Design Requirements and Objectives

One of the first steps in airplane design is the establishment of design requirements and objectives. These are used to formally document the project goals, ensure that the final design meets the requirements, and to aid in future product development. The specific DR&O's are based on customer requirements, certification requirements, and company policy (often in the form of a design standards manual). They have evolved from rather simple letters to very complex system engineering documents.

Early aircraft were developed in response to very simple requirements as demonstrated by the Army's contract with the Wright brothers. The agreement shown below requests one (1) heavier than air flying machine to be delivered in 6 1/2 months -- although even then fine print was included in the Signal Corps Specification Number 486. (Click on the image below for a readable version.)

Twenty five years later, a letter from Transcontinental and Western Air brought about the birth of the DC-1 through a page list of specifications shown below.

Today, complex sets of requirements and objectives include specification of airplane performance, safety, reliability and maintainability, subsystems properties and performance, and others. Some of these are illustrated in the table below, based on a Boeing chart

Transport Aircraft Design Objectives and Constraints
Issue Civil Military

Dominant design criteria Economics and safety Mission accomplishment and survivability

Performance Maximum economic cruise
Minimum off-design penalty in wing design
Adequate range and response
Overall mission accomplishment

Airfield environment Moderate-to-long runways
Paved runway

High -level ATC and landing aides

Adequate space for ground maneuver and parking
Short-to-moderate runways
All types of runway surfaces

Often spartan ATC, etc.

Limited space available

System complexity and mechanical design Low maintenance- economic issue
Low system cost

Safety and reliability

Long service life
Low maintenance- availability issue
Acceptable system cost

Reliability and survivability

Damage tolerance

Government regulations and community acceptance Must be certifiable (FAA, etc.)
Safety oriented

Low noise mandatory
Military standards
--Performance and safety --Reliability oriented

Low noise desirable

--Good neighbor in peace --Dectability in war

**Transport Aircraft Design Objectives and Constraints**
Issue	Civil	Military
Dominant design criteria	Economics and safety	Mission accomplishment and survivability
Performance	Maximum economic cruise Minimum off-design penalty in wing design	Adequate range and response Overall mission accomplishment
Airfield environment	Moderate-to-long runways Paved runway High -level ATC and landing aides Adequate space for ground maneuver and parking	Short-to-moderate runways All types of runway surfaces Often spartan ATC, etc. Limited space available
System complexity and mechanical design	Low maintenance- economic issue Low system cost Safety and reliability Long service life	Low maintenance- availability issue Acceptable system cost Reliability and survivability Damage tolerance
Government regulations and community acceptance	Must be certifiable (FAA, etc.) Safety oriented Low noise mandatory	Military standards --Performance and safety --Reliability oriented Low noise desirable --Good neighbor in peace --Dectability in war

A list of some of the typical high-level design requirements for an example supersonic transport study project are given in the table below.

Design Requirements for a Transpacific Supersonic Transport
Payload 300 passengers at 175 lbs. and 40 lbs. of baggage each.

Crew 2 pilots and 10 flight attendants at 175 lbs. and 30 lbs. of baggage each.

Range Design range of 5,500 nm, followed by a 30 min. loiter

Cruise Mach 2.5 at 65,000 ft. Outbound and inbound subsonic cruise legs at Mach 0.95, 45,000 ft

Take-off and Landing FAR 25 field length of 12,000 ft. Standard days, Wl_and= 0.85 W_take-off

Fuel JP-4

Materials Advanced aluminum where applicable

Themal Protection As required, rely on passive systems when feasible, use active systems only when necessary

Certification Base FAR 25, FAR 36 (noise requirements)

**Design Requirements for a Transpacific Supersonic Transport**
Payload	300 passengers at 175 lbs. and 40 lbs. of baggage each.
Crew	2 pilots and 10 flight attendants at 175 lbs. and 30 lbs. of baggage each.
Range	Design range of 5,500 nm, followed by a 30 min. loiter
Cruise	Mach 2.5 at 65,000 ft. Outbound and inbound subsonic cruise legs at Mach 0.95, 45,000 ft
Take-off and Landing	FAR 25 field length of 12,000 ft. Standard days, Wl_and= 0.85 W_take-off
Fuel	JP-4
Materials	Advanced aluminum where applicable
Themal Protection	As required, rely on passive systems when feasible, use active systems only when necessary
Certification Base	FAR 25, FAR 36 (noise requirements)

Many of the design requirements are specified by the relevant Federal Air Regulations (FAR's) in the U.S. or the Joint Airworthiness Requirements (JAR's) in Europe. These regulations are divided into portions that apply to commercial aircraft, general aviation, sailplanes, and even ultralight aircraft. The applicable regulations for aircraft with which we will be dealing depend on the aircraft category and are grouped as described in the tables below:

Aircraft Categories
Characteristic General Aviation Normal Transport

Maximum takeoff weight, lb <12,500 <12,500 Unrestricted

Number of engines > 0 > 1 > 1

Type of engine All Propeller Only All

Minimum crew:
Flight crew
Cabin attendants One
None Two
None for < 20 pax Two
None for < 10 pax

Maximum number of occupants 10 23 Unrestricted

Maximum operating altitude, ft 25,000 25,000 Unrestricted

**Aircraft Categories**
Characteristic	General Aviation	Normal	Transport
Maximum takeoff weight, lb	<12,500	<12,500	Unrestricted
Number of engines	> 0	> 1	> 1
Type of engine	All	Propeller Only	All
Minimum crew: Flight crew Cabin attendants	One None	Two None for < 20 pax	Two None for < 10 pax
Maximum number of occupants	10	23	Unrestricted
Maximum operating altitude, ft	25,000	25,000	Unrestricted

FAR Applicability
Regulations Covering: General Aviation Normal Transport

Airplane airworthiness standards Part 23 Part 23 Part 25

Engine airworthiness standards Part 33 Part 33 Part 33

Propeller airworthiness standards Part 35 Part 35 Part 35

Noise Part 36
Appendix F Part 36
Appendix F Part 36

General operation and flight rules Part 91 Part 91 Part 91

Large aircraft / airline operation -- -- Part 121

Agricultural / Travel clubs / Air taxi Part 137 Part 135 Part 123

**FAR Applicability**
Regulations Covering:	General Aviation	Normal	Transport
Airplane airworthiness standards	Part 23	Part 23	Part 25
Engine airworthiness standards	Part 33	Part 33	Part 33
Propeller airworthiness standards	Part 35	Part 35	Part 35
Noise	Part 36 Appendix F	Part 36 Appendix F	Part 36
General operation and flight rules	Part 91	Part 91	Part 91
Large aircraft / airline operation	--	--	Part 121
Agricultural / Travel clubs / Air taxi	Part 137	Part 135	Part 123

In addition to the regulatory requirements, the primary airplane design objectives include a specification of the number of passengers or cargo capability, target cruise speeds, and ranges. These are often established by extensive marketing studies of target city pairs, current market coverage and growth trends, and customer input.