9. October 2015

A jour­ney through an ex­haust plume – DLR flight tests for al­ter­na­tive fu­els

Flying behind the A320 ATRA with the Falcon
Fly­ing be­hind the A320 ATRA with the Fal­con
Image 1/7, Credit: DLR (CC BY-NC-ND 3.0)

Flying behind the A320 ATRA with the Falcon

For these ex­per­i­ments with al­ter­na­tive fu­els, two DLR re­search air­craft flew at a typ­i­cal cruis­ing al­ti­tude of be­tween nine and twelve kilo­me­tres, one be­hind the oth­er in for­ma­tion, in a spe­cial­ly re­strict­ed airspace. Lead­ing the for­ma­tion is the A320 ATRA, close­ly fol­lowed by the DLR Fal­con.
ATRA be­fore the flights
Image 2/7, Credit: WTD61.

ATRA before the flights

To con­duct the test flights, the DLR re­search air­craft ATRA us­es var­i­ous ap­proved ful­ly or par­tial­ly syn­thet­ic al­ter­na­tive fu­els.
The Fal­con 20E DLR re­search air­craft
Image 3/7, Credit: DLR (CC-BY 3.0).

The Falcon 20E DLR research aircraft

The DLR Fal­con 20E re­search air­craft was se­lect­ed as the most ap­pro­pri­ate air­craft for mea­sure­ment flights. The Fal­con has a full range of in­stru­ments to record flight dy­nam­ics and a nose boom that records the lo­cal in­ci­dence an­gle at the front of the air­craft in an undis­turbed air­flow.
Re­fu­elling ATRA be­fore flight
Image 4/7, Credit: WTD61.

Refuelling ATRA before flight

Be­fore the flight, the ATRA is fu­elled with a mix­ture of up to 48 per­cent of an al­ter­na­tive fu­el and con­ven­tion­al Jet A-1 fu­el.
Take-off from Manch­ing air­port
Image 5/7, Credit: WTD61.

Take-off from Manching airport

For three weeks, the A320 ATRA will be a guest at the Ger­man Armed Forces Tech­ni­cal and Air­wor­thi­ness Cen­ter for Air­craft (Wehrtech­nis­che Di­en­st­stelle für Luft­fahrzeuge und Luft­fahrt­gerät der Bun­deswehr [Wehrtech­nis­che Di­en­st­stelle 61]; WTD 61) in Manch­ing. The WTD61 al­so sup­ports DLR with the lo­gis­tics for the im­ple­men­ta­tion of the flight tests.
ATRA en­gine
Image 6/7, Credit: WTD61.

ATRA engine

The ATRA is a medi­um-range pas­sen­ger jet which has been con­vert­ed in­to a test air­craft and is an ide­al rep­re­sen­ta­tive re­search sub­ject for the sci­en­tists. It us­es V2500 se­ries en­gines, thou­sands of which are in use around the world.
Mea­sure­ments on the ground
Image 7/7, Credit: WTD61.

Measurements on the ground

Af­ter each flight, sup­ple­men­tary ex­haust mea­sure­ments are per­formed on the ground af­ter each flight. Two mea­sure­ment probes are vis­i­ble in the fore­ground.

Alternative fuels have the potential to support the environment- and climate-friendly developments in air transport. At present, global air traffic contributes towards almost five percent of global warming. In addition to the greenhouse gas carbon dioxide, condensation trails and the resulting cirrus clouds lead to a significant climate impact. In a three-week series of flight tests lasting until 9 October 2015, the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR) is investigating how to reduce the impact of air transport on the climate by using alternative fuels. A possible reduction in carbon particulate emissions – and with this a change in the properties of condensation trails – plays an important role.

ATRA and Falcon in formation flight

For these experiments, two DLR research aircraft flew at a typical cruising altitude of between nine and 12 kilometres, one behind the other in formation, in a specially restricted airspace. "Leading the formation is the twin-engined Airbus A320 Advanced Technology Research Aircraft (ATRA), which was previously fuelled with a mixture of up to 48 percent of an alternative fuel and conventional Jet A-1 fuel," says the Head of DLR Flight Op­er­a­tions (FB), Oliver Brieger. "This is a medium-range passenger jet which has been converted into a test aircraft and it is an ideal representative research subject for the scientists. It uses V2500 series engines, thousands of which are in use around the world." The Falcon, which is equipped with numerous instruments, follows behind. It takes off from the DLR facility in Oberpfaffenhofen and measures the exhaust gas composition and condensation trail properties at distances ranging from 100 metres to 20 kilometres behind the ATRA. In addition, supplementary exhaust measurements are performed on the ground after each flight.

