special considerations in configuration layout
DESCRIPTION
Aircraft DesignTRANSCRIPT
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
AERODYNAMIC CONSIDERATIONS
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
AERODYNAMIC CONSIDERATIONS
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.
AERODYNAMIC CONSIDERATIONS
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.
AERODYNAMIC CONSIDERATIONS
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.
T-38
AERODYNAMIC CONSIDERATIONS
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
AERODYNAMIC CONSIDERATIONS
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.
AERODYNAMIC CONSIDERATIONS
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
STRUCTURAL CONSIDERATIONS
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
STRUCTURAL CONSIDERATIONS
STRUCTURAL CONSIDERATIONS
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.
STRUCTURAL CONSIDERATIONS
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.
STRUCTURAL CONSIDERATIONS
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 CONSIDERATIONS
STRUCTURAL CONSIDERATIONS
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
STRUCTURAL CONSIDERATIONS
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.
STRUCTURAL CONSIDERATIONS
STRUCTURAL CONSIDERATIONS
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
STRUCTURAL CONSIDERATIONS
STRUCTURAL CONSIDERATIONS
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
STRUCTURAL CONSIDERATIONS
STRUCTURAL CONSIDERATIONS
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
STRUCTURAL CONSIDERATIONS
STRUCTURAL CONSIDERATIONS
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.
STRUCTURAL CONSIDERATIONS
STRUCTURAL CONSIDERATIONS
WING CARRYTHROUGH STRUCTURE:Strut-BracedMostly used by light aircrafts and slower transport aircrafts
Has a substantial drag penalty at higher speeds
STRUCTURAL CONSIDERATIONS
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
STRUCTURAL CONSIDERATIONS
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
loads
STRUCTURAL CONSIDERATIONS
STRUCTURAL CONSIDERATIONS
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
STRUCTURAL CONSIDERATIONS
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 CONSIDERATIONS
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
RADAR DETECTABILITY
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
B-2
F-23
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.
Special Considerations
VISUAL DETECTABILITY
Special Considerations
Camouflage paints
Fake Canopy
Swept forward wings
AURAL SIGNATURES (NOISE)
Caused by airflow shear layers, primarily due to the engine exhaust.
Special Considerations
Special ConsiderationsAURAL SIGNATURES
(NOISE)
Chevron
VULNERABILITY CONSIDERATIONS
Ability of the aircraft to sustain battle damage, continue flying, and return to base.
Special Considerations
Special Considerations
Vulnerable AreaProjected area of aircraft components (sq. ft or sq. m) times probability of aircraft to be lost if component was strucked.
VULNERABILITY CONSIDERATIONS
Special Considerations
Special 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.
Special Considerations
CRASHWORTHINESS CONSIDERATIONS
Special Considerations
de Havilland Comet
CRASHWORTHINESS CONSIDERATIONS
Special Considerations
PRODUCIBILITY CONSIDERATIONS
It is often said that aircrafts are bought “by the pound” -that aircraft cost is most directly related to weight.
Special Considerations
PRODUCIBILITY CONSIDERATIONS
Special Considerations
Forgings
PRODUCIBILITY CONSIDERATIONS
Special Considerations
Routing of Electrical Wirings, Cooling Ducts and Hydraulic Lines
PRODUCIBILITY CONSIDERATIONS
Special Considerations
Joining ofPartsRiveting
Bolting Welding
Bonding
PRODUCIBILITY CONSIDERATIONS
Special Considerations
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.
Special Considerations
MAINTAINABILITY CONSIDERATIONS
Special Considerations
F-4 Phantom
V/STOL AV-8B Harrier
Special Considerations
The use of common sense can help avoid problems but
careful design is mandatory. Don’t wish to learn the hard
way!