panorama Green Freighter Green Freighter development of an eco-friendly freighter at HAW Hamburg DIPL.-ING. KOLJA SEECKT; PROF. DR.-ING. DIETER SCHOLZ, MSME Department of Automotive and Aeronautical Engineering, Hamburg University of Applied Sciences
» ”Elegant“, ”quiet“, or ”green“ are not normally of the project is to research unconventional cargo catchwords associated with cargo aircraft. In fact, aircraft confi gurations and to compare these to discarded passenger aircraft that have been con- conventional ones. verted for a second life as a freighter are the fi rst thought to cross one‘s mind: robust workhorses, Unconventional Confi gurations: Practically often the last of their kind at European airports. all of today‘s transport aircraft have the con- But for these aircraft, the air is getting thinner. ventional confi guration, or tail-aft as it is also Prof. Dr.-Ing. Nighttime and noise restrictions, rising fuel pri- called. It is defi ned by three main features: a Dieter Scholz ces and the coming emission-related taxes are fuselage which accommodates the payload, a graduated in increasing the pressure on logistics companies to wing attached to the fuselage that produces the mechanical operate more modern aircraft. Today, brand new lift, and an empennage, also called a tailplane engineering at cargo aircraft are already available and in opera- or just a tail, at the aft end of the fuselage for Purdue Univer- sity Lafayette tion. Others, like the Boeing B747-8F or the Air- stability and control. and at Univer- bus A380-800F, are about to follow to meet the Any aircraft confi guration that differs from sity Hannover. fast-growing demand for new freighter aircraft. this in one or more features is unconventional. He worked at The size of the world freighter aircraft fl eet is Unconventional aircraft may be grouped as illus- Airbus and TU expected to double to 4000 aircraft by 2025. trated in Figure 1. The fi rst row depicts aircraft Hamburg-Har- burg and taught This is the tentative date for the entry into service with an arrangement of wing(s) and tail(s) other at Queen’s Uni- of the Green Freighter, which is the subject and than the described confi guration: canard, tan- versity Belfast. title of a current joint aircraft design research pro- dem or biplane, three-surface, joined-wing, and Since 1999 he ject under HAW leadership on environmentally delta-wing. The second row shows aircraft with has been a pro- fessor at Ham- friendly and economic aircraft operation. more than one fuselage: twin- or multi-fuselage. burg University This research includes technical aspects such The third row illustrates aircraft merging fuselage of Applied as low fuel consumption, future fuels (liquid and wing: lifting fuselage and blended wing body Sciences. hydrogen [LH2], synthetic fuels, biofuel), low (BWB). The fourth row displays aircraft with no
noise levels, low emissions (CO2, NOx, . . .), and extra fuselage but only a wing (with or without low operating costs (zero-pilot operation, little tails): fl ying wing, oblique fl ying wing (OFW), or no environmental control system). The pro- and EKIP (Russian acronym for „ecology and ject was launched at the end of 2006 and has a progress“; fl ying wing with a low aspect ratio; duration of three years. HAW‘s partners in this also see http://www.ekip-aviation-concern.com). project are the Institute of Aircraft Design and The columns indicate groups of different Lightweight Structures (IFL) at Technical Univer- arrangements of one, two or three lifting and sity of Braunschweig (TUBS), Airbus‘ Future Pro- control surfaces. jects Offi ce (FPO), and Bishop GmbH. The aim But why look at new confi gurations at all? Today‘s aircraft are a safe and reliable means of transportation, and over the last couple of years fl ying has become very cheap, particularly due to the launch of many low-cost airlines. Flying is often not only the fastest and safest way of traveling inside Europe, but also the cheapest. What are the driving factors that justify the huge efforts cur- rently underway to research and develop
