ANGLE AND DIRECTION OF FLIGHT.—In order to become familiar with terms which will be frequently used throughout the book, care should be taken to distinguish between the terms angle and direction of flight. The former has reference to the up and down movement of an aeroplane, whereas the latter is used to designate a turning movement to the right or to the left.
WHY SHOULD THE ANGLE OF THE BODY CHANGE? —The first question that presents itself is, why should the angle of the aeroplane body change? Why should it be made to dart up and down and produce a sinuous motion? Why should its nose tilt toward the earth, when it is descending, and raise the forward part of the structure while ascending?
The ready answer on the part of the bird-form advocate is, that nature has so designed a flying structure. The argument is not consistent, because in this respect, as in every other, it is not made to conform to the structure which they seek to copy.
CHANGING ANGLE OF BODY NOT SAFE.—Furthermore, there is not a single argument which can be advanced in behalf of that method of building, which proves it to be correct. Contrariwise, an analysis of the flying movement will show that it is the one feature which has militated against safety, and that machines will never be safe so long as the angle of the body must be depended upon to control the angle of flying.
Fig. 11a Monoplane in Flight.
In Fig. 11a three positions of a monoplane are shown, each in horizontal flight. Let us say that the first figure A is going at 40 miles per hour, the second, B, at 50, and the third, C, at 60 miles. The body in A is nearly horizontal, the angle of the plane D being such that, with the tail E also horizontal, an even flight is maintained.
When the speed increases to 50 miles an hour, the angle of incidence in the plane D must be decreased, so that the rear end of the frame must be raised, which is done by giving the tail an angle of incidence, otherwise, as the upper side of the tail should meet the air it would drive the rear end of the frame down, and thus defeat the attempt to elevate that part.
Fig. 12. Angles of Flight.
As the speed increases ten miles more, the tail is swung down still further and the rear end of the frame is now actually above the plane of flight. In order, now, to change the angle of flight, without altering the speed of the machine, the tail is used to effect the control.
Examine the first diagram in Fig. 12. This shows the tail E still further depressed, and the air striking its lower side, causes an upward movement of the frame at that end, which so much decreases the angle of incidence that the aeroplane darts downwardly.