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Home arrow Engineering arrow Small Unmanned Fixed-Wing Aircraft Design. A Practical Approach

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Adapting the Design to Local Conditions

An obvious application of this idea, from a UAV concept design point of view, is to design systems or subsystems that can move the optimum operating point such that it matches the current conditions and mission. For example:

  • • A hybrid propulsion system that always operates at its “sweet spot” - such as an internal combustion engine that always operates at its most efficient throttle setting for a given altitude, ambient temperature, and so on, and which drives a generator, which, in turn, charges the battery that powers the electric motors that turn the propellers;
  • • Constant-speed, variable-pitch propellers;
  • • Morphing aircraft: could the wing shape change, for example, to enable the aircraft to operate efficiently at a range of flight conditions (landing flaps are a classic example).

Asymmetry

Subtle asymmetries abound in the vast majority of aircraft, including UAVs (at the very least, the positioning of the components of the avionics tends to be asymmetrical), but these are unlikely to be considered at the layout design phase. In these early stages, the engineer may wish to consider what advantages could be gained from higher level asymmetries. When sketching out early concepts, our pencil seems to instinctively default to a symmetrical layout and there is much to commend this (cost minimization through part commonality is easier to achieve, design effort is likely to be lower, etc.).

It is, nonetheless, worth considering whether anything in the design brief and the early sketches may warrant significant asymmetries. Only a handful of manned aircraft featured high-level asymmetries, and they were experimental designs (such as the Messerschmitt P.1109, a series of Blohm & Voss WWII designs, the NASA AD-1 and its precursor, the OWRA - shown in Figure 10.4, and two Rutan aircraft, the Boomerang and the ARES), but this may have much to do with public acceptability considerations. A few prompts the reader may wish to consider:

NASA oblique-wing research aircraft (images courtesy of NASA). Could your design benefit from asymmetry?

Figure 10.4 NASA oblique-wing research aircraft (images courtesy of NASA). Could your design benefit from asymmetry?

  • • On twin-engine designs, would an asymmetrical arrangement help mitigate one engine out thrust asymmetry concerns? (This was the main reasoning behind Rutan’s Boomerang.)
  • • Could the fuselage be used to shield sensitive instruments from the heat and the exhaust of a single engine mounted on one side (in much the same way as the fuselage of the Rutan ARES shielded the intake of its single engine from the smoke of a machine gun mounted on its other side)?
  • • Would the desire for improved high-Mach-number performance warrant an oblique wing (like the NASA AD-1 and the OWRA shown in Figure 10.4)?

Incidentally, asymmetric aircraft are one example of the principle that topological concepts that have not found favor in the world of manned aircraft are worth revisiting in UAV design, as some of the reasons for their unpopularity may well have something to do with having passengers and/or the pilot on board.

 
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