1 Aircraft Confi gu- 1 new aircraft concepts ration matrix [6] and technologies?
34 panorama Green Freighter
Considering the different types of 2 aircraft that have been built over the course of more than one century, there have been major developments. The fi rst fl yers were homemade apparatuses able to make small, more or less controlled hops. But very soon after these initial fl ights, substantial progress was made. Flying became „higher, faster, further,“ as well as safer, more weather- and daylight-independent and more comfortable. In short, aviation became a mature technology. ability to handle 16 million fl ights per year, and a In 1954, 51 years after the Wright brothers‘ 30% reduction in the cost of air transport. Other fi rst sustained powered fl ight in December 1903, aims include a reduction of passenger waiting the Boeing B707 was rolled out: a modern jet air- times at the gate to less than 15 min for short- liner capable of carrying more than 150 passen- haul and less than 30 min for long-haul fl ights gers on transatlantic routes at a speed of more and an improvement in punctuality. As a result, than Mach 0.8. This aircraft may be regarded as less than 5% of all fl ights should be delayed by the prototype of most of today‘s airliners traveling 15 min or more. Last but not least, there are at high subsonic speeds: a pressurized fuselage plans to reduce noise emissions by 50%, while Dipl.-Ing. Kolja with a cylindrical midsection and the empennage CO and NO emissions should be lowered by Seeckt 2 x graduated at its rear end, a tricycle landing gear and a swept 50% and 80% respectively. in 2006 in wing carrying pylon-mounted jet engines. The development over the last decades aeronautical In 2005, another 51 years later, the similar- shows that these goals cannot be achieved solely engineering at looking Airbus A380 was rolled out. Nothing through evolutionary improvements to conventi- Hamburg Uni- versity of Ap- essential has changed, though there are two onal confi gurations. For example, many airports plied Sciences, main decks and many improvements regarding have reached their maximum possible number of started working noise, fuel burn, safety, passenger comfort, etc. departing and arriving fl ights. The only way for at Airbus in But all of these improvements are based on inter- many of these airports to further increase their the same year nal advancements. For example, modern turbo- amount of passengers is to increase the number and has been a postgraduate fan engines have a much higher bypass ratio, of passengers per fl ight. Hence, larger aircraft are at the KTH in which means considerably more air is accelerated needed, but regarding size, the A380 almost sets Stockholm since by the fan than the amount of air that passes the an upper limit for conventional aircraft due to 2007. In 2006 core engine. This, in combination with modern rather simple physical reasons. he joined the engine control and new materials, reduces noise, When scaling up a body of any shape, the Green Freighter project as a sci- fuel burn, and the production of smoke. body‘s volume rises to the power of 3 while its entifi c assistant. Further improvements to conventional-con- surface rises only to the power of 2; this basic fi guration aircraft are only possible on a small principle is called the square-cube law. If, for scale and involve large research and development example, a cube with an edge length of 1 is 2 Sketches of efforts. Furthermore, there are still high aims scaled up to an edge length of 2, its volume rises different possible that need to be achieved in order to tackle the from 1 to 8 (2 × 2 × 2), while its 6 surface areas engine arran- × gements on the demands of the continual growth in air traffi c. only rise from 1 to 4 (2 2). Therefore, the weight- planned new Particularly in light of global warming and other related pressure on the base area rises from 1 (1/1) model of HAW’s current environmental and social demands, the to 2 (8/4). Transposed to aircraft design, the AC20.40 [4] calls are getting louder for sustainable air traffi c square-cube law says that the simple geometri- and green aircraft. cal scale-up of an aircraft leads to an increased In 2001, the major European stakeholders wing loading, which of course cannot rise arbi- in aviation, including the European Commis- trarily for physical reasons. Some unconven- sion, national ministries, research agencies, and tional aircraft confi gurations, like the blen- aircraft and engine manufacturers, formulated ded wing body, offer more potential to realize a vision for the European air transport system: larger aircraft. Vision 2020. Vision 2020 sets several goals to be achieved by 2020 in order to reach the two The Blended Wing Body Confi guration: The top-level objectives of „Responding to Society‘s blended wing body‘s fuselage is shaped like an Needs“ and „Securing Europe‘s Global Leader- airfoil and therefore contributes to the overall ship in Aeronautics.“ lift of the aircraft. Furthermore, the fuselage and Some of the direct goals are the reduction wing merge smoothly into each other. The BWB‘s of accidents in air transport by a factor of 5, the main benefi ts compared to the conventional ›
35 panorama Green Freighter
confi guration are a lighter airframe structure and These aircraft often bear features, safety mar- improved aerodynamics. gins, and equipment which are not needed for Regarding hydrogen-powered aircraft, the freighter use but have to be paid for and carried BWB offers further advantages as it provides a on each fl ight. This shows that there is a gene- huge internal volume. As described below, hydro- ral need for dedicated freighter aircraft. Further- gen requires about four times the storage volume more, there are inevitable facts like depleting for the same amount of energy as kerosene does. crude oil resources and the need to reduce global
