Lift
An aircraft is a vehicle capable of achieving flight. In order for anything to achieve flight, it must create or use some force that propels it upward. This force is known as lift. Lift can be created in many different ways. Lighter-than-air craft use elements that have less mass than the air they are surrounded by to float on air by being lighter than it. Heavier-than-air craft take a different approach.
The most simple way to describe how any heavier-than-air craft creates lift is the displacement of air downward. This works by simply displacing the air’s mass downward to create a counter force pushing the airplane upward. The process by which the aircraft does this varies. In a rocket, for example, propellant is ignited to create hot gasses that are shot down ward to push the rocket up. Just like the recoil force you feel from a pressure washer shooting water in the opposite direction at high speeds.
Airplanes and helicopters use a different form of air displacement. Airplane wings are designed to direct air downward in multiple ways depending on its shape, which is called an airfoil. Helicopters use nearly flat blades that spin rapidly using their pitch as an angle of attack to force air downward and thereby push the aircraft upward. Pitch is a term used to describe the angle a propelling blade makes from being parallel to the blade’s direction of motion.
An aircraft is a vehicle capable of achieving flight. In order for anything to achieve flight, it must create or use some force that propels it upward. This force is known as lift. Lift can be created in many different ways. Lighter-than-air craft use elements that have less mass than the air they are surrounded by to float on air by being lighter than it. Heavier-than-air craft take a different approach.
The most simple way to describe how any heavier-than-air craft creates lift is the displacement of air downward. This works by simply displacing the air’s mass downward to create a counter force pushing the airplane upward. The process by which the aircraft does this varies. In a rocket, for example, propellant is ignited to create hot gasses that are shot down ward to push the rocket up. Just like the recoil force you feel from a pressure washer shooting water in the opposite direction at high speeds.
Airplanes and helicopters use a different form of air displacement. Airplane wings are designed to direct air downward in multiple ways depending on its shape, which is called an airfoil. Helicopters use nearly flat blades that spin rapidly using their pitch as an angle of attack to force air downward and thereby push the aircraft upward. Pitch is a term used to describe the angle a propelling blade makes from being parallel to the blade’s direction of motion.
Airfoils are shapes that cause moving fluids to flow around them in a specific way. Wing airfoils are specifically used to generate lift for airplanes and are very important in the process of creating efficient and performing aircraft. Airfoils are often described to generate lift because they force the air on top to move faster than the air on the bottom, however this is not entirely true. Although the air moving on the top of a wing does move faster than the air on the bottom, this is only a bi-product of the underlying cause.
When a generic airfoil is propelled through air, the forward end splits the air, pushing equal parts of air upward and downward, compressing both sides equally. Although this does nothing initially due to the balancing forces, the trailing end of the wing creates a generous amount of lift. The bottom end of the trailing edge is usually flat, or even curved downward which either maintains the high pressure from the leading edge or even forces even more air downward due to the nature of the curvature. The top of the trailing edge tapers downward which creates a sudden low pressure area on the top of the wing. High pressure air above and in front of it rushes to fill the gap. The downward moving air sucks the airplane upward while pulling air rearward, forcing it to move faster.
Angle of attack dramatically assists the airfoil’s generation of lift by changing the angle at which the airfoil is propelled through the air. Theoretically, a flat surface can generate lift using angle of attack. A ceiling fan, for example, pushes air by simply angling flat surfaces and propelling them so that they strike air downward, and create lift as a by-product. Airplanes can create the same effect by angling the front of the airfoil up in the front, thereby redirecting more air downward. High angles of attack may produce more lift, however it also produces a lot of drag and a high angle of attack may cause a stall.
Air acts cohesive, meaning it likes to stick together. When the angle of attack increases, the air moving over the top is forced to move downward to replace the gap more and more rapidly. Eventually, the air coming off the leading edge can no longer follow the air that was being sucked downward and off the trailing edge and becomes spoiled. spoiled air moves randomly and causes a complete lack of lift on the top end of the wing, which forces the airplane to stall. When an airplane is stalling, the wings are no longer providing sufficient lift, and the airplane falls from the sky. Due to air’s cohesive nature, the recovery for a stall requires a dramatic change in the angle of attack. While an airplane can fly with an angle of attack just a few degrees from going into a stall, in order to recover from that stall the aircraft must pitch the nose back down to a nearly negative angle. Once the angle of attack is reduced sufficiently, the wing will be forced to regain proper airflow, and the angle of attack can then be increased to provide lift again.
