GREASED LIGHT­NING

The science be­hind NASA’S in­sane tilt-wing con­cept

Flying - - Front Page - By Peter Gar­ri­son

When it is not oc­cu­pied with ex­trav­a­gan­zas like send­ing peo­ple to Mars, NASA does some in­ter­est­ing work on ter­res­trial aero­dy­nam­ics. Three years ago, I wrote about a NASA project called LEAPTECH (Lead­ing Edge Asyn­chronous Pro­pel­ler Tech­nol­ogy) that in­volved string­ing 18 elec­tric mo­tors and pro­pel­lers along the lead­ing edge of a very skinny wing.

The idea was to blow high-speed air over the wing dur­ing take­off and land­ing, aug­ment­ing lift and al­low­ing the wing area (and weight, and drag) to be re­duced by two-thirds. In cruise, 16 of the mo­tors would shut down and their pro­pel­lers would fold flush with the sides of their na­celles; thrust would then be pro­vided by two mo­tors at the wingtips. Th­ese would ro­tate top-blade out­ward, weak­en­ing the tip vor­tices and shav­ing off some in­duced drag. A full-scale pi­loted test item is now be­ing built, us­ing the fuse­lage and em­pen­nage of a Tec­nam P2006T, an Ital­ian-built light twin. It is ex­pected to fly in 2018 or 2019, and will, as NASA mod­estly states, “al­low engi­neers to com­pare the per­for­mance of the flight demon­stra­tor with that of the orig­i­nal P2006T.”

In the mean­time, an­other NASA group, this one at the Lan­g­ley Re­search Cen­ter in Vir­ginia, has been de­vel­op­ing a con­cept that is sim­i­lar in some re­spects to LEAPTECH, but in oth­ers more rad­i­cal. It is called GL-10. The GL stands for Greased Light­ning, not be­cause it is ex­pected to be ex­cep­tion­ally fast, but be­cause in its fi­nal form it will use a hy­brid power sys­tem con­sist­ing of an in­ter­nal com­bus­tion en­gine burn­ing fos­sil fuel or dis­carded french-fry oil — hence the “greased” — and driv­ing a gen­er­a­tor to sup­ply power to elec­tric mo­tors — hence the “light­ning.”

The 10 stands for the num­ber of mo­tors and pro­pel­lers. Each wing has four mo­tors, spread out so that the en­tire wing is bathed in prop wash; in that re­spect, its tech­nol­ogy is sim­i­lar to LEAPTECH’S. Two more mo­tors are on the tail. One project en­gi­neer de­scribed it as a tan­dem-wing de­sign, as op­posed to wing-and-sta­bi­lizer, be­cause the CG is some­what far­ther aft than it nor­mally would be and so the hor­i­zon­tal sta­bi­lizer/aft wing con­trib­utes about a sixth of the lift in cruis­ing flight. The rea­son for this de­sign choice will be­come ap­par­ent shortly.

What is un­usual about Greased Light­ning is that it’s a VTOL — ver­ti­cal take­off and land­ing — ma­chine that con­verts into an air­plane for cruis­ing flight. Un­like the tiltro­tor V22 Osprey, whose wing is fixed while its en­gines and ro­tors tilt, Greased Light­ning is a tilt-wing. Its mo­tors and pro­pel­lers are fixed with re­spect to the wing, which tilts up to a ver­ti­cal in­ci­dence for hover.

This con­fig­u­ra­tion has been around a long time. Canadair stud­ied ex­per­i­men­tal tilt-wing mod­els in the late 1950s, and the 1964 Ling-tem­coVought XC-142 was quite a big thing, with four in­ter­con­nected 2,850 hp tur­bo­props and a max VTOL weight

of 45,000 pounds. Al­though nei­ther air­plane went into pro­duc­tion, both, along with other fly­ing test-beds, ex­e­cuted many tran­si­tions to and from hover un­der di­rect pi­lot con­trol. A no­table char­ac­ter­is­tic of both was an un­usu­ally low power load­ing. The XC-142’S was about 4 pounds per horse­power; the Canadair’s 2. For com­par­i­son, power load­ings of con­ven­tional util­ity trans­ports like the C-130 Her­cules or CASA CN-235 are in the range of 8½ to 9½ pounds per horse­power. The ex­cess power is needed for the ver­ti­cal phases of flight; as any bird knows, it takes far more power to lev­i­tate in place than to fly for­ward.

