Thrust Required (level flight)

In straight, steady, level flight, the airplane is not accelerating, so the horizontal forces must balance: thrust equals drag. Since the aerodynamic efficiency is measured by the lift-to-drag ratio, D = W/(L/D) when lift equals weight, which leads directly to T = W/(L/D). This shows that a higher L/D means less thrust is needed to keep the aircraft flying level at the same weight.

Use this formula for cruise or other steady level-flight situations when lift is approximately equal to weight and thrust is balancing drag. It is especially useful for estimating engine thrust needs, comparing aircraft efficiency, or checking how weight changes affect cruise performance.

• Using this formula for climb, descent, or acceleration, where thrust and drag are not balanced in the same way
• Forgetting that the lift-to-drag ratio must be for the actual flight condition, not a generic aircraft handbook value
• Plugging in mass instead of weight, or using L/D with the units attached even though the ratio is dimensionless