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The Immelmann turn refers to two different aircraft maneuvers:
- In World War I aerial combat, an Immelmann turn was a maneuver used after an attack on another aircraft to reposition the attacking aircraft for another attack.
- In modern aerobatics, an Immelmann turn (also known as a roll-off-the-top, or simply an Immelmann) is an aerobatic maneuver that results in level flight in the opposite direction at a higher altitude.
Historical combat maneuver 
After making a high speed diving attack on an enemy, the attacker would then climb back up past the enemy aircraft, and just short of the stall, apply full rudder to yaw his aircraft around. This put his aircraft facing down at the enemy aircraft, making another high speed diving pass possible. This is a difficult maneuver to perform properly, as it involves precise control of the aircraft at low speed. With practice and proper use of all of the fighter's controls, the maneuver could be used to reposition the attacking aircraft to dive back down in any direction desired.
As a practical combat tactic, the Immelmann had already fallen somewhat into disfavor by 1917/1918.
Aerobatic maneuver 
In modern aerobatics, an Immelmann turn (also known as a roll-off-the-top, or simply an Immelmann) is an aerobatic maneuver of little practical use in aerial combat. Essentially, the aerobatic Immelmann comprises an ascending half-loop followed by a half-roll, resulting in level flight in the exact opposite direction at a higher altitude.
To successfully execute the aerobatic Immelmann turn, the pilot accelerates to sufficient airspeed to perform a loop in the aircraft. The pilot then pulls the aircraft into a climb, and continues to pull back on the controls as the aircraft climbs. Rudder and ailerons must be used to keep the half-loop straight when viewed from the ground. As the aircraft passes over the point at which the climb was commenced, it should be inverted and a half loop will have been executed. Sufficient airspeed must be maintained to recover without losing altitude, and at the top of the loop the pilot then executes a half-roll to regain normal, upright aircraft orientation... As a result, the aircraft is now at a higher altitude and has changed course 180 degrees.
Not all aircraft are capable of (or certified for) this maneuver, due to insufficient engine power, or engine design that precludes flying inverted (usually piston engines that have an open oil pan). In fact, few early aircraft had sufficiently precise roll control to have performed this maneuver properly.
The Immelmann turn has become one of the most popular aerobatic maneuvers, being commonly used in airshows all over the world. However, the aerobatic maneuver is of little use in modern dogfighting.
Sometimes called a "reverse" or "inverted" Immelmann is the Split S maneuver, which is a half-roll followed by a descending half-loop, resulting in level flight in the exact opposite direction at a lower altitude. This is more useful in a combat situation as the tradeoff of lost altitude results in an increase of airspeed, thus potentially allowing the aircraft performing the maneuver, a better chance of disengaging from an unfavorable situation (a tactical retreat.)
See also 
- "Immelmann Turn". Retrieved 8 January 2012.
- Wheeler (1965) pp. 68-71
- Wheeler, Allan H, Building Aeroplanes for "Those Magnificent Men", London, Foulis, 1963