Airplane Autopilot Development

A. Saccomanno Airplane Autopilot Development
The third and last phase consists in the controllers tests which are carried out by using 
Microsoft
®
 Flight Simulator 2004. Each controller is tested against a wide range of 
flight conditions. 
The main contribution of this work is to design a simple autopilot system with low 
development costs and by using an open architecture. The proposed autopilot is 
expected to have a performance comparable to the autopilot  that Microsoft
®
 Flight 
Simulator 2004 implements in his airplanes. 
INTRODUCTION 2
CHAPTER 1 
RUDIMENTS OF AIRPLANE FLIGHT CONTROL SYSTEMS 
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A. Saccomanno Airplane Autopilot Development
1.1 Introduction
Flight control has advanced considerably in last the years. In this chapter we will 
explain the basic concepts relative to the flight control systems. It contains the basic 
principles of aircraft motion (translational and rotational motion) and explains the 
aircraft primary controls and how the set of controls operate on the primary control 
surfaces called elevators, ailerons and rudder. In this chapter there is a short 
explanation about the difference between fighters and airliners. Then, it presents an 
illustration of the possible flight control system configurations. 
Inside this chapter,  the aircraft navigation systems useful for obtain the exact position 
of aircraft relative to the Earth (altitude, longitude, latitude) and all the flight 
information like air speed, vertical speed, angle of incidence, bank angle and heading 
will be presented. The most accurate system is the GPS and it will be the main 
measurement instrument of this thesis work because it is integrated on our test 
aircraft. 
Last, this chapter will be treats the operating concepts of modern autopilot systems. 
1.2 Principles of flight control 
All aircrafts are governed by the same basic principles of flight control, whether the 
vehicle is the most sophisticated high-performance fighter or the simplest model 
aircraft. 
The motion of an aircraft is defined through the translational motion and the 
rotational motion around a fixed set of pre-determined axes. For an orthodox aircraft 
there is only one direction in which translational motion occurs, that is the direction 
in which the aircraft is flying which is also the direction in which it is pointing.
[1]
The rotational motion relates to the motion of the aircraft around three defined axes, 
pitch, roll and yaw.
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A. Saccomanno Airplane Autopilot Development
Fig 1.1: Definition of aircraft coordinates and flight control surfaces 
Fig 1.1 shows the direction of aircraft velocity vector in relation to the pitch, roll and 
yaw axes. For most of the flight an aircraft will be flying straight and level and 
velocity vector will be parallel with the surface of earth and proceeding towards a 
heading that the pilot has chosen. If the pilot wishes to climb the flight control system 
is required to rotate the aircraft around the pitch axis in a nose-up sense to achieve a 
climb angle.
[1]
Upon reaching the new desired altitude the aircraft will be rotated in a nose-down 
sense until the aircraft is once again flying straight and level. In most fixed wing 
aircraft, if the pilot wishes to alter the aircraft heading then he will need to execute a 
turn to align the aircraft with the new heading. During a turn the aircraft wings are 
rotated around the roll axis until a certain angle of bank is attained. In a properly 
balanced turn the angle of roll (often called the bank angle) is maintained. This 
change in heading is actually a rotation around the yaw axis. The difference between 
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A. Saccomanno Airplane Autopilot Development
the climb (or descent) and the turn is that the climb only involves rotation around one 
axis whereas the turn involves simultaneous coordination of two axes. 
At certain times during flight the pilot may in fact be rotating the aircraft around three 
axes, for example during a climbing or descending turning manoeuvre. The aircraft 
flight control system enables the pilot to control the aircraft during all portion of 
flight. The system provides control surfaces which allow the aircraft to manoeuvre in 
pitch, roll and yaw. The system has also to be designed in order to provide stable 
control for all flight conditions in the aircraft flight envelope: this requires a complete 
understanding of the aerodynamics and dynamics motion of the aircraft. As will be 
seen, additional control surfaces are required during approach and landing phases of 
flight. The flight control system has to give the pilot considerable physical assistance 
to overcome the enormous aerodynamic forces on the flight control surfaces.
[1]
1.3 Flight control surfaces 
The requirements for flight control surfaces vary between different aircrafts, 
depending on the role, range and agility needs of the vehicle. These diffrent 
requirements may best be summarized through two types of aircraft: an agile fighter 
aircraft and a typical modern airliner. 
1.3.1 Fighter aircrafts 
The flight control surfaces of an Eurofighter 2000 “Typhoon” (EF2000) are shown in 
Fig 1.2 and Fig 1.3. The EF2000 represents the state-of-the-art fighter aircraft as 
defined by European manufactures at the beginning of the 1990s. 
The EF2000 is developed by the four nation consortium comprising Alenia
Aeronautica (Italy), British Aerospace (United Kingdom), CASA (Spain), EADS
(Germany). 
