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1
AE 465
Aircraft Design Configuration • Conventional Configurations
– Variations regarding powerplant & intake location, vertical wing position,
tail unit layout and landing gear.
• Unconventional Layouts
– Biplanes, variable sweep, canard designs, twin booms, multi-hulls, span-
loaders, joined wing and blended wing body designs.
• Special Configurational Issues
– Short Take-Off & Vertical Landing, stealth, waterborne operations.
Conventional Configurations • Cantilevered monoplane wing.
• Separate horizontal and vertical tail surfaces.
• Control via ailerons, elevators and rudder.
• Discrete fuselage to provide volume and continuity to airframe.
• Retractable tricycle landing gear.
• Minimum number of powerplants needed to meet power and operational
requirements.
Within the category of conventional aircraft there are many variations from the standard
to be considered:
• Powerplant Location – nose, wing podded, rear fuselage podded, internal.
• Intake Location – nose, side, ventral, dorsal.
• Wing Vertical Location – high, low, mid.
• Tail Unit Arrangements – variable incidence, all-moving, T-tail, multi-finned,
butterfly.
• Tricycle Landing Gear Configuration – numbers of legs, bogeys and wheels.
Nose-Mounted
• Most logical position for any single tractor propeller engine aircraft.
• Advantages include – symmetry of layout, good propeller clearance, access and
maintainablity
Airbus A330 C-130 Hercules
Supermarine Spitfire P-51 Mustang
2
Wing-Mounted (Outer Wing)
• Many uses:
– Large aircraft with propellers, turbojets or turbofans.
– For jets/fans, these will be podded and mounted onto under-wing pylons.
– For props, these will be mounted directly onto the wing structure.
• Advantages include:
– Versatility – use of alternative engines.
– Compact overall layout.
– Inertial relief – reducing required wing structural mass.
– Ease of access for maintenance.
• Also several drawbacks and necessary considerations:
– Ground clearance may be a problem in which case high wings may be
used (with tall landing gear) or possibly top-wing mounting (e.g. BAe
748) with aerodynamic penalty.
– Spanwise location – should depend on prop diameter or statistical analysis
of fan burst trajectory and impact on neighbour.
– Typical values are 30% and 55% semi-span for a 4-engine design; large
values give big engine-out yaw problems and larger rudder sizes.
Over Wing-Mounted
Wing-Mounted (Inner Wing)
• Some aircraft have housed the powerplants in the wing root area with significant
structural disadvantages.
Lockheed Constellation
Boeing 767 B-52
Shorts SD360 BAe 748
B-1 Lancer DeHavilland Comet
3
Rear Fuselage-Podded
• Used on many moderate sized transport aircraft of the past and also many modern
small business jet aircraft.
• Advantages
– Reduced engine-out yaw smaller rudder size.
• Disadvantages
– Rearwards movement of CG stability problems.
– Structural acoustic fatigue.
Wing-Podded Fuselage-Podded
Ground Clearance Possible problem Good
Internal Noise Fair Good
Acoustic Fatigue Possible problem for wing &
flaps
Possible problem for fuselage
Crash Safety Good Possible problem
Propulsive Efficiency Good OK if well positioned
Longitudinal Stability Good Problems due to aft CG & short tail arm
Tip Stall Good Possible problem
Asymmetric Thrust Poor Good
Weight Good Poor
Engine Maintenance Good High off ground
Wing Aerodynamic Efficiency Problems from cut-outs Very good
Fuel Feeds to Engines & Wing
Anti-Icing
Good Ducts and lines through cabin
Internally Housed
• Used on many single and twin turbojet/turbofan engine aircraft such as military
trainers and fighters.
• Advantages
– Compact layout.
– Reduced drag.
• Disadvantages
– Engine removal and maintenance problems.
– Structural acoustic fatigue due to jet efflux.
– Jet pipe length minimized by moving engine rearwards but this affects
CG, stability and control.
A-10 Thunderbolt
MD-11
4
Nose Intake
• Used on many early jet fighters with mid-fuselage mounted engines.
