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```NOMENCLATURE FOR AERONAUTICS 13

airfoils being always less than the angle at which the airfoils stall.

dead rise- In a cross section of a float or flying-boat hull, the amount by which the height of the chine differs from that of the keel.

decalage- The difference between the angular settings of the wings of a biplane or multiplane. The decalage is measured by the acute angle between the chords in a plane parallel to the plane of symmetry. The decalage is considered positive it the upper wing is set at the larger angle (See fig. 5.)

deflation sleeve- See SLEEVE, DEFLATION.

density altitude- See ALTITUDE, DENSITY.

derivatives, resistance- Quantities expressing the variation of the forces and moments on aircraft dur to disturbance of steady motion. They form the experimental basis of the theory of stability, and from them the periods and damping factors of aircraft can be calculated. In the general case there are 18 translatory and 18 rotary derivatives.
lateral resistance derivatives-Resistance derivatives expressing the variation of moments and forces due to small changes in the lateral, yawing, and rolling velocities.
longitudinal resistance derivatives-Resistance derivatives expressing the variation of moments and forces due to small changes in the longitudinal, normal, and pitching velocities.
rotary resistance derivatives-Resistance derivatives expressing the variation of moments and forces due to small changes in the rotational velocities of the aircraft.
translatory resistance derivatives-Resistance derivatives expressing the variation of moments and forces due to small changes in the translational velocities of the aircraft.

diaphragm, ballonet- The fabric partition between the gas an air compartments of the envelope of a nonrigid or semirigid airship or kite balloon.

dihedral angle- See ANGLE, DIHEDRAL.

directional gyro- A gyroscopic instrument for indicating direction, containing a free gyroscope which holds its position in azimuth and thus indicates angular deviation from the course.

directional stability- See STABILITY, DIRECTIONAL.

discharge header- The duct through which the air is conducted from the supercharger to the engine.

displacement, engine- The total volume swept by the pistons of all the cylinders during one complete stroke of each piston.

displacement, float or hull- The total volume, or total weight, of water displaced by a seaplane float or hull.

dive- A steep descent, with or without power, in which the air speed is greater than the maximum speed in horizontal flight.

dive (stress analysis)- A design condition for the wings representing a steady state of flight characterized by high speed and an angle of attack approximately that of zero lift (cf. inverted flight and pull-up, sudden).

dock- A large shed used for housing airships.

dock- To haul an airship into its dock.

dope:
airplane dope- The liquid material applied to the fabric surfaces of airplanes to increase their strength, to produce tautness by shrinking, and to act as a filler for maintaining airtightness.
fuel dope- Any material added to the fuel in small quantities for the purpose of preventing detonation.

downwash- The air deflected perpendicular to the direction of motion of an airfoil.

downwash angle- See ANGLE, DOWNWASH.

drag- The component of the total air force on a body parallel to the relative wind. (See fig. 9.)
induced drag- That part of the drag induced by the lift.
parasite drag- That portion of the drag of an aircraft exclusive of the induced drag of the wings.
profile drag- The difference between the total wing drag and the induced drag.
profile drag, effective- The difference between the total wing drag and the induced drag of a wing with the same geometric aspect ration but elliptically loaded.

drag direction (stress analysis)- The direction of the relative wind (cf. beam, chord, lift, and side directions).

drag force, or component (stress analysis)- A force, or component, in the drag direction; i.e., parallel to the relative wind (cf. beam, chord, lift, and side forces).

drag rope- A long rope which can be hung overboard from a balloon so as to act as a brake and a variable ballast in making a landing. Sometimes called "trail rope" or "guide rope."

drag strut- A fore-and-aft compression member of the internal bracing system of an aircraft. (see fig. 5.)

drift angle- See ANGLE, DRIFT/

drip flap- A strip of fabric attached by one edge to the envelope of an aerostat so that rain runs off its free edge instead of dripping into the basket or car. It also assists in keeping the suspension ropes dry and nonconducting. Also called "drip band" or "drip strip."

dry weight of an engine- See ENGINE, DRY WEIGHT OF.

dynamic factor (stress analysis)- The ratio between the load carried by any part of an aircraft when accelerating and the corresponding basic load.

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