Part 13 (2/2)

It would be better to use the centre line of the aeroplane rather than the butts of the spars It is not practicable to do so, however, as the centre line probably runs through the petrol tanks, etc

THE DIHEDRAL BOARD--Another le of incidence, is byto use, but leads to many errors, and should not be used unless necessary The reasons are as follows:

The dihedral board is probably not true If it must be used, then it should be very carefully tested for truth beforehand Another reason against its use is that it has to be placed on the spars in a position between the struts, and that is just where the spars may have a little permanent set up or down, or some inaccuracy of surface which will, of course, throw out the accuracy of the adjust it is as follows:

[Illustration]

The board is cut to the sale 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 ive 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 onally measured as already explained

YET ANOTHER METHOD of finding the dihedral angle, and at the sale of incidence, is as follows:

A horizontal line is taken from underneath the butt of each spar, and the set le 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 onally afterwards

[Illustration]

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 botto position The set measurement is obtained as follows:

[Illustration]

Plue of the top surface wherever struts occur, and also near the fuselage The set e to the plumb-lines Ita 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

If aline, it may result in a difference of perhaps 1/4 inch or er, with the certain result that the aeroplane will, in flight, be nose-heavy or tail-heavy

After the adjuster have been secured, it is as well to confir the last adjustment, the first one may have been spoiled

OVER-ALL ADJUSTMENTS--The following over-all check measurements should now be taken

[Illustration: The dotted lines on the surface represent the spars within it]

The straight lines AC and BC should be equal to within 1/8 inch The point C is the centre of the propeller, or, in the case of a ”pusher”

aeroplane, the centre of the nacelle The points A and B are marked on the main spar, and must in each case be the saer should not attempt to make A and B merely the sockets of the outer struts, as they may not have been placed quite accurately by the manufacturer The lines AC and BC must be taken from both top and bottom spars--two measurements on each side of the aeroplane

The two measurements FD and FE should be equal to within 1/8 inch F is the centre of the fuselage or rudder-post D and E are points marked on both top and bottom rear spars, and each must be the same fixed distance from the butt of the spar Two measurements on each side of the aeroplane

If these over-all measurements are not correct, then it is probably due to soht or too slack

Itout of truth, but of course the rigger should havethe rest of thewires within the lifting surfaces not being accurately adjusted, but of course this should have been seen to before covering the surfaces with fabric

FUSELAGE--The e is laid down in the aeroplane's specifications After it has been adjusted according to the specified directions, it should then be arranged on trestles in such a way as to make about three-quarters of it towards the tail stick out unsupported In this way it will assu conditions, and when it is in this position the set measurements should be confirmed If this is not done it ht when supported by trestles at both ends, as, in such case, its weightupon the trestles

THE TAIL-PLANE (EMPENNAGE)--The exact angle of incidence of the tail-plane is laid down in the aeroplane's specifications It is necessary to make sure that the spars are horizontal when the aeroplane is in flying position and the tail unsupported as explained above under the heading of Fuselage If the spars are tapered, then make sure that their centre lines are horizontal