Liberty Lifter WIG Wing-In-Ground-effect X-plane
On 18 May 2022 DARPA launched the Liberty Lifter project to demonstrate a leap in operational logistics capabilities by designing, building, and flying a long-range, low-cost X-plane capable of seaborne strategic and tactical lift. The new vehicle concept seeks to expand upon existing cargo aircraft by proving revolutionary heavy air lift abilities from the sea. The envisioned plane will combine fast and flexible strategic lift of very large, heavy loads with the ability to take off/land in water.
The point of novelty of this concept is that it would be capable of operating in WIG [Wing-In-Ground-effect] mode when possible, providing very long range, and operating as a free flight when and where WIG mode operations were not feasible. Its structure will enable both highly controlled flight close to turbulent water surfaces and sustained flight at mid-altitudes. In addition, the plane will be built with a low-cost design and construction philosophy.
Although current sealift is very efficient in transporting large amounts of payload, it is vulnerable to threats, requires functional ports, and results in long transit times. Traditional airlift is much faster, but has limited ability to support maritime operations. Additionally, today, such aircraft suffer payload limitations or require long runways.
“This first phase of the Liberty Lifter program will define the unique seaplane’s range, payloads, and other parameters,” said Alexander Walan, a program manager in DARPA’s Tactical Technology Office. “Innovative advances envisioned by this new DARPA program will showcase an X-plane demonstrator that offers warfighters new capabilities during extended maritime operations.”
To address the shortcomings of existing vehicles and operational concepts, the Liberty Lifter program focuses on addressing three main challenges.
- Extended Maritime Operations: Emphasis will be placed on operating in turbulent sea states by creating high-lift abilities at low speeds to reduce wave impact load during takeoff/landing, and innovative design solutions to absorb wave forces. In addition, the project will address risks of vehicle collision during high-speed operation in congested environments. Finally, the aim is for the vehicle to operate at sea for weeks at a time without land-based maintenance activities.
- Full-Scale Affordable Production: Construction will prioritize low-cost, easy-to-fabricate designs over exquisite, low-weight concepts. Materials should be more affordable than those in traditional aircraft manufacturing and available to be purchased in large quantities.
- Complex Flight and Sea Surface Controls: Advanced sensors and control schemes will be developed to avoid large waves and to handle aero/hydro-dynamic interactions during takeoff/landing.
There is a history of attempting to develop aircraft created to fly with “wing-in-ground effect,” which means the aircraft is flying no more than the length of its wingspan above ground or water. The most well-known examples are the Soviet “ekranoplans.” These vehicles were high speed and runway- independent, but were restricted to calm waters and had limited maneuverability.
Aircraft and avian wings are subject to parasitic drag which originates at their wingtips. Higher pressure below a wing may slip around the wingtip to a lower pressure region atop the wing, which for a left and a right wing operating as a pair, create two counter-rotating vortices that propagate rearward in cones of expanding diameter. When a wing operates near a surface such as ground or above a body of water, the diameter of the cone is perforce limited and the drag on the wing is reduced. The resulting increase in glide ratio of the craft is called "ground effect."
Aircraft may be designed to lift off from contact with the ground or water and as long as the wings remain low enough to remain in ground effect the aircraft may be propelled using a more modest and economical power source.
A hovercraft creates a pressurized air cushion contained beneath itself to rise above ground or water leaving an air gap around the perimeter of the craft. Air which escapes through the gap is usually replaced by ducted fans drawing air from above or ahead of the craft. These ducts may be shaped so that forward motion of the craft helps scoop air and direct it to support the underside of the craft. The perimeter of a hovercraft usually includes a skirt which helps trap air beneath the craft. Skirts are flexible so as to withstand impacts with uneven ground features or wave crests over water, then return to shape so as to maintain the air cushion. Although perimeter membranes or rims of semi-rigid material may be used, many hovercraft use an inflatable bag skirt. Most hovercraft skirts define an air cushion area mostly or substantially equal to the footprint of the entire craft.
Wing in ground effect vehicles offer the experience of flight but at such very low altitude that the vertical component of velocity in the event of a crash due to collision, loss of power, or novice piloting errors is negligible.
The WIG configuration that has reached the highest level of technical maturity is the Russian "ekranoplan." A typical example of the "ekranoplan" configuration is embodied in the Russian Orlyonok. In this prior art WIG, turbofan engines are located on either side of the fuselage. These engines are used for underwing blowing Power Augmented Ram (PAR) to increase the lift of the wing during take-off and landing thereby reducing take-off and landing speeds. The turbo prop engine provides efficient thrust for cruise. The horizontal stabiliser controls the pitching moment. A hydro ski can be lowered to reduce hull impact pressures on landing. The endplates help contain the pressure under the wing to provide increased PAR lift during take-off and landing. Because the endplates do not extend below the lowest part of the fuselage the effective air gap between the endplates and the water is no less than the gap between the lowest part of the fuselage and the water. The ability of the endplates to reduce the induced drag is therefore limited.
Because the height of land varies so much it is normal to fly WIGs over water. All existing WIGs fly entirely above the water at the height of the highest wave expected to be encountered plus a margin of safety. This is because of the extremely high wave impact forces that would be incurred at cruise speed. The "Wingship Investigation" (Advanced Research Projects Agency, Sept. 30, 1994) report concluded that designing basic structure and mission loads to tolerate impact with large waves is probably impracticable.
