How is human flight possible? How do all those aeroplanes in the sky above us get there and what keeps them up there. In this article we will discuss that how this interesting machine takes flight. Understanding how airplane flies require us to familiarize ourselves with the forces that act on them as they slice through the air. There are four of these forces that act on an aeroplane while in flight four forces are lift, thirst, weight and drag. The natural forces are the weight of the plane or the force of the gravity and wind resistance or drag which is a form of fraction exerted by different gases molecules in the air.
Modern aeroplanes are truly engineering marvels surmounting these effects requires forces known as lift and thrust. Each of which is delivered by a unique piece of technology found on any aeroplane. They are heaviest, loudest and perhaps the most familiar part of the flight experiences the engine. They overcome highly turbulent and unpredictable current in the air and complete their flights by undertaking many complex manoeuvres.
Design of the aeroplane:
One interesting thing we will notice is that they are not made as a single solid piece. The wings and tails of the aeroplane have many movable parts. The most fascinating thing about the whole wing and the different part of it is that it forms a very special shape in fluid mechanics that is the air foil shape. Just by understanding the physics behind this simple shape will allow you to completely understand aeroplane physics.
We might have learned that the curve shape of an air foil produces a lift force when moved relative to the air which gives ability to float upward air first diverges at the front of the wing part of it rolling over the curved section and the rest along with the straight edge because the distance between the two points. In this case the front and back of the wing is a straight line air along the bottom tends to want to travel faster so to catch up air molecules above the wing have to travel even more quickly to meet with the same air packet that diverged at the front of the wing. Fast moving air above creates low pressure while slower moving air below creates higher pressure so the overall effect is upward movement. The mechanism seems to make intuitive sense the thing is it is not quite right. There is no physical law that dictates the air molecules have to meet up after being curved apart by an aeroplane wing but shape does play integral part in flight. Air tends to want to move in a straight line the curved portion of the air foil however deflects air such as the same number of the air molecules are smeared over large volume then those travelling along the bottom edge this creates a pressure differential low pressure above and high pressure below and viola that creates the lift .
This lift force makes an aeroplane fly. Now we will discuss how this lift force is produced. The air foil produces downwash which causes a pressure at the top and bottom of the air foil and hence produce air lift. Generally higher the angles of attack the greater will be down wash and therefore the lift force. A greater air speed also increases the lift force significantly.
Interestingly in mankind first successful flight the Wright flyer also made use of this same air foil principle. Even though their air foil were in simple curved shape. It was sufficient to produce a good down wash.
How to create lift:
Basically the aeroplanes fly because it has wings. Now we will discuss what the wings actually doing which causes an aeroplane to fly. So firstly we need a wing whose shape is given below
The cross section of an aircraft wing also called an air foil. One of the most common bits in correct theories which is taught something called the longer path or equal transit theory which is based on the Bernoulli principle so the explanation of this theory gives is that the airflow the air which is hitting the wing split as it hit the front of the wing. The air passes over and under the wing and re-joins at the back of the wing at the same time. So for this to happen the air flowing over the top of the wing has further distance to cover over the curved surface therefore it speed up as the air speeds up it causes an area of low air pressure above the wing. The air which is passing underneath the wing slow down because it has shorter distance to travel and this is causes high air pressure below the wing. The end result this theory states that the pressure difference above and below the wing which is causes the wing to move upwards and this upwards force is commonly called lift. Now there are few aspects of this theory correct but it does not tell us the full story. Lift is created because a wing or air foil is turning the airflow and directing it downwards. It is not a pressure difference which creates the lift. The airflow hits the front of the wing and splits the same as before also Bernoulli was correct when he talked about the difference in pressure above and below the wing and he was also correcting saying that the air which flows over top of the wing speeds up now the air which passes over the wing it kind of sticks to the top of the wing and follows the curve of the upper surface. This sticking phenomenon is known as the Coanda effect as the airflow leaves the wing at the back it leaves a slight downward angle because leaves a slight downward angle because of that curve that downward deflection also affect the air which passes under the wing and also turn it in a downward direction. It is the turning of the airflow which causes lift we can generate more lift in two ways. The first is by increasing the speed at which the air passes around the wing this is generally attained by simply flying faster.
