Introduction

The Sig Four Star 40 has been available for a number of years in its home country of America, but has only recently been imported in numbers into the UK. It has an excellent reputation in America, and is often cited in Internet forums as being the perfect first low wing aeroplane for the learner flyer looking to move on from the high wing trainer stage. Simple construction and solid, stable flight performance have earned the model this strong reputation.

On opening the box, it is obvious that this is a very complete kit. All of the wood is superb, with careful selection for purpose, and all of the sheet components are laser cut. A good hardware pack includes two piece glass/ plastic engine mount, formed aluminium landing gear, nylon control snakes, together with all the necessary screws, nuts and threaded rods, leaving the builder to find wheels, tank, engine and radio gear. The plans also show optional spats which can be bought from Sig as an extra.

Clipped wings and increased rudder area are evident in this shot

Design of the Kit

The fuselage is constructed mainly from 3mm liteply, with full length sides running from nose to tail. Intricately designed doublers run from the nose to the rear of the wing seat, providing slots which tabs on the main formers fit into. Behind the wing, the fuselage sides themselves are slotted for tabbed formers, and a tapered bottom sheet from the rear of the wing to the tail makes building a straight fuselage virtually foolproof. The firewall and landing gear mount are 6mm birch ply, and three balsa stringers form the rear deck behind the canopy. The tail components are all simple 6mm sheet balsa and sheet balsa is used for the front decking.

The wing is not the usual ‘D’ box design, but uses spruce main spars with shear webs, and three balsa spars, two in front the main spar, and one behind. There is no leading edge sheeting, but the trailing edges have 25mm wide sheeting, and there is 100mm of centre section sheeting which runs front to back. The ribs are recessed to accept the TE sheeting, and that’s it; no ‘D’ tube and no capping strips on the 3mm ribs. Full strip ailerons are operated by torque rods which are housed in machined basswood blocks glued to the centre trailing edge. All of this combines to make a wing which is very simple and light, and which once covered with suitable film becomes very strong. The wing section is semi-symmetrical with a flat bottom from the main spar backwards, which makes the wing build even easier.

Wing Construction

A common modification to the Four Star is to make the wing one bay shorter on each side to reduce the wing area and increase the roll rate, and I duly incorporated the mod into my build. With the shear webs in place, the upper spars and the square section leading edge can be added followed by the top centre section sheeting. This leaves the wing strong enough to be taken off the board, allowing the front lower spar and the underside centre section sheeting to be added. The second wing panel is then built in the same way, and when everything is set, the wings can be joined.

The wings are joined with a plywood dihedral brace which is fitted into each wing half once the centre ribs have been cut away to allow the aileron servo to fit. With the wings joined, the servo bay is lined, and the aileron torque rods can be fitted, using the grooved basswood blocks. On my kit, these blocks were poorly machined, and did not make a matching pair. A fair amount of fiddling and sanding was necessary to get them in reasonable shape. This was the only niggle with the entire build, so I can't be too hard on Sig.

This completes the wing ready for fitting to the fuselage.

The wing laid out and ribs glued in place

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