Aeronautics and astronautics – Aircraft control – Automatic
Reexamination Certificate
2000-07-20
2001-12-04
Barefoot, Galen L. (Department: 3644)
Aeronautics and astronautics
Aircraft control
Automatic
C244S227000, C701S006000
Reexamination Certificate
active
06325333
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to flight control systems for aircraft, and more particularly, to an apparatus and method for controlling aircraft elevator commands using a pitch-axis stability and command augmentation system.
BACKGROUND OF THE INVENTION
On some new airplanes, the static stability of the plane has been purposefully relaxed to include instances of flight wherein the plane has neutral static stability. A significant benefit in fuel-efficiency, weight-savings, and drag-reduction can be realized with such a system, due to an allowable reduction in required tail size. An airplane with relaxed static stability may have some undesirable and unacceptable handling quality characteristics, though.
One characteristic is that a neutrally stable airplane will not naturally return to its previous airspeed after a change in pitch attitude. On a trimmed positive static stability airplane, the pitch attitude changes according to the pilot pushing forward or pulling back on the control column. When the column is released, the airplane will continue to fly at its new pitch for a short time. As the airspeed of the plane changes, the airplane will have a natural tendency to nose up or nose down, returning to its previous trim speed. This tendency is an expected handling quality of conventional commercial aircraft, and one to which pilots virtually automatically respond in flying their craft.
Neutrally stable planes have no tendency to return to their trim position and airspeed after an adjustment of pitch. For these aircraft, changes in pitch are maintained and airspeed varies accordingly. During pitch increases, airspeed decreases. During pitch decreases, airspeed increases. To return the aircraft to its trim position and speed, the pilot must return the plane to its trim pitch, by repositioning the elevators with the control column.
A characteristic that is common to planes with or without relaxed static stability is that the amount of elevator input required to accomplish a certain pitch is sensitive to changes in weight and location of center of gravity relative to the mean aerodynamic center. An aircraft that is light in weight and has a center of gravity near the mean aerodynamic center, requires only a small change in elevator angle of attack to produce a large change in airplane pitch. Conversely, a heavy aircraft having a forward center of gravity requires much more elevator deflection to produce a like change in airplane pitch. Without control augmentation, the pilot must move the control column much farther in the heavy case in order to get the same pitch response as in the light case. This can be an undesirable characteristic because it requires the pilot to adjust his or her column input based on the weight and location of the plane's center of gravity—pieces of information not readily perceptible.
A third characteristic, though not related to relaxed stability aircraft, concerns the manner in which pilots expect the airplane to respond when the control column is moved. It is fairly well known that at low airspeeds, pilots expect movement of the control column to produce a change in pitch rate. At high airspeeds, pilots expect movement of the control column to produce a change in normal acceleration. This situation was recognized in the 1960's and the C* criterion was developed as a way to express optimal airplane response taking it into consideration. The C* criterion is discussed in detail below.
Thus, there exists a need for a superior pitch attitude control system capable of improving the control and handling characteristics of an aircraft. Optimally, this pitch attitude control system should provide such an airplane with handling characteristics (from the pilot's point of view) similar to an ideal conventional airplane with positive static stability, in order to reduce the amount of variation in flying technique required. The ideal control system should accomplish this goal by meeting the pilot's expectation regarding the airplane's short and long term responses to elevator command; and by responding to pilot pitch commands in a similar manner, regardless of weight or center of gravity shifts. The ideal control system should further accommodate the pilot's expectations to control pitch rate during low airspeed maneuvers and control to normal acceleration during high airspeed maneuvers. As will be appreciated by a reading of the following description, the present invention is directed to providing such a superior pitch attitude control system.
SUMMARY OF THE INVENTION
The present invention provides a pitch-axis stability and command system for augmenting aircraft elevator commands. The present invention uses a criterion herein referred to as the C*U criterion to provide ideal pitch response airplane characteristics and long-term speed stability in the pitch axis. The C*U criterion allows the present invention to accomplish the previously mentioned goals by optimizing the pitch rate and normal acceleration response of the airplane and by providing the desirable speed stability characteristics of a conventional airplane.
The present invention uses a feedback system configured to receive a control column input and convert the input into elevator command signals. The feedback portion of the system processes signal representative of current airplane data which is formed in response to a previous elevator command. Augmentation of the system is accomplished by converting the column input into a pilot-requested C*U command signal and comparing that command signal with a computed C*U command signal generated on the basis of the current airplane state. The error signal, which represents the difference between what the pilot is commanding and how the aircraft is responding, is integrated and the result is added to the elevator command signal. Therefore, the pitch-axis stability and command augmentation system of the present invention zeros any difference between what the pilot has requested and what the airplane is performing.
In accordance with the present invention, the pitch-axis stability and command augmentation system includes a pitch command processor that converts a pilot column input into a feedforward command signal that is one of two signals used to generate an elevator pitch command. The second signal component is discussed below. The command processor additionally supplies a corrected column position signal to a commanded C*U processor that converts the corrected column position into a C*U pitch command signal representative of the movement of the control column that requests pitch change.
In accordance with further aspects of the present invention, a computed C*U processor forms a computed C*U signal that is based on the current state of the aircraft. The computed C*U signal of the currently preferred embodiments includes at least three components: a normal acceleration signal, a pitch rate signal, and a speed error signal. In the preferred embodiment, the normal acceleration and pitch rate signals are earth-referenced and are supplied by a normal acceleration complementary filter and a pitch rate complementary filter, respectively.
The speed error signal is supplied by a speed stability processor. The speed stability processor includes phugoid damping in its formation of the speed error signal. The speed stability processor further implements a method that allows the pilot to set a reference airspeed by using a trim device.
In accordance with other aspects of the present invention, three C*U compensation and protection signals are supplied to further enhance the handling and response characteristics of the airplane. An underspeed protection signal is supplied by a stall protection processor which includes the additional features of establishing a minimum reference airspeed below which the pilot may not trim and enhances the airplane response and handling characteristics during underspeed operation. A flare compensation signal is supplied by a flare compensation processor to cause the aircraft to perform as
Anderson Dorr Marshall
Bleeg Robert J.
Breuhaus Richard S.
Coleman Edward E.
Evans Monte R.
Barefoot Galen L.
Christensen O'Connor Johnson and Kindness PLLC
The Boeing Company
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