# Fundamentals of Flight

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Fundamentals of Flight
A Basic Introduction to Aerodynamics

The Four Forces of Flight
The four forces act on the airplane in flight and also work against each other.

The Four Forces of Flight
Since the Wright brothers first flew in 1903, people have created a multitude of airplane types. But every one of them has dealt with the same four forces--lift, weight, thrust, and drag. Lift is the most complex, so let’s tackle it first. Picture from: (See Internet Resources Guide) The four forces act on the airplane in flight and also work against each other.

- the force most familiar to us because we feel it everyday
- always directed toward the center of the earth - for the a/c to fly, enough lift must be generated to counteract the weight of the a/c + fuel + payload/passengers - weight acts through CG-center of gravity (lift acts through the CP-center of pressure) - it’s not an aerodynamic force, but a field force; it’s acting on the aircraft even when it’s not in flight Weight is a force that is always directed toward the center of the earth. The magnitude of the force depends on the mass of all the airplane parts, plus the amount of fuel, plus any payload on board (people, baggage, freight, etc.). The weight is distributed throughout the airplane. But we can often think of it as collected and acting through a single point called the center of gravity. In flight, the airplane rotates about the center of gravity, but the direction of the weight force always remains toward the center of the earth. During a flight, the airplane's weight constantly changes as the aircraft consumes fuel. The distribution of the weight and the center of gravity can also change, so the pilot must constantly adjust the controls to keep the airplane balanced. Weight is the force generated by the gravitational attraction of the earth on the airplane. We are more familiar with weight than with the other forces acting on an airplane, because each of us have our own weight which we can measure every morning on the bathroom scale. We know when one thing is heavy and when another thing is light. But weight, the gravitational force, is fundamentally different from the aerodynamic forces, lift and drag. Aerodynamic forces are mechanical forces and the airplane has to be in physical contact with the the air which generates the force. The gravitational force is a field force; the source of the force does not have to be in physical contact with the object (the airplane).

Weight counteracts lift.
The earth’s gravity pulls down on objects and gives them weight. Weight counteracts lift. Remember, the four forces work on the aircraft and against each other – weight acts against lift. If my airplane weights 1700 pounds, in the simplest sense, I’d need 1700 pounds of lift generated by the wings to get it off the ground. OK – somebody do me a favor a jump up out of your chair. Thanks! Now, can anyone tell me why he/she didn’t float away?

What’s it take to create lift?
Air and motion. How do we explain lift? Newton’s Laws of Motion and Bernoulli’s Principal are used to explain lift. You need a fluid (air acts like a fluid) and motion. You need air and you need the wing to be moving through the air (or air to be moving over the wing). ***So, if the lift off speed of a small aircraft is 50 kts, will it try to fly in a strong wind? You bet it will – that’s why we always tie airplanes down! Laws/principals proposed by Bernoulli & Newton are used to explain lift. (although neither of them proposed the theories for that reason)

Newton’s Second Law: force causes a change in velocity which in turn generates another force.
Newton’s Third Law: net flow of air is turned down resulting in an ‘equal and opposite’ upward force. Newton’s Second Law states that a force will cause a change in velocity and a change in velocity will generate a force. Also, the net flow of air around the wing is turned down resulting in an ‘equal and opposite’ upward force (Newton’sThird Law). From NASA’s Glenn Research Center: Lift occurs when a flow of gas is turned by a solid object.(Newton’s Second Law – a force will cause a change in velocity and a change in velocity will generate a force.) The flow is turned in one direction, and the lift is generated in the opposite direction, according to Newton's Third Law of action and reaction. For an airfoil, both the upper and lower surfaces contribute to the flow turning. Neglecting the upper surface's part in turning the flow leads to an incorrect theory of lift. For more detail, visit the Glenn Research Center site:

Newton’s Third Law states that for every action there is an equal and opposite reaction.
Illustration from Plane Math: (See Internet Resources) .

Venturi Tube Bernouli’s first practical use of his theorem
Where are venturi tubes used today? Understanding a Venturi tube is essential to understanding lift. As velocity in the constriction increases, pressure must decrease.

Hold two sheets of paper together, as shown here, and blow between them. No matter how hard you blow, you cannot push them more than a little bit apart! Also: Make two stacks of books, three or four tall. Place them next to each other with a small gap between. Place a sheet of paper over the books. Move the two stacks close enough to each other that the paper doesn’t sag down into the gap. Now blow between the books, under the paper. You might expect the ‘wind’ to blow the paper up, but it the lower pressure will suck the paper down into the gap.

