Iíve always been fond of Tiger Moths and I suspect Iím not alone in this respect. Twenty odd years ago I built a 1/5 scale version from the Practical Scale kit which was superb and still available from Toni Clark at Practical Scale in Germany. This was powered by an inverted O.S. 90 four-stroke. I left the hobby for a few years and when I re-kindled the interest some years later I was keen to explore the possibilities of electric flight.
About four years ago a plan of a 1/5 scale Tiger Moth designed by Gordon Whitehead featured in a magazine. This was powered by a Maxcim brushless motor with its own speed controller running off twenty cells. I had a little experience of current electric flight technology but not at this level of power, having always powered my models the oily way (or according to some of our club members the only way!). This Tiger project would be something of a learning curve for me as well as an interesting and fun to build.
The construction uses mainly light ply and balsa with birch ply in high stress areas such as the undercarriage mounting and engine firewall. This results in a very light and strong structure, far lighter than the Practical Scale model mentioned. The construction involves a great deal of fretwork as the formers have lightening holes cut in them. The battery box forms a central part of the structure and the fuselage is constructed in a modular fashion with the engine mount built first and joined with the battery box to the rest of the structure. A considerable amount of metalwork is required as the undercarriage is made of brass tube and fully sprung. It is also necessary to make all the strut fixings etc. out of brass sheet.
Probably the most tedious part is to simulate the fuel tank corrugations. This is done by sticking down thin strips of card (with photo mount spray) to the fuel tank balsa sheeting. This is then given copious coats of sanding sealer which smoothes out the sharp edges of the card. The result when painted looks quite realistic. The cabane struts are made of brass tube squeezed to an oval, which gives a very authentic look to the finished model. Wings and tail surfaces are thoroughly conventional .The original design used a pushrod and belcrank system to operate the ailerons but I decided to use a couple of the excellent Hitec HS125 thin wing servos primarily designed for glider wings and so I could use the differential aileron settings on my FF9 transmitter - this resulted in a very solid and slop free mechanism with more than adequate power.
I had decided to use the Maxcim motor with a monster box gearbox and built the motor mount accordingly but then discovered it was quite an old design and in fact was no longer sold to the model trade - I had to find an alternative that would fit the slim cowl. A bit of research and few comparison checks revealed that the Hacker C50 10L inrunner fitted with the acro 6.7:1 gearbox would fit perfectly so all that was required was to make a suitable motor mount. This was built by wrapping several layers of thin ply around a glass jar former so creating a cylindrical mount and then fitting a stout birch ply front former cut to fit the acro gearbox. The firewall was milled out with a Dremel tool to take this modified mount. This has proved to be more than adequately strong with the rear of the motor supported with a simple brass strap.
The top cowl is removable to facilitate battery changing and is held in place by large dress snaps, as is the front cowl. The speed controller sits behind the front cowl and gets a nice blast of cooling air from the 16" prop. Cockpit detail is kept to a minimum but I fitted a set of instruments and compass from Inzpan, these are superb and so is their service. The model is covered in Solatex and painted with Holts car paints in the colour scheme of G- ACDC, the oldest airworthy Tiger based at Headcorn in Kent.
In all this project took nearly two years on and off but I'm pleased with the result considering my somewhat mundane building skills. In view of the powerful electric motor system and the amount of wire work and control cables in the model, I decided to fit a new Spectrum 2.4 GHz radio module in my FF9 transmitter which means I donít have to worry about reception problems which may (or may not) have affected a conventional 35Mhz receiver.
Several weeks passed by before a suitable calm evening presented itself and the Tiger was lined up for its first flight. I had already dialled in some rudder/aileron coupling. The first take off was uneventful except that the model became airborne a lot sooner than I expected, probably because I did not get the tail up quickly enough and I had misjudged the power of this motor. The model climbed steeply due to the phenomenal urge from that Hacker motor, but quite a few bleeps of down trim sorted things out and she looked great in the air.
The elevator is very sensitive and the model will land very slowly. If I try hard I can pull of a passable three-pointer but not every time! In fact itís better and safer to land with a reasonable amount of speed to maintain good control authority. Ten minute of flying time is easily achievable on 20xNiMH cells and considerably more if a 5000mAh 5S Li-Po is used with a bit of lead to compensate for the lighter battery to get the C of G right. In fact the 18.5 volts provided by the Li-Po is more than enough. The only scary moment was during a subsequent flight in a stiff wind, I tried a tight turn too slow and down wind so she dropped into a spin but fortunately there was just enough height to recover, a lesson learned - it may have lots of wing but itíll still bite if miss-handled. The satisfaction of seeing two years of work finally get airborne takes some beating especially when it all comes back in one piece!
The Tiger Moth is a lovely aeroplane that takes just that little bit more skill to handle but this only adds to the fun and it teaches you to fly properly. So having built and flown this thing, am I sold on electric power? Well fundamentally yes, because the power available is very impressive and the fact that you can simply connect and go is very appealing. Iíll still use i.c. power for larger models because the cost of batteries at this level is very high and after all, who can resist the sound of an idling Laser four-stroke?
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