Up, Up and Away....
But how? It is with the help of the aerodynamic principles that an aircraft is able to fly up and stay flying in the air. Aerodynamics is a very vast and complex subject but if you know the basic/ fundamental principles then you will be able not just to understand the topic but also learn it with a keen interest in it. Here we are learning about the the aerodynamic principles applied to an aircraft. This will teach us how and why an airplane flies. The challenge to learn how an aircraft flies begins with learning the four forces of flight.
In my earlier lesson “Aviation for Beginners” I had explained the very basics of aircraft structure. To give a quick recap, I had touched uopn the history of the aviation industry starting from the Wright brothers continuing into the details of the machine that can fly in the air i.e. the Aircraft, the parts of the aircraft like the empennage, fuselage, wings of the airplane, landing gear, powerplant, ailerons, and flap wings.
Back to this lesson, you will learn how an aircraft
actually flies in the air i.e you can learn the science behind it. You
will learn the fundamental aerodynamic principles and forces acting on the
aircraft in flight. By
studying the way the air flows around the plane engineers can accordingly
design the shape of the plane. The wing, tail and the main body i.e. fuselage
of the plane all affect the way the air will move around the plane. By studying
aerodynamic principles, you can learn the ability to handle an airplane with accuracy and efficiency on both-
ground and air.
Aerodynamics has come from a Greek word ‘aerios’- concerning the air and dynamic which means force. It is a branch of dynamics (study of forces and their effect on motion) concerned with studying the motion of the air. In simpler words it is the force of the objects as they move through the air.
Aerodynamics effect both small and large objects. Lift, drag, weight and thrust are all factors that affect the ability of an object to move through the air and the speed at which it moves. However, the four factors i.e. lift, drag, weight and thrust also known as the four forces of flight will be explained in detail further in this lesson. This however helps to explain the principles of the flight of an aircraft. Humans have been making use of aerodynamic forces for thousands of years with sailboats and windmills. Two important concepts in aerodynamics i.e. continuum (measurement) and drags (forces acting on solid) appear in the work of Aristotle.
Aristotle was a Greek philosopher who had covered writings in many subjects. He was the first to create a comprehensive system of western philosophy. Although observation of aerodynamic forces such as drag were recorded by Aristotle, Leonardo da Vinci and Galileo Galilei, very little effort was made to develop a quantitative theory of air flow prior to 17th century. Sir Isaac Newton was the first to develop a theory of air resistance making him one of the pioneers of aerodynamics.
Have you ever played with a Frisbee? It flies because of the four forces. These same four forces help in flying a plane. The four forces as mentioned earlier are lift, thrust, drag and weight. Taking the example of the frisbee, it flies through the air as lift holds it up, the thrust is provided by your arm, the drag from the air lowers the speed of the Frisbee and the weight of the Frisbee helps to bring down the Frisbee back on the earth. Similarly, wings keep the airplane up in the air but the four forces are what make it happen. They push a plane up, down, forward or slow it down. Let’s understand each of them better:
Lift: It is an upward force that is created by the effect of the airflow as it passes over and under the wing of the plane. The airplane is flying in the air with the help of the lift. It is one of the key aerodynamic forces.
Thrust: This is a forward force which pushes the airplane through the air. It varies with the amount of engine power being used. The air is pulled in the engine and then thrown out in the opposite direction, thus called thrust. An easy example is the household fan.
Drag: The opposite force is called a drag. This helps in reducing the speed of an aircraft. This is caused due to differences in air pressure.
Weight: Just as lift helps in flying the aircraft in the air, the weight acts in an opposite manner. It pushes the plane downward due to the gravitation force. All objects that go up have to come down due to gravitation.
In unaccelerated flight, the four forces are in equilibrium. Unnaccelerated flight means that the airplane is maintaining a constant airspeed i.e. it is neither accelerating nor decelerating. The ways the four forces act on a plane make the plane do different things. Each force has an opposite force like lift works opposite to weight and thrust works opposite to drag.Movement of air over and under the wing of an airplane is necessary even before the aerodynamic principles can become effective. This can be better understood by some of the basic principles of motion explained in the next section. .
Newton’s Law of force and motion: The
motion of an aircraft through the air can be explained and described by
physical principals discovered over 300 years ago by Sir Isaac Newton. Newton
worked in many areas of mathematics and physics. He developed the theories of gravitation
in 1666, when he was only 23 years old. Some twenty years later, in 1686, he
presented his three laws of motion in the "Principia Mathematica
Newton’s first law: A body at rest tends to remain at rest, and a body in motion tends to remain moving at the same speed and in the same direction. E.g. an airplane at rest will remain at rest unless a force is applied to make it move. This is normally taken as the definition of inertia.
Newton’s second law: When a body is acted upon by a constant force, its resulting acceleration is inversely proportional to the mass of the body and it’s directly proportional to the applied force. The law defines a force to be equal to change in momentum (mass time velocity) per change in time. This law may be expressed by the formula: Force= mass * acceleration (F=ma).Thus, a force will cause a change in velocity and likewise a change in velocity will generate a force.
