AVIATION FANS

In this Article we will talk about the Basics of Flight Dynamics. There are four forces that effect flight; two that help and two that fight against you. Thrust and Lift are essential for flight, but are forced to overcome Drag and Weight.

Thrust:

Thrust is the force that causes an aircraft to move through the air. This can be produced by anything from a jet engine to a rocket motor to a propeller pulling your aircraft through the air. The measurement of thrust is usually in pounds or newtons. It would seem pretty obvious that the more thrust an aircraft engine produces, the faster an aircraft can travel. The faster an aircraft can travel, the faster the air can be moved across the wings, and thus the more lift that can be generated.

The power of fighter aircraft engines are expressed in a thrust to weight ratio. The ratio compares the thrust of the engine to the weight of the aircraft. The higher the ratio, the more powerful the aircraft. Most combat aircraft have had between 0.7 to 0.9 thrust to weight ratio. The F-15 and F-16 fighter models actually have a thrust to weight ratio of greater than 1.0 which allows them to climb vertically. The Soviet built twin engine MIG-29, with no weapons has a greater than 1.0 thrust to weight ratio … with one engine turned off.

Lift:

We briefly touched on lift. Lift is the force generated by air moving across the surface of the wing. To be more precise, it is the force generated by the unbalanced movement of air across the top as opposed to across the bottom of the wing. Due to the curvature of the top of the wing being greater than the curvature of the bottom of the wing, air flowing across the top of the wing must move faster, to cover the greater distance, if it is to meet the air moving across the bottom of the wing at the trailing edge of the wing at the same time.

The principal at work is known as Bernoulli's Law. Bernoulli, an 18th century Italian scientist, discovered that the faster a gas travels, the lower it's pressure. So if the air moving over the wing is moving faster than the air moving under the wing, there is more pressure below than above. This allows the higher pressure below the wing to "push" up and "lift" the wing.

The faster the aircraft travels, the faster the airflow across the wing. The faster the airflow over the wing, the more pressure differential there is between the top and bottom of the wing. A simplified example: if the pressure at 100 knots is 95 PSI on the top of the wing and 100 PSI on the bottom, there is a 5 PSI pressure differential.

At 200 knots, the pressure would be 200 over 190, for 10 PSI differential, or twice the lifting force. It should be noted that there are those that claim the Bernoulli theory is incorrect and that Newtonian theorems should be used. I have used the current most accepted explanation of Lift.

We also must take into account the AOA, or Angle of Attack, of the wing. Initially the amount of lift increases with the AOA. But there comes a point when the AOA is too high for the air to flow over the wing. Without airflow, there is no pressure differential. With no pressure differential, there is no lift. When this happens, the aircraft stalls. While AOA can cause a stall, flying too slow can cause a stall as well. When you fly slowly, you decrease the amount of airflow over the wings, which in turn decreases lift. When your lift drops too low to keep the aircraft aloft, you stall.

Drag:

Drag is the opposite of thrust. Drag is the force that slows the aircraft down. Drag is basically friction, the resistance of the air against the structure of the aircraft. This may be a bit difficult to understand. If you put your hand out the window of a moving car, the wind pushes against it and tries to push it back. This is drag.

Aircraft designers try to eliminate as many drag inducing features as possible. Bumps, rivet heads, paint, antennae, bombs, missiles, drop tanks, even control surfaces (rudders, canards, etc) all cause drag. The smoother an aircraft's surface is, the less drag will be induced. But you can never completely eliminate drag.

It is very interesting to note the Soviet design concept. The Soviets make the front of the aircraft as aerodynamic, thus less drag inducing, as possible. Nose, wing leading edges, anything that comes into contact with the air first is made as smooth and flawless as possible. The areas behind these parts of the aircraft are not considered as critical. By this time the air is "dirty", or is in a somewhat turbulent state, since the front of the aircraft has already pushed through and disturbed the airflow. Because of this, they feel there is not much purpose to spending the time or money to make these surfaces smooth. The Western countries do not follow this same train of thought and make the entire aircraft as smooth as possible.

Weight:

Weight is the opposite of lift. Weight is the effect of the gravitational pull of the earth on the aircraft. We can always add more thrust, or create a more efficient wing, or even reduce drag as far as possible, but we can do nothing to counteract gravity. Gravity will always win in the end (unless you hop on the Space Shuttle and blast yourself out of the reach of the gravitational pull of the earth but then, that would not be flying anymore). If thrust or lift become too low, or drag becomes too high, weight and gravity will persevere.