# Four Forces of Flight

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There are four fundamental forces involved and acting on the airplane at all times in flight.

## The Four Forces of Flight

### Thrust

Thrust is created by a propeller, jet engine, or rocket; and moves the aircraft in the direction of its motion. Thrust is generated through pulling air in and pushing air out in the opposite direction.

### Drag

Drag is caused by friction and differences in air pressure. Think about the last time you stuck your hand out of a moving car.  Air, as any other substance, has mass. The faster you move against that mass, the more resistance you will feel, and the more drag is created.

### Weight

Weight is a result of Earth’s gravitational pull. The more mass the airplane has, the more lift will be necessary to overcome the weight of the airplane.

### Lift

Lift is primarily created by the wings and must overcome weight for the aircraft to become airborne.

### When are the Four Forces Equal?

• In straight and level flight with a constant airspeed
• In a steady-state climb (constant airspeed and constant rate of climb)
• In a steady-state descent (constant airspeed and constant rate of descent)

### When are the Four Forces not Equal?

• When you begin a climb or begin a descent
• When you level off from a climb or descent (a changing vertical speed or rate of climb/descent)
• Anytime there is ACCELERATION in a vertical or horizontal direction
• When you are changing airspeed

#### Remember:

• Thrust opposes drag, and lift opposes weight.
• For an airplane to fly, thrust must overcome drag, and lift must overcome weight.

Watch the video above for a more thorough explanation.

#### TLDR:

IN UNACCELERATED FLIGHT, ALL FORCES ARE IN BALANCE (or equal).  Even if the airplane is climbing or descending. The only thing that makes the forces unequal to each other is ACCELERATION!