Lesson 3: AIRCRAFT AIRFOIL CHARACTERISTICS

Basic structure of an airplane

Lesson 3:  AIRFOIL CHARACTERISTICS






structure of an airplane



Tags;

 chord Line, AIRFOIL CHARACTERISTICS, Camber, Angle of Attack, Angle of Incidence, Dihedral, Anhedral, Static Pressure, Total Pressure, BERNOULLI’S 

AIRFOIL CHARACTERISTICS.

Chord and Chord Line.

AIRFOIL CHARACTERISTICS.



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Click here for the previous lessons, to learn about: Principle of Airframe; Principles of Aerodynamics; Airfoil Characteristics; Primary Flight Control Surfaces; Description and Operation of Helicopter; Miscellaneous Components of an Aircraft…

The wing chord, figure 1-3, is the distance from the leading edge (forward) to the trailing edge (aft) of the wing, parallel to the fuselage. It is measured in a direct manner as opposed to around the upper or lower surfaces of the wing. If the wing is not rectangular in shape, the chord length will vary at different points on the wing. The average of these chords, or Mean Aerodynamic Chord, is used for balance calculations.

Camber.

The term camber is analogous to curvature. Camber is defined as the ratio of the greatest distance between the chord line and the mean camber line (next paragraph) to the length of the chord. In general, the greater the amount of camber and the thicker the airfoil, the greater the coefficient of lift will be. A high camber and large thickness usually causes the airfoil to stall gradually at high angle of attack. Conversely, a thin, flat airfoil loses lift abruptly at low angles of attack.

Mean Camber Line.

Figure 1-3 shows an imaginary line drawn from the leading edge to the trailing edge of the wing that is equidistant from the upper and lower surfaces of the wing. Used to determine the curve of the wing.



Airfoil characteristics
Figure 1-3, Airfoil characteristics

ANGLE OF ATTACK (AOA).

Defined as the angle between the relative wind and the chord of an airfoil, (Figure 1-4). AOA directly affects the amount of lift being produced on an airfoil and managing AOA is extremely vital to aircraft performance. This is accomplished by both the aircrew in flight, as well as by aircraft design.

Angle of Attack of aircraft

Figure 1-4, Angle of Attack


ENGINEERING DESIGNS.

Aerodynamic engineers use a number of design parameters to obtain certain effects from an airfoil. These can be manipulated to allow for different aircraft types or functions.

Angle of Incidence.

This is the angle between the chord line and the longitudinal axis. Engineers determine the angle of incidence, figure 1-5, using two factors. First, the aircraft fuselage should be moving through the air at a zero angle of attack. Secondly, the wing should be at its maximum lift ratio for the average operating condition.

 

Angle of Incidence of an aircraft

Figure 1-5, Angle of Incidence 

Dihedral and Anhedral.

These terms refer to the angle between the wing chord plane and the lateral axis. See figure 1-6. The purpose of the dihedral angle is to affect roll stability when the aircraft is in a skid or slip such as while in a banked turn. The dihedral will cause an increase in the angle of attack on the low wing since the wind is hitting that wing more on the underside. This will cause more lift on the low wing that will tend to return the aircraft to straight and level. Thus, the dihedral design promotes lateral, or roll stability.

 

Dihedral angle on a KC-135

Figure 1-6, Dihedral angle on a KC-135

 

While a dihedral configuration angles the wing tips up from the fuselage, an anhedral configuration angles the wing tips down. This creates the opposite effect of dihedral by decreasing lateral stability, making the aircraft more likely to roll. Even though it is less stable, some aircraft are designed with anhedral wings and/or flight control surfaces to increase maneuverability. The F-16 in figure 1-7 utilizes a slightly anhedral design on its wings and a very noticeable anhedral on the aircraft’s tail section. These together give it incredible maneuverability, but at the expense of inherent stability.

 

Anhedral angle on an F-16

Figure 1-7, Anhedral angle on an F-16


AIR PRESSURES EXERTED ON AN AIRFOIL.

Lift is a result of pressure exerted on the surfaces of the wing of an aircraft. In aerodynamics, there are two kinds of pressure: Static and dynamic.

Static Pressure (Ps). Very simply, static pressure is the pressure exerted on a surface by unmoving air. Static pressure decreases as dynamic increases.

Dynamic Pressure (Pd). Dynamic pressure is exerted when the airfoil or the air around it begins to move. Dynamic pressure increases with the velocity of the air that moves over the top of the airfoil.

Total Pressure (Pt). Total pressure is the sum of static and dynamic pressure. It remains constant in a given set of conditions.

Formula: Pt = Ps + P

BERNOULLI’S PRINCIPLE.

Daniel Bernoulli, a Swiss scientist, advanced the following theory in 1738: As the velocity of a gas or liquid increases, the pressure decreases; and as the velocity decreases the pressure increases. See figure 1-8. This principle allows engineers to accurately predict such forces as lift and drag. A venturi tube, a tube with flared ends and a constricted middle, illustrates this principle. However, certain assumptions must be made for the purposes of the experiment:

The gas

- Is incompressible

- Has a constant rate of flow

- Passes through the tube without turbulence

Obviously, the amount of air entering the tube equals the amount of air exiting the tube. If the area is the same at each end, then the speed of the air will be the same at each end. At the throat of the venturi, where the area is smaller, the velocity must be higher in order to pass the same amount of air as at the ends. Because air has mass, a force must be exerted to accelerate it. The force in the venturi tube comes from a difference in the static and dynamic pressures. The pressure difference at the narrow section of the venturi will be greater than that at the entrance. The relationship between velocity and pressure is known as Bernoulli’s principle.