Bernoulli’s Theory in Action
The sideward tug you feel on your car when you pass a large truck going in the opposite direction is caused by air pressure. The passing vehicles form a constriction that speeds up the flow of air, reducing the air pressure between them. (It makes no difference which is moving--the air or the vehicles. The result is the same.) The higher air pressure on the other side of the car pushes it toward the truck during the split-second as they pass. Air speeds up in the constricted space between the car & truck creating a low-pressure area. Higher pressure on the other outside pushes them together.

What is a wing? A wing is really just half a venturi tube.
Venturi tubes describe what happens over a wing. A wing acts like half a venturi tube. A wing is really just half a venturi tube.

A fluid (and air acts like a fluid) speeds up as it moves through a constricted space
Bernoulli’s Principle states that, as air speeds up, its pressure goes down. We’ve covered how Newton’s Laws of Motion are used to explain lift. Let’s talk now about how Bernoulli’s Principle helps. When moving air encounters an obstacle--a person, a tree, a wing--its path narrows as it flows around the object. Even so, the amount of air moving past any section of the path must be the same, because mass can be neither created nor destroyed. The air must speed up where the path narrows, in order to have the same mass flowing through it. So air speeds up where its path narrows and slows down where it widens.

One of the many simple illustrations of Bernoulli’s Principle
One of the many simple illustrations of Bernoulli’s Principle. Here a couple more follow.

Bernoulli's Principle: slower moving air below the wing creates greater pressure and pushes up.
We’ve covered how Newton’s Laws of Motion are used to explain lift. Let’s talk now about how Bernoulli’s Principle helps. The air above a wing tends to move faster than the air below it. According to Bernoulli's Principle, slower air has higher pressure than faster air. That means that the air pressure pushing up on the bottom of the wing is greater than the pressure pushing down, so the wing goes up.

Bernoulli’s Principle: Air moving over the wing moves faster than the air below. Faster-moving air above exerts less pressure on the wing than the slower-moving air below. The result is an upward push on the wing--lift! A wing is shaped and tilted so the air moving over it moves faster than the air moving under it. Bernoulli’s Principle says that as air speeds up, its pressure goes down. The faster-moving air above exerts less pressure on the wing than the slower-moving air below. The result is an upward push on the wing--lift! Illustration from “How Things Fly” (See Internet Resources)

Bernoulli’s Principal: pressure variation around the wing results in a net aerodynamic pushing up.
Bernoulli’s Principal explains how pressure variation around a wing results in a net aerodynamic force. Air moving more quickly over the top of the wing creates lower pressure above. For more detail, visit the Glenn Research Center site:

A wing creates lift due to a combination of Bernoulli’s Principal & Newton’s Third Law
The super-simple explanation!

Interactive Wright 1901 Wind Tunnel

Wing Shape Internal ribs define the wings shape
These Civil Air Patrol Cadets are making ribs out of building styrafoam that will shape a horizontal stabilizor. How will the airfoil be placed for the horizontal stabilizer? Lift pointing up or lift pointing down? Lead in to weight and balance discussion.

These are the main wings.

This bush plane has a very pronounced curve in the airfoil shape. Why?

What other aerodynamic devices can you see?
This US Navy Carrier Jet has a very small wing, how can it fly? Can you see the airfoil? Why is the wing small? What other aerodynamic devices can you see? This US Navy Carrier Jet has a very small wing, how can it fly? Can you see the airfoil? Why is the wing small? What other aerodynamic devices can you see?

How can an airplane fly upside down?
Answering “how do airplanes fly inverted” is difficult if you stick only with the ‘curve over the top of the wing makes the air move faster” explanation. The curve (or camber) of many wings is greater on the top and facilitates the pressure difference between top and bottom. However, with no camber difference between top and bottom or even with no curve at all in the wing, pressure difference can be created by tilting the wing into the airflow – angle of attack. As long as the wing is tilted into the oncoming airflow (relative wind) at a great enough angle, the wing will produce lift. It doesn't matter which surface of the wing-- top or bottom--is facing "up." Aerobatic airplanes, which are built to fly upside down, have wings whose upper and lower surfaces are equally curved. Wings shaped this way make it easier to fly upside down because they don't need to be tilted as far to produce enough lift.

