SIG 'Monocoupe'

Written for 'Aero Modeller' November 1977.


When I first saw the American SIG kits or the 'Mr Mulligan' and 'Monocoupe' and decided that I wanted to build both of them, it was fairly obvious that the relatively simple 'Mister Mulligan' should be tackled first. This would prepare me for the much more complex 'Monocoupe' which has a construction that is much closer to scale. As all the writer's toy aeroplane projects of recent years have been more akin to carpentry than anything else, I wanted the return to real aeromodelling to be as painless as possible. Unfortunately, in between one and t'other I discovered Cyanoacrylates! Don't get me wrong, Cyanowatsits are great, but it's almost impossible to use too little of the stuff, and if you are a fumble fingers like me you invariably wind up using too much. This together with the very generous wood sizes in the 'Monocoupe' kit lead to the model being much heavier than I would have liked. Anyway, that's my excuse and I'm sticking to it!

One more thing before we get on with the kit is that I decided to power the model with a Telco CO2 motor. In retrospect, I should add that if I had paid more attention to Eric Coates' articles on this motor in previous Aero Modellers, I would almost certainly have decided that the model was too large for it.

So, back to the kit. This, like the 'Mr Mulligan', was notable for the high quality of the wood and the low quality of the die-cutting. On the 1/32in. sheet parts in particular, the wood was in some cases quite badly crushed across the grain, yet along the grain it was only partly cut through. Luckily, the most badly affected parts were those for sheeting the front of the fuselage and these were somewhat oversize.

The basic fuselage structure has 3/32in. square longerons and cross pieces. The longerons have a rather tricky double curvature which captures the character of the original rather well, but is very difficult to get right and still finish up with a symmetrical fuselage! Curved formers are then added to the sides, top and bottom of the structure. One small problem here was that the side formers are not symmetrical, and there is no indication as to which way up they should be fitted. Parts DS obviously should have the bulkier part of the curve at the bottom, whereas parts CS would seem to need fitting with the bulkier part at the top. After fitting the wire undercarriage parts, side stringers of 1/8in. x 1/16in. and top and bottom stringers of 1/16in. square are added. The nose and cabin area are then sheeted with 1/32in. sheet all of which produces a very rigid structure, but seems to be all rather hefty for a 24in. span model.

At this point the Telco motor was fitted by bolting it to a piece of 1/16in. ply glued between the front uprights of the basic fuselage. The tank was located below the leading edge of the wing with its bottom end held by a piece of sheet glued between two formers, the top end being restrained by the copper pipes. The filling valve (and this was to be a major mistake!) was built into the bottom of the fuselage between the undercarriage legs. I did not want the valve hanging outside the model so I bolted it to a small piece of 1/16in. ply epoxied between two bottom formers, with just the spout protruding from the bottom sheeting. Unfortunately, after being fitted (and after the fuselage had been covered) the filler began to leak, and it was necessary to cut open the model to fit a replacement!

By careful bending of the copper pipe at the cylinder head and careful hollowing of the cowl, the motor was completely enclosed. The throttle adjustment is concealed inside the noseblock which is removeable as on the rubber powered version.

The cowl and wheel fairings are built up in the time honoured manner using numerous laminations of 3/32in. sheet, and blocks are supplied for carving the fairings at the top of the u/c legs. All of this wood is of a much harder quality than I would have chosen for this job (my only criticism of the wood). At this stage, I was beginning to worry about the weight, but as the motor/fuselage unit seemed to be very nose heavy, and as the Telco unit is fairly light, I managed to convince myself that it was all an illusion (relativity, and all that).

The tail unit is constructed from 1/16in. square and 1/16in sheet parts in the usual manner, and presents no problems. As I like building wings, these were left until last (things get finished much quicker that way). Scale rib spacing is used with the majority of the ribs from 1/32in sheet and with 1/16in. ribs at the root and at strut locations. However, the leading and trailing edge sizes - 1/8in. x 3/32in. and 1/4in. x 3/32in. respectively - again seem rather excessive when coupled with two 1/8in. x 1/16in. spars and 1/32in. L.E. sheeting! Despite this, the finished product is certainly very pretty, and well worth the effort

Covering was not as tricky as expected. In fact, it is possible to cover the curvatious fuselage with four pieces of tissue. In order to keep a uniform colour on the fuselage and undercarriage, the u/c was covered with tissue. This bit was rather tricky and, on a model of this size was probably as heavy as one coat of coloured dope. The struts too were covered with tissue (well, I always was a glutton for punishinent), although it did occur to me later that these could easily have been coloured with a felt pen. This method was tried on the tailplane struts, and proved quite effective. All tjssue covering on the open areas was water shrunk and the entire model given one coat of well thinned clear dope.

After applying the various transfers and adding panel lines etc., with a Rapidograph pen, the model was assembled, using epoxy to avoid distortion. Now, at last, it could be weighed. Help! - 2.25 ounces and still nose heavy. A small amount of lead was added to the tail to bring the CG to the point shown on the plan and this brought the weight up to a little under 2.5 ounces.

Some time elapsed before a suitably calm evening appeared for test flights, so I became somewhat impatient and eventually ventured out on an evening which was rather on the chilly side for good C02 performance, and a little breezy too. Test glides showed the model to be very under-elevated (perhaps it was nose heavy!) and a judicious amount of up elevator was bent into the tailplane until an acceptable, but rather fast, glide was obtained. Now for the moment of truth. A charge from a half empty Sparklet bulb, and the model was away into a tight, steeply banked, left hand circuit and managing to maintain height! Back to the tail bending to add some right rudder, and the next attempt produced a much safer gentle left power turn and a gain in height. Feeling much better, I spent the remaining time until dark making short flights to improve the trim. However, one flight attempted on a nearly new bulb produced a moment of panic with the model climbing steadily and disappearing into the gathering gloom.

One thing which was impressive about the model was its considerable stability in what were relatively windy conditions. The rather high flying speed may have contnbuted to this. I have no doubt that with a little effort (and rather more sense) it would be quite possible to build the model down to around 1.5 ounces which would produce a very good flying performance by any standards. Anyone who wants to try should keep the following points in mind:
(a) Use balsa cement.
(b) Put the tank on the CO. Mine was under the front decking In order to have an uncluttered cabin.
(c) Make everything as light as possible and do not be tempted to add extra detail etc. to the tail to compensate for expected nose heaviness.

So, in conclusion, an excellent kit which can survive much abuse and still produce a very stable flyer. And, it's so pretty.


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