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Curtiss Type JN4-B Primary Trainer

Types of Flying Machines

The sustenation of a Heavier Than Air Machine is due to an entirely different application of forces. Forces in motion (Dynamic Forces) are essential to the support of a heavier than air machine, and it is the resultant of these forces that performs the actual lifting operation, this resultant corresponding to the buoyant force of the aerostat. "Dynamic" flight is obtained by an apparatus in which an arrangement of surfaces are moved in such a way as to cause an upward component of the forces generated by the impact of the air on the surfaces. The surfaces drive the air down and when the force necessary for the continuous downward deflection of air becomes equal to the weight of the machine it is sustained in flight. Dynamic flight therefore depends on the continuous downward deflection of masses of air, and when this motion ceases, sustentation also ceases.

An aeroplane is provided with a deflecting surface that is fixed rigidly in regard to the body of the machine, and the motion necessary for its support is provided by driving the machine forward, the forward motion being produced by the horizontal pull of air screws or propellers. It is at once evident that the forward horizontal motion of the aeroplane must be maintained for its support, for the surfaces are fixed and there is no other possible way of producing a relative motion between the wings and the air.