special considerations in configuration layout

Special Considerations in

Configuration Layout

Aerodynamics

Structures

Detectability

Vulnerability

Producibility

Maintainability

AERODYNAMIC CONSIDERATIONS

Minimization of wetted area

Wetted area is the area which is in contact with the external

airflow

Wetted area affects the friction drag

Most powerful aerodynamic consideration for virtually all

aircraft

Fuselage Layout: Wetted AreaMinimized by tight internal packaging and a low fineness

ratio (i.e., a short, fat fuselage)

Questair Venture

Excessive tight packaging should be avoided for maintainability considerations

A short, fat fuselage will have a short tail moment which increase the required tail areas

It has high supersonic wave drag

Fuselage Layout: Maintenance of smooth longitudinal contours

Use of smooth longitudinal control lines

Longitudinal breaks in contour should follow a radius at

least equal to the fuselage diameter at that point.

Fuselage Layout: Aft-FuselageThe aft-fuselage deviation should not exceed 10-12 degreesAir inflow induced by a pusher-propeller will prevent

separation despite contour angles of up to 30 degrees or more.

A lower-surface upsweep of about 25 degrees can be tolerated provided that the fuselage lower corners are fairly sharp.

Base AreaUnfaired, rearward-facing blunt surfaceCauses high drag due to the low pressure experienced by

the rear-ward facing surfaceA base area between or very near to the jet exhausts may

be “filled-in” by the pressure field of the exhaust, partially alleviating the drag penalty.

Interactions between different componentsA canard should not be located such that its wake might

enter the engine inlets at any possible angle of attack.It can stall or even destroy a jet engine

A separated vortex can be expected at high angle of attack if an aircraft’s forebody has a sharp lower corner.

Could be ingested by the engine inletsUnpredictability affect the wing or tail surfaces

Supersonic AircraftsThe greatest aerodynamic impact upon the configuration

layout results from the desire to minimize supersonic wave drag—a pressure drag due to the formation of shocks.

The Sears-Haack body has the lowest wave-drag.

Usually impossible to exactly or even approximately match the Sears-Haack shape

Major drag reductions can be obtained by smoothing the volume distribution shape.

This design technique is referred to as “area-ruling” or “coke bottling” and can reduce the wave drag by as much as 50%

STRUCTURAL CONSIDERATIONSThe primary concern in the development of a good

structural arrangement is the provision of efficient "load paths"-the structural elements by which opposing forces are connected.

The primary forces to be resolved are the lift of the wing and the opposing weight of the major parts of the aircraft, such as the engines and payload.Locating these opposing forces near to each other will

minimize the size and weight of the structural members.

STRUCTURAL CONSIDERATIONS

SpanloadingWeight would be distributed along the span of the wing

exactly as the lift is distributedEliminates the need for a heavy wing structure to carry the

weight of the fuselage to the opposing lift force exerted by the wing

LONGERONSIf the opposing lift and weight forces cannot be located at

the same place, then some structural path will be required to carry the load. The weight of structural members can be reduced by providing the shortest, straightest load path possible.

Prevents fuselage bending

LONGERONSThe lightest longeron structure occurs when the upper and

lower longerons are vertically far apart from each otherIn some designs similar to Fig. 8.5 the lower longerons are

placed near the bottom of the aircraft. A kink over the wing box is avoided by passing the longeron under or through the wing box.

This minimizes weight but complicates both fabrication and repair of the aircraft.

STRINGERSFor aircraft such as transports, which have fewer cutouts

and concentrated loads than a fighter

Distributed around the circumference of the fuselage

Weight is minimized when the stringers are all straight and uninterrupted.

KEELSONA large beam placed at the bottom of the fuselage

frequently used to carry the fuselage bending loads through the portion of the lower fuselage which is cut up by the wheel wells.

STRUCTURAL CUTOUTSRequired structural cutouts include the cockpit area and a

variety of doors(passenger, weapons bay, landing gear, engine access, etc.)

