Lesson One: What forces will take a plane in the air?

Return to Mini-Course: How do planes fly?

Introduction

Human flight has become a tired fact of modern life. At any given moment, roughly 5,000 airplanes crisscross the skies above the United States alone, amounting to an estimated 64 million commercial and private takeoffs every year. Consider the rest of the world's flight activity, and the grand total is incalculable.

It is easy to take the physics of flight for granted, as well as the ways in which we exploit them to achieve flight. We often glimpse a plane in the sky with no greater understanding of the principles involved than a caveman.

How do these heavy machines take to the air? To answer that question, we'll start from the various forces at work in any given flight.

What forces will take an airplane in the air?

Any textbook that talks about airplanes always discusses the four forces affecting an airplane: lift, weight, thrust and drag. Three of those forces are obvious to us.

1.Weight

Everyone weighs himself or herself on the bathroom scale every morning, so weight is easy to understand. Weight is a force caused by gravity which keeps us on the ground. Every object on Earth has weight, a product of both gravity and mass. A Boeing 747-8 passenger airliner, for instance, has a maximum takeoff weight of 487.5 tons (442 metric tons), the force with which the weighty plane is drawn toward the Earth.

2.Thrust

Thrust, whether caused by a propeller or a jet engine, is the aerodynamic force that pushes or pulls the airplane forward through space. Household fans produce thrust (at least a little) in the same way a propeller does, so that concept is familiar.

3.Drag

If you stick your hand out of a car window while moving, you'll experience a very simple demonstration of drag at work. The amount of drag that your hand creates depends on a few factors, such as the size of your hand, the speed of the car and the density of the air. If you were to slow down, you would notice that the drag on your hand would decrease.

We see another example of drag reduction when we watch downhill skiers in the Olympics. Whenever they get the chance, they'll squeeze down into a tight crouch. By making themselves "smaller," they decrease the drag they create, which allows them to zip faster down the hill.

A passenger jet always retracts its landing gear after takeoff for a similar reason: to reduce drag. Just like the downhill skier, the pilot wants to make the aircraft as small as possible. The amount of drag produced by the landing gear of a jet is so great that, at cruising speeds, the gear would be ripped right off the plane.

4.Lift

But lift - there's something uncommon. If lift were easy we could stick out our arms and fly like a bird! But obviously it is not that easy.

Question:

1.Have you ever taken an airplane before? How do you feel in the air?

2.Have you ever been curious about how an airplane can fly?

3.Share your ideas with your classmates in the discussion forum.

References

The 4 forces of flight: https://www.mansfieldct.org/Schools/MMS/staff/hand/flight4forcesoverview.htm

NASA Education: http://www.nasa.gov/audience/forstudents/k-4/stories/ames-how-do-planes-fly-text.html