If it is too great, then the rear spar must be warped up until it is right, and this is done by slackening all the wires going to the top of the strut, and then tightening all the wires going to the bottom of the strut.
If the angle is too small, then slacken all the wires going to the bottom of the strut, and tighten all the wires going to the top of the strut, until the correct adjustment is secured.
Never attempt to adjust the angle by warping the main spar.
The set measurement, which is of course stated in the aeroplane's specifications, should be accurate to 1/16 inch.
Lateral Dihedral Angle.—One method of securing this is as follows, and this method will, at the same time, secure the correct angle of incidence:
The strings, drawn very tight, must be taken over both the main and rear spars of the top surface. They must run between points on the spars just inside the outer struts. The set measurement (which should be accurate to 1/16 inch or less) is then from the strings down to four points on the main and rear spars of the centre-section surface. These points should be just inside the four centre-section struts; that is to say, as far as possible away from the centre of the centre-section. Do not attempt to take the set measurement near the centre of the centre-section.