PITOT TUBE: A tube with an end open square to the fluid stream, used as a detector of an impact pressure. It is usually associated with a coaxial tube surrounding it, having perforations normal to the axis for indicating static pressure; or there is such a tube placed near it and parallel to it, with a closed conical end and having perforations in its side. The velocity of the fluid can be determined from the difference between the impact pressure and the static pressure, as read by a suitable gauge. This instrument is often used to determine the velocity of an aircraft through the air. PLANE: One of the main supporting surfaces of an airplane or of a wing. (Thus the upper or lower plane of an airplane or the upper right plane or lower right plane of the right wing.) PONTOONS: See Float. PRESSURE NOZZLE: The apparatus which, in combination with a gauge, is used to measure speed through the air. PUSHER: See Airplane. PYLON: A mast or pillar serving as a marker of a course. RACE OF A PROPELLER: See Slip stream. RATE OF CLIMB: The vertical component of the flight speed of an aircraft—i. e., its vertical velocity with reference to the air. RELATIVE WIND: The motion of the air with reference to a moving body. Its direction and velocity, therefore, are found by adding two vectors, one being the velocity of the air with reference to the earth, the other being equal and opposite to the velocity of the body with reference to the earth. RIGHT-HAND ENGINE: An engine designed to drive a right-hand tractor screw. RIGHTING MOMENT: A moment which tends to restore an aircraft to its previous attitude after any rotational disturbance. RIP CORD: The rope running from the rip panel of a balloon to the basket, the pulling of which causes immediate deflation. RIP PANEL: A strip in the upper part of a balloon which is torn off when immediate deflation is desired. ROLL, A: An aerial maneuver in which a complete revolution about the longitudinal axis is made, the direction of flight being maintained. RUDDER: A hinged or pivoted surface, usually more or less flat or stream lined, used for the purpose of controlling the attitude of an aircraft about its normal axis—i. e., for controlling its lateral movement. _Balanced._—A rudder having part of its surface in front of its pivot. RUDDER BAR: The foot bar by means of which the rudder is operated. SEAPLANE: A particular form of airplane in which the landing gear is suited to operation from the water. (_a_) _Boat seaplane_ (or _flying boat_).—A form of seaplane having for its central portion a boat which provides flotation. It is often provided with auxiliary floats or pontoons. (_b_) _Float seaplane._—A form of seaplane in which the landing gear consists of one or more floats or pontoons. SERPENT: A short, heavy guide rope. SIDE SLIPPING: Sliding downward and inward when making a turn; due to excessive banking. It is the opposite of skidding.