Aircraft General Knowledge - PPL

Stress

Internal force per unit area (MPa) mega pascal

σ = F/A

A = mm^2

hence N/mm^2

Strain

Deformation caused by the action of stress on a material

ε = △L/L

△L = extension (m)

L= original length (m)

Strain is unite less

Tension & Compression

generated when a force is applied along the cross-section of a material

- pulling force

Load/force must be applied axially for it to be tension

Compressive load occurs when inward force is applied to a body

- both are normal stresses

Shear

a force that deforms the shape of the member while maintaining its length

- results of stretching or compressing two objects that are not exactly in line with each other

Shear stress action is parallel to the direction of the force acting on a plane

Bending

The deformation of a body as a result of the bending moment

- involves a change in the curvature of its axis

Material on inside of bent element is compressed

Outside it is stretched

e.g rods, beams

- they have at least one dimension significantly larger than the others

Buckling

a sudden change of shape under critical load

- can happen before material failure due to exceeding the material strength

e.g rods, shells, aircraft skins

Stresses on aircraft

Struts (wheel holder) are subjected to stress on the ground due to compression and in-flight due to tension

Wings are bent in flight due to aerodynamic forces

Static loads

independant of time, stay constant

- wheels grounded during parking

dynamic loads

dependent on time

e.g short burst of high load experienced by aircraft during bird strike or manoeuvring

- when passing through a turbulent region

cyclic loads

continuous and repeating

- can repeat once every flight = cabin pressurisation

- or up to several times a second = engine vibration

Fatigue

where material strength decreases over time due to cyclic loads

- results in growth of localised structural damage and micro cracks

For extended time:

- structure can fail even under loads lower than the ultimate load

S-N diagram

alternating stress (S) as a function of the number of cycles to failure (N)

- every material has one curve

fatigue development factors

- stress level

- number of cycles

- types of flight manoeurvres

- corrosion = lowers the number of cycles at the same load it can withstand before failure occurs

- level and quality of maintenance = installing crack stoppers and applying protective coatings decreases chance of fatigue failure

steel can withstand lower loads without experiencing fatigue failure

- such stress level is called the endurance limit

influence of maneouvres on fatigue

- high G turns

- abrupt pitch-ups

produce high loads

During cruise, wind gusts are a factor

uniform surface corrosion

transforms metal into chemically stable compounds e.g oxide, hydroxide, sulphide

5 types:

1) Uniform surface attack

- uniformly distributed over the surface of an aircraft

2) Intergranular corrosion

- corrosion along the grain boundaries

- grain boundary separates crystals inside metal crystallite structure

- rare and difficult to detect

- affected part must be replaced

3) Stress corrosion

- found in stress-prone areas e.g landing gear, crankshaft

4) Crevice/deposit corrosion

- when water gets trapped between surfaces

5) Filiform corrosion (Worm-like)

- occurs under the paint; due to poorly prepared surface before painting

- forms bubbling and flaking

Corrosive fluids

- Hydraulic fluid (highly corrosive)

- Coolant

- Engine oil (ester-based)

- Defrosting fluid (glycol-based)

can damage the protective coating

- spillage of refreshment can affect floor structure over time

stress prone areas

landing gear

- pilot must adhere to the descent rate limit during landing to not damage it

propeller

unit of strain

unitless

aluminium, steel, titanium

most used materials in aircraft

Aluminium

- airframe construction

- good strength to weight ratio

- good formability

- low cost

- wing and fuselage components, aircraft skins, control surfaces

Titanium

- for parts exposed to high temps

Aluminium

- used for structural elements e.g ribs and spars

composite materials

materials made of 2 elements

- can have different properties than those of original elements

- differ from alloys = elements stay separate instead of mixing like in alloys

- consist o bulk material called the matrix, and a reinforcing material

used in modern smaller aircraft

Carbon Fibre reinforced polymer (CFRP) used in aircraft skin and control surfaces

wing configurations

1) High-mounted wing

- better flight stability in flight

- limits pilot's upward visibility

2) Low-mounted wing

- good upward visibility

- ground effect reduces the take-off distance

- extend landing distance

tailplane configurations

1) Low-mounted tail

- tail attached to fuselage

2) Mid-mounted tail

- tail attached to fuselage

CONS:

