# Lift or Drag? The Physics of Flight Quiz

Staff

Image: refer to hsw

Long before the Wright brothers guided their fixed-wing aircraft over the North Carolina dunes, daredevils and engineers worked to decode the secrets of sustained heavier-than-air human flight. What do you know about the physics of flight?

# Which of the following is NOT a force acting on airplanes?

An airplane flying straight and level at a constant speed has four forces acting on it: lift, drag, weight and thrust. Weight and mass are not the same thing. Mass is a measure of the amount of matter in an object, while weight factors in the downward pull of gravity on an object.

# What part of an airplane is responsible for creating thrust?

In light aircraft, piston engines drive propellers. Propellers deflect air backward, and this air pushes back, creating thrust. The same principle applies to jet engines, which blast hot, expanding gases to the rear of the plane and, in turn, get pushed back by those same gases.

# Which force acts in the opposite direction of thrust?

The forces acting on a plane work in opposing pairs. Weight opposes lift, drag opposes thrust. During steady, level flight, the pilot adjusts the engine power and various control surfaces to keep the opposing forces in balance.

# What part of an airplane is responsible for creating lift?

Birds figured it out long before humans: You gotta have wings if you're going to fly. Wings create lift, the upward-acting force that gets your feet off the ground.

# What's the scientific name for a wing?

Wing sounds so simple, but airfoil soars with sophistication. Technically speaking, an airfoil is the shape of the wing -- a curved surface with a rounded leading edge and a sharp trailing edge.

# Which well-known scientific principle describing the pressure of moving fluids is often invoked when discussing how an airplane generates lift?

Daniel Bernoulli and his famous principle get a lot of attention when it comes to lift. A lot of aviation enthusiasts would argue, however, that Bernoulli is only part of the lift story though.

# What controversial book, first published in 2001, rejects the conventional wisdom about airplane wings and lift?

In 2001, David Anderson and Scott Eberhardt published "Understanding Flight" as a practical and intuitive guide to mechanical flight. In the book's opening paragraph, they made a bold statement that many took as heretical: "Forget Bernoulli's theorem."

# A different, more modern view of airplane flight relies on another famous scientific principle. Which one?

Move over Bernoulli, Newton wants to fly this plane. According to authors Anderson and Eberhardt, Newton's third law of motion is perfectly capable of explaining how a wing works: Grossly simplified, it says that the wing pushes the air down, so the air pushes the wing up.

# When air flows over an airplane wing, it sticks to the surface. What’s the name of this process?

When a fast-moving fluid encounters an airfoil, the fluid sticks to the surface and follows the curve. This is known as the Coanda effect, named after Henri Coanda, a Romanian scientist who first described the phenomenon in the 1930s.

# What term describes the degree to which an airfoil is curved?

On an airfoil, the amount of curvature is determined by the camber line. Airfoils with positive camber -- the upper surface curves more than the lower surface -- generate better lift.

# What are the two main types of drag?

Drag falls into two categories. The first, known as parasitic drag, is related to the friction between the aircraft and the air. The second, known as induced drag, arises as a resistance to lift. Who knew flying could be such a, well, drag?

# This term sounds threatening, but it merely describes the position of the wing relative to airflow.

For an airfoil to work, the leading edge of the wing must be inclined upward. The more it's inclined, the greater the angle of attack. Put another way, the angle of attack is the angle between the chord, or midline, of an airfoil and the direction of the surrounding undisturbed flow of gas or liquid.

# What's the relationship between the angle of attack and lift?

The angle of attack is related to the amount of lift. Lift will increase as the angle of attack is increased -- up to a point (called the critical angle of attack). For most aircraft, lift will be maximized if the angle of attack remains below 17 degrees.

# What happens when a plane reaches the critical angle of attack?

While it's true that increasing the angle of attack increases lift, it's also true that you can have too much of a good thing. When the angle of attack becomes too steep, the wing can't generate lift, and the aircraft stalls.

# What part of an airplane controls pitching (tilting the nose up or down)?

Elevators are hinged flaps located on the tail of the plane. Raising the elevators deflects air downward, which pushes the tail down (and the nose up). Lowering the elevators pushes the tail up (and the nose down).

# What part of an airplane controls rolling?

A roll results in an up and down movement of the wings. Such motion is controlled by two hinged flaps, located on each wing, known as ailerons. When the starboard aileron is raised, it deflects air upward, which produces a downward force on the wing, causing the wing to fall. The opposite occurs when the port aileron is raised.

# What part of an airplane controls yawing (turning the nose left or right)?

No, not "yawning" -- that's controlled by something entirely different. Yawing describes an airplane that's turning about its vertical axis. And the aircraft control responsible for that is the rudder. Turn the rudder to starboard, and you'll aim the plane's nose in that direction.

# Turning an airplane smoothly requires a pilot to operate two controls simultaneously. Which ones?

A smooth, graceful turn looks so easy, but it takes a practiced pilot working the rudder and the ailerons simultaneously.

# Supersonic flight changes aerodynamics drastically. Which two forces are affected most?

When an aircraft approaches the speed of sound (assuming 660 mph per second at 30,000 feet above sea level), its interaction with the surrounding air changes. That's because the air flowing over the plane is going faster than the plane itself. One of the most important effects is a very large increase in drag, as well as a reduction in lift.

# What number can be determined by dividing the speed of an aircraft by the speed of sound in air?

Scientists appreciate convenience as much as anyone else. Because the ratio of aircraft speed to the speed of sound has such important implications for flight, aerodynamicists came up with a convenient way to express the relationship -- the Mach number. Flight that’s faster than Mach 1 is classified as supersonic. Flight that’s faster than Mach 5 is classified as hypersonic.