Retracts come in basically two forms, the fixed rigid system and the sprung or dampened system allowing the unit to move in its mountings and absorb the bounce of rough terrain

It is important that the retract is fitted carefully, in order to guarantee a trouble free operation. Some units also contain fixing holes to allow servos to be added to the chassis

Part 1

Positioning the retract.

With most modern sailplanes the undercarriage lowers perpendicularly beneath the leading edge of the wing. In practice the model sailplane may, if fitted the same way, tilt forward onto the nose when on aerotow or landing. Because of this it is best to position the wheel centre when in the fully down position, slightly ahead of the leading edge of the wing.

Fuselage cut-out and retract doors.

The width of the cut-out should be 2mm smaller than the width of the chassis whilst the length of the cutout depends on the size of the wheel used. Determine the position of the front edge of the leg in the down position and measure the length for the door aperture to clear the wheel.

For marking out the cutting line on the fuselage, make a card template, which is then stuck onto the fuselage. Scratch a line deeply into the fuselage around the template which will allow the saw to follow much easier. Cut out the doors using a fine saw such as an Xacto and then split this cut-out piece exactly in the centre to make the two doors.

For hinging the doors, a piece of thin inner tube of a racing cycle can be used. Cut off 2 cm wide strips of appropriate length and punch holes with a chamfered brass tube into the centre of the rubber strip (see above). These hinges are very durable and adapt to the curvature of the fuselage. They are then stuck from the inside with impact adhesive across the hinge line of the fuselage and door leaving a gap of 1mm on the hinge line. Two small tabs made from sheet metal , Epoxy plate or plywood are then made and fixed to the inside of the opening to prevent the doors closing into the fuselage . The flap closing can be tightened, if necessary, with small rubber bands attached from inside the fuselage to the doors onto which small wire hooks have been epoxied.

Another method of hinging is to use the plastic hinges (Picture right) which have the metal pin for the hinge. If you use these, it is important that the cut out for the hinge is made on the fuselage side and not half door & half fuselage else the door may restrict the retract from lowering.

Producing the mounting formers and their installation.

Difference between front & rear formers

Make the formers out of 8-12mm thick plywood from a template of the fuselage cross-section.

To construct an accurate symmetrical templates proceed as follows :-

1. Fold a rectangular cardboard template in half.
2 Cut it roughly to the shape of the inside fuselage
3 Open the template and hold it at the appropriate position in the fuselage to check for shape

Repeat step 1 to 3 until the template corresponds to the fuselage. After templates for front and rear formers have been made, their outlines can be transferred to the plywood. Also transfer the fold line of the template which will serve as the centreline for the retract. Now the frames can be sawn out.

By lowering the retract unit, calculate the mounting height of the unit inside the fuselage, usually 1-5cm when measured from lower edge of the unit. Cut out the mounting holes in the formers for the retract unit and assemble the retract to the formers and check for fit in the fuselage, making sure that the unit fit is exactly horizontally and vertically aligned. The formers are now bonded in position with thickened Epoxy resin.

Retract assembled with formers

After hardening re-inforce the joint between the retract and fuselage with strips of fibreglass tape epoxied into position..

On the front former make a retainer for the retract out of strong ply, which is fit on top of the former and screwed into place with four wood screws as in the picture above left. An alternative method is to make an aluminium sheet metal retaining bar, 2-3 mm thick, which is bolted through the front face of the former as in the picture above right.