- affects behaviour in a spin = shields the rudder from the airflow, the tailplane creates a dead air region

- rudder authority is substantially decreased; resulting much harder to exit the spin

3) T-tail

- tail attached to vertical stabiliser

- allows tailplane to stay free of disturbed air coming from the wings

- increases pitch authority and decreases drag

CONS:

- requires much stronger vertical stabiliser

- occurrence of deep stall in aeroplanes with T-tail

- at high AOA, tailplane is in the stream of highly disturbed air coming off from the wing = limits control authority

wing structural components

as the wings experience high loads throughout the flight, their structure needs to be strong and durable

- main load is bending caused by the lift force

Structural components

Spars - absorbs bending moment caused by the lift force

- consists of 2 main components: web of the spar, top and bottom caps of the spar (girders)

- most load is absorbed by gaps; web allows the spar to maintain its shape

Skin - takes loads generated by the pressure differences or by the fuel in the fuel tanks if those are present

- directly interacts with airflow

- skin has to be smooth

Stringers - spanwise components of the wing structure, giving wing additional rigidity by supporting the skin

Ribs - maintains airfoil shape of wing

- supports the skin and all structural elements against buckling

- designed with cutouts to make them lighter and stiffer

- every wing consists of multiple ribs

- reinforced ribs are used to pass the concentrated loads from landing gear, engines and control surfaces to spars and into the skin

torsion box

lightweight structure absorbs high torsion and bending loads generated by the wings

flutter

a vibration that can affect both fixed surfaces and control surfaces of plane

- aeroelastic vibration

- caused by interaction of aerodynamic forces, inertia forces, properties of the structure

- destruction of plane

resonance

if plane subjected to a gust, wings will shake up and down

- cause wings to flap at a certain frequency

If frequency is similar to that of the structure of the wings, resulting resonance can overt stress the wing = failure of structure

bending flexibility

reduces loads at the attachments of the wings and control surfaces

- stabilises the wing, increasing its resistance to flutter

A stiff wing increases the stress in its attachments, leading to damage under resonance

plane design must ensure balance between flexibility and stiffness

torsional flexibility

torsionally flexible wing will also stabilise itself against vibrations

- increases stress in its attachments

- more susceptible to flutter

fuselage cross-section shapes

rectangular & circular

rectangular:

- simple and cheap

- used only in unpressurised planes

- low strength to weight ratio

circular:

- even spread of hoop stresses

- simple to manufacture

- inefficient use of volume

fuselage construction types

monocoque

- loads are supported by the skin

- internal frames give structure to the required shape

- any damage to skin weakens whole structure; all openings in skin need to be additionally reinforced

semi-monocoque

- not strong enough for increased loads in larger planes

- additional load-bearing components (stringers) need to be added to structure to strengthen it

- skin it attached to the frames/formers and stringers by riveting or with adhesive

truss

- rigid framework made up of beams, struts, and bars to resist deformation by applied loads

- used in small/lightweight aircraft

- made of welded steel tubes

- skins made of steel or aluminium alloy sheets, or fabric used to cover the structure

stringers, longerons, stiffeners

stringers

- strengthen skin by stiffening it and carrying loads along their length

- short stringers called stiffeners

longerons

- longer beams in fuselage which are fitted longitudinally from nose to tail

- takes the main bending loads acting on the fuselage like spars in wings

frames

support skin and give it shape

- can take major loads

- designed as oval elements that are open in the centre with other components attached to their outer edge

bulkheads

supports skin and give it shape

- are solid

- may have access doors in them

firewall

seperates flight deck and cabin from the engine

- in event of fire, they protect crew long enough for pilot to make an emergency landing

- constructed from a variety of heat resistant materials e.g stainless steel, and titanium

doors in unpressurised planes

simple and light bc they do not have to resist the pressure differential

What is the main function of the stringers?

