This interactive activity from NOVA allows you to explore the important forces of lift and drag and how they affect everything from airplanes to wind turbines. In order for an object to move through the air in a stable manner, it must balance four forces: lift, gravity, thrust, and drag. Lift and drag arise as air moves over and past an object like an airplane wing. The special shape of a wing that enables it to fly is called an airfoil. Airfoil design varies depending on the purpose of the object. Learn about the wings used on different types of planes, as well as airfoils used on helicopters, wind turbines, and even race cars.
All lift and drag data for the airfoils in this interactive activity were generated using NASA's FoilSim III software, version 1.3. Data assume average cruise speed for each type of aircraft.
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Here are suggested ways to use this interactive resource and activity ideas to engage students with this topic.
Airplanes and Lift and Drag
To get an airplane airborne or climbing in flight, its wings must produce more lift than the total weight of the aircraft. One popular explanation for how this happens is based on Bernoulli's principle, which describes the relationship between the velocity and pressure exerted by a fluid in motion. It states that as the velocity of a fluid increases, the pressure exerted by that fluid decreases, and vice versa. For most planes, the wings, viewed in profile, are curved on top and flat on the bottom. As air—behaving as a fluid —passes over the wing, it has further to travel and moves faster than the air passing beneath the wing. According to Bernoulli’s principle, this creates a difference in pressure that results in a net upward force.
But that explanation doesn't explain why air moving above and beneath a wing must meet at the wing's trailing edge. A complementary explanation calls on Newton's third law of motion: For every action, there is an equal and opposite reaction. The tilt of the wing, called its angle of attack, also influences airflow. As a wing is tilted upward, it generates more lift. This is because more air molecules strike the bottom surface of the wing and get deflected downward. Given Newton’s third law, this in turn transfers upward momentum to the wing. Thus, if the angle of attack is increased, the plane rises; if it is decreased, the plane descends.
Airfoil is a term for the cross-section of an airplane wing. While thick airfoils provide lots of lift, they also produce lots of drag, a force that tends to slow the motion of a plane through the air. For this reason, planes with thick airfoils are not well suited for high-speed or long-duration flight. Thin airfoils, by contrast, minimize drag and are both fast and fuel-efficient.
Fighter Jets and Helicopters
Fighter jet wings are almost symmetrical, with the curve of the upper surface nearly identical to the curve of the bottom surface. This results in less lift compared with other wing profiles. To compensate, the plane has to move through the air at high speed to stay aloft. In contrast, the spinning blades above a helicopter—long, thin wings that produce lift as they rotate—allow a helicopter to hover in place, but not fly very fast.
Unlike airplanes, which have wings that help them lift up, racecars[GJ1] have inverted wings that help them cling to the track during high-speed races.
By the middle of the 20th century, engineers understood airfoils so well that they could design wings precisely suited to the needs of different planes, from fighter jets to crop dusters. Racecar designers, however, didn't make the connection between planes and cars right away. At first, they focused mostly on reducing drag—streamlining cars so they would meet less air resistance and move faster.
But fast, streamlined racecars have a problem, especially around turns: If they are going too fast, the car will generate lift and fly off the track. Racecar designers began to borrow from airplane designers and put wings on their cars, only their wings were upside down. On most planes, the wings, viewed in profile, curve out on the top in smooth, rounded humps. The inverted wings on racecars are just the opposite: they are concave scoops that look more like shovels than wings. Most racecars today have at least two inverted wings: a front wing close to the ground and a rear wing in the air. Rather than lifting the car up, these create downward pressure—or "negative lift"—to press the car down toward the ground. This downward pressure creates more traction between the tires and the track, so the driver can turn, accelerate, and stop the car more quickly.
The same physical principles apply to other familiar items. Consider exploring the Lift and Drag properties of boat propellers, wind turbines, or even bird feathers.
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