Pitch Around the Lateral Axis

Elevator Controls Pitch
The ELEVATOR controls PITCH. On the horizontal tail surface, the elevator tilts up or down, decreasing or increasing lift on the tail. This tilts the nose of the airplane up and down.

Roll Around Longitudinal Axis

Ailerons Control Roll The AILERONS control ROLL. On the outer rear edge of each wing, the two ailerons move in opposite directions, up and down, decreasing lift on one wing while increasing it on the other. This causes the airplane to roll to the left or right.

Yaw Around the vertical Axis

Rudder Controls Yaw The RUDDER controls YAW. On the vertical tail fin, the rudder swivels from side to side, pushing the tail in a left or right direction. A pilot usually uses the rudder along with the ailerons to turn the airplane.

Vectors: Two Kinds in Aviation
Vectors to final approach – instructions to a pilot to steer a specific course “Turn left heading 270, vectors to final approach course Grand Junction.” A physics term to define magnitude and direction. Vectors are confused in aviation. One is just a steering command from ATC (Air Traffic Control). The other is used in determining how much lift is left on an airplane during a turn, groundspeed vs airspeed calculations, how wing will affect cross country flying, etc.

Vectors 20 Direction: 045 Magnitude: 20 45 o
A physics term to define magnitude and direction. 20 Direction: 045 Magnitude: 20 Basic definition of a vector. A vector is an arrow with direction and length. A number without direction is called a scalar. 45 o What?

Vectors 20 What Units? Some unit of distance, force, acceleration, time, etc. What units: typically distance as in inches, feet, miles, kilometers. In lift equations, vector magnitude is a force with units of pounds. In wind calculations, vector magnitude is a velocity with units of rate or distance per time, mph, knots, kmph.

Vectors The dog’s leash is a vector. It extends in a direction with a specific length.

Vectors Two leashes could provide the same action on the dog in both vertical and horizontal distances. They would act on the dog with the same force and as smart as this dog looks, he would not know the difference between wether there was one leash or two.

What good are they? Or, “I was told there would be No Math!” They help us find out what happens! Vector math is a very special kind of math. To find the resultant you add the vectors tail to nose. The resultant is then drawn from the origin to the end of the second or last vector. This can be derived by drawing vectors carefully and ensuring that they are parallel when placed tail to nose. Alternatively, vectors can be solved with trigonometry or Pythagorean theorem. Adding Vectors together = Resultant

Vectors Lift Therefore, any “vector” can be “analyzed” or broken down into horizontal and vertical components Lift is always produced on top of the wing and is perpendicular to the horizontal axis. This is problematic in a turn. See how the vertical component lift is reduced in order to accommodate the horizontal component. Gravity or weight always points to the center of the earth, straight down.

Vectors: “The MATH” Pythagorean Properties of right triangles
Here is the real math that allows one to solve vectors. Pythagorean Theorem is basic high school geometry. Properties of right triangles involve trigonometric functions from advanced high school mathematics and pre calculus.

Which of these airplanes will speed up? Which will slow down?
The top one will slow down (drag is greater than thrust); the bottom will speed up (thrust is greater than drag).