The ARPA Report also concludes that the induced drag increases and the Power Augmented Ram (PAR) lift decreases with the height of the endplates above the water. PAR directs the jet from engines located forward of the wing under the wing to provide added lift at slower speeds. Because of this there is an advantage for WIG endplates to penetrate the waves so that there is no gap at the wave trough between the bottom of the endplate and the water. The existing prior art has not taken advantage of the above as it has been assumed to be impossible to design wave piercing endplates that would (i) have a low enough drag in the water and (ii) be stable at expected angles of yaw at design cruise speed.
As a result, the endplates of existing WIGs usually resemble slender hull shapes similar to high speed racing catamarans, some of which include steps to reduce water friction on take-off. Because these designs are still relatively thick they would incur severe wave impact pressures at cruise speed as well as high drag. Consequently, these endplates are designed to be no lower than the lowest part of the fuselage of the WIG. As a result there is always an air gap greater than the wave height between the wing tip or endplate and the trough of each wave. This restricts their ability to reduce the induced drag.
Studies of ground ("wing-in-ground") effect and construction of wing-in-ground effect craft have more than 70 years in existence. However, no widely-used wing-in-ground effect craft still exists that could offer safety and/or cargo efficiency and/or ease of operation compared to those of conventional aircraft or ships, since no acceptable integral engineering solution has been proposed to meet main challenges associated with operations of WIG craft, that is, longitudinal stability, seaworthiness and amphibian properties, implemented in design combining ease of use and maintenance.
A key distinctive feature of wing-in-ground effect craft operation consists in that main operating height, i.e., height of flying in ground effect mode, is less than length of wing mean aerodynamic chord (MAC) in actual flight (0.1-0.3), while airspeed varies from 150 to 600 km/h that corresponds to aircraft speeds. Besides, aerodynamic forces and moments affecting pitch control feature somewhat complicated nature of dependence on flight parameters and, what is more important, they boast higher gradients of change.
Widely-used methods of providing longitudinal stability in tight time flight nearby water or ground surface minimizing decision-making interval may and, in fact, do cause crashes. This is due to development of emergent conditions in fractions of seconds at external disturbances or in faulty control over craft. Most known emergencies and crashes of wing-in-ground effect craft, both in flight tests and in service, both light and heavy craft, have been in some way related with longitudinal stability and controllability.
Airplane-type configuration with low-positioned wing and pressure boost, on which WIG craft KM, Orlyonok and Lun' are based, does not provide capability of restoring original balance of moments in relation to center of gravity after impact of occasional disturbance factors, and this means progressing instability of pitch control. In other words, stable flight near to ground effect mode can be only realized by a method where crew should manually counteract external disturbances by way of adjusting WIG craft's balance with elevator deflections either manually or using Automated Control System (ACS).
Another important characteristic of WIG craft is seaworthiness. On the one hand, it is limited by emergence of high impact loads during take-off and landing in heavy sea conditions which may lead to damage and disruption of structure, and also to development of forces and moments that prevent the craft from reaching liftoff speed and disrupt hydrodynamic conditions that are necessary for safe completion of take-off or landing. On the other hand, seaworthiness is limited by effective relative height of ground effect flight which depends on WIG craft geometry, that is, overall length and width of its airfoil (length of MAC). That is, solution to both the problems of seaworthiness and that of longitudinal stability consists in selecting methods of generating hydrodynamic forces and selecting aerodynamic configuration.
Aerodynamic configuration determines operational capabilities of wing-in-ground effect craft as an alternative transportation system. Full amphibiousness enables transportation services to areas that are inaccessible to conventional vessels and airplanes and thus provides a more cost-efficient ahternative to helicopters and also offers an increased flight range. Additional advantage is ease of practical use and maintenance.
As part of Phase 1 of the program, Aurora Flight Sciences is designing a 213-ft wingspan demonstrator capable of carrying up to 50,000 lbs. of cargo. The Liberty Lifter X-plane is designed to operate in standard flight at altitudes up to 10,000 ft and in ground effect very close to the surface of the ocean, extending its unrefueled range. The technologies demonstrated and tested on the X-plane would be applicable to future aircraft with cargo capacity similar to that of a C-17, 180,000 lbs. Aurora partnered with leading naval architecture and marine engineering company Gibbs & Cox, a Leidos company, on the project. The team aims to develop a flying boat capable of taking off and landing in up to Sea State 4, operating in ground effect up to Sea State 5, and that employs low-cost manufacturing techniques from the ship building industry to demonstrate affordability.
“Liberty Lifter fills a critical gap between today’s airlift and maritime transport capabilities,” said Mike Caimona, president and CEO of Aurora Flight Sciences. “Development in this space will advance strategic operations at sea, and we’re proud to be working with DARPA, Boeing, and our partners to move this technology forward.”
Phase1B of the program included testing activities and concludes in a preliminary design review. Tow tank testing has been completed in up to Sea State 4 at the Stevens Institute of Technology and Virginia Tech to develop and validate hydrodynamic models and seakeeping performance. Propeller performance characterization testing was completed in preparation for wind tunnel testing of a scale model in early 2025. Additionally, the team is constructing a cockpit simulation lab for human-factors testing of pilot interaction with an advanced control system for flying in ground effect over high sea states.
The next phase of the program, Phase 2, includes continued development leading up to critical design review. Phase 3 of the program, projected to begin in 2026, includes manufacturing the demonstrator followed by flight testing starting in 2028.
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