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G l o s s a r y
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Aeroelastic Divergence: Aeroelasticity is the science that studies orces causing structural
bending and exing o aircrat components such as wings. When a orward-swept wing produces
lit, structural bending causes the orward edge o the wing to increase incidence (it bends
upward). This in turn urther increases the aerodynamic lit until at some speed (or value o
lit) the aerodynamic orces overcome the structural restoring orces o the wing. This is called
aeroelastic divergence and causes the wing to bend up until it breaks.
Afterburner: On some jet engines, a tailpipe section at o the turbine where additional thrust
is produced. Aterburner operation is not continuous but can be selected by the pilot, normally
by moving the throttle lever orward through a detent to the aterburner position. Additional
uel is then injected into the hot exhaust gases and combustion signifcantly increases the thrust
compared to that o the basic engine. Unortunately, uel consumption is even urther increased.
Also reheat [UK], augmented thrust, or max thrust.
Aileron: A movable section on a wing used or controlling the roll attitude o the airplane.
Ailerons are normally located on the outboard trailing edges o each wing and are interconnected
so as to move in opposite directions. Downward aileron movement on one wing increases thatwings lit while a corresponding upward aileron movement on the opposite wing decreases lit;
the unbalanced orces cause a roll towards the wing with upward aileron deection. Ailerons are
controlled by the pilots control stick or wheel, which is moved in the direction o desired roll.
(From aleron, French or wing tip.)
Airfoil: A solid surace designed to move through air and obtain a useul orce called lit.
Examples are wings, sails, propeller blades, and helicopter rotor blades. In the study o
aerodynamics, airoil usually reers to the cross-sectional shape obtained by the intersection o
the wing with a perpendicular plane.
Raising ailerondecreases
lift and lowers wing
Ailerons controlroll
Normal lift Lowering aileronincreases
lift and raises wing
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Airspeed: The speed o an airplane relative to the surrounding air (groundspeed would be the
airplanes speed relative to the ground; the two would be equal in the absence o wind). Airspeed
is usually measured by a Pitot-static system, which reacts to air pressure caused by movement
o the airplane through the air.
Airspeed Indicator: An instrument in the pilots cockpit giving continuous indication o
airspeed. Usually, airspeed indicators show the speed in units o knots (1 knot = 1 nautical
mile per hour where 1 nm = 6000 eet). However, many o the smaller airplanes have airspeed
indicators calibrated in mph (1 mph = 1 statute mile per hour where 1 sm = 5280 eet).
Angle-of-Attack (AOA, alpha, a): The angle o incidence o the airplanes wing (or other
reerence) with the airfow direction. The airfow direction relative to the airplane is oten termed
the relative wind.
Attitude: An airplanes pitch, roll, and/or yaw angle relative to the Earths horizon or another
reerence. Attitude is displayed to the pilot on an instrument called an attitude indicator.
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Bank Angle: Airplanes angle o bank relative to the Earths horizon (see roll).
Bleed Air: Air under pressure that is bled rom a jet engine or purposes other than producing
thrust. These uses include cockpit or cabin pressurization and air conditioning or, in the case o
the Harrier, air to power the puer jet reaction control system.
Canard: A horizontal control surace mounted in ront o the wing. Also, oreplane.
Control (or Command) Augmentation System (CAS): An electrical circuit between the pilots
controller (stick, wheel, sidestick) and the control surace, which essentially boosts the pilots
initial control orce and makes fying the airplane easier and more precise; power steering.Sensors in the CAS circuit provide eedback signals (typically load actor, pitch rate, or roll rate)
to a computer, which compares them to the pilot command signal to make the airplane respond
as desired. In this manner, aircrat response is consistent over most o the fight envelope, and
unwanted motions are heavily damped.
Center of Gravity (CG, c.g.): Point through which the orce o gravity acts and about which the
airplane is balanced. An airplanes longitudinal, lateral, and vertical axes intersect at the c.g. The
location o the c.g. is normally given in inches rom a datum or expressed in terms o the wings
mean aerodynamic chord (%MAC).