In simple terms, the relationship states that the dynamic pressure will gain any loss in static pressure because the total pressure remains constant. The increased velocity at the throat of the tube causes the loss in static pressure, which results in an increased dynamic pressure. This differential pressure is the foundation for lift produced by an airfoil.

A wing is designed so that the distance from the leading edge to the trailing edge of the wing is different around the upper and lower surfaces. It is further across the top of the wing than it is across the bottom. As air moves across a wing, it must move faster across the top surface than across the bottom surface in order to maintain its integrity at the trailing edge. If you apply Bernoulli’s principle, the slower moving air on the bottom of the wing creates more pressure than the faster moving air on the top. The net effect is called lift.

 

bernoulli's principle illustration

Figure 1-8, Bernoulli’s principle illustrated

 

FACTORS AFFECTING LIFT.

Angle of Attack.

If you recall, the angle of attack is the angle between the relative wind and the chord of an airfoil. As the AOA increases, lift increases with it until a point of diminishing return is reached. At this critical point, the wing begins to stall and lift decreases dramatically. This phenomenon is explained in detail in the section on Boundary Layer Separation.

Airfoil Shape.

As camber, or curvature increases, so does the lift created by the airfoil. This property is used when employing flight controls. Flight controls increase or decrease lift in a given direction thus causing the airfoil to move in that direction.

Air Density.

As air density increases, lift increases and vice-versa. Air density is dependent on atmospheric conditions such as humidity and temperature. Increased humidity lowers air density and as a result, decreases lift. Temperature also affects density – higher temperature lowers density and therefore lifts generation.

True Airspeed.

As an aircraft sits still, the only pressure exerted on the airfoil is static pressure. In order to achieve a static pressure differential, the aircraft must move through the air to create dynamic pressure. As the airspeed increases, the static pressure differential on the airfoil increases resulting in increased lift. Similar to AOA, there is a point where the effect of TAS is negligible.

Wing Area.

For a given airfoil shape and aircraft weight, the greater the wing area then the greater the amount of lift produced by that airfoil. This is a result of fewer pounds per unit of area of wing surface.

AIRFLOW ABOUT AN AIRFOIL.

Any discussion about airflow must begin with the subject of boundary layers. Before reading on, be sure you understand skin friction as introduced in this study guide’s section on drag. The boundary layer is that thin layer of air in direct contact with the surface of an airfoil.

Laminar and Turbulent Airflow.

Because of skin friction, which exists between the airfoil and the boundary layer, the boundary layer is subject to very powerful decelerating forces. In order to maintain airflow, fast flowing layers of air must pass over slower layers beneath them. If this flow pattern is smooth, the boundary layer is said to be laminar. On the other hand, if the air is unable to travel smoothly the flow is said to be turbulent.

Boundary Layer Separation and Stall.

The usual tendency is for airflow to remain laminar from the leading edge until it begins to grow turbulent at a point near the trailing edge. The point at which the turbulence begins is considered the start of boundary layer separation. See Figure 1-9. Increasing the AOA until it reaches 16 degrees will continuously increase the lift produced by a conventional shaped airfoil. Beyond 16 degrees, the airflow over the upper surfaces is subject to forces too great to allow laminar flow. When the BLS transition point occurs far enough forward on the upper surface of the airfoil the result is separation to a point where it decays lift. At this point, the airfoil is said to stall. Beyond this stall angle, called the critical angle of attack, the airfoil suddenly produces insufficient lift to keep the aircraft aloft and the results can be disastrous.

Boundary Layer Separation


Figure 1-9, Boundary Layer Separation

Boundary Layer Control.

A variety of devices have been invented to ensure that the airflow remains laminar. Though there are more, we will discuss some of the more common ones here.

Slots and Slats.

Slots and slats are secondary flight control surfaces located on the leading edge of the wing. They actually allow air to flow through the leading edge of the wing in certain areas. This helps to maintain laminar airflow at a higher AOA.

 

Wing Fences.

Wing fences, figure 1-9, are vertical, fin-like structures on the upper surface of the wing. They help control the airflow over the flaps and near the wing tips. On swept wing airplanes, they are located about two-thirds of the way out towards the wing tip and prevent the drifting of air toward the tip of the wing at high angles of attack. On straight wing airplanes, they control the airflow in the flap area. In both cases, they give better slow speed handling and stall characteristics.

 

Wing Fences

Figure 1-9, Wing Fences

Vortex Generators.

The vortex generator, figure 1-10, is a small vane set at an angle to the airflow on the upper surface of the wing, although not an airfoil with camber, the resulting rotation of air produces a high drag phenomena, transfers energy downstream. This reduces overall drag and buffeting and maintains boundary airflow. Vortex generators are also placed on the aft fuselage and on the empennage to reduce buffeting.

 

Vortex Generators

Figure 1-10, Vortex Generators

Control Surface Effects

 As an aircraft moves through the air, the pilot makes inputs to the flight controls through various means. As the control surfaces are manipulated, the aircraft moves about any of three axes of rotation or most likely a combination of the three. These axes all intersect at the center of gravity of the airplane. See Figure 1-11.

 

LONGITUDINAL AXIS.

 An imaginary line extending from the nose to the tail of the airplane. Motion about this axis is called roll.

LATERAL AXIS.

The lateral axis is the horizontal reference line passing from wing tip to wing tip. It is perpendicular to the longitudinal axis. Movement about this axis is called pitch.

VERTICAL AXIS.

The vertical axis is the reference line that is perpendicular to both the other axes. Rotation around this axis is called yaw.

 

Axes of Rotation on aircraft

Figure 1-11, Axes of Rotation