Unmanned Aircraft Design. Mohammad Sadraey H.

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СКАЧАТЬ data, evaluate each of the candidate under consideration, perform a sensitivity analysis to identify potential areas of risk, and finally recommend a preferred approach. Only the depth of the analysis and evaluation effort will vary, depending on the nature of the component.

      Trade-off analysis involves synthesis which refers to the combining and structuring of components to create a UAV system configuration. Synthesis is design. Initially, synthesis is used in the development of preliminary concepts and to establish relationships among various components of the UAV. Later, when sufficient functional definition and decomposition have occurred, synthesis is used to further define “hows” at a lower level. Synthesis involves the creation of a configuration that could be representative of the form that the UAV will ultimately take (although a final configuration should not be assumed at this early point in the design process).

      One of the most effective techniques in trade-off studies is multidisciplinary design optimization. Researchers in academia, industry, and government continue to advance Multidisciplinary Design Optimization (MDO) and its application to practical problems of industry relevance. Multidisciplinary design optimization is a field of engineering that uses optimization methods to solve design problems incorporating a number of disciplines. Multidisciplinary design optimization allows designers to incorporate all relevant disciplines simultaneously. The optimum solution of a simultaneous problem is superior to the design found by optimizing each discipline sequentially, since it can exploit the interactions between the disciplines. However, including all disciplines simultaneously significantly increases the complexity of the design problem.

1.11 PRELIMINARY DESIGN

      Four fundamental UAV parameters are determined during the preliminary design phase: (1) UAV maximum take-off weight (W_TO), (2) wing reference area (S), (3) engine thrust (T) or engine power (P), and (4) autopilot preliminary calculations. Hence, four primary UAV parameters of W_TO, S, T (or P), and several autopilot data are the output of the preliminary design phase. These four parameters will govern the UAV size, the manufacturing cost, and the complexity of calculation. If during the conceptual design phase, a jet engine is selected, the engine thrust is calculated during this phase. But, if during the conceptual design phase, a prop-driven engine is selected, the engine power is calculated during this phase. A few other non-important UAV parameters such as UAV zero-lift drag coefficient and UAV maximum lift coefficient are estimated in this phase too.

      Figure 1.6 illustrates a summary of the preliminary design process. The preliminary design phase is performed in three steps: (1) estimate UAV maximum take-off weight; (2) determine wing area and engine thrust (or power) simultaneously; and (3) autopilot preliminary calculations.

      In this design phase, three design techniques are employed. First, a technique based on the statistics is used to determine UAV maximum take-off weight. The design requirements which are used in this technique are flight mission, payload weight, range, and endurance.

      Next, another technique is employed based on the UAV performance requirements (such as stall speed, maximum speed, range, rate of climb, and take-off run) to determine the wing area and the engine thrust (or engine power). This technique is sometime referred to as the matching plot or matching chart, due to its graphical nature and initial sizing. The principles of the matching plot technique are originally introduced in a NASA technical report and they were later developed by Sadraey [37]. The technique is further developed by the author in his new book on UAV design that is under publication.

      Figure 1.6: Preliminary design procedure.

      In general, the first technique is not accurate (in fact, it is an estimation) and the approach may carry some inaccuracies, while the second technique is very accurate and the results are reliable. Due to the length of the book, the details of these three techniques have not been discussed in details here in this section. It is assumed that the reader is aware of these techniques which are practiced in many institutions.

1.12 DETAIL DESIGN

      The design of the UAV subsystems and components plays a crucial role in the success of the flight operations. These subsystems turn an aerodynamically shaped structure into a living, breathing, unmanned flying machine. These subsystems include the: wing, tail, fuselage, flight control subsystem, power transmission subsystem, fuel subsystem, structures, propulsion, landing gear, and autopilot. In the early stages of a conceptual or a preliminary design phase, these subsystems must initially be defined, and their impact must be incorporated into the design layout, weight analysis, performance/stability calculations, and cost benefits analysis. In this section, the detail design phase of a UAV is presented.

      Figure 1.7: Detail design sequence.

      As the name implies, in the detail design phase, the details of parameters of all major components (Figure 1.7) of a UAV is determined. This phase is established based on the results of conceptual design phase and preliminary design phase. Recall that the UAV configuration has been determined in the conceptual design phase and wing area, engine thrust, and autopilot major features have been set in preliminary design phase. The parameters of wing, horizontal tail, vertical tail, fuselage, landing gear, engine, subsystems, and autopilot must be determined in this last design phase. To compare three design phases, the detail design phase contains a huge amount of calculations and a large mathematical operation compared with other two design phases. If the total length of a UAV design is considered to be one year, about ten months is spent on the detail design phase.

      This phase is an iterative operation in its nature. In general, there are four design feedbacks in the detail design phase. Figure 1.4 illustrates the relationships between detail design and design feedbacks. Four feedbacks in the detail design phase are: (1) performance evaluation, (2) stability analysis, (3) controllability analysis, and (4) flight simulation. The UAV performance evaluation includes the determination of UAV zero-lift drag coefficient. The stability analysis requires the component weight estimation plus the determination of UAV center of gravity (cg). In the controllability analysis operation, the control surfaces (e.g., elevator, aileron, and rudder) must be designed. When the autopilot is designed, the UAV flight needs to be simulated to assure the flight success.

      As the name implies, each feedback is performed to compare the output with the input and correct the design to reach the design goal. If the performance requirements are not achieved, the design of several components, such as engine and wing, might be changed. If the stability requirements are not met, the design of several components, such as wing, horizontal tail and vertical tail could be changed. If the controllability evaluation indicates that the UAV does not meet controllability requirements, control surfaces and even engine must be redesigned. In case that both stability requirements and controllability requirements were not met, the several components must be moved to change the cg location.

      In some instances, this deficiency may lead to a major variation in the UAV configuration, which means the designer needs to return to the conceptual design phase and begin the correction from the beginning. The deviation of the UAV from trajectory during flight simulation necessitates a change in autopilot design.

1.13 DESIGN REVIEW, EVALUATION, AND FEEDBACK

      In each major design phase (conceptual, preliminary, and detail), an evaluation should be conducted to review the design and to ensure that the design is acceptable at that point before СКАЧАТЬ