• Requires use of long inlet ducts and jet pipes – gives low flow distortion but high
total pressure losses.
• No need for boundary layer diverters.
• Occupies large amount of internal volume.
• Only small radome may be housed in shock cone centre-body.
Side Intake (Below Wing)
• Used on the majority of modern high-wing strike and combat aircraft designs.
• Leaves the nose area free for radar equipment installation.
• The wing is often extended above the intakes to improve high- performance.
• Flow diverters are needed to accommodate fuselage boundary layer growth.
Side Intake (Above Wing)
• Used on many low-wing design trainer and combat aircraft.
• Wings may be used to shield the intakes and reduce the manoeuvring .
• Any sharps bends have to be avoided to prevent flow distortions.
• Short intake lengths are possible with low overall volume requirements.
Panavia Tornado Sepecat Jaguar
MiG-19 Farmer English Electric Lightning
Tornado ADV Dassault Breguet F1 Mirage
T45A Goshawk BAe Hawk 100
5
Ventral Intake
Situated on underside of fuselage - an increasingly common position for high
performance combat aircraft.
• Gives very good high- manoeuvrability.
• Prone to FOD and debris ingestion.
• Complicates nose wheel positioning/stowage.
• Restricts carriage of under-fuselage stores.
• Low flow distortion and pressure losses into intake.
Dorsal Intake
Situated on top-side of fuselage.
• Only tends to be used on 3-engine airliners with 3rd engine buried in the rear
fuselage/fin area with a few exceptions.
• Gives poor performance at high- due to separated flow ahead of intake.
Vertical Location of Wing High Wing
• Gives an efficient spanwise lift distribution leading to low lift-induced drag.
• Improves lateral static stability.
• Preferred for most freight and military transport aircraft:
– Low floor line for easy loading & unloading.
– Good all-round vehicular access when on ground.
– Wing fuel load away from ground when landing with failed landing gear.
– Good ground clearance for powerplants, especially props.
Eurofighter 2000 F-16 Fighting Falcon
McDonnell Douglas DC-10 Lockheed L1011 Tristar
6
Low Wing
• Improves lateral manoeuvrability.
• Preferred for most passenger transport aircraft:
– Wing structure conveniently passes below floor.
– Volume free fore and aft of wing structure for cargo holds, luggage and
landing gear stowage.
– Minimizes landing gear length and mass.
– Wing provides buoyancy when ditching into water and also a platform for
emergency evacuation.
Tail Unit (Empennage) Conventional Layout
• Approximately 70% of aircraft in service have a “conventional” arrangement
comprising separate fixed horizontal stabiliser and vertical fin surfaces for
stability and moving elevator and rudder sections attached to fixed surfaces for
control.
• This is the simplest solution & provides optimum overall performance in the
majority of cases.
Conventional Primary Control Surfaces
Lockheed C-130 Hercules G-222
Boeing 777 Boeing 737
7
Variable Incidence Tailplane
• Here the forward (main) section of the horizontal surface is not fixed but is
capable of rotation through a small range of angles of attack.
• As such, it is generally used to adjust pitch trim rather than using the conventional
elevators.
• It is especially useful for countering the effects of significant pitching moment
increments caused by deployment of powerful high lift devices.
• Elevators are still used for pitch control.
All-Moving (Slab) Tailplane
• Whole of the horizontal tailplane surface is used for both pitch control and trim
(with no separate hinged elevator).
• This offers significant advantages at transonic and supersonic speeds when
effectiveness of conventional trailing edge surfaces is dramatically reduced.
• Universally adopted for supersonic fighter designs.
• Most also use differential movement of opposite sides to improve roll rate (then
known as tailerons).
• Powered controls are necessary due to the large control force requirements.
T-Tail
• Horizontal tailplane mounted on top of fin.
• Often used on large high-mounted swept-wing designs and also smaller low-wing
aircraft.
T-Tail - Advantages
• Provides substantial “end-plating” effect to fin, improving its effectiveness and
reducing the fin size requirement.
