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# Airfoils, Lift and Bernoulli’s Principle

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Airfoils, Lift and Bernoulli’s Principle
Airfoils and Lift Gateway To Technology® Unit 5 – Lesson 5.2 – Aeronautics Airfoils, Lift and Bernoulli’s Principle

History Ancient Discoveries Camber Airfoil Chinese and kites
Airfoils and Lift Gateway To Technology® Unit 5 – Lesson 5.2 – Aeronautics History Ancient Discoveries Chinese and kites Early flight explorers Lilienthal and Cayley in the 1800s Camber A measure of the curvature of the airfoil Airfoil a part with a flat or curved surface, such as a wing or rudder, specifically designed to keep an aircraft in the air. The ancient Chinese discovered that kites with curved surfaces flew better than kites with flat surfaces. Lilienthal and Cayley demonstrated that a curved surface produces more lift than a flat surface. This led to the conclusion that a wing must have a curve, known as camber. The top must be slightly curved like a hump. The bottom is left flat or straight. An object with this shape is called an airfoil.

Newton’s Laws and Bernoulli’s Principle
Gateway To Technology® Unit 5– Lesson 5.2– Aeronautics Airfoil A cut through the wing perpendicular to the leading and trailing edges will show the cross section of the wing. This side view is called an airfoil, and it has some geometry definitions of its own as shown at the lower left. The straight line drawn from the leading to trailing edges of the airfoil is called the chord line. The chord line cuts the airfoil into an upper surface and a lower surface. If we plot the points that lie halfway between the upper and lower surfaces, we obtain a curve called the mean camber line. For a symmetric airfoil (upper surface the same shape as the lower surface) the mean camber line will fall on top of the chord line. But in most cases, these are two separate lines. The maximum distance between the two lines is called the camber, which is a measure of the curvature of the airfoil (high camber means high curvature). The maximum distance between the upper and lower surfaces is called the thickness.

Newton’s Laws and Bernoulli’s Principle
Gateway To Technology® Unit 5– Lesson 5.2– Aeronautics Airfoil Trailing Edge Leading Edge The front of the wing (at the bottom) is called the leading edge; the back of the wing (at the top) is called the trailing edge. The angle of attack is the angle at which relative wind meets an airfoil. The angle of attack is the angle that is formed by the chord of the airfoil and the direction of the relative wind or between the chord line and the flight path. The angle of attack changes during a flight as the pilot changes the direction of the aircraft. It is one of the factors that determines the aircraft's rate of speed through the air. Angle of Attack

Bernoulli’s Principle
Newton’s Laws and Bernoulli’s Principle Gateway To Technology® Unit 5– Lesson 5.2– Aeronautics Bernoulli’s Principle The pressure of a fluid decreases as the speed of the fluid increases The principle was first discovered in the 1700s. When a fluid moves quickly, its pressure decreases. Since air is considered a fluid, this principle can be applied to the design of a wing. Lift can be explained in part by the Bernoulli Principle. A Swiss mathematician named Daniel Bernoullli discovered that as the speed of a fluid increases, its pressure decreases. Air is a fluid so as it flows over a wing or airfoil, the wing’s shape and angle of attack cause the air to speed up above the wing’s surface. As the air speeds up, its pressure goes down, creating a low pressure area above the wing. This low pressure area creates lift, drawing the aircraft upward.

Lift Bernoulli’s Principle
Airfoils and Lift Gateway To Technology® Unit 5 – Lesson 5.2 – Aeronautics Lift Bernoulli’s Principle When a fluid moves fast, its pressure decreases. Since air is considered a fluid, this principle can be applied to the design of a wing.

Bernoulli’s Principle
Newton’s Laws and Bernoulli’s Principle Gateway To Technology® Unit 5– Lesson 5.2– Aeronautics Bernoulli’s Principle Both streams must meet at the end of the wing at the same time. Stream A has farther to go; therefore, it must travel faster. The wings of a plane are curved in such a way that the air moving over the top of the wing has a greater distance to travel than the air moving below the wing. Think of the air splitting into two different streams, A and B. Stream A traveling over the top of the wing has a greater distance to travel, and therefore must travel faster to arrive at the back of the wing at the same time as Stream B.

Bernoulli’s Principle
Newton’s Laws and Bernoulli’s Principle Gateway To Technology® Unit 5– Lesson 5.2– Aeronautics Bernoulli’s Principle Bernoulli proved that as fluids move faster, their pressure decreases. This principle explains why a plane rises. Bernoulli proved that when fluids move faster, their pressure decreases, so air pressure above the wing will be less than the air pressure below the wing. The greater pressure below the wing lifts the plane into the air.

Factors that Affect Lift
Airfoils and Lift Gateway To Technology® Unit 5 – Lesson 5.2 – Aeronautics Factors that Affect Lift Many factors affect the flow of air, which creates lift. We can group these factors into (a) those associated with the object, (b) those associated with the motion of the object through the air, and (c) those associated with the air itself. The Object: Shape and Size The Motion: Velocity and Angle of Attack The Air: Mass, Viscosity, Compressibility

Factors that Affect Lift
Airfoils and Lift Gateway To Technology® Unit 5 – Lesson 5.2 – Aeronautics Factors that Affect Lift The Object – Wing geometry Airfoil shape Wing size Aspect ratio: Mathematical relationship between the wing span (overall length) to the wing area Object: Aircraft wing geometry has a large effect on the amount of lift generated. The airfoil shape and wing size will both affect the amount of lift. The ratio of the wing span to the wing area also affects the amount of lift generated by a wing.

