ABSTRACT

This paper will attempt to discuss the basic aerodynamic principles of flight. It will be based on Module 3, Learning Objective 1: ‘For a typical aircraft, describe the functions of the structure and the flight controls. Apply aerodynamic principles to explain how flight controls control pitch, roll, and yaw’. It will also outline the basic control surfaces of an aircraft and the primary and secondary effects of each of them. In addition, it will also explore Bernoulli’s Principle and the forces acting on an aircraft in flight.

Table of Contents
Introduction5
Main Components of An Airplane 6 * Fuselage * Wings * Empennage
The Wing and the Aerofoil7
Aerodynamics of Flight (Bernoulli’s Principle) 8
The Forces in Flight 9 * Lift * Thrust * Drag * Weight
The 3 Axes of Rotations 10 * Longitudinal Axis * Lateral Axis * Vertical Axis Main Control Surfaces11 * Ailerons * Elevators * Rudders
Secondary Effects of Control Surfaces12 Conclusion14

List of Figures Figure 1: The Magic of Flight 3 Figure 2: Main Components of an Airplane 4 Figure 3: The Wing And Aerofoil 5 Figure 4: Aerofoil Nomenclature 6 Figure 5: Bernoulli’s Principle 7 Figure 6: Forces on An Airplane in Flight 8 Figure 7: The 3 Axes of Rotation 9 Figure 8: 6 Degrees of Freedom 10 Figure 9: Control Surface: Ailerons 11 Figure 10: Control Surface: Elevators 11 Figure 11: Control Surface: Rudders 11
Introduction

Figure 1: The Magic of Flight
Man has come a long way since the advent of the first heavier-than-air flight by the Wright brothers back in 1903. In the span of merely a century and a decade, we have achieved an astounding feat of aeronautical capability. From the biplanes of WW2 to the successful launch of NASA’s Space Shuttle orbiter, Man’s thirst for the quest of achieving airborne flight with the birds has always been a source of great admiration and fascination. Those of us who were “bitten” with the ‘flying bug’ from a very young age can vouch for the sheer thrill and adrenaline rush of rolling down the runway in an insane but fulfilling roller coaster ride as we defy the clutches of Gravity and rise up into the clouds; a step closer to the heavens. It is, truly, a magical feeling to be able to soar in the skies in absolute freedom, throwing our Earthly worries away and entering a whole new world altogether. And while the picture above is a humorous attempt in showing how airplanes fly, it does not really explain the Physics behind the Laws of Aerodynamics. The ‘magic’ of flight as shown above can actually be explained and involves juxtapositions of Daniel Bernoulli’s principles, who, in 1738 investigated the forces present in a moving fluid; as well as Newton’s 3 famous Laws that govern most of Physics. The basic principles of flight are outlined in this paper, and are generally similar for many different kinds of fixed wing aircraft, big or small, that you may see flying out of the airports today.
Main Components of an Airplane

Figure 2: Main Components of an Airplane.
The figure above shows the main components of an airplane and its respective parts. The airplane can generally be divided into 3 main parts, namely the Fuselage, Empennage and the Wing.
Fuselage - The fuselage is also known as the main body of the airplane. This houses the cabin where the passengers sit, and in some airplanes, also houses the storage space for luggage, goods, and other forms of baggage on board a flight.
Empennage - The empennage is the term used to denote the entire tail end of the airplane (circled in black). It consists of components such as the rudders, elevators, vertical stabilizers as well as horizontal stabilizers. Some of the control surfaces critical for the aircraft to function and manoeuvre are located here. We will be looking at some of these control surfaces and their functions later.
Wing - The wing is probably one of the most important parts of an airplane. It is what gives the airplane its wingspan, and it is an important component in determining an aircraft’s ability to fly. The wing consists of parts such as the ailerons, flaps, spoilers, and slats. We will be looking at some critical components on the wing in greater detail next.
The Wing & the Aerofoil

Figure 3: The Wing and Aerofoil
The figure above shows a section of the wings typical to an airplane. If we were to cut the wings into half, the cross-sectional area of the wing is depicted in the shaded area outlined above, and is commonly termed as an aerofoil. The figure below shows a closer view of an aerofoil and its associated terminologies.