The aim of a “tilt” de­sign, be it tiltro­tor or tilt-wing, is to com­bine the range, speed and ef­fi­ciency of a con­ven­tional air­plane with VTOL ca­pa­bil­ity. In its cruis­ing con­fig­u­ra­tion, Greased Light­ning — a 10-foot-span, 62-pound ra­dio-con­trolled half-scale model — con­se­quently looks pretty much like a con­ven­tional air­plane. Un­like the Osprey, whose huge pro­pel­lers only per­mit it to take off and land ver­ti­cally, or nearly so, Greased Light­ning could, if its pi­lot wished, op­er­ate from a run­way like an or­di­nary air­craft, par­tic­u­larly be­cause the vari­able-in­ci­dence wing would al­low its fuse­lage to re­main hor­i­zon­tal dur­ing flare or ro­ta­tion.

Pitch and roll are con­trolled in hover by sep­a­rately vary­ing the speeds, and there­fore the thrust, of the mo­tors. Ailerons, blown by the wing mo­tors, con­trol yaw. The rea­son for the aft lo­ca­tion of the CG should now be ap­par­ent: For the two aft mo­tors to bear their share of the load dur­ing take­off and hover, the CG must be located at about 20 per­cent of the dis­tance be­tween them. Aft load­ing is detri­men­tal to lon­gi­tu­di­nal sta­bil­ity, how­ever, and so the wing is mildly swept. When it ro­tates to a hor­i­zon­tal po­si­tion its cen­ter of lift moves aft, re­duc­ing the dis­tance to the CG.

Dur­ing the tran­si­tion from hover to cruise, the wing grad­u­ally tilts for­ward so that some of the thrust be­gins to ac­cel­er­ate the air­plane hor­i­zon­tally. Wing-tuft video of Greased Light­ning, which can be found on Youtube, shows that the wing is en­tirely stalled while the air­plane gains speed with the wing partly tilted. The air­flow on the wing is a mix of pro­pel­ler-driven and am­bi­ent flows mov­ing in dif­fer­ent di­rec­tions, and the pro­pel­lers them­selves op­er­ate at an an­gle to the lo­cal air­flow. “There’s a lot go­ing on,” says project man­ager Robert Mcswain. Some sense of the sheer power re­quired for hover and tran­si­tion can be gleaned from one statis­tic: The power that GL-10 re­quires to hover is 16 times the power it re­quires to fly in air­plane mode.

The con­trols used in hover are dif­fer­ent from those used in cruise; the segue from one to the other re­quires man­age­ment of many vari­ables. This is the sort of sit­u­a­tion that cries out for a fly-by-wire con­trol sys­tem, which Greased Light­ning has. An in­ex­pen­sive off-the-shelf hob­by­ist’s con­troller trans­lates the pitch, yaw, roll and power out­puts from a con­ven­tional RC trans­mit­ter into the com­bi­na­tions of com­mands to mo­tor, wing and con­trol sur­faces that make tran­si­tion pos­si­ble.

The tar­get of the pro­gram is an au­ton­o­mous 20-foot-span sur­veil­lance air­plane that would cruise 200 miles at 100 knots, loi­ter at 21,000 feet for 20 hours or so and re­turn to base. The cur­rent test ar­ti­cle, which was pre­ceded by sev­eral smaller ones, is half-scale; it is bat­tery-pow­ered, and its en­durance is lim­ited. The full-size air­plane would use liq­uid fuel to power its en­gine-gen­er­a­tor unit, or genset. The en­ergy con­tent per pound of liq­uid fuel is much greater than that of bat­ter­ies, and al­lows long flights — which are the rea­son for a fast cruiser in the first place — at a weight that still per­mits ver­ti­cal take­off and land­ing. Surge power for take­off, hover and tran­si­tion would be stored in bat­ter­ies that the genset would recharge while cruis­ing.

A NASA tech­ni­cal mem­o­ran­dum sum­ma­riz­ing re­sults of the GL-10 ex­per­i­ments thus far con­cludes by sug­gest­ing three po­ten­tial roles for such air­craft: search and sur­veil­lance, pack­age de­liv­ery and, in a 3,000-pound ver­sion, on-de­mand per­sonal trans­porta­tion for four peo­ple over greater dis­tances, and at higher speeds, than a pure ro­tor­craft could achieve.

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