Takeoff Stall Protection Using Pitch Rate Saturation Control
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Solution Overview
Problem
Conventional flight control systems during takeoff are prone to over-rotation, leading to a risk of aerodynamic stall, especially with side-stick controllers, as they provide inadequate control authority and rely on ground-dependent stall information, which is not effective during lift-off.
Innovation Solution
A flight control system that generates a damped pitch rate command by calculating a pitch angle saturation limit based on aircraft speed and flight path angle, applying a factor to the pitch rate command, and coupling it to the aircraft control surface to prevent over-rotation and transition to in-air control laws after lift-off.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If a conventional direct-gain pitch controller with small-displacement side-stick is used, then the control system is simple and responsive, but the pilot tends to over-rotate the airplane during takeoff, creating stall risk
Solution Approach 1:
The control system preemptively limits the pitch rate command before over-rotation can occur. The pitch rate limiter applies saturation logic that prevents the pitch rate from exceeding safe thresholds during takeoff, thereby preventing stall conditions before they develop rather than reacting after over-rotation occurs.
Solution Approach 2:
The pitch rate limiter dynamically adjusts control authority based on flight phase and conditions. During takeoff, the system transitions from full control authority to limited authority mode, allowing adequate response for normal operations while preventing excessive pitch rates that would lead to stall. The control system adapts its characteristics to the current flight situation.
2Reliability
If control authority is decreased or inceptor control forces are increased to prevent over-rotation, then stall protection improves, but the ability to overcome stabilizer mis-trims or control surface jams is reduced
Solution Approach 1:
The control system dynamically modulates pitch rate limiting based on flight conditions. During takeoff when stall risk is highest, the limiter is active with appropriate thresholds. In other flight phases or when abnormal conditions are detected (such as stabilizer mis-trim or control surface jam), the system can adjust or deactivate the limiting to restore full control authority when needed.
Solution Approach 2:
The system changes the pitch rate command parameter through saturation logic rather than permanently reducing control authority. The saturation function applies temporary limiting only when pitch rate exceeds safe thresholds, while maintaining full control range availability for other maneuvers and abnormal condition recovery.
3Reliability
If ground-dependent stall information is used for stall protection, then protection is adequate during ground effect, but the protection becomes ineffective during lift-off when aerodynamics change
Solution Approach 1:
The pitch rate limiter transitions dynamically from ground-dependent to flight-path-based control logic as the aircraft lifts off. During ground effect, traditional stall information remains valid. As lift-off occurs and aerodynamics change, the system switches to using flight path angle and pitch rate commands that remain effective regardless of ground proximity, ensuring continuous protection across all phases.
Solution Approach 2:
The control system changes the basis for stall protection from ground-dependent parameters to flight-path-based parameters. Instead of relying solely on angle of attack information that changes with ground effect, the system uses pitch rate commands and flight path angle that provide consistent protection whether the aircraft is on the ground, just lifting off, or in flight.
Data Source
AI summary
Flight control systems, flight control laws, and aircraft are provided. An flight control system includes an input configured to receive a pitch rate command, a processor operative to receive the pitch angle command, to calculate a pitch angle saturation limit, to compare the sum of the pitch rate command, the scaled pitch rate, and the scaled pitch angle to the pitch angle saturation limit, to convert the pitch rate command system to the pitch angle command system in response to the sum exceeding the pitch angle saturation limit value to limit the pilot pitch-up pitch rate command, and to couple the pitch rate command to an aircraft control surface for the failure case of one of control surface, and the aircraft control surface configured to adjust an aircraft control surface setting in response to the pitch rate command and/or pitch angle command to protect an aircraft from being in stall condition.