They support the skin

Which of the following materials is the most common in aircraft construction?

aluminium

hydrostatic pressure

if fluid is in equilibrium, then pressure exerted by it at a given point that is within the fluid is called hydrostatic pressure

- open containers = pressure depends on height of fluid

- fluid will have the same pressure in containers of different shapes provided they have the same height of fluid

pascal's law

a pressure change in one part of the confined and incompressible fluid will be transmitted throughout the fluid without loss

- same change occurs everywhere

- pressure change causes force employed during the operation of the hydraulic system

hydraulic press

consist of two connected cylinders with pistons

- areas of both pistons are different

Force of piston A = 1000N

Area of piston B = 0.005m^2

weight of up to 5,000 N can be lifted with the larger piston

P = F/A

W = F x d

1000 x 0.5m = 500J

F(piston A) = F(piston A) x Distance (piston A)/ F(piston B)

Force of larger piston always bigger than force applied to the smaller piston

Displacement of large piston always smaller than displacement of smaller piston

If there is a body/weight equal 5,000 N put on larger piston, both pistons are balanced and come back to their initial state

properties of hydraulic fluids

- relatively incompressible

- thermally stable

- lubricant

- strongly vicious

- wide range between boiling and freezing points

- non-corrosive

- chemically inert

- flashpoint above 100C and be non-flammable

- low volatility

- easily storable

- available and reasonably priced

- be free from foaming and sliding

types of hydraulic fluids

1) Synthetic

2) Mineral

Choice affected by material used for glands, seals, rings, seats, and other elements of the hydraulic system

incompressibility of hydraulic fluids

incompressible at pressures up to 276.7 bar

When 346 bars applied:

Liquid: 1% volume reduction

Air: 99% volume reduction

seals

prevent leakage of hydraulic fluid by being squeezed between two surfaces

2 types:

1) Static (gaskets, packing)

- fitted between two stationary surfaces

2) Dynamic

- fitted between two sliding surfaces

reservoirs

- storage space for hydraulic fluid

- necessary to store extra volume of hydraulic fluid in a reservoir = small leaks compensation

- actuators have less capacity when contracted than when extended, and volume of fluid may change with temp = reservoirs should provide sufficient air space to allow for any variations of fluid

filters

To keep hydraulic fluid free from foreign bodies which may damage system components

- filters usually fitted on both sides of the pump

Suction filter:

- pump protection

Pressure filter:

- providing hydraulic fluid cleanliness

sometimes there is also a return filter fitted in the return line to the reservoir:

- removes particles that appeared during the operation

Individual components often have their own filters fitted to their inlets

filters materials and cleaning

- paper

- felt

- gauze

- metal wire

- combinations of these materials

only filters made of metal wire are not discarded when removed

- usually cleaned by ultrasonic process (cleaner)

- also in trichloroethane as a temporary measure

types of accumulators

used to store hydraulic fluid under pressure

Cylindrical:

To separate two parts of this accumulator, a floating piston is used

Spherical:

A flexible diaphragm used to keep gas and liquid apart

landing gear

- energy absorption while landing

- protection of the aircraft against uncontrolled contact with the ground

- ensure sufficient braking performance

- enable ground manoeuvring

- ensure low movement resistance

2 configurations:

1) Tricycle (nose wheel)

- two main undercarriage units behind the CG

- one smaller wheel under the nose

- 80%-90% of weight applied to the aft wheels

- most used in modern airplanes

2) Tail dragger (Tail wheel)

- two main wheels forward of CG

- one on tail

- 80%-90% of weight applied to the front wheels

- popular in small propeller-driven planes

fixed landing gear

3 types:

1) Rubber cord

- no longer used in modern aircraft

- rubber cord undercarriage usually in the form of tubular struts that direct the landing force over a series of coils of rubber cord

2) Spring steel leg

- used in main landing gear of small aircraft

- leg consists of spring steel tube or strip that absorbs energy by flexing

3) Oleo-pneumatic strut

- used in most fixed nose landing gears and all retractable landing gears

- highest efficiency and damping-to-weight ratio compared to other shock absorbers

- absorbs shock loads using gas and hydraulic fluid (fluids)

Oleo-pneumatic strut - design

- piston

- cylinder

- gas

- Orifice

Piston travels up/down in the cylinder; and compresses gas in the upper chamber, acting as a spring