• Lifts the horizontal tail clear of any propwash & the wing wake during cruise
flight, therefore reducing buffet and fatigue.
• Allows engines to be mounted on the aft-fuselage, if required.
T-Tail - Disadvantages
• Gives a large mass penalty to the empennage due to the higher loading and
aeroelastic effects.
• Increased likelihood of “deep stall” – puts tail in wake of stalled wing, making
recovery difficult or even impossible.
Multi-Finned
• If fin-sizing exercise results in large single fin dimensions then sometimes
preferable to use two (or more) smaller fins instead.
• Allowed Constellation to operate from existing hangars.
• Also produces desirable “end-plating” effect to horizontal tailplane, reducing its
size requirements.
• Fins have to be positioned far enough apart so that undesirable mutual
aerodynamic interference effects are not too severe.
Beechcraft Duchess
Lockheed C-5A Galaxy
8
• If fins are positioned in slipstream of propellers rudder performance is improved
at low speeds.
• Difficult to avoid fin stall at high sideslip angles.
• Not generally used nowadays for single-boom layout transport aircraft.
Twin Fin Fighter Aircraft
• Twin fins nowadays more associated with supersonic fighters.
• More compatible with twin-engine aircraft (F14/F15/F18) than single (F16) due to
“engine-out” sizing considerations.
• Special benefit of supersonic application is that interference effect disappears
providing fin Mach lines do not intersect.
• Can also provide infrared shielding of engine exhaust to improve stealth,
especially if canted (F22).
• Resultant reduced fin height improves aeroelastic behaviour.
Butterfly Tail
• In this case the conventional tail surfaces are combined into a pair of inclined
surfaces.
• The separate roles of the tailplane/elevator and fin/rudder are combined.
• Advantages include:
– Less interference drag; smaller total surface area; improved stealth
characteristics.
• Disadvantages include:
– Cross-coupling of stability/control characteristics; handling difficulties;
need for fully automatic flight control system.
MiG-29 Fulcrum F-15 Eagle
Beech Bonanza
Lockheed F-117 Nighthawk
9
Landing Gear Layout Tricycle Gear Configuration
• The most conventional, comprising:
– Pair of main legs behind aircraft CG.
– Single nose leg ahead of CG.
• Each leg incorporates:
– Shock absorber to dissipate vertical landing energy.
– Single or two side-by-side wheels or multiple bogie arrangement.
• Only main wheels are generally fitted with brakes.
• Only the nose wheel is usually steered for ground manoeuvring.
• For effective steering, nose leg should support between 6 and 10% of the aircraft
mass.
• Provision must be made for attachment and stowage of landing gear units.
• Lateral positioning (track) dictated by need to prevent overturning during ground
manoeuvring – mainly a function of height of CG, track distance & shock
absorber characteristics.
Tricycle Gear Configuration – Number of Wheels
• As the aircraft mass increases, operations from runways of given strength dictate
need for more wheels to spread the load – many possible variants:
– Two-axle bogie
– Three-axle bogie
– Three or four main legs
– Multiple legs on single axes
Two-Axle Bogie
• The main legs are split into two-axle bogies, with usually two wheels per axle.
• Such as arrangement is generally necessary if the aircraft mass is between about
90 and 200 tonnes.
• It is common to many civil and military transport aircraft types.
BAe Hawk Cessna 172N
Airbus A330
Airbus A310
10
Three-Axle Bogie
• For very large aircraft (e.g. > 210 tonnes), the load has to be spread even further –
one option is to use a 3-axle bogie arrangement.
On the Boeing 777, the extra axle is put in the centre of the bogie.
On the C-5 the extra axle is put side-by-side with the rear axle – the aircraft has 28
wheels in total!
Both have main bogie steering to reduce turn radius & tyre scrubbing.
Three Main Legs
• Some large aircraft use an additional main leg to spread the load, e.g. Airbus A-
340:
2-wheel nose gear and 3 main gear, each of double-wheel 2-bogie – 14 wheels in
total.
