Theory of Flight

Four forces acting on an aircraft in flight.

The three Axis of Rotation

Control Surfaces

a. Aircraft. Any weight-carrying device designed to be supported by air, either by buoyancy or by aerodynamic reaction.
b. Airfoil. A surface so designed as to produce an aerodynamic
reaction to its direction of motion.
c. Airspeed. Speed of the aircraft in relation to the surrounding air.
d. Angle of Attack. The angle between the relative air flow and the chord of the airfoil.
e. Angle of Incidence. The angle formed between the chord and the longitudinal datum line.
f. Aspect Ratio. The ratio between the span and the chord.
g. Camber. The curvature of the wing.
h. Centre of Gravity. The balance point.The point through which all weight acts downwards.
j. Centre of Pressure. The point along the airfoil chord of the body axis through which the resultant aerodynamic force acts.
k. Chord. An imaginary line from the leading edge to the trailing edge of an airfoil.
m. Dihedral. The angle each wing makes with the horizontal.
n. Drag. The total resistance to the aircraft in flight.
p. Equilibrium. Balance between forces, when opposing forces are equal.
q. Fin. Vertical stabilizer.
r. Groundspeed. The relation between the speed of the aircraft and a point on the ground.
s. Lateral Axis. An imaginary line running from wing tip to wing tip through the centre of gravity.
t. Longitudinal Axis. An imaginary line extending through the fuselage from the nose to the tail.
u. Mainplane. Main supporting airfoil (wing) of the aircraft.
v. Pitching. Movement around the lateral axis.
w. Rolling. Movement around the longitudinal axis.
x. Span. Measurement from wing tip to wing tip.
y. Stable. An object is stable if, when disturbed, it returns to its original position.
z. Stabilizer. Any surface of airfoil shape whose primary function is to correct instability of an aircraft in flight.
aa. Stalling Angle. The angle of attack of an airfoil where the smooth airflow breaks away and becomes turbulent.
ab. Sweepback. Outward and backward angle of the leading edge of the mainplane.
ac. Tailplane. Horizontal stabilizer.
ad. Turbulence. Disturbed air flow.
ae. Unstable. An object is unstable if, when disturbed, it continues to move farther and farther from its original position.
af. Venturi. A variable section tube wider at each end than in the middle.
ag. Vertical Axis. An imaginary line running at right angles to the longitudinal and lateral axes through the centre of gravity.
ah. Wing Tip Vortices. Spiralling air at the wing tips caused by high-pressure air from the lower surface of the airfoil moving into the low-pressure air on top of the airfoil.
ak. Yawing. Movement around the vertical axis.

What makes airplane fly?

Almost everyone today has flown in an airplane. Many ask the simple question "what makes an airplane fly?" The answer one frequently gets is misleading and often just plain wrong. We hope that the answers provided here will clarify many misconceptions about lift and that you will adopt our explanation when explaining lift to others. We are going to show you that lift is easier to understand if one starts with Newton’s laws rather than the Bernoulli principle. We will also show you that the popular explanation that most of us were taught is misleading at best and that lift is due to the wing diverting air down. Most of this diverted air is pulled down from above the wing.
Let us start by defining three descriptions of lift commonly used in textbooks and training manuals. The first we will call the Mathematical Aerodynamics Description of lift, which is used by aeronautical engineers. This description uses complex mathematics and/or computer simulations to calculate the lift of a wing. It often uses a mathematical concept called "circulation" to calculate the acceleration of the air over the wing. Circulation is a measure of the apparent rotation of the air around the wing. While useful for calculations of lift, this description does not lend themselves to an intuitive understanding of flight.

The second description we will call the Popular Description, which is based on the Bernoulli principle. The primary advantage of this description is that it is easy to understand and has been taught for many years. Because of its simplicity, it is used to describe lift in most flight training manuals. The major disadvantage is that it relies on the "principle of equal transit times", or at least on the assumption that because the air must travel farther over the top of the wing it must go faster. This description focuses on the shape of the wing and prevents one from understanding such important phenomena as inverted flight, power, ground effect, and the dependence of lift on the angle of attack of the wing.

The third description, which we are advocating here, we will call the Physical Description of lift. This description of lift is based primarily on Newton's three laws and a phenomenon called the Coanda effect. This description is uniquely useful for understanding the phenomena associated with flight. It is useful for an accurate understanding the relationships in flight, such as how power increases with load or how the stall speed increases with altitude. It is also a useful tool for making rough estimates ("back-of-the-envelope calculations") of lift. The Physical Description of lift is also of great use to a pilot who needs an intuitive understanding of how to fly the airplane.