The 4 Forces of Flight

The 4 forces of flight:

In level flight, lift equals weight and thrust equals drag when the plane flies at constant velocity.

Maintaining a steady flight requires a balance, often described as an equilibrium of all the forces acting upon an airplane. Weight, lift, thrust and drag are the acting forces on an airplane. Assuming a straight and level flight, lift must be equal to weight and drag must be equal to thrust. This is what happens if this equilibrium is violated:

If lift becomes greater than weight, then the plane will accelerate upward.

If the weight is greater than the lift, then the plane will accelerate downward.

When the thrust becomes greater than the drag, the plane will accelerate forward.

If drag becomes greater than the thrust a deceleration will occur.

Acceleration is best explained by using Newton's Second Law of Motion.

The proportion between weight and thrust is determined by the airplane designer depending on the anticipated missions. For example, if by design an airplane must be able to accelerate vertically upwards then the thrust must be greater than the weight and drag combined. In small aircraft the weight/thrust ratio is about. 10:1.

Thrust opposes drag. The engine creates thrust and moves the plane forward. (Gravity provides the thrust for a glider.) The engines push air back with the same force that the air moves the plane forward; this thrust force-pair is always equal and opposite according to Newton's 3^rd Law. When thrust is greater than drag, the plane accelerates according to Newton's 2nd Law. When the plane flies level at constant velocity, thrust equals drag. When the plane flies level at constant velocity, all opposite forces of flight are equal: drag = thrust and weight = lift. How the 4 forces of flight interact

Drag opposes thrust. Imagine sticking your hand out the window of a moving car and flying your hand. The force that pushes your hand back is called "drag". As your hand pushes on the wind, the wind also pushes against your hand. Isaac Newton would say that force of your hand pushing on the air is always equal to the force of the air pushing on your hand; this is his third law. When the plane flies level at constant velocity, weight = lift! When the engines of a plane quit, drag slows the plane down according to Newton's 2nd Law. How the 4 forces of flight interact

Legs of birds and wheels of planes are tucked in to reduce drag. Drag is unwanted because it makes the plane or bird inefficient. Planes with more drag require more thrust to fly successfully. To reduce drag and increase efficiency, planes are streamlined. Planes also use camber and high aspect ratios to reduce drag.

Drag versus Friction

How the 4 forces of flight interact

Lift opposes weight. Newton's Laws and Bernoulli's Principle generate lift. A plane that sits on a runway doesn't have any lift, but it does have weight. Lift is proportional to the square of the velocity of an airplane and as a plane goes faster, its lift increases. As a plane moves forward, its lift force increases until it equals its weight. When lift equals weight, the plane can fly. In level flight, lift equals weight as the plane flies at constant velocity.

When a plane accelerates upward, lift is greater than weight. See Newton's 2nd Law.

In contrast, lift = weight when the plane flies level at constant speed. See Newton's 3rd Law.

Imagine sticking your hand out the window of a moving car and flying your hand. As you tilt your hand up slightly, lift is the force that pushes your hand up. (Actually, lift is perpendicular to the direction of movement.) Lift is equal to the weight as your hand flies level at constant velocity. When a plane stalls, lift is lost! Stalling can occur due to insufficient air velocity or an excessive angle of attack.

How the 4 forces of flight interact

	Why can't a plane keep climbing in altitude? It runs out of air.
	Fun fact: Geese fly in formation because the vortex provides lift to the bird behind.

Weight opposes lift. Weight and lift are equal when a plane flies level at constant velocity. Because excess weight requires more lift, and therefore more thrust, heavy planes are more difficult to get off the ground as compared to lighter planes. Planes with less weight require less thrust. Thus, planes are designed to be as light as possible.

How the 4 forces of flight interact

The opposite forces of flight are equal only when a plane flies level at constant velocity.

The opposite forces of flight are not always equal.

For instance, as a plane climbs, its weight is equal to a portion of the lift force and a portion of the thrust force. In this situation, the opposite forces of flight are no longer equal to one another. However, according to Newton's 3rd Law, the force of air pushing on the plane is still equal to the force of the plane pushing through the air.

And, if a plane flies straight up, Thrust = Weight and Drag.

Flight Notes

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