Lift

Why do planes fly? It can be dangerous for a car to travel 120 mph, but you would rarely expect to see it take off.rarely expect to see it take off What characteristics do planes have (and cars lack) that allows planes to fly?

Wings… of course

Lift Lift counteracts weight When weight > lift, plane stays on ground or accelerates towards ground. When lift > weight, the plane accelerates upward. Weight Lift

Requirements for lift Lift requires… 1.a solid object moving through a fluid 2.a change in the direction the fluid flows No fluid, no lift No motion, no lift Fluid = gas (like air) or liquid (like water)

Factors of wing shape affect lift Speed of and orientation to relative wind 1.Engineers control angle at which wings are mounted on plane 2.Pilots control speed and angle at which plane travels.

Factors of wing shape affect lift Ratio of length of wing to its width Aspect ratio

Factors of wing shape affect lift ‘curviness’ of wing Camber

Factors of wing shape affect lift So many…. Strategies to reduce drag

Prior knowledge of wings On what type of aircraft do you find longer wings? What type of wings do you expect to find on highly- maneuverable aircraft like fighter jets? How are propellers and helicopter rotors like wings?

Incorrect explanations Explaining lift is complicated. In an effort to make it understandable, well-intentioned people have been misinforming their audiences for generations. Incorrect theory #1: Bernoulli Incorrect theory #2: Newton Incorrect theory #3: Venturi

Incorrect Theory #1: Equal Transit Times THEY SAY: Air traveling over the top of the wing travels faster than air traveling underneath the wing so that the particles reach the end of the wing at the same time.

Incorrect Theory #1: Equal Transit Times BUT… o Air molecules traveling above the wing do NOT reach the trailing edge at the same time as the air molecules that travel below the wing o the plane would have to fly VERY fast to generate a large enough pressure differential to lift the wing Aircraft would have to travel faster than the speed of sound to generate enough lift to get off the ground!

Incorrect Theory #2: Skipping Stones THEY SAY: Air bounces off the bottom of the wing, pushing the wing (and the plane) upward.

Incorrect Theory #2: Skipping Stones BUT…then the shape of the top of the wing shouldn’t make any difference at all; we know that the shape of the ENTIRE wing makes a huge difference in the amount of lift.

Incorrect Theory #3: Venturi Effect THEY SAY: The airfoil acts like a nozzle, compressing air above the wing, leading to lower pressure above the wing, leading to lift

Incorrect Theory #3: Venturi Effect BUT: The airfoil is like only 1/2 of a nozzle. Without the other 1/2, it is NOT a nozzle and does NOT compress air enough to generate sufficient lift to lift a plane

So what causes lift? The short answer: The shape of the wing changes the direction and speed of air as it flows around the wing

Physics of lift Air as individual particles Air as a fluid Step 1: Understand that air behaves like a fluid, comprised of moveable parts that interact. Do you need convincing? Schlieren optics might help.Schlieren optics

Physics of lift Step 2: Understand that air has mass. Massive objects resist changes in motion. Do you need convincing? Breaking a ruler might help. Suppose a particle of air comes barreling towards an airfoil. The particles that are already there resist a change in motion. As a result, air pressure builds up on the lower side of the leading edge of wing.

Physics of Lift You can map pressure around an airfoil and create a “pressure field” Step 3: Map the pressure around the wing. Low pressure High pressure

Physics of Lift Bernoulli’s principle:  speed   pressure  speed   pressure Step 4: Relate speed to pressure Low pressure High pressure

Physics of Lift Fluids are pushed from areas with high pressure to areas with low pressure. Step 5: Relate pressure to forces NOTE: force on air acts perpendicularly to the lines of the pressure field

Physics of Lift Stream of air is forced together above airfoil. Stream of air is forced apart below the airfoil and at the top of the trailing edge. Step 6: Relate forces to motion

Physics of Lift When air passes around an airfoil, it is deflected downward. Because the airfoil deflects air downward, the air deflects the airfoil (and the plane to which it is attached) upward. Step 7: Connect the pieces

Implications Design a wing that slows the air down at one part on the airfoil. Increase the pressure Decrease the speed Orient the aircraft to slow the air down at one part on the airfoil. Increase volume of downward deflected air around wing.

Automotive applications Spoilers on cars are inverted airfoils, designed to keep cars on the ground.