Principle of Flight

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1. Forces

We live in a world full of forces. Lets consider a man running on a sports track as shown below. There are many types of forces that are acting on him as he runs.

He experiences a force between his shoes and the ground and this is called "Friction". Friction slows him down but it surely does prevent him from slipping. He provides himself with some force to move his leg forward and backward. This is his internal force and is gained from body fuel ¡V food and water. And there is also his body Weight, which prevents him from moving quickly. Weight is also a force. If the man is extremely fat, he will experience great difficulty moving and vice-versa, if he is thin, he will be able to move quickly. Another hidden force that acts on him is the "Gravity". Gravity prevents the man from jumping too high and always ensures that should his feet leave the ground, they must touch the ground again. Gravity is a force that is exhibited by our planet on every body. We cannot escape gravity unless we leave this planet and enter space. In space, there is no 'gravity'. Let consider one more example before we start analysing forces. If the man in our example decided to run on a windy day, he will face another force. This force will prevent him from moving forward or it may even make him move faster. This very much depends on the wind direction. This force is exerted by Wind flow.

Now that we have seen many examples of force, let's further investigate the mechanisms behind forces. Forces may be loosely regarded as 'energy created by the movement of particles in some media'. We cannot touch force but we surely can feel it. There are many types of forces in this world and depending on our view, some are considered 'good' and others 'bad'.

Let's try our first experiment.

Experiment 1.

Cut a tiny rectangular strip of paper and paste some soap on of its end. Next place it in a any vessel containing water and observe its movement. Can you explain why the paper moves? Which direction does it move?

Let me now explain what happening to our tiny paper strip. When the strip is placed in water, you will observe some random movement. The soap breaks up the water surface tension and causes the paper strip to move forward. Surface tension is an important force and many insects in our world need it. For example, a dragonfly uses surface tension to stay afloat. A mosquito also uses this force to move on water.

But why random movement? To explain this, let's have a microscopic picture of our experiment. Water is full of molecules and these molecules move random. They have no order and definitely don't move " 2 by 2 " !! As you can see from the above diagram, as soon as we place our paper strip in the water, it gets bombarded by water molecules and as mentioned before, since the movement of these molecules is random, the paper strip also moves in a random order. One water molecule will hit another molecule and another molecule will hit the paper strip and eventually the paper strip will move a random direction.

There are other places where you can similar phenomenon. When you go and watch a movie in you local cinema, take a look at the movie projector. You will see many dust particles suspended in front of the projector's light. They all move in a random order as they are bombarded with air molecules. These dust particles are also experiencing forces.

2. Aircraft Forces

In the previous action, you have seen some of the natural forces that exist in our world. Lets now turn our attention to the different type of forces that act upon an aircraft. When an aircraft is kept safely in a hangar or when it is taxied near a runway, it only experiences one force - "Weight". No other force acts on it. However, when an aircraft is airborne i.e. flying, things become different - instead of one downward force, four main forces start to act upon the aircraft and they are:

Now that you know that there are four main forces that act upon an aircraft, lets investigate each force and learn how these forces are created.

3. Lift

Lift is a force that not only moves an aircraft upwards into the sky but also help an aircraft to maintain altitude. In order to achieve lift, aeroplanes are designed with special wings. These special wings are known as aerofoils.

Let's see what happens when air flows around an aerofoil.

When air hits the front edge of the aerofoil - 'Leading Edge', it separates into two layers and flows both on the upper and lower surface. Air around the upper surface travels a longer distance while air around the lower surface travels a shorter distance. Aerofoils of such behaviour are knows as cambered aerofoils i.e. they have differences in lengths on both upper and lower surfaces. It is due to this camber effect, that air around the lower surface will experience 'high pressure' while air around the upper surface will experience 'low pressure'. Another way of proving this pressure distribution is the Bernoulli's theorem (explained in the appendix section).

Let's have a look at an aerofoil and some of terms associated with an aerofoil.

3.1 NACA

NACA stands for 'National Advisory Committee for Aeronautics' - a committee which was based in USA. Their primary job was to issue standards for aerofoils. All aerofoils are numbered by an international standard known as ¡¥NACA Number¡¦. There is a 4 digit standard, 5 and a 6 digit standard (out of our scope).

In a 4 digit NACA number such as NACA 4138:

The digit '4' represents the 'Maximum camber of an aerofoil';

The digit '1' represents the 'Maximum camber position';

The last 2 digits '38' represent the 'Thickness of the aerofoil'.

