Aircraft Theory of Flight

Before a technician can consider performing maintenance on an aircraft, it is necessary to understand the pieces that make up the aircraft. Names like fuselage, empennage, wing, and so many others, come into play when describing what an airplane is and how it operates. For helicopters, names like main rotor, anti-torque rotor, and autorotation come to mind as a small portion of what needs to be understood about rotorcraft. The study of physics, which includes basic aerodynamics, is a necessary part of understanding why aircraft operate the way they do.

Four Forces of Flight

During flight, there are four forces acting on an airplane. These forces are lift, weight, thrust, and drag. [Figure 1]

 Figure 1. Four forces acting on an airplane

Lift is the upward force created by the wing, weight is the pull of gravity on the mass, thrust is the force created by the airplane’s propeller or turbine engine, and drag is the friction caused by the air flowing around the airplane.

All four of these forces are measured in pounds. Any time the forces are not in balance, something about the airplane’s condition is changing. The possibilities are as follows:
1. When an airplane is accelerating, it has more thrust than drag.
2. When an airplane is decelerating, it has less thrust than drag.
3. When an airplane is at a constant velocity, thrust and drag are equal.
4. When an airplane is climbing, it has more lift than weight.
5. When an airplane is descending, it has more weight than lift.
6. When an airplane is at a constant altitude, lift and weight are equal.

Bernoulli’s Principle and Subsonic Flow

The basic concept of subsonic airflow and the resulting pressure differentials was discovered by Daniel Bernoulli, a Swiss physicist. Bernoulli’s principle, as we refer to it today, states that “as the velocity of a fluid increases, the static pressure of that fluid will decrease, provided there is no energy added or energy taken away.” A direct application of Bernoulli’s principle is the study of air as it flows through either a converging or a diverging passage, and to relate the findings to some aviation concepts.

A converging shape is one whose cross-sectional area gets progressively smaller from entry to exit. A diverging shape is just the opposite, with the cross-sectional area getting larger from entry to exit. Figure 2 shows a converging shaped duct, with the air entering on the left at subsonic velocity and exiting on the right. Notice that the air exits at an increased velocity and a decreased static pressure when looking at the pressure and velocity gauges, and the indicated velocity and pressure. The unit leaving must increase its velocity as it flows into a smaller space, because a unit of air must exit the duct when another unit enters.

 Figure 2. Bernoulli’s principle and a converging duct

In a diverging duct, just the opposite would happen. From the entry point to the exit point, the duct is spreading out and the area is getting larger. [Figure 3] With the increase in cross-sectional area, the velocity of the air decreases and the static pressure increases. The total energy in the air has not changed. What has been lost in velocity, which is kinetic energy, is gained in static pressure, which is potential energy.

 Figure 3. Bernoulli’s principle and a diverging duct

In the discussion of Bernoulli’s principle, a venturi shown in Figure 4. In Figure 5, a venturi is shown again, only this time a wing is shown tucked up into the recess where the venturi’s converging shape is. There are two arrows showing airflow. The large arrow shows airflow within the venturi, and the small arrow shows airflow on the outside heading toward the leading edge of the wing.

 Figure 4. Bernoulli’s principle and a venturi

In the converging part of the venturi, velocity would increase and static pressure would decrease. The same thing would happen to the air flowing around the wing, with the velocity over the top increasing and static pressure decreasing.

In Figure 5, the air reaching the leading edge of the wing separates into two separate flows. Some of the air goes over the top of the wing and some travels along the bottom. As the air flows over the upper surface of an airfoil, its velocity increases and its pressure decreases; an area of low pressure is formed. There is an area of greater pressure on the lower surface of the airfoil, and this greater pressure tends to move the wing upward.

 Figure 5. Venturi with a superimposed wing

For the wing shown in Figure 5, imagine it is 5 ft. wide and 15 ft. long, for a surface area of 75 ft2 (10,800 in2). If the difference in static pressure between the top and bottom is 0.1 psi, there will be 1⁄10 lb of lift for each square inch of surface area. Since there are 10,800 in2 of surface area, there would be 1,080 lb of lift (0.1 × 10,800).

