Technicalities: Little Pies in the Sky

One of the staple features of cities of the future, if we can believe the artists and filmmakers who provide us with images of them, is the airborne taxi that whisks people from one strangely shaped building to another, heedless of the — presumably congested — streets below.

These agile and convenient people­ pods, levitated, like Marty McFly’s hover board, in unspecified ways, combine the best features of Manhattan’s swarming yellow cabs and the helicopters that carry a select subset of weekend commuters from downtown to the Hamptons.

The Jetsons had one. Maybe two.

The idea is not new. It occurred to the U.S. Army 60 years ago, and three firms — Chrysler, Curtiss-Wright and Piasecki — produced competing test-beds for a “flying jeep” that was presumably expected to be an improvement over the rolling one.

Two of the resulting designs used a pair of large ducted rotors; the third used four unducted propellers, two on each side of a long beam at the end of which an uneasy pilot perched. Unlike the road jeep, whose 60 hp “Go Devil” engine could propel it 300 miles on a tank of gas at up to 65 mph, the flying versions required around 500 hp, moved slowly and could not go far.

In 2007, at the prodding of Mark D. Moore (who has since gone to Uber, where he is studying an airborne version of the firm’s ride-hailing service), NASA began to fund studies and competitions aimed at developing a more or less new class of PAV, or personal air vehicle. Originally, the definition of a PAV consisted of little more than the obverse of common criticisms of personal aircraft: A PAV would be affordable, quiet, efficient, capable of being safely operated by a minimally skilled pilot in all weather conditions, and would require only small areas for takeoff and landing.

This sounded a bit like a Piper Cub, but NASA had something more innovative in mind. The goal, however, was elusive, and a good deal of prize money was squandered on products and projects that excelled under one or another of the defining criteria, but not under all, or even two or three.

Progress came, as it often does, from outside. The breakthrough technology, developed by industry (with no help from NASA), is the multicopter drone, most commonly seen as a quadcopter, which has soared into popularity so dramatically that a whole generation is growing up believing that the word “drone” means not a pilotless jet or an unemployed bee but a spidery plastic radio-controlled Christmas present. Most of the multicopter drones represent the happy convergence of several technologies—GPS, solid-state gyros and high-energy-density batteries—which are cheap because they are incorporated in billions of consumer products. Self-stabilizing, aware of location and capable of being endowed with practically any desired degree of autonomy by mapping, navigation and collision-avoidance software, multicopter drones have opened the door to the realization of the PAV dream: the autonomous, pilotless aerial taxi that would come when summoned and whisk you to your destination with no more effort on your part than it takes to say (be sure to speak clearly!) where you want to go.

All practical multicopter drones today run on electric power, which is easily capable of the subtle, instantaneous modulation of motor speed — the propellers are invariably fixed-pitch — by which the platform is controlled. Energy is usually supplied by batteries, although in a couple of instances hybrid power systems using electric motors and an internal­ combustion generator are proposed.

Quite a few man-carrying multi­copters have been built, some by homebuilders, some by industrial firms even including Airbus. Some achieve flight, and relative economy of construction, by using large numbers — e.g., 18 — of small rotors. Others use as few as four, although it is not clear to me how a four-rotor system survives the loss of one motor. Several designs, including Airbus’ Pop.Up, provide two motors and propellers on each of four arms, taking the quadcopter design and giving it the functionality of an octocopter.

The Airbus scheme is a good example of the ingenuity, not to say playfulness, being brought to this new technology. It envisions three modules: a four-wheel rolling chassis, a two-seat passenger pod and an autonomous rotor set. For flight, the rotor module flies itself to the passenger pod, sinks its claws into the roof like the roc of Arabian Nights fame and carries the pod away, leaving the chassis behind. At the destination, the rotor-roc places its prey gently upon another chassis, and the reconfigured vehicle, about the size and shape of a Smart car, drives off.

A simpler concept from EHang, a Chinese manufacturer of consumer drones, is similar in general configuration to the Pop.Up, but omits the roadable portion and the fan shrouds (which double, by the way, as people protectors). It was claimed a while ago that you would be able to hail an EHang 184 PAV in Dubai in 2017. I doubt this has come to pass, but as Pliny would have said, something new is always coming out of Dubai. A fundamental problem of multirotor designs is the conflict between compactness and disk loading. The single-seat EHang fits within a 15-foot square. Compare the EHang design to the Robinson R22, its conventional-technology equivalent — bearing in mind that the R22 is a two-seater. The R22’s 25-foot rotor sweeps 497 square feet and the helicopter’s gross weight is 1,370 pounds, for a disk loading of 2.76 pounds per square foot. This is on the low end for helicopters but allows the R22, which uses a 360-cubic-inch Lycoming engine derated to 124 hp continuous, to perform well with relatively little power. The R22’s power loading for takeoff is 10 pounds per horsepower.

The EHang 184, which is a single­seater, weighs about 800 pounds, including payload. Although it has eight 63-inch propellers, they operate in contrarotating pairs, so its disk loading is 9.2 pounds per square foot. Its maximum power is 191 hp, for a power loading for takeoff of 4.2 pounds per horsepower.

The fact that the EHang needs 50 percent more power to hoist 40 percent less weight highlights an inescapable fact about multirotor configurations. One large rotor is more efficient, within a given footprint, than several small ones. The multirotor suffers additional losses because, unlike the blades of a helicopter, its fan blades do not change pitch cyclically to compensate for forward motion. In exchange for the lost efficiency, however, you get the multirotor’s compactness, cheapness, reliability, ease of manufacture, and the great asset of electronic management of stability and control.

A persistent embarrassment, as with everything electric, is range. Avgas delivers about 1.8 hp-hr per pound to the propeller; the best current batteries do a tenth as well. The EHang 184 claims a cruising duration of 25 minutes at 54 knots, presumably including takeoff, a vertical climb to an unspecified altitude, a vertical descent and landing. The R22 goes 250 nm at 90 knots, using 100 pounds of fuel. An automotive analogy might be a Tesla sedan that goes 250 miles between charges, using 1,200 pounds of batteries. A conventional sedan consumes 50 pounds of gasoline for the same trip.

Self-flying electric PAVs might not work perfectly — yet — but they do work. Brace yourselves. We pilots might just go the way of elevator operators.

Peter Garrison taught himself to use a slide rule and tin snips, built an airplane in his backyard, and flew it to Japan. He began contributing to FLYING in 1968, and he continues to share his columns, "Technicalities" and "Aftermath," with FLYING readers.

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