Monoplane (double). An aeroplane with two aerofoils, one behind the other or two main planes, tandem-wise.
Biplane. An aeroplane with two aerofoils, one below the other, or having two main planes superposed.
Triplane. An aeroplane having three such aerofoils or three such main planes.
Multiplane. Any such machine having more than three of the above.
Glider. A motorless aeroplane.
Helicopter. A flying machine in which propellers are employed to raise the machine in the air by their own unaided efforts.
Dihedral Angle. A dihedral angle is an angle made by two surfaces that do not lie in the same plane, i.e. when the aerofoils are arranged V-shaped. It is better, however, to somewhat extend this definition, and not to consider it as necessary that the two surfaces do actually meet, but would do so if produced thus in figure. BA and CD are still dihedrals, sometimes termed "upturned tips."
Dihedrals.
Span is the distance from tip to tip of the main supporting surface measured transversely (across) the line of flight.