THE THEORY OF FLIGHT INTO THE FREE SKIES by Captain Savas USKENT
The theory of flight is as old as the age of the Universe. Excellent balance throughout the Universe thanks to amazing physics has been dominating since the Big Bang.
Once the Milky Way was created, the Earth, one of the planets of the solar system enlivened, the theory of flight was still there to be discovered by genius humankind, letting thrown rocks, spears, and arrows to projectile and helping birds to fly.
The theory of flight is almost unlimited. Complete discovery is hardly possibly. The description of flight can be done mainly in two categories: 1. Atmospheric flights. a. Powered flights. (1) Subsonic (2) Transonic (3) Supersonic b. Powerless flights. (1) Glider, balloon and any kind of lighter than air flights. c. Ballistic flights. 2. Space flights. Space flights are thoroughly in different category. Once a spacecraft exits the Earth's atmosphere, physics flies and steers her. There remains no relationship with neither fluid mechanics and nor aerodynamics until the craft returns into the atmosphere. Slow flights, subsonic, transonic, supersonic flights, very high altitude (VHA) or very low altitude (profile) flights, acrobatics, dog fights, vast variety of military aircraft and all kind of missile flights, rocket firings, bombs release, air to subsurface torpedo firings and commercial and private flights are examples of Atmospheric flights.
As you see, there is vast variety of flights, which, as technology increase so does, the abundance of variety.
Since man first saw the birds, he has yearned to fly alongside them. At the end of a continued quest, the first man who was able to fly was an Ottoman scientist known as "Hazerfan Ahmet Celebi".
In 1674, he departed from the Galata Tower on the European side of Istanbul, and arrived to the Asian side, flying over the Strait of Bosporus wearing a pair of special wings.
As a reward for his great scientific and courageous achievement, the Ottoman Emperor jailed him. In spite of the unfortunate fate of the hero, this was a big step ahead for the conquest of the skies by humankind in the 17th century.
As humankind envied the birds, on the quest of flying, Leonardo Da Vinci was one of the frontiers on the discovery of human flight even before Hazerfan. Leonardo Da Vinci (1452-1519), a well-known Italian artist and scientist, is not merely the creator of the "Mona Lisa" and the "Lady with an Ermine", the Archimedean ingenuity, which pervades his entire work, recognized also by his contemporaries, is remarkable for its strict scientific foundations, based on a profound analysis of ancient and traditional sources.
"A bird is an instrument working according to mathematical law, which instrument it is within the capacity of man to reproduce with all its movements, but not with a corresponding degree of strength"
After the lengthy but unsuccessful studies on artificial flight conducted in Milan, Leonardo dedicated himself to studying the flight of birds in 1500. He compiled comprehensive observations on the behavior of birds in relation to the wind, many of which he collected in a small codex that also contains notes on mechanics, hydraulics and architecture.
At around 1505, with the idea of a glider, a flying machine launched for a flight from the top of Monte Ceceri, at Fiesole near Florence.
In 1550, Gerolamo Cardano quoted on Leonardo's experiments with flight: "He tried to fly but in vain. However, he was an excellent painter".
With the beginning of 20th century, in 1903 aeronauts Orville and Wilbur Wright made their first flight at Kitty Hawk and revolutionized the world into the powered flight era.
Today, we are living in the era of "discovery and conquest of the Universe"
There are no limits to the discovery of flight. Flights may take place in the atmosphere or in the space... They both are conducted within the limits of physics.
As far as atmospheric flights of airplanes are considered, "aerodynamics" as a part of "fluid mechanics" takes precedence and principally, there are four forces acting on an airplane.
1. THRUST 2. DRAG 3. LIFT 4. WEIGHT due to GRAVITY
When thrust and drag are equal, the airplane is in a state of equilibrium. In such case, she continues to fly level and horizontal at her present speed.
If either of these forces becomes greater than the force opposing it, the state of equilibrium is lost.
Whenever thrust is greater than drag, an airplane accelerates and speeds up.
If drag is greater than thrust, the airplane shall decelerate, lose speed, and consequently loose her ability to fly and stall. Similarly, when lift and weight are equal, the airplane shall be in a state of equilibrium. If lift is greater than weight, the airplane shall climb. Whenever weight is greater than lift, she sinks.