Compressibility: Phenomenon o air moving over a curved aircrat surace at speeds close to
local sonic speed, where air can no longer be considered as incompressible (as it is assumed to
be or most subsonic aerodynamics applications). Eects include the appearance o shockwaves,
rapid increase in drag, rearward shit o the lit orces, and, or improperly designed aircrat,
stability and control problems.
Control Laws: The sequence o calculations an electronic fight control system uses to control
aircrat response to pilot inputs and external disturbances such as wind gusts.
Damper: An electrical circuit in a fight control system that senses uncommanded motion and
moves a control surace to stop unwanted oscillations. Typically, pitch, roll, and/or yaw dampersare used. (See Stability Augmentation System.)
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Drag: Retarding orce acting upon an airplane parallel to the direction o motion. Parasite
drag is caused by the riction o the air moving over the airplane. The other basic type o drag
is related to the wings production o lit and is called induced drag. Induced drag is aected
by the wings shape, the angle at which the air strikes the wing, and the speed and density o the
airow.
Down Force
Resulting Motion
Dynamic Stability: An airplanes response over time to a disturbance rom the trim condition
(a state o equilibrium). The response may diverge rom the trim condition, converge back to
trim, or oscillate about trim. An oscillatory response may continue to increase in amplitude, stay
about the same, or decrease in amplitude (damp out).
Elevator: A movable control surace hinged to the trailing edge o a horizontal stabilizer that is
connected to the pilots controls (stick, wheel, sidestick). At motion o the pilots control causes
the elevator to deect and produce a nose-up pitch response; orward control motion causes a
nose-down pitch response.
Elevons: A hinged control surace on the trailing edge o a wing that is connected to the pilots
controls combining the unctions o elevators and ailerons. Elevons are oten used on delta-wing
aircrat.
Feedback Control System: Circuit o a ight control system that regulates aircrat response
through eedback compensation techniques. The pilots control input becomes the systems
command; resultant aircrat motions are measured, transormed into electrical signals, and then
conditioned and amplifed or summing with the input command. This orms a closed loop,
illustrated below. The error between the output and input signal becomes an electrical commandto the ight control suraces until output equals input.
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Flight Controls: Devices the pilot uses to steer and control the trajectory o an airplane; also
reers to the external suraces responding to the pilots fight control movements or inputs. The
pilots controls, the external suraces, and all mechanical and electrical linkages, computers, and
sensors in between make up the fight control system. Pilots controls usually include a center
stick, control wheel, or sidestick; fap lever; speed brake handle or actuation button; and nozzle
angle position lever in the case o the Harrier.
Flight Control Surfaces (or Effectors): The external suraces that respond to pilot inputs to
control the airplanes trajectory. Primary suraces produce direct response in pitch, roll, and yaw.
These include elevators, stabilators, canards, elevons, tailerons, ailerons, rudders, and thrust
vectoring nozzles. Secondary fight control suraces augment the primary suraces by modiying
the lit and drag characteristics o the wings and airplane. These include wing faps (usually
on the trailing edge but sometimes used on the leading edge), wing slats, spoilers, and speed
brakes (UK = air brakes). The recent terminology or primary fight controls is fight control
eectors, as the eector may not be a conventional control surace. For example, NASAs F-15
ACTIVE research aircrat is said to have nine fight control effectors: let and right canards, letand right ailerons, rudder (the two rudders move together and are treated as one eector), let
and right stabilators, and pitch or yaw TV (thrust vectoring).
