Aerodynamics and stability Glossary.pdf

7/29/2019 Aerodynamics and stability Glossary.pdf

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Exploring the Extreme: An Educators Guide EG-2002-10-001-DFRC64

G l o s s a r y


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65Exploring the Extreme: An Educators Guide EG-2002-10-001-DFRC

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.

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