At what condition is the induced drag of an aeroplane at constant mass in un-accelerated level flight greatest?

The induced drag of an aircraft primarily arises from the generation of lift. In unaccelerated level flight, induced drag is influenced by the angle of attack and the efficiency of the wing design. As an aircraft flies slower, the angle of attack typically increases to maintain level flight since lift must equal weight.

When considering the lowest achievable speed in landing configuration, the aircraft is usually at a relatively high angle of attack to generate sufficient lift while maintaining lower airspeeds. This higher angle of attack at a low speed results in greater induced drag. Therefore, at this condition, the induced drag is greatest because more lift is required at a slow speed, leading to a significant increase in the induced drag due to the respective higher lift coefficient needed.

In contrast, the other speeds such as design manoeuvring speed, stalling speed in clean configuration, and maximum operating limit speed typically align with different flight conditions where the induced drag may not reach the same maximum as it does at the lowest speed in landing configuration. This emphasizes the importance of understanding how induced drag behaves in relation to operating conditions, particularly when approaching the limits of slow flight in a landing scenario.