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

 

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, Wland= 0.85 Wtake-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

 

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.