Airframes Revision Notes

8/8/2019 Airframes Revision Notes

1/16

361 (Gateshead) Squadron

Airframes Revision Notes

Airframe Design Features

An airframe consists of four main components:

Mainplanes or wings

Fuselage or body Tail unit (or foreplanes for a canard type aircraft)

Undercarriage

The wings are the main components of an airframe.

In most aircraft, the wings carry most, if not all fuel, and the undercarriage. In military aircraft they often carry a

substantial part of the weapons load and stores.

The fuselage serves a number of functions

It forms the body of the aircraft, housing crew, passengers and cargo (payload) and the aircraft systems

hydraulic, pneumatic, electronic and electrical.

It forms the link between the wings and tail or foreplanes. The loads from these items can try to bend

and twist the fuselage so it must be strong enough to resist these forces.

Engines may be attached to the fuselage and the forces from these can be very high

It must withstand pressurisation forces

It must be strong enough t withstand all these forces for many flying hours.

The tail unit consists of a fixed vertical fin with moveable rudder and either fixed horizontal surfaces and

moveable elevators, or an all-moving horizontal surface. Some modern combat aircraft use a canard at the front

instead of a horizontal tail at the back.

The undercarriage serves two purposes:

To support the aircraft on the ground and allow it to move or taxi on the ground.

To absorb landing shocks.

It also must resist braking and side loads, and as it is not needed in flight it must be as small and light aspossible. It is normally retracted in flight.

Weight is critical and aircraft are designed to be as light as possible and the most common way to save weight is

to use a combination of different materials.

1


2/16

For best performance drag needs to be minimised by good design.

Airframe Design Features Structures

There are four main types of structural member in airframe construction:

Ties are members subject purely to tension.

Struts are members in compression

Beams are members that carry loads at an angle (often at right angles) to their length and take bending

loads. Beams in an airframe include spars and stringers. The fuselage and wings themselves are beams.

Webs take shear (like tearing paper). Ribs and aircraft skin are shear webs.

Materials that are weak in one direction can be strong in another so selection is very important.

A cantilever is a structure that is supported at only one end. It is widely used in aircraft construction, (e.g.wings).

2


3/16

Where all the strength of an airframe is derived from the skin it is known as a monocoque structure, but the skin

would need to be very thick to take all loads.

To reduce weight usually a Stressed skin construction is used which consists offrames, skin and stringers.

(Stringers are stiffeners that run along the inside of the skin to provide extra strength.

Airframe Design Features Shape

Shape is a very important aspect of aircraft design. It is determined by the role of the aircraft.

Wing loading is the weight of the aircraft divided by the wing area. It can vary in flight depending upon g-forces. To determine a standard wing loading the maximum take-off weight (MTOW) is often used. Light

aircraft normally have the lowest wing loading and fast jets the highest, with transport aircraft somewhere in

between.

A monoplane has one wing. Some light aircraft have a diagonal brace to help support the wing. These are

known as braced monoplanes.

Most monoplane wings have no bracing and are therefore cantilevers. Wings can be low, mid or high mounted

on the fuselage.

Due to the additional aerodynamic forces on an aircraft, wings need to be able to carry greater weights than thatof the MTOW of the aircraft. (A Boeing 747 weighing 350 tonnes needs to be able to support loads of over 1000

tonnes.)

Designers of aircraft must try to achieve the best strength to weight ratio.

Type of wing is determined by the role and speed of the aircraft.

Swept-back wings are used for high speed, but are not as efficient as straight wings at low speed.

Swing-wings allow the best of both worlds aerodynamically, but add to weight.

3


4/16

Delta wings allow high speed and good turn rates and are becoming more popular for agile fighter

aircraft, often used with canards.

Aspect ratio is the ratio of the wing span to its average chord. Another way of defining it is:

aspect ratio = span2

area

So if wing has an area of 80 square metres and span of 20 metres, the aspect ratio is 202/80 = 5

Materials

Airframes are built using materials that have a high ratio of strength to weight (SWR). The following groups

of materials come into this category:

Aluminium and magnesium alloys (light alloys)

Steels

Titanium and titanium alloys

Plastics and composites

There are other things to consider apart from the SWR

A material must be consistent and predictable in its properties so that we know what to expect from its

behaviour.

It must be homogeneous i.e. have the same properties throughout. It must not suffer badly from corrosion.

It should be non-inflammable (although magnesium alloy does burn fiercely this does not normally

cause a problem).

