The Immelmann turn refers to two quite different aircraft maneuvers.
The maneuver nowadays usually called an "Immelmann" has, in fact, no connection with the World War I German flying Ace Max Immelmann.
Similar to a 'Split-S' manuver (reverse of).
In modern aerobatical parlance, 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, and is a different maneuver altogether from the original dogfighting tactic of World War I from which it derives its name. 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.
It should be stressed that 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).
The Immelmann turn has become one of the most popular aerobatic maneuvers in the world, being commonly used in airshows all over the world. However, the aerobatic maneuver is of little use in modern dogfighting, because modern high thrust fighters can quickly initiate sustained vertical maneuvering from level flight, and slow targets are highly vulnerable to air-to-air missiles. The aerobatic maneuver also involves rapid "energy loss" (loss of airspeed) even if the nose is pushed down sharply as the maneuver is completed (This maneuver, an aerobatic Immelman followed by a dive back to the original altitude is another aerobatic maneuver called the "Half-Cuban-Eight").
The Immelmann is contrasted with 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
Historical Combat Manuver
In any case, the World War I "Immelmann turn" was a far less polished maneuver. This attacking maneuver was probably used by Max Immelmann and may have been originated by him. It was certainly used by other World War I fighter pilots. After making a high speed diving attack on an enemy, the attacker would then zoom climb back up past the enemy aircraft, and before stalling, used full rudder to yaw (maneuver around the aircraft's "normal axis") 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 re-position the attacking aircraft to dive back down in any direction desired. This form of "Immelmann turn" was called Renversement by French pilots. The modern aerobatic maneuvers that most resemble the World War I Immelmann are the "wingover", and the "hammer-head turn".
As a practical combat tactic, the Immelmann had already fallen somewhat into disfavor by 1917/1918, as it became obvious that the zooming aircraft presented an easy target as it hung nearly motionless at the top of the maneuver - provided the aircraft under attack was sufficiently powerful and well armed to follow his adversary, or was fitted with flexible forward and upward firing guns. The Immelmann could still be used in World War II by fighters attacking unescorted bombers that could not follow the fighter up into the climb, as long as the zoom climb took the fighter beyond the range of the bomber's defensive guns