Assembly Mechanisms and Access Hatches
When working with a fully molded CFRP-type fuselage, it is quite normal for the bulk of the fuselage to be formed in a single piece. The use of SLS nylon as a structural material tends to work against this approach: first, we typically want structures that are larger than can be printed in the available laser sintering machines; and second, we commonly wish to break the fuselage down into parts for access and portability reasons. We have found two straightforward means of dealing with these issues: either we use bayonet-type joints designed directly into the SLS nylon parts or we thread a series of parts onto longitudinal tension rods that pull the components into a single hull. Sometimes we adopt both approaches in a single fuselage. Figure 4.16 shows an aircraft built in this way (see also Figure 4.6, where again tension rods and bayonets are used). When using bayonet systems in fuselage parts, it is wise to print several
Figure 4.16 Aircraft with SLS nylon fuselage formed in three parts: front camera section attached by bayonet to rear two sections joined by tension rods. Note the steel tension rod inside the hull just behind bayonet in the right-hand image.
Figure 4.17 Example hatches in SLS nylon fuselages. Note the locking pins and location tabs on the right-hand hatch.
trial parts to get these to work as desired before committing to large and expensive prints. In particular, we find that with SLS nylon, the first few operations of a bayonet will slightly polish the mating surfaces so they need to be quite tight when first made if they are not to become too loose when used repeatedly.
In most cases, the fuselages and nacelles used in UAVs should not be open to the elements since they contain sensitive avionics components, wring looms, batteries, and so on. The only exceptions tend to be for cooling requirements around engines, motors, and speed controllers. In consequence, some means of internal access on the airfield is then almost always necessary. Bayonets form one means of achieving this, and we often do this for payload items where we may have multiple payloads capable of fitting onto a single fuselage or other bayonet. The alternative is some form of hatch and cover. Hatches can be readily SLS-printed with closely conforming covers and duplicate locking mechanisms, see the examples in Figure 4.17. When designing hatches, we ensure structural continuity by forming a suitable flange around the opening, and we also sometimes add neoprene O-rings to these to provide watertight closure.