introduction to aircraft design - 2

8/3/2019 Introduction to Aircraft Design - 2

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Introduction to Aircraft DesignIntroduction to Aircraft Design

Airworthiness RequirementsAirworthiness Requirements

--

T. G. A. SimhaT. G. A. Simha


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Excerpts from Structural Load Analysis for Commercial Transport Aircraft Theory

and Practice

Ted L. Lomax.

The Elements of Aircraft Preliminary Design

Roger D. Schaufele.

FAR PART 25.

References


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Contents:

Introduction to Airworthiness Requirements and Loads

Introduction

Authority and Regulations

Compliance

FAR PART 25

Weights

Aircraft Performance

Structures

Requirements

Flight Loads

Ground Loads

Design and Construction

Systems Requirement


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Background

Pioneers built and flew airplanes.

Growth in aircraft utilization.

Accidents and lessons learnt.

Evolution of Airworthiness Requirements.


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Objective

To ensure safety when the aircraft is used as intended


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The Authority

The Regulatory authority is a government body.

Civil Airplanes

Ministry of Aviation

Military Airplanes

Ministry of Defence

Civil Aviation:

U.S.

Federal Aviation Administration.

INDIA

Director General of Civil Aviation.


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Regulations

U.S.

FEDERAL AVIATION REGULATIONS (FAR)

FAR PART 23

Normal, Utility, Acrobatic, Commuter

FAR PART 25

Transport

FAR PART 33

Engines

FAR PART 35 Propellers

FAR PART 36

Noise

FAR PART 91

General operating and flight rules

FAR PART 121

Domestic and Flag commercial operators of large A/C

FAR PART 123 Air Travel clubsFAR PART 135 Air TaxiFAR PART 137 Agricultural A/C


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Regulations

EUROPE

Joint Airworthiness Requirement (JAR)

MILITARY REQUIREMENTS

U.S.

MIL-A-8860 and a host of specification

U.K.

Def-Stan-970


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FAR Part 23 Applicability

CATEGORY Normal Utility Acrobatic Commuter

No. of Pax


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Proof of Compliance (General)

Demonstrate requirements are met by

Test ( Static and flight tests )

Approved Analysis methods

Systematic Investigation


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Mandatory Requirements:

E.g.

Max load factor

Emergency provisions etc.

Optional:

E.g.

No. of Passengers

No. of Engines

A/C configuration etc.

General Methodology for compliance:Define operating limits and envelops

Demonstrate compliance at all critical points of the envelope

Demonstrate that aircraft flies within the envelope

Proof of Compliance (General)


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FAR Part 25

Subpart

A : GENERAL

Subpart

B : Flight

Subpart

C : Structure

Subpart

D : Design and Construction

Subpart

E : Power plant

Subpart

F : Equipment

Subpart

G : Operating Limitations and Information


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Weight and Center of Gravity Limits

Compliance must be demonstrated at all conditions within.

Maximum Weight

Minimum Weight

Extreme Center of Gravity limits

Weight definitions:

MTW

Maximum Taxi Gross Weight

MTOW Maximum Take off Gross Weight

MLW

Maximum Landing Weight

MZFW

Maximum Zero Fuel Weight

OEW Operating Empty Weight

MEW

Manufacture Empty Weight


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Weight and Center of Gravity - Envelope


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Relationship between Design Airspeeds and Operational Envelope


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Aircraft Performance - Definitions

Stall speed

CLmax

definition.

Take off

Smooth, wet, dry and hard runways.

Head wind and tail wind.

Take off speeds

Definitions.Safe speed for take off.

Take off speed with critical engine failed etc.

Speed to provide adequate climb rate.

Accelerate stop distance

Runway condition.

Brake conditions, friction parameters.

Climb and take off path

Climb in landing configuration.

Climb with critical engine failure.

Landing distance

50 ft. above to complete halt.

head and tail winds, runway conditions etc.


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Summary of Air Speeds - Definitions

VS

Stalling Speed.

VA

Maneuver speed or full control deflection speed.

VB

Design speed for max gust intensity.

VFE

Design flaps extended speed.

VLE

Design Landing gear extended speed.

VLO

Design Landing gear operating speed.

VC

Design cruise speed.

VMD

Max operating limit speed.

VFC

Maximum speed meeting flight characteristic requirements.

VD

Design dive speed greater than VC/0.8 or speed reached in 7.5 deg dive for

20sec followed by 1.5g recovery.

VEF

Speed at which critical engine is assumed to fail.

VMC

Speed at which it is possible to maintain control of Aircraft after the critical

engine is made inoperative.


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Accelerate Stop Distance

Accelerate with full power from start to VEF.

At VEF,

assume critical engine fails.

Accelerate to maximum reject take off speed.

Come to a full stop.Conditions:-

Dry Runway

-

Wet Runway

-

Specified brake friction


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Take Off Field Length

The take off field length is greater of

The all engine take off multiplied by 1.15..

Take off distance with critical engine failed at most critical point.

