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). 

Pitch of propeller: 

Endurance at full and half power. 

Static ceiling. 

Dynamic lift under specified conditions. 

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. 

