42. Angle of Incidence.—The angle at which a wing is inclined to the line of flight.
43. Decalage.—Difference in angle of incidence between any two distinct aerofoils on an airplane.
44. Chord.—Distance between the entering edge and trailing edge of a wing measured on a straight line touching front and rear bottom points of a wing.
45. Camber.—The depth of the curve given to a sustaining surface such as a wing. Thus it will be observed that the planes are not straight in cross-section but are concave slightly upward. The depth of this concavity is the camber. Another way of expressing this is that camber is the greatest distance between the surface of a wing and its chord line.
46. Gap.—The distance between the lower and upper wings of a biplane.
47. Spread.—The distance over all from one wing tip to the other wing tip.
48. Aerofoil.—A general name applied to any wing or lifting surface of an airplane.
49. Deadhead Resistance.—Each part of an airplane against which the wind strikes offers a resistance against being moved through the air. This is called the deadhead resistance or the parasite resistance. It is for the purpose of lessening this resistance that the parts of a machine are stream-lined. Remember that force or power must be applied to overcome this resistance and the lessening of such resistance decreases the power necessary. A parallel illustration is to think of the power necessary to push a board sideways through water.
50. Drift.—When the air strikes the inclined wing of an airplane its force has two components. One part called the lift (see 52) acts up and tends to lift the machine. The other part, called drift, tends to push the machine backward. This drift must also be overcome by applying power enough to drive the machine forward.
51. Total Resistance.—Sometimes called drag. (49) Deadhead resistance added to (50) drift, gives the total forces opposing the forward movement of the airplane. This is called the total resistance and is overcome by the thrust of the propeller.