Bernoulli's Principle It explains why and airplane gets lift What is that? It explains why and airplane gets lift It explains why a curve ball curves It can impress your friends

Theory of Flight Aerodynamics Bernoulli principle “As the air velocity increases, the pressure decreases; and as the velocity decreases, the pressure increases.” A major part of the knowledge base needed in the design and development of aircraft. Contributed to the work of G.B. Venturi, an Italian scientist, who first noted the effects of constricted channels on the flow of fluids. A round tube, such as a nozzle or jet engine, designed to increase the speed of flowing gases and liquids is called a venturi. Bernoulli principle A principle of nature discovered by Swiss mathematician, Daniel I. Bernoulli. He observed that moving air exerts less pressure than still air. The Bernoulli principle states "As the air velocity increases, the pressure decreases; and as the velocity decreases, the pressure increases." A major part of the knowledge base needed in the design and development of aircraft. Contributed to the work of G.B. Venturi, an Italian scientist, who first noted the effects of constricted channels on the flow of fluids. A round tube, such as a nozzle or jet engine, designed to increase the speed of flowing gases and liquids is called a venturi.

Why a curve ball curves? What do the laces do? What does faster/slower rotation do? Demonstrate?

Theory of Flight The four forces in balance with one another hold the plane in the air. The four forces are lift, weight, thrust, and drag. The Forces of Flight A plane is in the air and four forces in balance with one another hold it there. The four forces are lift, weight, thrust, and drag. These forces operate in pairs: thrust and drag; lift and weight. Weight is a measure of gravity, which is the attraction of the Earth for all bodies on or near it. Lift operates to overcome weight, and weight serves to keep the aircraft from rising any higher than the pilot wants it to go. Thrust is a force that gives forward motion to the aircraft. The propeller or the jet engine produces the thrust. Drag is the force that is opposed to thrust. It opposes the forward motion, of the aircraft. The resistance of the air to the aircraft passing through it causes drag. When weight and lift are equal, the aircraft flies level. When thrust and drag are equal, the aircraft flies at a constant rate of speed. Each of these four forces is both an asset and a liability. They are forces to use and forces to overcome. The thrust of the engine produces the drag of air rushing past the aircraft. Without this drag, an aircraft would be like a car without brakes or steering equipment. Weight, too, can be an asset. It provides stability and control. Fuel capacity and payload (generally, passenger or cargo) contribute to weight. Thrust and lift, the two helpful forces, must also be kept within the limits of usefulness and safety. An aircraft can be designed with decreased drag, but this decreased drag may also decrease lift.

Video Demonstration Video Presentation / Bernoulli’s Principle

The Four Forces of Flight According to the Bernoulli Principle, there is an increase in the velocity of air as the airflow around an airfoil shape; therefore, there is an increase of the relative wind as it flows above and below the surface of the airplane wing. The Four Forces of Flight Lift According to Bernoulli’s Principle, there is increase in the velocity of air as the air flows around an airfoil shape. Because the camber of the upper wing surface is greater than that of the lower surface, air flowing above the wing will be increased more than air flowing beneath the wing. The Bernoulli’s Principle also states that an increase in the velocity of a fluid, such as air, results in a decrease of pressure within that fluid. As a result, the reduction in air pressure above the wing will be greater than the pressure reduction along the lower wing surface.

The Four Forces of Flight Lift can be increased in two ways 1. Increasing the forward speed of the airplane. (Increase Relative Wind) 2. Increase the angle of attack. The pilot can increase the forward speed of the aircraft by applying more power (Thrust). Lift can be increased in two ways: by increasing the forward speed of the airplane or by increasing the angle of attack. The pilot can increase the forward speed of the aircraft by applying more power. This increases the speed of the relative wind over the airfoil.

The Four Forces of Flight Lift Variables The pilot must have some way to control the amount of lift the airfoils generate. There are variables acting on the amount of lift generated. Lift Variables The pilot must have some way to control the amount of lift the airfoils generate. If the pilot didn’t, the aircraft would either constantly stall or climb. There are variables acting on the amount of lift generated. Angle of attack Velocity of relative wind (speed of the aircraft) Air density Airfoil shape Wing area Airfoil platforms High-lift devices.

