The Physics of Flight

There are four basic forces at work when an aircraft is in flight:
         -Lift
         -Thrust
         -Gravity
         -Drag

Of these four forces, only gravity is constant (unchanging), the remaining three forces can be altered or
affected by the pilot.
When an aircraft is flying level at a constant speed, all four of these forces are in balance or equilibrium.

















Lift          

Lift is achieved through the cross-sectional shape (airfoil design) of the wing.
As the wing moves through the air, the airfoil's shape causes the air moving
over the wing to travel faster
than the air moving
under the wing. The slower airflow beneath the wing generates more pressure, while
the faster airflow above generates less. This difference in pressure results in
lift.

Lift will vary dynamically depending on the speed an aircraft is traveling at.




















Angle of Attack

The angle at which the airfoil meets the airflow also greatly affects the amount of lift generated. This angle is
known as the
Angle of Attack (AoA). It is commonly thought that AoA is the angle of the aircraft relative to
the ground - this is
incorrect. The AoA is the angle of the wing relative to airflow, which can be a very
different angle, depending on the attitude of the aircraft.

For example, if you are flying at 300 mph on a level course, your AoA is normally close to zero (actually
about 5°) since your wing is pointed in the same direction as your mass is traveling. Picture an aircraft on a
landing glide. The pilot maintains a nose-up attitude to help slow the aircraft, while the actual direction the
aircraft is traveling is in a slope down toward the runway.

Thus AoA is the angle between where the wing is pointed and the glide slope the plane is on.

Why is AoA important? Angle of Attack is critical to all planes because the AoA greatly effects the flow of air
across the wings. Since planes have different wings, planes also have different AoA limits that they must fly
within. If you exceed your maximum AoA, you interrupt the flow of air over one or both wings and you induce
a stall. This is NOT just at low speeds. The Focke-Wulf Fw 190 series were well known to be susceptible to
high speed stalls if the AoA was exceeded. Despite flying at 300 mph, you can pull the aircraft into a turn
which interrupts airflow and will quickly cause a dangerous stall.

















Thrust          

When the propeller on the aircraft engine rotates, it pulls in air from in front of the aircraft and pushes it back
towards the tail. The force generated by this is
thrust. Thrust gives the aircraft forward momentum, and in
turn, creates lift on the lifting surfaces (mainly the wings). Generally, the greater the thrust, the greater the
airspeed. Thrust is controlled by raising or lowering the revolutions-per-minute (rpm) of the engine by using
the throttle.


Drag          

As an aircraft is propelled forward by thrust, an undesirable effect is also created: resistance. When the
aircraft travels through the air, its frontal area pushes against the air in front of it, and air flowing over the
aircraft causes friction. This is known as
drag.
For any given aircraft, drag can be increased and decreased depending on the conditions. For example, a
more streamlined aircraft will reduce drag, while other factors may increase drag. These include increased
AoA, lowering flaps and/or landing gear, and carrying external stores, such as bombs and rockets.

Altitude          

Air density varies with altitude; at lower altitudes, it is thicker, while higher up, the air is thinner. The density
of the air directly affects drag and thrust.
For example, at lower altitudes the thicker air increases thrust by supplying the propeller with more mass to
move. However, that mass also increases drag.
The lesser amount of oxygen associated with the thinner atmosphere of higher altitudes reduces the power
output of the engine, thereby reducing thrust. However one benefit of thinner atmosphere is that it creates
less drag.

G-Forces          

Gravity effects all objects within the Earth's gravitational field - G-force. When a person is standing still on
the earth, they are experiencing
One G (one times the force of gravity). When a pilot in an airplane changes
its orientation rapidly (tight turns, loops, etc.), the aircraft will undergo additional G-forces. These may be
positive or negative G-forces.

Positive G-Forces
Positive G's are generated when an aircraft pitches upwards (the nose pulls up). For example, when the
aircraft turns quickly or pulls up sharply. A World War II fighter may be capable of generating 7 G's or more.
The physical effect of Positive G's on a pilot is a possible
blackout, usually preceded by greyout (a less
severe effect).
This is caused by the increased effort the heart must generate to counter the G-forces and still supply the
brain with sufficient blood. When the G-forces are too great, the pilot will slowly lose vision due to this lack of
blood supply. When prolonged, the blackout can cause a loss of consciousness.

Negative G-Forces
Negative G's are generated when an aircraft pitches downwards (the nose goes down). For example, a
sharp dive or similar maneuver that
unloads the aircraft of the force of gravity. Excessive Negative G's will
cause a pilot to
red out.
This is the effect of excessive blood being pumped to the pilot's brain, causing distorted vision. Red out is
usually preceded by
pink out. This signals the onset of excessive negative G's.



Compressibility          

When an aircraft approaches the speed of sound, the airflow over the wings of the aircraft can actually
exceed 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 a dramatic loss in control efficiency and is known as
compression. Compression usually occurs between Mach 0.7 to 0.9. Mach 1.0 is the speed of sound. The
actual speed of sound varies at different altitudes, depending on air density.

The practical effect of compression on an aircraft is a lack of control. The ailerons and/or elevators seem to
lock up, and moving the joystick has little effect on the aircraft. If you experience compression in a dive, you
may not be able to recover.

For a World War II aircraft to attain these speeds, a high-speed dive would be required. To counter
compression, speed must be reduced. Increasing drag and decreasing thrust will slow the plane. Once the
aircraft slows, control will be regained.

Note that some aircraft compress at slower speeds, such as the A6M Zero and Messerschmitt Bf 109.
These aircraft are lighter than most others, and sustained high speeds in level fight can begin to compress
their control surfaces.