FRICTION DRAG: How Things Fly: Friction is the resistance to motion that occurs when two things rub together. Air rubbing against the surface of an airplane creates a force of resistance, known as friction drag. NASA Glenn Research Pages: One of the sources of drag is the skin friction between the molecules of the air and the solid surface of the aircraft. Because the skin friction is an interaction between a solid and a gas, the magnitude of the skin friction depends on properties of both solid and gas. For the solid, a smooth, waxed surface produces less skin friction than a roughened surface. For the gas, the magnitude depends on the viscosity of the air and the relative magnitude of the viscous forces to the motion of the flow, expressed as the Reynolds number. PRESSURE DRAG OR FORM DRAG: How Things Fly: Air flowing past an object pushes harder against the upstream side than against the downstream side. This pressure difference between front and back creates a backward force called pressure drag. Streamlining an object can dramatically reduce pressure drag. NASA Glenn Research Pages: We can also think of drag as aerodynamic resistance to the motion of the object through the fluid. This source of drag depends on the shape of the aircraft and is called form drag. As air flows around a body, the local velocity and pressure are changed. Since pressure is a measure of the momentum of the gas molecules and a change in momentum produces a force, a varying pressure distribution will produce a force on the body. We can determine the magnitude of the force by integrating (or adding up) the local pressure times the surface area around the entire body. The component of the aerodynamic force that is opposed to the motion is the drag; the component perpendicular to the motion is the lift. Both the lift and drag force act through the center of pressure of the object. VORTEX DRAG OR INDUCED DRAG: How Things Fly: The higher-pressure air below a wing spills up over the wing tip into the area of lower-pressure air above. The wing's forward motion spins this upward spill of air into a long spiral, like a small tornado, that trails off the wing tip. These wing tip vortices create a form of drag called vortex drag. Tilting the airplane's wings upward makes the vortices stronger and increases vortex drag. Vortices are especially strong during takeoff and landing, when an airplane is flying slowly with its wings tilted upward. NASA Glenn Research Pages: There is an additional drag component caused by the generation of lift call induced drag. This drag occurs because the flow near the wing tips is distorted as a result of the pressure difference from the top to the bottom of the wing. Swirling vortices are formed at the wing tips, and there is an energy associated with these vortices. The induced drag is an indication of the amount of energy lost to the tip vortices. The magnitude of induced drag depends on the amount of lift being generated by the wing and on the wing geometry. Long, thin (chordwise) wings have low induced drag; short wings with a large chord have high induced drag.

Drag is the force of resistance an aircraft ‘feels’ as it moves through the air.
When you put your hand out the wind of a car, you can feel the resistance of the air. The airplane ‘feels’ this resistance as it moves through the air too. Run your hand across the smooth top of your desk. It moves pretty easily, right? Now – close your eyes and imagine your desk covered with sand paper. What would it be like to run your hand across the desk now? You’d feel friction, right? Air molecules ‘feel’ friction against each little things that sticks out from the airplane’s skin – antenna, wheels, even tiny rivets. “How Things Fly” has lots of explanation about drag and flight at supersonic speeds. (See Internet Resources) The “Plane Math” lesson has a simple explanation that’s animated. (See Internet Resources)

This Gulfstream IV is making lift
This Gulfstream IV is making lift. Lift rolls off from underside the wings at the wing tips making wing tip vortices. Wing tip vortices of larger aircraft are a problem to smaller aircraft.

For an airplane to take off, lift must be greater than weight.
For an airplane to speed up while flying, thrust must be greater than drag.

Engines (either jet or propeller) typically provide the thrust for aircraft. When you fly a paper airplane, you generate the thrust. Now going back to the simple ingredients for creating lift – air and motion. Something needs to get that wing moving through the air before it can create lift. That’s where thrust comes in.

A propeller is a spinning wing that generates lift forward.
Why the twist? As a propeller blade spins, its tip slices through the air faster than the part near its hub. This rotary motion, combined with the airplane's forward motion, changes the effective direction of the oncoming air at different points along the propeller blade. Twisting the blade makes it meet the air at about the same angle across its entire length. This provides the most thrust and the least drag. The angle of attack of the propeller (the angle at which the blade meets the oncoming air or relative wind) is greater near the hub because that’s where it’s spinning the slowest.

Propeller-Produced Thrust
For the forty years following the Wright brothers first flight, airplanes used internal combustion engines to turn propellers to generate thrust. Most general aviation or private airplanes are still powered by propellers and internal combustion engines, much like your automobile engine. The engine takes air from the surroundings, mixes it with fuel, burns the fuel to release the energy in the fuel, and uses the heated gas exhaust to move a piston, which is attached to a crankshaft. In the automobile, the shaft is used to turn the wheels of the car. In an airplane, the shaft is connected to a propeller. Propellers as Airfoils On this slide, we show pictures of a P-51 propeller-powered airplane from World War II and a propeller being tested in a NASA Glenn wind tunnel. The details of propeller propulsion are very complex, but we can learn some of the fundamentals by using a simple momentum theory. The details are complex because the propeller acts like a rotating wing creating a lift force by moving through the air. For a propeller-powered aircraft, the gas that is accelerated, or the working fluid, is the surrounding air that passes through the propeller. The air that is used for combustion in the engine provides very little thrust. Propellers can have from 2 to 6 blades. As shown in the wind tunnel picture, the blades are usually long and thin. A cut through the blade perpendicular to the long dimension will give an airfoil shape. Because the blades rotate, the tips moves faster than the hub. So to make the propeller efficient, the blades are usually twisted. The angle of attack of the airfoils at the tip is lower than at the hub.