Lift makes an airplane fly, and engine powered thrust is needed for continuous lift, excluding gliders and the similar lighter than air stuff.
Lift is easy to understand by Sir Isaac Newton laws. He said: 1. Things that are moving like to keep moving and, things those are still like to keep still.
It is hard to start pushing a heavy box across the floor, but once you start it moving, it seems easier.
2. To make an object start or stop moving, a force must be used.
You are the force that pushed the box across the floor, and friction is the force that makes it stop moving. If no force acted on it, the box would stay at the same place forever.
3. If you apply a force to an object, you shall get a reaction that is equal and opposite.
If you fire a gun, it recoils, moving backwards in your hands. The bullet moves forward, and the gun moves opposite. The force is applied to the bullet to make it go forwards, so an equal and opposite reaction happens backwards, making the gun recoil.
There are indeed, mainly three descriptions of lift commonly used within aviation community.
Aeronautical engineers use MATHEMATICAL AERODYNAMICS DESCRIPTION.
This description uses complex mathematics and computer simulations to calculate the lift. These are powerful design tools for computing lift but do not provide an intuitive understanding of flight.
The second description is a widely used explanation that is based on the BERNOULLI PRINCIPLE.
The primary advantage of BERNOULLI DESCRIPTION is its simplicity. It has been widely taught for many years and used to describe lift in most manuals since many years.
The third description is the PHYSICAL DESCRIPTION of lift and is based on NEWTON'S LAWS. It is useful for understanding flight, accessible to all that are curious and gives a clear, intuitive understanding of flight phenomena.
Students of physics and aerodynamics are taught that airplanes fly because of BERNOULLI'S PRINCIPLE, which says that; "if air speeds up, the pressure is lowered". "A wing generates lift because the airs goes faster over the top creating a region of low pressure, and thus lift".
This explanation usually satisfies the curious and few challenge the conclusions. Some may wonder why the air goes faster over the top of the wing and this is where the Bernoulli explanation of lift falls apart.
In order to explain why the air goes faster over the top of the wing, many have resorted to the geometric argument that the distance the air must travel is directly related to its speed. The usual claim is that when the air separates at the leading edge, the part that goes over the top must converge at the trailing edge with the part that goes under the bottom. This is the so-called "principle of equal transit times".
Let us assume that this argument were true. From Bernoulli's principle, we can then determine the pressure forces and thus lift. If we do a simple calculation, we would find that, "in order to generate the required lift for a typical small airplane, the distance over the top of the wing must be about 50% longer than under the bottom". Now, can you imagine what a jetliner's wing would have to look like! Do you think this was the reason for Spruce Goose's failure to fly?
When we look at the wing of a small plane, which has a top surface that is 1.5 - 2.5% longer than the bottom, we discover that a Cessna 172 would have to fly at over 400 mph to generate enough lift.
Something in this description of lift smells flawed.
On the case of "principle of equal transit times", wing tunnel tests show that the air that goes over the top of the wing gets to the trailing edge considerably before the air that goes under. In fact, the air going under the wing is slowed down from the "free-stream" velocity of the air. Another fundamental shortcoming of the Bernoulli description is that it ignores the work done. Lift requires power. An understanding of power is a key to the understanding of lift phenomena.
How does a wing generate lift in the light of Newton's laws? In order to generate lift a wing must do something to the air. What the wing does to the air is the action while lift is the reaction.
As the laws suggests, the wing must change something in the air to be lifted. Changes in the air's momentum will result in forces on the wing. To generate lift a wing must divert air down.
The lift of a wing is equal to the change in momentum of the air it is diverting downward. Momentum is the product of mass and velocity. The lift of a wing is proportional to the amount of air diverted down, times, the downward velocity of that air.
To the pilot, the air comes off the wing at roughly the angle of attack (AOA). To the observer on the ground, it would be coming off the wing almost vertically.
Vertical velocity (VV) increases with the AOA.
Likewise, for the same AOA, as the speed of the wing increases, so does the VV. Both the increase in the speed and the increase of the AOA, increase the length of the vertical arrow. This VV lifts the wing.
In the mathematical aerodynamics description of lift, rotation of the air around the wing gives rise to the "bound vortex" or "circulation" model. The advent of this model, and the complicated mathematical manipulations associated with it, leads to the direct understanding of forces on a wing. Nevertheless, the mathematics required typically takes students in aerodynamics some time to master.
