Home Engineering Small Unmanned Fixed-Wing Aircraft Design. A Practical Approach
Main Design Drivers
Having set out an outline catalog of the component parts of our UAVs, and before going on to consider each aspect in detail, it is useful to consider the key design drivers that control our medium and small fixed-wing UAVs. Inevitably, the payload mass and the range/endurance/cruise speed of the aircraft are the fundamental inputs to any UAV platform design - in fact, they are the key inputs to any vehicle design whether they be airborne, seaborne, or landborne. The next key aspect to consider in fixed-wing UAVs turns out to be the landing speed - this controls the size and complexity of the wings, since low landing speeds require large wings and/or increasingly sophisticated high-lift devices. In small UAVs, the emphasis tends to be on large wings and simple flaps, while in medium-sized aircraft it becomes viable to consider more complex high-lift devices. Since we are concerned with UAVs under 150 kg MTOW and often have to operate from less than perfect landing fields, low landing speeds strongly impact on the likelihood of damage and wear and tear on the airframe. Ideally, landing speeds would be 12m/s or less, but are often as high as 20m/s. While this is still very low by comparison with large jet aircraft, it can still be frighteningly fast when things go wrong. High landing speeds can also lead to the need for long runways unless braking systems are used, which can be heavy, complicated, and expensive.
For low-cost designs, the next important area to consider is propulsion, since engines or motors have to be purchased off the shelf. When selecting electric motors, a very wide range of choices is available, but for internal combustion engines, particularly for larger airframes, the choice is much more restricted. It then often turns out that only one or two particular engines are suitable for the configuration under consideration, and this can significantly impact on the resulting design. One can be caught between selecting an engine that is slightly too small and risks giving an underpowered result, or one that is rather too large giving an over-heavy design. Sometimes we find that it is sensible to revisit payload and mission choices in the light of available engines rather than dogmatically insisting that these numbers are fixed. As ever in design, one is seeking a balanced result that acceptably trades a range of characteristics off into an harmonious whole. Given such information, it is possible to consider the boundaries of the wing loading (W/S) versus thrust-to-weight ratio (T/W) design domain. These boundaries are representations of the basic constraints that enforce the adherence of the design to the numbers specified in the design brief.
Armed with this information and a catalog of possible engines or motors, it is then relatively easy to start to build some form of concept model from which many of the principal aircraft parameters can be derived, starting with wing loading and principal dimensions and working through to a weight summary and control requirements to ensure that a balanced design can be achieved. Although such trade-off studies can be carried out using the proverbial back-of-an-envelope methods, engineers typically reach for their collection of computer-based tools at this stage - quite commonly the ubiquitous spreadsheet during initial concept definition: in Part III, we use Microsoft Excel. We will illustrate our approach with the concept design of several aircraft we have built and flown. Before heading in this direction, however, it is useful to consider the airframe components in more detail so as to understand the whole toolset that will be needed in the design process and how these various tools will be linked together. Having run through the airframe components and demonstrated a concept design process, we then follow this with CAD and physics-based analysis of one of our aircraft, together with details of its manufacture, regulatory approval, trials, and documentation before setting out how we operate and maintain our UAVs.
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