It should be easily available at a reasonable cost, and should be easy to work using standard processes.

It should not suffer badly from fatigue, or be used in places where this does not cause a problem.

Aluminium and Magnesium Alloys

Pure aluminium and magnesium cannot be used as structural materials, but when alloyed they form the most

common airframe materials. Alloying metals include zinc, copper, manganese, silicon and lithium. Where

aluminium is coated onto its alloys it is known as Al-clad.

Advantages of Aluminium and Magnesium allots are:

High strength/weight ratios

A wide range of alloys to suit different uses

4


5/16

Light, so more can be used to resist buckling

Available in many standard forms sheet, plate, tube, bar, extrusions, etc.

Easy to work

Can be super-plastically formed (SPF) This involves heating to a precise temperature where the metal

can be blown into a mould to form intricate shapes

Reasonable electrical and magnetic shielding

Disadvantages

Subject to corrosion

Steel and its Alloys

Steel is an alloy of Iron and other metals. Steel always includes carbon and alloying metals include chromium,nickel and titanium.

Advantages of steel

Cheap and readily available

Consistent strength

Wide range of properties dependant upon choice of alloy and heat treatment

High strength where space is limited

Some stainless steels are highly corrosion resistant

High-tensile steels have high SWR Hard surface so resistant to wear

Suitable for use at higher temperatures than aluminium alloys but not as good as titanium alloys

Easily joined by welding

Very good electrical and magnetic screening

Disadvantages

Poor SWR except high tensile steels

Heavy

Titanium Alloys

Its properties are similar to steel but it is superior in strength at high temperatures. It can be SPF. It ca bediffusion bonded, i.e. when heated to a precise temperature, two pieces of titanium can be pressed together and

they will fuse into one piece.

Advantages

High SWR

Maintains strength at high temperature

Higher melting point and lower expansion than other materials

Can be SPF

Disadvantages

Expensive

Can be difficult to work, especially machining

Poor electrical and magnetic shielding A hard scale forms on the surface at high temperature

Plastics and Composites

Pure plastics have little structural use. However, composites where plastics are reinforced with fibres such as

glass, carbon or Kevlar are now commonly used.

Advantages

Very high SWR and low weight

Non-corrodible (although Kevlar can absorb water if damaged)

Easily available in wide range of forms

Can be used to make complex shapes easily

Low resistance to radar and radio waves so can be used for radomes and antenna covers

5


6/16

Disadvantages

Needs special manufacturing and repair techniques

Strength and stiffness is not the same in all directions

Poor electrical screening

Can cause corrosion to normal metals due to galvanic action

Fatigue

Fatigue is a materials tendency to break under a high number of relative stresses such as take-offs and landings

or vibration. Next to human errors it is the chief cause of aircraft accidents.

To guard against such failures, aircraft life is often quoted in flying hours. E.g. an RAF training aircraft may

have a life of 5,000 flying hours and Concorde 45,000 hours. Most RAF aircraft have fatigue meters to recordloads and aircraft manufacturers carry out tests to determine when problems are likely to occur. By comparing

the fatigue meter records with the manufacturers data, remaining airframe life can be monitored.

Wings

Wings consist of two parts the internal structure, such as spars and ribs and the external skin, which may be

fabric, metal or composite.

Fabric covered wings consist of the front andrear spars which are the main structural members. Ribs give the

wing its shape and join the two spars and transfer the load from the skin to the spars. The leading edge

structure is attached to the front spar and the trailing edge structure, which includes the ailerons and flaps, is

attached to the rear.

Stressed skin wings are used for higher speeds as the load on an airframe increases as the square of the speed.

(e.g. the load at 400 knots is four times that at 200 knots.) A thicker metal skin can share the loads taken by the

structure underneath which can then be made lighter. The two main spars are still the main strength members.

Wing tips, ailerons and the leading edge can be made of composites. Ribs may have large holes to reduce

weight. The skin may be fixed to the internal structure by rivets, or by bonding (gluing). The volume between

the front and rear spars may be used for fuel storage.

Spars are given depth to resist bending. Sometimes the skin of the aircraft can be machined internally to form

part of the spar structure. Normally two spars are used, but where the wing is thin in very high-speed aircraft

multi-spar construction is used.

6


7/16

Most modern aircraft have two spars with stressed skin construction between them that forms a torsion box. The

leading and trailing edge structures are then attached to this box. The box can be used to carry fuel. The skin

needs to be stiffened with stringers to prevent buckling.

