Per­sonal air ve­hi­cles are on the way

THE FU­TURE IS AL­MOST HERE 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

Flying - - Contents - By Peter Gar­ri­son

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 in­ter­nal­com­bus­tion 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 360cu­bic-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.

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