Rudder
Right Canard
Left Canard
Pitch and Yaw
Nozzles
Left Aileron
Right Aileron
Right Stabilator
Left Stabilator
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Flight Envelope: The boundaries o speed, Mach, altitude, load actor, and angle-o-attack
within which a particular airplane design can sustain ight. A typical ight envelope is presented
as a two-dimensional plot with altitude on the y-axis and speed/Mach on the x-axis.
x 10 3
60
50
4 5 10
A 6O40 A
0
3Altitude, - 10
0 13
ft g1 2 3 12
30 020
11300
400
1 500
20 600
700
800
900 sf
1000
1100
s9 7 8 12
00
1300
ure,
p
1400
sre
10 1500
p
1600 m
icany
D
00 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0
M a c h N u m b e r
ACTIVE envelope
Fly-By-Wire: The generally accepted terminology when the pilots control commands are
processed by a computer and sent to the ight control surace actuators with electrical signals
(or fber-optic signals, sometimes called y-by-light) rather than mechanical linkage. NOTE:
power-by-wire is a recent term or ight control surace actuators that are electrically powered,
as opposed to the conventional hydraulic-powered actuator.
g Loading (load factor, g, n, Nz): The increase in apparent weight o the pilot and aircrat
when an airplane is maneuvered in ight, caused by inertia orces. Load actors are measured
in reerence to the vertical (Z) axis by accelerometers and expressed in g units. In level ight
with no turning, the load actor is at 1g and the load on the structure is equal to the weight o the
structure. I the aircrat turns or pulls up at 2 gs, the load on the structure and pilot is twice their
weight. Since wing lit must be increased to eect a turn or other maneuver, it can be shown that
the load actor equals the ratio o lit to weight: n = L/W.
Head-Up Display or Heads-Up Display
To improve ight saety and reduce pilot workload in high-perormance aircrat, critical param-eters such as aircrat attitude, altitude, airspeed, and navigation inormation are viewed directly
by the pilot via a plasma display embedded into the windshield or canopy o the aircrat. This
reduces the need or the pilot to divert his attention to the instrument panel inside the cockpit,
especially or those parameters he views directly on the windshield.
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Horizontal Stabilizer: A horizontal airoil at the rear o an airplane providing stability in pitch.
The stabilizer may be fxed or have a small angular adjustment or trim only. Elevators may be
mounted on the back. In some designs, especially supersonic airplanes, the entire stabilizer is
pivoted and moves or pitch control. Also, tailplane.
Lift: The useul orce produced by a wing as air ows over it. Air travels aster over the curved
upper surace than along the bottom, which creates lower pressure on the top and higher pressure
on the bottom. The pressure dierential creates a vacuum suction on the upper surace, tending
to lit the wing. The pressure dierential can be represented by a single orce acting at a location
called the center o pressure and perpendicular to the relative wind. The amount o lit is
aected by the shape o the wing, the angle at which the wind strikes the wing, and the speed and
density o the airow.
Mach Number: Ratio o aircrats airspeed to the speed o sound: M= V/a where V is true
airspeed and a is the speed o sound which varies with air temperature. At Mach 2, an aircrat
is traveling at twice the speed o sound. The term is named ater Ernst Mach (1838-1916), an
Austrian physicist.
Maneuverability: The ability o an aircrat, as commanded by its pilot, to change trajectory. A
pilot must be able to hold an airplane in a maneuver, and the designer has to provide adequate
ight control eectors appropriate to the role o the aircrat. Maneuverability is opposed by
stability; that is, the less stable an airplane, the more maneuverable it is. However, some degree
o stability is required in order or the pilot to maintain the desired trajectory.
90
90
Lift
Drag
Relative Wind
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Nozzles: The aft section of a jet engine tailpipe through which the exhaust gases escape. Usually,
the nozzle is designed with interleaved metal sections (nozzle leaves), which allow the nozzle
exit area to be varied. If the nozzle exit area is decreased, the exhaust velocity is increased by
the relationship V2
= (A1/A
2) V
1. A nozzle controller continuously varies the exit area to provide
optimum exhaust gas velocity and internal tail pipe temperature. Recent nozzle design provides
the ability to vary the nozzle and exhaust gas angular direction to provide thrust vectoring.
