4.6 Fuselage Design

The third component that considerably impacts UAV aerodynamics is the fuselage. The primary function of the fuselage is to accommodate/house the payload, and autopilot. This section is devoted to the aerodynamic design of a fuselage.

4.6.1 Fuselage Design Fundamentals

Since a UAV has no human onboard, there will be no cockpit and cabin; hence, the human factor is not considered in the fuselage design. Moreover, a pressurized cabin/place is often not required. However, some avionic and payload may not operate efficiently at low pressure; thus, sometimes a pressurized compartment is required. For instance, the Global Hawk fuselage is made out of aluminum and contains pressurized payload and avionics compartments to house the heat-generating avionics. Two cooling techniques of convection and conduction require a pressurized compartment. Radiation heat transfer is operational in vacuum (or low pressure) but will not be effective in cooling a high-temperature sensor.

By definition, payload is the useful load that the aircraft is intended to carry. For a non-military UAV, the payload basically includes items such as autopilot, measurement devices, sensors, and communication equipment. For a military UAV, the payload will include items such as rockets, missiles, and bombs. Fuel is not considered as a payload.

In addition, in order to reduce aircraft drag, a few other major components and systems such as landing gear, engine, fuel system, and power transmission system are highly likely to be enclosed by the fuselage.

There are a few expectations recommended to be considered during the fuselage design process. These include low weight, low drag, positive contribution to lift generation, external symmetricity, and safety against environmental hazards such as lightning. The fuselage drag usually contributes 30–50% of the aircraft zero-lift drag (C Do). Furthermore, the fuselage may be aerodynamically designed such that it provides as much as 20% of the total lift. Interestingly, in a basic configuration, only about 5% of the aircraft lift is produced by the fuselage.

Two major fuselage parameters that must be determined during the design process are: (a) fuselage length (Lf); and (b) maximum diameter (Df). The fuselage configuration as well as these two parameters are functions of several design requirements.