Once in possession of a design brief, the engineer can proceed to the next step, which is to convert the design specification into algebra, and in the case of aircraft design, this typically means drawing up a clear, constrained optimization problem.
Such clarity is relatively easily achieved in the case of some of the fundamental performance constraints, such as not allowing the design to exceed a given landing run; we shall deal with finding the chunk of the design space left feasible by these constraints in Section 10.3. Other constraints require more subtlety. Their formulation may be hindered by the fact that, at this stage, we do not have a layout.
A typical example may be that the aircraft should fit into a case no larger than l x w x d for transportation purposes. There may be a number of ways of solving this. The trivial solution is to limit the bounding box of the design to the size of the case. We may, however, wish to consider an aircraft that can easily be disassembled into a number of components that can be accommodated by the specified transportation case. We may also end up breaking the mission down into subtasks executed by a half a dozen small aircraft, which would all have to fit into the case (there may well be scope here for interesting 3D packing layout optimization - essentially a game of Tetris integrated into the concept design process!).
Because of the enormous range of possible missions unmanned aircraft could be designed for, an all-encompassing design specification template will always end up being- ? ? well, not all-encompassing. The following list of questions is simply meant to be a series of prompts to assist in drawing up the brief, the overarching document that will guide the design process.
1. Regulatory framework. Which document governs the operation of the aircraft? Which certification standard(s) will have to be met?
2. Take-off constraints. How will the aircraft be launched? Is a strip available for normal rolling takeoff? If so, will it always be a hard surface or can it also be a grass strip? What is the maximum expected cross-wind at takeoff?
3. Landing constraints. Is a runway available? (Ship-based operations are typical of this not being the case.) What is the maximum allowable landing roll? What is the maximum safe touch-down speed that will ensure damage-free landings? Will the aircraft always land into a headwind? (This is quite common when the landing roll is very short compared to the size of the airfield.)
4. Climb constraints. What should the rate of climb versus payload/fuel mass trade space look like? (Typically, the most stringent number is specified here.) What is the minimum service ceiling/maximum operating altitude?
5. Turn performance. What is the minimum acceptable rate of turn (constant velocity and instantaneous)?
6. Mission profile. This is a catch-all term for the fundamental performance figures of almost any aircraft: still air range, endurance in the hold/loiter, contingencies/diversion, cruise speed/Mach number.
7. Gliding and “unpremeditated descent”1 What is the minimum rate of descent the aircraft would have to maintain without propulsion? (Note that this may vary with density and hence altitude.)
1 Regulatory bodies can always be trusted to supply engaging euphemisms for “crash.”
8. Stall performance. What should the maximum allowable clean/dirty stall speed be constrained at? Is the aircraft expected to have “benign” stall characteristics? (The latter is a relatively rare requirement in the world of autonomous aircraft.)
9. Stability and handling. Constraints on static stability, control forces, dynamic stability, and general handling characteristics may be considered, though in a system designed for autonomous operations these are generally of less importance than in the case of manned aircraft.
10. Engine out constraints. Is the aircraft expected to safely become airborne in case of an engine failure at takeoff? Is the aircraft expected to climb following the failure of one or more of its powerplants?
11. Ditching. Is the aircraft expected to float on water for a significant amount of time? Does it have to be operational immediately following recovery?
12. Constraints imposed by the payload. What are the environmental extremes the payload may be subjected to? What are the dynamic extremes (e.g., maximum acceleration)?
13. Ground transportation/handling constraints. What is the bounding box the aircraft will have to fit into? What ground handling overloads can be expected? This is to differentiate between a surveillance drone carried in a soldier’s backpack to be deployed in the heat of battle and a science platform that is carefully wheeled out of the lab on a calm day by operators wearing white gloves. Is there a maximum mass constraint dictated by transportation considerations? Will one person have to be able to pick it up?
14. Aerosols, pollutants, and operation in harsh environments. Will the aircraft be expected to operate in polluted environments, for example, in airborne sand, volcanic ash? Will the aircraft have to operate in extreme heat/cold or be exposed to nuclear fallout? Is there any risk of airframe/engine icing?
15. Environmental impact. What are the constraints on the environmental impact of the aircraft in terms of noise (very important, e.g., in covert surveillance applications or wildlife monitoring) and emissions? This may end up setting the agenda in terms of propulsion system design. Is there a significant chance that the aircraft will land in a remote area and not be recoverable - if so, which materials, battery types, and so on, should not be used?
16. Ground personnel. What is the maximum number of personnel available to operate the system? How many people will be available to unpack/assemble the aircraft and conduct preflight checks? Will the aircraft be operated by qualified, experienced personnel? The answer to the latter is generally “yes,” but there may be exceptions, for example, a delivery/cargo drone that may have to be unloaded by the recipient.
Once the design brief is complete, the stage is set for the first “real” design task: the definition of the topology (or layout) of the airframe. This is what we turn our attention to next.