FAA-H-8083-25C, ch. 5

What are the four forces of flight?

Every aircraft in flight is acted on by thrust, drag, lift and weight. Here is what each force does, the exam traps and practice questions, with the FAA source under each answer.

Part 107, explained · Last reviewed October 25, 2026 against the eCFR and faa.gov. UAV Campus is not affiliated with the FAA.

What are the four forces of flight?

The four forces on an aircraft in straight-and-level, unaccelerated flight are thrust, drag, lift and weight. Thrust is the forward force from the propeller or rotor and opposes drag. Lift is produced by air acting on the airfoil and opposes weight. Weight is the combined load of aircraft, fuel and cargo, pulled down by gravity.

Source: FAA-H-8083-25C ch. 5, Forces Acting on the Aircraft

What the FAA handbook says

FAA-H-8083-25C ch. 5, Forces Acting on the Aircraft

Thrust—the forward force produced by the powerplant/ propeller or rotor. It opposes or overcomes the force of drag.

FAA-H-8083-25C ch. 5, Forces Acting on the Aircraft

It does not mean the four forces are equal. It means the opposing forces are equal to, and thereby cancel, the effects of each other.

Sources: FAA-H-8083-25C ch. 5, Forces Acting on the Aircraft · FAA-H-8083-25C ch. 5, Forces Acting on the Aircraft

Exam traps

  • In steady, straight flight the sum of the opposing forces is always zero, so there are no unbalanced forces.
  • The four forces are not all equal: the opposing forces are equal to, and cancel, each other.
  • Lift acts perpendicular to the flight path, and in level flight it opposes the downward force of weight.
  • Weight acts vertically downward through the aircraft's center of gravity (CG), and drag acts rearward parallel to the relative wind.

Try it: practice questions

Similar to the FAA test, not the actual test questions.

Practice question 1

As the angle of attack increases, induced drag will

  1. A remain the same.
  2. B increase.
  3. C decrease.
Show the answer
Answer: B. increase.

FAA source: FAA-H-8083-25C ch. 5, Induced Drag

Practice question 3

How would high density altitude affect the performance of a small unmanned aircraft?

  1. A Decreased performance.
  2. B Increased performance.
  3. C No change in performance.
Show the answer
Answer: A. Decreased performance.

Density Altitude is the altitude corrected for non-standard pressure and non-standard temperatures. A high density altitude means your UAS will “feel” like it’s operating at a higher altitude, hence the performance will be diminished.

FAA source: FAA-H-8083-25C ch. 11, Density Altitude

Does thrust equal drag in level flight?

In steady, straight, level flight, thrust and drag are equal to each other and lift and weight are equal to each other. The handbook adds that the lift and weight pair is also greater than the thrust and drag pair.

Source: FAA-H-8083-25C ch. 5, Forces Acting on the Aircraft

What is weight made of?

Weight is the combined load of the aircraft itself, the crew, the fuel, and the cargo or baggage. It pulls the aircraft downward because of gravity.

Source: FAA-H-8083-25C ch. 5, Forces Acting on the Aircraft

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