The Four Forces of Flight Angle of Attack Angle of Attack Again Changing the angle of attack can change the amount of lift generated as the airfoil moves through the air. Airflow over an airfoil is normally smooth with no turbulence. In the case of an airfoil with a flat or approximately flat undersurface and when the lower surface is parallel to the relative wind, there is no impact pressure on the lower surface. The whole lift force comes from reduced pressure along the upper surface (pressure-differential lift). When the wing is tipped up so that the lower surface makes an angle of 5 with the relative wind, the impact pressure on the undersurface contributes about 25 percent of the total lift. When it is tipped up to 10, the impact pressure on the lower surface produces about 30 percent of the total lift. A small force acts on each tiny portion of the wing. This force is different in magnitude (size) and direction from the force acting on other small areas of the surface farther forward or rearward.

The Four Forces of Flight Angle of Attack The sum of all the tiny forces over the surface of the wing is called the resultant. It is possible to mathematically add all of these small forces, taking into account their magnitude, direction, and location. The sum of all the tiny forces over the surface of the wing is called the resultant, since it results from adding all the forces together.

The Four Forces of Flight Angle of Attack This resultant has magnitude, direction, and location. The point of intersection of the resultant with the chord of the wing is called the center of pressure (C/P). This resultant has magnitude, direction, and location. The point of intersection of the resultant with the chord of the wing is called the center of pressure (C/P).

The Four Forces of Flight Angle of Attack The angle at which lift stops increasing and begins to decrease is called the burble point. The angle at which lift stops increasing and begins to decrease is called the burble point. This angle may also be called the stalling angle or the angle of maximum lift. When the angle of attack is increased beyond the burble point, the resultant decreases in magnitude and its angle back from the vertical becomes bigger. At the various angles just described, the direction of the resultant has had an upward and backward direction. As the angle of attack is increased, more and more lift is generated. This increase in amount of lift continues up to a certain angle of attack (the burble point, mentioned previously) which depends on the type of wing design. Most aircraft wings have a burble point of somewhere between 15 and 20°, but this is built into the aircraft. When the air no longer flows smoothly over the top surface of the airfoil it is called burbling. When burbling is taking place on a surface, there can be no decrease in pressure below the atmospheric pressure.

The Four Forces of Flight Angle of Attack The point at which the amount of lift generated is no longer sufficient to support the aircraft in air is called the STALLING POINT. Typically 15 degrees The point at which the amount of lift generated is no longer sufficient to support the aircraft in air is called the stalling point, and the maneuver in which the pilot does this is called the stall.

The Four Forces of Flight Velocity of Relative Wind The velocity of the airfoil through the air is a factor in determining the amount of lift generated. If an airfoil is made to travel faster through the air, greater pressure differences between the lower and upper surfaces of the airfoil result. Increase Relative Wind – Increase Lift Another Lift Variable - Velocity of Relative Wind The velocity of the airfoil through the air is another important factor in determining the amount of lift generated. If an airfoil is made to travel faster through the air, a greater pressure difference between the lower and upper surfaces of the airfoil results.

The Four Forces of Flight Air Density and Lift Lift varies directly with density. If flying at 18,000 feet where the density is about half that at sea level, an aircraft will need to travel 1.414 times as fast as it would at sea level to maintain altitude. If something reduces the lift by half, we will have to increase the speed so that the square root of the new velocity is twice the square of the original velocity. Air Density and Lift Air density is another variable factor that can influence lift. The first thing to note is that lift varies directly with density. For instance, at 18,000 feet, where the density is about half that at sea level, an aircraft will need to travel 1.414 times as fast as it would at sea level to maintain altitude. The number 1.414 is the square root of 2. If something reduces the lift by half, it has to increase the speed so that the square of the new velocity is twice the square of the original velocity.

The Four Forces of Flight Airfoil Shape It is extremely important to preserve the characteristic curve that the designers built into the airfoil. Dents, mud, and ice are three common things that can spoil the built-in shape of the airfoil and interfere with the performance of the entire aircraft. Airfoil Shape as a Variable Up to a certain point, the greater the camber, the greater the lift. It becomes extremely important once an airfoil has been designed, to preserve the characteristic curve that the designers build into the airfoil. Dents, mud, and ice are three common things that can spoil the built-in shape of the airfoil and interfere with the performance of the entire aircraft.

The Four Forces of Flight Wing Area and Lift The greater the surface area of the wing, the greater the amount of lift that will be generated. Gliders and sailplanes are very good examples of how a large wing surface generates lift. Wing Area and Lift If the pressure differential is only 2½ ounces per square inch (a very small amount of differential pressure), this will produce a lifting force of more than 20 pounds per square foot (144 square inches/square foot x 2½-ounces/square inch). The greater the surface area of the wing, the greater the amount of lift that will be generated, within practical limitations if the proportions of the wing and the airfoil section stay the same. Gliders or sailplanes are very good examples of how a large wing surface generates lift. Lighter, stronger, materials are being developed, so that today’s aircraft can be built to withstand tremendous strains and yet not be heavy.