Different aerofoils have different lift characteristics. Some provide greater lift than others and some less. Some operate at high speeds and some at low speeds.

NACA is now part of NASA and all aerofoil standards are issued issued by NACA division.

3.2 Wind Tunnel

A wind tunnel is an experimental tunnel where aerofoils of different shapes are tested. A wind tunnel plan view is shown below. An aerofoil is suspended in the test-bed section of a wind tunnel and is then subjected to a windflow. In a typical aerodynamics (study of air flow over aeroplanes), the windflow is kept constant and only the angle of attack is varied. The angle of attack is varied from ¡V2^o to 32^o and the pressure experienced over the upper and lower surfaces of the aerofoil are measured. In a NACA4212 foil pressure analysis, it is found that, as the angle of attack is increased, lift increases. The lift continues to increases until a specific angle of attack is reached. Beyond this angle, no lift is produced, no matter how much the angle of attack is increased.

Pressure measurements obtained from wind tunnel tests show that lift at various part of an aerofoil surface are unequal. All lift forces are perpendicular (90^o) and two-thirds of the lift is obtained from the top surface of the aerofoil. Shown below is a lift distribution round an aerofoil.

3.3 Centre of Pressure

When we add up all these lines of small lift forces together, we come up with one straight line of force which represents all the lift the aerofoil is producing. This straight line is drawn at a specific point where all forces balance. This average point at which all lift forces seem to act is know as the 'centre of pressure'.

We did mention that lift increases with angle of attack and then begins to fall at some angle. Let's have a closer look at this phenomenon. At ¡V2 degrees angle of attack, the aircraft with the NACA412 moves in a straight line. The lift forces acting on the aircraft are equal on both upper and lower surfaces.

At 2 degrees angle of attack, the aircraft begins to experience lift and the lift force is mainly on the upper surface.

At 5 degrees of angle of attack, the aircraft continues to experience lift, far greater than that at 2 degrees. The flow around the aerofoil is smooth. The lift forces increases greatly at 10^o.

At 14 degrees of angle of attack, an interesting phenomenon takes place. A bubble know as ¡¥separation bubble¡¦ forms on the upper surface of the aerofoil near the leading edge. The flow aft of this bubble starts to separate and turbulent flow begins to show sign.

At 15 degrees of angle of attack, the bubble bursts and turbulent flow appears on the upper surface of the aerofoil. Lift decreases drastically and this is known as 'Stalling'.

3.4 Factors affecting lift

Angle of attack is not the only factor which affects lift ¡Vthere are other factors and they are:

3.4.1 Air Density

Before understanding air density factor, we must first understand our atmosphere. Our atmosphere appears as a blanket of air that covers us, however in reality, its structure is much more complex. There are in fact seven main layers in our atmosphere as shown here. The lowest layer is known as 'Troposphere' and the highest layer is known as 'Thermosphere'. As we move upwards from troposphere towards the fourth layer, stratosphere, air density falls, in other words, air gets thinner. We can explain this phenomenon. At high altitude, there are not many air molecules present in the atmosphere as compared in the troposphere.

Imagine a box of volume (1m³) filled with 10 small balloons (air molecules), each 100grams and using the density formula,

Density = mass / volume

Therefore, mass = 10 * 100grams = 1000grams = 1Kg

Thus density = 1000 / 1m³ = 1000grams/m³

Now imagine the same scenario with half the balloons, what we have now is,

Mass = 5 * 100grams = 500 grams

Thus density = 500/1m³ = 500grams/m³

The density has been halved

Lift greatly relies on air source and if air density decreases, lift decreases.

3.4.2 Airspeed

Lift is also affected by airspeed. At ground level, before take-off, an aircraft experiences zero lift. As the throttle is opened, the aircraft experiences forward momentum and gather speed. This speed further increases as the engine power increases. An increase in airspeed of an aircraft results in an increment in lift and this lift continues to build up and eventually the aircraft is airborne. The pilot at this point will continue to ascend his or her aircraft before levelling off.

The relation between airspeed and lift is given the lift formula,

Where C_L is the coefficient of lift (stated in NACA), r is the density of the air, V is the airspeed and A, the aspect ratio

3.4.3 Wing Area and Shape (Aspect Ratio)

There are many types of aerofoils of various shapes as stated by NACA and of various length. The aspect ratio (length of the aerofoil multiplied by the cross-sectional area). The greater the aspect ratio, the greater the lift. The factors are determined and fixed by aeronautical engineers.

