Aerospace Structures: Chapter 1 (Introduction)

8/22/2019 Aerospace Structures: Chapter 1 (Introduction)

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CHAPTER 1

INTRODUCTION

1.1 Loads on Aircraft and Spacecraft

1.2 Design Loads

1.3 Aerospace Materials

1.4 Description of Aircraft Structures

1.5 Description of Launch Vehicles and Spacecraft Structures

(To be discussed in class)


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1.1 Loads on Aircraft and Spacecraft

Loads on aircraft

Flight loads

- Maneuver- Gust- Buffet- Flutter- Pressurization

Power plant

- Thrust- Torque

Takeoff and landing

- Catapult- One wheel- Arrested- Braking

Ground operation

- Taxing (bumps and turning)- Towing

Example: Aircraft in pull-up maneuver

M: vehicle mass2V

aR

: acceleration

V : velocity R : radius of circular path

Newtons second law:

WLMa

)( gaMMgMaWMaL o

L

a

W


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Substructure Loads

Fuselage Maneuver, Braking, Pressurization

Wing Maneuver, Gust Loads,

Vertical Tail Yaw Maneuver, Lateral Gust

Horizontal Stabilizer Pitch Maneuver, Vertical Gust

Control Surfaces Max Control Deflection

Engine Pylon Thrust, Vibration, Reverse Thrust

Main Landing Gear One Wheel Landing, Crash Landing

Nose Gear Landing, Taxi, Towing

Loads on rockets and spacecraft

Axial load due to acceleration

Shear force and moment due to aerodynamic loads

Wind load gust at low altitude and jet stream at 30,000 to 40,000 ft

Dynamic loads (that varies rapidly with time) during launch cause structural vibration

that induces additional stress in the structure.

The main sources of dynamic loads are:Acoustic - Rocket engine gases passing through the nozzle at high velocity mix

turbulently with the surrounding air.

Shock - engine ignition, engine shutdown, staging, pyrotechnic devices.

Additional sources of vibration are engine pulsation due to uneven burning,turbine vibration, fuel sloshing and control forces.

Loads in orbit at zero gravity the loading environment is benign and the spacecraft inorbit can be of lightweight and flexible.

Reentry vehicle aerodynamic loads and heating during reentry


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1.2 Design Loads

Each part of an aerospace structure is designed based on the greatest loads acting on that

part.

Limit load: The largest load that a structure is expected to experience during its lifetime.

Safety factor: A safety factor (SF) is specified to account for uncertainties in material

properties, loading environments etc. 1.5 for inhabited craft and 1.25 formissiles.

Ultimate load (or design load) = Limit load x SF

A structure is designed such that the ultimate strength (or the failure load) of the structureis equal to or slightly above the ultimate load.

Ultimate margin of safety =ultimate strength ultimate load

ultimate load


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Example:

A 600-lb payload is mounted in the upper stage of a launch vehicle. During the boostedvertical flight phase, a peak acceleration of 9gis reached. The payload is mated to the

booster by four bolts loaded in shear, each of which has shear strength of 2,126 lb. Thespecified factor of safety is 1.25.

Determine

(a) the limit load per bolt,(b)the ultimate load per bolt, and(c) the ultimate margin of safety.Solution:

(a)

a : acceleration

F: Total (shear) force from the bolts to thepayload

Newtons second law

ma F mg

( )F ma mg m a g

max 9a g

? u

u

F m g m gW

gg

gg lb

( ) ( )

,

9 1 10 10

60010 6 000

Limit shear load per bolt: 6 000 4 1500, , lb

(b) Ultimate load per bolt: 1500 125 1 875, . ,u lb

(c) Ultimate margin of safety: ( , , ) , .2 126 1 875 1 875 0 134

payload

F

mg

a

payload nose cone


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1.3 Aerospace Materials

Typical materials are aluminum, titanium, steel alloys and composite materials. Important

considerations in the selection of structural materials are specific stiffness (i.e. Young's

modulus per density), specific strength (ultimate strength per density), fatigue resistance,damage tolerance, corrosion, high temperature property, cost etc.

Table: Specific stiffness and strength & other material properties

material

specific

stiffness( )E U

(m2/s2)x106

specific

strength( )V Uu

(m2/s2)x103

elastic or

Youngs

modulusE(GPa)

density

U

(g/cm3)

tensile

yield stress

Vy(MPa)

tensile

ultimate

stress Vu

(MPa)

Aluminum

2024-T3

7075-T6

25.9

25.54

161.5

193.5

72

71

2.78

2.78

324

490

449

538

TitaniumTi-6Al-4V 24.66 207.4 110 4.46 869 925

SteelAISI4340

300M

25.64

25.64

229.5

238.5

200

200

7.8

7.8

1483

1520

1790

1860

*Values from Marks Handbook

Note:(1)Steel is susceptible to corrosion.(2)Titanium is used where temperature is high e.g. the leading edge of supersonic

aircraft wing. Titanium costs more than Aluminum.

(3)Composite materials will be discussed later.


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1.4 Description of Aircraft Structures

1.4.1 Wing and Tail Structures

A wing consists of skin, spars and ribs.

Skin and spars form a closed wing box to carry bending and torsional loads.Ribs maintain airfoil shape.

Stiffeners are used to stiffen skins and provide additional bending stiffnessHorizontal and vertical tail structures are similar to wing structure.

The leading edge part of a wing section consists of a slat and the actuators.

The trailing edge part of a wing section consists of flaps and ailerons and the actuators.

The wing box occupies 40 50 % of the wing chord.

.


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Wing-to-fuselage

bulkhead frameCarry-through section

Wing

High wingLow wing

Mid-wing

Wing

Carry-through section

The left part and the right part of the wing are joined via a carry-through or ring frames

The loads (force and moment) from the wing are transferred to the fuselage through thebulkheads.


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Variable-incidence mount

Aft

fuselage Bulkhead

Hinges

Jackscrew

Horizontal tail

Aft fuselage

Hinges

All-moving tail (flying tail): transport

Hydraulics

Horizontal tail


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Aft fuselage BulkheadRight tail

Left tail

TaileronRight tail

Left tail

.

Flying tail

.

Joint

Vertical tail box

Bulkhead

Flying tail or taileron mount: fighter


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1.4.2 Fuselage Structures

Frames maintain the cross-sectional shape of the fuselage.

Stringers, longerons and stiffeners carry axial loadsSkins carry shear stresses

Frames also provide supports to prevent stringers or longerons from prematurely

buckling

Semi-monocoque structures refer to shells or skins that are reinforced with stiffenersand/or spars. Fuselage or wing can be considered semi-monocoque structures.

frameskin

stiffener or stringer or

longeron


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A B

P (Load due to moment)P

Keel beam

A B

A B

P (Load due to moment)P

Keel beam

A B

Section A-A

Keel beamWing

Fairing

Keel beamMain landing

gear

Fuselage

bulkhead

Section B-B

Fuselage

bulkhead

Section A-A

Keel beamWing

Fairing

Keel beamMain landing

gear

Fuselage

bulkhead

Section B-B

Fuselage

bulkhead

The wing carrythrough and wheel wells introduce discontinuities in the fuselage structureand a loss of bending stiffness.

The keel beam is introduced to compensate for the lost bending stiffness.

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Aerospace Structures: Chapter 1 (Introduction)