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Forces Acting on an Airplane in Flight
There are four forces acting on an airplane in flight. These four forces are called thrust, drag, lift and weight.
1. Thrust: The force exerted by the engine and the propeller(s) which pushes air backwards with the object of causing a reaction, or thrust, in the forward direction.
2. Drag: The resistance to forward motion directly opposed to thrust.
3. Lift: The upward force that sustains an airplane in flight.
4. Weight: The downward force, due to gravity, directly opposed to lift.
When thrust and drag are exactly equal, the airplane is in a state of equilibrium. This means that the airplane is not speeding up or slowing down. This can happen when the airplane is on the ground not moving (no thrust, no drag, so the forces are equal), or when the airplane is flying in the air, but not accelerating or decelerating. If thrust is greater than drag, or if drag is greater than thrust, the aircraft is not in a state of equilibrium. When thrust is greater than drag, the aircraft will accelerate. When drag is greater than thrust, the aircraft will decelerate.
Think of being in a car, driving down the highway. If you (or your mom) give the car alot of gas, it will speed up rapidly. This is acceleration, and the thrust of the car is greater than the drag. If you tke your foot completely off the gas (take away the thrust) the car will slow down, especially if you are going up a hill. This is deceleration, and the drag acting on the car is greater than the thrust. If you are driving down the highway at exactly 100km/h, not slowing down or speeding up at all, then the car is in a state of equilibrium. There is thrust and there is drag, but they are equal, and they cancel each other out.
Thrust and Drag are a "couple", they are paired together because they take effect on each other. The other couple is Lift and Weight.
Lift and weight are also opposite to each other, just like thrust and drag. If they are equal then the plane will not climb or descend. If lift is greater than weight, the airplane will climb. If weight is greater than lift, the airplane will descend.
It is important to understand that none of these couples is ever constant. Throughout a flight they are constantly changing. If the pilot adds extra power and accelerates, then thrust is greater than drag. At the same time, more lift will be created by the extra speed (you will soon understand why) and the airplane may climb (lift is greater than weight). As the pilot approaches to land, he may take off power, the airplane will decelerate (drag is greater than thrust) and as it slows, less lift will be produced so the airplane will descend (weight is greater than lift).
Lift
An airplane has wings that are specially designed so that when they move through the air they produce a reaction with the air called lift. This special design is called an "airfoil". An airfoil is any surface designed to obtain a reaction from the air through which it moves (lift). The best shape for doing this is a curved shape, which we call "cambered".
The camber of an airfoil is the curvature (rounded) of the upper and lower surfaces. Usually the top of the airfoil is more cambered than the bottom.
How is Lift Created??
The scientist Sir Isaac Newton (the guy who supposedly had an apple fall on his head) discovered several Laws of Motion that are important to understanding how lift is created.
Newton said (basically):
1. Things that are moving like to keep moving, and things that are still like to keep still.
Ever notice that it is hard to start pushing a heavy box across the floor, but once you start it moving, it seems easier? Then you have experienced this law.
2. To make an object start or stop moving, a force must be used to make it stop or start.
You are the force that pushed the heavy box across the floor, and friction is the force that made it stop moving. If no force acted on it, the box would stay there forever.
3. If you apply a force to an object you will get a reaction that is equal and opposite.
This law is probably hardest of all three to understand. Think of a gun like you have seen in movies. If the actor shoots the gun, the gun recoils, or moves backwards in his hands. If you have ever fired a shot gun, you know that the gun will push backwards into your shoulder when you pull the trigger. The bullet moves forward, and the gun moves backwards. The force is applied to the bullet to make it go forwards, so an equal and opposite reaction happens backwards, making the gun recoil (jump backwards).
So what does all this have to do with lift? Everything, even air follows these rules. Air that flows around an airfoil has motion, and like Newton's First Law saws, it wants to stay in motion. The airfoil is where Law Number Two comes into play. The airfoil is the force that changes the way the air is moving. The airfoil changes the direction that the air is flowing, a lot like a pipe changes the way water flows. This change in direction causes an equal but opposite reaction (Newton's Third Law) which is lift!
As air goes over the leading edge of the wing (front end) towards the trailing edge (back end), the air goes backwards and down. This is called downwash. The bottom of the wing guides the air downwards. Wings work in a lot of ways like a surf board. The surf board pushes down on the water, and the water pushes up on the surf board (Newton's #3).