3.5 Stalling

As mentioned in section 3.3, lift continues to increase as the angle of attack increases. However there comes a point in every NACA aerofoil at which the lift no longer appears to increases and this point is known as the stalling angle. Different aerofoils have different stalling angles and it very much depends on the aircraft¡¦s role and aerodynamics needs. A commercial aircraft like an airborne 747 should not stall at any cost as loss of lift may cause severe structural damage, however a F-14 USAF (United States Air Force) jet may stall at tight manoeuvres. Manoeuvre is a technical term used to describe the movement behaviour of an aircraft.

Most aircraft have built in alarms to tell the pilot if the aircraft is approaching the stalling angle. Some rely of pre-entered aeronautical values and while sophisticated aircraft rely on the movement of the separation bubble. The separation bubble moves towards the leading edge as the angle of attack is increased.

Most low speed aircrafts, stalling takes place at 15^o and this angle does not vary. Airspeed does however affect the stall and this is known as the 'Stalling speed'. For example, if a pilot in a level flight decides to switch off his aircraft¡¦s engines while flying with normal load - then continues to maintain the same lift by steadily easing the control column back (increasing the angle of attack), the aircraft will eventually stall.

3.6 Factors affecting stall

3.6.1 Weight

An increase in weight increases the stalling speed.

3.6.2 Engine Power

The higher the power used, the lower the stalling speed and vice-versa.

3.6.3 Flaps

With flaps lowered, the stalling speed lowers. Flaps increase the aerofoil camber.

3.6.4 Ice / Damaged Wings

Both ice and damaged wings reduce lift and increase the stalling speed.

3.6.5 Manoeuvres

In tight manoeuvres, stalling speed increases.

Do remember that stalling can occur at any attitude and it is the angle of attack that determines the occurrence of a stall.

4. Thrust

Aircrafts have engines which provide enough power to lift them from the ground. An aircraft will only take off if the lift is greater than its weight and once airborne, the power generated by the engines is re-deployed for forward motion and other manoeuvres. This forward momentum experienced during take-off, ascent flight, level flight and other manoeuvres is known as 'Thrust'. Thrust is well covered in aerodynamics topic 'Powerplant'. Powerplant is the study of engines, engines maintenance/design and electrical/electronic equipment power distribution in an aeroplane.

5. Drag

With thrust, we get a 'negative' force ¡V drag. Drag is created as a result of air resistance caused by the shape of aircraft. Various parts of an aircraft such as wings, fuselage, tail unit, undercarriage, engines and other parts contribute to drag.

The total drag of an aircraft is made up of three parts:

5.1 Form Drag

This type of drag is created by the shape of the aircraft. 'Form' means shape. If the shape of an aircraft is very flat and big, it will experience a lot of form drag. Form drag reduces the fuel efficiency of an aircraft. Aeroplane designers, better known as aeronautical engineers reduce form drag by streamlining the aircraft¡¦s body. Streamlining is a lengthy process which involves designing aeroplanes which allow for smooth and fast airflows around its body. In streamlining, protruding parts are removed or embedded in the aircraft to minimise air resistance that leads to form drag.

A good streamlined shape has a fineness ratio of about 4 (length) to 1 (breadth).

Form drag increases with air speed - double the airspeed, four times the form drag (approx.)

5.1.1 Skin Friction

This type of drag is caused by the roughness of aircraft surface and viscosity of air. When air flows over the surface or the 'skin' of an aircraft, there is always a narrow layer of air close to the skin which flows beneath the main airflow. This layer can be imagined to be composed of many very thin layers. The thin layer immediately next to the skin is 'stationary', the thin layer above it has some movement, the next layer more, and so on until the top layers which travels at the same speed as the main flow. Air has viscosity (stickiness) and each layer slows up the layer above causing a drag known as 'Skin Friction'. The collective term for all the layers is ¡¥Boundary layer¡¦. If the flow in the boundary layer is smooth, we call it 'Laminar flow' and if the flow is choppy, then it we call it 'Turbulent flow'. When the flow is turbulent, the thickness of the boundary layer increases and causes more drag than a laminar flow.

