[03] STRUCTURES AND DESIGN TERMS

Airplane Design

is the intellectual engineering process of creating on paper (or on a computer screen) a flying machine to

(1) meet certain specifications and requirements

established by potential users (or as perceived by the

manufacturer) and/or

(2) pioneer innovative, new ideas and technology.

Phases of Aircraft Design

Conceptual Design

Preliminary Design

Detail Design

Structural System

Any deformable solid body which is capable of carrying loads and transmitting these loads to other parts of the body

Bar elements

Are one-dimensional structural members which are capable of carrying and transmitting bending, shearing, torsional, and axial loads or a combination of all four

Axial Rod or Two Force Members

Bars which are capable of carrying only axial loads are referred to as

Trusses

Structural systems constructed entirely out of axial rods are called ______

Plate elements

Are two dimensional extensions of bar elements.

Membranes

Plates made to carry only in-plane axial loads are ________.

Shear Panels

Plate which are capable of carrying only in plane shearing loads

Shell elements

Are curved plate elements which occupy a space. Fuselages, building domes, pressure vessels, etc., are typical examples

Surface Loads

Those loads that are produced by surface contact. Example are dynamic and static pressure.

Body Loads

Loads which depends on body volume. Example are inertial forces, magnetic and gravitational forces.

Dynamic Loads

These are time dependent whereas static loads loads are independent

Thermal loads

Are created on a restrained structure by a uniform and/or nonuniform temperature change.

Bending moment

may be defined as a force whose vector representation lies in and parallel to the plane of the cut.

Torque

is a force whose vector representation is normal to that cut.

Shear load

is a force which lies in and is parallel to the plane of the cut

Axial loads

is a force which acts normal to the plane of the cut

Statically Determinate Structure

All of its external reactions and internal loads on its members can be obtained by utilizing only static equations of equilibrium.

Statically Indeterminate Structure

REDUNDANT STRUCTURE

A structure or body which is over-constrained such that there are more unknown supports than there are equations of static equilibrium.

Limit loads/ Applied loads

the limit loads used by civil agencies or applied loads used by military agencies are the maximum anticipated loads in the entire service life-span of the vehicle.

Ultimate loads

Commonly referred to as design loads, are the limit loads multiplied by a factor of safety (FS)

Factor of Safety (FS)

Ultimate load / Limit load

Limit load factor

is a factor by which basic loads on a vehicle are multiplied to obtain the limit loads.

Ultimate load factor

is a factor by which basic vehicle loads are multiplied to obtain the ultimate loads; in other words, it is the product of the limit load factor and factor of safety

Positive High Angle of Attack (PHAA) condition

Obtained in a pullout at the highest possible angle of attack on the wing. The lift and drag forces are perpendicular and parallel respectively, to the relative wind, which is shown as horizontal.

Positive Low Angle of Attack (PLAA) condition

the wing has the smallest possible angle of attack at which the lift corresponding to the limit-load factor may be developed. For a given lift on the wing, the angle of attack decreases as the indicated airspeed increases, and consequently the _________ condition corresponds to the maximum indicated airspeed at which the airplane will dive.

This condition represents an upward acceleration at its design gliding speed Vg

Negative High Angle of Attack (NHAA) condition

Occurs in intentional flight maneuvers in which the air loads on the wing are down or when the airplane strikes sudden downdrafts while in level flight.

In this condition usually the wing is assumed to be at the negative stalling angle of attack for steady flow conditions.

Negative Low Angle of Attack (NLAA) condition

Occurs at the diving-speed limit of the airplane. This condition may occur in an intentional maneuver producing a negative load factor or in a negative gust condition.

This condition allows for the effect of a sudden decrease in angle of attack while flying at the speed of Vg

Primary Structure

• A critical load-bearing structure on an aircraft

• If this structure is severely damaged, the aircraft

cannot fly

Secondary Structure

• Structural elements mainly provide enhanced aerodynamics

• Fairing, for instance, are found where the wing meets the body or a various locations of the leading edge or trailing edge of the wing

Truss Type

a rigid framework made up of members such as beams, struts, and bars to resist deformation by applied loads

Monocoque

• The skin carries all the load

• Unstiffened Shell. Must be relatively thick to resist

bending, compressive, and torsional loads

• Consist of skin and frames/formers/ bulkhead

Semi-Monocoque

Construction with stiffening members that may also be required to diffuse concentrated loads into the cover

• More efficient type of construction that permits much thinner covering shell

Skin

• Reacts the applied torsion and shear forces transmits

aerodynamic forces to the longitudinal and transverse

supporting members

• Acts with longitudinal members in resisting the

applied bending and axial loads

• Acts with the transverse members in reacting the

hoop, or circumferential, load when the structure is

pressurized

Spar

Resist Bending and Axial Loads

Forms the wing box for stable torsion resistance

Stiffener and Stringers

• Resist bending and axial loads along with the skin

• Divide the skin into small panels and thereby increase its buckling and failing stresses

• Act with the skin in resisting axial loads caused by pressurization

Longeron

Main longitudinal member of a fuselage or nacelle

Tie Rod (Tension Rod)

Member taking a tensile load

Strut

Member taking a compression load

Stressed skin

Structure where loads are shared between skin and framework.