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A. Saccomanno Airplane Autopilot Development
Fig 1.2: EF2000 (BAe, Alenia Aeronautica, EADS, CASA) – http://richard.ferriere.free.fr 
Fig 1.3: EF2000 (BAe, Alenia Aeronautica, EADS, CASA) – http://richard.ferriere.free.fr  
Primary flight control in pitch, roll and yaw is provided by the control surfaces 
described below. Pitch control is provided by the moving canard surfaces, or 
foreplanes, as they are sometimes called, located either side of the cockpit. 
These surfaces provide the very powerful pitch control authority required by an agile 
high performance fighter aircraft. The position of the canard in relation to the wings 
destabilizes the aircraft. Without the benefit of an active computer driven control 
system the aircraft would be uncontrollable. While this may appear to be a fairly 
drastic implementation, the benefit in terms of improved maneuverability experienced 
by the pilot overcomes the engineering efforts required to provide the computer 
controller or “active” flight control system. 
[1]
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A. Saccomanno Airplane Autopilot Development
Roll control is provided by the differential motion of the foreplanes. In order to roll to 
the right the left foreplane leading edge is raised relative to the airflow generating 
grater lift then before. Conversely, the right foreplane moves downwards by a 
corresponding amount relative to the airflow thereby reducing the lift generated. The 
resulting differential forces cause the aircraft to roll rapidly to the right. To some 
extent roll control is also provided by the differential action of the wing trailing edge 
flaperons (sometimes called elevons). However most of the roll control is provided by 
the foreplanes.
 [1]
Yaw control is provided by the single rudder section. For high performance fighter 
aircraft yaw control is generally less important than for conventional aircraft due to 
the high levels of excess power. There are nevertheless certain parts of the flight 
envelope where the control yaw (side-slip) is vital to prevent roll-yaw divergence.
 [1]
1.3.2 Commercial aircraft 
An example of flight control surfaces of a typical commercial airliner is shown in Fig. 
1.4 and Fig. 1.5. Although the example is for the McDonnell Douglas MD-11, similar 
airliners produced by Boeing or Airbus Industries have the same control surfaces. 
The controls used by this type of aircraft are described below: 
Pitch control is exercised by four elevators located on the trailing edge of the tailplane 
(or horizontal stabilizer). Each elevator section is independently powered by a 
dedicated flight control actuator, powered in turn by one of several aircraft hydraulic 
power systems. This arrangement is dedicated by the high integrity requirements 
placed upon flight control systems. The entire tailplane section itself is powered by 
two or more actuators in order to trim the aircraft in pitch. In a dire emergency this 
facility could be used to control the aircraft, but the rates of movement and associates 
authority are insufficient for normal control purposes.
 [1]
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A. Saccomanno Airplane Autopilot Development
Roll control is provided by two aileron sections located on the outboard third of the 
trailing edge of each wing. Each aileron section is powered by a dedicated actuator 
powered in turn from one of the aircraft hydraulic systems. At low airspeed the roll 
control provided by the ailerons is augmented by differential use of the wing spoilers 
mounted upon the upper surface of the wing. During a right turn the spoilers on the 
inside wing of the turn, that is the right wing, will be extended. This reduces the lift of 
the right wing causing it to drop, hence enhancing the desired roll demand.
 [1]
Yaw control is provided by three independent rudder section located on the trailing 
edge of the fin (or vertical stabilizer). These sections are powered in a similar fashion 
to the elevators and ailerons. On a civil airliner these controls are associated with the 
aircraft yaw dampers. These damp out unpleasant dutch roll oscillation which can 
occur during flight and which can be extremely uncomfortable for the passenger, 
particularly those seated at the rear of the aircraft. 
[1]
Fig 1.4: McDonnell Douglas MD-11 – http://richard.ferriere.free.fr  
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A. Saccomanno Airplane Autopilot Development
Fig 1.5: McDonnell Douglas MD-11 – http://richard.ferriere.free.fr  
1.4 Flight control systems
Flight control system (FCS) supplies the interface between pilot and flight control 
surfaces. To control the airplane, pilot has to actuate the flight control surfaces by 
moving the cloche and rudder pedals. 
Flight control systems are classified in Mechanical, Hydromechanical and Fly-By-
Wire systems. The following paragraph explains those operating principles.
1.4.1 Mechanical systems 
The most basic designs are mechanical flight control systems. These systems were 
used in the early aircrafts and currently in the ultra-lights airplanes where the 
aerodynamic forces are not high. This kind of flight control systems are composed by 
a lot of mechanical parts such as rods, cables, pulleys and sometimes chains to 
transmit the pilot’s input to the control surfaces.  
In the last years, the tendency to design bigger and faster aircraft implied an increase 
in control surface areas and then, a large increase in the forces needed to move them. 
For this reason, these aircraft needs of sophisticated actuator systems in order to make 
the forces required acceptable to the pilots. This arrangement is implemented on the 
bigger or high performance propeller aircrafts. 
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