AIRCRAFT DESIGN – UNIT - I

INTRODUCTION:
The process of design of a device or a vehicle, in general involves the use of knowledge in
diverse fields to arrive at a product that will satisfy requirements regarding functional aspects,
operational safety and cost. The design of an airplane, which is being dealt in this course,
involves synthesizing knowledge in areas like aerodynamics, structures, propulsion, systems
and manufacturing techniques. The aim is to arrive at the configuration of an airplane, which
will satisfy aforesaid requirements.
The design of an airplane is a complex engineering task. It generally involves the following.
a) Obtaining the specifications of the airplane, selecting the type and determining the geometric
parameters.
b) Selection of the power plant.
c) Structural design and working out details of construction.
d) Fabrication of prototype.
e) Determination of airplane performance, stability, and structural integrity from flight tests.

STAGES IN AIRPLANE DESIGN:

The design process can be divided into the following three stages.
a) Project feasibility study.
b) Preliminary design.
c) Design project

Project Feasibility Studies:

The aim of this study is to evolve a complete set of specifications for the airplane. It involves
the following steps.
1) Comprehensive market survey to assess the number of airplanes needed.
2) Study of the operating conditions for the proposed airplane. These conditions include (a)
landing field length, (b) type of landing field, (c) weather conditions in flight and near
landing sites and (d) visibility.
3) Study of the relevant design requirements as laid down by the civil and military regulating
agencies. Some of the regulating agency for civil airplanes are : FAA (Federal Aviation
Administration) in USA; EASA (European Aviation Safety Agency) in Europe; DGCA
(Director General of Civil Aviation) in India.
The military airplanes are governed by more stringent regulations called MIL specifications
in USA.
4) Evaluation of existing designs of similar airplanes and possibility of incorporating new
concepts.
5) Collection of data on relevant power plants.
6) Laying down preliminary specifications which may consist of the following.
a) Performance: Maximum speed, maximum rate of climb, range, endurance, rate of turn,
radius of turn, take-off and landing field lengths.
b) Payload.
c) Operating conditions at the destinations.
d) Maneuverability.
Preliminary design
This stage of design process aims at producing a brochure containing preliminary drawings
and stating the estimated operational capabilities of the airplane. This is used for seeking
approval of the manufacturer or the customer. This stage includes the following steps.
(i) Preliminary weight estimate.
(ii) Selection of geometrical parameters of main components based on design criteria.
(iii) Selection of power plant.
(iv) Arrangement of equipment, and control systems.
(iv) Aerodynamic and stability calculations.
(vi) Preliminary structural design of main components.
(vii) Revised weight estimation and c.g. travel.
(viii) Preparation of 3-view drawing.
(ix) Performance estimation.
(x) Preparation of brochure. Section 10.3 deals with the items included in the brochure. It is
also called aircraft type specification.

Design project
After the preliminary design has been approved by the manufacturer / customer. The detailed
design studies are carried out. These include the following stages.
1) Wind tunnel and structural testing on models of airplane configuration arrived after
preliminary design stage. These tests serve as a check on the correctness of the estimated
characteristics and assessment of the new concepts proposed in the design.
2) Mock-up: This is a full scale model of the airplane or its important sections. This helps in
(a) Efficient lay-out of structural components and equipments, (b) checking the clearances,
firing angles of guns, visibility etc.
Currently this stage is avoided by the use of CAD (Computer Aided Design) packages which
provide detailed drawings of various components and subassemblies.
3) Complete wind tunnel testing of the approved configuration. Currently CFD
(Computational Fluid Dynamics) plays an important role in reducing the number of tests to
be carried-out. In CFD, the equations governing the fluid flow are solved numerically. The
results provide flow patterns, drag coefficient, lift coefficient, moment coefficient, pressure
distribution etc. Through the results may not be very accurate at high angles of attack, they
are generally accurate near the design point. Further, they provide information on the effects
of small changes in the geometric parameters, on the flow field and permit parametric studies.
4) Preparation of detailed drawings.
5) Final selection of power plant.
6) Calculations of (a) c.g. shift (b) performance and (c) stability.
7) Fabrication of prototypes. These are the first batch of full scale airplane. Generally six
prototypes are constructed. Some of them are used for verifying structural integrity and
functioning of various systems. Others are used for flight testing to evaluate performance and
stability.
Classification of airplanes
At this stage, it is helpful, to know about the different types of airplanes.
The classification is generally based on
(a) The purpose of the airplane,
(b) The configuration and
(c) Design Mach number (e.g. subsonic, supersonic and hypersonic).

