1. Airframes and Environmentals
Chapters 1 & 11

2. Aim
To review principals of Airframes and Environmental systems

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

4. 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

5. 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

6. 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

7. 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

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

9. 2. Components and Construction
Airframe Components
Fuselage
Mainplanes
Empennage

10. 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

11. 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

12. 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

13. 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

14. 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

15. 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

16. 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

17. 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.

18. 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

19. 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.)

20. Questions?