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Balance
For an aircraft to be stable during flight, the most important part is proper balance. for any aircraft to maintain proper balance it must have a well placed center of gravity. The term center of gravity is used to describe the position of any object at which that object’s mass is spherically proportionate in all directions. For example, a uniform sphere made of the same material throughout, has a center of gravity in its dead center. A ‘T’ shaped object on the other hand, like a hammer, will have a center of gravity on the handle, however it will be located significantly closer to the head, due to the extra mass in that location. |
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Anything that is traveling through the air has a center of aerodynamics. This is the location from the leading edge of the object that the drag from the forward side of this position is balanced from the rearward side. For a perfect rectangular wing, this is in the dead center of the wing. There are methods for calculating the center of aerodynamics on swept wings and non uniform objects, that will not be given in this paper.
For an airplane, helicopter, or rocket to maintain stable flight, it must have a center of aerodynamics that is located behind its center of gravity. This way the airplane’s momentum will lead, while the airplane’s drag will trail behind it, pulling the tail in the rearward direction. This is known as a negative feedback response, which is good. It is called a negative feedback response because when the aircraft is destabilized, the drag will respond negatively, pulling the aircraft back into a stable position.
For an airplane, helicopter, or rocket to maintain stable flight, it must have a center of aerodynamics that is located behind its center of gravity. This way the airplane’s momentum will lead, while the airplane’s drag will trail behind it, pulling the tail in the rearward direction. This is known as a negative feedback response, which is good. It is called a negative feedback response because when the aircraft is destabilized, the drag will respond negatively, pulling the aircraft back into a stable position.
Finally, there is the center of lift. This is the location at which the aircraft’s combined lift is positioned. For rockets, this is located on the very bottom of the craft, making it very important to correctly balance the center of gravity and center of aerodynamics. For helicopters, this is on the very top, making them very stable due to the low center of gravity. For airplanes, due to the wing’s shape and how it produces lift, the center of lift is often located behind the center of gravity. Because the main upward force is located behind the main downward force, the wing naturally forces the airplane to pitch downward forcefully. (This picture shows the weight being too far aft, making it pitch upward.)
Stabilizers and control surfaces
Often called stabs, these are the multiple fins you see on various aircraft to stabilize them in flight. Control surfaces are the multiple different flaps you see integrated on the stabilizers and wings that control the aircraft’s orientation in flight. The control surfaces are able to change the aircraft’s orientation by directing air in the opposite direction of the desired change. The horizontal stabilizer, otherwise known as the elevator, helps the airplane counter the downward pitch caused by the wing. This also doubles as the primary control surface for the up and down pitch of the aircraft. The vertical stabilizer helps to counter any difference in drag and stabilizes the aircraft’s yaw axis. Although there does not seem to be any direct need for this stabilizer, without it, there would be no way to control and maintain the yaw axis if there was any difference in left or right drag. This is very important because is air moves over the airfoil while the yaw is off by a few degrees, it will foil and the aircraft will begin to slip. Intentional slip will be discussed later in this paper. The rudder is also located on the vertical stabilizer, which is the control surface in charge of the yaw axis.
Often called stabs, these are the multiple fins you see on various aircraft to stabilize them in flight. Control surfaces are the multiple different flaps you see integrated on the stabilizers and wings that control the aircraft’s orientation in flight. The control surfaces are able to change the aircraft’s orientation by directing air in the opposite direction of the desired change. The horizontal stabilizer, otherwise known as the elevator, helps the airplane counter the downward pitch caused by the wing. This also doubles as the primary control surface for the up and down pitch of the aircraft. The vertical stabilizer helps to counter any difference in drag and stabilizes the aircraft’s yaw axis. Although there does not seem to be any direct need for this stabilizer, without it, there would be no way to control and maintain the yaw axis if there was any difference in left or right drag. This is very important because is air moves over the airfoil while the yaw is off by a few degrees, it will foil and the aircraft will begin to slip. Intentional slip will be discussed later in this paper. The rudder is also located on the vertical stabilizer, which is the control surface in charge of the yaw axis.
There are two main control surfaces on the wings that are important to flight. The first are the ailerons, which are located toward the tips of the wings. These actually move in opposite directions so that one wing is pushed up, while the other is pushed down. This changes the roll axis of the airplane. It allows the airplane to turn as well by putting in a bank with the assistance of the rudder. The second control surface on the wings are the flaps. These are the surfaces more toward the fuselage of the craft. They only move downward to change the airfoil shape and angle of attack mid-flight without changing pitch. This is incredibly useful during landing because it helps to slow down the airplane while creating more lift. It is also useful during takeoff to allow the airplane to take off at lower speeds.
