Mechanics

Flying and mechanics

The flight and manoeuvres of an aeroplane provide glorious examples of the principles of mechanics. However, this is not a book on mechanics. It is about flying, and is an attempt to explain the flight of an aeroplane in a simple and interesting way; the mechanics are only brought in as an aid to understanding. In the opening chapter I shall try to sum up some of the principles with which we are most concerned in flying.

Force, and the first law of motion

An important principle of mechanics is that any object that is at rest will stay at rest unless acted upon by some force, and any object that is moving will continue moving at a steady speed unless acted upon by a force. This state­ment is in effect a simple statement of what is known as Newton’s First Taw of Motion.

There are two types of forces that can act on a body. They are:

(1) externally applied mechanical forces such as a simple push or pull

(2) the so-called body forces such as those caused by the attraction of gravity and electromagnetic and electrostatic fields.

External forces relevant to the mechanics of flight include the thrust produced by a jet engine or a propeller, and the drag resistance produced by movement through the air. A less obvious external force is that of reaction. A simple example of a reactive force is that which occurs when an object is placed on a fixed surface. The table produces an upward reactive force that exactly balances

the weight. The only body force that is of interest in the mechanics of flight is the force due to the attraction of gravity, which we know simply as the weight of the object.

Forces (of whatever type) are measured in the units of newtons (N) in the metric SI system or pounds force (lbf) in the Imperial or Federal systems. In this book, both sets of units are used in the examples and questions.