monly used only of such flows as are not eddying, but the dis› tinction should be made clear . by the context. 

Streamline flow.-Steady flow past a solid body, that is, a flow in which the direction at every point is independent of time. 

Strea;mlirw form.-Solid body which produces approximately stream› line flow. 

Surface, control.-Movable airfoil designed to be rotated or other› wise moved by the pilot in order to change attitude of airplane or airship. 

Tait group (or tail unit).-Stabilizing and control surfaces at rear end of aircraft, including stabilizer, fin, rudder, and elevator. (Also called "empennage.") 

Tau heavy (airship) .-Condition in which in normal flight the after end of an airship tends to sink and which requires correction by means of the horizontal controls. In this condition an airship is said to "trim by the stern.’’ It may be due to either aerody› namic or static conditions, or to both. 

Thrust, static.-Thrust developed hy propeller when rotating at a fixed point. 

Tractor propeller.-Propeller mounted on forward end of engine or propeller shaft. (It is usually forward of fuselage or wing nacelle.) 

Trailing edge.-Rearmost edge of airfoil or propeller blade. 

5. Types of airships.-a. Airships are divided into three general classes in accordance with their method of construction. These three classes are 

(1) Nonrigid. 

(2) Semirigid. 

{3) Rigid. 

b. The names describe means by which shape of the envelope is maintained. In the nonrigid, gas in the envelope is kept under suffi› cient pressure to keep the hull shape by this means alone. In the semirigid a central keel is provided which carries the loading and is itself swung by suspensions from the top of the envelope. Due to its rigidity, the keel assists the internal pressure in maintaining f;hape of the envelope. In rigid construction a metal structure is provided to maintain shape of the hull. Usually the gas is at at› mospheric pressure, although in some cases a slight superpressure is maintained. 

c. All types .have control and power plant cars and control sur› faces. 

