ANGLE OF INCIDENCE.—One method of finding the angle of incidence is as follows:
First place the aeroplane in its flying position. The corner of the straight-edge must be placed underneath and against the CENTRE of the rear spar, and held in a horizontal position parallel to the ribs. This is secured by using a spirit-level. The set measurement will then be from the top of the straight-edge to the centre of the bottom surface of the main spar, or it may be from the top of the straight-edge to the lowest part of the leading edge. Care should be taken to measure from the centre of the spar and to see that the bubble is exactly in the centre of the level. Remember that all this will be useless if the aeroplane has not been placed accurately in its flying position.
This method of finding the angle of incidence must be used under every part of the lower surface where struts occur. It should not be used between the struts, because, in such places, the spars may have taken a slight permanent set up or down; not, perhaps, sufficiently bad to make any material difference to the flying of the machine, but quite bad enough to throw out the angle of incidence, which cannot be corrected at such a place.
If the angle is wrong, it should then be corrected as follows:
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.