Two sets of wings and a 4-in-1 rocket engine ac­cel­er­ates a Chi­nese plane to up to seven times the speed of sound. The de­sign kick-starts a new era of hy­per­sonic air­craft.

Science Illustrated - - CONTENTS -

Olde-timey flight was all about biplanes, then we aban­doned that idea. But maybe we need to re­visit it, to go even faster...

Three planes leave Bei­jing for New York at the same time. An or­di­nary Air­bus A320 air­liner, the pen­sioned-off su­per­sonic Con­corde, and the new Chi­nese I-plane de­sign, which is a biplane with two sets of wings. In the imag­ined sce­nario, the Air­bus A320 and the Con­corde will land in New York 14 and six hours after take-off, re­spec­tively. A busi­ness­man, who trav­elled on the I-plane, is in a meet­ing in New York after only two hours, and he can get back to Bei­jing again and get a good night’s sleep, be­fore the Air­bus A320 ever reaches New York. The trick be­hind the I-plane ’s hy­po­thet­i­cal feat is a new engine type com­bi­na­tion, which can ac­cel­er­ate the plane to hy­per­sonic speeds, i.e. seven times the speed of sound: Mach 7.

The sta­bil­ity of the I- plane at such ex­tremely high speeds is due to the two sets of wings, which also in­crease the load ca­pac­ity and seems to min­i­mize the su­per­sonic booms caused by speeds of Mach 1+. The com­bi­na­tion of in­no­va­tive de­sign and the TRRE engine makes the Chi­nese air­craft quite a sen­sa­tion. If the I-plane takes off as planned in 2025, it will be at a speed of 8,600+ km/h.

Plane surfs on shock wave

Hy­per­sonic air­craft are planes that can fly at speeds of Mach 5+ or what cor­re­sponds to about 6,200 km/h at sea level. Nor­mally, the speed will be reached in thin­ner air lay­ers, where the speed of sound varies slightly de­pend­ing on tem­per­a­tures, etc. At such high speeds, fly­ing is haz­ardous, and so, the speeds have pri­mar­ily been con­fined to drones and rock­ets. How­ever, the I-plane de­sign al­lows for a hy­per­sonic pas­sen­ger plane race.

The most suc­cess­ful air­craft de­sign for hy­per­sonic flight is the wa­verider de­sign, which func­tions ac­cord­ing to the same prin­ci­ple as a surfboard. Any craft trav­el­ling through the air pushes air in front of it. At su­per­sonic speeds, the air drag turns into a wave that causes a boom of up to 200 deci­bels, as the plane passes through it. The higher the speed, the more pow­er­ful the pres­sure wave and the drag that it makes up. In other words, a wa­verider plane surfs on the drag by point­ing its nose up­wards at a spe­cific an­gle and main­tain­ing the pres­sure un­der the craft on a care­fully de­vel­oped, flat, or slightly in­wardly curved subsurface.

The I-plane also takes ad­van­tage of the wa­verider prin­ci­ple, but the Chi­nese sci­en­tists be­hind the con­cept con­cluded that the de­sign in­volves a num­ber of weak­nesses. In­tense shock waves make aero­dy­nam­ics at hy­per­sonic speeds too un­pre­dictable to make a sta­ble wa­verider de­sign, and wave-rider planes are very thin and frag­ile and leave too lit­tle space for pas­sen­gers and other cargo. So, the Chi­nese sci­en­tists added two ex­tra wings, which tack­led many of the ma­jor chal­lenges of hy­per­sonic flight in re­cent wind tun­nel tests.

Dou­ble wing bal­ances air flow

The I-plane still needs to pass a long se­ries of tests, be­fore the craft can take off, but if the two ex­tra wings per­form just as ef­fi­ciently in real life as in the first tests, the plane will be sta­ble, able to carry more load, and is seem­ingly less noisy than other su­per­sonic air­craft.

The wings were added, when the sci­en­tists no­ticed that at hy­per­sonic speeds, com­pressed air flowed up along the top side of the plane. They con­cluded that the flow could be used, so they placed an­other set of wings at the top of the plane, and sub­se­quent tests showed that the de­sign was much more sta­ble and gen­er­ated more lift than the clean- cut wa­verider de­sign.

