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. 

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. 

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. 