Machined skin is machined from a single piece of alloy (called a billet) using CNC (Computer Numerically

Controlled) machines. It is possible to make the whole span of a wing skin in one piece, however up to 90% of

the metal may need to be removed so the process is very expensive, although the finished structure can be lighter

and stronger than a fabricated one.

Advantages of a machined skin are:

No riveting so smoother surface

Lighter and stronger structure

CNC machining makes mistakes and faults less likely

Easy inspection during manufacture and service

No maintenance

Easy sealing of fuel spaces

Disadvantages:

High cost

Difficult battle damage repair

Careful design needed to limit fatigue cracking.

Delta and swept wings may need different types of construction due to their shape.

Fuselages and Tail Units

Most fuselages are semi-monocoque structures where the skin takes a share of the loads. This leaves space for

crew, passengers and cargo. Semi-monocoque structures are also easier to design to withstand pressurisation,

which typically exerts a force of up to 5600 kilogrammes per square metre of fuselage skin.

The fuselage also is:

The interconnecting link between the other structural units (wings, tail, etc.).

Has the undercarriage loads transmitted to it

Carries the air loads and weight of the tail unit and/or foreplanes

Often carries the engines, so must withstand the loads from these

Carries most of the aircraft systems (hydraulics, electronics, radar, etc) and also some or all of the

weapons in the case of a military aircraft.

7


8/16

There are three distinct parts of a fuselage:

Nose section

Centre section

Aft or rear section

The centre section carries most of the loads as the wings are attached to it.

Semi-monocoque construction consists offrames, skin and stringers, with the frames protruding be about 100

150 mm into the fuselage. The skin normally takes approximately half of the loads.

Pressure bulkheads are fitted at the nose and tail to withstand loads caused by pressurisation and strengthening

needs to be added at door and window cutouts.

Tail units and foreplanes are usually built in the same way as wings. On large aircraft the fin may be used to

carry fuel.

Rudders, elevators and foreplanes may be made from composites.

Engine Installation

Engine installation depends upon type and number of engines, required performance and the role of an aircraft.

An aircraft with only one piston or turboprop engine will normally have its engine fitted in the nose.

An aircraft with more than one engine can have its power units in a variety of places (e.g. in or under/over its

wings, in the fuselage, at or in the tail, etc.).

8


9/16

To minimise problems if an engine fails it is preferable to place the engines as close to the fuselage as possible.

If this is not possible a large rudder is required.

Prop-fan engines have an un-shrouded fan which looks a little like a propeller.

Twin or multi-engined propeller aircraft have their engines spaced along the wing as there has to be clearance

between the propellers and the fuselage, and also between each propeller.

Disadvantages of wing mounted engines

Maximum yaw if engine fails

Space taken neutralises some of the wing area so that span has to be increased

Wing must be protected fro engine heat

Using engines mounted in pods can eliminate some of these disadvantages and advantages are:

Airflow over wing not disturbed

Thin high-speed wing section can be used

Shorter wing span is possible More space in wing for fuel

No heat insulation in wing required

Reverse thrust is more easily achieved without damage to wing

Engines easily accessible for servicing

It is easy to use different types of engines

Reduction in bending forces on the wing in flight

Disadvantages are:

Yaw problems if engine fails

Heavy loads on wings if violent manoeuvres are necessary during flight and landings.

More easily damaged by ground debris and foreign objects (FOD)

Taller undercarriage required to keep engines clear of ground

Undercarriages

An undercarriage is required to:

Support the aircraft on the ground

Absorb the shock of landing and provide smooth taxying Withstand side forces during cross wing take-offs and landings

Give minimum rolling friction on ground and minimum drag in the air

Withstand braking loads on landing

9


10/16

Be as light as possible as it is dead weight during flight.

Advantages ofnose wheel undercarriage:

Ground manoeuvring easier with steerable nose wheel

Improved pilots view

Aircraft horizontal on ground

Aerodynamic drag reduced on take-off giving better performance

Directional stability on ground improved

Braking is more straightforward and brake parachutes can be used Less tendency to float and bounce on landing, making landing easier

Disadvantages

Nose wheels need to be stronger and heavier than tail wheels

More damage to aircraft if nose wheel collapses

Shock absorbers prevent damage to the structure when an aircraft lands. (The force on landing can be up to

three times the weight of an aircraft on land and up to eight times on an aircraft carrier.)

