Unmanned Aircraft Design. Mohammad Sadraey H.
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 proceeding with the next stage. There is a series of formal design reviews conducted at specific times in the overall system development process. An essential technical activity within the design process is that of evaluation. Evaluation must be inherent within the systems engineering process and must be invoked regularly as the system design activity progresses. When conducted with full recognition of design criteria, evaluation is the assurance of continuous design improvement. The evaluation process includes both the informal day-to-day project coordination and data review, and the formal design review.
The purpose of conducting any type of review is to assess if (and how well) the design configuration, as envisioned at the time, is in compliance with the initially specified quantitative and qualitative requirements. A design review provides a formalized check of the proposed system design with respect to specification requirements. In principle, the specific types, titles, and scheduling of these formal reviews vary from one design project to the next. The following four main formal design reviews are recommended for a design project.
1. Conceptual Design Review (CDR)
2. Preliminary Design Review (PDR)
3. Evaluation and Test Review (ETR)
4. Critical (Final) Design Review (FDR)
Figure 1.6 shows the position of each design review in the overall design process. Design reviews are usually scheduled before each major design phase. The CDR is usually scheduled toward the end of the conceptual design phase and prior to entering the preliminary design phase of the program. The purpose of conceptual design review (CDR) is to formally and logically cover the proposed design from the system standpoint. The preliminary design review is usually scheduled toward the end of the preliminary design phase and prior to entering the detail design phase. The critical design review (FDR) is usually scheduled after the completion of the detail design phase and prior to entering the production phase.
The evaluation and test review is usually scheduled somewhere in the middle of the detail design phase and prior to production phase. The ETR accomplishes two major tasks: (1) finding and fixing any design problems and the subsystem/component level, and then (2) verifying and documenting the system capabilities for government certification or customer acceptance. The ETR can range from the test of a single new system for an existing system to the complete development and certification of a new system.
1.14 QUESTIONS
1. What are the five terms which are currently used for unmanned aircraft?
2. What are the primary design requirements for a UAV?
3. Describe features of a Tier II UAV in the Air Forces.
4. Describe the features of a micro UAV.
5. What is the main objective for the feasibility study?
6. What is the size range for mini UAVs?
7. What do MALE and HALE stand for?
8. What is the operating altitudes for HALE UAVs?
9. What is the endurance range for MALE UAVs?
10. What are the wingspan and MTOW of Global Hawk?
11. What are the cruise speed and endurance for Predator (RQ-1A)?
12. What was the major setback during Phase II flight testing of the Global Hawk on March 29, 1999? What was the reason behind that?
13. Describe the fundamentals of systems engineering approach in UAV design.
14. What are the main four formal design reviews?
15. What are the UAV main design groups?
16. Describe conceptual design phase.
17. Describe main outputs of the preliminary design.
18. Describe process of detail design.
19. Describe trade-off analysis process.
20. From systems engineering approach, what are the main design phases?
CHAPTER 2
Design Disciplines
2.1 INTRODUCTION
There are several design disciplines which work in parallel within a UAV design project. Some examples are: (1) aerodynamic design, (2) structural design, (3) propulsion system design, (4) power transmission system design, (5) mechanical system design, and (6) control surfaces design, (7) ground station, and (8) launch and recovery system. This chapter briefly covers the first six topics disciplines; the other two are presented in Chapters 8 and 9, respectively. Due to the limited volume of the book, only the basic fundamentals are presented. The interested reader should refer to Sadraey [37] for more details. Table 2.1 shows the UAV major components and their primary functions.
Table 2.1: UAV vehicle major components and their functions
In a UAV design process, some UAV parameters must be minimized (e.g., weight), while some other variables must be maximized within constraints (e.g., range, endurance, maximum speed, and ceiling), and also others must be evaluated to ensure that they are acceptable. The optimization process must be accomplished through a systems engineering approach. In some cases, the design of the