In cruise, an airplane increases power to accelerate and increase speed. What happens to the angle of attack if the aircraft maintains altitude?

• A. Angle of attack increases
• B. Increase angle of attack until stable, and then decrease
• C. No change
• D. Angle of attack decreases

Answer: (D)

In level cruise, lift must balance weight. Lift is proportional to dynamic pressure (which rises with speed) times the lift coefficient, and the lift coefficient increases with angle of attack. When you increase thrust to accelerate and you maintain altitude, the airplane’s speed goes up, so the dynamic pressure increases. To keep the lift the same (equal to weight) you don’t need as high a lift coefficient, so you reduce the angle of attack. Since reducing angle of attack lowers the lift coefficient, the angle of attack must decrease as speed increases in steady, level flight.