Newton’s third law: For every action there is an equal and opposite reaction. This principle applies whenever two things act upon each other, such as the air and propeller, or the air and the wing of an airplane.
Bernoulli, a Swiss mathematics, expanded on Newton’s idea and further explored
the motion of fluids in his 1738 publication Hydrodynamica.
It was in this text that Bernoulli’s equation, which describes the basic principle of airflow pressure differential first appeared. Bernoulli’s principle is a concept of fluid dynamics (how fluids i.e. liquids and gases work) and states that, as the velocity of fluids i.e. liquids and gases increases, its internal pressure decreases. His principle is derived from Newton’s second law of motion. Bernoulli's principle is the principle that allows wings to produce lift. It works on the idea that as a wing passes through the air the its shape make the air travel more over the top of the wing than beneath it. This creates a higher pressure beneath the wing than above it. The pressure difference cause the wing to push upwards and lift is created.
One way you can imagine Bernoulli’s principle is to imagine air flowing through a tube which is narrow in the middle as seen in the above diagram. As the air enters the tube it is travelling at a known velocity and speed. When the airflow enters the narrow portion, the velocity increases and the pressure decreases. Then, when the air flows to the end of the tube which is a wider portion, both the velocity and pressure come back to their original values. Thus we can say that whenever there is increase in velocity there is decrease in pressure.
With the help of the Newton’s law and Bernoulli’s principle learned above we can now use these laws and principles to learn how does these affect the flying of an aircraft and also what happens when these principles are not followed properly.
Air Foil: A part
or surface, such as a wing, propeller blade, or rudder. The aerodynamic cross section of a body such
as a wing that creates lift as it moves through the air. Airfoils
typically have teardrop shape. The shape of the airfoil strongly affects
the amount of lift it produces and its flight characteristics.
An airfoil is any surface, such as wing which provides aerodynamic force when in contact with the moving stream of air. Movement of air about the airfoil is one of the important factors in the generation of lift. The principles and laws learned in section 4 help to explain the circulation of air around the wing and the pressure difference on the wing surface. A combination of these forces thus provides a lift. The airplane’s wing shape is designed to take advantage of both Newton’s law and Bernoulli’s principle.
As seen in the above diagram the upper part of the wing is more curved to the lower part of the wing. Thus the airflow is higher above the wing leading to low air pressure. The airflow is lower under the wing due to its more flatten shape thus air pressure is higher. In simpler words, the air flows faster on the above of the airfoil as it has to travel a longer distance due to the curve on the upper body of the airfoil but has to meet the lower airflow at the trailing edge thus travels fast.
Hence according to Bernoulli’s principle as the speed increases the pressure decreases. Following the same principle thus there is high air speed on the upper part of the airfoil leading to low air pressure. The under part of the airfoil being almost flat the speed of the airflow is low thus leading to high pressure. As high pressure tends to move upwards towards the low pressure thus a lift is created.
In the above diagram you can see the parts of an airfoil:
Leading Edge: The part of the airfoil which meets the airflow first.
Upper Chamber: Also called Upwash. It’s the deflection of the oncoming airstream upward and over the wing.
Lower Chamber: Also called Downwash. It’s the downward deflection of the airstream as it passes over the wing and moves towards the trailing edge.
Trailing Edge: The portion of the airflow where the airflow over the upper surface rejoins the lower surface airflow.
Mean Chamber: The curves of the upper and lower body of the airfoil.
Chord “C”: It is an imaginary line drawn through the airfoil starting from the leading edge going up till the trailing edge.
Angle of attack: This is the relative angle formed by the wing. This is the angle formed between an airfoil and the oncoming wind. The chord lines and relative wind is called the angle of attack. As air circulates around the wing’s surface where the pressure is less than atmospheric, and regions where the pressure is greater than atmospheric. This specific pressure distribution varies the angle of attack.
As the angle of attack grows larger, the lift reaches a maximum at some angle; increasing the angle of attack beyond this critical angle of attack causes the air to become turbulent and separate from the wing; there is less deflection downward so the airfoil generates less lift. The airfoil is said to be stalled.
Stalls: Sufficient airspeed must be maintained in flight to produce enough lift to support the airplane without requiring too large an angle of attack. At a specific angle of attack, called the critical angle of attack, air going over a wing will separate from the wing causing the wing to lose its lift (stall). The airspeed at which the wing will not support the airplane without exceeding this critical angle of attack is called the stalling speed. This speed will vary with changes in wing configuration (flap position).
Excessive load factors caused by sudden manoeuvres, steep banks, and wind gusts can also cause the aircraft to exceed the critical angle of attack and thus stall at any airspeed and any attitude. Speeds permitting smooth flow of air over the airfoil and control surfaces must be maintained to control the airplane.To recover from a stall attack you must restore the smooth airflow by decreasing the angle of attack to a point below the critical angle of attack so that you can allow the wings to regain lift.