Figure 4: Aerofoil Nomenclature
Some of the common terms associated with the aerofoil are as follows:-
Wing Camber – The curvature of the surface of the wing. This is important when explaining Bernoulli’s Principle.
Chord line – The imaginary straight line joining the leading and trailing edges of the wing. Its length, referred to as the chord, is the width of the wing.
Angle of Attack – The angle between the wing chord line and the relative wind passing through the wing as the aircraft is in flight.
Aerodynamics of Flight (Bernoulli’s Principle)

Figure 5: Bernoulli’s Principle

After understanding the technical terms involved in an aircraft’s wing, it is time to explore the concept of Bernoulli’s principle. It states that as the air passes through the surface of the wing in flight, it is split into two different routes. Part of the air travels on top of the wing and part of it travels below the wing. Due to the camber, or curvature of the wing design, the air travelling on top of the wing has to travel a greater distance in order to meet the air travelling below the wing at the same time on the trailing edge. Hence, the air travelling above the wing needs to travel at a faster speed than the air travelling below the wing. Bernoulli’s principle states that as the speed of a fluid increases, the pressure in the region decreases. The region on top of the wing now has a lower pressure, and the region below the wing now has a higher pressure. This difference in pressure is what creates a resultant Lift force acting upwards on the airplane wings. The more Lift being generated, the longer the aircraft is able to stay aloft. This is the fundamental concept that Bernoulli theorised, and so far it has held true for all fixed wing heavier-than-air planes that are flying around the world today.

The Forces in Flight

Figure 6: Forces on an Airplane in Flight
The diagram above shows the distribution of forces acting on an airplane in flight. The forces are Thrust, Lift, Drag and Weight.
Thrust – The force generated by the power plant/ propeller or rotor that propels the aircraft forward. It tries to oppose or overcome the force of drag.
Lift – It is a force produced by the effect of airflow on the aerofoil, and acts perpendicular to the flight path through the center of lift. It opposes the downward force of weight.
Drag – A rearward force caused by disruption of airflow by the wing, rotor, fuselage, and other protruding objects. Drag opposes thrust, and acts rearward parallel to the relative wind.
Weight - The combined load of the aircraft itself, the crew, the fuel, and the cargo or baggage. Weight pulls the aircraft downward because of the force of gravity. It opposes lift, and acts vertically downward through the aircraft’s center of gravity (CG).
Maintaining a steady flight on a straight-and-level attitude often requires a balance of these 4 forces, often described as ‘equilibrium’ between them. It is important for the pilot to have an understanding of how these 4 forces interact, as any failure in recognizing the onset of these 4 forces could result in potentially catastrophic consequences, such as an aerodynamic stall or loss of airspeed/power.
The Axes of Rotation

Figure 7: The 3 Axes of Rotation Figure 8: 6 Degrees of Freedom

Most aircraft operate on the 3 axes of rotation, and have 6 degrees of freedom. They can move up or down, left or right, and forwards or backwards (thrust reversers). The 3 axes of rotation are namely the Longitudinal Axis, Lateral Axis and Vertical Axis, shown above.
Longitudinal Axis – Also known as the roll axis, this is an imaginary line acting through the body of the aircraft that causes it to follow a circular rolling motion in the air.
Lateral Axis – Also known as the pitch axis, this is an imaginary line acting through the wings of the aircraft, causing it to follow a downward or upward pitching motion.
Vertical Axis – Also known as the yaw axis, this is an imaginary line that acts vertically through the centre of the wings, and causes it to follow a sideways left and right skidding motion in the air.
It is important to note that all 3 lines go through the centre of gravity of the airplane. The centre of gravity is the balance point where all the weight of the airplane can be taken to act on that particular spot. In the next section, we will investigate the control surfaces on the aircraft that will enable it to execute the Pitch, Roll and Yaw motions.

