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Military

LESSON 2

BASIC AERODYNAMICS
Critical Tasks:

OVERVIEW

LESSON DESCRIPTION:

In this lesson you will learn basic aerodynamics.

TERMINAL LEARNING OBJECTIVE:

ACTION: You will know basic aerodynamics.
CONDITION: You will be given the information provided in this text, a list of reference manuals, and charts provided in the text.
STANDARD: You will correctly answer all the questions in the practice exercise at the end of lesson 3.
REFERENCES: Listed in appendix I.

INTRODUCTION

2.1.  GENERAL

Aerodynamics involves the motion of air and other gases and the forces acting on objects in motion through the air. Put another way, aerodynamics deals with aircraft, wind movement, and atmosphere. A working understanding of aircraft flight must start with a basic knowledge of flight theory as it pertains to conventional aircraft.

2.2.  BERNOULLI'S THEOREM

At any point in a tube through which a liquid is flowing, the sum of pressure energy, potential energy, and the energy of motion is constant. This theory was discovered by Daniel Bernoulli, a Swiss mathematician and physician. How the theory works is illustrated in figure 2.1. An American engineer, Clemens Herschel, invented the venturi tube and named it in honor of Giovanni Venturi. If the same amount of air that enters the airflow inlet is to leave the airflow outlet, then the velocity of the air must increase while passing the venturi throat. As the velocity increases, the air has less time to push against the sides of the tube, thereby exerting less pressure. Figure 2.1 shows the decrease in pressure gages. Because there is no change in the velocity of the air about the open end of the tube, there is no change in pressure. The differential pressure on the ends of the tubes attached to the venturi throat causes the fluid to move toward the end of the tube that has the least pressure.

Figure 2.1.  Venturi Tube.
Figure 2.1. Venturi Tube.

2.3.  NEWTON'S LAWS

Sir Isaac Newton formulated the three laws of motion upon which classical dynamics are based. These laws are directly applicable to modern aerodynamics and are discussed in the subparagraphs that follow.

  a.  The first law. A body at rest remains at rest, or if in motion it continues to move in the same direction with the same speed, unless a force acts upon it. To accelerate an airplane, for example, the engine must deliver a thrust greater than the existing drag or resistance force. For unaccelerated flight, the thrust must be exactly equal to the drag.

  b.  The second law (two parts). (1) When different forces are allowed to act upon moving bodies, the rates at which the momentum changes are proportional to the forces applied. For example, two forces are judged equal if they produce a change of momentum at equal rates. One force is twice as great as another if it changes the momentum at equal rates. One force is twice as great as another if it changes the momentum at twice the rate. (2) The direction of the change in momentum caused by a force is that of the line of action of the force. For example, when a rope operates over a pulley, the force is always combined with one or more auxiliary forces, resulting in the changed direction of the momentum.

  c.  The third law. For every action there is an equal and opposite reaction. For example, a propeller can bite into the air forcing the air rearward and thereby producing a force sufficient to propel the airplane forward.

2.4.  FORCES ACTING ON AN AIRCRAFT

Figure 2.2 shows the four forces that act on an aircraft in flight; they are weight, lift, thrust, and drag. The weight of the aircraft and its occupants, fuel, and cargo must be lifted against the force of gravity. In designing aircraft the lightest and strongest materials possible are used.

Figure 2.2.  Four Forces Acting on an Aircraft.
Figure 2.2. Four Forces Acting on an Aircraft.

Lift is the force that overcomes gravity. Lift is obtained through the action of air moving past the wings or rotor blades of an aircraft. How to get maximum lift is a major problem in wing and rotor-blade design.

Thrust is the force that puts the aircraft into motion relative to the ground and brings the force of lift into existence. Conventional aircraft are pushed or pulled forward by one or more reciprocating or turbine engine-driven propellers or jet engines.

Drag, the resistance to forward motion, is created by the flow of air over the surface of the aircraft. Figure 2.3 shows how different shaped objects are affected by airflow. The two kinds of drag are induced and moving through the air. While the aircraft is flying, high-pressure air below the wing tends to flow into the low-pressure area above the wing. The two pressures mix at the wing tip and create a vortex (whirlpool). The vortex creates a suction effect at the ends of the wing and causes induced drag that varies directly with the angle of attack. Parasite drag is created by the entire aircraft, excluding induced drag. It is caused by protrusions such as landing gear, rough surfaces, and air striking on the aircraft's frontal surfaces.

