The Aeroplane Speaks. Fifth Edition

The board is cut to the same angle as that specified for the upward inclination of the surface towards its wing-tips. It is placed on the spar as indicated above, and it is provided with two short legs to raise it above the flanges of the ribs (which cross over the spars), as they may vary in depth. A spirit-level is then placed on the board, and the wires must be adjusted to give the surface such an inclination as to result in the bubble being in the centre of the level. This operation must be performed in respect of each bay both front and rear. The bays must then be diagonally measured as already explained. YET ANOTHER METHOD of finding the dihedral angle, and at the same time the angle of incidence, is as follows: A horizontal line is taken from underneath the butt of each spar, and the set measurement is either the angle it makes with the spar, or a fixed measurement from the line to the spar taken at a specified distance from the butt. This operation must be performed in respect of both main and rear spars, and all the bays must be measured diagonally afterwards. Whichever method is used, be sure that after the job is done the spars are perfectly straight. STAGGER.--The stagger is the distance the top surface is in advance of the bottom surface when the aeroplane is in flying position. The set measurement is obtained as follows: Plumb-lines must be dropped over the leading edge of the top surface wherever struts occur, and also near the fuselage. The set measurement is taken from the front of the lower leading edge to the plumb-lines. It makes a difference whether the measurement is taken along a horizontal line (which can be found by using a straight-edge and a spirit-level) or along a projection of the chord. The line along which the measurement should be taken is laid down in the aeroplane's specifications.