The pow­er­ful lift al­lows the plane to bulge on the top side, leav­ing space for the craft to one day be­ing able to load up to five tonnes of goods or some 50 pas­sen­gers. So far, wa­verider planes have only been able to carry one sin­gle per­son.

More­over, the I-plane’s ex­tra wings will al­ter the pres­sure, split­ting up and min­i­miz­ing the sound of the su­per­sonic boom. So, su­per­sonic and hy­per­sonic air­craft may one day be al­lowed to fly above land, which is now il­le­gal in China, the US, and ma­jor parts of the world. The ban was vi­tal for the Con­corde never be­com­ing a ma­jor suc­cess.

Pre­dictable hy­per­speed

Hardly any biplanes have been made since the 1930s due to in­creased weight, air drag, and poor fuel econ­omy, but plane de­sign­ers' ex­per­i­ments will now ap­par­ently al­low biplanes a come­back in the shape of the I-plane.

Aero­dy­nam­ics and plane de­sign are no­to­ri­ously hard to cal­cu­late, par­tic­u­larly at hy­per­sonic speeds, as count­less fac­tors af­fect the plane’s lift and re­sis­tance. At or­di­nary speeds of less than that of sound, an ob­ject such as a car, a plane, or a wing will split the air into flows around the ob­ject. In the case of a plane, suc­cess­ful aero­dy­namic de­sign will ac­cel­er­ate the air across the craft, forc­ing it in a down­ward, back­ward direc­tion. The mo­tion causes un­der­pres­sure above the plane, al­low­ing the air flow pass­ing the subsurface to push the ob­ject up­wards.

As a plane ac­cel­er­ates to su­per­sonic speeds, the rules of lift and aero­dy­nam­ics be­come un­pre­dictable, and hy­per­sonic speeds are even more com­pli­cated. At around Mach 5, the laws of aero­dy­nam­ics hardly ap­ply any­more. At al­most 6,200 km/h, the re­sis­tance does not come from the air, but rather from the waves that the plane pro­duces.

As the I-plane’s ex­tra wings seem to coun­ter­act su­per­sonic booms and re­duce the ex­tent of waves that could cause tur­bu­lence, etc., sci­en­tists have found a de­sign that can man­age the ex­tremely high speeds.

Su­per­sonic aero­space ac­tiv­i­ties

The fu­ture ex­is­tence of the I-plane also re­quires that its TRRE engine passes a se­ries of tests in 2025. The engine is the first of its kind to com­bine four engine types in the same sys­tem, and it could pave the way for hy­per­sonic air­craft. So, the I-plane de­sign might be in­ter­est­ing in sev­eral ways.

Much of the hy­per­sonic race is aimed at space. Tak­ing off from a run­way, car­ry­ing goods, as­tro­nauts, or tourists into space, and land­ing safely on the ground again is a chal­lenge that hy­per­sonic planes hope to han­dle in the fu­ture. If the I-plane can func­tion as a re­us­able lift­ing mod­ule, the plane will re­duce the costs of aero­space ac­tiv­i­ties con­sid­er­ably, per­haps al­low­ing more and more fre­quent space mis­sions.

How­ever, the Chi­nese and Amer­i­can mil­i­taries also closely ob­serve the de­vel­op­ment of hy­per­sonic planes, whose high speed re­duces the dis­tance be­tween the great pow­ers, i.e. the tech­nol­ogy con­sti­tutes a threat that they must be pre­pared for.

In re­la­tion to the gen­eral pub­lic, the I-plane and its com­peti­tors could kick-start a new era, in which air traf­fic makes the world even smaller than to­day. The de­mand for su­per­sonic pas­sen­ger flight is ris­ing in an in­creas­ingly glob­al­ized world. The Con­corde flew at Mach 2.2, but was scrapped in 2003 after a tragic crash and sub­se­quently years of losses. The I-plane prom­ises to be suc­cess­ful, so the dream of trav­el­ling to re­mote cor­ners of the world in only three hours is more re­al­is­tic than ever.

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