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Multiple Functions

The minimization of weight, complexity, and part count are almost universal concerns in aerospace engineering, and the design of unmanned aircraft is no exception. One way of achieving this - as early as the concept design phase - is by assigning multiple functions to as many components as possible. Here is a brief list of some ideas that may be considered at this point:

  • • the use of a V-tail instead of the more conventional stabilizer plus fin layout, replacing three surfaces with two and replacing two elevators and a rudder with two ruddervators;
  • • reducing the number of other control surfaces by merging their functions. This is where most aircraft-design-related portmanteaus (e.g., spoilerons, flaperons) live! Reciting them in the context of the concept layout design of the control system may be a good way of ensuring that the number of control surfaces (and thus ancillary equipment, like servos) is minimized. Of course, the trade-off is usually against redundancy, both in terms of merging the roles of control surfaces and sharing servos between multiple control surfaces;
  • • Should the fuselage generate some of the lift? This is not very common in manned transports for a variety of reasons (cylindrical cross-section fuselages tend to have to cruise at low angles of attack - meaning low CL - to reduce the slope of the aisle, and blended wing bodies have a plethora of issues related to the “self-loading freight”[1]). Almost none of these reasons has any relevance to unmanned aircraft and all of the positive aspects of lifting fuselages do (chiefly, aerodynamic efficiency), so blended wing body or lifting body type configurations might well be worth considering. The typical trade-off is against manufacturing difficulties: blended wing bodies often require special tooling (though additive manufacturing can mitigate this issue at certain scales);
  • • Can large, external structural components (e.g., a wing brace) better earn their keep? For example, can they also generate lift or house sensors or fuel/batteries?
  • • Can any of the avionics components - for example, a printed circuit board (PCB) - have a structural role? This is a question worth asking especially when the size of the PCBs is the limiting factor in the miniaturization of a micro air vehicle.

Figure 10.5 Multifunctionality: the 3D-printed fuel tank (highlighted) of the SPOTTER unmanned aircraft does not only hold the fuel (with integral baffles) but also generates lift and it has a structural role too, see also Figures 2.3, 2.6, and 3.14.

• The cost of multifunctionality is often geometrical complexity. One way to solve this problem is the use of additive manufacturing techniques, the cost of which is independent of complexity. Consider, for example, the fuel tank of the SPOTTER aircraft, highlighted in Figure 10.5: it contains integral baffles, stiffeners, and a filler; it is a structural member connecting the two engine nacelles and the payload pod; and it generates a significant amount of lift. Such loading of a single part with a variety of functions may reduce weight, it reduces part count and facilitates assembly.

  • [1] The passengers, to use a more conventional term.
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