Pitch, Pitch Attitude: Angular rotation about the airplanes lateral (Y) axis, usually termed
nose up or nose down. The angle between the airplanes longitudinal (X) axis and the
Earths horizon is called pitch attitude. Pitch is produced by the elevators (or canards, or pitch
thrust vectoring).
Side Force
Resulting Motion
Positive Static Stability: The level of static stability of an airplane is determined by the relation-
ship or location of the aerodynamic center of pressure (ac) with respect to the airplanes center
of gravity (cg). Typically the more forward the center of gravity from the aerodynamic center,
the more positive static stability an airplane will have. This means that following application and
removal of a disturbance, the airplane will return to its initial undisturbed equilibrium positionrelatively quickly.
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Quadraplex: To provide security in the event o certain critical complex airplane system ail-
ures, such as a fight control computer, our (quadraplex) computers are utilized to perorm
similar simultaneous unctions. Each computer checks on the others to veriy that they all agree
by comparing output commands. I one computer or system command output is ound to di-
er, it can then be deactivated or ignored and the others will continue their normal unction, thus
avoiding the possibility o a catastrophic situation.
Reaction Control System: In the absence o atmospheric aerodynamic pressure acting on a
space planes control suraces (elevator, ailerons, or rudder), to change its attitude or position
with respect to another object, such as the Earth, another means must be employed. Such a sys-
tem must be utilized in space where no aerodynamic orce can be produced. In the vacuum o
space, small on/o rocket motors (thrusters) must be used by a spacecrat to produce the orce to
change vehicle attitude or position. The jet thrust o small reaction engines (thrusters) is devel-
oped by ejecting a substance, such as a stream o gases, rom burned uel (see diagram below).
This type oreaction control system (RCS) is controlled by a pilot or computer and is usually
either on or o; variable levels o thrust are usually not available. The density o the atmospheredecreases as a unction o altitude. Approximately 50 percent o the Earths atmosphere is below
18,000 eet (5,486 m). As altitude increases the number o molecules or mass per unit volume
Roll thrusters on
Pi
Thrusters
wings
Yaw thrusters
tch thrusters
(a) X-15 reaction controls
(b) Space Shuttle reaction controls
Examples of reaction controls
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decreases, resulting in a decrease in atmospheric pressure. As altitude increases, aerodynamic
pressure on aircrat control suraces decreases to a point where reaction control thrusters must
be blended with aerodynamic control to maintain adequate control. As altitude increases ur-
ther, aerodynamic controls must be phased out and reaction controls phased in. Once out o the
atmosphere, reaction control thrusters are then the sole means o orienting or maneuvering the
vehicle. In the return to Earth, the control system must phase out the thrusters and phase in aero-
dynamic control.
Relaxed Static Stability: As the location o the center o gravity (c.g.) with respect to the air-
planes aerodynamic center o pressure (a.c.) is decreased, as the case when the c.g. is allowed
to move at, the airplanes static stability will be relaxedor decreased. This means that ollow-
ing application and removal o a disturbance, the airplane will return to its initial undisturbed
equilibrium position less quickly. I the c.g. is allowed to move at o the a.c., the airplane will
become unstable (negative static stability). This is the situation in which, when disturbed rom a
condition o equilibrium by some orce, the airplane will not return to its initial undisturbed posi-
tion or attitude ollowing removal o the disturbance orce, but will continue to diverge rom thatposition.
Roll, Roll Attitude, Bank Angle: Angular rotation about the airplanes longitudinal (X) axis,
usually called right/let roll or right/let bank. The angle between the aircrats vertical (Z)
axis and the Earths vertical is the roll attitude or bank angle. Roll is produced by the ailerons (or
spoilers, tailerons, or elevons).
