Home Engineering Small Unmanned Fixed-Wing Aircraft Design. A Practical Approach
Fuselages and Tails (Empennage)
After the wings, the next most fundamental parts of the aircraft are the fuselage and tail/empen- nage. The fuselage generally houses the avionics and payload, supports the engines or motors (sometimes in the form of nacelles), and provides attachment points for all the flying surfaces and undercarriage elements. The tail surfaces provide flight control and stability. All must be engineered for low drag and light weight while being robust and readily maintainable.
Main Fuselage/Nacelle Structure
In most aircraft, there is a well-defined main fuselage or nacelle structure that is elongated in the direction of flight, suitably streamlined and with some means of access to the interior. These provide the space to house the myriad components needed for flight control and payload operation. They can be made in a wide range of materials, always bearing in mind the desire for light weight, high strength, and good rigidity. The most common construction materials are plywood, carbon-fiber-reinforced plastic (CFRP), rigid foam, and polymers. We have tried all types, including space-frame and monocoque methods and now prefer to use selective laser-sintered (SLS) printed nylon monocoques. Although SLS nylon has an inferior strength-to-weight ratio when compared to CFRP, the ability to adopt complex geometrical reinforcing allows competitive structures to be produced, see Figure 4.1. Also such 3D-printed structures do not require molds or other tooling for their construction and permit the inclusion of fittings for bayonets, hatches, switch gear, and sensors with comparative ease, see Figure 4.2.
When very light structures are needed, the use of rigid foams can be attractive, though special care then has to be taken to provide hard points for fitting highly loaded components such as engines or undercarriage - typically by gluing load-spreading plates to the foam, usually made from thin sheets of plywood or plastic (see Figure 4.3) - this can also be necessary when using nylon for the main fuselage material, and even some CFRP structures include metal hard points within the molding process. Foam can, however, be used for the entire structure of lightweight designs, see, for example, Figure 4.4. One way of combining the attributes of CFRP, foam, and SLS nylon is in the form of a foam-covered space frame that is made of CFRP tubes joined together with SLS nylon clamps, see Figure 4.5.
Figure 4.1 SPOTTER SLS nylon engine nacelle/fuselage and interior structure.
Figure 4.2 Bayonet system for access to internal avionics (a) and fuselage-mounted switch and voltage indicators (b).
Figure 4.3 Load spreader plate on Mylar-clad foam core aileron.
Figure 4.4 Commercially produced model aircraft with foam fuselage (and wings).
Figure 4.5 Space frame structure made of CFRP tubes with SLS nylon joints and foam cladding.
In cases where experimental aircraft are being developed, we sometimes opt for modular fuselage sections that allow the length of the fuselage to change. This allows the longitudinal position of the payload and avionics to be widely varied during development. In our experience, good control of the longitudinal center of gravity (CoG) can be difficult in a rapidly changing research airframe, so this capability can be very useful - note the repeated modules used in Figure 4.6, here held together by longitudinal tension rods that can easily be varied in length.
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