Lift and Newton’s Third Law

Newton’s third law identifies that for every force there is an equal and opposite reacting force. In addition to Bernoulli’s principle, Newton’s third law can also be used to explain the lift being created by a wing. As the air travels around a wing and leaves the trailing edge, the air is forced to move in a downward direction. Since a force is required to make something change direction, there must be an equal and opposite reacting force. In this case, the reacting force is what we call lift. In order to calculate lift based on Newton’s third law.

Newton’s second law and the formula “Force = Mass × Acceleration” would be used. The mass would be the weight of air flowing over the wing every second, and the acceleration would be the change in velocity the wing imparts to the air.

The lift on the wing as described by Bernoulli’s principle, and lift on the wing as described by Newton’s third law, is not separate or independent of each other. They are just two different ways to describe the same thing, namely the lift on a wing.

Airfoils

An airfoil is any device that creates a force, based on Bernoulli’s principles or Newton’s laws, when air is caused to flow over the surface of the device. An airfoil can be the wing of an airplane, the blade of a propeller, the rotor blade of a helicopter, or the fan blade of a turbofan engine. The wing of an airplane moves through the air because the airplane is in motion, and generates lift by the process previously described. By comparison, a propeller blade, helicopter rotor blade, or turbofan engine fan blade rotates through the air. These rotating blades could be referred to as rotating wings, as is common with helicopters when they are called rotary wing aircraft. The rotating wing can be viewed as a device that creates lift, or just as correctly, it can be viewed as a device that creates thrust.

 Figure 6. Wing terminology

In Figure 6 an airfoil, or wing, is shown, with some of the terminology that is used to describe a wing. The terms and their meaning are as follows:

Camber

The camber of a wing is the curvature which is present on top and bottom surfaces. The camber on the top is much more pronounced, unless the wing is a symmetrical airfoil, which has the same camber top and bottom. The bottom of the wing, more often than not, is relatively flat. The increased camber on top is what causes the velocity of the air to increase and the static pressure to decrease. The bottom of the wing has less velocity and more static pressure, which is why the wing generates lift.

Chord Line

The chord line is an imaginary straight line running from the wing’s leading edge to its trailing edge. The angle between the chord line and the longitudinal axis of the airplane is known as the angle of incidence.

Relative Wind

The relative wind is a relationship between the direction of airflow and the aircraft wing. In normal flight circumstances, the relative wind is the opposite direction of the aircraft flight path.
• If the flight path is forward then the relative wind is backward.
• If the flight path is forward and upward, then the relative wind is backward and downward.
• If the flight path is forward and downward, then the relative wind is backward and upward.
Therefore, the relative wind is parallel to the flight path, and travels in the opposite direction.

Angle of Attack

The angle between the chord line and the relative wind is the angle of attack. As the angle of attack increases, the lift on the wing increases. If the angle of attack becomes too great, the airflow can separate from the wing and the lift will be destroyed. When this occurs, a condition known as a stall takes place.

There are a number of different shapes, known as planforms that a wing can have. A wing in the shape of a rectangle is very common on small general aviation airplanes. An elliptical shape or tapered wing can also be used, but these do not have as desirable a stall characteristic. For airplanes that operate at high subsonic speeds, sweptback wings are common, and for supersonic flight, a delta shape might be used.

The aspect ratio of a wing is the relationship between its span, or a wingtip to wingtip measurement, and the chord of the wing. If a wing has a long span and a very narrow chord, it is said to have a high aspect ratio. A higher aspect ratio produces less drag for a given flight speed, and is typically found on glider type aircraft.

The angle of incidence of a wing is the angle formed by the intersection of the wing chord line and the horizontal plane passing through the longitudinal axis of the aircraft. Many airplanes are designed with a greater angle of incidence at the root of the wing than at the tip, and this is referred to as washout. This feature causes the inboard part of the wing to stall before the outboard part, which helps maintain aileron control during the initial stages of a wing stall.

Boundary Layer Airflow

The boundary layer is a very thin layer of air lying over the surface of the wing and, for that matter, all other surfaces of the airplane. Because air has viscosity, this layer of air tends to adhere to the wing. As the wing moves forward through the air the boundary layer at first flows smoothly over the streamlined shape of the airfoil. Here the flow is called the laminar layer.

