e of Incidence and also enter an area of compressed air,
and so produce more Drift than those of us on the other side of the
Aeroplane, which are deflected upwards into an area of rarefied air due
to the _suction_ effect (though that term is not academically correct)
on the top of the Surface. If there is more Drift, _i.e._, Resistance,
on one side of the Aeroplane than on the other side, then of course it
will turn off its course, and if that difference in Drift is serious, as
it will very likely be if there is no wash-out, then it will mean a good
deal of work for the Rudder in keeping the Aeroplane on its course,
besides creating extra Drift in doing so."

"I think, then," said Efficiency, "I should prefer to have that
wash-out,[7] and my friend the Designer is so clever at producing
strength of construction for light weight, I'm pretty sure he won't
mind paying the price in Lift. And now let me see if I can sketch the
completed Aeroplane."

[Illustration]

"Well, I hope that's all as it should be," she concluded, "for to-morrow
the Great Test in the air is due."

[Footnote 4: Morane parasol: A type of Morane monoplane in which the
lifting surfaces are raised above the pilot in order to afford him a
good view of the earth.]

[Footnote 5: Skin friction is that part of the drift due to the friction
of the air with roughness upon the surface of the aeroplane.]

[Footnote 6: Banking: When an aeroplane is turned to the left or the
right the centrifugal force of its momentum causes it to skid sideways
and outwards away from the centre of the turn. To minimize such action
the pilot banks, _i.e._, tilts, the aeroplane sideways in order to
oppose the underside of the planes to the air. The aeroplane will not
then skid outwards beyond the slight skid necessary to secure a
sufficient pressure of air to balance the centrifugal force.]

[Footnote 7: An explanation of the way in which the wash-out is combined
with a wash-in to offset propeller torque will be found on p. 82.]

PART III

THE GREAT TEST

It is five o'clock of a fine calm morning, when the Aeroplane is wheeled
out of its shed on to the greensward of the Military Aerodrome. There is
every promise of a good flying day, and, although the sun has not yet
risen, it is light enough to discern the motionless layer of fleecy
clouds some five thousand feet high, and far, far above that a few filmy
mottled streaks of vapour. Just the kind of morning beloved