Factors that Affect Lift
Airfoils and Lift Gateway To Technology® Unit 5 – Lesson 5.2 – Aeronautics Factors that Affect Lift Motion – Move the object Velocity (speed) Angle of attack (how the object is tipped toward the wind) Motion: To generate lift, we must move the object through the air. The lift then depends on the velocity of the air and how the object is inclined to the flow.

Factors that Affect Lift
Airfoils and Lift Gateway To Technology® Unit 5 – Lesson 5.2 – Aeronautics Factors that Affect Lift Air – Mass Viscosity Stickiness Compressibility Springiness Density Altitude Density of air molecules at a given altitude Air: Lift depends on the mass of the flow. The lift also depends in a complex way on two other properties of the air: the viscosity, or stickiness, of the air and the compressibility, or springiness, of the air. As an object moves through a gas, the gas molecules stick to the surface. This creates a layer of air near the surface, called a boundary layer, which, in effect, changes the shape of the object. Aerodynamic forces also depend in a complex way on the compressibility of the gas. As an object moves through the gas, the gas molecules move around the object. If the object passes at a low speed (typically less than 200 mph), the density of the fluid remains constant. But for high speeds, some of the energy of the object goes into compressing the fluid and changing the density, which alters the amount of resulting force on the object. This effect becomes more important as speed increases. Near and beyond the speed of sound (about 330 m/s or 700 mph on earth), shock waves are produced that affect the lift and drag of the object. Density Altitude may be one of the most important factors affecting lift. Think about moving in water, versus moving in steam. In which would it be easier to move? Steam – because the water molecules in steam are less dense than the water molecules in water. At higher altitudes and higher temperatures, the density of the air molecules is less, which will reduce the amount of lift.

Airfoil Shapes and Lift
Airfoils and Lift Gateway To Technology® Unit 5 – Lesson 5.2 – Aeronautics Airfoil Shapes and Lift Wing or Airfoil? Airfoil is a shape designed to generate as much lift as possible while incurring as little drag as possible. The wing is attached to a plane and must take into consideration drag and the lift created.

Angles and Airfoils Angle of Incidence
Airfoils and Lift Gateway To Technology® Unit 5 – Lesson 5.2 – Aeronautics Angles and Airfoils Angle of Incidence The tilt of the wing with respect to its attachment to the body of the airplane (fuselage)

Angles and Airfoils Angle of Attack
Airfoils and Lift Gateway To Technology® Unit 5 – Lesson 5.2 – Aeronautics Angles and Airfoils Angle of Attack The tilt of the airfoil with respect to the airflow

Angles and Airfoils Airflow must remain smooth
Airfoils and Lift Gateway To Technology® Unit 5 – Lesson 5.2 – Aeronautics Angles and Airfoils Tilt with respect to airflow Airflow must remain smooth Small angle of attack; air flows smooth on the surface However, there can be too much of a good thing. The airfoil's ability to create lift is dependent on the airflow remaining smooth. Think of a stream flowing gently around a rock. The water's flow changes direction to go around/over the rock, but it remains smooth – it doesn't get jumbled or choppy – and it hugs the rock as it flows around it. Now, if that rock were a larger rock, the water would hit the rock, get all jumbled up, and then eventually move on. The flow around the larger rock would not be smooth. The same thing happens with a wing. Up to a certain angle of attack, the air will flow smoothly along the surface. The wing acts like a small rock.

Angles and Airfoils Angle of attack becomes too steep
Airfoils and Lift Gateway To Technology® Unit 5 – Lesson 5.2 – Aeronautics Angles and Airfoils Angle of attack becomes too steep Airflow is disrupted and the airplane loses lift or stalls Critical Angle of Attack is the point where it stalls If the angle of attack becomes too great, an effect similar to throwing a big rock in a stream is created. The air will get all jumbled up and will not flow smoothly around the airfoil. If this happens, lift will not be generated. We say the wing "loses its lift" or "stalls".

Angles and Airfoils Causes of stall Flying at too steep an angle
Airfoils and Lift Gateway To Technology® Unit 5 – Lesson 5.2 – Aeronautics Angles and Airfoils Causes of stall Flying at too steep an angle Real-life flying situations Sudden gust of wind decreases the plane’s forward speed, decreasing the airflow over the wing Flying too slow (indicated airspeed) Wing icing Changes the shape of the airfoil

Airfoils and Lift Gateway To Technology® Unit 5 – Lesson 5.2 – Aeronautics Amount of Lift Amount of lift produced by an airfoil depends on may factors Angle of attack Lift devices used (flaps, etc.) Density of the air Area of the wing Shape of the wing Speed at which the wing is traveling

Now follow along as we prove Bernoulli’s Principle is True!

Airfoils and Lift Gateway To Technology® Unit 5 – Lesson 5.2 – Aeronautics Image Resources Aerospaceweb.org. (2009). Ask-Us – Parts of an airplane. Retrieved June 26, 2009, from Microsoft, Inc. (2008). Clip art. Retrieved June 26, 2009, from National Aeronautics and Space Administration (NASA). (n.d.). Virtual skies: Aeronautics tutorial. Retrieved June 24, 2009, from National Aeronautics and Space Administration (NASA). (n.d.). FoilSimU beta version 1.5a. Retrieved June 19, 2009, from

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