness, and sanded to a high finish, panel lines need to be about 0.5mm deep.

A stuff called Vivac is good and tough but PVC will do but is fairly easily breakable and styrene goes brittle with time, different materials require different temperatures - I use about 130°C - an expert has told me to heat the plastic (in the oven and in clamping frame) till there is 3 inches of sag, push the frame down onto the seal and it's done, be quick though. Can I add that the judicious use of a heat gun (used with care though) can help persuade the plastic to close contact with the mold. The plug should also be heated (quite warm to the touch) to prevent the plastic cooling too quickly.

A limiting factor of size would be the size of the oven. Mine is made to fit the largest frame I can get in the oven.

Malcolm's EPP Whisper Jet which has quite a few molded parts