The Machine

I[Motions and Principles]

When an airplane flies, it must first overcome two primary forces--weight and drag. Weight is the force of gravity acting to pull the plane to the ground, and it is overcome through lift. Lift results in the plane rising into the air. Drag is created by the force of air particles striking and flowing around the airplane, and it is overcome through thrust. If you've ever tried running or riding a bike into a strong headwind, then you have directly experienced how hard drag can be to overcome at times (in the case of riding a bicycle, you create thrust by pedaling). Thrust is the push of the plane in a forward direction.

The shape of an airplane's wings is what makes lift possible. Just like the wings on a bird, the top of an airplane wing is curved. Due to the curve in the wing, air must travel faster over the top of the wing in order to get to the back of the wing.Bernoulli's Principle when the velocity of air is increased, then the air pressure in that region is decreased. Therefore, an airplane's wings have a low pressure area directly over their upper surfaces. This causes the higher air pressure underneath each wing to push the plane into the air. This rising of the airplane due to Bernoulli's Principle is known as "induced lift."

The thrust of an airplane is created by the use of either jet engines or propellers. Jet engines use the principle of actions and reactions outlined in Newton's Laws, while propellers function under Bernoulli's Principle just as the wings do. Propellers are curved on the top and flat on the bottom, creating "lift" in a forward direction as they are rapidly turned by an engine. In essence, propellers don't pull an aircraft forward; they cause the plane to be pushed through the air.

Click here to view a detailed diagram about the parts of a plane.

II[Axes and Vertical Damping]

The purpose of this section is to examine how the airplane responds to pure vertical motions and to pure rolling motions. We will see that (except near the stall) the airplane vigorously resists such motions. For a non-aerodynamic object like a pompom, if you wave it through the air, it will resist the motion, due to ordinary air friction. An airplane has friction, too, but we will see that there is another process ("aerodynamic damping") that is enormously more powerful than friction. This strong aerodynamic damping should not be taken for granted, since you can certainly get an airplane into situations where the damping goes to zero or becomes negative. This is why the airplane is hard to fly near the stall. We will discuss how such situations.

Normally, the airplane is in equilibrium and all forces are in balance.Let's consider the vertical forces in particular, and see how the airplane maintains its equilibrium. To see how the wing reacts initially to eliminate any unbalanced vertical force, consider this scenario.Initially, the airplane is buzzing along in straight-and-level flight and is nicely trimmed. Vertical forces are in balance. Then we imagine there is sudden change in the weight of the airplane, relative to the lift. A sudden excess of lift over weight could happen in several ways, such as the departure of a skydiver. Conversely, a sudden excess of weight over lift could happen in at least three ways:

The lift decreases if you lose airspeed because of a sudden wind shear.
The load on the airplane (the effective weight) increases in a steeply banked turn.
The weight increases if an albatross flies in the window and sits on the seat beside you.

For a brief instant after the weight increase, there will be an unbalanced downward force. According to Newton's second law, this will result in a downward acceleration. This in turn means the airplane will begin to descend. If the downward force remained unbalanced, the airplane would continue to accelerate downward. It would not just go down, it would go down faster and faster and faster. This is not what happens, for a very interesting reason. As soon as the wing picks up an appreciable downward velocity, its angle of attack will be different. The angle of attack is just the angle at which the air hits the wing. In the figure see that the air hits the wing at a larger angle during the descent; the pitch attitude of the airplane has not changed, but the relative wind is coming from a new direction, ahead of and below the airplane. This increase in angle of attack normally results in an increase in coefficient of lift. The extra lift balances the new weight, and equilibrium is restored. This phenomenon is called vertical damping.

III[Stalls or loss of Vertical Damping]

Let's repeat the previous experiment, but this time let's imagine that the airplane was flying at a rather low airspeed (higher angle of attack) when it picked up the added weight. Note the higher angle of attack when compared with the previous figure.

As before, the added weight causes a downward acceleration.

As before, this leads to downward velocity.

As before, this causes increased angle of attack.

Surprise! the increased angle of attack causes no increase in upward force, because the coefficient of lift does not increase forever as a function of angle of attack.

Equilibrium is not restored. The airplane continues to accelerate, descending faster and faster....

The situation just described is called a stall

Quick Read:

The stall occurs at the critical angle of attack, which is the point where a further increase in angle of attack does not create a further increase in coefficient of lift.
Lift does not go to zero at the stall. In fact, the coefficient of lift reaches its maximum at the stall.
Vertical damping goes to zero at the stall.
The airplane is very ill-behaved near the stall because of the loss of vertical damping
Extending the flaps allows you to fly slower, since you can achieve a higher coefficient of lift without stalling. Secondarily, extending the flaps increases the angle of incidence. This improves your ability to see over the nose. Thirdly, extending the flaps increases drag.
It is possible to fly at an angle of attack above the critical angle of attack, but it is not possible to maintain steady flight at an airspeed below the stalling airspeed.