A journey through an exhaust plume
Video "A journey through an exhaust plume"

Down­load „A jour­ney through an ex­haust plume“ (MP4, 1080p, 587 MB)

Varying fuel compositions

"During each of the measurement flights, we use an alternative fuel with varying composition," explains the director of the Emission and Climate Impact of Alternative Fuels (ECLIF) project, Patrick Le Clercq. The two ATRA V2500 engines are operated simultaneously using conventional Jet A-1 as well as various approved fully or partially synthetic alternative fuels. "In doing this, we vary the proportion of cyclic hydrocarbons over a range of between 10 and 19 percent and measure the changes in the exhaust plume," says Le Clercq, who works at the DLR Institute of Combustion Technology in Stuttgart. Cyclic hydrocarbons – otherwise known as aromatics by researchers – are largely responsible for the formation of carbon particulates during combustion in the engines. These particulates supply condensation nuclei in the aircraft exhaust gas for the formation of condensation trails under appropriate meteorological conditions. Comparative test flights with the pure conventional Jet A-1 fuel were also conducted.

Ice crystals in focus

The Falcon atmospheric research aircraft is equipped with a large number of instruments that measure the number and size of the carbon particulates as well as the quantity and shape of the resulting ice crystals. "The number, size and shape of the ice crystals determine the effects that the condensation trails have on radiation," explains Hans Schlager from the DLR In­sti­tute of At­mo­spher­ic Physics in Oberpfaffenhofen. "We want to find out how the composition of the various fuels changes the optical properties of the ice crystals created by the jet engine." The instruments on board the Falcon allows a full assessment of the emissions in the exhaust plume and of the ice crystals that are formed over the complete size range of particles. For this purpose, an additional laser-based particle detector has been used on the Falcon for the first time; it can detect individual ice particles in condensation trails and in the resulting cirrus clouds.

In the long term, findings from the current flight tests could be used to design improved aircraft fuels. "It might be possible, for example, to further develop the synthesis of fuels from renewable energy sources for aviation with respect to more climate-friendly emissions," comments Le Clercq. "To address the carbon dioxide footprint while having societal acceptance, biomass sources such as camelina, jatropha and algae are of particular interest, as these do not compete with the need to grow food."

Previous flight tests with NASA

Today, the biofuel HEFA (Hydro-processed Esters and Fatty Acids) – approved for aviation – exhibits a more favourable environment and climate compatibility when compared to conventional kerosene. This was demonstrated by the joint DLR and NASA flight tests in 2014. In this case, the DLR Falcon, together with the NASA DC-8 and Falcon, conducted test flights from Palmdale, California using HEFA. Measurements of exhaust gas plumes and condensation trails require a lot of experience, as well as specialist measuring equipment. DLR has been developing these measurement techniques in recent years. Since 2000, the DLR Falcon has been used in several measurement campaigns for the study of emissions and condensation trails behind commercial airliners.

The ECLIF project

The emissions produced by alternative fuels are being analysed in the ECLIF project using the full range of methods available at DLR – from combustion analysis in the laboratories of the DLR Institute of Combustion Technology and tests in the combustion chamber test facilities at the DLR Institute of Propulsion Technology, through to the exhaust gas measurements conducted by the DLR Institute of Atmospheric Physics now taking place during the flight trials. Scientists from NASA's Langley Research Center and the University of Oslo are taking part in the supplementary ground measurements during static tests using the A320 ATRA at the German Armed Forces Technical and Airworthiness Center for Aircraft (Wehrtechnische Dienststelle für Luftfahrzeuge und Luftfahrtgerät der Bundeswehr [Wehrtechnische Dienststelle 61]; WTD 61 ) in Manching. The WTD61 also supports DLR with the logistics for the implementation of the flight tests.

  • Falk Dambowsky
    Ger­man Aerospace Cen­ter (DLR)

    Com­mu­ni­ca­tions and Me­dia Re­la­tions
    Telephone: +49 2203 601-3959
    Linder Höhe
    51147 Cologne
  • Patrick Le Clercq
    Head of De­part­ment Mul­ti­phase Flow and Al­ter­na­tive Fu­els
    Ger­man Aerospace Cen­ter (DLR)
    In­sti­tute of Com­bus­tion Tech­nol­o­gy
    Telephone: +49 711 6862-441
    Fax: +49 711 6862-578
    Pfaffenwaldring 38-40
    70569 Stuttgart
  • Dr Hans Schlager
    Ger­man Aerospace Cen­ter (DLR)
    In­sti­tute of At­mo­spher­ic Physics
    In­sti­tute of At­mo­spher­ic Physics
    Telephone: +49 8153 28-2510
    Fax: +49 8153 28-1841
    Münchener Straße 20
    82234 Oberpfaffenhofen
  • Oliver Brieger
    Head Flight Ex­per­i­ments
    Ger­man Aerospace Cen­ter (DLR)
    Flight Ex­per­i­ments
    Telephone: +49 531 295-2800
    Fax: +49 8153 28-1347
    Lilienthalplatz 7
    38108 Braunschweig
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