2. Fuselage

2.1 Fuselage considerations

The purpose of the fuselage is to attach the wings and empennage, as well as the cockpit, motor, payload, and landing gear. The challenge with designing a agricultural aircraft is the requirement for the payload to be located directly on the centre of gravity to ensure that when the payload is released, there are no significant changes in the stability of the aircraft. The other design consideration is to ensure that the required components of the aircraft can all fit within the fuselage.

For the agricultural aircraft, the required components include the cockpit, the motor, the front and rear landing gear, the wing attachment, tail attachments, and the payload and payload distribution system. As the landing gear, wing location and the payload location are all determined by the location of the centre of gravity, determining the size and layout of the fuselage is an iterative process.
A fuselage is the body of an aircraft, which houses passengers, cargo, and usually the pilots of the craft as well. Fuselages vary widely in shape, size, and style, depending on the purpose of the aircraft that they are attached to. They are hollowed out to reduce the overall weight of the craft, and they provide the structural framework to which the wings, tail, and other features are attached.
Construction of a fuselage starts with assembling a framework which accounts for specific needs in the plane design, and then covering the framework with a lightweight metallic skin. The plane may be insulated to help control the temperature inside, and then the inside of the fuselage is fitted out in accordance with the plane's purpose.

2.1.1 Aerodynamic considerations

The fuselage is responsible for a large percentage of the overall drag of most airplanes: 25 – 50 percentages. Since it is desirable to have as little drag as possible, the fuselage should be sized and shaped accordingly.

Fuselages generate the following types of drag:

1. Friction drag
2. Profile drag
3. Base drag

For more detailed data on the fuselage drag, can be found in chapter three of Roskam’s Airplane Design Part III: Layout Design of Fuselage.

Drag

CDparasite = CDskin + CDform + CDinterference

In order to shift the separation point towards the rear and reduce the wake, the fuselage has been designed to resemble a large aerofoil with an optimal exposed surface area for the nose of the plane giving a drag coefficient of only 0.4.

Skin friction drag results from roughness on the surface causing an earlier transition from laminar to turbulent boundary layers and typically comprise about 50% of the total drag on an aircraft at cruise (Barnes & McCormick, 1979). There are three key factors affecting skin friction drag that can be altered in order to minimise the resistance; reducing the surface area of the aircraft (Sw), reducing the roughness of the material (Cf) and reducing the airspeed to decrease dynamic pressure (q).
Df = qCfSw
(Brandt et al, 2004)
Taking this into account, whilst the aircrafts speed cannot be reduced, the design process will aim to minimise the surface area of the fuselage and select smooth materials and joints. The fuselage design must consider the boundary layer formations at the leading edge and aim to maintain a laminar velocity profile and delay the transition to turbulence (Roskam, 2004).

This criterion is addressed through the selection of aluminium alloy 2024, a relatively smooth material, and the implementation of flush riveting and polishing on the body. The fuselage can also be tailored to reduce the amount of interference drag affecting flight. As interference drag generates vortices when two surfaces meet at sharp angles, this can be easily prevented by specifying that all angles on the plane, such as those between the fuselage and wings, are to consist of fairings that alter them into fillet junctions (Roskam, 2004). 2.1.2 Fuselage sizing

The fuselage sizing was based on statistical data