GUY NORRIS / LOS ANGELES
One of the most unusual concepts to emerge from Boeing's Phantom Works is the huge Pelican wing-in-ground-effect transporter - but how real is it?
Soaring over the ocean, the Pelican seems to cruise effortlessly above waves that appear insignificant beneath the span of its vast wings. Yet this Pelican is no bird. It is a 2.7 million kg (5.9 million lb), 152m (500ft)-span flying ship carrying nearly 200 containers at 250kt (460km/h).
The Pelican Ultra (ultra-large transport aircraft) is a concept being studied by Boeing's Phantom Works as a potential transoceanic cargo and strategic military transport for the 2015 timeframe. With a payload of almost 1.28 million kg and the ability to fly non-stop across the Pacific, the company believes this "bird" could transform US military airlift capability as well as scoop a substantial chunk of the ever-growing international cargo shipping market. As well as military needs, and assuming current cargo trends, Boeing predicts more than 1,000 Pelican-type aircraft could be needed by 2020, even if the aircraft takes only a 2% share of the civil market.
The premise assumes that a there is a potential "mid-market" between ships and aircraft, the only two means of transoceanic cargo transport. Aircraft are used when the value of speed is sufficient to offset the high cost. For this reason, says Boeing, less than 1% of cargo is sent by air. Ships therefore take the lion's share of all containerised cargo because of their low cost.
The Pelican is designed to address this gulf between ships and aircraft in terms of speed and cost. The Phantom Works believes that a "mid-speed" solution could spawn a newer market, which itself could flourish and, in turn, support a fleet of dedicated new vehicles such as the Pelican that combine the best of both worlds. However, it is military requirements that look most likely to launch Pelican - particularly since the shortcomings of the current airlift infrastructure were again highlighted in the build-up to the recent war in Iraq.
First news of the Pelican study emerged in early 2002, when initial artists' concepts of a massive wing-in-ground-effect (WIG) vehicle were unveiled. Unlike previous WIG vehicles such as Russia's Ekranoplan, the Pelican was designed from the outset as a conventional, land-based aircraft able to operate from runways and be compatible with cargo-handling systems.
"The Pelican is land-based, and that's where we are garnering most military support," says Deborah Beron-Rawdon, who is helping steer advanced airlift and tankers strategic development within the Phantom Works project. Pelican project manager Blaine Rawdon adds: "It seems to have gained a lot of traction recently within the Defense Department. Whether or not there is a civil interest, our focus is on a military version for strategic deployment."
All three main branches of the US forces are showing interest in the craft, although the US Navy has been drawn towards the Joint Chiefs of Staff (J-4 Mobility Division) investigation into lighter-than-air hybrid ultra-large aircraft.
"We'd like to turn their heads, because we've studied lighter-than-air and come to different conclusions," says Rawdon. Meetings are scheduled between the "user community", possibly at the end of June, and the Phantom Works hopes that study contracts could emerge in 2004. "The USAF is proposing a sort of programme in the 2015 timeframe to have technology in place to build something like a WIG/Ultra. To get there we have to go at 90mph," adds Rawdon.
To arrive at the current version of the Pelican, Boeing has considered and dismissed several other high-capacity cargo options ranging from fast ships to hybrid airships and sea-based WIGs. Although the speed of the largest container ships is being increased to around 26kt with a new generation of vessels, Boeing's study indicated this trend was limited. Any further dramatic increases in speed by boosting the size of the vessel would, the study argued, be held back by the size of ports and the longer load transfer time at the dock. Furthermore, it says, ships have one insurmountable shortcoming versus the Pelican - they cannot operate on or over land.
Airships, although efficient at low- speed, loiter and hover, were shown to have several drawbacks compared with an Ultra. The study concluded that they were challenged by their large size and low speed. They had difficulty with headwinds, were vulnerable to weather and posed big questions over the design and development of cargo-handling systems. Sea-based WIGs, Boeing believes, are also compromised by the design requirements for operating from the water. Typical features include a low aspect-ratio wing, large horizontal stabiliser, massive propulsion requirements and routine contact with water. The result, it says, is "poor" free-air performance.
A land-based WIG, argues the Phantom Works, is therefore the best solution as the availability of highly reliable, augmented flight-control systems allows designers to avoid necessarily operating the vehicle from water. This in turn, it says, means the airframe can be lighter, the wings can be given a greater aspect ratio, and engine power requirements can be significantly reduced for take-off.
To define clearly the merits of the Pelican against alternatives, the Phantom Works contrived a standard, single measure of speed and fuel efficiency. Cruise speed was easily adapted for the comparison, whereas fuel efficiency was derived from the ratio of payload to fuel burn. This integrated aerodynamic, structural and propulsive efficiency and, even in the case of the non-aerodynamic FastShip Atlantic (a proposed semi-planing 10,000t payload mono hull), allowed one-to-one comparisons to be drawn.
The results indicate that the mid-size version of three conceptual Pelican WIGs (with gross weights of 1.6, 2.7 and 4.5 million kg), has an efficiency in ground-effect (at 20ft) about equal to comparable airships, but at more than twice the speed. Out of ground-effect at 20,000ft (6,000m), the mid-size WIG has the efficiency roughly equal to the Boeing 747-400F, at about 80% of the speed. The 747-400F, the Phantom Works adds, is an efficient air-cargo aircraft in its own right, being able to deliver 0.5kg of cargo per kilogramme of fuel at a cruise speed of 490kt. It even acknowledges that an aircraft such as the larger Airbus A380-800F "can improve on these values", but only slightly.
