Aircraft design process

Aircraft design process
The aircraft design process is the engineering design process by which aircraft are designed.
These depend on many factors such as customer and manufacturer demand, safety protocols,
physical and economic constraints etc. For some types of aircraft the design process is regulated
by national airworthiness authorities. This article deals with powered aircraft such as airplanes
and helicopter designs.
Aircraft design is a compromise between many competing factors and constraints and accounts
for existing designs and market requirements to produce the best aircraft.


1 Design constraints
o 1.1 Purpose
o 1.2 Aircraft
o 1.3 Financial
factors and
o 1.4
o 1.5 Safety
2 Design optimization
3 Computer-aided
design of aircraft
4 Design aspects
o 4.1 Wing design
o 4.2 Fuselage
o 4.3 Propulsion
o 4.4 Weight

o 4.5 Structure
5 Design process and
o 5.1 Conceptual
o 5.2 Preliminary
design phase
o 5.3 Detail design
6 See also
7 References
8 External links

Design constraints

The design process starts with the aircraft's intended purpose. Commercial airliners are designed
for carrying a passenger or cargo payload, long range and greater fuel efficiency where as fighter
jets are designed to perform high speed maneuvers and provide close support to ground troops.
Some aircraft have specific missions, for instance, amphibious airplanes have a unique design

that allows them to operate from both land and water, some fighters, like the Harrier Jump Jet,
have VTOL (Vertical Take-off and Landing) ability, helicopters have the ability to hover over an
area for a period of time.[1]
The purpose may be to fit a specific requirement, e.g. as in the historical case of a British Air
Ministry specification, or fill a perceived "gap in the market"; that is, a class or design of aircraft
which does not yet exist, but for which there would be significant demand.
Aircraft regulations

Another important factor that influences the design of the aircraft are the regulations put forth by
national aviation airworthiness authorities.[2][3]
Airports may also impose limits on aircraft, for instance, the maximum wingspan allowed for a
conventional aircraft is 80 m to prevent collisions between aircraft while taxiing.[4]
Financial factors and market

Budget limitations, market requirements and competition set constraints on the design process
and comprise the non-technical influences on aircraft design along with environmental factors.
Competition leads to companies striving for better efficiency in the design without
compromising performance and incorporating new techniques and technology.[5]
Environmental factors

An increase in the number of aircraft also means greater carbon emissions. Environmental
scientists have voiced concern over the main kinds of pollution associated with aircraft, mainly
noise and emissions. Aircraft engines have been historically notorious for creating noise
pollution and the expansion of airways over already congested and polluted cities have drawn
heavy criticism, making it necessary to have environmental policies for aircraft noise.[6][7] Noise
also arises from the airframe, where the airflow directions are changed.[8] Improved noise
regulations have forced designers to create quieter engines and airframes.[9] Emissions from
aircraft include particulates, carbon dioxide (CO2), Sulfur dioxide(SO2), Carbon monoxide (CO),
various oxides of nitrates and unburnt hydrocarbons.[10] To combat the pollution, ICAO set
recommendations in 1981 to control aircraft emissions.[11] Newer, environmentally friendly fuels
have been developed[12] and the use of recyclable materials in manufacturing[13] have helped
reduce the ecological impact due to aircraft. Environmental limitations also affect airfield
compatibility. Airports around the world have been built to suit the topography of the particular

region. Space limitations, pavement design, runway end safety areas and the unique location of
airport are some of the airport factors that influence aircraft design. However changes in aircraft
design also influence airfield design as well, for instance, the recent introduction of new large
aircraft (NLAs) such as the superjumbo Airbus A380, have led to airports worldwide redesigning
their facilities to accommodate its large size and service requirements.[14][15]

The high speeds, fuel tanks, atmospheric conditions at cruise altitudes, natural hazards
(thunderstorms, hail and bird strikes) and human error are some of the many hazards that pose a
threat to air travel.[16][17][18]
Airworthiness is the standard by which aircraft are determined fit to fly.[19] The responsibility for
airworthiness lies with national aviation regulatory bodies, manufacturers, as well as owners and
operators.[citation needed]
The International Civil Aviation Organization sets international standards and recommended
practices for national authorities to base their regulations on [20][21] The national regulatory
authorities set standards for airworthiness, issue certificates to manufacturers and operators and
the standards of personnel training.[22] Every country has its own regulatory body such as the
Federal Aviation Authority in USA, DGCA (Directorate General of Civil Aviation) in India, etc.
The aircraft manufacturer makes sure that the aircraft meets existing design standards, defines
the operating limitations and maintenance schedules and provides support and maintenance
throughout the operational life of the aircraft. The aviation operators include the passenger and
cargo airliners, air forces and owners of private aircraft. They agree to comply with the
regulations set by the regulatory bodies, understand the limitations of the aircraft as specified by
the manufacturer, report defects and assist the manufacturers in keeping up the airworthiness
standards.[citation needed]
Most of the design criticisms these days are built on crashworthiness. Even with the greatest

