AIRSHIP AERODYNAMICS  ’rM 1-320  4  Prope’!Ze1t blade area.- Area  of the blade face, exclusive of the boss  and the root, that is,  of  a portion which is usually taken as extend- ing 0.2 of maximum radius from axis of the shaft.  •  Propeller-caml;er  ratio.-Ratio of maximum thickness of proj)eller  section to its chord.  Propeller  efficien<n.J.-Ratio of thrust power  to  power input of pro› peller. Its symbol is  ’YJ•  Propeller,  pusher.-Propeller mounted to rear of engine or propeller  shaft.  (It  is usually behind the wing cell or nacelle.)  Pr9peller  rake.-Mean angle which the line joining the centroids of  the sections of propeller blade makes with a plane perpendicular  tO  the axis.  Propeller section.-Cross  section of propeller blade made at any point  by a plane parallel to axis of rotation of propeller and tangent at  • the centroid of the section to an arc drawn with the axis of rotation  as its center.  Propeller  th1’U8t.--Component parallel to propeller axis of the total  air force on the propeller. I ts symbolisT.  Propeller torque.- Moment applied to propeller by engine shaft. Its.  symbol is  Q.  Race  rotation.-Rotation produced by action of propeller of stream of  air passing through or influenced by propeller.  Revolutions, ma.a:imum.-Number  of revolutions per minute cone.  sponding to maximum horsepower.  Revolution.s,  normal.- Highest number of revolutions per minute that  may be maintained for long periods.  Righting rM11&ent  (or restoring moment).-Moment which tends to  restore aircraft to its previous attitude after any small rotational  displacement.  Rudde1’.-Movable auxiliary airfoil function of which is to impress  a yawing moment on aircraft in normal flight. I t is usually located  at rear of aircraft.  Skiln  frictiO'n.~Tangential  component of fluid force at point on  surface.  Vl  Rey nold ~  number.-Name  given the fraction  P-;lli  which- p= density of the air.  V  =relative velocity of the air.  l=  linear dimension  of  the body.  ,u.=coefficient of viscosity of the fluid.  285746"-41.-----