As noted in the previous chapter, we typically attach wings to the fuselage using carbon fiber spars to carry the principal loads, with pegs of some form to resist moments. This means
Figure 4.6 DECODE aircraft with modular fuselage elements.
Figure 4.7 Wing attachment on SPOTTER fuselage. Note the recess for square torque peg with locking pin between main and rear spar holes.
providing some form of reinforced tubular hole in the fuselage into which the wing spar slides. It is important that the sliding fit be a good one to ensure a wide load transfer surface that reduces stress concentrations. It is also important to diffuse the local stresses stemming from flight loads and running through the spars into the wider fuselage structure, particularly toward the main undercarriage pick-up points, since the connection between wing spars and undercarriage is probably the most highly stressed part of the fuselage, see Figure 4.7.
Engine and Motor Mountings
Engines and motors, of course, provide the thrust for powered flight, and this thrust must be carried into the airframe via the engine mount. Engines and motors are also, however, among the highest density items in the aircraft, and so due allowance must be made for the inertia forces they generate during flight such as during gust events. Finally, when dealing with internal combustion engines, allowance must be made for the heat and vibrations caused by engine operation. High temperatures can adversely affect polymer structures, while vibrations can lead to fatigue failures in any mountings made of material such as aluminum that do not have lower stress limits on fatigue (we try and avoid using aluminum components in tension near engines - steel is much preferred for such loads). We have found that, provided suitable care is taken to prevent heat transfer, engines and motors can be bolted directly to sufficiently thick SLS nylon structures without problems. We do, however, often include antivibration mountings between the engine and the main fuselage to insulate avionics and payloads from such vibrations. Figure 4.8 shows some typical engine and motor mounts.
Figure 4.8 Typical engine and motor mounts for SLS nylon fuselages and nacelles. Note the steel engine bearer in first view, engine hours meter in second image, and vibration isolation in third setup.