Pilots might be inclined to associate the phrase control system with only aircraft flight control systems. Although the control system theory of this course has a large application to flight control sysems, this material applies to any process or system in which control is exercised over some output variable. Examples of these controlled variables are: the speed of an automotile, the temperature of a room, the attitude of a spacecraft, ad infinitum.
The study of flight control systems is of central importance in the design and flight test of modern aircraft. Increasingly these aircraft are designed with performance considerations only and sophisticated flight control systems are required to correct a host of handling qualities deficiencies. Today's complex control systems are comprised of various elements, washout filters, prefilters, sensors, electronic compensators and structural filters, in full authority fly-by-wire systems. The purpose of this course is to provide a fundamental understanding of the basic types of aircraft control strategies. Simple mechanical systems are presented including the effect of control system elements, such as actuators, artificial feel systems, and mechanical elements. Stability and control augmentation systems are then discussed, stressing the advantages and disadvantages of each type of feedback commonly used. Electronic compensators are studied as further refinements to the augmented system. Modern analog and digital electronic flight control systems are then analyzed to gain an appreciation for their operation and potential handling qualities deficiencies. In addition, automatic flight control systems are discussed including both autopilots and the more sophisticated integrated systems for automated maneuvering. Aircraft sensors used in most of these flight control systems are also analyzed. Finally, flight control system test techniques are presented.
Dynamics is concerned with the time history of the motion of physical systems. An aircraft is such a system, and its dynamic stability behavior can be predicted through mathematical analysis of the aircraft's equations of motion and verified through flight test. During this study of aircraft dynamics, the solutions to both first order and second order systems will be of interest, and several important descriptive parameters will be used to define the dynamic response of either a first or a second order system.
This chapter studies the algebra and calculus of vectors and matrices, as specifically applied to the USAF Test Pilot School curriculum. The course is a prerequisite for courses in Equations of Notion, Dynamics, Linear Control Systems, Flight Control Systems, and Inertial Navigation Systems. The course deals only with applied mathematics; therefore, the theoretical scope of the subject is limited.
This chapter reviews the mathematical tools and techniques required to solve differential equations. Study of these operations is a prerequisite for courses in aircraft flying qualities and linear control systems taught at the USAF Test Pilot School. Only analysis and solution techniques which have direct application for work at the School will be covered.
The performance of an aircraft can adequately be described by assuming the aircraft is a point mass concentrated at the aircraft's center of gravity (cg). The flying qualities of an aircraft, on the other hand, cannot be described in such a simple manner. The flying qualities of an aircraft must, instead, be described analytically as motions of the aircraft's cg as well as motions of the airframe about the cg, both of which are caused by aerodynamic, thrust and other forces and moments. In addition, the aircraft must be considered a three dimensional body and not a point mass. The applied forces and moments on the aircraft and the resulting response of the aircraft are traditionally described by a set of equations known as the aircraft equations of motion. This chapter presents the form of the aircraft equations of motion used in the Flying Qualities phase of the USAF Test Pilot School curriculum.
Your study of flying qualities to date has been concerned with the stability of the airplane flying in equilibrium on symmetrical flight paths. More specifically, you have been concerned with the problem of providing control over the airplane's angle of attack and thereby its lift coefficient, and with ensuring static stability of this angle of attack. This course considers the characteristics of the airplane when its flight path no longer lies in the plane of symmetry. This means that the relative wind will make some angle to the aircraft centerline which we define as Beta. The motions which result from Beta being applied to the airplane are motion along the y-axis and motion about the x and z axes.
Static stability is the reaction of a body to a disturbance from equilibrium. To determine the static stability of a body, the body must be initially disturbed from its equilibrium state. If, when disturbed from equilibrium, the initial tendency of the body is to return to its original equilibrium position, the body displays positive static stability or is stable. If the initial tendency of the body is to remain in the disturbed position, the body is said to be neutral stable. However, should the body, when disturbed, initially tend to continue to displace from equilibrium, the body has negative static stability or is unstable. Longitudinal static stability or gust stability of an aircraft is determined in a similar manner. If an aircraft in equilibrium is momentarily disturbed by a vertical gust, the resulting change in angle of attack causes changes in lift coefficients on the aircraft (velocity is constant for this time period). The changes in lift coefficients produce additional aerodynamic forces and moments in this disturbed position. If the aerodynamic forces and moments created tend to return the aircraft to its original undisturbed condition, the aircraft possesses positive static stability or is stable. Should the aircraft tend to remain in the disturbed position, it possesses neutral stability. If the forces and moments tend to cause the aircraft to diverge further from equilibrium, the aircraft possesses negative longitudinal static stability or is unstable.
From the designer to the pilot, everyone associated with the flying qualities of high performance military aircraft, particularly the fighter or attack variety, is or should be aware of the importance of the high angle of attack flight regime. It is here that the aircraft will spend a significant amount of its time when performing the mission for which it was designed. It is here that the aircraft must display its most outstanding performance. It is also here that the aircraft, when pushed beyond its limits of controllability, can seemingly defy all laws of physics and principles of flight with which its surprised and often bewildered pilot is acquainted. The frequency of inadvertent loss of control at high angle of attack is such that many combat aircraft pilots are becoming firmly convinced that all pilots may be divided into two categories: those who have departed controlled flight, and those who will. Most thoroughly convinced are those pilots who fall into the former category. The unfortunate fact concerning departure from controlled flight at high angle of attack is that many aircraft and pilots are lost each year due to failure to recover from the out-of-control flight condition. The circumstances surrounding the losses are varied. Departures from controlled flight may occur unintentionally during high-g maneuvers or intentionally during a nose-high deceleration to zero airspeed in an attempt to gain an advantage over an opponent in combat maneuvering; the aircraft may spin and the gyration be identified too late for recovery or a steep spiral may be mistakenly identified as a spin, causing recovery controls to be misapplied. Whatever the circumstances, departures from controlled flight result all too often in catastrophe. For this reason, test pilots in particular must be familiar with every facet of the high angle-of-attack flight regime.
This chapter examines the problems associated with an engine failure and how engine out flight testing is accomplished. The discussion will begin with performance issues, how engine loss affects the takeoff and initial climb segments. Next, the equations of motion are introduced, modified for an engine failure. They are used to show how minimum control speeds are determined by design. Finally, flight test techniques are introduced for the evaluation of engine out flying characteristics of multiengined aircraft.
