n Surface, although set at the same angle on the
machine. It may then be said to have _in effect_ a less Angle of
Incidence. I'll just show you on the Blackboard."

[Illustration]

"And now," said Efficiency, "I have only to meet the Ailerons and the
Rudder, haven't I?"

"Here we are," replied the Ailerons, or little wings. "Please hinge us
on to the back of the Main Surfaces, one of us at each Wing-tip, and
join us up to the Pilot's joystick by means of the control cables. When
the Pilot wishes to tilt the Aeroplane sideways, he will move the stick
and depress us upon one side, thus giving us a larger Angle of Incidence
and so creating more Lift on that side of the Aeroplane; and, by means
of a cable connecting us with the Ailerons on the other side of the
Aeroplane, we shall, as we are depressed, pull them up and give them
a reverse or negative Angle of Incidence, and that side will then
get a reverse Lift or downward thrust, and so we are able to tilt the
Aeroplane sideways.

"And we work best when the Angle of Incidence of the Surface in front
of us is very small, for which reason it is sometimes decreased or
_washed-out_ towards the Wing-tips. The reason of that is that by the
time the air reaches us it has been deflected downwards--the greater the
Angle of Incidence the more it is driven downwards--and in order for
us to secure a Reaction from it, we have to take such a large Angle of
Incidence that we produce a poor proportion of Lift to Drift; but the
smaller the Angle of the Surface in front of us the less the air is
deflected downwards, and consequently the less Angle is required of us,
and the better our proportion of Lift to Drift, which, of course, makes
us much more effective Controls."

[Illustration: "Wash out" on both sides.]

"Yes," said the Lateral and Directional Stabilities in one voice,
"that's so, and the wash-out helps us also, for then the Surfaces
towards their Wing-tips have less Drift or 'Head-Resistance,' and
consequently the gusts will affect them and us less; but such decreased
Angle of Incidence means decreased Lift as well as Drift, and the
Designer does not always care to pay the price."

"Well," said the Ailerons, "if it's not done it will mean more work for
the Rudder, and that won't please the Pilot."

"Whatever do you mean?" asked Efficiency. "What can the Rudder have to
do with you?"

"It's like this," they replied: "when we are deflected downwards we gain
a larger Angl