Home Engineering Small Unmanned Fixed-Wing Aircraft Design. A Practical Approach
We have been building and flying small fixed-wing unmanned air vehicles (UAVs) since 2008. Figure 21.1 shows our very first student-designed and -built airframe. Since that time, we have built some 30 further aircraft and our methods of working have evolved as we have discovered the best ways of getting our designs into the sky, not always successfully, Figure 21.2.
Perhaps the most important single lesson we have learned is that good-quality detail design is of paramount importance. Even the very best concepts will be spoiled by poor detailing, while even quite poor concepts will often perform acceptably if all the detail design work is of good quality. Initially, this attention to detail also extended to the skills needed to carry out aircraft manufacture. The airframe in Figure 21.1 involved much labor-intensive construction using typical aero-modeler-type approaches with wings covered with aero-modeler film and glued wooden fuselage construction. Now, however, following our commitment to digital manufacture, this mainly concerns detail CAD-based design work backed up with appropriate simulations of the underlying physics and the mechanisms being deployed. It is thus very important that a well-rehearsed approach to the detail design phase and the supporting analyses and tests that go with it be deployed. One of the key reasons for writing this book was to ensure that our student teams be aware of these things from the outset of their project work.
Next, we would point out the importance of experiments and test flights. It is often the case that building a Mark 1 airframe early on in a project, so as to gain information that can inform a Mark 2 version, is a better approach that committing too much effort to analysis that may be of limited fidelity or based on incorrect assumptions. It is also almost impossible to get every thing “right first time,” even in the most sophisticated design organizations. Early prototypes can be invaluable in improving detail design work.
Thirdly, we would re-emphasize the critical importance of weight and center of gravity control and their impact on stall speed, pitch stability, and take-off and landing velocities. Once the wings have been sized, any increase in weight leads inexorably to an increase in stall speed and thus worsens take-off and landing performance. Such weight increases over the design weight budget stem from two main areas: first, it is all too easy to neglect vital components when making weight build-up assessments (in particular, wiring and servo linkages are often much heavier in total than one might image); second, there is always the temptation to add things; in particular, when designing the structure, it is easy to add small strengthening and
Small Unmanned Fixed-wing Aircraft Design: A Practical Approach, First Edition. Andrew J. Keane, Andras Sobester and James P. Scanlan.
©2017 John Wiley & Sons Ltd. Published 2017 by John Wiley & Sons Ltd.
Figure 21.1 Our first student-designed UAV.
Figure 21.2 Not all test flights end successfully! stress-relieving elements without being fully aware of their total impact on weight. At all times one should be looking for places that are overly strong and removing weight, rather than just worrying about areas that are too weak and adding to them. Provided sensible sized spars are in use and basic stress analysis has been carried out, in our experience small UAV airframes are generally too strong. In all our work, we have only ever had one flight-critical structural failure and that was due to poor manufacturing and not to poor design or analysis. It is also wise to allow for some internal heavy items such as batteries to be moved forward and aft in the airframe without major redesign. This allows fine-tuning of the longitudinal center of gravity and thus control of the static margin. If there is great uncertainty about the longitudinal center of gravity position during the early stages of design, it is sensible to design the fuselage in a modular manner so that its length can be changed. We have even built aircraft where the fuselage length can be changed depending on the payload being deployed, see Figure 21.3. On smaller aircraft, it is sometimes possible to arrange for the main wing attachment point to be adjustable to achieve the same effect.
Fourthly, we note that if a very conventional type of airframe is to be produced, then simply following previous satisfactory design concepts will likely ensure a successful outcome. Perhaps the best place to start is with the choice of payload, powerplant, and overall airframe topology. It is only when the designs become more radical or extreme performance is required that the full gamut of well-established aeroengineering calculations become critical. That is not to say that such calculations should not be carried out for all projects; rather it is to point out that by following a previous well-designed example, much of the knowledge accumulated over the last 100 years of flight will implicitly be embedded in the new design. However, when it comes to making any calculation, we would emphasize that the care that goes into the calculation should be related to how important the result is, not how technically difficult the calculation is. So, such simple but crucial calculations as the choice of wing area should be checked and double-checked before they are accepted by the design team. One does not want to get to the airfield before it dawns on one that the wings are out of proportion to the rest of the airframe (see below)!
Good team working is, of course, also vital to any project undertaken by more than a single individual. Good teams need a mix of both personal and technical skills. Not everyone can be the team leader and many engineers are not good at such roles. Equally, it takes a particular
Figure 21.3 Aircraft with variable length fuselage. (a) Fuselage split open. (b) Spare fuselage section.
sort of mindset to carefully and relentlessly control weight growth in a project. It is also not wise to form a team comprised of 10 aerodynamicists, just one structural engineer, and no avionics experts. As in all aspects of aircraft design and manufacture, balance is crucial.
Finally, and by no means least importantly, always carry out initial flight trials at a properly controlled test airfield and with an experienced test pilot. As already noted, we have for many years worked with Paul Heckles of the Paul Heckles Flight Centre. If your test pilot is not of the first grade, then any unexpected behavior is likely going to lead to a major crash and total loss of the airframe.
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