- absorbs the shock of the aircraft's vertical movement

Damping effect achieved by orifice in cylinder, reducing hydraulic flow and slowing down the piston movement

retractable landing gear

reduced fuel consumption bc improved aerodynamic characteristics

- high level of complexity, higher failure rate and higher maintenance costs

landin gear

common layout:

- 3 green lights = illuminates when correponding gear is down and locked

- 1 red/amber light = illuminates when gear in transit

tyres

designed to withstand extremely high loads for a short time

1) tubeless tire

- during puncture, air will leak slower

- do not cause heat from friction between tyre and tube

- more resistant to impact and creep damage

- 7.5% lighter than tube-type tyres

2) Inner tube (light/older aircraft)

- less expensive

- if punctured, cheaper to replace inner tube rather than whole tyre (tubeless)

- not sensitive to sidewall puncture

- only need to change the tube

- easy to fit on the rim; they do not have to be airtight against the rim

- carry entire mass of plane

- stability in high crosswind conditions

- responsible for most braking force

tyre tread

1) Rib tread

- used on paved surfaces

- has circumferential grooves around tyre for good water dispersion

2) Diamond tread

- unpaved surfaces

3) Combined

pre-flight tyre inspection

before flight check tyres for:

- proper inflation

- level

- pattern of tread wear

- damage cuts

- embedded foreign object

- flat spots

- bulges

- tyre creep

tyre wear

tread wear is normal process when using a tyre

- if tread is worn evenly and tread depth is within limit at every spot, tyre is fully operational

Green = good

yellow = borderline

red = replace

If tread unevenly worn, cause must be determined and corrected

- uneven wear caused by wheels or gear misalignment

- if cause cannot be corrected, demount the tyre and turn it around

cuts

remove tyre from service when depth of cut reaches the outer plies of the tyre

flat spot

area where tread is scraped due to skidding on runway surface

- occurs when brakes lock the wheel while aircraft is moving

If flat spot causes variation or fabric is exposed, tyre is removed

tyre pressure

90% of all tyre damage attributed to incorrect gas pressure

aircraft brakes

each of main wheels has a brake unit

- nose wheel/tailwheel has no brake

Aircraft wheel brakes function by using friction between braking surfaces, converting KE to heat

-slows aircraft down

- stationary during engine start-up

- provides steering during taxi

brakes control

controlled using rudder pedals

- top part of pedal activates brake on main wheel assembly

hydraulically actuated brakes

hydraulic system common for brake actuation system

- small and light aircraft

- consists of 2 hydraulic cylinder connected by a hydraulic line

- master cylinder acts as a pump

When brake pedal is pressed, a piston inside the master cylinder forces hydraulic fluid through a line to the piston in brake assembly

- results in pressing the friction elements against the brake disc

parking brake

operates a shut-off valve in the return line of the brake system

- traps pressurised fluid in brakes

Due to internal leakage of the valves when source of hydraulic pressure is no longer available

- hydraulic system can hold the brakes in place only for a limited amount of time

If park more than a few hours, place chocks on wheels

Which tire will lose pressure the fastest in the event of a puncture?

In a tire with an inner tube

flight control surfaces

elevators, rudders, ailerons

1) elevators

- rotation about lateral axis

- pitch control; longitudinal stability

- controlled by control column; aft movement of column causes up movement of elevator; aircraft pitches up

- but, some planes have all moving stabilator or canard foreplane instead

2) Rudders

- rotation about normal axis

- yaw control and directional stability

- controlled by right rudder pedal moved forward to move rudder right

3) Ailerons

- rotation about longitudinal axis

- roll control and lateral stability

- control column moved to left, left aileron goes up, right goes down. Aircraft rolls to the left

Fully manual control system - principle of operation

transmits forces applied to the cockpit

- controls directly to the control surface by mechanical linkage

Connection system consists of:

- tension cables

- pushrods

- pulleys

- counterweights

Turnbuckles used to adjust cable tension

Forces on control surface also transmitted to cockpit controls, so pilot feels them

jamming

control system jamming solutions:

1) 2 sets of interconnected control cables linked to control systems =

- can be decoupled by pulling a handle or applying excessive force to brake the interconnection

2) 2 or more control systems

- with separate hydraulic systems

- in case of jamming, the other takes over the control

gust lock (control lock)

manual flight control system

- when aircraft is on ground, risk of mechanical damage of control surfaces and cockpit controls due to wind gusts