The second way that an airfoil produces lift is explained by a principle (law) called Bernoulli's Principle. This principle basically says that if one part of a reaction gets bigger, the other must get smaller, and vice versa. What do I mean? Imagine that you are going running. If you run slowly, you will have more energy and be able to run farther, so:
slow=farther
But if you run as fast as you possibly can, you will probably only be able to run a very short distance, so:
fast=shorter
Air works in the same way, except we don't use speed and distance. Instead, we use speed and air pressure. Air pressure is a little like weight. You will understand better what air pressure is when we study the meteorology section. For now, imagine a balloon filled with air. If the air inside the balloon has a low pressure, or decreased pressure, it will go up. If the air inside the balloon has a high pressure, or increases pressure, it will go down. The wing of an airplane works the same way, except it is not filled by air, the air flows around it.
It is the special way that the wings make the air flow around them that creates lift. Remember that "cambered" (curved) shape that we mentioned earlier? That shape forces the air that flows around the wing to travel at different speeds. Because the air travels at different speeds, the pressure changes, and lift is created. The top surface of a wing usually has more area than the bottom surface, due to the camber. If you were to take a string, and stretch it from the leading edge to the trailing edge, first over the bottom of the wing, and then over the top of the wing, you would find that the top side of the wing is longer than the bottom side.
As the leading edge meets the air, it splits the air into two: the part that flows over the top of the wing, and the part that goes under. Imagine two tiny molecules of air at the leading edge of the wing. One is forced up and over the top of the wing, and the other along the bottom surface of the wing. Due to the laws of nature, these two molecules of air must meet again at the trailing edge at the exact same time.
Imagine that you live five blocks from school, and your friend lives three blocks from school. If both of you leave home at 8:00am, and you must both arrive at school at 8:15am, who has to travel faster? You do, because you have a farther distance to go in the same amount of time as your friend. Air works in a similar way.
The two air molecules must arrive at the trailing edge of the wing at the exact same time, and because the air that travels over the top of the wing has a farther distance to go, it must travel faster than the air flowing along the bottom of the wing.
Remember Bernoulli? He said that if one part of a system increases, the other part must decrease, and if one part of a system decreases, the other part must increase. In aviation, the two parts of the system are speed and pressure. About speed and pressure, Bernoulli said:
"As speed increases, pressure decreases, and as speed decreases, pressure increases."
For airplanes, this means that where the air is moving faster (along the top of the wing) there will be lower pressure, and where the air is flowing slower (along the bottom of the wing) there will be higher pressure. The wing is forced upwards by the high pressure under the wing, into the lower pressure above the wing. Presto, we have lift!
Definitions:
Relative Airflow: As a wing flies forward through the air, the air passes around the wing. Imagine a car parked in a parking lot on a breezy day. The wind may be coming from any direction, and will change direction over time. Once the car starts moving, thw ind seems to be blowing from the front of the car to the back of the car, parallel to the direction the car is driving, but opposite. This is relative airflow, the wind that is created when the airplane is moving. Relative airflow is always parallel, but in an opposite direction to the flight path or direction of travel. If a airplane wing is moving forwards and downwards, then the relative airflow is upward and backwards. If the wing is moving forward horizontally, the relative airflow is moving backwards horizontally.
Angle of Attack: The angle that the airfoil meets the relative airflow is called the angle of attack. As the angle of attack increases, the difference in pressure between the top and bottom of the wing continues to increase, creating more and more lift. However, the angle of attack can only increase so far. If the angle of attack becomes too big, the wing will stall, and lift will decrease and drag will increase.
Centre of Pressure: The centre of pressure is the resultant of all the forces of lift. This means the balancing point. You can balance a pencil on your finger if you find the exact point where all the forces are balanced. The point along the chord line where all the pressures (lift) are balanced is called the centre of pressure. As the angle of attack of an airfoil is increased up to the point of stall, the centre of pressure will move forward. Beyond this point, it will move back.
Weight
Weight is caused by gravity. The weight of an airplane is the force that acts straight downwards to the Earth's surface. The airplane has a balance poit called the centre of gravity, which is much like the centre of pressure, except that it is the point through which all weight acts, not pressure.
Thrust
Thrust is what allows the airplane to move forwards. Propellors are a common way, but some airplanes may also use jets or rockets to create thrust. Each of these devices forces air backwards....Remember Newton's 3rd Law? (If you apply a force to an object, you will get a reaction that is equal and opposite) The reaction to the air being forced backwards is that the airplane is forced forwards.