Roll Coupling: Coupling is what happens when a motion about one axis causes a motion
about another axis. Roll coupling is a divergence in pitch or yaw during rolling maneuvers. It is
caused by a complicated interaction o inertial and aerodynamic properties and can lead to loss
o control or structural ailure. Many high-perormance aircrat with y-by-wire ight control
systems incorporate a roll rate limiter to avoid roll coupling.
Rudder: A movable ight control surace hinged to the back o the vertical stabilizer, providing
yaw control. The rudder is connected to a set o pedals operated by the pilots eet; the right
pedal moves (yaws) the nose to the right, and let pedal moves the nose let. This yawing motion
pivots the airplane about the vertical (Z) axis. Since the directional stability o an airplane is
necessarily very high, ull rudder deection normally produces small angular displacements
compared to pitch or roll capability, perhaps 15 or less yaw angle at normal cruise speed.
Shockwave: The air pressure waves ahead o a supersonic airplane bunch, because the speedo propagation o the pressure waves is fnite (the speed o sound), and orm a surace o
discontinuity where the ow changes rom subsonic to supersonic. The shockwave orms a three-
dimensional conical surace, sometimes called a Mach cone. Shockwaves cause a change in the
airow pressure patterns around an aircrat. This will ultimately change its maneuverability,
stability, and control characteristics.
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Sideslip: When an airplane is not ying straight but has an angle between the uselage and the
direction o motion (ight path, or relative wind), it is said to be in a sideslip. This causes
additional drag.
Spin: A dangerous maneuver in which the aircrat has stalled and entered a sustained yawing
rotation about the vertical axis and a near vertical descent.
Spoiler: A small control surace on the top o a wing that decreases (spoils) the wings lit by
deecting the airow over the wing. This drag is used to augment roll control when spoilers
on one wing only (the down-going wing) deect or to provide quick airspeed reduction when
spoilers on both wings deect. The latter unction is called a speed brake (UK: air brake),
which the pilot controls by a speed brake extension lever. These may also be extended ater
landing to aid deceleration and reduce landing distance.
Stability: The property o a body, as an aircrat or rocket, to maintain its attitude or to resist
displacement, and i displaced, to develop orces and moments tending to restore the orginal
condition.
Stability Augmentation System (SAS): An auxiliary system to the basic manual vehiclecontrol system, whereby response o the control suraces to inputs by the pilot can be adjusted
to give a preselected vehicle response by selection o certain fxed gains in a standard eedback
loop on control-surace output.
Stabilator: Single-piece horizontal tail control surace used as primary pitch control,
especially on supersonic aircrat where shockwaves might reduce the eectiveness o an
elevator type control. Also stab and slab.
Static Stability: The tendency o an airplane (or body), when disturbed rom a undisturbed
position or attitude, ollowing removal o the disturbance orce. Airplanes possess a delicate
and important balance between aerodynamic and gravitational orces. A more stable airplane is
more difcult to move using controls, so i ease o maneuverability is desired, then decreasing
or relaxing its stability will achieve this objective.
Stall: A ight condition that occurs when the angle-o-attack becomes so high that the air
owing over the top o the wing no longer ows smoothly, but breaks away (stalls), causing
a rapid loss o lit. The associated angle-o-attack is called the stall angle-o-attack and is
always the same angle or a given wing shape.
Stick: A control stick is a pilot cockpit control and is a primary ight control device (a control
wheel may also be used in larger airplanes). A pull or push on the stick controls the airplanepitch attitude. Push results in nose down and pull results in nose up. Let stick or wheel deec-
tion rolls the airplane let, and right stick rolls to the right.
Swept Wing: The term or wings that are inclined towards the back o the airplane. Also
called sweepback, and the angle between the longitudinal axis and the wing leading edge (or
other reerence such as the quarter-chord line) is the sweepback angle.
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Taileron: A stabilator design where the let and right halves can move in unison or pitch
control or dierentially to produce roll; may produce both pitch and roll simultaneously. Also,
dierential tail.
Tricycle Landing Gear: Landing gear, or wheels o an airplane, consisting o two main
wheels and a nose wheel that provide support or the airplane while on the ground.
