the application of power. Heavier aircraft will require more power to change their energy state, so two aircraft with equal energy will not have the same maneuverability. This leads to the concept of "specific power" in the same fashion as specific energy. For any given operational condition, a selected speed and altitude for instance, any given aircraft will require a certain amount of power simply to maintain those conditions, due largely to the effects of drag. This gives rise to the concept of "specific excess power", the amount of additional power available to an aircraft over and above the power needed to maintain those flight conditions. Specific excess power is normally expressed for an aircraft flying straight and level. Turning requires an expenditure of energy, both to change the energy state of the aircraft, as well as due to the additional induced drag that is naturally created as a side effect of generating the lift force required to change direction. This implies that an aircraft with higher specific excess power has higher sustained maneuverability performance. This overall concept is known as "energy maneuverability". Maneuverability is not solely a factor of energy or specific power, many other factors like the efficiency of the wing planform at generating lift, or the load limits of the aircraft, can limit the maneuverability in ways that are not directly related to weight and power. This gives different aircraft very different types of performance under various maneuvers. For instance, an aircraft with high thrust for weight may have high specific excess power but nevertheless suffer from very high induced drag during turns - this was very common on delta wing aircraft for instance - in which case it will attempt to avoid turns and instead use climbs and dives to its advantage. Such aircraft are referred to as "energy fighters". Others, typically those with lower wing loading, may have less excess power but nevertheless be able to perform turns without losing as much energy, and are referred to as "angles fighters" or "dog-fighters". When two aircraft meet in combat, they may have different energy states and energy retention. Typically, the fighter with higher energy and better retention will make an "energy move", like a high yo-yo to maintain the energy advantage, while the fighter at an energy disadvantage (angles fighter) will make an "angles move" such as a break turn, trying to use the opponent's energy