General Loads on Aircraft Structure

 AIRCRAFT STRUCTURE-I (ASEG 331)

Main structural Components and Teir Functions Conventional aircraft usually consist of fuselage, wings and tail plane.  The basic functions of an aircraft's aircraft's structure are to transmit transmit   and resist resist   the applied loads; loads; to provide an aerodynamic shape and to protect passengers, payload, systems, systems, etc. from the environmental environmental   conditions encountered in ight.

!in"# • Spars • Stringers • Ribs • Skin

Main structural Components and Teir Functions Conventional aircraft usually consist of fuselage, wings and tail plane.  The basic functions of an aircraft's aircraft's structure are to transmit transmit   and resist resist   the applied loads; loads; to provide an aerodynamic shape and to protect passengers, payload, systems, systems, etc. from the environmental environmental   conditions encountered in ight.

!in"# • Spars • Stringers • Ribs • Skin

Contd.

S!"R# • $ongitudinal member in the %ing. • &enerally &ene rally %ing having T%o spars called ront ront spar (located at )*+ of %ing chord from leading edge and Rear spar (located at -+ of %ing chord from the leading edge. • &enerally Spar having / cross0section, because / section having ma1imum moment of inertia, hence 2ighest strength, for the same %eight. • Spar %ebs takes Torsional load (i.e. shear stresses and spar anges takes bending loads (i.e. bending stresses.

Contd

.

Stringer: • •

3sed for 4ending loads. &enerally having 5, $, T, channal and small %ings having rectangular cross0sections because of easy attachment to the skin and space and %eight advantage.

Contd

.

RIBS: •

•

•

•

 The dimensions of ribs are governed by their span0%ise location in the %ing (i.e. "irfoil shape and by the loads they are re6uired to support. 3sed for maintain the "irfoil shape through out the %ing section.  They also act %ith the skin in resisting the distributed aerodynamic pressure loads.  They distribute concentration loads (e.g. undercarriage and additional %ing store loads into the structure.

.

Contd

Skin: • •

•

•

 The outer cover of the %ing structure is skin.  The primary function of the %ing skin is to form an impermeable surface for supporting the aerodynamic pressure distribution from %hich the lifting capacity of the %ing is desired. Skin is e7cient for resisting shear and tensile loads. Skin buckles under comparatively lo% compressive loads. Stringers are attached to the skin and ribs thereby dividing the skin into panels and increasing the buckling stresses .

FUSELAGE  The fuselage of any aircraft has T89 main functions# :. Carries the payload# passenger  cargo. !?# •  This type of structure is still in use in many light%eight aircraft using %elded steel tube trusses. • " bo1 truss fuselage structure can also be built out of %ood@often covered %ith ply%ood.

"nt#$. %. &n!'ue stru!ture: it is possible to make a skin strong enough to carry all the loads %ithout the need for any supporting frame%ork. Consists of0 • Skin. • ormers. • 4ulkheads.

.

Contd

(. Simi mn!'ue stru!ture: /n this fuselage structure the skin is used to avoid buckling, it is common for the stress skin to carry about half of the total load carried by the skin and longerons together. the typical fuselage structure consists of series of hoops, or frames at intervals along the skin, %hich gives the fuselage its cross0sectional shape, connected by longerons that run the length of the fuselage. mainly consists of0 • • •

Skin 4ulkheadsA ormers (frames $ongerons#

TAIL )LA*ES  The tail0plane provides stability in !itch  >a%. •

•

$arge "ircraft having cross0section same as %ing structure. Small "ircraft having solid section.

Imprtan!e f stru!tural weight •

•

•

•

•

 The structure of an airplane must %ithstand the applied aerodynamic load and interior loads not only for the normal ight but also for e1treme conditions may be encountered very rarely.  The essential character of an aircraft structure is light %eight, because %eight plays such an important role in the performance and economics of an airplane.  The importance of empty %eight should be clear from the limitations placed on ma1imum takeo= %eight by the available run%ay. " pound more structural %eight is a pound less of payload.  The speciBc range is inversely proportional to the airplane %eight, so in increase in structural %eight raises the fuel consumption and the fuel cost.

Contd.

 The Brst cost of the airplane is generally found to be proportional to the empty %eight. • /f the payload and range cannot be reduced, a higher structural %eight re6uires a larger engine to meet the takeo= and landing re6uirement, thereby raising the structural %eight even further. or all these reason, the aircraft structural design has al%ays sought to meet the load re6uirements %ith a least possible %eight.  The potentially e=ect of an aircraft structural failure means that the structure must be designed for long life either %ith safe life or %ith fail safe #esign . Safe life: safe life means that the stresses in a components are so lo% that fatigue failure is not possible over the life of the airplane. •

Contd.

•

Fail safe:+ fail safe means that the structure has alternate loads paths so that no single failure %ill be e=ected to the aircraft. This can be achieved by designing so that no one component carries a large part of the load. Therefore, if one part fails, the reminder of the structure can still carry most of the ma1imum load.

General la#s n Air!raft 4efore the structural design of an airplane can be made, the e1ternal loads acting on the airplane in ight, landing and takeo= conditions must be kno%n. Limit la#: limit loads are the ma1imum loads anticipated on the airplane during its life time.  The airplane structure shall be capable of supporting the limit loads %ithout su=ering detrimental permanent deformations. Ultimate r #esign la#s: 3ltimate or design loads are e6ual to the limit load multiplied by a factor of safety. /n general the overall factor of safety is :.. •

Contd. •

 The board general category of e1ternal loads on conventional aircraft can be broken do%n into such classiBcations as follo%s# Air la#s:  –  –

ue to "irplane Daneuvers (under the control of the pilot ue to air gust (not under the control of pilot.

Lan#ing la#s:  –  –

$anding on land (friction on tyre $anding on %ater.

)wer plant la#s:  Thrust.  –  Tor6ue.  –

,eight an# Inertia Fr!es:

Contd.

•

,eight:  The term %eight is that constant force, proportional to its mass. 8hich tends to dra% every physical body to%ards the centre of the earth. Inertia Fr!es: Inertia Fr!es fr mtin f pure translatin f rigi# -#y

/f the unbalanced forces acting on a rigid body cause only a change in the magnitude of the velocity of the body, but not in the direction, the motion is called translation and from the basic physics# "ccelerating force  E D a rom the basic physics

Inertia fr!es n rtating rigi# -#ies: • " common airplane maneuver is a motion along a curved path in a plane parallel to the F5 plane of the airplane, and generally referred to the pitching plane. • " pull up from steady ight or a pull out from a dive causes an airplane to follo% a curved path.

/f at point " the velocity is increasing along its path, the airplane is being subGected to t%o accelerations# :. at, tangential to the curve at point " and e6ual in magnitude to a t E r a. •