Flight Control Stall Protection With Pitch and Load Factor Limits
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Solution Overview
Problem
Existing aircraft systems lack effective mechanisms to transiently limit pitch attitude and pitch rate to prevent stall and overload, particularly in modern fly-by-wire systems, which can lead to exceeding load factor limits and angle of attack constraints.
Innovation Solution
A nested loop flight control computer system that includes a stall protection circuit to compute attitude limits based on flight path angle and angle of attack, and a load protector circuit to compute pitch rate limits based on load factor and true airspeed, providing saturation on both pitch attitude and pitch rate to prevent stall and overload.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If fly-by-wire systems are used to improve aircraft control precision, then control precision is improved, but the risk of exceeding load factor limits and angle of attack constraints increases
Solution Approach 1:
The system performs preliminary calculations of maximum allowable pitch attitude (θmax) and pitch rate ({dot over (θ)})max before actual maneuvers occur. The stall protection circuit computes θmax as a function of flight path angle and angle of attack limit, while the load protector circuit computes {dot over (θ)})max as a function of load factor limit and true airspeed. These pre-computed limits are then applied to constrain pilot or autopilot commands, preventing exceedance of mechanical limits.
2Reliability
If pitch attitude and pitch rate limits are applied to prevent stall and overload, then aircraft safety is improved, but control flexibility is reduced
Solution Approach 1:
The system dynamically adjusts the pitch attitude and pitch rate limits based on real-time flight conditions. The stall protection circuit continuously computes θmax as a function of the current flight path angle (γ) and angle of attack limit (αmax), while the load protector circuit computes {dot over (θ)})max as a function of the current load factor limit (Nz,max) and true airspeed (ν). This dynamic adaptation allows the control system to provide maximum flexibility within safe boundaries, adjusting constraints as flight conditions change rather than applying fixed limits.
Data Source
AI summary
There is disclosed in one example an inner loop controller for an aircraft flight computer, including: a stall protection circuit to compute, for an attitude angle θ, an attitude limit θmax as a function of a flight path angle (γ) and an angle of attack limit (αmax); a transfer function circuit to convert θ to an attitude rate {dot over (θ)}, wherein {dot over (θ)} is a time derivative of θ; and a load protector circuit to compute a limit on {dot over (θ)} ({dot over (θ)}max) as a function of a load factor limit (Nz,max) and a true airspeed (v).