attention to airworthiness, accidents still occur. Crashworthiness is the qualitative evaluation of
how aircraft survive an accident. The main objective is to protect the passengers or valuable
cargo from the damage caused by an accident. In the case of airliners the stressed skin of the
pressurized fuselage provides this feature, but in the event of a nose or tail impact, large bending
moments build all the way through the fuselage, causing fractures in the shell, causing the
fuselage to break up into smaller sections.[23] So the passenger aircraft are designed in such a way
that seating arrangements are away from areas likely to be intruded in an accident, such as near a
propeller, engine nacelle undercarriage etc.[24] The interior of the cabin is also fitted with safety
features such as oxygen masks that drop down in the event of loss of cabin pressure, lockable
luggage compartments, safety belts, lifejackets, emergency doors and luminous floor strips.
Aircraft are sometimes designed with emergency water landing in mind, for instance the Airbus
A330 has a 'ditching' switch that closes valves and openings beneath the aircraft slowing the
ingress of water.[25]

Design optimization
Aircraft designers normally rough-out the initial design with consideration of all the constraints
on their design. Historically design teams used to be small, usually headed by a Chief Designer
who knows all the design requirements and objectives and coordinated the team accordingly. As
time progressed, the complexity of military and airline aircraft also grew. Modern military and
airline design projects are of such a large scale that, every design aspect is tackled by different

teams and then brought together. In general aviation a large number of light aircraft are designed
and built by amateur hobbyists and enthusiasts.[26]

Computer-aided design of aircraft

The external surfaces of an aircraft modelled in MATLAB

In the early years of aircraft design, designers generally used analytical theory to do the various
engineering calculations that go into the design process along with a lot of experimentation.
These calculations were labour-intensive and time consuming. In the 1940s, several engineers
started looking for ways to automate and simplify the calculation process and many relations and
semi-empirical formulas were developed. Even after simplification, the calculations continued to
be extensive. With the invention of the computer, engineers realized that a majority of the
calculations could be automated, but the lack of design visualization and the huge amount of
experimentation involved kept the field of aircraft design stagnant. With the rise of programming
languages, engineers could now write programs that were tailored to design an aircraft.
Originally this was done with mainframe computers and used low-level programming languages
that required the user to be fluent in the language and know the architecture of the computer.
With the introduction of personal computers, design programs began employing a more userfriendly approach.[27][not in citation given]

Design aspects
The main aspects of aircraft design are:


All aircraft designs involve compromises of these factors to achieve the design mission.[28]

Wing design
See also: Wing configuration

The wings of a fixed wing aircraft provide the necessary lift for take-off and cruise flight. Wing

geometry affects every aspect of an aircraft’s flight. The wing area will usually be dictated by
aircraft performance requirements (e.g. field length) but the overall shape of the planform and
other detail aspects may be influenced by wing layout factors.[29] The wing can be mounted to the
fuselage in high, low and middle positions. The wing design depends on many parameters such
as selection of aspect ratio, taper ratio, sweepback angle, thickness ratio, section profile, washout
and dihedral.[30] The cross-sectional shape of the wing is its airfoil.[31] The construction of the
wing starts with the rib which defines the airfoil shape. Ribs can be made of wood, metal, plastic
or even composites.[32]
Main article: Fuselage

The fuselage is the part of the aircraft that contains the cockpit, passenger cabin or cargo hold.[33]

Aircraft engine
Main article: Aircraft engine

Aircraft propulsion may be achieved by specially designed aircraft engines, adapted auto,
motorcycle or snowmobile engines, electric engines or even human muscle power. The main
parameters of engine design are:[citation needed]

Maximum engine thrust available
Fuel consumption
Engine mass
Engine geometry

The thrust provided by the engine must balance the drag at cruise speed and be greater than the
drag to allow acceleration. The engine requirement varies with the type of aircraft. For instance,
commercial airliners spend more time in cruise speed and need more engine efficiency. Highperformance fighter jets need very high acceleration and therefore have very high thrust

Main article: Aircraft gross weight

The weight of the aircraft is the common factor that links all aspects of aircraft design such as
aerodynamics, structure, and propulsion together. An aircraft's weight is derived from various

factors such as empty weight, payload, useful load, etc. The various weights are used to then
calculate the center of mass of the entire aircraft.[35] The center of mass must fit within the
established limits set by the manufacturer.