On light aircraft spring steel or rubber shock absorber are used, but on heavier aircraft a telescopic shock

absorber called an oleo leg is used.

An oleo leg uses oil or gas to dampen the shock by allowing the oil or gas to pass slowly though a hole in a

piston mounted in a cylinder.

Main wheels must be behind the centre of gravity to prevent the tail hitting the ground on take-off or landing. If

they are too far back the increased load on the nose wheel could cause it to collapse. Main units often retract into

the wing so must be small enough to fit a limited space.

There are many variations of main units, including single, double, tandem or bogie units. Heavier aircraft use

many wheels to spread the load and the Boeing 747 has 18 wheels four main units each a four-wheel bogie,

and a double nose wheel unit.

Jockey units are often used on transport aircraft that require rough field performance such as the Hercules.

10


11/16

To reduce drag in flight, an undercarriage is retracted in flight on most aircraft using a hydraulic jack.

Undercarriage doors can be attached to the legs or closed using separate jacks. A sequencer is used to ensure

that the doors open at the right time.

Some undercarriage units need to twist to fit into the bay. Emergency systems are installed to extend the

undercarriage in case of emergency, often using pressure bottles.

On the ground, ground locks prevent the undercarriage being retracted by accident.

There are two main types ofbrakes, drum and disc, but aircraft are usually fitted with disc brakes as thesedissipate heat more easily. Discs may be made from aluminium alloy, steel, carbon or other material gripped by

pads of friction material.

They are normally controlled by pressing toe pedals fitted to each rudder pedal to allow them to be operated

differentially to steer the aircraft by braking each side separately.

Aircraft brakes are fitted with Maxaret units to prevent skidding. These are similar to ABS units on cars.

In flight, some aircraft have air brakes which increase drag to slow the aircraft. On the ground, jet aircraft can

use reverse thrust which to deflect the jet exhaust forward. Propeller aircraft can use reverse pitch.

Controls

Elevators are hinged to the tailplanes spars, or all-flying tailplanes or foreplanes can be used. Rudders are

hinged to the fin spar and ailerons to the rear wing spar.

Fighter aircraft can use computers to control an unstable aircraft to give responsiveness and manoeuvrability.

The control column (stick) operates the elevators and ailerons. A yoke or sidestick may also be used. The

rudder bar or rudder pedals operate the rudder.

On light aircraft the controls are operated by cables or rods. On large or fast aircraft power assistance or power

operation is required.

Fly-by-Wire controls are where the cockpit controls are linked to the control surfaces by an electrical cable.

11


12/16

Fly-by-Light controls are where the cockpit controls are linked to the control surfaces by fibre optics.

Powered controls use actuators (hydraulic or electric) to operate the control surfaces

Controls have backup systems for safety, and Fly-by-Wire and Fly-by-Light systems use a voting system to

ensure safety (there are three computer systems and two can overrule one if there is a problem).

Elevons combine the functions of Elevators and Ailerons on delta wing aircraft (e.g. Concorde).

Tailerons combine the function of Elevators and Ailerons on aircraft with all moving tailplanes (e.g. Tornado).

Autopilot and Related Systems

The autopilot makes flying simpler for pilots flying a fixed course and on modern aircraft can even fly the plane

from take-off to landing.

Autopilots can use gyroscopes to detect disturbances from a set course. These send a signal to a corrector, which

sends a message to the control actuators to move the control surfaces in the appropriate direction.

More complex systems use other types of detectors to make corrections.

12


13/16

An Instrument Landing System (ILS) uses a signal from a transmitter near to the runway to give an indication

to the pilot when the aircraft is on the correct approach path.

Autoland allows aircraft to land without pilot intervention by using data from the ILS and other systems to

operate the flying controls. Automated take-offs are also possible in some cases.

Terrain following radar uses data from radar to control the aircraft using the autopilot.

Hydraulic Systems

Hydraulic systems use oil to help operate aircraft controls. Oil is fed from a pump to actuators (jacks) that move

the control surfaces. The oil is controlled by valves operated by the aircraft controls.

Systems are normally duplicated for safety.

Pneumatic Systems

Pneumatic systems use pressurised air to operate controls. Because air can be compressed, it can be stored in

pressure reservoirs for emergency use ore to provide assistance when heavy operations are required.

Pneumatic systems are not as effective as hydraulic systems.