We can also increasing something called the angle of attack. This is the angle of the wing relative to the air flow. So in this example we can see that the wing is pretty much pointed directly in to the air flow. However if we increase the angle of attack we can see that wing is tilted backwards this creates more lift because the airflow is turned downwards at a sharper angle. Now there is a danger to increasing the angle of attack if we increase that angle too much there will come a point where the airflow cannot turn and follow the top of the wing. When this happens the airflow will separate from the top of the wing. This is what happens when an aircraft stalls. There is no steady flow of air over the wing therefore the wing stops producing lift and the plane begins to literally fall out of the sky. Now there are couple of devices used on aircraft which help increases the amount of the lift that wing can generate. The main we have flaps the flaps on any craft are designed to increase the amount of the lift that a wing can produce by increasing the surface area of the wing. This allows a plan to take off and land at slower speed yet still produce enough lift for safe flight. The downside to flaps is that they create a lot of aerodynamic drag or we could think of it as wind resistance. Normally flaps are constructed and integrate in to overall shape of the wing then they extend downward at the back of the wing. This increases the curvature for airflow around the wing on modern aircraft we can likely see something called a slotted flap. When extended these have a small gap between the wing and the flap itself where the air can pass underneath the main wing through a small slots and over the flap. This helps the airflow over the main wing stay attached at higher angles of attack.
Now if we want to solidify to understanding of flaps when we look at the aircraft flaps we notice that the flaps are essentially mini wings. They have similar shape and structure to the main wing.
Now another device which is commonly found on the commercial jets is the leading edge slats this device sits over the leading edge or the front of the wing and can extend forward to increase the surface area of a wing again causing the wing to produce more lift at slower speeds. These can also have slots where air passes underneath the leading edge slats and then flow over the main wing as with flips these can be retracted to reduce the amount of drag or wind resistance allowing a plane to fly faster. Now there is one more device which is normally integrated in to a wing however its purpose is to reduce the amount of the lift that wing produces. These are spoilers so these are normally flats panels which extends up from the top of a wing they are called spoilers because they spoil the airflow over the top of the wing and reduce the amount of lift it produces. These are most often used on landing to ensure that an aircraft touches down safely and to prevent it from taking off again. They also have a secondary effect that they create a lot of drag or wind resistance assisting the plane with slowing down to a safe taxi speed on the ground. It may also be used in flight to help a plane descend faster however they will be only partially extended. The main issue with this is that they create a lot of turbulence error which causes the wings and the plane to vibrate. We also get a sort of rushing wind noise so both of these effects can be uncomfortable for passenger.
The force of the thrust upon the air craft forward this is created by the propellers so we can say that lift and thrust are two forces helping the airplane fly.
While the other two forces which contract these two those force would be weight and drag.
The material used to build this aircraft along with the pilot and baggage known as payload in the feel way is gross weight along with the gravity acting as weight pulling down the on the aircraft. In order for an aeroplane to fly the lift generated by the wings must be equal to or greater than the weight of the air craft. If the aircraft is over gross weight and the wings cannot produce enough lift to get off the ground. The results can be disastrous.
Opposite of the thrust is drag which is simply crated by the aircraft existence an object natural desire to resist moving through fluid such as air or water is called drag. The effect of the drag can be best be demonstrated by putting a camera on the tail of the aircraft. The camera will be shaking pretty violently this is because of the air pushing against the camera. The camera naturally wants to flow with the air and fall away from the air craft.
The aeroplane engine is an engineering marvel all its own the engine that powered the famous 1903 flyer designed by the wright brothers featured its custom built 12 horse power gasoline engine. It was essentially a powered bike chain that drove twin propellers. Today turbo jets engines are far cry from their predecessors weighting up to 8000 pounds and generating more than 30000 horse powers. Both however accomplish the same job to provide a force that thrusts airplanes forward on take-off and once they are in the air the sheer size and sound of today jet engines might make it feel like they are doing all the work to get us and our fellows passenger on ground but that forward motion is only part of the flight equation. The upward force required to lift the plane in to the air is provided by the plane wings.
