Aerospace Technology II – Stability & Control

This report presents findings from a research conducted about fundamentals of mechanics of flight. It focuses on different aspects of the stability and control of an aircraft, including effects of different components or devices on the stability and control of the aircraft.

AEROSPACE TECHNOLOGY II – STABILITY AND CONTROL
Name
Course
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University
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Date
Aerospace Technology II – Stability & Control Ali Talal Mohammed 1

Table of Contents
1. Introduction......................................................................................................................................3
2. Task 1................................................................................................................................................4
2.1. Body axes of an airplane......................................................................................................4
2.2. Types of airplane stabilities.................................................................................................5
2.3. Static margin............................................................................................................................7
2.4. Spiral and Dutch-roll modes................................................................................................8
2.5. Experimental data...................................................................................................................9
3. Task 2..............................................................................................................................................10
3.1. Stability and controllability.................................................................................................10
3.2. Control surfaces....................................................................................................................12
3.2.1. Tail....................................................................................................................................13
3.2.2. Wing.................................................................................................................................14
4. Conclusion.....................................................................................................................................15
References.............................................................................................................................................16
Aerospace Technology II – Stability & Control Ali Talal Mohammed 2

1. Introduction
This report presents findings from a research conducted about fundamentals of
mechanics of flight. As a member of a team working in a reputable aviation company, it
is very important to understand the basics of flight mechanics, which mainly comprises
of the stability and control of an aircraft. This report focuses on different aspects of the
stability and control of an aircraft, including effects of different components or devices
on the stability and control of the aircraft. Other areas discussed in the report include
different types of aircraft stability, static margin and its effect on stability and control of
an aircraft, spiral and Dutch-roll modes, stability and controllability of an aircraft, and
control surfaces of an aircraft. Information in this report is very essential in
understanding how different devices of an aircraft are used to produce forces that help
in controlling the aircraft and making it stable.
Figure 1 below is a sketch showing an airplane’s three body axes.
Figure 1: Body axes of an airplane
Figure 2 below is a sketch showing aerodynamic forces and positive angle of attack
(AOA) of an airplane
Aerospace Technology II – Stability & Control Ali Talal Mohammed 3

Figure 2: Aerodynamic forces and positive AOA
Figure 3 below is a sketch showing aerodynamic forces and positive sideslip angle
Figure 3: Aerodynamic forces and positive sideslip angle
2. Task 1
2.1. Body axes of an airplane
An airplane has three main axes: longitudinal axis, lateral axis and vertical axis.
There are also four main aerodynamic forces that act on the body of an airplane. These
are: thrust (the aerodynamic force pushing the airplane forward), drag (aerodynamic
force pulling the airplane backwards), lift (aerodynamic force pushing the airplane
Aerospace Technology II – Stability & Control Ali Talal Mohammed 4

upwards) and weight (aerodynamic force pulling the airplane downwards). Thrust acts in
the opposite direction of drag while lift acts in the opposite direction of weight.
2.2. Types of airplane stabilities
An airplane is said to be in static stability if the sum of all forces that are acting on
it and all moments created equals zero i.e. positive forces equal negative forces and
positive moments equal negative moments. In this state, the airplane experiences zero
accelerations and it continues flying in a steady condition. A deflection caused by
control surfaces or a gust of wind creates an unbalance of force or moment thus
disturbing the airplane and causing it to accelerate. When this kind of disturbance
occurs, the airplane will tend to return to its steady condition of flight. This tendency of
returning to the steady condition of flight after experiencing a disturbance is what is
referred to as static stability. The static stability can be positive, neutral or negative. An
airplane is said to have static stability if it tends to return to equilibrium or steady
condition of flight after disturbance. An airplane in negative static stability is the one that
continues flying in the direction or condition of disturbance. An airplane in neutral static
stability is the one that fly in the direction of disturbance but remains in equilibrium1.
Dynamic stability deals with an airplane continuing to fly in the resulting motion or
conditions with time after experiencing disturbance. This means that dynamic stability is
defined by the time history of the airplane’s resulting motion2. An airplane with positive
dynamic stability is the one whose motion’s amplitude decreases with time while the
one with negative dynamic stability is the one whose motion’s amplitude increases with
time. Negative dynamic stability is also referred to as dynamic instability. Neutral
dynamic stability is the tendency of an airplane to go back to the original flight condition
(level and direction) after experiencing a disturbance that put it in a new position.
An airplane is said to have longitudinal stability if it has the capability to keep a
constant angle of attack (AOA) in relation to relative wind (that is, there is no tendency
1 Flight Control, ‘Stability and Control’ (Flight Mechanic, (n.d.)) <http://www.flight-
mechanic.com/stability-and-control/> accessed 17 January 2019.
2 Agnieszka Kwiek, ‘Study on the Static and Dynamic Stability of a Modular Airplane
Systems’ (2016) 20 4 Aviation 160.
Aerospace Technology II – Stability & Control Ali Talal Mohammed 5