Hot air from the engines can be used for de-icing the leading edges of wings, tailplanes and engine cowlings. It

can also be used to operate cabin pressurisation and air conditioning systems. A heat exchanger is used to heat

cold incoming air.

Electrical Systems

Modern aircraft need large amounts of power to operate radio, radar, navigation aids, aircraft instrumentation,

weapon aiming and control and engine starting. Fly-by Wire systems need even more power.

13


14/16

Aircraft have normally have two generators driven by the engines and batteries to supply a small amount of

power to start the engines or power systems fro a short time in case of emergency.

A Ram Air Turbine (RAT) may be available for use in an emergency, and many aircraft have an Auxiliary

Power Unit (APU) to supply power on the ground or for emergency use.

Larger aircraft have two power systems:

115 volt AC, 400 Hz, 3 Phase for higher power systems

28 volt, DC to power the electronic systems and instrumentations

Any other voltages and frequencies that are needed are converted from these basic supplies

Fuel Systems

Aircraft in flight use a large amount of fuel. A very large aircraft on take-off can use as much as 10,000 gallons

per hour so large fuel tanks are necessary. Most is stored in the wings, but the fuselage and tail can also be used.Tanks may be sealed compartments or flexible fuel bags. Military aircraft can also use drop tanks which can be

jettisoned in an emergency or for combat.

Fuel can be pumped between tanks to maintain the centre of gravity as fuel is used, and tanks are pressurised to

prevent fuel from boiling off at high altitudes.

Most modern aircraft can be refuelled through a single refuelling point. Air-to-air refuelling can be used to

extend range.

Weapons

Weapons can be split into three main categories:

Fighter weapons such as guns and air-to-air missiles (AAMs)

Ground/Sea attack weapons, such as bombs, guns and air-to-surface missiles (ASMs)

Electronic weapons, such as electronic counter measures (ECM) and flares

14


15/16

The Cockpit

The aircraft is controlled from the cockpit. In a military aircraft the crew is confined to this area so every control

has to be provided there. All controls must be easily reached, easily operated and logically arranged. For a pilot

the cockpit is the most important part of an aircraft.

Displaying information in a clear way is vital. Most important instruments are grouped in front of the pilot with

other instruments and controls to the sides.

The four main flying instruments (attitude, altitude, speed and vertical speed) are normally positioned in a T-

shape.

Instruments in an aircraft include:

Attitude indicator, to show position of horizon relative to the aircraft

Horizontal situation indicator, to show aircraft heading (similar to compass) and may include a radio

compass system

Air speed indicator (ASI), to show speed of aircraft through the air. This measures the difference in

pressure between the air entering the pitot tube and that in the static vent.

Machmeter, which shows the speed of the aircraft compared to the speed of sound

Altimeter, which shows the height of the aircraft above a reference height by measuring the static air

pressure. A radio altimeter measures height by sending and bouncing a radio signal from the ground

and measuring the time taken for it to return.

Vertical speed indicator (VSI) shows whether the aircraft is climbing and descending.

A glass cockpit uses television-type displays to show a variety of information when needed. Information not

required is not displayed.

A head-up display (HUD) projects information onto a screen in front of the pilot so that there is no need to look

down at the instrument panel

15


16/16

Controls

Aircraft need to have many controls in addition to the stick and rudder pedals. They are positioned so that themost important ones are easily reached and the least used are tucked away. Many modern combat aircraft have

important switches mounted on the stick and throttle to allow the pilot to operate them without taking hands off

these main controls. This system is known as HOTAS (hands on throttle and stick).

Escape Systems

Jet aircraft use ejector seats (often known a bang seats) to allow crew to escape at high speed.

When activated by a handle or lever the sequence operates automatically:

1. The canopy is jettisoned is broken by a small explosive charge

2. Rockets or cartridges are fired to propel the seat clear of the aircraft

3. A small parachute stabilises the seat

4. At a safe height the crewmember is separated from the seat and the personal parachute deploys

In modern aircraft one crew member can operate the seat of another member to safely eject them even if they are

injured and modern seats have zero-zero capability so that the can be used at zero altitude and zero airspeed, so

can be used when the aircraft is static on the ground.

Future developments

Future aircraft may just use one computer display screen to show all information. Helmet mounted sights allow a

pilot to aim weapons by simply looking at them.

Voice activation of controls will be used.

To help pilots withstand high-g forces their seats will be reclined (but must be brought upright if they need to

eject).

16

Documents

Airframes Revision Notes