Classification of airplanes according to function

There are two main types of airplanes viz.

i) Civil aircraft and
ii) Military aircraft.

The civil airplanes are categorized as

a) Passenger,
b) Cargo,
c) Agricultural,
d) Sports and
e) Ambulance.

The military airplanes are categorized as

a) Fighter,
b) Bomber,
c) Interceptor,
d) Reconnaissance, and
e) Airplanes for logistic support like troop-carriers and rescue airplane.
The military aircraft are often designed to cater to more than one role e.g. fighter-bomber or
interceptor-fighter.
Influence of the function of airplane on specifications/design requirements
The specifications or design requirements of an airplane are decided by its function. It can be
mentioned that a passenger airplane should have:
(a) High level of safety in operation,
(b) Adequate payload carrying capacity,
(c) Economy in operation,
(d) Comfort level depending on range and cruising altitude,
(e) Ability to fly in weather conditions normally encountered on chosen routes and
(f) Ability to use airfields of intended destinations.
A bomber airplane should have:
(a) Range corresponding to the mission,
(b) Capacity to carry and deploy intended bomb load,
(c) High values of speed, endurance, and ceiling
(d) Adequate protection against accidental fire.
An interceptor airplane should have:
(a) Adequate thrust to give high
(i) Rate of climb,
(ii) Maximum flight speed and
(iii) Maneuverability
(b) Ceiling 3 to 4 km above that of contemporary bombers
(c) Ability to fly in adverse weather conditions and
(d) Appropriate armament.
 Civil & Military Aircraft

Classification by Configuration
Airplanes can be classified in accordance with their shape and structural layout, which in turn
will contribute to their aerodynamic, tactical and operational characteristics. It can be done by
classification of the following:
(a) Shape and position of the wing
(b) Type of fuselage
(c) Location of horizontal tail surfaces
(d) Type of landing gear
(e) Location and number of engines.

Classification of airplanes based on wing configuration

Early airplanes had two or more wings e.g. the Wright airplane had two wings braced with
wires. Presently only single wing is used. These airplanes are called monoplanes. When the
wing is supported by struts the airplane is called semi cantilever monoplane. Depending on
the location of the wing on the fuselage, the airplane is called high wing, mid-wing and low
wing configuration. Further, if the wing has no sweep the configuration is called straight
wing monoplane. The swept wing and delta wing configurations are shown in Figs.
(a) Shape and Position of the Wing
(1) Braced airplane – D.H. Tigermoth
(2) Braced sesquiplane – An- 2
(3) Semi-Cantilever monoplane, Pushpak, Piper Cub,
(4)Semi-cantilever parasol monoplanes…Baby Ace
(5) Cantilever low-wing monoplaneDC-3, HJT-16, IL-18, DH Comet
(6) Cantilever mid-wing monoplane …Hawker Hunter, Canberra
(7)Cantilever high-wing monoplane…An-22, Breguet 941, Fokker friendship
(8)Straight wing monoplane… F-104 A….
(9) Swept-wing monoplane… F-24, MIG-21, Lightning ……
(10) Delta-monoplane with small ARAvro-707, B-58 Hustler, Avro Vulcan.
 Types of Wings

Straight Swept
Wing Wing

Forward Swept Oblique

Wing Wing
 Types of Wings

Variable Sweep Wing Delta Wing

Biplane Triplane

Classification of airplanes based on fuselage

Generally airplanes have a single fuselage with wing and tail surfaces mounted on the
fuselage. In some cases the fuselage is in the form of a pod. In such a case, the horizontal tail
is placed between two booms emanating from the wings. These airplanes generally have two
vertical tails located on the booms. The booms provide required tail arm for the tail surfaces.
Some airplanes with twin fuselage had been designed in the past. However, these
configurations are not currently favoured.
(b) Type of fuselage
(1) Conventional single-fuselageHT-2, Boeing 707,
(2) Twin-fuselage design
(3) Pod and Boom construction. Fairchild Packet, Vampire
Classification of airplanes based on horizontal stabilizer
In a conventional configuration, the horizontal stabilizer is located behind the wing. In some
airplanes there is no horizontal stabilizer and the configuration is called tailless design. In
these airplanes, the functions of elevator and aileron are performed by ailevons located near
the wing tips.
When both ailerons (on left and right wings) move in the same direction, they function as
elevators and when the two ailerons move in opposite direction, they function as ailerons. In
some airplanes, the control in pitch is obtained by a surface located ahead of the wing. This
configuration is called canard configuration

