TM 1-320  4-5  • AIRSHIP AERODYN AMICS  monly used only of such flows as are not eddying, but the dis› tinction should be made clear . by the context.  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.  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.  11  Trailing  edge.-Rearmost edge of airfoil or propeller blade.  Tractor propeller.-Propeller  mounted on forward end of engine or  propeller shaft.  (It  is usually forward of fuselage or wing  nacelle.)  Thrust, static.-Thrust  developed hy propeller when rotating at  a  fixed point.  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.  Tait group  (or tail unit).-Stabilizing and control surfaces at rear  end of aircraft, including stabilizer, fin, rudder, and elevator.  (Also  called "empennage.")  Surface, control.-Movable  airfoil designed to be rotated or other› wise moved by the pilot in order  to  change attitude of airplane  or airship.  Strea;mlirw form.-Solid  body which produces approximately stream› line flow.  Streamline  flow.-Steady flow past a solid body, that is, a flow in  which the direction at every point is independent of time. 