Lesson 2: Aerodynamic Forces

In this lesson, aerodynamic forces will be discussed including: Thrust – the force that moves an object, Lift – the force that enables modern aircraft to stay in the air, Drag – various types of drag which is the resistance of movement of air, and Weight – the weight of the object in flight. Introductory dimensional analysis will be introduced in this section.

Section 1 - Forces Acting on an Airplane

There are four aerodynamic forces acting on an airplane: DRAG, LIFT, THRUST, and WEIGHT. Drag is the resistance of the movement of air and is invisible. When an aircraft moves at Mach 2 or two times the speed of sound, it is pushing aside an enormous volume rate, and thus air pushes back in the form of drag.

TYPES OF DRAG

There are three main types of drag affecting performance of an aircraft: Induced drag, skin friction of parasitic drag, and wave drag.

Wave drag is only found in jet fighters or supersonic aircraft. When a plane moves at supersonic speeds, it builds up a tremendous front. A huge amount of energy is required to move through these waves and this resistance is called wave drag. When the resulting shock wave hits the ground, it is experienced by people of the ground in the rattling form of a “sonic boom.”

Skin friction or parasitic drag is a simple kind of drag that results from wind resistance to the rough surfaces of an aircraft. For example, when an F-16 is loaded up with weapons and fuel tanks, the plane’s aerodynamics are complicated. This creates drag which will affect flight performance and G forces – that is, forces of acceleration that pull a pilot when he or she is in motion.

Induced drag is the most important form of drag because it occurs as a result of the force of lift which enables a plane to remain airborne. The backward force on the plane is the induced drag force.

LIFT

Lift enables aircraft to stay in the air. When an aircraft moves quickly over an arched surface, the air pressure above the surface drops. Air speeds up and pressure drops above the wing. This air pressure difference results in the force that creates lift in a wing.

The size, shape and thickness of a wing all determine that amount of resulting lift. The velocity of the air above the wing and the air pressure or density of the air are also factors that affect lift.

Essentially, lift can be generated by any part of the airplane, but most of the lift is generated by the wings. It is an aerodynamic force produced by the motion of a fluid past an object.

Lift is a mechanical force that is generated by the interaction and contact of a solid body with a fluid (liquid or gas). It occurs when a flow of gas is turned by a solid object. The flow is turned in one direction, and the lift is generated in the opposite direction. This refers directly to Newton’s Third Law of action and reaction. Because air is a gas and the molecules move about freely, any solid surface can deflect a flow. For an airfoil, both the upper and lower surfaces contribute to the flow turning.

We can therefore say that lift is generated by the difference in velocity between the solid object and the fluid.

There must be motion between the object and the fluid. If there is no motion, there is no lift. Lift acts perpendicular to the motion whereas drag acts in the direction opposed to the motion.

The development of the theory of lift has come about in the last century, extending to around the 1920’s. This theory corresponds with rapid advancement in aviation.

A simple way to view and experiment with the action of lift is by flying a kite. You can actively feel the process of lift as you pull on the kite’s string. The component of the pull acts as a right angle to the wind direction. This component is lift and supports both kite and string.

When the upper wing surface of an aircraft is curved, there is a dramatic increase in lift. We can divide the lift by the drag to obtain the lift-over-drag ratio which is a measure of the efficiency of wing design.

In modern airplanes, the lift-over-drag ratio is usually close to 20. Lift is a function of angle of attack which is the angle that the wing cuts through the air. This is also referred to as AOA. As AOA increase, do does lift. But if an AOA is too high, the flow of air over the wing may be interrupted resulting in a stall or temporary loss of control of an aircraft. Here, the lift force drops below the weight of the airplane, and the airplane begins to lose altitude. The pilot of the aircraft can correct stall by recovering an altitude sufficient for lift.

In the early days of flight, greater lift was achieved by biplanes with two wings mounted one over the other, connected by supports, pylons, and wires. Biplane were flown the Wright Brothers and remained popular for many years even as fighter planes during World War I.

An ideal example of lift would be one in which an airplane could sustain a constant speed and level flight in which the weight would be balanced by the lift, and the drag would be balanced by the thrust. Perhaps, the best example of this condition is a cruising airliner. Here, while the weight decreases due to fuel burned, the change is very small or minute in relation to the total aircraft weight. The aircraft will maintain a constant cruise velocity as described by Newton’s First Law of Motion.

If the forces become unbalanced, the aircraft will move in the direction of the greater force.

To summarize, if the weight of a plane is decreased while the lift is held constant, the airplane will rise. If the lift is decreased while the weight is constant, the plane will fall. Increasing the thrust while the drag is constant will cause the plane to accelerate, and increasing the drag at a constant thrust will cause the plane to slow down.

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