Longitudinal stability
The static longitudinal stability describes the property of an aircraft disturbed in its longitudinal flight attitude to counteract this disturbance with a restoring and thus stabilizing pitching moment . The dynamic longitudinal stability , on the other hand, describes the extent to which the disturbed longitudinal flight attitude develops or regresses over time. If an aircraft is statically and dynamically longitudinally stable, it has the property of automatically moving back to a stable original position in the event of a disturbance in the longitudinal flight attitude. Such a disruption of the longitudinal flight attitude, that is to say a change in the angle of attack α, can occur, for example, through turbulence or gusts. Since, with a few exceptions, most aircraft are constructed mirror-symmetrically to their x / z plane, the longitudinal stability can be considered separately from the roll and yaw stability . All considerations on longitudinal stability are based on rotational movements around the aircraft-fixed transverse or pitch axis as well as on translatory movements in the aircraft-fixed xz plane. Both static and dynamic longitudinal stability are elementary criteria in aircraft design.
Stable design
The criterion for the static longitudinal stability of an aircraft is This term shows that any change in the angle of attack always results in a resulting pitching moment (shown as a coefficient) with the opposite sign.
The aerodynamic stability of non-powered aircraft is determined by the position of the aerodynamic pressure point in relation to the center of gravity. In order to fly stable, the focus will be on the pressure point. If the direction of flow is disturbed, a pressure point migration to the rear creates a torque that counteracts the change in the angle of attack. The aircraft is turned back around the transverse axis in the original direction of flow.
In the case of motor-powered aircraft, the position and the power of the drive also play an essential role in terms of stability and controllability. However, since most aircraft should also be able to fly and steer without propulsion, they are designed to be aerodynamically stable according to the above principle.
Unstable design
The aerodynamics of modern combat aircraft are designed in such a way that at subsonic speed the center of gravity is behind the pressure point, which leads to unstable aerodynamic behavior. Due to the pressure point in front of the center of gravity, a tilting moment acts during the flight, which tends to turn the aircraft around the transverse axis. This overturning moment must be permanently compensated for by interventions in the rudder. The rudders are controlled by an attitude computer , which constantly and actively corrects the attitude using data from flow and attitude sensors. In tight turns, the rudder does not have to push the aircraft against the return torque of the pressure point because of the tilt. Instead, the aircraft's tendency to swerve is used to maneuver particularly quickly, which leads to superior maneuverability compared to stable aircraft.
The control of the rudders as a reaction to undesired movements of the aircraft must take place much faster than a human pilot can do due to his biologically determined reaction time. The aircraft is therefore dependent on the flight attitude computer to function correctly. If it fails, the ejection seat is automatically triggered.
See also
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- Götsch, Ernst: Aircraft technology . Motorbuchverlag, Stuttgart 2003, ISBN 3-613-02006-8 .
- Fichter, Walter: Static stability of the longitudinal movement. (PDF; 640 KB) Short manuscript. Institute for Flight Mechanics and Flight Control, University of Stuttgart, 2015, accessed on March 7, 2017 .