Four Main Legs
• This will generally be
the case for very large
civil transports (> 300
tonnes) with low
wing designs (e.g.
Boeing 747).
• It poses significant
problems for airframe
C-5 Galaxy
Boeing 777
11
attachment & stowage.
Multiple Main Legs with
Single Axles
• Good option for
heavy high wing
military transports
with retraction into
fuselage blisters
The Antonov An124 Condor
has 24 wheels – two side-by-
side 2-wheel nose legs and
ten main legs (5 each side),
each with 2 wheels.
Tail Wheel Configuration
• Here the two main wheels are located forward of the CG and a tail wheel or skid
provides the third support point.
• This is a simpler, lighter and cheaper design than a tricycle layout but has
significant disadvantages:
– Difficult ground manoeuvring and take-off/landing due to inhibited
visibility.
• This was the norm for many early aircraft but its application is nowadays limited
to simple light aircraft where emphasis is on simplicity and low cost – often with
fixed (rather than retractable) legs
Single Main Gear Leg
• Sometimes advantageous to concentrate the main load into a single main leg
rather than two.
For Harrier, tricycle main units difficult to
accommodate in fuselage (because of powerplant)
or wing (because of wing trailing edge controls
and underwing pylons).
• Ground roll stability obtained from pair of light weight , lightly-loaded
outriggers, located near to wing tips.
Bicycle Configuration
• This is a specialized form of the single main leg configuration but with the rear
leg significantly further back.
Curtiss P-6 Hawk
Hawker Sea Fury
BAe Harrier
12
• This results in the nose leg carrying a similar proportion of the mass as the rear
leg.
• Advantage is an uncluttered wing and long length of available fuselage space (e.g.
for a bomb bay).
• Disadvantages are:
• Highly loaded nose leg makes ground manoeuvring very difficult.
• Specialized landing technique needed , especially if in cross-winds.
• Outriggers needed for ground roll stability.
• The configuration is not recommended unless there is no viable alternative.
Biplane
• The norm for the first 30 years of aviation.
• Early aerofoils were very thin requiring external bracing so that biplanes gave
best structural efficiency.
• Many penalties of use, especially at higher speeds – increased total mass, drag
and aerodynamic interference.
• Aerodynamics and materials advances have led to increased wing loadings (W/S)
so that biplanes are mostly redundant nowadays – main exception is aerobatics
aircraft where low W/S is an advantage and specialized aircraft such as crop-
sprayers.
Unconventional Configurations Variable Sweep (Swing-Wing)
• Design Problem:
– High sweep usually needed for transonic/supersonic speed designs but this
affects low speed performance.
– Possible solution is to use variable sweep wings.
• This gives a better matched performance over a wide speed range and offers an
aircraft multi-role capabilities over subsonic and supersonic speed ranges.
Variable Sweep - Disadvantages
• Increased mass over conventional design due to heavy actuation system.
Boeing B47E Stratojet
B-52 Stratofortress
DH-82 Tiger Moth
Pitts S1-S
13
• Increased system complexity and costs.
• Increased drag due to interaction between fixed and moving parts of the wing.
• Trim and stability/control problems due to movements of aerodynamic centre and
CG
Canard Layout
• The conventional aft horizontal tailplane is replaced by a foreplane (or canard)
while the main wing is then moved rearwards for stability purposes.
• Two main categories:
– Lifting canard – canard provides substantial lift as well as longitudinal
trim and control.
– Control canard - longitudinal trim and control only.
• This is not a new idea – the original Wright Flyer was a control canard
configuration
Canard Layout – Configurational Advantages
• Negligible trim drag penalty, usually a download on the rear tail surface on a
conventional layout.
• More rapid pitching manoeuvre response as initial change is in required direction.
• Possible layout advantage (e.g. aft-located wing passes behind the cabin).
• Better provision for escape from “pitch-up” (associated with tip-stall on highly
swept wings).
Canard Layout – Configurational Disadvantages
• Airflow interference from the canard over the main wing surface.
