Discovering a way for people to take flight is undoubtedly one of the most awe-inspiring feats of human ingenuity the world has known to date. Although the first airplane was invented over a century ago, watching planes lift off and land still seems like something that defies reality.
In order for an airplane to fly, engineers had to master the balance and control of four forces: lift, gravity, thrust, and drag, also known as the "four forces of flight." By adjusting these forces, pilots are able to speed up, slow down, lift off, and land.
This module is designed to demystify each of these forces and examine how they contribute to flight.
- Describe the four forces involved in flight.
- Determine which forces to adjust in order to change flight speed or height.
- Identify adaptations that make flying possible for birds.
- Describe how air acts as a fluid.
- Predict how flight controls will affect the direction of an aircraft.
A Brief History Of Flight
As stories and legends tell us, people have always wanted to fly, but it's only fairly recently that the dream has been achieved.
The earliest flying machines were kites, invented thousands of years ago in Asia. Although they're tethered to the ground, these lightweight wings have carried weapons, cameras and even people. They're used for sport, ceremonies, carrying messages and fishing, and they have taught us a lot about the physics of flight.
In the 1480s, Leonardo da Vinci applied his artistic and engineering genius to the problems of flight. His ornithopter allowed the passenger/pilot to flap giant wings like a bird, and his helicopter featured a screw-shaped sail. His machines were never built.
The Hot Air Balloon
Lighter-than-air flight happened when brothers Joseph Michel and Jacques Etienne Montgolfier invented the hot air balloon. They used a lightweight cloth bag to catch the smoke from a fire. They thought the smoke was lifting the balloon; they didn't know that it was the hot air, lighter for its size than cold air, which caused the balloon to float. In 1783, the first passengers in the colorful balloon were a sheep, rooster and duck. The first people took flight on November 21, 1783. Interestingly, the French word for a hot air balloon is a montgolfière, named after its inventors .
Starting in 1799 and continuing for 50 years, English engineer George Cayley designed many different gliders. He changed the shape of the wings to experiment with the way the air flows over the wings. He designed a tail to help with stability. He tried a biplane design to add strength. Based on his experiments, he identified the four forces—weight, lift, drag, and thrust—which are in effect on any flying machine. His gliders were the first to successfully carry a person into the sky.
Cayley's book On Aerial Navigation showed that a fixed-wing aircraft with a power system for propulsion and a tail to assist in the control of the airplane would be the best way to allow people to fly.
Several other inventors continued to improve the glider design and experiment with adding power to their prototypes, including Otto Lilienthal (1891), Samuel P. Langley (1891).
Orville and Wilbur Wright are celebrated as the first to create a powered flying machine that could carry a person. After a great deal of research and experiments with kites, they spent a lot of time testing different glider shapes and learning about how gliders could be controlled. They designed and used a wind tunnel to test the shapes of the wings and the tails of the gliders. Once they found a successful glider shape, they turned their attention to how to create a propulsion system that would create the lift needed to fly.
The "Flyer" lifted from level ground to the north of Big Kill Devil Hill, at 10:35 a.m., on December 17, 1903. Orville piloted the plane, which weighed 274 kg. This first heavier-than-air powered airplan traveled 36.5 metres in 12 seconds.
Modern airplanes are complex machines. However, despite their size and thousands of parts, they need the same four forces identified by George Cayley to take off and stay in flight: gravity, lift, thrust and drag.
Gravity is the force that pulls objects towards the centre of the earth.
Lift is the force that opposes gravity. This is the force that gets the plane into the air and keeps it from crashing to the ground.
Thrust is the force that moves the airplane forward in flight. Thrust is usually created with engines or propellers.
Drag opposes thrust and slows the airplane down. It is the force of the air resistance as the airplane moves through the air.
When the thrust and drag are equal and opposite, an airplane will continue to move forward at the same uniform speed. If thrust is greater than drag, the aircraft will accelerate. If drag is greater than thrust, the aircraft will lose speed.
When the lift is equal and opposite to the force of gravity, the airplane neither rises nor falls. If the lift is greater than the force of gravity on the plane, the aircraft will climb. If the force of gravity is greater than the lift, the airplane will descend.
How does the airplane wing (known as an airfoil) create lift?
First, an airplane can only take off if its wings are at an angle. The air hits the front edge and the bottom. As the bottom of the wing pushes the air downward, the air also pushes the wing upward, creating lift. But there's more to it than that. The shape and angle of the airplane wing affects the flow of the air over top of the wing. There are a lot of forces at play, but the end result is that the air on the top of the wing moves faster than the air beneath. The faster air has a lower pressure than the slower air (due to Bernoulli's Principle), and the pressure difference contributes to lift.
Scientists are not in agreement about how much the Bernoulli Principle versus Newton's Third Law of Motion contributes to lift in an airplane. As an extension activity for keen students, they may like to research the arguments and present them to the class, or find some way of building a model to test the theories. Links can be found in the References section below.
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