Airframes and Environmentals Chapters 1 & 11

Aim To review principals of Airframes and Environmental systems

Objectives State the effect of loading on aircraft components Describe the aircrafts major components and their construction methods Describe environmental control systems

1. Airframe Loading Aircraft Loads When an aircraft is designed the weight must be kept as low as possible whilst still maintaining strength The 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 Loading Effect of Loads All loads will place extraneous force on the aircraft structure, this is called stress and is measured as force per unit area All 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 strain Structures which distort easily are flexible, those that are harder to distort are referred to as rigid

1. Airframe Loading Effect 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 Compression Load Lift Tensile Load

1. Airframe Loading Effect 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 More Lift being produced Less Lift being produced 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

2. Components and Construction Airframe Components Spinner Propeller Cowling Ailerons Flaps Flaps Ailerons Fuselage Horizontal stabilizer Elevator Vertical stabilizer or Fin and Rudder

2. Components and Construction Airframe Components Fuselage Mainplanes Empennage

2. Components and Construction Fuselage 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 Construction Truss Fuselage Construction Similar to the construction method used in power pylons or cranes Longerons are assembled in a box-like structure with struts and braces Without struts and braces any loading would be felt in the construction points making the structure weak Early truss fuselages were wooden with wires or rods for cross bracing, later models such as seen in the Piper Cub used steel tubing The fuselage framework is typically covered in fabric or canvas All the strength in this form of construction is in the framework

2. Components and Construction Monocoque Construction Can be seen as the opposite of truss type construction All loading is absorbed by the skin with any internal framework playing no part, an example of this design is an egg shell The airframe gains its strength from its rounded shape This 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 collapse Skins and bulkheads are used to keep the required shape The skin must be very thick to take the loads, therefore it is heavy and expensive to construct This form of construction is typically used in large aircraft around doors and openings

2. Components and Construction Stressed Skin Construction Also known as semi-monocoque construction Blend of a truss and monococuque construction, used on the majority of light aircraft Internal structure consists of a series of frames or bulkheads held together with Longerons A skin of either aluminium or composite material is then fastened to the frame and where necessary stiffened with stringers The load is shared between the internal frame and the stressed skin

2. Components and Construction Wing 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 ground The 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 spar Ribs are used to give the wing shape and transfer loads to the spar

2. Components and Construction Layout of Controls In most light aircraft controls are manipulated from the cockpit via a system of cables and pulleys Aerodynamic 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. Environmentals Cabin Ventilation and Heating Most light aircraft use air scoops to channel fresh air into the cabin Heating can be achieved by ducting the fresh air from outside the aircraft around the exhaust shroud before it enters the cabin Larger aircraft can use gasoline heaters to heat the air With 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. Environmentals Pressurisation In pressurised piston aircraft pressurisation is achieved by bleeding air from the turbo compressor Before 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 limitation Pressure 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. Environmentals Pressurisation If the pressure differential is too high the airframe will not have sufficient strength to overcome this and structural failure can occur. Typically the cabin altitude will be set around 8000ft, some newer composite aircraft can maintain a cabin altitude of 6000ft On some aircraft the maximum altitude can be limited by the maximum pressure differential The 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

3. Environmentals Oxygen Systems At higher altitudes supplemental oxygen is required, larger aircraft will have a fixed installation however backup systems are still required Systems consist of a pressurised canister and mask Pure oxygen will spontaneously combust if it comes to contact with oil or grease Industrial oxygen must not be used as it contains impurities Medical 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.)

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