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

The Morphology of a UAV

All heavier-than-air UAVs contain many similar and well-understood components that the

design team need to consider:

  • 1. Lifting surfaces. Traditionally wings or rotors but this can include blended wing-bodies - certainly it is common for fuselages to generate lift. As this book is concerned with fixed-wing UAVs, rotorcraft are not considered further.
  • 2. Control surfaces or their equivalents. Typically elevators, rudders, ailerons, and perhaps flaps and air-brakes (sometimes a single surface provides multiple functions).
  • 3. Fuselages. To house systems, but these may be subsumed into a blended wing-body configuration or engine nacelles.
  • 4. Internal structure to support all the loads seen by the vehicle and to connect the components together).
University of Southampton SPOTTER UAV with under-slung payload pod

Figure 2.2 University of Southampton SPOTTER UAV with under-slung payload pod.

  • 5. Propulsion system. Normally propeller-, turbo-fan-, or jet-based. Here, the focus is on propeller-driven aircraft that use piston engines or electric motors.
  • 6. Fuel tanks or other energy sources for propulsion and possible on-board generation. Here, they are generally JP8 kerosene, gasoline, methanol, or LiPo batteries.
  • 7. Command, communication, and control systems and associated on-board power system (generally supported by (LiFe) battery, generator, or the main engine).
  • 8. Payload. Commonly sensors or munitions but sometimes emergency aid, medicines, or other lightweight high-value goods.
  • 9. Take-off and (normally) landing gear. Generally wheels with suspension and steering, sometimes retractable (this can include catapult attachment points or landing hooks).

Figures 2.1 and 2.2 show the University of Southampton SPOTTER aircraft with most of these components including an external under-slung payload pod; Figure 2.3 shows the integral center fuel tank and main spars for this aircraft. It is noticeable that even in the most sophisticated vehicles, payload volume and mass are still a small percentage of the total, though they are a larger fraction than a manned aircraft would permit because all of the life-support systems and accommodation spaces can be dispensed with: on SPOTTER, the maximum payload mass is 5 kg excluding fuel, while the remaining airframe weight is 23 kg.

There are, of course, a considerable number of ways these basic elements can be laid out to form an aircraft, even if canard designs are not considered. First, the number and type of engines must be chosen - here we have considered only single-, twin-, and occasionally three-engine/propeller configurations. Second, we must choose whether to use tractor or pusher propellers, or perhaps a tractor/pusher combination. This naturally leads to a single

Integral fuel tank with trailing edge flap and main spars

Figure 2.3 Integral fuel tank with trailing edge flap and main spars.

central fuselage, a central fuselage with twin wing-mounted engine nacelles, or twin fuselages that incorporate the nacelles. Third, the type of wing has to be chosen: generally low, middle, or high wing monoplane designs are adopted but biplane configurations can have some advantages. Next, one must consider the type of tail-plane from simple “T,” inverted-“T,” “V” or inverted-“V,” and various forms of “U” or “H.” It will immediately be clear that may tens of possible configurations can adopted, and any brief survey will show that almost all possible combinations have been tried at some time or other. Finally, one must choose the undercarriage layout assuming one is to be fitted - this is generally either a nose-wheel or tail-wheel configuration with the main undercarriage under the wings.

Ideally, one would systematically consider all possible combinations of these choices at the concept stage. However, to do so requires that the concept model can adequately distinguish between the various options, and unless one has a large quantity of prior data, this is unlikely to be possible - accurately estimating the impact of tail choice on structural weight is a nontrivial calculation, for example. At Southampton we have built and flown the following combinations:

  • 1. Single-engine tractor configurations with conventional undercarriages and tails.
  • 2. Single-engine pusher configurations with a range of tails, with and without undercarriage with low, middle, and high wing locations.
  • 3. Twin-engine, dual-fuselage tractor designs with conventional “H tails,” middle wings, and tail wheel undercarriage.
  • 4. Hybrid designs with tractor gasoline and twin electric propulsion.

We next set out in general terms the approaches adopted at Southampton in the various areas of the aircraft before going on to more detailed descriptions in Part II. Clearly, there are many alternatives in each case that we have not considered - those described here reflect the overall approach already set out, their typical (low) costs, and their ready availability.

 
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