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Weight Force & the Lift Equation

Gemini Sparkle

Key Takeaways:

  • To maintain straight-and-level flight and altitude when an airplane's weight increases, the total lift force generated by the wings must increase proportionately.
  • Pilots can increase the wing's lift force primarily by increasing the coefficient of lift (through an appropriate increase in Angle of Attack and/or airspeed), assuming wing area and air density are constant.
  • The total lift force is determined by four interrelated factors—coefficient of lift, air density, wing surface area, and airspeed—and an increase in one or more of these factors is required to increase lift.
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If the airplane’s weight force increases to 2150 lbs as described in the main text, then to maintain altitude and straight-and-level flight, the total lift force generated by the wings must increase proportionately using the equation in Table 1, at right. According to the lift equation, assuming wing area cannot be increased in flight (e.g., increasing wing surface area by deploying Fowler flaps) and air density is constant, a pilot may increase the wing’s lift force by increasing the coefficient of lift. This is accomplished by appropriately increasing AoA and/or airspeed.

The equation presented in Table 2 describes how four basic factors—coefficient of lift (CL) air density, wing surface area and airspeed—determine the total lift force generated by the wings. (The coefficient of lift is a measure of a given wing’s efficiency in transforming dynamic pressure into lift and is directly affected by AoA.) All four interrelated factors act simultaneously and in the presence of each other to affect lift. For lift force to increase, one or more of the four factors on the right side of the equal sign of the equation in Table 2 must increase.

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