Bulkhead

- A partition within the structure. Usually lateral but can be longitudinal. If it forms the boundary of pressurized structure it is called a pressure bulkhead

Crack stopper

A reinforcing member normally placed at right angles to the path of an anticipated crack which will reduce the rate of further propagation

Gussets

A flat sheet triangular in shape used to reinforce the corners of structure.

Keelson/Keel beam

structural element frequently used to carry the fuselage bending loads through the portion of the lower fuselage which is cut up by the wheel wells.

Profile/ Form Drag

Drag produced by the shape of the aircraft.

This type of drag increases as speed increases

Interference Drag

is generated by the mixing of airflow streams between airframe components, such as the wing and the fuselage, or the landing gear strut and the fuselage

Exceedance Drag

Drag produced due to surface roughness. Could be reduce by installing flush fasteners or using flush repair or application of filleting sealants

Frames

Just as for the ribs in the wing structure, the primary function these are:

1. Maintain the shape of the fuselage

2. To sustain concentrated loads imposed

3. Serve as attachments for equipment,

flooring, and the like

4. Transmit the loads to adjacent structural

members

Simple Frames

1. Serve mainly to maintain the shape of the

fuselage

2. These are not subjected to stress unless distortion of the entire adjacent structure has taken place

Intermediate Frames

1. Serve to act as anchorage for medium

weight equipment, control system and the

like.

2. Similar simple frames but must be reinforced

locally to carry the load and reduce

deflection to a minimum

3. Additional brackets may have to be

introduced and tied in with the longitudinal

stingers as well as the frames

4.. These are not subjected to stress unless

distortion of the entire adjacent structure has

taken place

Main Frames

1. To which large external loads are supplied

through the landing gear, powerplant, or

wing structure.

2. These are usually two in number, spaced a

small distance apart and designed so as to

take fittings to serve as carry through

members

3. Act as the main transverse load carrying

member

Stringers

are the main bending elements, they should be continuous and therefore pass through the transverse frame

Wings

is essentially a beam that is subjected to shear, bending, and torsion imposed upon it by aerodynamics and inertia loads.

Mean Aerodynamic Chord

The chord of the equivalent rectangular wing.

An imaginary chord on which all the wing area may be considered concentrated without changing the lift of the moment of the air forces on the airplane

Taper Ratio

The ratio of the tip chord to the root chord.

A wing with taper is a trade-off between elliptical (least induced drag, difficult to manufacture) and a rectangular wing (more induced drag, easy to manufacture).

Lightening Holes

This is a misnomer, for the decrease in weight due to removal of material is secondary

The primary purpose is

1. To increase the in-plane stability by means of the raised lip around the hole

2. To provide a passageway for control systems, wiring, tubing

3. to provide an access in another part of the interior.

Beading

Are used to increase rigidity of an otherwise flat surface

ONE HORSE SHAY

Each part fails at the same time

Stiffeners

Small, relatively short-length angles may be attached to an otherwise large flat sheet to increase its shear carrying ability and to reduce or eliminate any tin-canning tendencies.

three view

consist of top view or plan view

Front View or Front Elevation

Side View or side Elevation

Stringers

Main Bending Element of the structure (Fuselage)

These members are added for greater rigidity and load carrying capability.

Stinger Spacing (Longitudinal members)

6 inches to 12 inches

Sweepback

For aircraft operating at high subsonic speed incorporating this in the wing planform would increase the CRITICAL MACH NUMBER.

Advantage of Sweepback

▪ Delays drag divergence effects

▪ Used for balance

▪ Used for stability (dihedral effect)

▪ Better ride through turbulence characteristics

Disadvantage of Sweepback

▪ Contributes to pitch up characteristics

▪ Performs less during take-off and landing

▪ Reduces subsonic lift

▪ Significant weight penalty

▪ Liable to tip stall

Dihedral

The mounting of wings so that the wingtips and higher than the wingroot.

Increase lateral stability. Angle varies around 3 to 8 degrees

Anhedral

Decrease lateral stability/ Angle varies around 3 to 6 degrees Also known as negative dihedral or drooped wing

Wing Loading (W/S)

▪ Affects [a] take-off and landing field length, [b] cruise

performance (L/D), [c] ride through turbulence, and [d]

weight

▪ For a short field length, a large wing / low wing

loading is required

▪ Wing can be kept small by using flaps

▪ For cruise at (L/D)max, a high wing loading is required

▪ For flight at high altitudes and at low speeds, a large

wing is required.

