Which factors influence optimum cruise speed (Mopt)?

Which factors influence optimum cruise speed (Mopt)?

• A. Only weight matters
• B. Mopt is fixed and not affected by weather
• C. Weather does not affect fuel burn
• D. Weight and ambient temperature influence Mopt, as do winds

Answer: (D)

Optimum cruise speed is the Mach number that minimizes fuel burn per distance in cruise. It isn’t fixed; it shifts with operating conditions. Heavier weight means more lift is needed, which increases induced drag and changes the drag versus speed balance, so the speed that yields the lowest fuel per mile moves. Ambient temperature changes air density and engine/propulsive efficiency, which also alters the drag and thrust needed at a given Mach, nudging the optimal speed. Winds affect the energy required to cover a given ground distance—the same airframe must work differently depending on headwinds or tailwinds—so the economically best speed can change with the wind profile. In short, weight, temperature, and winds all influence Mopt.

Optimum cruise speed is the Mach number that minimizes fuel burn per distance in cruise. It isn’t fixed; it shifts with operating conditions. Heavier weight means more lift is needed, which increases induced drag and changes the drag versus speed balance, so the speed that yields the lowest fuel per mile moves. Ambient temperature changes air density and engine/propulsive efficiency, which also alters the drag and thrust needed at a given Mach, nudging the optimal speed. Winds affect the energy required to cover a given ground distance—the same airframe must work differently depending on headwinds or tailwinds—so the economically best speed can change with the wind profile. In short, weight, temperature, and winds all influence Mopt.