Conventional aircraft using gaseous fuel, such as emissions of CO2 and other pollutants which also the Tupolev Tu-155, have to sacrifi ce valuable apply to air traffi c. Noise reduction is another key volume inside their fuselage, or the hydrogen has requirement, especially for cargo aircraft, as they to be carried in voluminous tanks located above are most often used during the night and have to the passenger compartment or in external tanks face increasing noise restrictions. beneath the wings, as in the case of the Cryo- On the other hand, unconventional confi gu- plane project. This increases both the wetted rations, and fi rst and foremost the blended wing area and the friction drag of the aircraft. body, reveal great prospects for economic and A large number of institutions are currently very effective future aircraft operation. None- carrying out studies which deal with BWB aircraft theless, there are still challenges, especially for design. Two of these are HAW‘s AC20.30 student passenger versions, like the rectangular pres- project and the X-48B of Boeing, NASA, and the sure vessel and maneuver loads on passengers U.S. Air Force Research Laboratory (AFRL), see on the outboard seats. Some of these key issues Figures 2 and 3. In general, the BWB appears to play little or no role for freighter aircraft. Conse- be the most probable next leap in aircraft design. quently, the Green Freighter may be regarded as However, the BWB confi guration still poses some a means of developing a knowledge base regar- challenges that need to be solved before such an ding these aspects, just like the Boeing/NASA aircraft may enter into service: X-48B, which is also intended as a step towards a BWB freighter aircraft. • Stability and control • Emergency passenger evacuation Selection of Reference Aircraft: As already men- • No outside-view for many/all passengers tioned, the blended wing body confi guration is • Maneuver accelerations at outboard seats well suited for large aircraft. Moreover, for use • Cabin pressurization as a transport aircraft, a certain size is more than • Airport infrastructure just a possibility-it‘s a must. In order to accom- • Certifi cation modate a passenger cabin or a cargo deck, the fuselage must have a minimum thickness and be Why an Unconventional Freighter Aircraft? As of at least passenger or cargo deck height plus mentioned in the beginning, the global freigh- some extra space for installations and structure. 3 Boeing X-48B unmanned blen- ter market is a very fast-growing aircraft mar- Of course, this is true for every fuselage on every ded wing body ket. Both Boeing and Airbus expect the world aircraft, but in the case of the BWB, the fuse- demonstrator [3] freighter aircraft fl eet to more than double to lage is shaped like an airfoil and therefore has to extend to the front and especially to the rear. As a result, high freight like cargo pallets or passen- 3 gers will increase the length of the BWB, whe- reby the length relates directly to the wing span if a desired aspect ratio is to be obtained. Ulti- mately, civil and military transport blended wing bodies need to and will be large aircraft. Previous investigations at the Institute of Aircraft Design and Lightweight Structures at Technical Univer- sity of Braunschweig revealed a minimum size for transport BWBs in the 50 t payload class. In April 2007, the project partners agreed on key requirements regarding range, payload, and a design cruise speed of 4800 nm (9000 km), 109 t, and Mach 0.84. These values roughly equate to the Boeing B777F, which therefore about 4000 aircraft by 2025 [1], [2]. Up until represents the conventional reference aircraft. now, many freighter aircraft have been conver- It was decided that the comparative unconven- ted passenger aircraft. They are converted after tional aircraft shall be two blended wing body being decommissioned as passenger aircraft, or aircraft, one meeting the same requirements as they are newly manufactured aircraft that were the conventional freighter and one using propel- originally designed as passenger versions and ler engines and hence cruising at a lower speed of have been trimmed down for use as a freighter. about Mach 0.5. The cruise speeds for both con-
36 panorama Green Freighter
fi gurations are only starting fi gures that may and will be optimized later. Additional features will 4 include single-engine cruise, zero-pilot operation, and the application of a hydrogen and hydrogen- biofuel hybrid propulsion system.
Liquid Hydrogen as a Fuel: First of all, hydrogen is not a real fuel in the classic defi nition of an energy source; in fact, it is an energy carrier com- parable to a battery. Energy must be employed fi rst to obtain hydrogen in a pure state, e.g., by means of electrolysis. This process is very energy demanding, and only a fraction of the energy previously spent can be retrieved afterwards. Nowadays, hydrogen is most often produced from natural gas because this process is much cheaper than electrolysis. If environmentally produced, hydrogen offers the potential of extremely low emissions (zero
CO2 and very low NOx) by lean combustion. As an alternative to kerosene, H2 needs to be cooled down to -253 ºC in order to be stored in liquid form (LH2). Based on the same energy content,
LH2 has only one-third of the mass of kerosene Current State: There are currently several student 4 Preliminary but four times greater volume. Safety analyses projects being undertaken at HAW, mostly dea- Aircraft Design have shown that H is at least as safe as con- ling with general issues of cargo, environmental and Optimiza- 2 tion Program ventional hydrocarbon fuels. One of its biggest aspects of air traffi c, and the installation and PrADO for advantages is its gaseous state. In the event of use of PrADO. More projects will follow soon. evaluation of a leakage and/or fi re, it evaporates quickly and The topics of upcoming projects will include kerosene-/LH2- does not form a (burning) carpet. the improvement of an initial aircraft layout in freighters [5] PrADO and the calculation of propeller effi ciency Tools: The central tool in the Green Freighter within PrADO. « project is the multidisciplinary aircraft design tool References known as PrADO (Preliminary Aircraft Design and Optimization) developed by the IFL. The tool [1] Airbus S.A.S. Corporate Information: Global Market Fore- is an arrangement of subroutines that acquire and cast 2006-2025 [online]. 2006 [cited 2007-07-16]. Available from Word Wide Web: share information by means of a common data
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