• Increased pitching moment effect with wing flap deployment due to large moment
arm – so sophisticated high lift devices may not be used with consequent low-
speed performance penalty.
Long-Coupled Canard Layout
• Small canard located far enough forward so that interference effects are small.
• Particularly suited to long-range supersonic aircraft designs (bombers, transports,
etc.).
• Foreplane effect is beneficial for cruise trim drag reduction and at low speed,
particularly for take-off rotation
MiG-23
General Dynamics
F-111
F-14 Tomcat
14
Short-Coupled Canard Layout
• Foreplane placed just ahead of (& usually above) wing.
• Careful location enables lift effectiveness of pair to exceed that of sum of isolated
lifting surfaces.
• Most applicable to high agility combat aircraft designs.
Canard with Forward Sweep
• Rearward sweep usually preferable as it gives better compromise of aerodynamic
characteristics – especially stability/control.
• Forward swept wings also more prone to aeroleastic divergence – overcome with
associated mass penalty.
• Method could give overall layout advantages, e.g. by allowing wing carry-through
structure to pass through rear of aircraft and avoid main section.
Tu-144 Concordski
Rockwell B-70 Valkyrie
Dassault Rafale
Saab Gripen
Grumman X-29A
Su-47 Berkut
15
Three Surface Aircraft
• Employs both a foreplane and a
tailplane.
Advantages
• Stabilizing effect of tailplane.
• Favorable trim & control functionality
of foreplane.
Disadvantages
• Fuselage mass penalty.
• Increased interference drag and also skin
friction due to increased total wetted
surface area.
Twin-Boom Layout Aircraft
• Several possible reasons for being
adopted:
– Allows engine to be mounted close to CG – particularly pusher-prop types
& early jets.
– Over-riding requirement for aircraft to have unrestricted access to rear of
freight hold.
– Visibility for rear gunner/bomber crew.
• Results in use of twin fins.
• Disadvantages include: increased wing mass, increased interference drag and less
usable volume.
Span-Loaders
• Closely related to flying wing designs whereby the payload held in main wing box
structure.
• Small central fuselage pod sometimes used to house flight deck and central
services.
Advantages
– Spreads the payload across the wing, rather than the fuselage.
– This gives inertial relief to the wing structure.
– Most of aircraft then comprises wing (with higher lift/drag than
conventional fuselage).
– Gives typical 10% reduction in take-off mass.
Northrop P-61 Black
Widow
Cessna C337
Skymaster
Piaggio Avanti
16
Span-Loaders - Disadvantages
• Difficult emergency passenger evacuation
procedures.
• Structural layout problems.
• Fuel location.
• Pressurization of wing section.
• Increased moments of inertia leading to
poor roll rates.
• Complicated flight control system.
Flying Wing (Blended Wing-Body) Layout
• Similar to spanloaders – optimum
aerodynamic solution sought - wing is
most efficient means of lift generation so
fuselage is dispensed with altogether.
Advantages
– As for spanloader – inertial relief
of wing gives lower wing structure
mass and lower costs.
– Potential for increased passenger
cabin volume and improved
comfort levels.
– Major opportunity for using laminar flow technology – easier to apply to
wing than a fuselage.
BWB Aircraft - Disadvantages
• Passenger wariness of unconventional (more feasible to military & cargo
transports).
• Unfamiliar structural layout & design.
• Complex aerodynamic interference effects.
Special Configurational Issues
• An aircraft’s specifications and requirements may include some special provision
which could then have a dominant influence over the resultant configuration.
• These include:
– Short Take-Off & Vertical Landing (STOVL).
– Stealth.
– Waterborne Operations.
STOL & STOVL Aircraft
• Short Take-Off (& Vertical) Landing Aircraft.
• Two classes of military aircraft sometimes have a need for STOL or STOVL
capabilities.
– Freight.
– Combat.
Military Freight STOL Airlifters
• Often required to operate to and from airstrips of short length and poor surface
strength.
• No major effect upon configuration selection (unless tilt-rotor/wing technology
adopted) but increased emphasis on:
– High installed thrust.