What will happen when the fire-fighting plane drops its load of water?
Weight will decrease – then the amount of lift being generated will be greater than weight and the airplane will gain altitude.

AIRPLANE PARTS The most important part is the wing – remember, this is how the airplane generates lift. What does it take to generate lift again? Motion! The prop and engine are necessary to get the airplane moving through the air so the wings can do their job and create lift. The fuselage is where you go! That’s the body of the airplane – where the pilot, passengers and baggage ride. Ailerons, elevators and rudder are control surfaces. These are how the pilot steers the airplane.

The wings generate most of the lift to hold the plane in the air
The wings generate most of the lift to hold the plane in the air. To generate lift, the airplane must be pushed through the air. The jet engines, which are located beneath the wings, provide the thrust to push the airplane forward through the air. Some airplanes use propellers for the propulsion system instead of jets. To control and maneuver the aircraft, smaller wings are located at the tail of the plane. The tail usually has a fixed horizontal piece (called the horizontal stabilizer) and a fixed vertical piece (called the vertical stabilizer). The stabilizers' job is to provide stability for the aircraft, to keep it flying straight. The vertical stabilizer keeps the nose of the plane from swinging from side to side, while the horizontal stabilizer prevents an up-and-down motion of the nose. (On the Wright brother's first aircraft, the horizontal stabilizer was placed in front of the wings. Such a configuration is called a canard after the French word for "duck"). At the rear of the wings and stabilizers are small moving sections that are attached to the fixed sections by hinges. In the figure, these moving sections are colored brown. Changing the rear portion of a wing will change the amount of force that the wing produces. The hinged part of the vertical stabilizer is called the rudder; it is used to deflect the tail to the left and right as viewed from the front of the fuselage. The hinged part of the horizontal stabilizer is called the elevator; it is used to deflect the tail up and down. The outboard hinged part of the wing is called the aileron; it is used to roll the wings from side to side. Most airliners can also be rolled from side to side by using the spoilers. Spoilers are small plates that are used to disrupt the flow over the wing and to change the amount of force by decreasing the lift when the spoiler is deployed. The wings have additional hinged, rear sections near the body that are called flaps. Flaps are deployed downward on takeoff and landing to increase the amount of force produced by the wing. On some aircraft, the front part of the wing will also deflect. Slats are used at takeoff and landing to produce additional force. The spoilers are also used during landing to slow the plane down and to counteract the flaps when the aircraft is on the ground. The next time you fly on an airplane, notice how the wing shape changes during takeoff and landing. The fuselage or body of the airplane, holds all the pieces together. The pilots sit in the cockpit at the front of the fuselage. Passengers and cargo are carried in the rear of the fuselage. Some aircraft carry fuel in the fuselage; others carry the fuel in the wings.

Airplane Parts Why are the wheel pants shaped the way they are?
A vertical stabilizer, or tail fin, keeps the airplane lined up with its direction of motion. Air presses against both its surfaces with equal force when the airplane is moving straight ahead. But if the airplane pivots to the right or left, air pressure increases on one side of the stabilizer and decreases on the other. This imbalance in pressure pushes the tail back into line. Like the vertical stabilizer, the horizontal stabilizer helps keep the airplane aligned with its direction of motion. If the airplane tilts up or down, air pressure increases on one side of the stabilizer and decreases on the other, pushing it back to its original position. The stabilizer also holds the tail down, counteracting the tendency of the nose to tilt downward--a result of the airplane's center of gravity being forward of the wing's center of lift. To help make turning easier, an airplane is usually less stable along its roll axis than along its pitch and yaw axes. Several factors help the pilot keep the wings level: the inclined mounting of the wings, the position of the wings above or below the fuselage, the swept-back shape of the wings, and the vertical stabilizer. As an airplane rolls, it tends to slip to the side, changing the direction of relative wind on the wings and tail. These design features help the pilot restore the airplane to its upright position. Flaps change a wing's curvature, increasing lift. Airplanes use flaps to maintain lift at lower speeds, particularly during takeoff and landing. This allows an airplane to make a slower landing approach and a shorter landing. Flaps also increase drag, which helps slow the airplane and allows a steeper landing approach.