Two seasons on, whilst this model is still flying well, it isn't getting as much air time as I'd hoped. It was originally designed for flat field thermal soaring so is not built to stand the rigours of being uncermoniously dumped into the side of the hill when the lift dies so I'm very reluctant to launch it off if the conditions are marginal.

On the other hand, its light wing loading and domewhat draggy shape mean that it's not a fast model either, so penetrating in anything more than a moderate breeze can be difficult. I have fitted a more streamlined canopy and made provision for up to 1lb of ballast which has helped a bit, but the inescapeable fact is that we don't get very many days for which this is the right model.

A look around at other scale models on the slopes confirmed that I'm not alone in this quandry and more and more scale gliders seem to be sprouting folding propellers on their snouts. These 'sustainers' are there to provide just the insurance policy I'm looking for, ensuring that, if the lift dies, the model can still be brought up the hill for a safe landing on the top.

Back or Front?
There is a motorised version of the AV36, designated AV45, with a pusher motor at the back so my first thought was to emulate this arrangement, perhaps with the motor mounted on the cg driving the prop through an extension shaft. A quick feasibility check though showed that the shaft would have to pass through the wing which would make removing the wing rather problematic! Also, I took the trouble to finish my AV36 with the colour scheme of the one at Old Warden so there's no point really in trying to pretend that it's an AV45. So the decision was made - a removable tractor prop at the front.

The noseblock on the model is hollowed out to take a 3s x 2200mah LiPo battery and a polythene bag holding 11oz of lead shot. A quick look at likely motors suggested that a suitable motor and ESC might weigh in at around 9oz, i.e. a couple of ounces less than the lead ballast I was removing. On this basis, I reckoned that the 2200mah LiPo could be swapped out for a 3300mah one without causing any serious balance issues.

Nose Job
With a motor chosen (Turnigy PropDrive 35-48 900kv), the patient was marked up for surgery, the plan being that, with the motor hanging off the back of a 1/8in ply mounting plate, the end of the shaft should be flush with the re-fitted noseblock.

I used a cardboard template to ensure my pencil mark was coplanar, and that the thrustline was straight, with maybe a little downthrust. Then it was just a case of taking the hacksaw - and a deep breath!

The next job was to make a hole for the motor. a 36mm hole saw bit proved to be the ideal tool for this. A 3mm ply nose plate was then roughly cut out and drilled to take the motor, four location pegs and, potentially, two magnets. The motor was temporarily wrapped a cardboard tube to centre it in the cavity. Then sharpened cocktail sticks were pushed through the pre-drilled holes into the balsa.

With the motor removed again, I used epoxy to attach the nose plate. Once the epoxy had dried, the locating pegs were sawn off flush with the nose plate.

Although it was tempting to open up the original F1 to help with getting the motor in and out, I opted to leave it untouched. In the event that central crossbar came in handy later for fitting the battery box. The nose plate was sanded back to align with the fuselage and the edges covered in cream Oracover. This last step proved to be somewhat premature as the covering inevitably picked up a few scuffs when re-profiling the noseblock, so it will have to be tidied up again later.

The next question was how to fit the battery. I had initially thought of putting it on the floor under the pilot's seat but with a motor in the equation, easy access to the battery becomes more important than for a pure glider. After trying a few positions, I was delighted to find that the battery could stand vertically behind F1. The leads protrude above the fuselage sides but there is enough room behind the instrument panel to accommodate them. So a battery box was sketched out and built.

The box is secured with two c/s head screws through the holes just visible in the pic above. The screws go through the hardwood bar shown in the picture above right and into the pre-existing bar glued to F1. A couple of balsa blocks stop the bottom of the box from shifting around.

The ESC is trapped against the fuselage side and the battery held in its box by rubber bands. All this is happily well out of sight when the fuselage top is fitted. The pilot has a little less room for his wellies but otherwise is not really inconvenienced.

At this stage I assembled the model to check the cg. With no propeller fitted the model balanced just slightly forward of the mark with the nano-tech 3300mah battery in the picture, and just rear of the mark with a Turnigy blue 3000mah pack.

The main remaining job was to tidy up the front end. The next two shots show the reshaped noseblock, backed with 1/32in ply, re-fitted, with and without the propeller in place. I now had a choice- either fit this block semi permanently (e.g. by covering over the join with the fuselage) and make some sort of plug or collar to fill the gap left by the prop driver or make a second noseblock for use when the prop isn't fitted. In the end I opted for the latter approach and made up a new noseblock.

Magnets were fitted at the top and bottom of the firewall in the holes provisioned earlier and also in matching locations in each of the two blocks. The two blocks were then covered and the scuffed covering round the front of the fuselage replaced. The joint is not exactly invisible but hopefully won't seem too intrusive.

All that remained now was to assemble the model one more time and finalise the balance. A small lump of lead was wrapped in foam to be stowed in the rear of the fuselage to restore the balance when the prop is fitted (although the cg shift is pretty small). In glider mode, the model now weighs in at just under 7lb with no ballast on board. With the prop and tailweight added it goes up to 7lb 3oz which is pretty much the same as it was when last flown as a glider with 8oz of ballast on board so hopefully there will be no nasty surprises when it is re-maidened.

With the Aeronaut 14" x 8" folding prop fitted, the motor draws around 45amps at full throttle, giving a power loading of 70w/lb. Hopefully that should be sufficient to bring the model safely home when needed. Time will tell!