Distance Learning Module: Types of Flight
This week we’re looking at different aspects of flight and ways that we can explore those at home.
Our world is full of things that fly! Birds, bats, insects, and other animals have bodies that allow them to soar, glide, and buzz through the air. Humans achieve air travel through all types of vehicles: jets and airplanes, helicopters, blimps, gliders, and even hot air balloons. In this module, we’ll break down the big topic of aviation and aeronautics by exploring the different types of flight and categories of human-made aircraft.
Note: Adult supervision is required for today’s hands-on activity, which involves flame!
Buoyant Flight
Buoyancy is the force that allows an object to float in a fluid. Think about a boat floating in the ocean: buoyancy is what keeps the boat from sinking in the water. We discussed this idea in a past post about aquatic astronaut training, as well as the Saturday STEM challenges of building a raft and keeping a fish underwater.
International hot air balloon festival in Leon Guanajuato, Mexico (image credit: Tomas Castelazo, www.tomascastelazo.com / Wikimedia Commons)
Those past lessons were focused on water—and in everyday life, people often use the term “fluid” as another word for “liquid.” However, in science, a fluid is any substance that flows under force: that might be a liquid, gas, or even plasma. Have you ever held a helium balloon? The balloon probably floated above your head—or floated away entirely if you let go of the string! Buoyancy is what makes the balloon float in the air around it—just like a boat floating in water.
The force of buoyancy works against the force gravity: when an object is suspended in fluid, buoyancy pulls the object UP and gravity pulls it DOWN. If the object is less dense than the fluid around it, buoyancy will make the object float. This is how hot air balloons work. The air on the inside of the balloon is heated with a burner, which makes it less dense than the cooler air on the outside of the balloon. The air inside the balloon rises, pulling the hot air balloon up into the air. Watch this nifty demonstration from the New York Hall of Science:
Airships (aka dirigibles)—like rigid zeppelins or non-rigid blimps—also float through the air using buoyancy. Instead of heating air, these vehicles fill their body with a gas (like helium, which is safer than super-flammable hydrogen) that is already lighter than air. Airships can move forward through the air, hover in place, travel in all kinds of weather, and stay afloat for days at a time. In contrast, hot air balloon pilots can control the direction of the balloon up and down by turning the burner off and on (cooling or heating up the air inside the balloon), but they rely on outside air streams to move forward or side-to-side and cannot do any tight steering.
Vehicles like these, that are powered by changes in air density and rely on the force of buoyancy, are also called aerostats, because another term for buoyancy is aerostatic lift.
illustration of various dirigibles from the Brockhaus and Efron Encyclopedic Dictionary, 1890-1907 (image credit: wikimedia commons)
Aerodynamic Flight
Another type of force that helps things fly is aerodynamic lift. Unlike aerostatic lift (buoyancy), aerodynamic lift requires the rising object to also be moving through the air. In mechanical aircraft, this forward-moving thrust is achieved with the help of an engine. Aerodynamic lift occurs in a direction perpendicular to the thrust. In other words, if a vehicle is traveling forward, lift will move it upward.
Lift is one of the four forces at work on an airplane—along with thrust, drag, and weight. (image credit: nasa)
Airplanes are a good example of a human-made aircraft that uses aerodynamic lift. An airplane’s wings help it to rise into the air. It’s a complicated process, but basically: the curved shape of the wings causes air to travel at different speeds above and below the wing, and causes air to move in a path that generates lift on the underside of the wing. (Mirka discussed this topic at greater length in yesterday’s post about Bernoulli’s Principle and the Coanda Effect.) Airplane wings are a type of airfoil—a structure that’s curved on one side, which creates lift when air flows across its surface.
A helicopter is another type of mechanical aircraft that uses rotating (spinning) wings called blades to fly. Like an airplane’s wing's, helicopter blades are airfoils, with a shape designed to control the flow of air around the wings and create lift. A helicopter's rotor (blades plus the attached control system) allows it to do things that an airplane cannot. While an airplane must fly at a high speed to move enough air over its wings to provide lift, a helicopter achieves lift by spinning its blades to move air over the rotor. The fact that a helicopter can achieve lift without moving quickly through the air means that it can hover in place in the air, fly backwards or sideways, and even take off and land without a runway. This capability makes helicopters useful for loading supplies onto ships, transporting people in medical emergencies, conducting aerial surveillance (such as with news channels’ traffic choppers), and even fighting fires.
An MH-60S Seahawk helicopter participates in a training exercise near the aircraft carrier USS Carl Vinson In the Pacific Ocean, Jan. 29, 2017 (image credit: wikimedia commons/US Department of Defense/Seaman Jake Cannady)
We can even see the principles of aerodynamic flight at work in nature!
For instance, a bird flaps its wings (another kind of airfoil) to achieve lift and thrust (forward motion). Birds angle their wings to change the flow of oncoming air, allowing them to adjust their pitch (upward or downward motion) during flight. (Katie will be posting a whole module about this later in the week—check back Thursday!)
