The Next X-plane

Air & Space Smithsonian - - Viewport - BY RICHARD WHIT­TLE

Tilt ro­tor, lift fan, tail sit­ter: What’s the best tech­nol­ogy for both ver­ti­cal lift and high speed?

de­vel­op­ment, pro­duc­tion, and op­er­at­ing these V/STOL con­cepts,” writes Mike Hirschberg, who de­vel­oped a mod­ern V/STOL Wheel of Mis­for­tune while work­ing on the F-35 pro­gram and is now the ex­ec­u­tive di­rec­tor of AHS. The only four to go into pro­duc­tion are the Har­rier, the Yak-38 and F-35B jet fight­ers, and the V-22 Osprey tiltro­tor troop trans­port, which swivels two large wingtip “pro­pro­tors” up­ward to take off and land like a he­li­copter and for­ward to fly like an air­plane. That en­ables the Osprey, now in ser­vice with the United States and, soon, Ja­pan, to cruise at 275 knots, roughly dou­ble the cruis­ing speed of most mil­i­tary heli- copters. Though the Osprey can hover, it doesn’t do so very ef­fi­ciently, in part be­cause speedy for­ward flight re­quires the pro­pro­tors’ blades to be short and heav­ily twisted, and in part be­cause the craft has high disk load­ing (the to­tal weight of the air­craft di­vided by the area of the cir­cle its ro­tor blades de­scribe). “Nearly all of the air­craft shown on the V/STOL Wheel—all of which were built and flown—could have been fielded, given enough time, money, and will­ing­ness to ac­cept com­pro­mise,” says Hirschberg. “But they needed to have a com­pelling ca­pa­bil­ity over less ex­pen­sive, less com­pli­cated con­ven­tional air­craft of their day.” What the VTOL X-plane pro­gram, also known as the VXP, is try­ing to do is to come at the prob­lem by com­bin­ing the lessons learned over the past 75 years with new tech­nol­ogy and new ideas.

DARPA’S Ba­gai says a key VXP re­quire­ment is “the abil­ity to hover and to per­form sus­tained hover. That’s an at­tribute of ver­ti­cal flight that’s im­por­tant. That’s why we have he­li­copters to­day, be­cause noth­ing else can do those mis­sions that he­li­copters can do.” Speed, he says, can be got­ten pretty much just by adding power: “We know how to fly barn doors fast.” He should know. When at Siko­rsky, he was prin­ci­pal engi­neer on the X2 high-speed tech­nol­ogy demon­stra­tor, a com­pound he­li­copter—coax­ial ro­tors with a pusher pro­pel­ler—that won the Col­lier Tro­phy in 2011 for set­ting an unof­fi­cial he­li­copter speed record: 250 knots in level flight.

In March 2012 Ba­gai came to DARPA and started the VTOL X-plane pro­ject, the sort of ex­per­i­men­tal pro­ject DARPA was made for. The agency doesn’t pro­duce mil­i­tary equip­ment; in­stead, it funds farsighted, some­times far-fetched re­search to help keep the mil­i­tary on the cut­ting edge of tech­nol­ogy. If DARPA’S projects bear fruit, the re­sults are handed over to the mil­i­tary for pos­si­ble de­vel­op­ment. For ex­am­ple, the agency did some early work on the lift fan tech­nol­ogy used in the F-35B, but has no di­rect role in F-35 pro­duc­tion.

The VTOL pro­ject kicked off in 2013, when the agency awarded pre­lim­i­nary de­sign con­tracts of $11 mil­lion to $17 mil­lion each to four com­peti­tors. Two of the awards went to some of Amer­ica’s top mil­i­tary avi­a­tion man­u­fac­tur­ers: Boe­ing and a Siko­rsky–lock­heed Martin team. The other con­tracts went to smaller com­pa­nies headed by two of the coun­try’s most in­ven­tive aero­nau­ti­cal engi­neers: Aurora Flight Sciences, whose chief de­signer, John Lang­ford, got his start head­ing a team that set a world record for hu­man-pow­ered flight, and Karem Air­craft, founded by Abra­ham Karem, who among other achieve­ments in­vented the Preda­tor drone.

The four con­tes­tants are of­fer­ing dra­mat­i­cally dif­fer­ent so­lu­tions—all un­manned, an op­tion DARPA of­fered to speed the work.

says, adding that Phan­tom Swift uses a “four-post lift ap­proach” for ver­ti­cal flight and hover. Af­ter ver­ti­cal take­off, lou­vers at the bot­tom of the lift fans in the fuse­lage can be used for ma­neu­ver­ing in a hover or closed en­tirely for for­ward flight. The body lift fans then shut down while the wingtip thrusters tilt for­ward to make the craft fly like a con­ven­tional air­plane.

“We have the abil­ity to con­trol thrust [in­de­pen­dently] on all four fans,” Rit­ter says. “We have lou­vers in the bot­toms of the body lift fans which can pro­vide di­rec­tional con­trol. We have the abil­ity to ro­tate our wingtip thrusters in­de­pen­dently and help pro­vide di­rec­tional con­trol. And then, by con­trol­ling the thrust out of each of those fans, all of those dif­fer­ent things com­bined pro­vide the means for low-speed di­rec­tional con­trol.” At high speed “you’ve got full fly­ing tails and flap­er­ons on the air­craft as well,” Rit­ter adds. “So we’ve got ma­neu­ver­abil­ity and agility, in ad­di­tion to just the wingtip thruster con­trol in for­ward flight.”

Power for Phan­tom Swift comes from two GE CT7-8 en­gines, a vari­ant of GE’S 700 se­ries, which are used in Siko­rsky’s S-92 multi-role he­li­copter. Boe­ing built no sub­scale demon­stra­tor to test its con­cept in flight, but Rit­ter says some “young engi­neers” at the Phan­tom Works built a six-foot model as “a mar­ket­ing tool dur­ing the pro­posal stage”; a video of it fly­ing in a lab can be found on Youtube.

It would be “very easy” to build manned or un­manned ver­sions of the Phan­tom Swift, Rit­ter ven­tures, scaled down from the VXP’S re­quired 10,000 to 12,000 pounds to as lit­tle as 4,000 pounds. Scal­ing Phan­tom Swift up to 24,000 pounds also could be done, he says, though “it’s go­ing to take some en­gi­neer­ing de­sign.” Nev­er­the­less, Rit­ter adds, “when we look at this, we see a great X-plane and fun op­por­tu­nity to demon­strate new tech­nol­ogy—but also the mak­ings of a new prod­uct line.”

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