Figure 2.3.  How Different Objects are Affected by Airflow.
Figure 2.3. How Different Objects are Affected by Airflow.

2.5.  AIRFOILS

An airfoil is any surface, such as a wing or rotor blade, designed to produce lift when air passes over it. Air passing over the upper surface of a foil produces two-thirds of a foil's lift by creating a lower pressure. One third of the foil's lift is produced by the higher pressure of air on the foil's under surface.

Relative wind is the air flowing opposite and parallel to the direction of airfoil motion. When an aircraft is at rest, relative wind does not exist, only wind created by nature. Relative wind, shown in figure 2.4, is created by the motion of the aircraft traveling through the air using its own power to reach its desired speed.

Figure 2.4.  Relative Wind.
Figure 2.4. Relative Wind.

A symmetrical airfoil is designed to have equal cambers on both sides. This kind of airfoil has the characteristics of limiting center-of-pressure travel.

An asymmetrical airfoil is designed to have unequal cambers. This type of airfoil has the characteristic of a rapid movement of center-of-pressure travel. Figure 2.5 shows the contrast between a symmetrical and an asymmetrical airfoil.

Figure 2.5.  Symmetrical and Asymmetrical Airfoils.
Figure 2.5. Symmetrical and Asymmetrical Airfoils.

2.6.  ANGLE OF ATTACK

The angle of attack is the angle at which an airfoil passes through the air. This angle is measured between the chord of the airfoil and the relative wind, as shown in figure 2.6. The chord is an imaginary line from the leading edge to the trailing edge of an airfoil. Increasing the angle of attack deflects the airstream and causes an upward pressure on the underside of the airfoil. This in turn increases the speed of the airflow over the topside of the airfoil. As air-flow-speed increases, pressure on the foil's top side is further reduced. The upward pressure on the foil's underside and pressure reduction on the top side combine to furnish lift.

Figure 2.6.  Angle of Attack.
Figure 2.6. Angle of Attack.

The angle of attack and angle of incidence are measured angles.

Angle of incidence (fixed-wing aircraft) is the angle between the airfoil chord line and the longitudinal axis or other selected reference plane of the aircraft.

Angle of incidence (rotary-wing aircraft) is the angle between the chord line of a main or tail rotor blade and the plane of rotation (tip path plane). It is usually referred to as the blade pitch angle. For fixed airfoils, such as vertical fins or elevators, the angle of incidence is the angle between the chord line of the airfoil and a selected reference plane of the helicopter.

2.7.  STALL

As the angle of attack is increased, lift is also increased up to a certain angle. Beyond this angle airflow can no longer follow the contour of the airfoil's upper surface, as shown in the sequences in figure 2.7. After the burble point the airfoil goes full stall.

Figure 2.7.  Stall Angle.
Figure 2.7. Stall Angle.

2.8.  AILERONS AND FLAPS

Figure 2.8 illustrates both ailerons and flaps. The aileron is the surface control mounted on the trailing edge near the wing tip that allows the pilot to change the angle of bank as desired.

The flaps are mounted inboard of the ailerons and are probably the most used lift device in service. They increase the lift capability of the airfoil to the maximum attainable. This means that an aircraft can become or remain airborne at lower speeds with flaps extended. They also permit a shorter ground run on landing when used as airbrakes.

When the flaps are extended, the curvature (camber) of the wing is increased. On a high lift aircraft, the ailerons are interconnected to the flaps. In this arrangement, as the flaps are extended, the ailerons droop to add more lift and better control response at slower speeds. Flaps and ailerons are shown in figure 2.8.

Figure 2.8.  Parts of a Fixed Wing Aircraft
Figure 2.8. Parts of a Fixed Wing Aircraft

2.9.  ASPECT RATIO

Paragraph 2-9 Aspect Ratio DELETED

Figure 2.9.  Motion of an Aircraft about its Axes.
Figure 2.9. Motion of an Aircraft about its Axes.

