| Themes > Science > Physics > Fluid Dynamics > Flying the flag for fluid dynamics > Aerodynamics > Hydro- and Aerodynamics of Animal Swimming and Flight > How to build a small-scale flying machine? |
There probably should be a question mark at the end of this section heading...in any event, now that small-scale engineering technology is becoming more commonplace, there is renewed interest in constructing small-scale flying machines, mostly as manouevrable, and stealthy, information-gathering devices. Applications can easily be envisioned in both military and civilian life. Leaving aside for now the justifiable concerns about the possible infringement of liberty when the borders between these two areas are blurred, the engineering objectives of designing and constructing a small flying machine are fascinating, and bring us into head-on collision with many of the topics discussed above. The requirements for a Microscale Air Vehicle (MAV) are that it should be able to fly from 0-10 m/s (note that 0 m/s includes hovering), with a range of 10-20 km. The wingspan will be 15-20 cm, and the total takeoff mass will be 100g, or less. A quick calculation or two shows us that the specifications point to a slightly overweight starling, which is otherwise a perfectly average bird. Should it then flap its wings? What is the benefit of doing this? An invited talk entitled Are small-scale flying machines practicable? was given at a symposium in Lund, Sweden, addressing these issues. As described in more detail in the abstract of the following paper Technical Aspects of Microscale Flight Systems [SL98], the principal areodynamical difficulties of flight at this scale stem from the low Reynolds numbers involved, where standard aerofoil performance is quite poor. This consideration includes the lifting and propulsive efficiencies of both the wings and the propeller (if there is one), and one possible argument for a flapping wing design is that combining the two functions decreases the performance hit on the tiny propellers that would otherwise be used. On the other hand, the engineering requirements of designing and constructing an efficient reciprocal oscillator drive system at these small scales may outweigh (literally!) any potential benefits. In any event, the power requirements of this micro-scale device will be formidable, and currently-available battery technology is insufficient. The figure of merit here is the energy density available from a power source, measured in J/g. Gasoline contains about 4.7 x 10^4 J/g, which is sufficient to power the MAV, but no-one has built an efficient gas-powered engine at this scale. Aeromodellers will also attest to the distinct lack of stealthy qualities in existing devices. It is an interesting fact that the fat metabolised by vertebrates to fuel muscle activity has about the same energy density as gasoline. Perhaps it would be easier after all to strap a camera to the head of a (very) well-trained hummingbird... However, the challenge has been set, and the similarities and differences that emerge between human-engineered and naturally-evolved flight systems will continue to provoke the humans into energetically searching for improvements. This is how we make progress, in understanding, and building. |
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