Gliders
Some airborne animals and vehicles glide, rather than fly. Unlike a powered aircraft like a plane or helicopter (or a bird that’s flapping its wings), a glider is acted on by just three forces—lift, drag, and weight—but has no thrust. A glider generates lift by soaring through the air. Since there is no engine creating thrust, there is no force working to counteract drag as the glider moves forward. The glider naturally slows down until it can no longer generate enough lift to oppose its weight—at which point it gently “coasts” down to ground-level again.
image credit: wikimedia commons
You’ve probably made a very basic glider yourself before—a paper airplane! Balsa wood and Styrofoam are other common materials used to make toy gliders. Hang-gliders are a type of aircraft consisting of cloth wings and a minimal metal frame, which allow people to glide (sometimes very long) distances for sport. The Wright Brothers gained piloting experience through a series of glider flights from 1900 to 1903, before inventing the first motorized airplane. Interestingly, U.S. Space Shuttles even acted as gliders during reentry and landing (all fuel would be jettisoned either during ascent or before reentry into Earth’s orbit, so that the shuttle engines were present but not creating thrust).
Many types of animals have evolved gliding capabilities—from “flying” squirrels to various species of birds, insects—even frogs, snakes, and fish! Some large birds, like eagles, conserve energy by alternating periods of gliding with propelling the body by flapping their wings. Mammalian gliders like the flying squirrel achieve lift by launching themselves from trees or other tall surfaces, spreading their limbs, and catching the air on special gliding membranes.
special membranes along the side body of the sOUTHERN FLYING SQUIRREL (GLAUCOMYS VOLANS) help it to glide on the air after launching from a high perch (image credit: Nebraska Game and Parks Commission /Joe Mcdonald)
Ballistic Flight
Ballistics is the field of mechanics concerned with the use and behavior of projectiles—objects that move by being launched or thrown (like a cannonball from a cannon). Items in true ballistic flight are impacted only by weight—there is no lift, thrust, or drag acting on their movement. We would encounter these conditions on the Moon, where there is no atmosphere to produce drag. However, on Earth, most objects achieve ballistic-like flight, in which a projectile moves mainly due to momentum and gravity, but is not acted upon by lift.
launch of The Space Shuttle Discovery from Kennedy Space Center, 2007 (image credit: nasa)
Aeronautical and astronautical engineers use ballistics to help design the rockets that launch spacecraft—such as satellites, crewed and un-crewed capsules, the International Space Station, or the Space Shuttles—into orbit. Rocket engines produce thrust by burning fuel and projecting the exhaust backwards, lifting the craft up and away from the ground. It takes a lot of thrust to launch a rocket into space—so again, the launch of a spacecraft is not strictly ballistic flight. Rockets are capable of accelerating very rapidly but efficiently.
After the initial rocket boost, a spacecraft achieves orbit when its forward momentum equals out the force of gravity, so that the object constantly “falls” in a circle around the outside of the Earth’s perimeter. Some spacecraft actually have rocket boosters that allow them to apply additional thrust as needed in orbit, to maintain the appropriate flight trajectory. Orbital mechanics—also called flight mechanics or astrodynamics—concerns the study and application of the motion of rockets and spacecraft. The motion of these objects is usually calculated from Newton's laws of motion and law of universal gravitation.
Katherine johnson working at nasa in 1966 (image credit: nasa)
Bonus Note:
If the term “orbital mechanics” sounds familiar and you’re not sure why—perhaps you’ve seen the film Hidden Figures? The movie, released in 2016, features Taraji P. Henson as the pioneering African American NASA mathematician Katherine Johnson, whose work calculating trajectories was instrumental to the first American space flight in 1961 by Alan Shepard, the Apollo 11 mission to the Moon, and other important missions in the U.S. space program. See NASA’s web page dedicated to Katherine Johnson here.
Hands-On Activity: Make a Teabag Act Like a Hot Air Balloon
Materials
Paper teabag—this needs to be the kind that has a string & a label attached to the top—they’re usually individually wrapped. Make sure it’s a paper bag, and not cloth or a synthetic material!
Non-flammable plate
Scissors
Match or long-bodied candle-lighter (DO NOT USE FLAMES WITHOUT ADULT PERMISSION AND SUPERVISION!!)
Safe location with nothing flammable nearby (in the bathtub was the best spot in my apartment!)
Procedure
Cut straight across the top of the tea bag (cutting off the part where the string is attached), and unfold the remaining bag (you may dispose of the tea)
Use one finger to help form the bag into a tube shape (see image)
Stand the tube on one end on the plate. (Make sure the plate is resting on a flat surface, so your “balloon” doesn’t tip over!)
Use a lighter or match to set the top of the teabag tube on fire—then stand back. It will only take a few seconds for the entire tube to burn into a large cohesive ash.
What’s Happening?
You should see the ashes of the teabag fly into the air.
Lighting the top of the teabag tube heats the air inside. The heated air molecules start to move more quickly and spread out to take up more space. As the air molecules spread out, the air inside the tube becomes less dense. Warm, less dense air rises above cool, dense air.
The ash of the teabag does not have much weight—so it doesn't require much force to lift it. As the warm, less dense air rises, it has enough force to lift the ash of the teabag right into the air. This is similar to the functioning of a hot air balloon—in which a burner beneath the balloon heats air inside, which becomes less dense than the surrounding air and rises, pulling the balloon upward.