The aircraft structure focuses not only on strength, stiffness, durability (fatigue), fracture
toughness, stability, but also on fail-safety, corrosion resistance, maintainability and ease of
manufacturing. The structure must be able to withstand the stresses caused by cabin
pressurization, if fitted, turbulence and engine or rotor vibrations.[36]

Design process and simulation
The design of any aircraft starts out in three phases[37]
Conceptual Design

Conceptual design of a Breguet 673

The first design step, involves sketching a variety of possible aircraft configurations that meet
the required design specifications. By drawing a set of configurations, designers seek to reach the
design configuration that satisfactorily meets all requirements as well as go hand in hand with
factors such as aerodynamics, propulsion, flight performance, structural and control systems.[38]
This is called design optimization. Fundamental aspects such as fuselage shape, wing
configuration and location, engine size and type are all determined at this stage. Constraints to
design like those mentioned above are all taken into account at this stage as well. The final
product is a conceptual layout of the aircraft configuration on paper or computer screen, to be
reviewed by engineers and other designers.

Preliminary design phase

The design configuration arrived at in the conceptual design phase is then tweaked and
remodeled to fit into the design parameters. In this phase, wind tunnel testing and computational
fluid dynamic calculations of the flow field around the aircraft are done. Major structural and
control analysis is also carried out in this phase. Aerodynamic flaws and structural instabilities if
any are corrected and the final design is drawn and finalized. Then after the finalization of the
design lies the key decision with the manufacturer or individual designing it whether to actually
go ahead with the production of the aircraft.[39] At this point several designs, though perfectly
capable of flight and performance, might have been opted out of production due to their being
economically nonviable.
Detail design phase

This phase simply deals with the fabrication aspect of the aircraft to be manufactured. It
determines the number, design and location of ribs, spars, sections and other structural elements.
All aerodynamic, structural, propulsion, control and performance aspects have already been
covered in the preliminary design phase and only the manufacturing remains. Flight simulators
for aircraft are also developed at this stage.


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External links

Aircraft Design : Synthesis and Analysis

Basic principles of Crashworthiness
Basic Construction of Aircraft

A typical study

A typical design and analysis task is described here under. It is obvious that the OAD may
achieve in its entirety or partially, depending on customer needs.

Initial discussion with the customer to understand his needs and expectations
Definition of the general configuration of the aircraft to meet the initial specifications
Analysis of the preliminary cost (market price)
Adjustment of the specifications to minimize the costs
Analysis of the market & the competitor
Design of the optimal configuration (iterative process)
o Total wetted area
o Propulsion, (definition of the propeller characteristics : diameter, pitch anle, …)
o Sizing the lifting surfaces (airfoil selection)
o Sizing the high lift devices
o Sizing the landing gear
o Weight analysis
o Longitudinal stability analysis, lateral and directional analysis
o Calculation of the stability derivatives
o Calculation of the CG position
o Calculation of the CG range
o Sizing the control surfaces
o Calculation of the performances (cruise, best range and endurance, stall, climb,
takeoff and landing) for different wing loading.
o Check the accordance with the selected airworthiness requirements
o Calculation of the moment of inertia for different flight weights
o Calculation of the dynamic stability
Design and integration of the different systems
o Propulsion
o Landing gear

Electrical system
Hydraulic system
Control system
Fuel system
Validation by comparison with existing aircraft and virtual flight on a flight simulator
Detailed analysis of the manufacturing process
Detailed cost analysis, design, manufacturing and operational.
Generating a 3D Model of the aircraft
Load analysis according to the selected airworthiness requirement
Load analysis & structural design
Wind tunnel testing
Load testing
Detailed drawings
Prototype manufacturing
Flight test program
Optimization of the flight characteristics of the airplane
Accompanying during the certification process
Tooling design and manufacturing

Comprehensive study
Thanks to our extensive network of partners, OAD can offer a high quality comprehensive
service. We can help with the whole project, from defining the specifications to the flight tests
for the prototype:

Flight test