Most jet propelled airplanes use a turbo fan design. The turbofan can be taught of as a high tech propeller inside of a duct called a diffuser driven by a gas generator. The core of a jet engine is a gas generator that creates high pressure gas to power a turbine. This setup has compressor, combustor and turbine sections.
Compressed air makes for much more powerful combustion reaction relative to engine size. Compression happen in stages that force incoming air in to an increasing narrow chamber a single compressor stage is comprised of a spinning rotor paired with a ring of stationary stator vanes which are attached to the core casing. Rotor blades swirl the air as they force it through the compressor. Stator vane slow this swirling momentum in exchanging for increased air pressure. This compressor has four low pressure and ten high pressure stages.
In the combustor air is mixed with fuel and ignited as it passes through the combustor releasing a jet of super high powered gas. Compressed air enters the inlet nozzles. Each nozzle is coupled with a fuel injector and is designed to swirl with the incoming fuel and air for an even mix. A couple of ignitor plug not like the spark plugs found in car engines ignite this mixture and the reaction spreads evenly around the ring. Once started the combustion continues as long as air and fuel are supplied
The turbine as the rear of jet engine is powered by exhaust gases exiting the combustor. Much of the turbine power is used to turn the fan while a smaller percentage powers the compressor stages. Turbine fins get extremely hot some air from the compressor is diverted for cooling and special coating are used to keep temperature down.
The exhaust cone is specially shaped to mix and accelerate exhaust streams. It also covers sensitive internal engine parts.
Early jet engines were turbo jets where all incoming air flows through the core. Most modern winged aircrafts engines are turbofans where only a fraction of air enters the core or gas generator and resulting power turns a special designed fan. Again the fan can be thought of as high tech propeller inside of a duct. Air that does not enter the core is called bypass air. High bypass engines are designed to move large quantities of air at slower cruising speed a range of about 310 to 620 mph. The exchange for high efficiency is engine size high bypass engine can be very large with massive fans compared to core size. Commercial air lines or military transport air craft are example application.
Exhaust velocity is a major factor in engine noise. High bypass engine surround fast moving core exhaust with large quantities of slower moving bypass air for quieter operation. Military fighter aircraft use low bypass engines which are more compact have high power to weight ratio, supersonic and after burner capabilities in exchange for things like poor noise control and high fuel consumption.
High performance engine may have afterburner capability. Additional fuel is sprayed in to a jet pipe section where it mixes with exhaust gas and is ignited, producing a stage of combustion. Since afterburner is fuel inefficient it is generally used in short bursts during take-off, climb or combat manoeuvres. The exhaust nozzle is adjustable for maximum exhaust acceleration and to avoid undesirable back pressure which can harm forward engine parts.
Horizontal Stabilizer and Elevator:
The horizontal stabilizer is designed such that it can be deflected to a certain angle.
The elevator which is also installed on the horizontal stabilizer will deflect based on inputs from the pilot or the autopilot the movement of the horizontal stabilizer depends on some additional factors to understand this we need to look at what is meant by trimming an aircraft?
An aircraft trim refers to an aircraft maintaining a particular attitude such as straight and level flight a constant climb or a constant descent without any control surface inputs.
