ne this is
secured by packing up the machine so that the engine foundations are
perfectly horizontal both longitudinally and laterally. This position is
found by placing a straight-edge and a spirit-level across the engine
foundations (both longitudinally and laterally), and great care should
be taken to see that the bubble is exactly in the centre of the level.
The slightest error will assume magnitude towards the extremities of
the aeroplane. Great care should be taken to block up the aeroplane
rigidly. In case it gets accidentally disturbed while the work is going
on, it is well to constantly verify the flying position by running
the straight-edge and spirit-level over the engine foundations. The
straight-edge should be carefully tested before being used, as, being
generally made of wood, it will not remain true long. Place it lightly
in a vice, and in such a position that a spirit-level on top shows
the bubble exactly in the centre. Now slowly move the level along the
straight-edge, and the bubble should remain exactly in the centre.
If it does not do so, then the straight-edge is not true and must be
corrected. _This should never be omitted._

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:

[Illustration]

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 m