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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

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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

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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