This is achieved by deflection of a secondary flight control such as trim tabs or the trimmable horizontal stabilizers. These trim surfaces reduce the pilot’s work load assist in flying the aircraft smoothly and reduce the stresses acting on the primary flight controls such as the elevator. So when the aircraft is in the crew’s face the trim surfaces are adjusted so that the aircraft maintains that altitude if the aircraft was climbing the trim surfaces may be adjusted so that the aircraft maintains a constant rate of climb and if the aircraft was descending the trim surfaces may be used to achieve a constant rate of descend this is called as pitch trim. There are other trim surfaces installed on the aircraft in order to maintain a constant yaw angle or a constant bank angle. Now let’s see what is a trimmable horizontal stabilizer and when it is used the THS is mainly used to trim an aircraft such that it maintains a particular attitude as seen before the THS will be effective over the entire operating speed of that aircraft and will be available from takeoff to landing. The THS will be deflected up or down depending on the inputs from the pilot or the autopilot the pilot may adjust the THS by a control wheel in the cockpit and the autopilot will adjust the THS as required depending on the phase of flight if the THS is deflected downwards the aircraft will maintain a nose up attitude and if the THS is deflected upwards the aircraft will maintain a nose down attitude another advantage of the THS is that it reduces the drag acting on the tail section of an aircraft because of elevator deflection so whenever the elevator is moved for trimming the aircraft the THS is also moved to the same angle to reduce the drag the THS is also helpful when the flaps are extended the flaps are called as high lift devices since they increase the lift generated by the wings if the lift increases to make the aircraft longitudinally stable more downward force should be generated at the tail section the elevator alone cannot generate this downward force continuously as it may be required for pitch up or pitch down maneuvers so the THS is deflected to create more downward force at the tail section instead of the elevator usually the THS movements are controlled by the autopilot through the pitch control computers but the THS can also be controlled manually using a trim wheel in the cockpit this would have more priority and is linked directly with the THS the wheel should be operated carefully because even small movements on the THS will result in severe changes in the pitch attitude of the aircraft normally the trim wheel is adjusted before takeoff depending on the aircraft’s loading after takeoff the autopilot takes control of the THS.
Now the vertical stabilizer of the air craft is given in the figure which will create a movement called yaw. Now the vertical stabilizer which include rudder will help for the yaw movement.
Now we will check that how this vertical stabilizer will helpful for the stability. There are two types of air foils which are:
- Symmetrical air foil
- Cambered air foil
When there is zero angle of attack symmetrical air foil will create no lift and when there is positive angle of attack it will produce lift. Now as shown at the top view of the air craft the cross section vertical stabilizer which symmetrical air foil. In relative air it will not create angle of attack so there is no unbalance force.
When there is disturbance in the wind it will create angle of attack and lift will be create. This will create either in clockwise or anticlockwise force. With the help of vertical stabilizer it will come to original position.
Angle of attack:
It is important to understand what the angle of attack is as it directly relates to the lift production and stalling characteristics of an aeroplane take the cross section of a typical wing when you slice the wing precisely in half from front to back you get the mean camber line along this line the thickness of the top half of the wing matches the thickness of the bottom half the point. Where the mean camber line starts is called the leading edge of the wing and the other end is called the trailing edge when you connect these two points together you get the cord line.
In flight the error plane passes through air particles in the atmosphere. These particles form a common path that indicate the relative motion between the aeroplane and the atmosphere this is called relative airflow and it acts in the opposite direction to the flight path direction of the aeroplane. The angle of attack is the angle between the chord line and the relative airflow the angle of attack changes as the attitude of the aeroplane changes for example the angle of attack changes if the aeroplane pictures up and down in level flight in a steady climb. However the relative airflow is no longer horizontal but assumes the direction of the arrow planes flight path even if the noise is pointing higher up the angle of attack may be the same as if it was in level flight similarly in a descend the direction of the relative air flow assumes the arrow planes flight path direction towards the ground but the angle of attack remains the same.
Fuselage is normally a semi monocoque design monocoque derives its name from the French word single shell. In a pure monocoque structure the skin is responsible for all the loads placed upon it the shape provides its strength unsurprisingly given its name eggs are an excellent example of a monocoque structure.
However as transport aircraft became larger with increasing loads the pure monocoque structure lacked sufficient strength thus a semi monocoque structure was introduced a semi monocoque structure is fitted with additional structural members known as stringers. These stringers run lengthwise along the fuselage joining the frames together this enables the construction of a much more robust assembly. The purpose of the stringers is to provide rigidity to the skin and carry the load along its length the skin is attached to the frames and stringers by riveting or bonding cutouts are made to provide access panels and passenger windows. These are reinforced by doublers or backing plates to summarize a jet transport aircraft requires doors and windows and access panels thus making the purely monocoque design impractical this is why a semi monocoque design is preferred.