What is the main difference between a forward slip and a sideslip?

The main difference between a forward slip and a sideslip lies in their purposes and how they manipulate the aircraft's flight path. A forward slip is primarily used to lose altitude while maintaining the aircraft's desired airspeed, typically during approaches to landing when a quick descent is needed without gaining airspeed. In this maneuver, the pilot uses full aileron input to bank the aircraft and opposite rudder input to maintain a straight flight path, creating a sideways airflow over the wings that provides additional drag.

On the other hand, a sideslip is used to maintain a heading during flight while compensating for adverse conditions such as crosswinds, particularly during approach and landing phases. In this maneuver, the pilot banks into the wind while using rudder to keep the nose of the aircraft pointed down the runway. This alignment balances the aircraft’s trajectory with the effects of the wind, aiding in control without necessarily changing altitude significantly.

The correct answer highlights this fundamental distinction in purpose: a forward slip is focused on losing altitude, while a sideslip's main objective is to maintain the aircraft's heading.