Top surface of wing does much more to move air than bottom. Therefore, the top is the more critical surface. Thus, airplanes can carry external stores, such as drop tanks, under the wings but not on top where they would interfere with lift. That is also, why wing struts under the wing are common but struts on the top of the wing have been historically rare. A strut, or any obstruction, on the top of the wing would interfere with the lift.
When a moving fluid, such as air or water, meets a curved surface, it will try to follow that surface. The tendency of fluids to follow a curved surface is known as the Coanda effect. From Newton's first law we know that for the fluid to bend there must be force acting on it. From Newton's third law we know that the fluid must put an equal and opposite force on the object which caused the fluid to bend.
There are different types of wings. In all cases, a wing forces the air downwards. AOA is common for each type of wings with respect to the oncoming air.
AOA is the primary parameter in determining lift.
The inverted wing can be explained by its angle of attack, despite the apparent contradiction with the Bernoulli principle.
Pilots adjust the AOA to adjust the lift for the speed and load.
Typically, the lift begins to decrease at an angle of attack of about 15 degrees. The forces necessary to bend the air to such a steep angle are greater than the viscosity of the air will support, and the air begins to separate from the wing. The separation of the airflow from the top of the wing is a stall.
Lift requires power. Power is supplied by the airplane's engine or by gravity and thermal activity for a sailplane.
The power needed for lift is the energy per time-unit and is proportional to the amount of air diverted down, times, the square velocity of diverted air.
As the speed of a plane doubles so does, the amount of air diverted down... The AOA must be reduced to give a VV that is half the original to give the same lift.
The power required for lift becomes less as the airplane's speed increases.
The power associated with lift, is often called the "induced" power.
Power is also needed to overcome what is called "parasitic" drag, which is the drag associated with moving the wheels, struts, antenna, etc. through the air.
The energy an airplane imparts to an air molecule on impact is proportional to the speed squared. The number of molecules struck per time is proportional to the speed and density altitude.
At low speed, the power requirements of flight are dominated by the induced power.
The slower wing flies less air diverted and therefore the AOA is increased to maintain lift.
Practice of flying on the backside of the power curve to recognize the AOA and required power correlation to sustain flight are essential.
As cruising speed increases, the power requirement is dominated by parasitic drag. Since this goes as the speed cubed, an increase in engine power does little to improve the cruise speed of an airplane.
As you see, power requirements vary with speed.
Slow airplanes and gliders are designed to minimize induced drag, which dominates at lower speeds. Fast airplanes are more concerned with parasitic drag.
There is a close relationship between wing loading and power.
At a constant speed, whenever wing loading is increased the VV is increased to compensate. This is done by an increase of the AOA.
When the total load factor on an airplane is doubled with a 2g turn, the VV of the air as well is doubled compensate for.
The induced power is proportional to the load times the VV of the diverted air.
Thus, the induced power requirement has increased by a factor of four!
The same theory would be true if the airplane's weight were doubled by adding more fuel, passengers, etc.
The increase in the AOA with increased load can go up to a climax that is critical AOA or critical ALPHA angle. Thereafter adding more power shall not help. Eventually stall occurs when air can no longer follow the upper surface. The angle of attack at which the plane stalls is constant and is not a function of wing loading.
The stall speed increases as the square root of the load. Thus, increasing the load in a 2-g turn increases the speed at which the wing will stall by 40%.
An increase in altitude will further increase the angle of attack in a 2-g turn.
For any speed, there is a load factor to induce a stall.
Guess what a downwash from a wing looks like. The downwash comes off a wing as a sheet in accordance with the load distribution on the wing.
The distribution of load changes from the root to the tip.
Thus, the amount of air in the downwash also changes along the wing.
The wing near the root scoops up more air than the tip.
Since the root diverts much air, the downwash sheet begins to curl outward around itself, just as the air bends around the top of the wing due to the change in the air velocity. This is called wing vortex.
The tightness of the curling of the wing vortex is proportional to the rate of change in lift along the wing. At the wing tip, the lift rapidly becomes zero causing the tightest curl.
Wing tip vortex is just a small, though often most visible part of the wing vortex.
Winglets, small vertical extensions on the tips of some wings, are used to improve the efficiency of the wing by increasing the effective length of the wing, blocking, and reducing the negative effects of vortices.
