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Concept Design

At the outset of any design process, a series of initial choices and calculations are necessary to bootstrap the whole process. These typically span the design brief and the selection of overall design topology as well as a sequence of preliminary sizing decisions. A number of specialist aircraft concept tools are available to support this process, both commercially as well as the many empirical processes detailed in various text books that can be set up in simple programming environments such as spreadsheets, see, for example, ACSYNT [14], ADS,[1] Gudmundsson [15], PaceLab APD,[2] OpenVSP,[3] Raymer [11], Stinton [5], Torenbeek [1], and so on. There are also some specialist software development environments that can be used to build concept design tools that combine some form of limited geometrical modeling along with the ability to build a linked series of aircraft analysis modules (PaceLab Suite[4]). Initial concept calculations do not generally need detailed geometrical information and so can be considered outside of any CAD-based model. They do, however, often consider geometrical quantities, so some form of geometric sketching to support concept development remains important. In most of our work, we proceed from basic principles, since small and medium-sized UAVs are not so common that the available tools are well suited to this class of aircraft, although some empirical relationships were drawn from the aforementioned texts at the outset, before our first series of aircraft had been built and flown and from which more detailed data could be taken. It is also possible to make very crude aircraft sketches using the limited drawing capabilities of some spreadsheet tools; here we do just that using Microsoft Excel.

To enable rapid prototyping of the concept model, our initial work is carried out using a simple constraint analysis followed by the use of an Excel spreadsheet that contains several likely topologies, which are then balanced using the internal “solver” optimization tool within Excel to satisfy a series of design requirements and constraints. The basic design calculation that we chose to tackle at this stage in the design process is the maximization of range given a fixed total maximum take-off weight (MTOW) at a given cruise speed. The fixed weight is achieved by simply adding fuel to the base airframe. The principal design variables are chosen as the main wing area and the cruise lift coefficient along with the longitudinal positions of the front bulkhead and tailplane (to control trim). To this are added a range of constraints that ensure a feasible overall concept emerges. By way of example, this basic design task is illustrated in Table 9.1 for UAVs of 7-20 kg MTOW. Note that inside Excel, the solver optimizer will be used in global mode to balance the design by finding a wing area and coefficient of lift that match the take-off weight. Moreover a table of available engine types is provided that the design process selects from as the overall size and performance requirements change. As set up at Southampton, our concept process gives a reliable initial estimate of leading dimensions for UAVs in the 2-150 kg category, and assumes conventional propeller-based propulsion and ele- vator/rudder/aileron control surfaces. Airframe weights are based on a collection of data built up by the designers at Southampton over a number of years and assume carbon-fiber main spars, foam-cored wings, and selective laser-sintered (SLS) nylon or foam and carbon-fiber space-frame fuselages.

Table 9.1 Concept design requirements.

Item

Lower

Typical

Upper

Units

Type

Comment

limit

value

limit

Range

100

km

Goal

Goal to be maximized

Wing area

0.5

1.2

3

2

m2

Design

Sets size of main lifting

variables

surface

CL cruise

0.1

0.25

0.5

Defines style of wing section

M fuel

0

0.5

kg

Mass of fuel (ex reserve)

X forward

200

650

1000

mm

Position of forward bulkhead

bulkhead

in front of main spar

X tail spar

500

900

1500

mm

Position of tail spar behind main spar

MTOW

20

20

kg

Constraints

Fundamental design requirement

CL/CD

15

15

Prevent unrealistically

cruise

optimistic wing performance

L/D cruise

4

8

CL landing

1.5

1.5

Thrust

60

Installed

N

Allow for limits of selected

takeoff

static thrust

power plant

Power max.

1.5

Installed

kW

power

Available

take-off

rotation

15

18

To allow take-off

LCoG

25

25

mm

Pitch stability

Static margin

0.25

0.3

  • [1] http://www.pca2000.com/en/pca2000/main.htm.
  • [2] http://www.pace.de/en - PaceLab Aircraft Preliminary Design (APD).
  • [3] http://openvsp.org/.
  • [4] http://www.pace.de/en - PaceLab Suite.
 
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