Air combat maneuvering or ACM

The term Immelmann turn, named after German World War One Eindecker fighter ace Leutnant Max Immelmann, 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.

An Immelmann trades airspeed for altitude during a 180 degree change in  direction. The aircraft performs the first half of a loop, and when  completely inverted, rolls to the upright position. The Immelmann is a  good offensive maneuver for setting up a high-side guns pass against a  lower altitude, slow moving opponent, going in an opposite direction.  However, an Immelmann is a poor defensive maneuver, turning the defender  into a slow moving target. 

 To successfully execute the aerobatic Immelmann turn, the pilot accelerated to sufficient speed to perform a loop in the aircraft.  The pilot then pulls the plane into a climb, and continues to pull back  on the controls as the aircraft climbs. As the plane passes over the  point at which the climb was commenced, it should be inverted and a half  loop will have been executed. Sufficient speed 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 fighter is now at a higher altitude and  has changed course 180 degrees. 

 A Chandelle, also called a Pitchback, is an Immelmann that is executed  in some plane other than the vertical. Basically just a pitch turn, the  fighter will be at some angle of bank before performing the half loop  and roll. Unlike the Immelmann, a Chandelle depletes less kinetic energy  and is harder for an adversary to track. 

 A chandelle is a climbing turn, performed in the clean configuration,  with a 180-degree change in course. It begins near the maneuvering speed  of the aircraft and finishes close to its stalling speed. The first  half of the chandelle features a constant bank attitude and increasing  pitch attitude. During the second half of the maneuver, the pitch  attitude stays constant and the bank continuously decreases to a level  attitude. 

The scissors are a series of turn reversals and flight path overshoots intended to slow the relative forward motion (downrange travel) of the  aircraft in an attempt to either force a dangerous overshoot, on the  part of the defender, or prevent a dangerous overshoot on the attacker's  part. The defender's goal is to stay out of phase with the attacker, trying to prevent a guns solution, while the  attacker tries to get in phase with the defender. The advantage usually  goes to the more maneuverable aircraft. There are two types of scissor  maneuvers, called flat scissors and rolling scissors. 

Rolling Scissors:  Rolling scissors, also called vertical scissors, tend to happen after a  high-speed overshoot from above. The defender reverses into a vertical  climb and into a barrel roll over the top, forcing the attacker to attempt to follow. The advantage  lies in the aircraft that can pull its nose through the top or bottom of  the turn faster. In battles with aircraft that have a thrust-to-weight ratio of less than one the aircraft will quickly lose altitude, and crashing into the ground becomes a possibility. According to author Mike Spick, "Disengagement from a vertical rolling scissors is best made with a split-s and a lot of hope. 

Flat Scissors:  Flat scissors, also called horizontal scissors, usually occur after a  low-speed overshoot in a horizontal direction. The defender reverses the  turn, attempting to force the attacker to fly out in front and to spoil  aim. The attacker then reverses, trying to remain behind the defender,  and the two aircraft begin a weaving flight pattern.

 The opposite of an Immelmann is the Split-S. This maneuver consists of  rolling inverted and pulling back on the stick, diving the aircraft into  a half loop, which changes the aircraft's direction 180 degrees. The  split-s is rarely a viable option in combat as it depletes kinetic  energy in a turn and potential energy in a dive. It is most often used  to set up a high-side guns pass against a lower but fast moving opponent  that is traveling in the opposite direction. Also, the split-s is  sometimes used as a disengagement tactic.

The Split S is contrasted with the Immelmann turn,  which is an ascending half-loop that finishes with a half-roll out,  resulting in level flight in the opposite direction at a higher  altitude. The Split S is also called a reversed Immelmann turn and can  also be written with a hyphen: Split-S. In basic terms, the  Immelmann and Split S are very similar, both accomplishing the same  reversal in course, but the Split S exchanges altitude to gain speed,  while the Immelmann turn exchanges speed to gain altitude. 

