TM 1-320  4  AIR CORPS  U seful.-Crew  and passengers, oil and fuel, ballast other than  emergency, ordnance, and portable equipment.  Nose heavy.-Condition  of an airship which when at rest in still air  trims with its axis inclined down by the bow.  The term "bow  heavy" is preferred to "nose heavy"  in  describing airships.  Oscillation,  stable.-Oscillation whose amplitude does not increase.  Oscillation,  ~tnstable .-O sc illation  whose amplitude incres.ses con› tinuously until an attitude is reached from which there is no  tendency to return toward the original attitude, the motion becom- .  ing a steady divergence.  P erformance CM’I’CUJteristics  (airship) .-In general:  Maximum speed at various altitudes.  Maximum altitude attainable with definite weight relations and  ballonet volume (if fitted).  Endurance at full and half power.  Static ceiling.  Dynamic lift under specified conditions.  Pitch of propeller:  Etfective.-Distance  which aircraft advances along its flight  path for one revolution of propeller.  Its symbol is  pa.  Geomet,rical.-Distance  which an element of a propeller would  advance in one revolution if it were moving along a helix of  slope equal to its blade angle.  Mean geometrical.- Mean  of the geometrical pitches of the sev› eral elements.  I ts symbol is  p$_{0}$ •  Standard.-Geometrical pitch taken at two-thirds of thE’ radius.  Also called "nominal pitch." Its symbol is  Ps•  Ze1’0  th?’U8t.-Distance which propeller would have  to  advance  in one revolution in order that there might be no thrust. Also  called "experimental mean pitch." Its symbol is  pv.  Zero  torque.-Distance which propeller would have to advance in  one revolution in order that the torque might be zero.  Its  symbol is  Pa•  Pitch  mtio.-Ratio of the pitch (geometrical unless otherwise stated)  to the diameter  pj D.  Pitch  speed.-Product of mean geometrical pitch by number of revo› lutions of propeller  in  unit time, that is, the speed aircraft would  make if there were no slip.  Propeller area,  proje~ted.-Total  area in the plane perpendicular to:  propeller shaft swept by propeller, except portion covered by the  boss and that swept by root of the blade. This portion is usually  taken as extending 0.2 of maximum radius from axis of the shaft.  8 