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DB Sport & Scale Cirrus Moth

By: Don Harvey

When I first saw the engine that David Boddington dropped into my hand, I felt I was looking at a well made scale reproduction that was primarily intended as a collectors piece, only to be displayed, certainly not run, and most certainly not flown in a model. I was looking at a rather superb piece of Russian model engineering in the shape of a four cylinder ‘Cirrus’ near enough

Cirrus

Good impression of the size of the Cirrus engine.

¼ scale four stroke engine. The only real give-away that it was meant to be a working engine was the typical R/C carburettor. It seemed sacrilegious to condemn this little gem to a shelf somewhere, it deserved to be put into it’s element; on the front of a DH Moth, a Cirrus Moth! What better excuse than to build the new DB Sport & Scale Cirrus Moth kit. It’s about the right size, just under quarter scale, and the right type. There are, and were, a number of variants of the full size which would show the engine off to best advantage, so no lack of prototypes for inspiration.

The building of the kit went at a fairly steady pace, starting with the wings and then on to the fuselage. Once the basic fuselage was completed attention was focused on the engine installation.

Bare construction.

Bare ‘Bones’. Trial fit of everything before covering. Look at all those ribs!

This was not exactly a standard fitting engine. It needed a specially made pair of brackets to be able to mount it onto the engine bearers. The hardwood bearers had to be raised so that the engine sat in a more scale-like position that also made it more visible. A fair bit of brain exercise took place to work out the best way of operating the throttle; this was achieved by using a bell-crank fitted to a small bracket on the engine mount. It was now full steam ahead to complete the model, which was done without any major hitches. A few alterations were made along the way to suit personal preferences (does anyone build a model straight from the box without changing something?).

To cover the model, I used red and silver Solartex, following the colour scheme of the full-size Moth I had chosen to replicate. Litho plate, suitably scribed and gently punched, was used to represent the top wing tank, footholds, and hinges.

Just about finished and sitting pretty.

Cockpit detail with pilot getting impatient.

DH MothOpen cockpits, especially on models of this size, cry out for some detail and at least one occupant. The instrument faces were created using my P/C and then glued behind a thin sheet of clear plastic (the glass) which, in turn, was glued behind a thin piece of ply that had previously been prepared with the appropriate holes and dyed ‘antique pine’ to make it look right. Rib stitching was simulated using the ‘thread-paper-pva-glue’ system that gives a very convincing representation. The fuselage and wing lettering was carefully masked and then spray-painted using car paint, the acrylic type. The same type of spray paint was also used on all the other surfaces that required paint. My trusty P/C was once again used to create the ‘Moth’ and ‘DH’ emblems.

DHThe rigging, which works for it’s living on this model, was made from heavy duty control line wire. All rigged up and ready to go the Cirrus Moth weighs in at 10lbs. not bad for an 80” wingspan biplane.

The day chosen for the test flight, or more accurately, engine test run and maybe flight, could not have been better; a warm, sunny February morning with little or no wind. Model assembled and checked; tank full of fuel and engine primed. Would the engine start and run? Would there be a loud bang with engine bits flying in all directions? Now would be the time to find out.

DH Cirrus Moth

Sniffing the air… ready for testing.

With David Bottington holding the model, the needle valve was opened three turns, followed by a few flicks of the propeller making sure there was no hydraulic lock, it was time to apply the electric starter. Utter amazement; the engine roared into life almost instantly, but not for long. The sound of those four cylinders running at a fair old pace was absolute magic. After a bit of fiddling around we managed to get a sustained run, albeit at no more than a tad above half throttle.

The decision was then made, so it was top up the tank and go for a test flight. We decided to get the engine to run at optimum revs. whilst holding on to the model and then releasing when ready, rather than risk the engine stalling by trying to use the throttle. We felt that we had just about enough power to reach flying speed before we ran out of field. Once more the little gem burst into life; all four cylinders doing their stuff with that lovely smile-making sound. Best revolutions achieved, the throttle position was noted; David held the model slightly tail high to give it a little more help and, on my signal, released the Moth with a slight forward shove. Off it trundled, very slowly at first, the temptation to give the throttle lever just a little nudge forward was immense, but resisted. Careful rudder inputs keep the model running straight whilst speed gradually built up. The tail was high and actually biting air, so a little up elevator was applied resulting in a gentle lift off at what seemed an incredibly slow airspeed. The Moth was airborne and the engine sounded magnificent. Time to make a 90-degree turn and throw in some trim adjustments at the same time. Still gaining a little height it was time again to make the second turn to bring the moth back towards us. It looked just great in the winter sunshine, coming towards us at a steady scale-like speed when suddenly there was silence; dead stick. A gentle turn into wind and in it came for a gentle landing, almost at our feet. Enormous grins all round!

Airborne and looking good.

Although, as test flights go, this was in no way long enough to evaluate the model, it did prove to us that the little four cylinder Cirrus was by no means a ‘glass cabinet’ engine. Since that first ‘hop’ several things have been tried. The engine now runs with greatly improved consistency, though still not quite up to the standard of the more ‘conventional’ engines. Further trials are planned and will be carried out at a steady pace. The model has had numerous flights and proved to be a real lady of a flier, and I can’t help but look forward to the next trial sessions.

By: Don Harvey