of the airplane’s nose going down and causing it to dive nor going up and causing it to
stall). In other words, an airplane attains longitudinal stability when it is in pitch motion.
The longitudinal stability is mainly controlled by the horizontal stabilizer whose action
depends on the aircraft’s AOA and speed.
Lateral stability refers to the tendency of an airplane to go back to its original
attitude from motion about longitudinal axis of the airplane (also known as rolling
motion). In the process of an airplane gaining lateral stability, the wing on one side rolls
up while the wing on the opposite side rolls down. This means that the wings are not
perpendicular (at right angle) to the direction of gravitational acceleration hence lift and
gravity are also not matching. The rolling creates a vertical lift component and a
horizontal side load that cause the airplane to slip on one side. The airplane is then said
to be laterally stable if it is able to return to its original flight condition by the help of the
sideslip load that is generated. Lateral stability can be attained using sweptback wings
or wings that are included upwards3.
Directional stability is the airplane’s stability about the vertical axis. Airplanes are
designed such that once they are in a steady condition flight-path, they can continue in
this direction and level even without the control inputs of the pilot (this is referred to as
auto-pilot mode). But even in this mode, the airplane can still experience a skid due to
external or natural factors that create aerodynamic forces. When an airplane
experiences this kind of imbalance when in auto-pilot mode and it automatically
recovers back to steady condition, it is said to have good directional balance
performance. The directional stability is primarily controlled by the vertical stabilizer.
Some of the design parameters sued to improve an airplane’s directional balance
include extended fuselage, sweptback wings and big dorsal fin.
2.3. Static margin
Static margin is the distance measured between aerodynamic centre or neutral
point of an aircraft and its centre of mass. This distance is expressed as a percentage
(%) of mean aerodynamic chord. This distance affects the aircraft’s static longitudinal
3 Aerospace Engineering, ‘Control and Stability of Aircraft’ (Aerospace Engineering, 23
January 2016) <https://aerospaceengineeringblog.com/control-and-stability-of-aircraft/>
accessed 17 January 2019.
Aerospace Technology II – Stability & Control Ali Talal Mohammed 6

stability. An aircraft always has a fixed neutral point that is determined by the design of
the aircraft and it can only attain static longitudinal stability if its centre of gravity is in
front of the neutral point. The static margin of an aircraft usually varies from 5 to 40%.
There different techniques that can be used to improve the static longitudinal stability of
an aircraft including moving the wingback or moving the centre of gravity in front. Static
margin affects stability of the aircraft. An increase in static margin causes a
corresponding increase in stability of an aircraft meaning that static margin is directly
proportional to an aircraft’s stability.
An aircraft with positive static margin is the one whose centre of gravity is in front
of the neutral point making it to be longitudinally stable. On the other hand, an aircraft
with negative static margin is the one whose centre of gravity is behind its neutral point
making it longitudinally unstable. Static longitudinal stability of the aircraft is attained
using control inputs of suitable control surfaces that will move the centre of gravity in
front of the neutral point. In general, the stability and controllability of an aircraft are
significantly affected by the position of centre of gravity of the aircraft in relation to its
neutral point. Large static margin means that the aircraft is unstable hence its degree of
response to the pilot control inputs is slow (i.e. it has low controllability – less
controllable). On the other hand, small static margin means that the aircraft is more
stable hence it has a high degree of response (quick response) to the pilot control
inputs (i.e. it has high controllability – more controllable). The pilot can attain the desired
static margin using control surfaces to move the aircraft’s centre of gravity in front of the
neutral point4.
2.4. Spiral and Dutch-roll modes
Spiral mode is a type of mode that is non-oscillatory in nature. The mode typically
consists of yaw movement with marginal roll. The half-life of spiral mode is in the minute
order. In most cases, the spiral mode in excited by sideslip disturbance that usually
follows a roll disturbance causing the wing to drop. For instance, assuming that an
airplane is in a straight-and-level flight-path and it experiences a disturbance that makes
4 Michael Carley, Some notes on aircraft and spacecraft stability and control (Cranfield
University 2012).
Aerospace Technology II – Stability & Control Ali Talal Mohammed 7