▪ Of course a large wing means more weight

Low Wing Loading

W/S:

High load Factor

Poor ride qualities

Low Stall Speed

Low L/D max

High Weight

High Wing Loading

W/S:

High Stalling Speed

Long Fieldlength

High L/D max

Good Ride qualities

Low Weight

High Aspect Ratio

AR:

Low induced drag

High lift curve slope

poor ride qualities

high wing weight

large wing span

Low Aspect Ratio

AR:

High induced drag

Low lift curve slope

Good ride qualities

Low wing weight

Small wing span

Thickness Ratio

▪ Most important geometric consideration when selecting and airfoil

▪ Higher thickness ratio, higher profile drag / wave drag

▪ Higher thickness ratio, lower weight

▪ Higher thickness ratio (up to 12-14%), higher Clmax

▪ Higher thickness ratio, greater fuel volume

Washout

tip airfoil has negative incidence relative to root airfoil.

delays tip stall

Large washout

Washout:

High Induced Drag

Good response to tip stall

Mildly lower weight of wing

Wash in

tip airfoil has positive incidence relative to root airfoil.

Geometric Twist

- one type of airfoil used, incidence is changing relative to root chord.

Aerodynamic Twist

Difference in the zero-lift angles of the root and tip airfoil. Same as geometric twist if one type of airfoil is used

Angle of Incidence

The angle between the chordline of the wing where the wing is mounted to the fuselage and a reference axis along the fuselage.

▪ Used to minimize drag at some operating condition, usually cruise.

▪ set the wing at an angel to the longitudinal axis of the fuselage corresponding to the angle at which minimum drag occurs.

▪ Used to improve attitude

▪ Usually at 1-3 degrees

High Wing

▪ Places fuselage closer to the ground; easier

loading/unloading; adapted by cargo aircraft

▪ Sufficient ground clearance for engine nacelle or

propeller; less landing gear height needed

▪ Wing tips less likely to strike the ground

▪ Usually less in weight (Semi-Cantiliver)

▪ A strutted wing usually presents less weight but struts

adds to drag.

▪ Struts for a high wing, that is struts below the wing,

offer less drag compared to struts above the wing

▪ Weight savings for placing wing box at the top; no

fuselage stiffening necessary; however, increased

frontal area adds to drag

▪ Prevents floating (ground effect is reduced) which

makes it hard to land on desired spot

Mid Wing

▪ Needs fuselage stiffening; means more weight

▪ Carry-through structure will limit space for a

passenger or cargo aircraft; difficult to incorporate in a

fighter aircraft in which most of the fuselage is

occupied by the jet engines and inlet ducts

Low Wing

▪ Landing gear can be attached to (and retracted into)

the wing which is already strong with no stiffening

(and no external blisters) necessary

▪ Allows for a shorter landing gear strut which means

less weight; however there still must be enough

ground clearance

▪ Given enough ground clearance, aft-fuselage

▪ upsweep can be reduced, reducing drag

▪ Commonly adapted by large commercial transports

which normally operate in well-equipped airfields;

loading and unloading is not a problem

▪ Ground clearance problems may be alleviated by a

dihedral; but too much dihedral can cause Dutch roll

tendencies.

▪ Placing the propellers higher above the wing

increases interference effects and cruise fuel

consumption.

Gap

The vertical distance between the two wing

Span Ratio

The ratio of the shorter to longer wing

Stagger

The longitudinal offset of the two wings relative to each other (positive, when upper wing is closer to the nose; negative, otherwise)

Decalage

Relative incidence between the two wings (positive, when upper wing has a larger incidence; negative, otherwise)

Sesquiplane

- Biplane that has smaller lower wing than the upper wing

Wing Spar

This is the main support structure of the wing itself.

a member having a relatively large material in the flanges, chords, or caps located at the top and bottom member, with a relatively thin shear web connecting the two.

Forms the wing box for stable torsion resistance

Location of Front Spar

12 - 17 % Chord

Location of Rear Spar

65 - 75 % chord

Usually 70% to accommodate 25% chord aileron

Spar Cap (flange)

These consist of the upper and lower flanges attached to the spar webs.

- carry the bending moment generated by the wing in flight.

The upper spar cap will be loaded in compression and the lower in tension for a positive load factor . The spar caps also form a boundary onto which wing skin is attached and support the wing skin against buckling

Spar Web

Maintains a fixed spacing between the spar caps.

Acts in pure tension and compression

Tension-field beam

- Allows the shear web to wrinkle, and this the transverse shear is resisted by tension in more or less the same way as wire-braced truss.

- Relatively light

Shear resistant Beam

- designed so that the shear web will not wrinkle under limit loads

- In order to increase the shear load that the web can withstand, stiffeners are attached at specified intervals

Ribs

1. to maintain the chordwise shape of the airfoil

2. Act as supports of the wing skin panel or envelope

3. Transmit the pressures on the wing to the spanwise

members

Rib Spacing

6 to 18 inches

Externally braced - Semi-Cantilever

- Reduction of the bending moment imposed produces Lighter structure

- Any gain in wing structure may be offset by the additional supporting structure

- Causes more drag

Internally braced/bolted - Cantilever

- No external brace

- May be heavier due wing junction will carry the bending moment

- Less drag

Wing Tips

Devices are intended to improve the efficiency of fixed-wing aircraft by reducing drag.

Such devices increase the effective aspect ratio of a wing without greatly increasing the wingspan.

Adds to wetted area, and therefore drag ▪ Might be better to just add to the span instead ▪ Solution to a short span requirement