There is a control surface located on the leading edge of the wing known as the leading edge slats. They are similar to flaps, however their purpose is to counterbalance the change in pitch that is often caused by flaps. They allow the airplane to change its airfoil even more without the natural pitch change from the flaps. If slats are used during flight without the use of flaps they create a positive feedback loop and generate violent pitch oscillations often leading to a crash.
On every orientational control surface is a trim tab. Trim is used on an airplane to make minor adjustments to the settings of control surfaces. Their main purpose is to even out forces so that the airplane naturally maintains a level heading without stick input. Trim tabs can be used in emergency situations to control any of the major control surfaces if a cable or hydraulic line snaps.
Weight, drag, and thrust
Weight is the opposing force of lift. Because most modern aircraft are heavier-than-aircraft, weight is one of the most important elements to overcome. Weight reduction is about as simple as it sounds. Remove useless material, and reduce weight. As materials have evolved, so have aircraft. Airplanes were once made of wood and fabric, and due to increasing technology they are now made from aluminum alloys and composite materials. Composite materials are the combination of two materials to make a new stronger, lighter material. This can include carbon fiber, fiberglass, and carbon nanotubes. Aircraft have also been structurally designed differently to require less material while also being structurally sound.
Weight is the opposing force of lift. Because most modern aircraft are heavier-than-aircraft, weight is one of the most important elements to overcome. Weight reduction is about as simple as it sounds. Remove useless material, and reduce weight. As materials have evolved, so have aircraft. Airplanes were once made of wood and fabric, and due to increasing technology they are now made from aluminum alloys and composite materials. Composite materials are the combination of two materials to make a new stronger, lighter material. This can include carbon fiber, fiberglass, and carbon nanotubes. Aircraft have also been structurally designed differently to require less material while also being structurally sound.
Drag is the forced caused by having air striking or rubbing against the airframe in the form of friction. Drag pulls the airplane backward, slowing it down. In some cases, drag is good. The horizontal and vertical stabilizers use drag to keep the airplane oriented properly during flight. Flaps and leading edge slats use drag to slow the airplane down during landing. When aircraft are trying to fly efficiently, however, drag is no longer favored. Many developments in aerospace have led to more drag reduced designs in aircraft, however it cannot be fully eliminated.
Although most drag is produced by the airframe directly striking air there is also something known as induced drag. Induced drag is drag caused by the suction of air that an airfoil naturally creates. Because of the low air pressure on the top rear portion of the wing, suction is created for lift, however, this suction also pulls on the back of the wing, causing induced drag. Another major source of induced drag is a phenomenon known as wingtip vortices. This is caused by high pressure air on the bottom of the wing rolling over the sides to the top of the wing to fill the low pressure. This is an incredibly significant source of induced drag and many engineers have devoted their lives to finding ways to prevent it. Wingtip vortices can completely counteract the lift created by more than 30% of the outer wing, making that portion of the wing useless.
There are many ways to produce thrust for an aircraft, however all of them include some form of air displacement. The most common form of thrust for aircraft comes from a propeller. Propellers create thrust by using angled blades to whack molecules of air in the opposite direction of desired thrust. It uses air to push off of its surrounding air to create thrust. Jet-turbine engines are used in a similar fashion, however they have replaced pistons with an air flow and combustion system that utilizes turbine fans to harness energy and reuse it to further propel itself. Air is first sucked into a funnel by a series of fans which are all connected to a main axle. The fans compress the air into a small chamber where it is mixed with fuel and ignited. The fans have created high pressure in the front, so the hot, energised air is forced out the back. Before the air reaches its exit, it is forced to pass through turbine blades which power the main axle that sucks more air in the front. This system is extremely effective due to its lack of a timing trigger, which is necessary for piston motors. Jet engines are able to reach nearly limitless speeds.
Along with the jet-turbine engine came many variations for different use. The first being the ramjet, which uses the final exhaust gasses from the turbine, combines them with fresh air, and reignites it in an afterburner. These are often used on fighter jet aircraft and consume a very large amount of fuel and produce a great amount of thrust. Jet-fan engines took the place of many of the older jet-turbine engines due to their extra efficiency. Jet-fan engines appear fatter then the old turbine engines, however their only difference is a single large fan in the front of the engine. They utilize the power from the turbines in the back to channel more air into the turbine, while also forcing air into a larger tube wrapped around the turbine, similar to a propeller. The final variant of the use of turbines is the turbo-prop engine. Turbo-prop engines use a jet turbine to power a propeller on the front of the engine.