Skin friction is reduced by polishing all the surface over which airflow passes. The better the polish, the less the skin friction. In sophisticated aircrafts, there are suction devices and suck turbulent air in and out of an aircraft.

5.1.2 Induced Drag

An aircraft produces lift because of pressure difference between the upper and lower surfaces. On the upper wing surface, there is a region of low pressure and on the lower wing surface, there is a region of high pressure and further away from the wings, we have the constant atmospheric air pressure. As the aircraft flies, surrounding atmospheric air rushes inwards over the upper wing surface and outwards beneath the lower wing surface. As a result, two streams of air moving in opposite directions are formed over the upper and lower surface of the wings. The two streams eventually meet and rotate at the trailing edge and weak 'Vortices' are formed. Vortices are circular airflow, very similar to a wine corkscrew. The weak vortices get attracted to the strong vortices and add up to form two main vortices at wing tips. These two main vortices are known as 'Wing tip vortices'. The wing tip votices disturb the airflow aft of the wing and a lot of energy is waster in this air disturbance. The loss of energy results in what we call 'Induced Drag'. Induced drag is reduced by using tapered wings or elliptical wings.

6. Aircraft Controls

So far what we have learned are key forces that act upon an aircraft and now we shall focus our attention to "Aircraft Controls". Controls are either mechanical or electrical devices that allow a pilot to alter an aircraft's course by controlling the forces that act on it. Before we can understand how aircraft controls operate, we first need that to know that there are six main movement co-ordinates in an aircraft.

An aircraft can move up and down - this movement is known as "Pitch".

An aircraft can roll from left to right ¡V this movement is known as "Roll"

An aircraft can move from left to right ¡V this movement is known as "Yaw"

To control this six movements, we have various control surfaces on an aircraft.

6.1 Elevator

An elevator is situated in the tail section and it is used to create the pitch movement. It has two surfaces on the left and the right as shown below. The elevator is controlled by the pilot with the help of a control column known as 'stick'. The 'stick' is usually located between the legs of a pilot.

When the pilot moves the stick backwards, the elevator moves upwards. This reduces the effective camber (less lift) and the aircraft nose begins to move upwards. Vice-versa, when the pilot moves the stick forward, the elevator moves downwards. This increases the effective camber (more lift) and aircraft nose begins to move downwards. (Use two fingers and hold a pencil somewhere near its end and use the other hand to move its rear end up and down. Notice what happens to the front)

6.2 Rolling

Ailerons are situated in aircraft wings and are used to control the rolling movement. Like elevator, there have two moveable surfaces on the left (portside) and the right (starboard) as shown below. However, the movement of the aileron is different from that of elevator.

When the pilot moves the stick to left, the left aileron moves upwards and the right aileron moves downwards. More camber (more lift) is created on the right wing compared to the less cambered (less lift) left wing. The force on the right wing makes the aircraft roll to its left.

When the pilot moves the stick to right, the left aileron moves downwards and the right aileron moves upwards. There is now less camber (less lift) on the right wing compared to the more cambered (more lift) left wing. The force on the left wing makes the aircraft roll to its right.

Do remember that in some aircrafts as in our example, both pitch and roll movement can be controlled by a single control column - 'stick'.

6.3 Yawing

Rudder is situated in the tail section and is used to control the yawing movement. There is only one moveable surface and it is controlled by the pilot using the rudder 'foot' pedals.

When the pilot moves his left feet forward, the rudder moves to the left. There is an effective increase in camber on the left side (more sideways force) and this force, pushes ¡¥yaws¡¦ the aircraft nose to the left about its normal axis. The aircraft continues to yaw in this direction until the pilot returns the rudder pedals to their central position.

When the pilot moves his right feet forward, the rudder moves to the right. There is an effective increase in camber on the right side (more sideways force) and this force, pushes ¡¥yaws¡¦ the aircraft nose to the right about its normal axis. The aircraft continues to yaw in this direction until the pilot returns the rudder pedals to their central position.