The Four Forces of Flight Weight There is a point in the relationship of airfoil to angle of attack where lift is destroyed and the force of gravity (weight) takes command. Some of the most powerful jet fighter types and aerobatic sport airplanes can, for a short time and distance, climb straight up without any significant help from their airfoils. Weight There is a point in the relationship of airfoil to angle of attack where lift is destroyed and the force of gravity (weight) takes command. Some of the most powerful jet fighter types and aerobatic sport airplanes can, for a short time and distance, climb straight up without any significant help from their airfoils, but these airplanes will eventually stall and start to fall toward Earth. The stalled condition is one from which recovery and continued flight is fairly easy.

The Four Forces of Flight Weight There is another situation where lift can no longer overcome weight. The atmosphere becomes less and less dense as altitude increases. The airplane must be constructed of the lightest weight materials that can be used. The weight of whatever the airplane carries (payload) also receives very careful consideration. There is another situation where lift can no longer overcome weight. This limit is called the aircraft's ceiling. At its ceiling, the aircraft's power plant is producing all possible power, and the airfoils are producing all possible lift just to equal the force of the aircraft's weight. The atmosphere becomes less and less dense as altitude increases. The aircraft's ceiling is that point in the atmosphere where the air is too thin to allow further increase in lift. The airplane must be constructed of the lightest weight materials that can be used. Most airplanes today are built of metal. Aluminum alloy is used extensively in aircraft construction because of its strength and light weight. The weight of whatever the airplane carries also receives very careful consideration. Each airplane has a total weight limitation called the maximum allowable gross weight above which the airplane is unsafe for flight.

The Four Forces of Flight Weight Where the weight, or useful load, is placed in the airplane is another factor that has a pronounced effect on how well an airplane will fly. The pilot has to subtract the empty weight from the maximum allowable gross weight to find out how many pounds may be loaded into the airplane. This is the useful load. Where the weight, or useful load, is placed in the airplane is another factor that has an effect on how well the airplane will fly. This is because the center of gravity of the airplane must be within certain limits prescribed by the manufacturer. These limits are based on where the center of lift (CL) of the particular design happens to be. If placement of the useful load moves the center of gravity too far forward or too far aft of the CL, the airplane will be difficult, if not impossible, to control while in flight

The Four Forces of Flight Thrust and Drag Thrust is the force that propels the aircraft forward. An airplane cannot gain altitude or maintain straight and level flight unless its engine is producing enough thrust. Without the needed thrust, weight has more influence than lift and pulls the airplane toward the ground. Thrust and Drag Thrust is the force that propels the aircraft forward. Thrust for aircraft is obtained from different types of engines. An airplane cannot gain altitude or maintain straight and level flight unless its engine is producing enough thrust to propel (pull or push) the airfoils fast enough to produce the needed amount of lift. Without this thrust, the airplane will continue to fly. It will not “drop out of the sky” as many people think, but its flight becomes a gradual descent toward the ground. Without the needed thrust, weight has more influence than lift and pulls the airplane toward the ground. Helping the force of weight is drag.

The Four Forces of Flight Thrust and Drag Drag is present all the time and can be defined as the force that opposes thrust. The friction of air particles rubbing against all parts of the airplane causes part of the total drag. The shape of something may create low-pressure areas and turbulence that retard the forward movement of the aircraft. Drag is present at all times and can be defined as the force that opposes thrust. Better yet, drag is the force that opposes all motion through the atmosphere and is parallel to the direction of the relative wind. The friction of air particles rubbing against all parts of the airplane causes part of the total drag. In fact, airspeed can be increased several miles per hour if the surfaces of the airplane are kept highly polished. The shape of something may create low-pressure areas and turbulence that retard the forward movement of the aircraft. Streamlining the aircraft will reduce form drag. Parts of an aircraft that do not lend to streamlining are enclosed in covers, called fairings (or cowling for an engine), that have a streamlined shape.

Summary 1. Theory of Flight 2. Airfoils and Flight 3. Relative Wind 4. Angle of Attack 5. The Four Forces of Flight In this lesson we discussed: 1. Theory of Flight 2. Airfoils and Flight 3. Relative Wind 4. Angle of Attack 5. The Four Forces of Flight

Summary As air velocity increases, pressure decreases As velocity decreases, pressure increase Pressure increase creates lift Curveball is a ball adjusting to pressure Bernoulli principle major part of wing design