sson which was learnt from these first two years of
experiment, was that where, as in a biplane, two surfaces are superposed
one above the other, each of them has somewhat less lift than it would
have if used alone. The experimenters were also still in doubt as to the
efficiency of the warping method of controlling the lateral balance
as it gave rise to certain phenomena which puzzled them, the machine
turning towards the wing having the greater angle, which seemed also to
touch the ground first, contrary to their expectations. Accordingly,
on returning to Dayton towards the end of 1901, they set themselves to
solve the various problems which had appeared and started on a
lengthy series of experiments to check the previous figures as to wind
resistance and lift of curved surfaces, besides setting themselves
to grapple with the difficulty of lateral control. They accordingly
constructed for themselves at their home in Dayton a wind tunnel 16
inches square by 6 feet long in which they measured the lift and 'drag'
of more than two hundred miniature wings. In the course of these tests
they for the first time produced comparative results of the lift of
oblong and square surfaces, with the result that they re-discovered the
importance of 'aspect ratio'--the ratio of length to breadth of planes.
As a result, in the next year's glider the aspect ration of the wings
was increased from the three to one of the earliest model to about six
to one, which is approximately the same as that used in the machines
of to-day. Further than that, they discussed the question of lateral
stability, and came to the conclusion that the cause of the trouble was
that the effect of warping down one wing was to increase the resistance
of, and consequently slow down, that wing to such an extent that its
lift was reduced sufficiently to wipe out the anticipated increase in
lift resulting from the warping. From this they deduced that if the
speed of the warped wing could be controlled the advantage of increasing
the angle by warping could be utilised as they originally intended.
They therefore decided to fit a vertical fin at the rear which, if the
machine attempted to turn, would be exposed more and more to the wind
and so stop the turning motion by offering increased resistance.

As a result of this laboratory research work the third Wright glider,
which was taken to Kill Devil Hill in September, 1902, was far more
efficient aerodynamically than either