The lift of a normal wing goes to zero at the tip, because the bottom and the top meet around the end.
The winglets block this communication so the lift can extend farther out on the wing.
Since the efficiency of a wing increases with length, this increases efficiency. Another commonly misunderstood phenomenon is the GROUND EFFECT. It is the increased efficiency of a wing when flying within "a wing length" of the ground.
A low-wing airplane will experience a reduction in drag by 50% just before it touches down.
There is a great deal of confusion about ground effect. Many pilots mistakenly believe that ground effect is the result of air being compressed between the wing and the ground.
To understand ground effect it is necessary to have an understanding of UPWASH. For the pressures involved in low speed flight, air is considered non-compressible. When the air is accelerated over the top of the wing, it must be replaced. Therefore, some air must shift around the wing to compensate, similar to the flow of water around a canoe paddle when rowing. This is the cause of up wash.
Up wash accelerates air in the negative direction for lift. Thus, a greater amount of downwash is necessary to compensate for the up wash and to provide the necessary lift.
Near the ground, the up wash is reduced because the ground inhibits the circulation of the air under the wing. Therefore, less downwash is necessary to provide the lift. The angle of attack is reduced and so is the induced power, making the wing more efficient.
In understanding the theory of flight, the importance of the notion of "the axes of an airplane" cannot be overlooked.
An airplane moves around three main axes. All pass through the airplane's center of gravity, which is the point of the center of the airplane's total weight.
The longitudinal axis extends lengthwise through the fuselage from the nose to the tail. Movement of the airplane around the longitudinal axis is known as roll and is controlled by movement of the ailerons.
To move the ailerons, the pilot turns the control wheel either clockwise or counter clockwise. This action lowers the aileron on one wing and raises the aileron on the other wing.
The down going aileron increases the camber of its wing, producing more lift and the wing rises. The up going aileron spoils the airflow on its wing, decreases the lift and the wing descends. The airplane rolls into a turn.
The lateral axis extends crosswise from wingtip to wing tip. Movement of the airplane around the lateral axis is known as pitch and is controlled by movement of the elevators. To affect a nose down attitude, the pilot pushes forward on the control wheel. The elevator deflects downward, increasing the camber of the horizontal tail surface and thereby increasing the lift on the tail. To affect a nose up attitude of the airplane, the pilot pulls the wheel toward him. The elevators are deflected upwards decreasing the lift on the tail, with a resultant downward movement of the tail. The movement around lateral axis is also coordinated by stabilizer trim, autopilot trim, and mach trim systems on high tech and high performance aircraft.
The vertical or normal axis passes vertically through the center of gravity. Movement of the airplane around the vertical axis is yaw and is controlled by movement of the rudder by pilots or on high performance aircraft by yaw damper or auto flight system computers. There is a distinct relationship between movement around the vertical and longitudinal axes of an airplane (i.e. yaw and roll). When rudder is applied to effect a yaw, the opposite wing, on the outside of the turn, moves faster than the inside, meets the relative airflow at a greater AOA and speed, producing more lift...The use of rudder, therefore, along with aileron can help to raise the wing and produce a better coordinated turn. In a roll, the airplane has a tendency to yaw away from the intended direction of the turn. This tendency is the result of aileron drag and is called adverse yaw. The up going wing, as well as gaining more lift from the increased camber of the down going aileron, also experiences more induced drag. The airplane, as a result, skids outward on the turn. Use of rudder in the turn corrects this tendency. These are usually automated maneuvers done by Yaw Damper Computers on transport category high performance jet aircraft.
Rudder controls an airplane around her vertical axis. Ailerons, aileron trim, and spoilerons control an airplane around her longitudinal axis. Elevators and elevator trim, stabilizer trim, control an airplane around her lateral axis.
Controls are dynamically balanced to assist the pilot to move them.
An airplane in flight may be subjected to forces that might disturb it from its normal flight path as in the cases of turbulence, wind shear, microbursts, innocent rising columns of hot air, down drafts, gusty winds etc. How the airplane reacts to such disturbances depends on its stability characteristics.
Stability is the tendency of an airplane in flight to return to its original attitude if displaced without any corrective action by the pilot.
Static stability is the initial tendency of an airplane, when disturbed, to return to the original position.
Dynamic stability is the overall tendency of an airplane to return to its original position, following a series of damped out oscillations.