Weight can be reduced by locating structural cutouts away from the wing• The wing provides the lift force, load-path distances can be

reduced by locating the heavy weight items as near to the wing as possible

Structural cutouts should be avoided altogether

FUSELAGE BULKHEADSCarries large concentrated loads such as the wing and

landing gear attachmentsBulkheads can be minimized by arranging the aircraft so

that the bulkheads each carry a number of concentrated loads, rather than requiring a separate bulkhead for each concentrated load.

WING CARRYTHROUGH STRUCTUREThe lift force on the wing produces a tremendous bending

moment where the wing attaches to the fuselagethis bending moment is carried across the fuselage is a key

parameter in the structural arrangementWill greatly influence both the structural weight and the

aerodynamic drag of the aircraft

WING CARRYTHROUGH STRUCTURE:Wing Box CarrythroughVirtually standard for high-speed transports and general-

aviation aircraftThe fuselage itself is not subjected to any of the bending

moment of the wing, which minimizes fuselage weightHowever, it occupies a substantial amount of fuselage

volume, tends to add cross-sectional area at the worst possible place for wave drag, and interferes the longeron load paths

WING CARRYTHROUGH STRUCTURE:Ring FramesRelies upon heavy bulkheads to carry the bending moment

through the fuselageThe wing panels are attached to the fittings on the side of

the fuselage bulkheadsMostly used for most modern fighters

Though usually heavier from a structural viewpoint, the resulting drag reduction at high speeds has led to its use

WING CARRYTHROUGH STRUCTURE:Bending BeamCan be viewed as a compromise between the wing box

carrythrough and ring framesThe wing panels are attached to the side of the fuselage to

carry the lift forces.The bending moment is carried through the fuselage by one

or several beams that connect the two wing panels.

WING CARRYTHROUGH STRUCTURE:Strut-BracedMostly used by light aircrafts and slower transport aircrafts

Has a substantial drag penalty at higher speeds

WING STRUCTURE:SparFront spar is located at about 20-30% of the chordRear spar is located at about 60-75% of the chordAdditional spars may be located between the front and rear

spars forming a “multispar” structureTypical for large or high-speed aircraft

WING STRUCTURE:Wingbox Formed if the wing skin over the spars is an integral part of

the wing structureProvides the minimum weightLanding gears in the wing will usually have the gear located

aft of the wing boxWith a single trailing-edge spar behind the gear to carry the flap

WING STRUCTURE:RibsCarry the loads from the control surfaces, store stations,

and landing gear to the spars and skinsA multispar wing box will usually have few ribs there major

load occurs

WING STRUCTURE:Multirib or Stringer panel boxHas only two spars, plus a large number of stringers

attached to the wing skinsNumerous ribs are used to maintain the shape of the box

under bending

STRUCTURAL CLEARANCEAmount of clearance between structural components

Typical airliners require 4 in. of clearance from the inner wall of the passenger compartment to the outer skin, a conventional fighter require about 2 in. while small general aviation aircraft require 1 in. or less may be acceptable

Type of internal component will affect the required clearance

There is no easy formula for the estimation of structural clearance.

RADAR DETECTABILITYRadar (acronym for Radio Detection and Ranging),

the primary sensor used against aircraft today, consists of a transmitter antenna that broadcasts a directed beam of electromagnetic radio waves and a receiver antenna which picks up the faint radio waves that bounce off objects illuminated by the radio beam.

During World War I, the only sensor in use was the human eye ball.

Radar was first used during World War II, “Chaff” was the first stealth technology.

Chaff drops bits of metal foil or metallized fibers to create many radar echos

RADAR DETECTABILITY

Radar Cross Section (RCS) The extent to which an object returns electromagnetic

energyThe largest contributions to airframe RCS occurs any time a

relatively flat surface of the aircraft perpendicular to the incoming radar beam

RADAR DETECTABILITY

Stealth Designs First-generation stealth designs relied upon faceted shaping

in which the aircraft shape is constructed of interlocking flat triangles and trapezoids.