Types of Tails

T Tail Twin Fin

Horizontal Tailless Vertical Tailless

(c) Location of horizontal tail surface

(1) Conventional design with horizontal tail located behind the wing Krishak, Avro-748,
(2) Tail-less design with no horizontal tail …Mirage IV, B-58 Hustier, Concorde
(3) Canard design with horizontal tail located ahead of the wing… (XB-70-A …)

(d) Type of landing gear

(1) Retractable landing gear… DC-9, TU-114, SAAB-35
(2)Non-retractable landing gear…Pushpak, An-14, Fuji KM-2

Fixed type of landing gear Retractable type of landing gear

Classification of airplanes based on number of engines and their location
Airplanes with one, two, three or four engines have been designed. In rare cases, higher
numbers of engines are also used. The engine, when located in the fuselage, could be in the
nose or in the rear portion of the fuselage. When located outside the fuselage the engines are
enclosed in nacelles, which could be located on the wings or on the rear fuselage. In case of
airplanes with engine-propeller combination, there are two configurations – tractor propeller
and pusher propeller. In pusher configuration the propeller is behind the engine. In tractor
configuration the propeller is ahead of the engine.
(e) Classification by Power Plants
a. Types of engines
Piston engines – Krishak, Dakota, Super Constellation, etc. Turboprop engines – Viscount,
Fokker Friendship, An-12, etc.
Turbo-jet engines – Turbo-fan, By-pass engines –HJT-16 (Kiran), Boeing 707 MIG-21, etc.
Ram-jet engines
Rocket engines – Liquid propellant – Solid propellant – X-15A

b.Number of engines
Single-engine – HJT-16, DH Chipmunk, Hawker Hunter, etc.
Twin-engine – HF-24, DC-3, Canberra, etc.
Multi-engine – An-22, Boeing 707, Belfastetc.

c.Engine Located
Propeller single engine located in fuselage nose (HT-2, YAK-9, BEAGLE A-109).
“Pusher”-engine located in the rear fuselage (Bede x BD-2)
Engines (jet) submerged in the wing
(a) At the root – DH-Comet, TU-104, Tu-16
(b) Along the span – Canberra, U-2, YF-12A
Jet engines in nacelles suspended under the wing by “POD” mountings - Boeing 747,
Airbus, Boeing 707, DC-8, Convair 880,
Boom and Pod construction - Fairchild Packet, Vampire Engines (jet) located on the rear
fuselage – Trident, VC-10, IL-62, Caravelle
Jet engines located within the rear fuselage – HF-24 (Marut),
Lightning, MIG-19,…

Engine Arrangement
Factors affecting the configuration
The configuration of an airplane is finalized after giving consideration to the following
factors.
(I) Aerodynamics
(II) Low structural weight
(III) Lay-out peculiarities
(IV) Manufacturing procedures
(V) Cost and operational economics
(VI) Interaction between various features