Thrust: The pushing or pulling orce o an aircrat engine that gives it the orward motion
through the air to create lit. In a jet engine, air ows in through the ront o the engine and is
mixed with uel and compressed. A spark plug makes the uel mixture burn, expand, and rush
out through the exhaust nozzle at the back o the engine to make the airplane go orward.
Thrust Reverse (Reverse Thrust): The simplest and most general use o thrust vectoring is
reverse thrust . This is typically used on large jet airliners to a stop in short distances. Propeller
blades may also be reversed on some airplanes to achieve the same objective. Reverse thrust
engines are equipped with a mechanism that turns the normal rearward thrust orce 180 to a
near-orward direction.
Thrust Vectoring: Thrust is a orce and as such has both magnitude and direction, whichmakes it a vectorquantity. Normally, the thrust o an airplane cannot be changed except by
turning the vehicle. I, however, some mechanism is provided to turn, deect, or rotate an air-
planes thrust vector, with respect to the vehicle, it is capable othrust vectoring. This type o
system is used to enhance maneuvering.
Trim: That state o airplane ight where all orces and moments are in balance and little, i
any, control is required to maintain the condition.
Triplex: Similar to a quadraplex system except the number o complex systems is only three
(triplex) rather than our.
Variable Geometry: Some highly specialized airplanes such as the B-1, F-111, and F-14 have
the ability to change wing sweep in ight. These are reerred to as variable geometry airplanes
due to their ability to change shape as ight conditions dictate. Typically they have low sweep
(wing orward) at low speed and high sweep (wing at) at high speed.
Variable Stability: Some experimental airplanes have complex computer systems installed so
that their stability and or ight characteristics can be artifcially altered in ight based on some
predetermined plan. These systems allow the simulation o increased or reduced (variable)
stability or duplicating the ight characteristics o other airplanes or study or pilot amiliariza-
tion purposes.Vertical Fin (Vertical Stabilizer): A fxed vertical airoil surace usually at the rear o an
airplane that provides directional stability. Rudder controls are hinged to the back o the
vertical stabilizer. In some supersonic designs, the entire vertical stabilizer can move, similar
to a horizontal stabilator. Also, fn.
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Force
The ruddercontrols yawing
Moving therudder to the
left forces thetail to the right
and the nose
to the left
Moving the
rudder to the
right forces thetail to the left
and the noseto the right
Wing: The airoil surace that produces the main source o lit or an airplane. The ront o a
wing is termed the leading edge while the back is called the trailing edge. The intersection o the
wing and uselage is called the wing root.
Wing Warping: During the development o the airplane the Wright brothers determined that the
ability to control or steer and airplane was as important as the wings ability to generate lit. The
construction o the frst airplanes was o wood, wire, and linen abric. The Wright brothers de-
termined that i they pulled down and twisted (warped) the outboard trailing edge o a wing, the
airplane would roll or bank in the opposite direction. That is, the right wing trailing edge down
the airplane would roll let, and vice versa. This method literally distorted or warped the wing
(hence the term wing warping). Without the ability to bank an airplane, it is virtually impossible
to accomplish a turn. Modern airplanes use ailerons to accomplish banked turns and roll maneu-
vering, and the wings cannot typically be warped.
Yaw: Angular rotation about an airplanes vertical (Z) axis. Yaw is produced by the rudder.
Undesired yaw may also be produced by the use o ailerons or other roll controls or banking.
This is called adverse yaw i the nose moves opposite the direction o roll and proverseyaw when the nose moves in the same direction. This undesired yaw is an example o coupling
(motion about one axis causing motion about another axis) and is due to the lit vectors o the
up-going and down-going wing being inclined at dierent angles during a roll. Thereore, a pilot
o a conventional airplane must oten use the rudder simultaneously with the ailerons to prevent
undesired yaw. This is oten termed coordinating a turn.