Once a particular project/model is decided upon, the modeller often needs to study scale drawings (which, in our experience, are not always correct) and photographs of the full size aeroplane, if accurate retraction geometry is to be determined. The following notes give a guide to calculating the required angles.
On the face of it, the retraction angle is that measured when viewing the front (or side in the case of twist and turn mechanisms), of the aeroplane from the extended position to the retracted position in the dihedral of the wing (not in relation to the ground datum). Whilst this is satisfactory if the wheel moves in an arc perpendicular to the fuselage /wing datum, it is not the case if the wheel scribes an arc at an angle to these datum’s, e.g. if the oleo leg is angled forwards when down but rearwards when retracted. This angle is not easy to calculate and we use a special gauge.
By way of example, the Spitfire has a retraction angle of 93.5° as measured from the front but requires an angle of 96° for accurate geometry. In other cases the oleo leg does not lie in line with the wing datum’s when retracted and this has to compensated for (for example the Douglas Dauntless).