This has advantage in ease of construction and signature analysis, but offers large number of sharp edges to create diffraction returns

Lockheed F-117

RADAR DETECTABILITY

Current stealth design begins by configuring the aircraft such that all big returns are aimed in just few directions

RADAR DETECTABILITY

RCS can also be reduced simply by eliminating parts of the aircraft

A horizontal tail that isn’t there cannot contribute to the radar return

Nacelles can be eliminated through the use of buried enginesEliminating the entire fuselage through he use of the flying-wing

concept

NorthropB-2

RADAR DETECTABILITY

Radar Absorbing Materials (RAM)Skin materials that absorb radar energyAre typically composites such as fiber-glass

embedded with carbon or ferrite particlesThese particles are heated by the radar electromagnetic waves

absorbing some of the energyReduces the radar return due to perpendicular bounce and

also reduce the surface currents

RADAR DETECTABILITY

Other RCS ContributorsRadomeA radome is a structural, weatherproof enclosure

that protects a microwave (e.g. radar) antennaCovers the aircraft’s own radarTransparent to radar

Therefore, the aircraft’s radar can magnify the threat radar. This can be reduced with a bandpass radome which is transparent only to the aircraft’s radar.

RADAR DETECTABILITY

Inlet and Exhaust CavitiesRadar energy gets into these cavities, bounces off

the engine parts, and sprays back out the cavity towards the threat radar

CockpitsProvides a radar returnRadar enters the cockpit, bounces around off the

equipment inside and reradiates outsideSolution: thinly coat the canopy with conductive metal such as

gold to reflect the radar away

RADAR DETECTABILITY

Aircraft’s WeaponsThese have natural corner reflectors, cavities and

surface discontinuitySolution: place the weapons behind closed doors

Electronic Countermeasures (ECM)Devices to trick the threat radarSends a deceiving signal back to the threat radar

INFRARED DETECTABILITYMany short-range air-to-air and ground-to-air

missiles rely upon infrared (IR) seekers.Modern IR sensors are sensitive enough to detect

radiation emitted by the engine exhaust and hot parts, aerodynamic heating by the whole aircraft skin at transonic and supersonic speeds, and IR radiation that reflects off the skin and cockpit transparencies (windows)

Continuation..

Special Considerations

VISUAL DETECTABILITY

Depends upon the size of the aircraft, its color and intensity of contrast with the surroundings.

VISUAL DETECTABILITY

Camouflage paints

Fake Canopy

Swept forward wings

AURAL SIGNATURES (NOISE)

Caused by airflow shear layers, primarily due to the engine exhaust.

Special ConsiderationsAURAL SIGNATURES

(NOISE)

Chevron

VULNERABILITY CONSIDERATIONS

Ability of the aircraft to sustain battle damage, continue flying, and return to base.

Vulnerable AreaProjected area of aircraft components (sq. ft or sq. m) times probability of aircraft to be lost if component was strucked.

VULNERABILITY CONSIDERATIONS

FIRE is the GREATEST DANGER to a

battle-damaged aircraft.

CRASHWORTHINESS CONSIDERATIONS

Airplanes crash; BUT careful design can reduce the probability of injury in a moderate crash.

de Havilland Comet

PRODUCIBILITY CONSIDERATIONS

It is often said that aircrafts are bought “by the pound” -that aircraft cost is most directly related to weight.

Forgings

Routing of Electrical Wirings, Cooling Ducts and Hydraulic Lines

Joining ofPartsRiveting

Bolting Welding

Bonding

Stereolithography

CAD/CAM

Structural Assembly

MAINTAINABILITY CONSIDERATIONS

The ease by which the aircraft can be fixed.

Accessibility to components must always be considered for ease in fixing.

MAINTAINABILITY CONSIDERATIONS

F-4 Phantom

V/STOL AV-8B Harrier

The use of common sense can help avoid problems but

careful design is mandatory. Don’t wish to learn the hard

special considerations in configuration layout

drag penalty

configuration layout

high supersonic wave

fuselage diameter

fuselage deviation

fuselage lower corners

structural elements

structural members

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