it develop a small positive roll angle. If the airplane is not checked, a small positive
sideslip velocity will develop producing lift that generates yawing moment, which can
make the aircraft to turn and start moving in the direction of sideslip. In other words,
spiral mode should be checked because it can become fatal. The pilot should always
control or stop spiral mode immediately it happens. However, spiral mode is usually
invisible when it starts developing. With time, the airplane continues to roll off its true
vertical to a point where the aircraft can no longer be supported by the lift produced.
This causes a significant increase in speed of the airplane and dropping of the nose. If
the situation is not controlled, spiral dive starts. Unchecked spiral mode causes the
airframe’s structural failure. The condition can be corrected automatically using true
horizon5. An airplane in spiral mode usually has more kinetic energy that should be
released using control surfaces. The characteristics of spiral mode mainly depends on
the airplane’s directional static stability and lateral static stability6. There are two types
of spiral modes: stable spiral mode and unstable spiral mode. The airplane should
always be in stable spiral mode to avoid complications that may turn fatal if unchecked
on time.
Dutch-roll is a lateral-directional motion of an airplane comprising of an out-of-
phase combination of roll and yaw oscillations. The Dutch roll can happen accidently or
naturally (as a result of directional stability). Dutch roll usually happens at high altitude
especially if the yaw dampener has been turned off. Therefore the first step to control
Dutch roll is to ensure that the yaw dampener is turned on. In other words, one artificial
way of increasing Dutch roll stability is by installing a yaw damper. The next step is for
the pilot to try and reduce yawing oscillations. This can be achieved by ensuring that the
rudder pedals are held in neutral position. The next step is to apply spoiler (aileron)
control in the opposite direction to the roll. The last step is to accelerate the airplane to
high speed or descend to where the air is denser so as to improve directional stability.
5 Stefan Bogos and Ion Stroe, ‘Similarity Criteria for "Full" and "Scale" Aircraft on the
Lateral Stability Analysis’ (2012) 74 4 UPB Scientific Bulletin, Series D 1.
6 Michael Cook, Flight Dynamics Principles: A Linear Systems Approach to Aircraft
Stability and Control (3rd edn, Butterworth-Heinemann 2013).
Aerospace Technology II – Stability & Control Ali Talal Mohammed 8