Accelerate to VEF

where engine fails.

Continue and take off and climb to 35 feet.


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Climb Gradient Requirements

All engines operative

Aircraft with landing gear and flaps extended.

Climb gradient > 3.2 %.

With one engine operative

For 2 engine aircraft greater than or equal to 2.4 %




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Landing Distance

Distance necessary to come to a complete halt.

Airplane in landing configuration

Speed VREF

at 50ft above the ground

Head wind and tail wind conditions


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Controllability and Maneuverability

Airplane must be safely controllable and maneuverable

During the entire flight take off -

Landing

Transition from one flight condition to another smoothly.

Not to exceed limit load factor.

One critical engine failed.

Two engines failed for aircraft with 3 or more engines.

Flaps and undercarriages are deployed.


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Pilot Forces

Force, in pounds, applied to the control wheel or rudder pedals Pitch Roll Yaw

For short term application for pitch and roll controltwo hands available for

control

75 50

For short term application for pitch and roll controlone hand available for

control

50 25

For short term application for yaw control 150

For long term application 10 5 20


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Maneuverability

Configuration Speed

Maneuvering bank angle in

a coordinated turn Thrust power setting

Takeoff V2 30 Asymmetric WAT-Limited.1

Takeoff V2 + XX 40 All-engines-operating climb.3

En route VFTO 40 Asymmetric WAT-Limited.1

Landing VREF 40 Symmetric for 3

flight path angle.

Maneuver at constant speed without stall at most forward C.G.


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Longitudinal Control

To pitch aircraft nose down to accelerate to trim speed.

Landing gear extended.

Flaps retracted and extended.

Power off and max power.

Operating forces within limits with rapid deployment of flaps

Extend and Retract.


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Directional and Lateral Control

Directional Control

-

Yaw into operative engine.

-

Sudden change of yaw 150

direction of inoperative engine.

-

Pedal force not to exceed 150 lbs.

-

Most unfavorable C.G., a/c retracted, flaps extended.

Lateral Control

-

Possible to make 200

banked turn

-

Towards and against inoperative engine.

-

Critical engine inoperative and others on full power.

-

Landing gear retracted and extended.

-

Flaps deployed for climb position.

-

To provide adequate roll rate with one engine inoperative.


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Stability Requirements

Static stability

Longitudinal

Lateral and directional

Dynamic Stability

Short period oscillations

Dutch roll-stability

Conditions are specified for demonstration of stability


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Structures - Requirements

GENERAL:

LIMIT LOAD

Max load expected in service

ULTIMATE LOAD (DESIGN LOAD)

Limit load *Factor of safety

Factor of safety 1.5 ( Special cases >1.5 )

Flexibility effects on loads

Equilibrium Inertia forces

Validation of load distribution


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Strength and Deformation

At Limit Load

No detrimental permanent deformation (safe operation).

At Ultimate Load

Support ultimate load for 3 seconds.

Effects of rate of loading

Transient stresses.

Vibration and buffeting

Structural vibrations up to Vc due to malfunction of control system.

PROOF OF STRUCTURES

Static or dynamic tests

Analysis

Similar structures-Proven methods


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Loads Specification

Loads are specified as:

Flight loads

Ground / Water loads


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Landing Case

Level Landing

Tail down Landing

One gear Landing

Ground handling

Taxi, Take Off, Landing roll

Braked roll

Turning

Tails/nose wheel steering

Pivoting

Towing

Jacking & Tie Down


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Ground Loads

Towing Loads

-

0.3W to 0.15 W (W = 30,000 lbs to 100,000 lbs)

-

Load applied horizontally and swiveled 45 deg

Jacking

-

Vertical Load 1.33 W

-

Horizontal Load 0.33V


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Emergency Landing Conditions

Ultimate Inertia forces acting independently

-

Upward 3.0g

-

Forward 9.0g

-

Side -

3.0g on the airframe and 4.0g on the seat

-

Downward 6.0g

-

Rearward 1.5g

Deformation should not impede subsequent evacuation

Seat and safety belts to withstand above forces

Passenger weight 170 lbs


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Emergency Landing Test Demonstration

30o

downward, downward velocity change 35ft/sec, peak floor deceleration 14g

Yawed 10o

with change in vertical velocity of 44 ft/sec, peak floor deceleration 16g.

Upper torso straps

force not to exceed 1750 lbs.

Max compressive load between pelvis and lumbar region 1500 lbs.


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Fatigue and Damage Tolerance

Damage may be due to fatigue, corrosion, manufacturing defect or

accidental.

Evaluation to demonstrate no catastrophic failure during operational life of the

airplane

-

Typical spectra including temperature and humidity.

-

Identify critical structural elements.

-

Analysis based on service history.

-

Establish Inspection requirements.


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Damage Tolerance

Single Load path structure.

Multiple load path, failsafe structures.

Assume an initial flaw.