As the boundary layer approaches the center of the wing, it begins to lose speed due to skin friction and it becomes thicker and turbulent. Here it is called the turbulent layer. The point at which the boundary layer changes from laminar to turbulent is called the transition point. Where the boundary layer becomes turbulent, drag due to skin friction is relatively high. As speed increases, the transition point tends to move forward. As the angle of attack increases, the transition point also tends to move forward. With higher angles of attack and further thickening of the boundary layer, the turbulence becomes so great the air breaks away from the surface of the wing. At this point, the lift of the wing is destroyed and a condition known as a stall has occurred. In Figure 7, view A shows a normal angle of attack and the airflow staying in contact with the wing. View B shows an extreme angle of attack and the airflow separating and becoming turbulent on the top of the wing. In view B, the wing is in a stall.

 Figure 7. Wing boundary layer separation

Boundary Layer Control

One way of keeping the boundary layer air under control, or lessening its negative effect, is to make the wing’s surface as smooth as possible and to keep it free of dirt and debris. As the friction between the air and the surface of the wing increases, the boundary layer thickens and becomes more turbulent and eventually a wing stall occurs. With a smooth and clean wing surface, the onset of a stall is delayed and the wing can operate at a higher angle of attack. One of the reasons ice forming on a wing can be such a serious problem is because of its effect on boundary layer air. On a high-speed airplane, even a few bugs splattered on the wing’s leading edge can negatively affect boundary layer air.

Other methods of controlling boundary layer air include wing leading edge slots, air suction through small holes on the wing’s upper surface, and the use of devices called vortex generators.

A wing leading edge slot is a duct that allows air to flow from the bottom of the wing, through the duct, to the top of the wing. As the air flows to the top of the wing, it is directed along the wing’s surface at a high velocity and helps keep the boundary layer from becoming turbulent and separating from the wing’s surface.

Another way of controlling boundary layer air is to create suction on the top of the wing through a large number of small holes. The suction on the top of the wing draws away the slow-moving turbulent air, and helps keep the remainder of the airflow in contact with the wing.

Vortex generators are used on airplanes that fly at high subsonic speed, where the velocity of the air on the top of the wing can reach Mach 1. As the air reaches Mach 1 velocity, a shock wave forms on the top of the wing, and the subsequent shock wave causes the air to separate from the wing’s upper surface. Vortex generators are short airfoils, arranged in pairs, located on the wing’s upper surface. They are positioned such that they pull high-energy air down into the boundary layer region and prevent airflow separation.

Wingtip Vortices

Wingtip vortices are caused by the air beneath the wing, which is at the higher pressure, flowing over the wingtip and up toward the top of the wing. The end result is a spiral or vortex that trails behind the wingtip anytime lift is being produced. This vortex is also referred to as wake turbulence, and is a significant factor in determining how closely one airplane can follow behind another on approach to land. The wake turbulence of a large airplane can cause a smaller airplane, if it is following too closely, to be thrown out of control. Vortices from the wing and from the horizontal stabilizer are quite visible on the MD-11 shown in Figure 8.

 Figure 8. Wing and horizontal stabilizer vortices on an MD-11

Upwash and downwash refer to the effect an airfoil has on the free airstream. Upwash is the deflection of the oncoming airstream, causing it to flow up and over the wing. Downwash is the downward deflection of the airstream after it has passed over the wing and is leaving the trailing edge. This downward deflection is what creates the action and reaction described under lift and Newton’s third law.

Axes of an Aircraft

An airplane in flight is controlled around one or more of three axes of rotation. These axes of rotation are the longitudinal, lateral, and vertical. On the airplane, all three axes intersect at the center of gravity. As the airplane pivots on one of these axes, it is in essence pivoting around the center of gravity (CG). The center of gravity is also referred to as the center of rotation.

On the brightly colored airplane shown in Figure 9, the three axes are shown in the colors red (vertical axis), blue (longitudinal axis), and orange (lateral axis). The flight control that makes the airplane move around the axis is shown in a matching color.

 Figure 9. The three axes intersect at the airplane’s center of gravity. The flight control that produces motion around the indicated axis is a matching color

The rudder, in red, causes the airplane to move around the vertical axis and this movement is described as being a yaw. The elevator, in orange, causes the airplane to move around the lateral axis and this movement is described as being a pitch. The ailerons, in blue, cause the airplane to move around the longitudinal axis and this movement is described as being a roll.

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