There are three main causes of Dutch roll mode: weaker yaw stability, strong roll
stability, and dihedral effect and sweepback. The main characteristic of a Dutch roll
mode is the airplane experiencing a series of out-of-phase turns. In this case, the
airplane usually rolls in one direction while at the same time yawing in the other
direction, making it unstable and less controllable.
2.5. Experimental data
The experimental data for coefficient of pitching moment, yawing moment and
rolling moment shows that the three aircraft models have longitudinal stability, lateral
stability and directional stability. An aircraft is said to have longitudinal stability if it has a
positive AOA and coefficient of pitching moment. An aircraft is said to have lateral
stability if it has a positive sideslip angle and coefficient of yawning moment7. An aircraft
is also said to have directional stability of it has a positive yaw angle and coefficient of
rolling moment. By looking at the experimental data obtained for the three aircraft
models and drawing the graphs of Cm against alpha, Cn against beta and Ct against
beta, the graphs provide nearly linear curves for each parameters. This means that
coefficient of pitching moment is directly proportional to alpha, coefficient of yawning
moment is directly proportional to beta, and coefficient of rolling moment is directly
proportional to beta. Therefore the three aircraft models have longitudinal, lateral and
directional stability.
3. Task 2
3.1. Stability and controllability
Stability is the ability of an airplane to return to the desired, steady or neutral
position after experiencing a disturbance or turbulence without the input of the pilot. The
airplane returns to the neutral position by the airframe itself. In other words, stability
enables the airplane to recover from different upsetting forces and maintain steady
flight-path without the input of the pilot. Different airplanes have varied levels of stability.
For example, a fighter airplane has a different level of stability with that of a training
7 Firdaus Mohamad, Wirachman Wisnoe, Rizal Nasir and Norhisyam Jenal, ‘Yaw
Stability Analysis for UiTM’s BWB baseline-ii UAV E-4’ (2013) 393 1 Applied Mechanics
and Materials 323.
Aerospace Technology II – Stability & Control Ali Talal Mohammed 9

airplane. A training airplane is designed to have high stability because the learners do
not have adequate skills to control the airplane and return it to stable conditions in case
it gets disturbed. In general, stability refers to the ability of an airplane to generate
appropriate forces that enable it to recover back to the original steady condition flight-
path after experiencing some disturbance that pushed it out of equilibrium.
Controllability is the response quality of an airplane to the pilot control inputs or
commands. The airplane must respond accordingly to the pilot’s control inputs for it to
record best performance8. The pilot controls the airplane by moving the flight controls
that creates aerodynamic forces to make the airplane follow a desired flight-path9. An
airplane with high controllability is the one that responds promptly and easily to the
movement of controls. It is also important to note that stability is always a compromise
for controllability. This means that an airplane with high stability has low controllability,
and vice versa.
Longitudinal control refers to the control of pitching motion of an airplane.
Pitching motion is basically where an airplane moves from side to side10. The control
surface of longitudinal control is the elevator (also referred to as stabilator)11. The pilot
can control pitching motion by deflecting the elevator. The elevator is usually fixed on
horizontal stabilizer on the tail of the airplane. When the elevator is deflected up or
down, it causes the lift generated on the tail of the airplane to either increase or
8 Davoud Nikbin, Malliga Marimuthu, Sunghyup Sean Hyun and Ishak Ismail, ‘Effects of
Stability and Controllability Attribution on Service Recovery Evaluation in the Context of
the Airline Industry’ (2014) 31 7 Journal of Travel & Tourism Marketing 817.
9 Yihua Cao, Guozhi Li and Qian Yang, ‘Studies of Trims, Stability, Controllability, and
Some Flying Qualities of a Tandem Rotor Helicopter’ (2009) 223 2 Journal of
Aerospace Engineering 171.
10 S N Deepa, ‘Longitudinal Control of an Aircraft Using Artificial Intelligence’ (2013) 5 6
International Journal of Engineering and Technology 4752.
11 S N Deepa and G Sudha, ‘Longitudinal Control of Aircraft Dynamics Based on
Optimization of PID Parameters’ (2016) 2323 2 Thermophysics and Aeromechanics
185.
Aerospace Technology II – Stability & Control Ali Talal Mohammed 10

