|
Organizers |
Propulsion Control and Trim Adjustment for an Impaired Large Transport Aircraft
by
Yoshimasa Ochi
National Defense Academy
Coauthors: Kimio Kanai
This paper describes three topics about failure accommodation of large transport aircraft, especially for control surface jam that can be caused by hydraulic system failure: First, we consider controlling the aircraft with hydraulic failure using engine thrust only; second, thrust vectoring is applied to the aircraft; third, feedforward step input is applied to slow effectors such as stabilizers and engines to counteract the disturbances caused by the stuck surfaces.
Since the hydraulic system of a large transport aircraft is redundant, it is rare that all the control surfaces of the aircraft becomes no use. However, several accidents caused by hydraulic failure have really happened, for example, the Boeing 747 of Japan Airline in 1985 and the DC-10 of United Airline in Sue-City in 1989. Particularly, in the latter case the pilots controlled the aircraft using thrust only to manage to fly to the airport, saving 187 people out of the 298 crew and passengers. This example illustrates the possibility of thrust control of aircraft. Since it is not easy for pilots to do this, introducing automatic control to thrust control will alleviate pilots? workload and enhance safety.
The aircraft controlled by thrust only is called Propulsion Controlled Aircraft (PCA). NASA Dryden Flight Research Center successfully conducted flight experiments of PCA using the F-15A in 1993. Following this, McDonnell Douglas and the center developed an autopilot using thrust control for the MD-11 and succeeded flight experiments, including approach and landing [1, 2]. Piloted simulation tests for a Boeing 747 jet transport were also conducted[3].
Reference 4 proposes a flight control system for longitudinal control of PCA, which is a model following control system. Thereby the outputs of PCA follow those of the reference model or the normal aircraft model. They apply H control to design of the autopilot. However, there are two problems with their method. One is that the order of the controller is 16 including the reference model, which is quite high. The other is that they modify the aircraft model, which is a fourth-order model of the L-1011, by making the distance between the x axis and the second engine ten times larger than the real one; then the distance becomes almost 20 m. The purpose of this modification is to increase the aircraft's pitching ability, but it is totally impractical.
We also design a similar flight control system, but it is based on H-infinity state feedback control. Although in Ref. 4 all the state variables are measured, their controller is basically an output feedback one with a dynamic compensator, which is a typical structure of H-infinity controllers. By contrast, we design the controller as a full-state feedback one; as a result, the order of the controller becomes 6 including the reference model. The second problem is removed by reducing the static stability margin. This modification allows the aircraft to pitch with smaller moment. It is more practical modification, and relaxed static stability is a trend of new transport aircraft such as the Boeing 777. Meanwhile, Ref. 5 suggests the use of thrust vectoring for civil aircraft. We applied thrust vectoring to the second engine to control pitching motion, assuming that the actuation system for thrust vectoring is still available after failure.
We conducted numerical simulation, and the results show that the aircraft with relaxed static stability is better controlled without input saturation than the original aircraft. The effectiveness of thrust vectoring is also confirmed by simulation. It is found that the vectored thrust is as effective as the elevators.
Hydraulic system failure sometimes causes control surface jam. Stuck control surfaces can generate significant disturbances, so that the failure may lead to a fatal accident, especially in large transport aircraft. We proposed a method to accommodate such failures by taking advantage of slow effectors such as stabilizers and engines[6]. The slow effectors are usually not used for attitude control along with fast effectors such as elevators, ailerons, and rudders; on the other hand, they can generate large force and moment to reject the disturbances. Since the slow effectors cannot move fast, control command to the effectors is given by step input. The command input is determined using a pseudo-inverse of the control derivative matrix in a linear aircraft model so that the force or moment produced by the stuck control surfaces can be counteracted by the slow effectors. This idea is the same as control mixer used in self-repairing flight control[7]. However, the method does not take account of the effect given by the failure through aircraft dynamics. In fact, the system derivative matrix is not used in derivation of the control law. For example, when the rudder is stuck, the yawing moment generated by the rudder cannot effectively be counteracted by any other effectors; however, the rolling moment can be by differential deflection of the horizontal stabilizers. Hence, the control command that directly counteracts the rolling moment is given to the stabilizers. On the other hand, the yawing moment produces a large yawing motion that results in a large rolling motion by the difference of the lift between the right wing and the left wing. Besides, the direction of the rolling motion is opposite to the one directly generated by the rudder. Thus, the control law for the slow effector is not necessarily good, when the slow effectors cannot effectively counteract the force or moment generated by the stuck surfaces. In such a case, we have to take dynamic effect of the control surface jam into account.
We proposes a solution to this problem, which is based on the steady state solution of the optimal regulator. Specifically, the method is composed of three steps: First, the optimal control law is designed for the slow effectors, assuming that the control inputs are the slow effectors only. Second, steady state of the closed-loop system is calculated in the case where control surface jam occurs. Third, steady-state inputs of the slow effectors are obtained by substituting the steady state variables to the control law. We use the steady-state inputs as command inputs to the slow effectors and use the steady state variables for modification of the trim point. Numerical simulation using a linear model of the Boeing 747 demonstrates that the proposed control law and trim adjustment provides smaller deviation of the state variables for the failure of rudder jam.
The three methods described above can be integrated into a flight control system to improve its failure accommodation ability.
Date received: February 1, 1998
Copyright © 1998 by the author(s). The author(s) of this document and the organizers of the conference have granted their consent to include this abstract in Atlas Conferences Inc. Document # caav-06.