A10 Thunderbolt II GWS kit

Specifications

For some time I had been intending to scratch-build an A10 powered with 'Blue Flame Blaster' Speed400 fans, but when GWS announced their A10 design I changed my plans. Interestingly, the dimensions of the GWS A10 are almost identical to the enlarged 3-view I had printed out. I was impressed with the kit's scale accuracy. The only obvious departure was the jet nacelle pylons, which project out horizontally instead of angling upwards, probably because this makes it easier to mold into the fuselage (the non-scale pylon angle is not noticable from a side view).

The 'Warthog' was GWS's first EDF model, and it had a mixed reception when it was introduced. There have been reports of marginal performance ('won't fly!') and motors burning out after a few flights. Therefore I decided to go for a lightweight setup, with no undercarriage and a 2 cell Lithium-Polymer battery. Initial CG testing showed that the Lipoly would have to fitted as far forward as possible, to avoid having to add nose weight. I built a balsa compartment to fit in the nose, so the battery could be inserted through the existing bay. This allowed for the option of using a conventional Nicad or NiMH battery if the model proved capable of handling the extra weight.

Construction

GWS kits are often described as ARF's (Almost Ready to Fly), but although the major parts of this kit are nicely pre-molded from expanded Polystyrene foam, there is still a bit of work to do to complete it. Control surfaces have to be cut out, shaped and hinged, and the parts glued together with GWS's special foam-friendly contact cement. I gave the fuselage and ducts a light sanding to ensure close mating of the two halves. I also covered up the empty wheel wells with some scrap foam. After assembly, I filled the entire surface with 'Red Devill' spackle, and sanded it to a smooth finish. To prevent damage on landing, I applied 0.75 oz glass cloth to the bottoms of the nose, wheel wells, and tail fins.

A slight problem occured when the wing was fitted, as the elevator servo arm interfered with the aileron linkages. I solved this by mounting the servo upside down (when looking from underneath).

Painting

I deliberately chose the unpainted (white) version, as I figured it would be lighter and easier to paint with a custom colour scheme. Tamiya acrylic paints were used, two thinned coats giving good coverage without adding too much weight. The matt finish looked excellent, but proved vulnerable to fingerprints, so I sealed the surface with a single coat of water-based polyurethane (Cabot's 'Crystal Clear Satin'). This darkened the colours slightly and gave a nice sheen. Unfortunately I had already stuck on some decals before applying the polyurethane, which caused a lighter colour under the clear parts of the decals (however this is un-noticable from a distance).

The paint job added about 20g of weight to the model, making for an AUW of 375g with the1200mAH lipoly battery installed. The CG ended up exactly 57mm from the wing's leading edge, which is the most rearward recommended in the instructions. From previous experience and reports of other A10 builders, I knew that this CG location would be good.

Flying

Would it fly, or would it fry? After the obligatory photo session and range test, a maiden flight was called for. With cameras rolling, I gave full throttle while Ian hand-launched the model. It climbed out smoothly and then... the motors died! I pulled it around for a dead-stick landing, and we tried another launch. Again the motors cut, but this time I quickly reset the throttle and continued at half stick. I did several circuits, low passes, and a loop and and roll - all at half throttle. Handling was very positive, with excellent stability and a fast roll rate. Landing on the spot was easy due to the low stall speed and good aileron response.

The motor cutout problem was caused by my modifiying the ESC for a Low Voltage Cutoff of 6V (the original 4.3V LVC had been fried by a previous owner). It was actually cutting off at 6.3V, and the Lipoly's voltage was dropping just below this level at full load current (about 9A). Since this current is right at the limit of an ET-1200's ability, I decided to try a NiMH battery.

I disabled the LVC, and switched to a 6 cell KAN650 NiMH pack. With this battery the model is little heavier at 419g, and requires a bit of up-trim for level flight. The top surface of the elevator is almost level with the stabilizer, which agrees with other flyers' experiences. When the battery gets to the end of its capacity there is a noticable drop in performance, indicating that it's time to land. Flight time is between 6 and 7 minutes.

Impressions

I like this model. It looks good, with lots of 'technical' bits, and a distinctive scale profile. The twin EDF motors sound great in the air. It flies gently like the park flyer that it is, but can pick up quite a bit of speed for swooping low passes. It also handles wind well. As with most ducted fan models, it doesn't have a lot of acceleration, and the climb rate is not amazing. But its light wing loading and good aerodynaimcs make for smooth confident flying and easy landings.

Time will tell how well the EDF motors hold up. An un-scientific 'wet finger' temperature measurement indicated that the motors are staying cool in the air. I expect that with sensible throttle usage they should last for at least 30 flights, but we shall see...

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