(I) Aerodynamic considerations – drag, lift and interference effects

The aerodynamic considerations in the design process involve the following.
(A) Drag
The drag of the entire configuration must be as small as possible. This requires (a) thin wings,
(b) Slender fuselage (c) smooth surface conditions, and (d) proper values of aspect
ratio (AR) and sweep (Λ).
(B) Lift
The airplane must be able to develop sufficient lift under various flight conditions including
maneuvers. The maximum lift coefficient also decides the landing speed. These
considerations require proper choice of (a) aerofoil, (b) means to prevent flow separation and
(c) high lift devices.
(C) Interference effects
In aerodynamics the flows past various components like the wing, the fuselage and the tail
are usually studied individually. However, in an airplane these components are in proximity
of each other and the flow past one component affects the flow past the others(components).
The changes in aerodynamic forces and moments due to this proximity are called interference
effects. The lay-out of the airplane should be such that increase in drag and decrease in lift
due to interference effects are minimized. These can be achieved in subsonic airplanes by
proper fillets at the joints between
(a) Wing and fuselage,
(b) Tail and fuselage and
(c) Wing and engine pods.
(II) Low structural weight
The weight of the aircraft must be as low as possible.
This implies use of
(a) High strength to weight ratio material,
(b) Aerofoil with high thickness ratio
(c) Wing with low aspect ratio
(d) Relieving loads (e.g. wing mounted engines) etc..
The airplane structure must be strong enough, to take all permissible flight loads and stiff
enough to avoid instabilities like, divergence, aileron reversal and flutter.
(III) Layout peculiarities
The specific function of the airplane often decides its shape e.g. the fuselage of a cargo
airplane generally has a rectangular cross section and a large cargo door. The height of
fuselage floor should be appropriate for quick loading and unloading
(IV) Manufacturing processes
During the detail design stage, attention must be paid to the manufacturing processes. The
cost of manufacture and quality control also must be kept in mind.
(V) Cost and operational economics – Direct operating cost (DOC) and
Indirect operating cost (IOC)
The total operating cost of an airplane is the sum of the direct operating cost (DOC) and the
indirect operating cost (IOC). The DOC relates to the cost of hourly operation of the airplane
viz. cost of fuel, lubricants, maintenance, overhaul, replacement of parts for airframe and
engine. IOC relates to crew cost, insurance cost, depreciation of airplane and ground
equipment, hangar rental, landing charges and overheads. Thus, for a personal plane lower
initial cost of the airplane may be more important whereas, for a long range passenger
airplane lower cost of fuel may be the primary consideration.
(VI) Interaction of various factors
Some of the considerations mentioned above may lead to conflicting requirements. For
example, a wing with an airfoil of relatively higher thickness ratio, has lower structural
weight but, at the same time has higher drag. In such situations, optimization techniques are
employed to arrive at the best compromise.

Primary requirements for Civil Aircraft

Passenger Aircraft
 High Safety level
 High payload carrying capacity
 Economy in operation
 Comforts
 Ability to fly in any Weather (All-weather flying) aerodromes
 Ability to use aerodromes of respective classes
Cargo Aircraft
 High payload carrying capacity
 Economy in operation
 Ability to fly in any Weather
 Suitable for civil aerodromes
Primary requirements for Military Aircraft
Strategic
 Long range (over 6000 km)
 High load carrying capacity - (2000-4000 kg)
 High speed
 High endurance
 High ceiling
  Adequate fire protection
Tactical
 Long range 3000-5000 km)
 High load carrying capacity (above 6000 kg)
 High speed
 High ceiling
 High endurance
 All-weather flying
 Ability to use field aerodromes
Primary requirements for Military Aircraft – FIGHTERS-Tactical
 High speed (300-400 kmph- more than contemporary bomber speeds)
 High ceiling (2-4 km than bomber more than contemporary bomber ceilings)
 Maneuverability
 Sufficient endurance
 High rate of climb
 Ability to launch repeat- ed attacks
 All-weather flying
 Ability to use field aerodromes
 Ease of dismantling and assembling
Primary requirements for Military Aircraft – FIGHTERS- Interceptor
 High rate of climb
 High ceiling (3-4 km contemporary ceilings)
 High speed (500-600 kmph more than contemporary bomber speeds
 High Maneuverability
 All-weather flying
 Ability to fire powerful single volley at target
 Ease of dismantling and assembling
Primary requirements of bomber
 Long range,
 High load carrying capacity,
 High speed ,
 High endurance,
 High ceiling and
 Adequate fire protection.
Primary requirements of interceptor
 High rate of climb,
 High ceiling (3 to 4 km above contemporary bombers),
 High speed ,
 High Maneuverability,
 Ability to fly in any weather and
 Appropriate armament.
Tailless and Flying Wing Design
A flying wing is a tailless fixed-wing aircraft which has no definite fuselage, with most of the
crew, payload and equipment being housed inside the main wing structure.
Advantages of tailless and flying wing design
 Small drag-reduction of the number of non-lifting surfaces and minimizing the
interference effects lead to much lower CD.
 Smaller structural weight. In this design the horizontal tail and that part of the
fuselage carrying the tail is eliminated. The bending moment is less
 Large thickness at the root of the wing .The structure can therefore be made lighter
due to the increased rigidity of the wing section
Disadvantages
 Restricted c.g travel due to the absence of horizontal stabilizer
 The wing tips are wash-out. This causes a negative lift at wing tips and hence a larger
wing area is required to develop the same overall lift.
 Poor directional stability
 Use of high lift devices results in large diving moments.
 Reversal of panel stresses at wing tips-Due to the negative lift at the tips of flying
wing some of the wing panels have to be designed for both tensile and compressive
loads
Canard design
In this special type of design the horizontal tail surfaces are located ahead of the main plane.
In the major part of the flight the canard tail assists the wing in sharing the weight. The
horizontal tail is effective as it is not in the downwash of the wing surface.
Advantages
 The forces acting on the horizontal tail is upwards and supplements the lift developed
by the wing. Hence for the same CL the wing area is reduced, and there is a decrease
in drag.
 The wing weight is lesser than the conventional aircraft wing
 At low speeds the horizontal tail stalls first, and automatically sets the airplane to a
smaller angle of attack. Wing stall is difficult.
 After reaching the critical Mach number the lifting stabilizer permits compensation of
the growing wing diving moment
 When the Mach number increases, the static stability of the canard also increases.
Disadvantages
 Shock stall may occur on the horizontal stabilizer before occurs in the wing
 At supersonic speeds the longitudinal stability is more difficult in the canard the
canard type than conventional airplane.
 The elevator power required for landing is very high.
 The landing characteristics are very poor.
The advantages of high wing design
 Increased lateral stability
 The possibility of engine failure due to suction of hard particles from the ground is 4
times less than in a low wing design.
 In passenger aircraft there is passenger comfort. The down view of the passenger is
undisturbed by the wing
 In cargo aircraft, loading and unloading are easier, on account of the lower position of
fuselage.
  The interference drag is lower as compared to low wing airplane.