Stability may be, "Positive", means the airplane develops forces or moments tending to restore its original position, "Neutral", means the restoring forces are absent and the airplane will neither recover nor worsen, "Negative", means she develops forces or moments which tend to worsen her unstable condition.
Negative stability is a condition of instability.
A stable airplane is one that will fly "hands off" and is pleasant and easy to handle. An exceedingly stable airplane, on the other hand, may lack maneuverability.
An airplane, which, following a disturbance oscillates with increasing up and down movements until it eventually stalls or enters a dangerous dive, would be said to be unstable, or to have negative dynamic stability.
An airplane that has positive dynamic stability does not automatically have positive static stability. The designers may have elected to build in, for example, negative static stability, and positive dynamic stability in order to achieve their objective in maneuverability. In other words, negative and positive dynamic and static stability may be incorporated in any combination in any particular design of airplane.
An airplane may be inherently stable, that is, stable due to features incorporated in the design, but may become unstable due to changes in the position of the center of gravity (fuel consumption, improper disposition of the disposable load, etc.).
Stability may be, Longitudinal, lateral or directional, depending on the pitch, roll, or yaw axes of an airplane.
LONGITUDINAL STABILITY is pitch stability, and means the stability around the lateral axis of an airplane.
The CG is very important in achieving longitudinal stability. If an airplane is loaded with a center of gravity too far aft, the airplane may assume a nose up rather than a nose down attitude. The inherent stability will be lacking and, even though down elevator may correct the situation, control of the airplane in the longitudinal plane will be difficult and perhaps, in extreme cases, impossible.
LATERAL STABILITY is the stability around the longitudinal axis or roll stability.
Lateral stability is achieved through dihedral angle, sweptback wing, keel effect, and proper distribution of weight.
The dihedral angle is the angle that a wing makes with the horizontal. The purpose of dihedral is to improve lateral stability.
Some modern airplanes have a measure of negative dihedral on the wings and/or stabilizer. The incorporation of this feature provides some advantages in overall design in certain type of airplanes. However, it does have an adverse effect on lateral stability.
Dihedral is more usually a feature on low wing airplanes although some dihedral may be incorporated in high wing airplanes as well.
Most high wing airplanes are laterally stable thanks to "Keel Effect". As such an airplane is disturbed, the weight acts as a pendulum returning the airplane to its original attitude.
A sweptback wing is one in which the leading edge slopes backward. When a disturbance causes an airplane with sweepback to slip or drop a wing, the low wing presents its leading edge at an angle that is perpendicular to the relative airflow. As a result, the low wing acquires more lift, rises and the airplane is restored to its original flight attitude.
Sweepback also contributes to directional stability. When turbulence or rudder application causes the airplane to yaw to one side, the right wing presents a longer leading edge perpendicular to the relative airflow. The airspeed of the right wing increases and it acquires more drag than the left wing. The additional drag on the right wing pulls it back, returning the airplane back to its original path.
Sweptback wing is very important for transonic and supersonic aircraft as well. The purpose is to keep the ailerons clear of shock wave disturbance as much as possible. Very recent supersonic aircraft projects and space shuttle simply omit ailerons, instead use combined "elevon" systems in one.
DIRECTIONAL STABILITY is stability around the vertical axis.
The most important feature affecting directional stability is the vertical stabilizer surface, that is, the fin and rudder. Keel effect and sweepback also contribute to directional stability to some degree.
As you see, flights may be conducted in subsonic, transonic, and supersonic regime. Each requires its own design parameters to be taken into consideration for production phases. The purpose of projected flights shall lead the design characteristics of the projected type of flying craft.
Nevertheless, the main "theory of flight into the free skies" will not change.
As the technology keeps running with vertiginous speed, so does the design features.
However, the theory shall continue to be within the easy reach of aviation technology.Anytime, anywhere. By: Captain Savas Uskent. August 03, 2003 Copyright (c) 2003 Commander Pilot ATPL/CFI Next Generation Boeing 737/800, B737/500-400, Airbus 310/300-200, BAe146/100-70 (Avro 100), Learjet-60, Learjet-55C, Learjet-35A, Challenger 601-3A, Caravelle SE-210 " target="geocities.com/uskent/index.html">">http://geocities.com/uskent/index.html">http://geocities.com/uskent/index.html uskent@yahoo.com
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