Strong winds cause control surface movement out of its deflection range, resulting in destruction

Solutions:

- gust lock/control lock

- system that blocks control surface movement

types of gust locks

may be fitted directly to control surfaces

- smaller aircraft

- cockpit controls = throttle; impossible to open throttle until lock is engaged

rudder deflection limitation

rudder deflection must be adjusted to airspeed

- prevents excessive loads on rudder and vertical stabiliser

- achieved by a mechanical stop

- limits rudder deflection

- provides sufficient yaw control within the flight envelope

secondary flight controls

improve performance characteristics

- reduce loads on control surfaces

Examples:

- lift augmentation devices = flaps, trim tabs

flaps

hinged portion of trailing edge of wing

- can be deflected downwards, increasing wing camber

- results in increased lift

Flaps used for take-off and landing to increase lift at low speeds, reducing take-off and landing distances

maximum flaps extended speed

flaps and slats having limited operating speed range

- with extended flaps and slats, structural loads could be exceeded

VFE (white part on ASI) is the maximum flaps extended speed

- upper limit of the white arc on airspeed indicator

Modern airliners, VFE indicated on a speed tape on PFD (primary flight display)

trimming

to maintain attitude and speed, necessary to hold the control surface deflected

- achieved by trim tabs

- fitted to the elevator and rudder

Rudder trim tab is usually fixed:

- it can only be adjusted on the ground to correct a permanent out-of-trim condition

- trimming in roll not usually employed on light aircraft

icing conditions

happens if water in the liquid state is present

- airframe temp < 0C

- ambient temp < 0C

- Ambient temp slightly above 0C e.g after rapid descent

types of ice

may accumulate:

1) clear ice or glaze caused by large supercooled droplets

2) Rime ice caused by small supercooled droplets

3) Mixed ice (mix of both listed above and frost) caused by water vapour freezing

icing effects

altered shape of airfoil, lift decreases, drag increases

- accreted ice causes a weight increase

- fuel consumption increases

- icing of probs and sensors lead to inaccurate indications

- windscreen freezing obstructs pilot's vision

- structural issues e.g problems with landing gear retraction or ice stuck underneath control surfaces

ice protection systems

2 categories:

1) Anti-icing systems

- prevents ice forming on the protected components

2) De-icing systems

- removing ice that has already accumulated

ice protection systems - protected components

- wing's leading edge

- tail's leading edge

- propellers

- instrument probes

ice protection systems - types

- thermal systems = hot air and electrical systems

- fluid systems

- using freezing point

- fluid

- decreasing liquid and mechanical systems

- using inflatable rubber boots

hot air system - protected components

- airfoils' leading edges

- engine air intakes

hot air system - supply

combustion heating process employs combustion heater

- small portion of main engine supply fuel, mixed with air, is burned in a sealed combustion chamber

- ram air is blown into the duct surrounding the chamber, where it is heated up by its walls

In heat exchanger method:

- engine's exhaust gases pass through the heat exchanger, heating the outside air

- heated air is then mixed with the appropriate amount of cool outside air to reach desired temperature

- solution is generally employed in aircraft powered by turboprop engines

Distribution:

- hot air is distributed through the ducts spreading along the leading edges and alongside engine air intakes

electrical system - protected components

- engine air intakes

- windscreen

- propellers

- leading edges

Operation:

- electrical ice protection system consists of resistant elements placed in between layers of glass laminate or rubber

- when voltage is applied, elements heat up

Propeller protection: (propeller de-icing)

- resistant wiring is placed along leading edges of propeller's blades

- high power alternating or direct current transferred to the wirings by stationary brushes and rotating slip rings attached to the propeller

- a power relay controlled by the cyclic timer is employed to adjust heating periods

- system can be turned on and off using the switch

Inner parts of blades are heated constantly, and further part are heated in cycles

- tips are usually not heated as they are no susceptible to icing due to their high velocity

Inner radius = constant heating

Middle section = cyclic heating

Tips = no heating

Propeller icing effects:

- accumulated ice alters airfoil's shape, changing aerodynamic characteristics, results in efficiency loss