6.3 Trimming Tabs

6.3.1 Moveable tabs

Nearly every aircraft experience some sort of load changes and these happen due to many reasons. One common reason is the consumption of aircraft fuel. For example, with a full tank (fuel), an aircraft will have a fixed load balance as the fuel is used up, its load balance will change. Eventually, the aircraft will start to experience forces on it due to these changes in the centre of gravity. Assuming that the fuel tanks are located near the rear end, and aircraft nose will begin to dip as the centre of gravity moves forward. At this stage, the pilot will pull the stick backward to keep the nose level. It can be extremely tiring for the pilot as he or she has to maintain the straight level flight by continuing holding the stick. This is where the trimming tabs come in use. Trimming tabs are 'mini control' surfaces that are embedded into the main control surfaces. By adjusting these controls, the pilot can relieve himself from the main controls and keep the aircraft in the desired path. Trimming tabs are only used for 'fine-tuning' aircraft movements and are not used to move the aircraft drastically.

Trimming tabs are found in the rudder and these are called 'Rudder trimming tab' and they are adjusted by the pilot in event of minor offset yaw movements. Trimming tabs are also found in the elevator ¡V "Elevator trimming tab" - and this is adjusted by the pilot for offset pitch movements. Last but not least the ailerons are also equipped with trimming tabs for preventive offset rolling movements. Aileron trimming tabs are known as 'Aileron trimming tab'. The three aforementioned tabs are adjusted by three separate trimming controls which are mounted directly on the cockpit panel. The pilot adjusts can use any three controls in flight to 'trim out' extra forces.

6.3.2 Fixed tabs

Some light aircrafts especially old ones have tendency to fly "one wing low" and this tendency can be cancelled out by "fixed tabs". Unlike moveable tabs, fixed tabs must be adjusted on the ground prior to flying. The exact setting is determined by trial and error during one or more test flights.

6.3.3 Balance tabs

These tabs are not under the direct control of a pilot and are deployed automatically during flight. When a control is moved one way, the balance tab moves in the other direction. The airflow acting on the tab assists the movement of the control surface, thereby reducing the effort required from the pilot.

6.4 Flaps

Do you remember when we talked about camber effect and lift in the earlier sections? If yes, you will know how this camber effect can affect an aircraft landing and taking-off characteristics. No fixed length wings can provide a same high performance while landing and taking-off. Usually wings of this type provide good performance while take off and an average performance while landing or vice-versa. Performance measurements taken during landing and take-off are extremely important as they can greatly affect the fuel efficiency of an aircraft. To combat this problem of fixed length wings, aeronautical engineers came up with the 'flaps' solution.

Flaps are hinged surfaces which are fitted to the trailing edges of the wings ¡V inboard of the ailerons. During take off, flaps are project outwards to increase the overall camber of the wings. This increment provides extra lift to an aircraft and in turn reduces the take off distance. During pre-landing, flaps are employed to reduce the stalling angle and landing distance. There are many types of flaps and each flaps have different landing and take-off characteristics. In general, a flap deployed at 15^o degree angle shortens the take-off run due to lift increment. A flap deployed at 30^o degree angle provides an increment in lift while take-off and equally an increment in drag while landing. A 60^o degree deployed flap does not produce much lift but does provide a great deal of drag, which is ideal for steeper and slower landing conditions. Lastly and rarely, 90^o degree deployed flap will only induce large amount of drag with little lift. Do remember that drag is not always evil as imagined.

Drag provides braking effect while landing and reduces the touch-down speed.

6.5 Slats

Like flaps, they are small auxiliary hinged surfaces. They are however located at the leading edge of an aerofoil unlike flaps. They are automatically deployed with flaps but in some aircrafts, they may be controlled by a pilot. Slats are used to delay tall and gain extra lift at low speeds. Stalling takes place at a fixed angle and soon after it is exceeded, turbulent flow forms on the upper layer of the wing. Prior to stall angle, a slat is deployed automatically to delay the oncoming stalling. Lets look this in detail.

As we approach stall, the centre of pressure builds up near the upper leading edge of the wing. This pressure automatically moves the slat forward. When the slat opens via a spring loaded mechanism, ¡¥imminent turbulent¡¦ airflow rushes through the gutter to form smooth airflow. This effectively increases the stalling angle. When the aircraft reverts to the normal attitude, the centre of pressure moves backwards pulling the slat inwards along with it to its original position ¡V closed slat.

Slats and flaps are usually interconnected and are operated from the cockpit. In interconnected slats and flaps, both the flap and the slat move simultaneously. During lift-off, the leading edge slats are operated to the fully open position and at the same time, the flaps are deployed to 20^o degrees. The end result is a shorter take-off distance. While airborne, both slats and flaps are closed and trimming tabs are used. During low speed and steep glide landing, both slats and flaps are fully extended.