Boeing BWB Airliner
17
– Complex high lift devices and wing technology.
– Low tyre pressures.
• Several civil variants also developed with perceived need.
STOVL Combat Aircraft
• For vertical landing the available vertical thrust component must exceed the
landing weight.
• Logical to also use this component for short take-off.
• STOVL thrust component provided by downward deflection of exhaust gases of
forward flight propulsion unit(s).
• Impractical to locate this thrust component immediately below CG at all times so
additional thrust provision needed for balance.
STOVL Combat Aircraft – Further Comments
Kawasaki NAL Asuka
Boeing YC-14
DHC Dash 7
Breguet 941
Canadair CL-84 Dynavert Vertol VZ-2A
Bell XV-15 Bell-Boeing V-22
Osprey
18
• Three standard methods available for providing vertical thrust component:
– Vectored bypass flow.
– Separate vertical lift engine.
– Remotely driven lift engines (using main powerplant as energy source).
• All methods require separate low-speed control capability, usually using reaction
jets supplied with bleed air from main engine compressor.
STOVL Fighter – Vectored Bypass Flow
• RR Pegasus engine has 4 nozzles, each rotating to vector efflux as required – rear
two exhaust hot gases and front two exhaust colder bypass air from behind fan.
• Results in compact system, though bulky and also has to be located about aircraft
CG.
• Several thrust augmentation methods are available (e.g. plenum chamber burning
where fuel is burnt in bypass air) but cause problems (e.g. hot gas ingestion &
ground erosion).
STOVL Fighter – Vertical Lift Engines
• Uses one or more dedicated lift
engines in addition to deflected
thrust from cruise engine.
• Allows engine to be located more
conveniently to aft of aircraft with
lift engines forwards, giving more
design flexibility.
• Disadvantage is extra mass of lift
engine – worthless in forward flight
mode.
STOVL Fighter – Remotely Driven Lift
Fans
• Lift fan driven remotely from main cruise
engine by either mechanical shaft drive (as
in X-35 JSF) or gas drive.
• Mechanical drive places restrictions
on fuselage layout.
• Compressed gas drive is bulky and
relatively inefficient.
• Total effective fuselage volume likely to be
more than for other two possible methods
Stealth
• Increasingly important for modern combat aircraft designs.
BAe Harrier
Rolls Royce Pegasus
Yak-141 Freestyle
Lockheed Martin X-35 JSF
19
• Final configuration depends heavily on overall priority of stealth against
performance.
Stealth – General Observations
• Foreplanes best avoided.
• Internal powerplants & weapons.
• Intakes with long curved ducts.
• Exhausts must be shielded.
• Avoid surfaces positioned at right angles to each other (e.g. use inclined fins).
• Minimize discontinuities in shape/surface.
• Surface edges parallel to each other.
• Difficulties with cockpit transparencies – use of
unmanned vehicles advantageous.
Waterborne Aircraft
• Very common in the early days of aviation.
• Can operate from anywhere with a large stretch of
reasonably calm water.
• Became less popular due to:
– More airfields available after WW2.
– Trend for using higher wing loadings -
• Results in higher take-off & landing
speeds and high water resistance forces.
• Use nowadays restricted to small aircraft operating in
coastal regions or in remote locations with many lakes
& rivers.
• Two basic categories – float planes & flying boats.
Float Planes
• Conventional landing gear replaced by large floats.
• Invariably propeller-driven.
• Usually direct conversions from land-based types.
• Usually only applicable to small aircraft (12 tonnes
max).
• Air drag of floats is high and gives large tail download
trim requirement.
Flying Boats
• Usually larger than float planes.
• Fuselage used as a hull for waterborne operations.
• Wing tip floats or fuselage sponsons used to provide waterborne roll stability.
• Some types also have conventional retractable landing gear and are then
amphibious.
B-2: Stealth is
primary design
driver
F-22: high performance
levels with stealth
DeHavilland Beaver
Consolidated OA-10 Catalina