Airframes and EnvironmentalsChapters 1 & 11

Aim

To review principals of Airframes and Environmental systems

Objectives

1.State the effect of loading on aircraft components2.Describe the aircrafts major components and

their construction methods3.Describe environmental control systems

1. Airframe LoadingAircraft Loads

When an aircraft is designed the weight must be kept as low as possible whilst still maintaining strengthThe loads which the airframe must be able to withstand include:

• Manoeuvring loads• Gust loads• Control surface loads• Pressurisation loads• Landing and take-off loads

1. Airframe LoadingEffect of Loads

All loads will place extraneous force on the aircraft structure, this is called stress and is measured as force per unit areaAll structures are flexible and as a result any stress place on them will distort the shape, the amount of distortion compared to the original shape is called strainStructures which distort easily are flexible, those that are harder to distort are referred to as rigid

1. Airframe LoadingEffect of Loads

Loads can be classified according to the effect they have on the structure:• Bending loads try to bend the structure. This can be seen if we compare

an aircraft on the ground to one airborne, on the ground the wings are creating a downwards moment, when airborne they are creating lift

On the ground Airborne

• Compression and tensile loads occur at the same time. Again if we consider a wing in flight the lift produced by the wing will produce compression loads on the top surface of the wing and tensile loads on the bottom

Lift

Tensile Load

Compression Load

1. Airframe LoadingEffect of Loads

• Torsional loading tends to twist a structure. This can be seen on our wing where most of the lifting force is being produced towards the leading edge

• Shear loading occurs when two airframe components slide against each other. It can be seen here on the flap of the C172SP where it has rubbed against the wing

More Lift being produced

Less Lift being produced

2. Components and ConstructionAirframe Components

Ailerons Ailerons

Spinner

Propeller

Flaps Flaps

Cowling

Fuselage

Horizontal stabilizer

ElevatorVertical stabilizer or Fin and Rudder

2. Components and ConstructionAirframe Components

Mainplanes

Fuselage

Empennage

2. Components and ConstructionFuselage Construction

There are generally three forms of fuselage construction• Truss• Monocoque• Stressed skin or semi-monocoque

Composite aircraft may use a mixture of these forms employing the use of modern composite material

2. Components and ConstructionTruss Fuselage Construction

Similar to the construction method used in power pylons or cranesLongerons are assembled in a box-like structure with struts and bracesWithout struts and braces any loading would be felt in the construction points making the structure weakEarly truss fuselages were wooden with wires or rods for cross bracing, later models such as seen in the Piper Cub used steel tubingThe fuselage framework is typically covered in fabric or canvasAll the strength in this form of construction is in the framework

2. Components and ConstructionMonocoque Construction

Can be seen as the opposite of truss type constructionAll loading is absorbed by the skin with any internal framework playing no part, an example of this design is an egg shellThe airframe gains its strength from its rounded shapeThis characteristic is easily demonstrated by a thin aluminium beverage can. You can exert considerable force to the ends of the can without causing any damage. If side loading occurs the can will easily collapseSkins and bulkheads are used to keep the required shapeThe skin must be very thick to take the loads, therefore it is heavy and expensive to constructThis form of construction is typically used in large aircraft around doors and openings

2. Components and ConstructionStressed Skin Construction

Also known as semi-monocoque constructionBlend of a truss and monococuque construction, used on the majority of light aircraftInternal structure consists of a series of frames or bulkheads held together with LongeronsA skin of either aluminium or composite material is then fastened to the frame and where necessary stiffened with stringersThe load is shared between the internal frame and the stressed skin

2. Components and ConstructionWing Construction

Wings must be made to withstand the bending and twisting loads created when they generate lift as well as the bending loads generated when they are on the groundThe wing spar is the main bearing component of the wing. There are typically two spars per wing one at the thickest part of the wing and the other towards the rear where the ailerons and flaps are connected. In some aircraft the spars will run through both wings and the fuselage will sit on the sparRibs are used to give the wing shape and transfer loads to the spar

2. Components and ConstructionLayout of Controls

In most light aircraft controls are manipulated from the cockpit via a system of cables and pulleysAerodynamic balancing is used to make it easier to manipulate the controls however if the surfaces are too large or the aircraft is traveling at too high a speed, hydraulic actuators or fly by wire systems will be utilised to aid the pilot

3. EnvironmentalsCabin Ventilation and Heating

Most light aircraft use air scoops to channel fresh air into the cabinHeating can be achieved by ducting the fresh air from outside the aircraft around the exhaust shroud before it enters the cabinLarger aircraft can use gasoline heaters to heat the airWith both of these systems there is a chance of exhaust gases mixing with the air entering the cabin, carbon monoxide detectors are used to prevent this from incapacitating the aircrafts occupants

3. EnvironmentalsPressurisation

In pressurised piston aircraft pressurisation is achieved by bleeding air from the turbo compressorBefore entering the cabin the hot bleed air must be cooled. When it is cooled any moisture in the air condenses reducing the humidity of the air, some aircraft are fitted with humidifiers to overcome this limitationPressure of the cabin is adjusted by controlling an outflow valve at the rear of the bulkhead. This will maintain the current pressure differential between the cabin pressure and the outside air pressure.

3. EnvironmentalsPressurisation

Typically the cabin altitude will be set around 8000ft, some newer composite aircraft can maintain a cabin altitude of 6000ftOn some aircraft the maximum altitude can be limited by the maximum pressure differentialThe cabin altitude must never be higher than the aircrafts altitude as this would produce loading in the opposite direction as to what it was designed to handle

If the pressure differential is too high the airframe will not have sufficient strength to overcome this and structural failure can occur.

3. EnvironmentalsOxygen Systems

At higher altitudes supplemental oxygen is required, larger aircraft will have a fixed installation however backup systems are still requiredSystems consist of a pressurised canister and maskPure oxygen will spontaneously combust if it comes to contact with oil or greaseIndustrial oxygen must not be used as it contains impuritiesMedical oxygen must not be used as it contains water vapour that can freeze in the regulator

For details on when oxygen is required see CAO 20.4 (Note: This is not required for CPL systems, however, is required for CPL Air Law.)

Questions?