The Aeroplane Speaks. Fifth Edition

In the case of aeroplanes fitted with engines of the rotary type, the "flying position" is some special attitude laid down in the aeroplane's specifications, and great care should be taken to secure accuracy. 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.