Boeing's Pelican concept is a massive, crude-looking, conventional wing-body-tail cantilevered monoplane. The payload is carried in standard sea-going containers located in the huge, unpressurised fuselage and within the bulk of the slab-like wing. The cavernous hull houses containers two-deep on the main deck, while an upper deck can be used to store a single layer of containers. The upper deck has a cargo-loading system which allows up to 20 containers to be stored in each wing.
The blunt-nose fuselage is around 120m long overall and has a large swing-nose door to allow for cargo loading and unloading. A conventional, pressurised flightdeck is located in a blister fairing above the nose. Beneath the fuselage is an undercarriage with 76Êwheels. The gear is configured with two long rows of retractable undercarriage legs, each gear supporting two tyres and being steerable. The all-wheel steering provides ground manoeuvrability, some cross-wind landing capability and a relatively low pavement loading similar to current runways.
The full-length landing gear arrangement means the Pelican cannot rotate conventionally, but must take-off by "levitating", like the Boeing B-52. This makes the flap design vital, and the full-span flap deflection is expected to be used for take-off and landing. These unusual take-off characteristics, plus the obvious requirements for the full-span flaps, also meant Boeing had to consider a conventional planform rather than flying-wing and blended-wing body shapes.
The outer section of the wing, which has a mean aerodynamic chord of almost 30.5m and optional spans between 120m and 190m, is designed to droop to enhance the WIG performance during cruise. The outer half of the outboard section is also hinged to enable it to be raised for take-off and landing. The section is also designed to be folded up about 90¡ for taxi and ground operations. "Span is the number one problem, and while an aircraft of this span may seem absurd on the face of it, there are quite a number of military airfields into which it could already operate," says John Skorupa, senior manager for strategic development for advanced airlift and tankers. The folding option is therefore a potentially critical aspect of the design.
Unlike the T-tail featured on other WIG designs, the Pelican is shown with a low horizontal tail to reduce structural weight and complexity. The T-tail is usually employed by WIGs to minimise the effect of destabilising variations in downwash angle with altitude, but advances in flight- control systems mean the Pelican can dispense with this precaution.
The Pelican would also provide engine makers with plenty of new business. Powered by eight 60,000-80,000shp (44,700-59,600kW) turboprops housed in four cantilevered nacelles, the Pelican would sport four sets of contra-rotating propellers.
As with the sea-based WIGs, much more power would be needed for take-off and climb-out than for ground-effect cruise, allowing some of the engines to be shut down during long overwater legs. Potential study candidates include General Electric's CF6-based LM6000 marine turbines or future turboshaft derivatives of the GE90, says Boeing.
The Pelican would spend most of its flying life at between 20ft and 50ft above the sea surface, but would have the ability to cruise at up to 20,000ft to avoid terrain and some weather. The benefit of WIG operations completely alters the cruise performance of the Pelican, which, in flight out of ground-effect, has an estimated free air lift-to-drag (L/D) ratio of around 21. Flying at around 20ft effectively changes the mid-sized Pelican's moderate wing aspect ratio of 5.4 to the equivalent of 15.8, while the effective span increases from 152m to 245m. Similarly, L/D leaps from around 21 to 36.
Although flying in the dense, low-altitude air reduces WIG cruise speeds, Boeing believes the direct operating costs (DOC) make it worthwhile. The long-recognised effects of WIG flying are fundamental to the operating economics of the Pelican over long ranges, but Boeing's studies suggest that even at a range as short as 5,550km (3,000nm) there is substantial DOC advantage to ground-effect flying. Conversely, if terrain or weather makes WIG flying impossible, the optimum solution is to fly as high as possible.
Not surprisingly, company studies suggest the DOC difference (in $/tonne km) begins to diverge significantly with increased ranges well above 11,100km. For long-range missions up to 18,500km, the most dramatic DOC increase occurs with an increase in cruise altitude from around 30ft to 80ft.
Against this theory, Boeing acknowledges that the technical and commercial validity of the Pelican concept is, at best, uncertain. It is therefore engaged in more research to get more answers. In particular, it is focused on developing better methods to obtain more reliable estimates of key parameters such as mass properties and maximum lift coefficient.
Further analysis of the stability and control of such a ponderous flying machine is also required, as is a more thorough investigation of ground-effect aerodynamics. In support of this, Boeing has proposed that WIG aerodynamics and the Pelican should be part of a forthcoming NASA Langley study into the maturation of advanced aerodynamics and structures technology for subsonic transport air vehicles. The company expected to know whether the Pelican would form part of the NASA study by the end of June.
The Pelican's sheer size could also offer Boeing the chance to improve its structural design and manufacturing processes. The lofting of the deep wing could be made simpler and, given the thick boundary layer and lack of pressurisation, the skin would not require the high level of costly manufacturing tolerances required on higher-speed aircraft designs.
Other areas of risk include dealing with the high possibility of numerous bird strikes. Although the Pelican is expected to be far more rugged than previous comparable aircraft, the Phantom Works is studying a number of bird-strike strategies, ranging from detection and avoidance systems (including tracking migration paths) to methods for scaring birds away from the flightpath.
Further studies of aspects such as the effect of sea state and the detection of "rogue waves" could also come under a proposed tie-up with ongoing studies by the Long Beach-based Center for Commercial Deployment of Transport Technology. These studies, for the US Navy's Space and Warfare Command, are also designed to arrive at a wider understanding of cargo throughput at ports.
Above all, the biggest studies remain focused on analysis of the military market and of the potential operation of a giant WIG aircraft within the current infrastructure. Like the aircraft itself, these are enormous areas for consideration and it will almost certainly be another decade before the Pelican, or an aircraft like it, graces the global sea lanes.
Source: Flight International