FAA Flight Instructor Airplane (FIA)Aerodynamics and Principles of FlightEasy
During straight-and-level, unaccelerated flight, a flight instructor asks a student to describe the relationship between the four forces acting on the airplane. Which statement is correct?
- ALift is less than weight, and thrust equals drag
- BLift equals weight, and thrust equals drag
- CLift equals thrust, and weight equals drag
- DLift is greater than weight, and thrust is greater than drag
Show answer & explanationAnswer & explanation
Correct answer: B. Lift equals weight, and thrust equals drag
In straight-and-level, unaccelerated flight, the airplane is in equilibrium, so the opposing forces must be equal: lift equals weight and thrust equals drag. Any imbalance would cause the airplane to accelerate, climb, descend, or decelerate.
Why the other options are wrong
- A. If lift were less than weight the airplane would be descending, not level.
- C. Lift and thrust are perpendicular forces and are not directly compared this way.
- D. This describes an accelerating climb, not steady level flight.
Four Forces of Flight
Lift, weight, thrust, and drag are the four forces acting on an airplane; in steady, unaccelerated flight they exist in equilibrium pairs.
- Lift opposes weight
- Thrust opposes drag
- Equilibrium exists only in unaccelerated flight
- Any imbalance causes acceleration in that force's direction
Memory trick: L-W-T-D: 'Lazy Weekends Take Discipline' balance in level flight