decrease. As a result, the nose of the airplane tilts either upwards or downwards. This is
what is referred to as horizontal control. During horizontal control, the pilot will continue
deflecting the elevator until when the airplane has returned to the original steady
condition flight-path or stabilizes in the resultant motion.
Roll control refers to the mechanisms used to stabilize the airplane when it
experiences rolling motion. It is a type of control that is performed to attain lateral
stability of an airplane. The main control surfaces of roll control are spoilers or ailerons.
These surfaces are found on the wing of the airplane. During roll control, the airplane
can be turned left or right by moving the spoilers or ailerons. Ailerons and spoilers are
fixed on both the left and right wing, and they move in opposite direction. This means
that when the left-wing aileron or spoiler is moving upwards, the right-wing aileron or
spoiler will be moving downwards, and vice versa. This opposite movement of the
aileron and spoiler causes a decrease in lift on one wing and an increase of lift on the
opposite wing simultaneously. This makes it possible for the airplane to roll either left or
right because of the unbalanced aerodynamic forces and moments. Just like
longitudinal control, the pilot accomplishes roll control by continuing to tilt the ailerons or
spoilers until when original steady flight conditions or desirable resultant motion
conditions are reached.
Yaw control is the mechanism of stabilizing an airplane when its nose is
experiencing a side to side movement. The yaw motion is caused by the rudder’s
deflection hence yaw control is also performed using rudder. Yaw is basically the
rotation of the airplane around the vertical axis. The yaw control is a directional stability
control that is performed by controlling the rudder to apply pressure either on the left or
right side of the aircraft. Upon application of pressure, the rudder tends to spin sideways
pushing the tail of the airplane to move either to the left or right direction. This left-right
movement of the tail also causes the nose and entire body of the airplane to move left-
right thus controlling yaw. When the airplane in in yaw motion, the pilot will continue
applying pressure on the rudder until when the airplane reaches the desired new motion
condition or regains its original steady conditions.
Aerospace Technology II – Stability & Control Ali Talal Mohammed 11

3.2. Control surfaces
Control surfaces are the devices or components used by the pilot for adjusting
and controlling the direction and level of an airplane (i.e. flight-path). These control
surfaces are used to move the airplane left, right, up or down. In other words, the
control surfaces are used in controlling the longitudinal, lateral and directional stability of
the airplane. Figure 4 below shows various devices of an airplane, including control
surfaces.
Figure 4: Control surfaces of an airplane12.
3.2.1. Tail
Some of the control surfaces associated with the tail of an airplane include the following:
Elevator: this is a primary control surface fixed on the tail of an airplane. It is used
for adjusting the airplane’s pitch angle and AOA. These two parameters are very
essential determinants of the stability and controllability of the airplane. They contribute
to the airplane’s pitch stability/control and orientation. The elevators move up when the
stick is pulled backward by the pilot and the elevators move down when the stick is
12 Federal Aviation Administration, Aviation Maintenance Technician Handbook –
General, Washington, DC, Federal Aviation Administration, 2013.
Aerospace Technology II – Stability & Control Ali Talal Mohammed 12

pushed forward. When the elevator is pushed up, the tail gets pushed down causing the
nose of the airplane to pitch up thus increasing the AOA of the wings. This generates
more drag and lift. When the stick is centered, the elevators go back to neutral thus
stopping change in pitch. When the elevator is pushed down, the tail gets pushed up
causing the nose of the airplane to pitch down thus reducing the AOA of the wings. This
generates less drag and lift.
Rudder: this is a primary control surface fixed on the tail of the airplane (usually
on the airplane’s vertical stabilizer). The rudder is used for yaw control along the
airplane’s vertical axis. When pressure is applied on rudder, the nose of the airplane
moves either to the left or right until the desired flight-path is achieved. When the pilot
pushes the right pedal, the rudder gets deflected to the right thus pushing the tail of the
airplane to the left and causing the nose to move to the right. When the pilot pushes the
left pedal, the rudder gets deflected to the left thus pushing the tail of the airplane to the
right and causing the nose to move to the left. When the pedal is centered, the rudder
goes back to neutral thus stopping yaw.
Stabilizer: there are two types of stabilizers – horizontal stabilizer and vertical
stabilizer. The two control surfaces are used for providing control and stability of the
airplane. The stabilizer are used in adjusting for changes in center of gravity or center of
pressure that is caused by changes in attitude and speed of the airplane, dropping
payload or cargo or fuel consumption. The stabilizers can also create a negative or
positive lift.
Trim tab: this is a secondary control surface that is fixed on the airplane’s trailing
edge and is used for counterbalancing the aerodynamic forces and stabilizing the
airplane until it attains the desired attitude with minimal use of control force.
3.2.2. Wing
Some of the control surfaces associated with the wing of an airplane include the
following:
Aileron: this is a primary control surface that is fixed on the airplane wing’s
trailing edge. The movement of the ailerons is in opposite directions (when the aileron
Aerospace Technology II – Stability & Control Ali Talal Mohammed 13

