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Figure 1: Maximum allowable wing loading as a function of altitude. This figure illustrates both the critical need for low wing loading in order to reach high altitudes and the aerodynamic margin that exists in present designs.


of payload requires from 5 to 20 unit masses of aircraft. Thus, when designing new platforms from scratch (as opposed to adapting existing ones) any reduction in payload mass has a large leverage on the overall system weight, which is in turn a primary determinant of aircraft cost. Thus we are driven technically and economically to use the smallest aircraft that will do the job. 

There are several ways to minimize the size of a high-altitude aircraft:

1. Minimize the weight of the payload. While it may be tempting to specify initially, that any future research aircraft be able to carry the same instruments that fly aboard the ER-2, this may in fact be far from the optimal approach. A more cost-effective solution can likely be obtained by spreading development dollars between instruments and airframes.

2. Remove the pilot. A human pilot with life support system adds at least 150 kg (and more likely, 500 kg) to any design, which is (or should be) a significant fraction of the payloads at issue here. Further, manned aircraft require higher levels of safety than unmanned aircraft, with consequent increases in structural weight fraction.

3. Minimize the time at high altitude to that required for the scientific measurements. It is important to distinguish between location of the measurement and the aircraft basing logistics. There are other ways of moving aircraft, such as using ferry flights at lower altitudes or making the aircraft small enough for disassembly and transport, when only logistics are being addressed. This distinction is almost irrelevant at lower altitudes, but the energy costs of high-altitude flight are so great that it is important to consider here.

Fortunately, a number of technological developments have converged to make the early 1990s a propitious time to develop an unmanned aircraft dedicated to scientific research.

Technology

Although pilotless aircraft date from at least the First World War, the vehicles under discussion here would have been impossible even a decade ago. Recent progress in four technologies has dramatically changed the prospects for a low-cost, scientifically viable unmanned aircraft. These technologies are:

• computational aerodynamics,

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