2.10.  STABILITY

For lateral stability, positive dihedral is designed into the wing. In simple terms, this means that the wing tips are higher than the wing roots and the aircraft's center of gravity is below the wing's mean center of pressure. Notice in figure 2.10 that the wing tips for the U-21 and U-6 aircraft are 7 and 2 higher than the wing roots. On the U-21 the tips of the horizontal stabilizer are also higher than the roots. A definition for dihedral then is: the spanwise inclination of a wing or other surface such as a stabilizer relative to the horizontal gives the wing or other surface dihedral. This angle is positive if it is upward and negative if it is downward.

Figure 2.10.  Wing Dihedral.
Figure 2.10. Wing Dihedral.

2.11.  CANTILEVER WINGS

Figure 2.11 shows a cantilever wing and a noncantilever wing. A cantilever wing has no external supports and its structural strength is derived from its internal design. The advantage of this kind of wing is it eliminates drag caused by wing struts. Its disadvantage is the added weight required to give the wing its strength.

Figure 2.11.  Cantilever and Noncantilever Wings.
Figure 2.11. Cantilever and Noncantilever Wings.

2.12.  BOUNDARY LAYER

Figure 2.12, Boundary Layer Flow, shows laminar and turbulent flow and the transition point in between. The boundary layer is the air close to the aircraft wings' upper surfaces. In the forward portion of the boundary layer, the air flows in layers, or separate sheets, called laminae. These layers slide over one another with little mingling of the air particles. However, behind the leading edge there is a transition point where the layer thickens, the airflow becomes turbulent, and one layer mixes with another. This results in increased drag. To aid in preventing boundary-layer turbulence, fences are installed as shown in figure 2.13.

Figure 2.12.  Boundary Layer Flow.
Figure 2.12. Boundary Layer Flow.

Figure 2.13.  Wing Fences.
Figure 2.13. Wing Fences.

2.13.  SPEED BRAKES

Figure 2.14 shows an extended speed brake. These brakes are manually or hydraulically operated flaps that project into the airstream. Generally, they extend from the sides of the aircraft. However, they can be located on the bottom centerline of the fuselage or on the wings' upper surfaces. The purpose of speed brakes is to retard an aircraft's speed. Such brakes are generally used on aerodynamically clean aircraft such as jets.

Figure 2.14.  Speed Brakes.
Figure 2.14. Speed Brakes.

2.14.  EMPENNAGE GROUP

The tail section of an aircraft consists of the horizontal stabilizer and elevator, vertical stabilizer, and rudder as shown in figure 2.8.

The horizontal stabilizer gives the pilot control about the aircraft's lateral (pitch) axis. Some aircraft have a flying tail in which there is no elevator, only one large movable surface called a stabilizer.

The vertical stabilizer acts the same as a keel surface on a boat, and it is needed for adequate directional stability. The rudder is also used for the same purpose as a rudder on a boat and it controls the aircraft around its vertical (yaw) axis.

2.15.  SUMMARY

Daniel Bernoulli, born 1700, discovered the principle bearing his name. Giovanni Venturi, in 1822, noted the effects of constricting a passage through which fluid flowed. The Venturi tube was invented by Clemens Hershel, an American engineer. He named it in honor of G. B. Venturi. Newton's three laws on force and motion are applicable to aerodynamics.

The four forces acting on an aircraft are weight, lift, thrust, and drag. Flight becomes possible when lift overcomes weight and thrust overcomes drag. The two kinds of airfoils used on Army aircraft are symmetrical and asymmetrical. An airfoil uses low-pressure air on top of the wing and high-pressure air under the wing to produce lift. When the wing's angle of attack is increased, the deflection of the airstream over the wing's upper surface creates more lift. However, if the angle of attack is too great the airstream breaks away from the upper surface and burbles. At this point, the aircraft can stall.

Ailerons control the aircraft about its bank or roll (longitudinal) axis. Flaps are used to increase the lift capability of a wing and are used mostly in landing and takeoff.

A true cantilever wing derives its strength from internal wing-design. The boundary layer is that air closest to the surface of an aircraft's wings, and flows in layers called laminae.


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Page last modified: 27-04-2005 07:23:27 Zulu