Main Control Surfaces
The ailerons are control surfaces attached to the trailing edges of the wings. They are controlled by turning the control columns left or right. Turning the control column right will cause the right aileron to be deflected upwards, and the left aileron to be deflected downwards. This lowers the right wing, raises the left wing, and causes the aircraft to initiate a roll to the right. (Fig 9.) Ailerons

Figure 9: Control Surface: Ailerons
The elevators are control surface attached to the horizontal stabilizers. They are controlled by pushing the control columns up or down. Pushing the control column up will cause the elevators to deflect downwards. This causes the tail of the aircraft to be pushed downwards, raising the nose of the aircraft and causing the aircraft to pitch upwards. (Fig. 10).
Elevators

Figure 10: Control Surface: Elevators
The rudders are control surfaces attached to the vertical stabilizers. They are controlled by the pilot’s feet sliding the rudder pedals from side to side. Sliding the left rudder pedal forward will cause the rudders to deflect to the left. This creates an aerodynamic force that acts to the right, causing the aircraft to yaw its nose sideways and to the left. This is useful in a crosswind landing situation when the aircraft needs to point its nose sideways while coming in to land. (Fig. 11)
Rudders

Figure 11: Control Surface: Rudders
Combining the effects of these control surfaces together, they enable the aircraft to manoeuvre up in the air and thus have the ability to generate the 6 degrees of freedom of movement. The next section will now investigate the secondary effects of control surfaces.
Secondary Effects of Control Surfaces

Ailerons
When we bank the aircraft around its roll axis by moving the stick to the left or to the right, the lift vector component which is always oriented at right angles to the airflow over the wing will also tilt as well. In other words, when the aircraft is banked, the weight and the lift are no longer exactly opposite to each other; an angle is formed between these two forces and a resulting force is generated. This resulting force acts towards the low wing and causes the aircraft to slip accordingly. This slipping motion generates lateral relative wind acting on the fuselage and on the vertical fin: as the major part of the fuselage, and particularly the vertical fin, is located behind the centre of gravity, the aircraft tends to rotate around the yaw axis towards the low wing. This yaw motion is known as weathervane effect and is the secondary effect of the ailerons.

Rudders
When we cause the aircraft to rotate around its yaw axis by means of the rudder, say for instance to the left, the right wing develops a higher angular velocity than the left one. As a higher velocity generates more lift, the right wing tends to move up, i.e. the aircraft banks to the left. In other words, an initial motion around the yaw axis generates a resulting motion around the roll axis, known as induced roll which is the secondary effect of the rudder. As the initial motion around the yaw axis causes the nose to move sideways, to the left in this example, a direction change to the left will obviously occur but, due to its inertia, the aircraft tends to maintain a rectilinear trajectory and skids towards the outside of the curve, in this example to the right.

Elevators
The elevator is said to have no secondary effects, although its use implies certain consequences: a) When the aircraft nose pitches up, the altitude will increase and the airspeed will decrease;
b) When the aircraft nose pitches down, the altitude will decrease and the airspeed will increase.
Although these consequences are rather obvious, they are not considered as secondary effects, because a control surface effect, whether a primary or a secondary one, is by definition a certain motion around a certain axis.

Conclusion

This concludes the aeronautical perspective paper for Module 3, giving a brief overview of the fundamentals of flight. Understanding the Physics of flight is crucial in determining why an airplane behaves in the way it does. Contrary to popular ‘belief’, there is no wizardry or ‘magic’ involved in flying at all. Hopefully, with this aeronautical perspective paper, it will give a basic insight to the technical aspects of flying and that this knowledge will be applicable to further studies of airplane aerodynamics in the future.

References

Anderson, D. F., & Eberhardt, S. (2001). Understanding flight. New York: McGraw-Hill

United States. Federal Aviation Administration. (2008). Pilot's handbook of aeronautical knowledge. Washington, D.C.: U.S. Dept. of Transportation, Federal Aviation Administration.

The Four Forces | How Things Fly. (n.d.). Retrieved from http://howthingsfly.si.edu/forces-flight/four-forces

Flight controls. (n.d.). Retrieved from http://www.free-online-private-pilot-ground-school.com/Flight_controls.html