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Is a carbon-free aviation industry really possible?

Fly easy with better aerodynamics

Analysis Cécile Michaut, Science journalist
On February 2nd, 2021 |
3 mins reading time

Marie Couliou
Marie Couliou
Research scientist at ONERA and temporary lecturer at the Institut Polytechnique de Paris
Key takeaways
  • Each kilogram of kerosene saved is equivalent to 3.16 kg less CO2.
  • Making planes more aerodynamic is one way to reduce fuel consumption and, therefore, carbon emissions.
  • Modifying the configuration of aircraft or flying in a “V” formation like a flock of birds, Marie Couliou, a researcher at aerospace lab ONERA, explains new avenues to improve 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.

Chang­ing plane configurations

Inter­est­ing­ly, ONERA has devel­oped a very inno­v­a­tive plane con­cept: Nova 1. 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.

NOVA: a more aero­dy­nam­ic air­craft con­cept © ONERA

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.

Con­den­sa­tion trails

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 emis­sions 3.

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.

2Lee, D. S., Fahey, D. W., Skowron, A., Allen, M. R., Burkhardt, U., Chen, Q., … & Get­tel­man, A. (2020). The con­tri­bu­tion of glob­al avi­a­tion to anthro­pogenic cli­mate forc­ing for 2000 to 2018. Atmos­pher­ic Envi­ron­ment, 117834.