At higher altitudes, lift-generating surfaces must be flown at what condition to produce the same lift as at lower altitudes?

At higher altitudes, the air density decreases, which results in a reduction in the amount of lift generated by lift-generating surfaces such as wings. To produce the same amount of lift that one would generate at a lower altitude, the aircraft must operate at higher speeds or angles of attack.

Increasing the speed enhances the airflow over the wings, thereby increasing the lift according to Bernoulli's principle. Similarly, raising the angle of attack allows the wings to capitalize on the reduced air density by increasing the lift coefficient, although doing so must be balanced to avoid stalling.

This adjustment is crucial because maintaining lift is essential for stable flight, especially in environments where the atmosphere is less supportive. Therefore, flying at higher speeds or increased angles of attack compensates for the diminished lift caused by the thinner air at altitude.