There are four basic forces at work, when an aircraft takes flight. LIFT, THRUST, GRAVITY and DRAG. Out of the four forces, only gravity is a constant, all other forces are subject to pilot control.

Lift

Lift is achieved throught the cross sectional shape (airfoil) design of the wing. As the wing moves through air, the airfoil's shape causes the air moving OVER the wing to travel faster than the air moving UNDER the wing. The faster airflow generates lower pressure than the slower air traveling under the wing. The resultant higher pressure is LIFT.

The angle at which the airfoil meets the air flow greatly affects the amount of lift generated. This angle is kown as the ANGLE OF ATTACK (AOA). It is commonly mistaken that this is the angle of the aircraft relative to the ground, this is INCORRECT! The AOA is angle of the WING relative to AIRFLOW, which can be a very large angle from the horizon to itself. When the AOA is too great, the airflow OVER the wing is disrupted and will cause a lost in lift, known as STALL. This condition usually occurs, when the aircraft is flying too slow, or too steep of an angle. Stalling will generally cause the aircraft to go into a dive. As you can see, when this happens at too low an altitude, it is usually fatal. If you have enought altitude, when the aircraft gains enought airspeed, you can regain control, once lift is re-established.

Thrust

When the propeller on the aircraft engine rotates, it pulls in air from the front and pushes it out the back. The force generated by this pushing is THRUST. Thrust gives the aircraft forward momentum, in turns, creates lift. Generally, the greater the thrust, the greater the airspeed. Thrust is controlled by the throttle.

Drag

As an aircraft is propelled forward by Thrust, an undesireable effect is also created: Resistance to air. When the wing travels through the air, it's surface area pushes the air infront of it to form a higher pressure area for it to travel through. This is known as DRAG. Therefore, the higher the AOA, the higher the Drag. Additionally the more streamlined the aircraft, the less the drag. For any given aircraft, drag can be increase and decrease depending on the conditions. For example, lowering your flaps and/or landing gear will INCREASE your drag. Same when an aircraft is loaded with exteranl stores, like bombs and rockets. The added stores, also add weight.

When an aircraft is flying level at a constant speed, all four forces discussed above are in balance.

Altitude

The higher up in the atmosphere you are, the thinner the air gets. This thinner air greatly affects an aircraft's performance. The thinner air, gives less mass for the propeller to grab onto and thus generates less thrust. The lesser amount of oxygen associated with thinner atomosphere, also reduces the power output of the engine. There is however one benefit of thinner atmosphere - it creates less drag.

These effects combined, to dictate a given performance envelope to each aircraft at a given altitude. Since this effect different aircraft differently, one airplane may be able to out perform another at a given altitude, but not able to, at a higher or lower altitude. The thinner atmosphere also decrease the effectiveness of an airplane's control surfaces.

Black Outs and Red Outs

When a person is at a stand still, he/she is experiencing One G (G-Force). That is the force exerted on this person by gravity. The G in G-Force, stands for Gravity. When a pilot in an airplane changes its orientation rapidly (Tight Turns, loops etc.) will experience additional G-Forces.

The are primarily two kind of G-Force a person can experience, Positive and Negative. Positive Gs are generated when an aircraft turns quickly or pulls up sharply. A World War II fighter may be capable of generating 7 Gs or more. The physical effect of Positive Gs on a pilot is Black Outs. Black-Outs are usually preceeded by Grey-Out. This is due to the human heart not being able to counter the forces exerted, to continue pumping blood to the brain.

Negative Gs are the results of sharp dives, or similar maneuvers. Excessive Negative Gs will cause a pilot to Red Out. This happens when the aircraft accelerates downwards faster then the acceleration of gravity. This in turns causes excessive blood being send to the brain, and lost of consciousness will occur.

Compressibility

When an aircraft attains speed approaching the speed of sound, the airflow over the wing of the aircraft can actually be over the speed of sound. This transonic airflow creates a shockwave and a barrier that disrupts the flow of air over the control surfaces. This causes dramatic lost in control effectiveness. This is known as Compressing. This condition usually occurs between Mach 0.7 to 0.9 (Mach is the speed of sound). The actual speed varies, since the speed of sound differs depending on air density. (i.e. Altitude)

For this condition to occur in World War II aircraft, usually requires it to enter into a high speed dive. To counter this condition, cut throttle, drop flaps, dive brakes (if available), and may be even landing gear (anything that will increase drag). Once the aircraft slows, controls will be regained. Since deceleration of this kind takes time, if compression occurs close to the ground, it's usually fatal.