Fly easy with better aerodynamics

You could be for­giv­en for think­ing that everything has already by done in aero­dy­nam­ics. We already know that redu­cing planes’ vapour trails allows them to con­sume less fuel, there­fore lim­it­ing release of green­house gases. To be exact, each unused kilo­gram of ker­osene saves 3.16 kg of CO₂ from being released. How­ever, pro­gress remains to be made. “There are still areas where we can con­tin­ue to reduce drag,” Mar­ie Couli­ou, an exper­i­ment­al aero­dy­nam­ics research­er at aerospace lab ONERA, says.

The goal is to con­trol air­flow over the plane. This can be done in pass­ive ways by adding append­ages to modi­fy the air­flow path or chan­ging the rough­ness of sur­faces. Or act­ive meth­ods can be used, by installing pulsat­ing jets that force air­flow to fol­low a cer­tain topo­logy, for instance. But where­as a lot of research has gone into the former – par­tic­u­larly at Boe­ing and NASA – they have been used on very few planes so far. 

Before­hand, tests are required to bet­ter under­stand how these append­ages age through­out the life of an air­craft. As for act­ive meth­ods, a major obstacle has presen­ted itself: the energy required. So, even if these act­ive sys­tems work well in wind tun­nels, imple­ment­ing them in real con­di­tions may be a whole oth­er story. How­ever, if suc­cess­ful, these flow con­trol sys­tems could save a few per­cent­age points of effi­ciency, which would rep­res­ent colossal amounts of fuel.

Inter­est­ingly, ONERA has developed a very innov­at­ive plane concept: Nova¹. It was designed to optim­ise aero­dy­nam­ics. Spe­cific­ally, the jet engines are partly integ­rated, instead of being loc­ated out­side the fusel­age. As a res­ult, the air enter­ing the engines comes from lay­ers run­ning off the fusel­age, which are there­fore slowed down by fric­tion. “This concept should decrease fuel con­sump­tion by 15–20% com­pared to cur­rent mod­els of medi­um-haul air­craft,” the research­er states.

Redu­cing vapour trails does not con­cern only single planes, it can also be used on groups. Why not imit­ate the “V” form­a­tion of migrat­ing birds? Air­bus believes that this concept could reduce fuel con­sump­tion by 5–10%. With each plane cre­at­ing wing­tip vor­tices, this accel­er­ated air would give extra lift to nearby planes. How­ever, they should not fly too close to one anoth­er, for safety reas­ons. The bal­ance between effi­ciency and safety is yet to be found.

Decreas­ing fric­tion is not the only aim of aero­dy­nam­ics research. One less­er known prob­lem is that of con­trails, those long white lines that can be seen after planes pass at high alti­tudes. They occur when ice crys­tals form around soot particles emit­ted by jet engines, and they can have a ser­i­ous impact on the cli­mate. Under cer­tain con­di­tions, they turn into cir­rus, wispy clouds that appear at high alti­tudes. Although the sun’s rays can pass through them, they partly reflect rays boun­cing from the earth back towards the ground, thereby con­trib­ut­ing to cli­mate imbal­ance ². This effect has been well known for a long time but, thanks to pro­gress in cli­mate sci­ence, we now know for cer­tain that this has a sig­ni­fic­ant neg­at­ive impact.

“It’s dif­fi­cult to meas­ure the effect of con­trails; there is still a lot of uncer­tainty, but they could have as big an impact as planes’ fuel con­sump­tion,” the research­er con­firms. Accord­ing to Ger­man cli­mate sci­ent­ist Bernd Kärch­er, con­dens­a­tion clouds are respons­ible for the vast major­ity of the radi­at­ive for­cing caused by the avi­ation industry. Radi­at­ive for­cing meas­ures the way in which a factor dis­rupts the Earth’s energy bal­ance. In the case of avi­ation, con­trails con­trib­ute more to this prob­lem than the industry’s CO₂ emis­sions³.

Pre­vent­ing these air­craft-induced cir­rus clouds is there­fore cru­cial. How­ever, we first need to bet­ter under­stand how they form and evolve. In addi­tion to water vapour, jet engines emit soot, which serves as the trig­ger for ice crys­tals to form. These crys­tals are then scattered by wing­tip vor­tices. “Under­stand­ing the con­di­tions in which these con­trails form and dis­perse, accord­ing to the pos­i­tion of the motor in rela­tion to the wing­tips, would mean we can reduce their impact on the cli­mate,” Ms. Couli­ou adds. There is still much pro­gress to be made in this area.