Size Matters – Small is Sexy in the UAV Microcosm

Richard B Gasparre discovers just how small a UAV can get.


When a new technology takes off, associated terms and names proliferate as well, usually with only a passing nod at overall consistency. In the unmanned aerial vehicle (UAV) world, technological advances promise to make current categories seriously incomplete if not altogether obsolete.

This may be a semantic problem for now, but changes in physical scale and mechanical characteristics will have major functional consequences down the line.

THE USAF'S WASP

Given that UAVs are by definition flying vehicles, let's use as a baseline the UAV taxonomy of the US Air Force. The USAF system has four tiers, and the lowest-altitude layer is the suggestively named tier N/A, which was carved out of tier 1 in prospective recognition of the unique qualities of mini-UAVs.

The current archetype of the tier N/A vehicle is the BATMAV (Wasp block 3) system, built by Aerovironment. Like most small planes, the Wasp gets lift from fixed wings and thrust from a propeller, which is powered by a 10W electric motor.

All three Wasp models have top speeds of about 40mph, outright ranges of around three miles, and endurance of almost an hour. But look at the Wasp's size evolution:

  • Block 1: 5in long, 13in span, 0.4lb
  • Block 2: 6in long, 16in span, 0.6lb
  • Block 3: 15in long, 29in span, 0.9lb

Size is directly proportional to payload here, but one of the Wasp's most useful attributes is that individual soldiers can launch it by hand. In contrast, the tier 1 Gnat is 500lbs, which leaves a lot of room between the current 1lb Wasp and the upper limits of throw launching. After all, even Olympic shot-putters toss only 16lb weights.

THE ARMY'S RAVEN

The lowest-altitude layer in the Army's taxonomy is class 1, the home of the highly praised RQ-11 Raven UAV, which is also built by Aerovironment. Like the Wasp, the Raven has standard fixed wings, a propeller, and an electric motor.

"The extra capability of the Raven's does require some extra size compared to the Wasp."

Unlike the Wasp, however, the Raven has a top speed that is 50% faster at around 60mph, twice the range at just over six miles (including 'over-the-hill capability), and up to double the endurance at 90 minutes, not to mention an operational altitude and ceiling an order of magnitude greater. An intraclass disparity this large doesn't exist in any standard military aviation category, such as fighter or cargo aircraft.

The Raven's extra capability does require some extra size compared to the Wasp. The B model has a length of 3.5ft, a wingspan of 4.25ft, and weighs 4.2lb. Relatively speaking, wingspan is the smallest of these dimensions, but even so, the Raven is almost double the width of the Wasp.

LET'S GET SMALL

Clearly, the Army's class 1 and the Air Force's tier N/A are most similar in structural design. This may sound like a low hurdle to jump given that prop-driven fixed-wing airplanes are a century old. Nevertheless, jets such as the variable-geometry F-14 and the vectored-thrust Harrier suggest that structural aviation design possibilities are much broader than the line-up at your local airport taxiway would indicate.

In fact, miniature UAV technology has crossed the engineering threshold from traditional aeronautics to biomimicry. Standard aviation technology, such as fixed wings and rotorised propulsion, does not scale down effectively for micro-UAVs smaller than the Wasp. Instead, researchers are using natural biological designs, such as flapping wings, to provide lift, thrust, and stability in one structural package.

The more prominent micro-UAVs emerging from labs are indeed the size of insects. At Onera, France's national aerospace lab, the Remanta programme has a self-imposed design constraint of a 15cm wingspan, which is just under 6in. This is not only a fifth of the block 3 Wasp's wingspan, but 1cm less than the smallest wingspan in Chiroptera, the bat order.

How small can small get? At Harvard University's Microrobotics Laboratory, founder and director Robert Wood is developing a robotic fly that currently weighs just over 2oz and has a wingspan of just over 1in. These dimensions are one-seventh and one-twenty-fifth those of the Wasp, but more critically, they are the measurements of your average housefly.

"A robotic fly is in development that currently weighs just over 2oz and has a wingspan of just over 1in."

Achieving true aerial autonomy at this physical scale would have major implications for operational use in many respects. In particular, small is stealthy, but stealthy is more than just small. The ability to fly like a bird, bat, or insect gives a UAV a form of natural camouflage – even if hostile troops see it, they may well misinterpret it as an animal.

So how should the military services characterise small UAVs? If the goal of military taxonomy is to capture functional differences, a single class 1 or tier N/A is clearly inadequate. The key question is the number of levels to create, and several organising principles are possible for this purpose. If visibility is a critical consideration, however, the first distinction should separate traditional aeronautical UAVs from the new biomimetic family.

Within the latter, given the proliferation of design features in nature, a simple scheme might focus on the ability of soldiers to recognise a UAV as such at a glance. Therefore, counting backwards from tier N/A, which is really tier 0, would yield the following:

  • Tier -1: recognisable when in flight
  • Tier -2: indistinguishable in flight, but recognisable at rest in a relatively small space, such as a living room – the Remanta products would probably fall into this range
  • Tier -3: indistinguishable at rest, but recognisable at arm's length, or by touch and inspection – Professor Wood's fly would fall into this category
  • Tier -4: recognisable only through inspection by CSI teams or their equivalents

If nothing else, the Air Force should rename the Gnat and the Army should rename the Wasp. Metaphoric names might have been acceptable a decade ago, but they amount to no more than false advertising when micro-UAVs really are as small as genuine gnats and wasps.