Home Engineering Small Unmanned Fixed-Wing Aircraft Design. A Practical Approach
Things that Have Gone Wrong and Why
We have learned these lessons the hard way: over the years we have had a number of “uncontrolled descents,” mostly of student-designed and -built aircraft (but not always). We try not to intervene too strongly into our student groups during their project work since we find they tend to learn more if allowed to make their own mistakes. What follows is a list of some the things that have gone wrong and why - it is a bit of a rogue’s gallery but we would not pretend that we only ever make perfect aircraft at the University of Southampton:
Figure 21.4 Student-designed flying boat with large hull volume forward and insufficient vertical tail volume aft.
Figure 21.5 Aircraft with split all-moving elevator. (a) Without dividing fence. (b) With fence.
Engine vibrations caused a power isolation switch to momentarily open and thus initiated the engine ignition safety cut-out system, forcing the pilot to glide the unpowered aircraft back to the landing strip.
• A sizing error during concept design led to an aircraft being built with wings that were too small given the installed power; this was not revealed until its first catapult launch, Figure 21.7.
Figure 21.6 Autopilot on vibration test.
Figure 21.7 Student UAV with undersized wings. The open payload bay also added to stability issues.
Figure 21.8 2SEAS aircraft after failure of main wheel axle.
• Repeated autonomous landings on a concrete runway led to low-cycle fatigue of a main wheel axle due to the significant impact loads experienced. Subsequent use of laser height finding mitigated these issues on automated landings, Figure 21.8.
Despite this apparent litany of failures, our aircraft fly successfully much more often than not; so we would encourage anyone setting out to design and build his/her own unmanned air system (UAS) to go right ahead. It is not nearly as daunting as one might initially think (even given the size of this book) and it can be a great deal of fun. Certainly, our student teams enjoy the experience enormously and clearly get a great thrill when “their” aircraft takes wing for the first time. Even those whose aircraft do not perform as desired learn a great deal from actually being directly involved in the whole process from first ideas to final flight.
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