Wing and stabilizer orientation
Throughout the design of aircraft many wild and interesting designs have made their way into aviation history with their mysterious ability to fly. Most of these designs incorporate bold wing and stabilizer orientation. This just means the location of stabilizers and wings, or lack thereof, on the airframe. The first feat to be performed in aircraft design was the mono-wing airplane. What is now the most popular design in aircraft history was known to be impossible less than 100 years ago. When airplanes were first invented, they incorporated a bi-wing, or biplane design for structural support. This meant one wing above another. there was not a strong, light material for airplanes to be able to utilize a mono-wing design.
Throughout the design of aircraft many wild and interesting designs have made their way into aviation history with their mysterious ability to fly. Most of these designs incorporate bold wing and stabilizer orientation. This just means the location of stabilizers and wings, or lack thereof, on the airframe. The first feat to be performed in aircraft design was the mono-wing airplane. What is now the most popular design in aircraft history was known to be impossible less than 100 years ago. When airplanes were first invented, they incorporated a bi-wing, or biplane design for structural support. This meant one wing above another. there was not a strong, light material for airplanes to be able to utilize a mono-wing design.
The next designs included different wing shapes including a swept design. Swept wings are shaped as if the wingtips have been tapered backward from their original position to make a more aerodynamic design. This eventually led to the delta wing design, which is a wing in the shape of a delta, or triangle. In the delta design, horizontal stabilizers are placed in the front of the airplane to reduce stability. This makes the airplane incredibly maneuverable. Along with the swept design came the elliptical wing shape, which is the most efficient design ever utilized. Elliptical wings taper off the wingtip exponentially, nearly eliminating the effects of wingtip vortices. Problems arose with the elliptical wing due to its difficulty to manufacture.
Flying wings were experimented with in the late 1940’s and became one of the most revolutionary designs to date. Flying wings, like the Northrop Grumman B-2, use a swept design with winglets to keep the airplane stable, while also providing lift without the need for stabilizers. Twin engine flying wings do not need winglets, as they can use computerized gyros to differentiate thrust from either side and maintain yaw stability.
Stabilizer placement on an airplane can make it perform very differently. if stabilizers are placed in the front of the aircraft, they cause a positive feedback loop. If the airplane dips in a different direction, even slightly, the stabilizers will actually force the airplane to go further in that direction, which is why it is called a positive feedback loop. The further the airplane pitches, the more force is put into getting the airplane to pitch more. This instability can be utilized with modern technology. Computerized fly-by-a-wire systems will automatically correct for any unwanted pitch in the aircraft by changing the deflection of the stabilizer itself to counteract the unwanted movement. This means that when a pilot wants to make a rigorous maneuver, he can.
Maneuvers
Throughout the history of aircraft, increasingly outrageous maneuvers have been performed. Aviation is filled with shows of wonder as airplanes carry out these crazy aerobatics. In the past airplanes could not perform the way they do now, and many maneuvers were known to be impossible. Some of the most notorious maneuvers performed throughout history are the inside and outside loops. These are simply loops that are performed by holding the elevator in the up or down position. The inside meaning which side the pilot is located relative to the loop. If the pilot was on the inside of the loop throughout the maneuver, it is known as an inside loop.
Throughout the history of aircraft, increasingly outrageous maneuvers have been performed. Aviation is filled with shows of wonder as airplanes carry out these crazy aerobatics. In the past airplanes could not perform the way they do now, and many maneuvers were known to be impossible. Some of the most notorious maneuvers performed throughout history are the inside and outside loops. These are simply loops that are performed by holding the elevator in the up or down position. The inside meaning which side the pilot is located relative to the loop. If the pilot was on the inside of the loop throughout the maneuver, it is known as an inside loop.
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The spin is another common maneuver that was performed by early pilots. The spin is carried out by reducing speed and shoving the rudder hard to one side. The airplane pitches down while spinning in the direction the rudder was pressed. The airplane can only be recovered if the rudder is pushed in the opposite direction then pulling up when the spinning is counteracted. If the elevator is pulled up during the spin, the airplane will enter a flat spin which cannot be recovered from. There are hundreds more examples of maneuvers including the cuban eight, tailslide, and hammerhead stall.
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