diagram.)

Pitch, Propeller--The distance a propeller advances during one
revolution supposing the air to be solid.

Pitch, to--To plunge nose-down.

Reaction--A force, equal and opposite to the force of the action
producing it.

Rudder--A controlling surface, usually hinged to the tail, the operation
of which turns an aeroplane about an axis which is vertical in normal
horizontal flight; causes an aeroplane to turn to left or right of the
pilot.

Roll, to--To turn about the longitudinal axis.

Rib, Ordinary--A light curved wooden part mounted in a fore and aft
direction within a surface. The ordinary ribs give the surface its
camber, carry the fabric, and transfer the lift from the fabric to the
spars.

Rib, Compression--Acts as an ordinary rib, besides bearing the stress of
compression produced by the tension of the internal bracing wires.

Rib, False--A subsidiary rib, usually used to improve the camber of the
front part of the surface.

Right and Left Hand--Always used relative to the position of the pilot.
When observing an aeroplane from the front of it, the right hand side of
it is then on the left hand of the observer.

Remou--A local movement or condition of the air which may cause
displacement of an aeroplane.

Rudder-Bar--A control lever moved by the pilot's feet, and operating the
rudder.

Surface--See "Aerofoil."

Surface, Detrimental--All exterior parts of an aeroplane including
the propeller, but excluding the (aeroplane) lifting and (propeller)
thrusting surfaces.

Surface, Controlling--A surface the operation of which turns an
aeroplane about one of its axes.

Skin-Friction--The friction of the air with roughness of surface. A form
of drift.

Span---The distance from wing-tip to wing-tip.

Stagger--The distance the upper surface is forward of the lower surface
when the axis of the propeller is horizontal.

Stability--The inherent tendency of a body, when disturbed, to return to
its normal position.

Stability, Directional--The stability about an axis which is vertical
during normal horizontal flight, and without which an aeroplane has no
natural tendency to remain upon its course.

Stability, Longitudinal--The stability of an aeroplane about an axis
transverse to the direction of normal horizontal flight, and without
which it has no tendency to oppose pitching and tossing.

Stability, Lateral--The stability of an aeroplane about its longitudinal
axis, and without which it h