Disadvantages of high wing design

 The area of the vertical stabilizer is 20-22% of the wing area as compared to 10-12 %
in low wing design. This increases weight and drag.
 The weight of the fuselage is increased as lower fuselage is to be reinforced for
protection in emergency landing.
 The weight of the fuselage structure is increased by the heavy frames needed for the
attachment of the landing gear to the fuselage. Landing gear weight also very high.
 If the bogies are attached to the fuselage the treads are narrow, which leads to poor
stability on the ground.
 Maintenance and inspection of engines are difficult
Location of engines
 Engine under pylons
 Inside the root portion of the wing
 Outside the rear fuselage
 Advantages in engine under pylon
 The weight of wing structure decreases by 15-20 %as the wing is relieved by the
weight of the engines
 The space inside the wing is utilized for fuel storage, maintenance, inspection and
replacements are facilitated
 The wing structure is free from the heat from the engines, and better fire safty can be
achieved.
Disadvantages
 Failure of outboard engine creates a large yawing moment. This moment has to
becounted by rudder deflection which results in higher drag.
 Smaller ground clearance increases the possibility of foreign particles entering the
engines.
 The noise level in the cabin is 5 db higher as compared to airplanes having engines on
the rear fuselage.

Engines located in the wing root

Advantages
 There is very little increase in frontal area due to installation of power plants.
 Almost the entire wing span can be used for ailerons and high lift devices
Disadvantages.
 The weight of the wing is high due to the cut portion in the wing spars to
accommodate engine
 The weight of the power plant is high due to the long air ducts.
 The intake is located at a place where the air flow is not clear.
 The space in the root cannot be used for fuel accommodation
 Engine located on the rear fuselageAdvantages
 There is less noise in the cabin and the fuselage
 The entire wing can be used for fuel storage
 The whole wing space can be used for high lift devices and ailerons
 Fire hazard is minimum.
 The actual wing does not deviate from the design profile and the wing is clean.
Disadvantages
 The fuel is located far from the engines, therefore the length of
the pipe line isincreased and special fuel pumps should be
provided.
 Due to power plant weight at the tail, longer fuselage is provided
ahead of the wingwhich requires larger vertical and horizontal
stabilizers.

********************************** END OF UNIT - I ******************************

Important Questions:
1. Explain the phases of Airplane Design.
2. How do you classify airplanes? Explain each one of them with necessary diagrams.
3. Explain the basic requirements for civil and military airplanes.
4. Explain the merits and demerits of high/low/mid wing configuration.
5. Write short notes on
(i) Tailless and flying wing design.
(ii) Canard Design
6. What are the factors which affect the configuration of airplane design? Explain.