Residual strength -

Structure must be capable of withstanding limit loads considered

as ultimate for

-Flight loads

-Ground loads

Fatigue Life -

Where damage tolerance analysis is impractical,

-

must show a crack free fatigue life with scatter factors.

Aircraft must successfully complete the flight-when a 4lbs bird hits at Vc at sea level

-when a 4lbs bird hits at 0.85*Vc at 8000ft.


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Design & Construction


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Design and Construction

Requirements for detail design and provision of various features

are defined

in this part.

General

Materials, accessibility, special factors etc.

Control Surface and System

Installation, Hinges, Stops, Operation Test etc. and System.

Landing Gear

shock absorption tests, wheels and brakes retracting mechanism etc.

Personal and Cargo

Pilot compartment, access doors, window, vision etc.

Emergency Provisions

Emergency Exits, markings, seat belts etc.

Ventilation, Pressurization, Fire protection etc.


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Materials

Approved materials. (Aircraft specifications).

-Effect of temperature and humidity

-A-Basis properties for critical structures (99% probability with 95% confidence)

-B-Basis properties for multiple load path structures (90% probability with 95%confidence)

Fabrication Methods-Approved methods.

-Process monitoring and control.

Fasteners

-Two separate locking devices -

critical fastener.


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Special Factors

Fitting Factor

1.15 applied to

-Fitting

-Attachment

-Bearing

Bearing Factors-Large enough bearing factor to account for clearance fit, vibration and pounding.

Casting Factor-Critical Castings

1.25

-100% inspection visual, radiographic etc.

-3 castings to be tested


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Bird Strike Requirement

Bird strike requirement

-

Aircraft and Engine

4lbs bird at Vc

-

Empennage

8lbs bird at Vc


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Emergency Exits

Type Size Max Passenger per Exit

A 42" X 72" 110

B 32" X 72" 75

C 30" X 48" 55I 24" X 48" 45

II 20" X 44" 40

III 20" X 36" 35

IV 19" X 26" 9

Maximum Seats abreast

An aisle to be provided for every 3 seats.

Aisle width required

15

up to 25

from floor and 20

above that.


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Control System

Control System

-

Provision of Stops

-

Limit Load Static Test

-

Operational Tests, free from jamming, excessive friction etc.

Landing Gear

-

Shock absorption test

10fps drop test at MLW.

12 fps reserve energy drop test.

-

Retracting mechanism: Emergency means for extending.

Seat, Seat belts

-

Pilot seats

Additional factor 1.33.

-

Passenger seats

-

Attendant seats

-

Energy absorbing seats.


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Pressurized Cabins

Cabin altitude not greater than 8000 feet.

Two pressure relief valves and reverse pressure differential valves.

Strength test.

Functional Test.

Proof Pressure Test to 1.5 pressure.

Burst Pressure Test to 2.0 pressure.

Sudden Decompression effects.


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Support E: Power Plant

Engine Installation and associated requirements

Fuel System and fuel tanks

Oil System

Engine induction and exhaust

Engine Controls


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Power Plant

Engine must be approved

corresponding category (Part 34)

Propeller must be approved.

Means for stopping rotation.

Re-start capability.

Propeller clearance -

Minimum 7 inches.

Engine thrust reversing system requirements.

-

Failure conditions

critical.


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Fuel System

To provide required flow to the engine.

Flame out or interruption not more than 20 seconds.

Vent requirements as per part 34.

Functional Test of Fuel System.

Lightening Protection

Fuel Tanks

-

Pressure test to 3.5psi

Max pressure from inertial force.

-

Vibration test

-

Expansion space of 2% minimum.


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Support F: Equipment

Instruments

Electrical System

Lights

Safety equipments

Miscellaneous equipments

-

Vacuum

-

Hydraulic

-

Pressurization

-

Oxygen

-

Etc.


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Instruments

Arrangement and visibility requirements

Warning caution and advisory lights

Flight and navigation instruments

Power plant instruments

Miscellaneous


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Flight and Navigation Instruments

Common

-

Free air temperature indicator

-

Clock

-

Direction indicator (Magnetic compass)

At each pilot station

-

An air speed indicator

-

Altimeter

-

Rate of climb indicator

-

Rate of turn indicator

-

A bank and pitch indicator

-

A Mach meter


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Other Systems

Electrical Systems

-

Generating system requirements

-

Installation

Interference with other systems.

Cable routing to minimize hazard

Proper installation of batteries (venting, sealing etc.)

-

Redundancy

Lights

-

Requirement for instrument lights

-

Position lights

Forward Left

RED (Wing Tip)

Forward Right

GREEN (Wing Tip)

Rear position

White (Fin Tip)

-

Landing Light

-

Anti-collision lights


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Miscellaneous Equipments

Miscellaneous Equipments

-

Ditching Equipment

-

Public address system

-

Vacuum systems

-

Hydraulic System

Ultimate pressure = 3 x Operating pressure

High Pressure accumulator and hoses

4 x Operating pressure

-

Oxygen System

-

Cockpit voice recorder

-

Flight recorders

introduction to aircraft design - 2

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