on the left wing moves down, the one on the right wing moves up, and vice versa). A
lowered aileron increases lift while a raised one reduces lift generated on the wing.
Spoiler: this is a secondary control surface that are used for disrupting flow of air
over the airplane wing thus reducing lift. The spoiler is very useful in reducing altitude
without having to gain undue airspeed.
Flap: this is a secondary control surface that is fixed on the trailing edge of each
wing of the airplane. Deflecting the flap downwards causes an increase in the wing’s
effective curvature. Flabs are used to increase the maximum lift coefficient thus
reducing the stalling speed of the airplane. The flaps are usually used when the airplane
is flying at low speed, when the airplane has a high AOA and during landing.
Slat: this is a secondary control surface that is used for increasing lift of the
airplane. The slats alter the flow of air over the wing thus decreasing the stalling speed.
Slats can either be fixed or retractable.
4. Conclusion
This report has discussed different aspects of stability and control of an aircraft.
The fundamental basic of mechanics of flight is that an aircraft has three main axes:
longitudinal axis, lateral axis and vertical axis. There are also three main forces acting
on the aircraft: lift, weight, thrust and drag. A pilot has to use different control surfaces to
balance these forces by changing the position of center of gravity relative to the neutral
point. There are also different types of control including pitching control, yawing control
and rolling control. Pitch is controlled using elevator, roll is controlled using ailerons or
spoiler, and yaw is controlled using rudder. There are also primary and secondary
control surfaces that are used in controlling and stabilizing the aircraft. The aircraft can
also be controlled automatically or manual through control inputs of the pilot.
Aerospace Technology II – Stability & Control Ali Talal Mohammed 14

References
Aerospace Engineering, ‘Control and Stability of Aircraft’,
https://aerospaceengineeringblog.com/control-and-stability-of-aircraft/, 2016, (accessed
17 January 2019).
Bogos S and Stroe I, ‘Similarity Criteria for "Full" and "Scale" Aircraft on the Lateral
Stability Analysis’ (2012) 74 4 UPB Scientific Bulletin, Series D 1.
Cao Y, Li G and Yang Q., ‘Studies of Trims, Stability, Controllability, and Some Flying
Qualities of a Tandem Rotor Helicopter’ (2009) 223 2 Journal of Aerospace Engineering
171.
Carley M, Some notes on aircraft and spacecraft stability and control (Cranfield
University 2012).
Cook M, Flight Dynamics Principles: A Linear Systems Approach to Aircraft Stability
and Control (3rd edn, Butterworth-Heinemann 2013).
Deepa S and Sudha G, ‘Longitudinal Control of Aircraft Dynamics Based on
Optimization of PID Parameters’ (2016) 2323 2 Thermophysics and Aeromechanics
185.
Deepa S, ‘Longitudinal Control of an Aircraft Using Artificial Intelligence’ (2013) 5 6
International Journal of Engineering and Technology 4752.
Federal Aviation Administration, Aviation Maintenance Technician Handbook – General
(Federal Aviation Administration 2013).
Flight Control, ‘Stability and Control’ (Flight Mechanic, (n.d.)) <http://www.flight-
mechanic.com/stability-and-control/> accessed 17 January 2019.
Aerospace Technology II – Stability & Control Ali Talal Mohammed 15

Kwiek A, ‘Study on the Static and Dynamic Stability of a Modular Airplane Systems’
(2016) 20 4 Aviation 160.
Mohamad F, Wisnoe W, Nasir R and Jenal N, ‘Yaw Stability Analysis for UiTM’s BWB
baseline-ii UAV E-4’ (2013) 393 1 Applied Mechanics and Materials 323.
Nikbin D, Marimuthu M, Hyun S and Ismail I, ‘Effects of Stability and Controllability
Attribution on Service Recovery Evaluation in the Context of the Airline Industry’ (2014)
31 7 Journal of Travel & Tourism Marketing 817.
Aerospace Technology II – Stability & Control Ali Talal Mohammed 16