The next step is to apply the maximum power in order to regain the lost airspeed and also try to reduce the lost flight altitude to its maximum. As airspeed has been regained and stall recovered the power should then be maintained as per desired flight condition. Straight and leveled flight should then be established with full coordinated use of controls.
Wings are one of the major reasons for an aircraft to fly. Learning to design wings become an important part because every wing design uses the aerodynamic principles in different ways and thus help to understand the aerodynamic principles used in an aircraft in a better way. Wing design is based on the anticipated use of the airplane, cost, and other factors. The main design considerations are wing planform, camber, aspect ratio, and total wing area. Wing weight is strongly affected by the thickness of the wing. The thicker the wing the lighter it is.
Wing planform: This refers to the shape of the airplane wing when viewed from above or below. Each planform design has its advantages and disadvantages. Some examples are below:
This type of wing is ideal for flight at low speeds since it provides a minimum drag. This type of planform is difficult to construct and its stall characteristics are not as favourable as rectangular wings.
They are not as efficient as elliptical wings, but it has a tendency to stall first at the wing root which provides adequate stall warning and aileron effectiveness.
This type of wing provides increase in lift and decrease in drag which is most effective in high speeds. A good form of an aircraft is a combination of both rectangular and tapered configurations. These are also cost effective.
They are efficient at high speed. Low speed performance is degraded by this design.
Camber: It affects the difference in the velocity of the airflow between the upper and lower surfaces of the wing. If the upper camber increases and the lower camber remains the same, the velocity differential increases. There is, of course, a limit to the amount of camber which can be used. After a certain point, air will no longer flow smoothly over the airfoil. Once this happens, the lifting capacity diminishes. The ideal camber varies with the airplane's performance specification, especially the speed range and the load-carrying requirements.
Aspect ratio: Aspect ratio is the relationship between the length and width of a wing. It is one of the primary factors in determining lift/drag characteristics. At a given angle of attack, a higher aspect ratio produces less drag for the same amount of lift. Thus an aspect ratio formula is:-
Aspect Ration = Total length of the wing / Average width of the wing
Total wing area: Wing area is the total surface area of the wing. Most wings don't produce a great amount of lift per square foot, so wing area must be sufficient to support the weight of the airplane. For example, in a training aircraft at normal operating speed, the wings produce only about 10.5 pounds of lift for each square foot of wing area. This means a wing area of 200 square feet is required to support an airplane weight of 2100 pounds during straight-and-level flight.
It’s not just the lift that is important in an aircraft but we also need to control the lift so that we can maneuver the aircraft as we want. The amount of lift generated by an airplane is controlled by the pilot as well as determined by aircraft design factors. For example, you can change the angle of attack and the airspeed or you can change the shape of the wing by lowering the flaps. Anytime you do something to increase lift, drag also increases. Drag is always a by-product of lift. Three ways to control lift during flight are: - change angle of attack, changing airspeed and changing both angle of attack and airspeed.
Changing angle of attack: You have direct control over the angle of attack. During flight at normal operating speeds, if you increase the angle of attack, you increase lift. Anytime you change the pitch of the airplane during flight, you change the angle of attack of the wings. At the same time you are increasing the lift.
Changing airspeed: The faster the wing moves through the air, the greater the lift. Actually, lift is proportional to the square of the airplane’s speed. For e.g. at 200 knots an airplane has four times the lift of the same airplane travelling at 100 knots, if the angle of attack and other factors are constant. On the other hand, if the speed is reduced by one-half, lift is decreased to one-quarter of the previous value. Although air speed is an important factor in the production of lift, it is only one of the several factors. The airspeed required to sustain an aircraft in flight depends on the flap position, the angle of attack and the weight.
Angle of attack and airspeed: The relationship between angle of attack and airspeed in the production of lift is not as complex as it may seem. Angle of attack establishes the amount of lift for the airfoil. At the same time, lift is proportional to the square of the airplane’s speed. Since you can control both angle of attack and speed so you can control lift too. Total lift depends on the combined effect of the airspeed and angle of attack. When the speed decreases you must increase the angle of attack to maintain the same amount of lift and vice versa.
·During flight, the four forces acting on the plane are lift, weight, thrust and drag.
·The four forces are in equilibrium during the unaccelerated flight.
· Lift is the upward force created by the effect of airflow it passes over and under the
· The airplane’s wing shape is designed to take advantage of both Newton’s laws and
·Planform, camber, aspect ratio and wing area are some of the design factors which
affects a wings lifting capability.
· A stall is caused by the separation of airflow from the wing’s upper surface. For a given
airplane stall always occur at the critical angle of attack, regardless of air speed, flight
altitude and weight.
· Weight is the force of gravity which acts vertically through the center of the airplane
towards the center of the earth.
· Thrust is the forward acting force which opposes drag and propels the airplane.
· Drag acts in the opposite direction of flight.
Source: Jeppesen Guided Flight Discovery