The Split S, being a descending maneuver, means that the pilot must  ensure that it is started high enough to complete the half-loop; the  exact minimum altitude depends on factors like the aircraft's speed,  weight and maneuverability, likewise the terrain below the plane.  Misjudgements can arise from a lack of situational awareness or from an error in reading instruments

 If the attacker has a significant altitude advantage, a high-side guns  pass is usually prudent. Sometimes called a "swoop", "boom and zoom",  "hit and split", plus a variety of other names, it consists of a powered  dive toward the rear quarter of a lower flying opponent. Shooting with  the cannons in a single, high-speed pass, the attacker uses excess  kinetic energy to disengage from the fight in a zoom climb back to a safe altitude, restoring the potential energy. This allows  the attacker to set up another attack and dive again. Surprise is often a  key element in this type of attack, and the attackers will often hide  in the sun or clouds, stalking their opponents until a good opportunity  is presented. A high-side guns pass is a very effective tactic against a  more maneuverable opponent, where the turning battle of a dogfight is  best avoided. 

Spotting an attacker approaching from behind, the defender will usually  break. The maneuver consists of turning sharply across the attacker's  flight path, to increase AOT (angle off tail). The defender is exposed  to the attacker's guns for only a brief instant (snapshot). The maneuver  works well because the slower moving defender has a smaller turn radius and bigger angular velocity,  and a target with a high crossing speed (where the bearing to the  target is changing rapidly) is very difficult to shoot. This can also  help to force the attacker to overshoot, which may not be true had the  turn been made away from the attacker's flight path. 

 The counter to a break is often a displacement roll called a barrel roll attack. A barrel roll consists of performing a roll and a loop, completing both at the same  time. The result is a helical roll around a straight flight path. The  barrel roll attack uses a much tighter loop than the roll, completing a  full loop while only executing 3/4 of a roll. The result is a virtual 90  degree turn, using all three dimensions, in the direction opposite of  the roll. Rolling away from the defender's break, the attacker completes  the roll with the aircraft's nose pointed in the direction of the  defender's travel.  

 This maneuver differs from the defensive high-g barrel roll in that a  great loss of speed to force an attacking fighter to overshoot is not  necessary. The g forces can therefore often be quite small. Closely  resembling the roll-away, the barrel roll attack is used to alter the  angle of approach to the defender without losing a lot of speed. It is  used when the attacker becomes aware that he is going to overshoot a  turning target. He rolls the wings level, pulls the nose hard up, then  rolls away from the direction of turn. This three- dimensional maneuver  is completed by sliding in astern of the target. 

 The high Yo-Yo is a very effective maneuver, and very difficult to  counter. The maneuver is used to slow the approach of a fast moving  attacker while conserving the airspeed energy. The maneuver is performed  by reducing the angle at which the aircraft is banking during a turn,  and pulling back on the stick, bringing the fighter up into a new plane  of travel. The attacker then rolls into a steeper pitch turn, climbing  above the defender. The trade-off between airspeed and altitude provides  the fighter with a burst of increased maneuverability. This allows the  attacker to make a smaller turn, correcting an overshoot, and to pull in  behind the defender. Then, by returning to the defenders plane, the  attacker restores the lost speed while maintaining energy. 

The low Yo-Yo is one of the most useful maneuvers, which sacrifices  altitude for an instantaneous increase in speed. This maneuver is  accomplished by rolling with the nose low into the turn, and dropping  into a steeper slice turn. By utilizing some energy that was stored in  the vertical plane, the attacker can quickly decrease range and improve  the angle of the attack, literally cutting the corner on the opponent's  turn. The pilot then pulls back on the stick, climbing back to the  defender's height. This helps slow the aircraft and prevents an  overshoot, while placing the energy back into altitude. A defender  spotting this maneuver may try to take advantage of the increase in AOT  by tightening the turn in order to force an overshoot. The low Yo-Yo is often followed by a high Yo-Yo, to help prevent an  overshoot, or several small low Yo-Yos can be used instead of one large  maneuver.

 A lag displacement roll, also called a "lag roll", is a maneuver used to  reduce the angle off tail by bringing the attacker from lead pursuit to  pure, or even lag pursuit. The maneuver is performed by rolling up and  away from the turn, then, when the aircraft's lift vector is aligned with the defender, pulling back on the stick, bringing the  fighter back into the turn. This maneuver helps prevent an overshoot  caused by the high AOT of lead pursuit, and can also be used to increase  the distance between aircraft. 

Superiority in the rolling plane could be used to defeat a better-turning opponent. Rolling away from the direction of the turn allowed the pursuing fighter to cut the corner. This was, however, a double-edged sword: American Thunderbolts often used the vector roll against Bf 109s.