This article also explains how to make a high torque starter motor for £5.00.  It also explains the basic steps to building a suitable large model.  

If your next model is going to be slightly larger than usual, then this project could be for you. I was paying about £12.00 per gallon for glow fuel. Having read a little about converting petrol strimmer engines, I decided to give it a try. £3.80 per gallon for a car is not good – but for a model it's fantastic!

Firstly, let’s mention wing loading. When most of us start model flying, we are sure that the only thing that keeps it in the air is the lack of weight. Wrong! As the years go by, you will learn that we can get away with a lot. We will be looking at a wing loading of about 1.6 lbs. per square foot. I have converted several strimmer engines, up to 40cc. These quite happily fly round with a wing loading of 2.5 lbs. per square foot. In this exercise I will refer to a Kawasaki 24cc. (A McCullock 22cc could be used but really, they just aren't powerful enough).

Where do you find the engine? Simple. These engines are readily available from garden centres with a garden machinery department. Look in the ‘good old Yellow Pages’ and have a ring round (perhaps one quiet day at work!). There are literally thousands of old strimmers sitting around in the dark dungeons of these centers. Often a strimmer goes wrong for a reason other than the engine. Commonly the pull start fails or the strimming cord unit smashes. The centres gives a quote of say £60.00 to repair; so most people buy a new one – part exchange. I went to one garden centre in Suffolk and the storeroom (out the back) had hundreds of engines just lying about on shelves! What an Aladdin’s cave. He gave me two Kawasaki’s for £20.00 with a guarantee! They are great engines. Slightly heavy (about 4lbs after conversion), but designed to be thrashed from cold and revved all day! They are very reliable and most importantly give a good power to weight ratio. The main bearings only cost a couple of pounds, but have never caused me any problems. The engine must have magneto ignition (most have) and the carb. needs to be pumped. There should be a reasonable compression and try to find one that looks fairly ‘modern’. (Remember Kawasaki have been going since before the war!)

Once home, make sure it runs. (I once spent hours converting an engine only to discover the con-rod had bent!) If you have bought a ‘pull-start failure’, set it in the vice and use a domestic drill to spin it, with a socket on the crankshaft nut (pull-start side). Guess what - most strimmer engines go the correct way for us model flyers! i.e. the correct way for a conventional prop.

You will by now be thinking – ‘How on earth is this slab of metal ever going to fly’?!!! The answer. Get out the hacksaw! ** I would advise against stripping down the engine completely – unless you have got hold of the relevant gaskets. (It takes ages to convert an old foil milk carton into a gasket)! Take off all ancillary parts – except the magneto, as the tolerance is critical. Next bung up any holes with paper cloth. (You don’t want any metal in the delicate cylinder head). The prop will be going on the flywheel and the engine will mount via the pull start casing. Taking this into account, work out what casting isn’t required. Think carefully and then cut away. File off any burs. Best not to touch the flywheel- it is critically balanced. It should now be looking more like an aero engine.

Next, clean the engine. Make a bath out of an old 1-gallon glow fuel container and clean with white spirit or something. You are now ready to join the prop to the engine. Unless you have access to a lathe, you will need to visit a local ‘Small Engineering Company’. Again look in the Yellow Pages. Basically you will need a spacer made which will bolt the flywheel on and then take a bolt for the prop.  (See pictures).  If you make a friend and wave cash, you should be charged between £10 & £15. You can have the adapter made from aluminium or steel. I use an 18x6 wooden prop. It might be an idea to leave the engine and prop with the engineering company, to help them.

The throttle linkage is tricky. In a nutshell, there is a hefty spring, which is far to powerful for a servo. The spring needs to be removed and you must use a plastic control cable inner and outer (for radio interference reasons). Eliminating all interference is vital. By learning from my mistakes, you should be ok. Firstly ask your dealer for a ‘resistive plug’. The plug number usually starts with an ‘R’. The receiver should be as far back as possible, i.e. at least 18 inches. The receiver can be rapped in foil, but put a 5mm plastic sleeve round the aerial wire through the foil. The servo wires should not be extended beyond their normal length. Next the outside of the H.T. lead to the plug needs to be earthed. Unwrap about 2 metres of TV coax outer sheath and coil it round the entire length of the H.T. lead, including the plastic cap. Then screw the end of the cable to the cylinder head. Rap insulation tape round the cable, to hold it all in place. Spend time doing extensive range checks. You shouldn’t have any glitching.

It is now time to get the prop spinning and see how much air it moves. Mount it somehow. If you are lucky your starter will turn over a nose cone in the normal way. If not you can loosen the plug to reduce the compression as a temporary measure. You can then get it started and hot, stop it and then tighten the plug. It should then turn and fire.

If you are thinking what a palaver and don’t have a high torque starter, then don’t panic. What I did was to visit my local scrap yard. You want one with lots of mud! Nice and cheap! After speaking to a very knowledgeable mechanic, it turns out that the best starter motor for the job was one from a mini metro. And guess what - the rotation direction is the correct way again! What luck! The gear mechanism comes off the long shaft by removing a circlip type washer. I was fortunate enough to have an old broken model starter and easily made a handle switch and nose drive.

Assuming you are happy with everything so far, it is time to talk a little about a model. If it is a change you were thinking about then you must remember that the model will be a little different too. It is unlikely that you will have a model suitable for this particular power unit. It is important to appreciate that the model needs to be built around the engine. (I always build my own designs; they are often semi-scaled up kits. If you aren’t keen on building your own design, see article on building own designs.  Play on the safe side and build a high wing, scaled up ‘trainer type model’, to move safely into this new area of the hobby. Strength is very important to its success, but don’t be tempted by paranoia. Aim for about 11 or 12 lbs all up weight. If the engine weighs 4lbs, the wing 2 ½ lbs, and the fuselage 2 ½ then you will still have 3 lbs for all the other little bits. A seven foot one piece wing (12-inch cord) should go in most cars. We built ours from wire-cut foam and gave it a Clark y section. There is a main spruce spar and it was covered with 3/32 balsa. The fuselage basically consists of  ¼ x 4 x 36 sides. There is a light ply doubler and a ply trebler! The firewall is ply total 12mm. This has 6 degrees of down thrust. Don’t be tempted to build a cowling until you have mastered the side thrust, about another 6 degrees. As a rule I set the initial C of G exactly between a quarter and a third of the average cord from the leading edge. (about 30%). The under carriage needs to be strong and I always use big wheels. Visit B&Q, they do a superb 4-inch wheel, which is very light.

After extensive engine checking, release your model on full power and after only a few feet it should climb with plenty of power. Get to a good height and the trim out. As soon as you dare reduce the throttle. It should glide like any other model. If you master this, your landing should be without incident. Remember the bigger and heavier a model gets the more gracefully it needs to be  flown and the more full size it will look.