Drag
As an airplane moves through the air, the air resists the movement a little bit. This force is called drag. Drag is a type of friction. If you try to drag a large heavy box across a carpet, it will be difficult, because the box creates alot of friction with the carpet. If you try to drag the same box across a skating rink, you will be better able to move the box because it creates less friction with the ice than with the rug.
Drag comes in two main types:
1. Parasite Drag
2. Induced Drag
Parasite drag is caused by all the parts of the airplane which don't help create lift. For example, the landing gear do not create lift, nor the struts, or fuselage. Parasite drag can be further divided into two types:
a. Form Drag
b. Skin Friction
Form drag is the drag created by the shape of an object as it moves through the air.
Skin friction is caused by some of the air sticking to the surface of the object that it is flowing over. Air especially sticks to dirty areas.
Parasite drag can never be completely destroyed, but many things can be done to avoid having a lot of form drag.
-landing gear can be made retractable
-wing struts are replaced by fully cantilevered wings (wings that support themselves, with no struts)
-skin friction is reduced when the airplane is clean and waxed.
The second main type of drag, called induced drag, is caused by all parts of the airplane that DO produce lift, such as the wings. The only way to reduce induced drag is by designing the aircraft to be very streamlined. A wing with a high aspect ratio (long span and narrow chord, so the wing is very long and thin) produces less induced drag than a wing with a shorter span and a wide chord (a short, fat wing).
Induced drag is proved by a phenomenon called wing tip vortices. Wing tip vortices occur because of the differences in pressure between the top and bottom of the wing. Air always flows from areas of high pressure to areas of low pressure. Because there is low pressure on the top of the wing, and high pressure on the bottom of the wing, air tends to flow around the tips of the wings, from the bottom to the top. (see diagram, below)
Induced drag is worst at low speeds, close to the stalling speed of the aircraft. This often occurs when the airplane is close to the ground for landing or taking off. When the airplane is close to the ground, the swirls produced by the wing tip vortices are blocked by the ground, and instead of swirling around the wing tips, hit the ground. When the air is not able to swirl from the bottom to the top, induced drag is decreased.
Other parts of the airplane also produce drag. Ailerons produce drag when they are moved to make an aircraft turn. The aileron that moves downwards into the air of higher pressure on the bottom of the wing creates more drag than the aileron that moves upwards into the air of less pressure on the top of the wing. The extra drag on the down-going aileron causes a yaw in the opposite direction of the bank. Two designs of ailerons attempt to reduce the drag that ailerons create. The first is called "differential ailerons". In this type of aileron, the down going aileron doesn't move as far as the up going aileron. In this way, the down going aileron created less drag than normal and the up going aileron creates more drag than normal. In this way, the amount of drag is less than on normal types of ailerons. In the second design, ailerons called "frise ailerons" are designed so that the front of the upgoing aileron sticks out into the air of higher pressure below the wing, while the down going aileron is streamlined. This again causes the forces to be balanced.
The Boundary Layer
Over the surface of the wing lies a very thin sheet of air. This air moves very smoothly over the surface of the wing. At this point the flow is called the "laminar layer". As the boundary layer approaches the centre of the wing, it begins to lose speed due to skin friction. The air becomes more turbulent and doesn't flow smoothly over the surface of the wing. Here it is called the "turbulent layer". The point where the boundary layer changes from laminar to turbulent is called the "transition point". As the angle of attack increases, the transition point moves forward along the wing (towards the leading edge). As the angle of attack decreases, the transition point moves backwards along the wing (towards the trailing edge).
Three methods have been developed to try to prevent the laminar layer from becoming turbulent too early and reduce skin friction.
1. Suction method: Thin slots in the wings run from the wing root to the wing tip. A vacuum sucks the air down through the slots, forcing the air to follow the curved surface of the wing.
2. Laminar Flow Airfoil: This is a specially designed airfoil that has a slightly different shape than traditional airfoils. The thickest part of a laminar flow airfoil is at 50% of the chord (in the middle of the wing) rather than at 25% of the wing like in traditional wings. This causes the transition point to be farther back than on the conventional type of airfoil.
3. Vortex Generators: These are small plates that stand in rows on the wing and generate vortices (swirls of air). These give extra energy to the air and keep the air from becoming turbulent.
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