- mass distribution of ice is uneven, leads to imbalance and vibrations, accelerating process of fatigue wear of propeller's mechanism

fluid system - protected components

used as the anti-ice system for the airfoil's leading edges and propellers

operation:

- prevents ice from forming on the protected surfaces by lowering freezing point temperature of the water surrounding them

- achieved by pumping Freezing Point Depressant (FPD) fluid to the distribution panels on the airfoil's leading edges

- FPD fluid seeps through the porous material covering the panels and is distributed by airflow

mechanical system - protected components

pneumatic system

- used for de-icing wings' leading edges and tailplane's leading edges

- used in piston-engine aircraft and some turbo-propeller aircraft

- which thrust loss due to bleed air would be too severe

Operation:

- de-icing effect achieved by filling the inflatable rubber boots with air

- boots consist of either span-wise or chord-wise rubber tubes

- by cyclic expansion, they reduce the adhesion of ice and consequently cause it to break off

- minimum thickness of ice for system to operate properly is 0.5-1.5 cm

- if layer is thinner, ice continues to accrete around the inflated boots

Considering aircraft icing, which of the following is considered to be the most serious consequence of ice accretion?

The stalling speed increasing substantially

Which parts of propeller blades are the most likely to accumulate ice?

Blade roots

fuel types

Piston engines:

1) AVGAS

2) Mogas (automotoive gasoline)

Aircraft diesel engines:

1) AVTUR )aviation turbine fuel)

piston engine fuels

AVGAS 100

- 100 octane fuel

- low amount of lead

- green colour

AVGAS 100LL

- 100 octane fuel

- lower amount of lead

- blue colour

AVGAS 115

- 100 octane fuel

- high amount of lead

- green colour

- used in high-performance engines having high compression ratios

Mogas

leadless automotive gasoline

- used on certified engines with power output not exceeding 150 HP

- on ultralight & experimental aircraft

Higher volatility causes:

- carburettor icing

- vapour locking

fuel properties

- octane rating

- density

- viscosity

- volatility = a measure of the ease of vaporization of gasoline

- vapour locking = partial or complete interruption of the fuel flow in an internal-combustion engine, caused by the formation of vapour or bubbles of gas in the fuel-feeding system.

- calorific value = total energy content produced in the form of heat when a substance is combusted completely with air or oxygen

- stability = resistance to changes during storage and distribution

- sulphur content

- lead content

octane rating

measure of a fuel's ability to withstand compression without detonating

- the higher octane number, the more compression fuel can withstand before igniting

- higher octane rating (more lead) used in high-performance petrol engines

- fuels with lower octane ratings are used in engines with low compression ratios

- use of fuel with low octane rating may lead to problem of engine knocking = when fuel burns unevenly in your engine

high octane rating

helps achieve:

- higher compression ratio with higher thermal efficiency (without autoignition superchargers)

- better fuel consumption

- high engine power

By the use of a supercharger:

- Higher induction pressure

- Much higher engine power

Supercharging is less commonly used in the 21st century, as manufacturers have shifted to turbochargers to reduce fuel consumption and increase power outputs.

Viscosity

determines internal friction resulting from sliding of fluid layers relative to each other when flowing

volatility

how readily a substance vaporises

In spark engines with carburettor:

- volatility affects efficiency which fuel delivered to the carburettor apportions and maxes with air to create an air-fuel mixture

vapour locking

boiling of liquid occurs when pressure of its vapour is greater than the atmospheric pressure at the surface of the liquid

- atmospheric pressure decreases with altitude increase

- at lower atmospheric pressure values, liquid boiling takes place at lower temperatures

If volatility of fuel is too high:

- at higher altitudes low-pressure boiling may occur blocking the fuel flow to the carburettor (called vapor locking)

calorific value

- amount of heat released during combustion

- varies with chemical composition

- higher values for fuels with higher hydrogen content

- measured in MJ/kg or BTU/lb (British thermal units per pound)

also referred to as fuel volume unit

- higher the fuel density, higher the calorific value per volume unit

fuel stability

- fuel compounds tend to perform oxidation

- oxidation changes the properties

- oxidation has negative impact on fuel quality

- to prevent oxidation, oxidation inhibitors are added to the fuel