Technology of designing high-quality flight control systems

V.G.Borisov, G.N.Nachinkina, B.V.Pavlov,A.M.Shevchenko

Trapeznikov Institute of Control Sciences of RAS

65, Profsoyuznaya, Moscow, 117997, Russia

Flight conditions of modern aircrafts differ widely and consequently the aerodynamic characteristics of the aircrafts vary significantly. This paper describes the through technique of designing of flight control systems for aircrafts with essentially nonlinear characteristics. The technique includes new theoretical results and their practical realization as part of the multifunctional semi-natural stand. New results are the energy approach to flight control and the original version of modal synthesis. The stand make possible to carry out optimization of system parameters, its verification and flight control system testing with pilot in the loop. The technique's efficiency was demonstrated with a full nonlinear model of the high-speed maneuverable aircraft. Modeling showed invariance of handling qualities in a wide range of flight conditions.

 

The creation of high-quality flight control systems has taken place throughout the history of aircraft evolution and continues up to this day.

The process of designing and preflight tryout of flight control systems consists of following stages:

-       analysis of static and dynamic properties of the object;

-       synthesis of control;

-       modeling tests and estimation of aircraft performance;

-       system tests at the flight simulator with the pilot in a control contour.

Each stage was usually carried out by different systems project teams often separated territorially, on different engineering tools. Such organizational management has resulted in unjustified material expenditure and the long lead time of system engineering. Analytical studies of modern/state-of-the-art and promising technological levels have reflected the conservatism or backwardness of this level. Scientific achievements were often ignored in consequent/current projects.

The aim of this work is development of the through technology of control systems designing. According to this approach all stages of researches, designing and debugging are made by means of uniform computer technology, on the same equipment.

Original scientific components of offered technology are the energy approach to motion control in space and the special version of modal synthesis of regulators with required quality.

There is a wide variety of flight control concepts.

From the very beginning of aviation the main controlled state variables were flight altitude and airspeed. It was quite natural that elevator deflection d_e and power lever angle (PLA) were used as control variables. With progress in the theory and practice of automatic flight control the burden of manual control was transferred to the autopilot and the auto throttle. Such channel division was somewhat advantageous for formal description of vehicle motion by a differential equation system. The classical motion equations were also divided into the dynamic and kinematic equations.

That is how the conventional two-channel structure of flight control system in the longitudinal plane has formed in aviation. Such channel separation has the following serious operational shortcomings:

-        control in any channel results in a coupled response in the other channel;

-        constraints of control power in one channel are disregarded in the other channel; this sometimes results in the stalling or loss of height and therefore collision with the ground;

-        aircraft response in automatic mode is not adequate to the manner of an expert pilot;

-        attempts to decouple the channels and ensure higher transient quality result in unwarranted cross-couplings in the system.

These and some other disadvantages of conventional approaches to flight control systems have fueled the search for new ways to satisfy the ever-growing requirements for flight control quality.

 

In conclusion we note.

The technique of high-quality flight control systems design is developed. The basic structural concept is the energy approach. The rational structure of the energy control system (ECS) is received on its basis.

The ECS has shown excellent handling qualities of transport aircrafts at landing approach in diversified conditions. However, the system with fixed coefficients could not provide acceptable control quality of maneuverable high-speed aircrafts over the operational area.

Therefore parameter adjustment was required for using the ECS over the whole flight envelope.

The version of a modal method for calculation of the feedback coefficients matrix has been developed. The control on each mode as the sum of available controls with weighting coefficients is offered for solution ambiguity eliminating.

Synthesis of the flight control system for a high-speed maneuverable aircraft has been carried out. Optimal feedback coefficients in discrete points of operational area have been found. The coefficients adjustment circuit has been introduced into the system structure. Modeling of the ECS with adjusted parameters has shown that transients had retained invariant in diversified flight conditions and had been practically identical at speed from 100 up to 500 m/s and altitude from 0 up to 10,000m.

 

This work was supported in part by Russian Foundation for Basic Research, Project 09-08-00313-a

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