New Benchmarks in Ground-Effect Flight Energy Efficiency

doi:10.21203/rs.3.rs-4707178/v1

Digital prototypes of a new ground-effect aircraft platform were evaluated to compare respective energy efficiencies to established alternatives. The platform uses a lower cavity similar to a hovercraft, but with design features to better utilize air’s dynamic pressure to sustain flight. Strategic technologies include lower-cavity fences to block lateral losses of lift pressures and a leading section stagnation point that splits air flow both to generate pressure in the cavity and to create induced thrust on the vehicle’s forward section.

Three-dimensional (3D) computational fluid dynamic (CFD) simulation of digital prototypes achieved lift-drag ratios (L/D) in excess of 60 for vehicles with aspect ratios less than 1.0. This paper details the technology and methods; thereby, establishing benchmark performances that provide a foundation for potentially advancing ground-effect flight technology.

Aeronautics and Astronautics

Efficiency

Ground Effect

Aircraft

transportation

fluid dynamics

LD-Efficiency

Multiple prototype and commercial aircraft have flown at L/D efficiencies twice that of commercial aircraft; however, factors like large wingspans have limit applications. Digital prototypes of a new ground-effect flight transit platform (GEFT) now identify low aspect ratio designs with significantly increased L/D flight efficiencies. As an initial phase of the development of these GEFT aircraft, benchmark L/D efficiencies are calculated and compared to current transportation options.

Figure 1 illustrates a base case platform design. The design starts with a hovercraft-like cavity which has an open frontal section to direct on-coming air into a cavity. Unlike hovercraft cavities that are based primarily on containing air within the cavity, the Figure 1 cavity develops air flow patterns that can result in very high L/D efficiencies. Multiple technologies supplement the performance, including a frontal section that generates induced thrust and a propulsion section referred to as Lift Span Tech.

This paper summarizes 2D and 3D computational fluid dynamics (CFD) studies on airfoils and digital prototypes that provide a basis identifying GEFT performance potential and areas showing the greatest prospect to increase the impact of the technology. The L/D efficiencies are compared to the efficiencies of existing transportation benchmarks. The summaries of this paper provide a vital reference that allows reports of future summaries of research to focus on supplementary technologies and matching capabilities with applications.

Many sources report energy efficiency in transportation. Table 1 reports numbers by sources that have had consistent trends through the decades; the Table 1 values are typically within 5% of the values reported by the U.S. Department of Energy Data Books and Statista [1, 2]. Energy was converted to emissions using a conversion factor of 0.072 having units of [g mile] / [Btu km] for a typical liquid hydrocarbon fuel.

Table 1

Benchmark values reported for vehicle efficiencies. Estimated error is +/-5%. Numbers are rounded-off averages of values reported in the US and Europe [1, 2].
Mode	Carbon Dioxide (g/passenger-km)	Energy (Btu/passenger-mile)
Short-Haul Flight	250	3472
Car	185	2569
Airliner	155	2153
U.S. Commuter Rail	114	1583
Bus	100	1389
Electric Car	50
National Rail	45	625
Ferry	19	264

A common metric for aircraft transit efficiency is the lift-drag ratio (L/D). It is particularly useful since it can be calculated using computational fluid dynamics. The numbers reported in Table 2 are L/D for cruising from range of sources.

Table 2

Benchmark L/D reported for aircraft efficiencies. Numbers are from a range of sources and are identified at best cruising L/D [3–7].
Multicopters	< 5
Helicopter	4.5 to 6.2
Cessna 172	11
Average Airliner	15 to 19
B2 Bomber	21
Physical Prototype Aircraft	25–37
ASH 31 Glider	56
Eta Glider	70

Values of L/D greater than 20 in Table 2 identify how flying wing and demonstration aircraft can attain a 20–100% increase in efficiency. The gliders use high wingspans to attain L/D efficiencies up to 300% greater than commercial airliners through use of aspect ratios of 31 and 51. No fundamental limitation prevents airliners from achieving a 300% increase in efficiency; rather, a typical constraint is the cost-effectiveness and application constraints of increasing aspect ratios from 10 for the Boeing 737 to 50 for the highest L/D gliders.

Strut-braced wings are one of several methods identified to increase commercial airliner L/D efficiency, other improvements including:

6 to 12% from winglets, riblets and other airframe retrofits.
~ 15% from braced wings where 30% is reported for the combination of braced wings and improved engines.
20% from increased fuselage lift and improved trailing section propulsion from designs like the double bubble where 35% is reported for the improved airframe combined with improved engines.
25% from blended wing body designs with distributed propulsion where 35% is reported with hybrid propulsion [8].

Other advances continuously improve efficiency of aircraft with improved engines, hybrid propulsion with electric power, and improved materials of construction. These other advances are applicable to most airframes. This paper is on airframe technology.

Digital Prototypes – The accuracies of 3D simulations are accepted as accurate [9–14]. The simulations provide detailed information beyond what is typically available from physical prototypes.

The authors’ 2024 TRB paper reported how pressure profiles generated by CFD simulations on model airfoils are able to elucidate how air flow generates aerodynamic lift [15]. Five principles, infra, identify how air flow generates aerodynamic lift and to L/D.

Principle 1. Air velocities impacting surfaces increase surface pressures.

Principle 2. Air velocities diverging from surfaces decrease surface pressures (by creating voids).

Principle 3. Air flows from higher to lower pressures (at the speed of sound); this pressure-driven flow joins with other air flows to form complex streamlines.

Principle 4. The L/D of a section of an airplane surface is approximately equal to 57° divided by the pitch of the surface in degrees for lower surfaces and − 57° divided by the pitch for upper surfaces. The pitch angle is relative to horizontal with the nose up as positive.

Principle 5. Surfaces can be used to block loss of lift pressures leading to increased L/D. Example surfaces are winglets on wings and fences under lifting bodies.

These continuum-level Principles 1–4 are validated on the discrete level of gas molecules through the following restatements in terms of the kinetic theory of gases:

Air molecules having random translational directions have increased velocities relative to an approaching airfoil; therefore, the momentum of the molecules relative to the leading edge are increased by a value proportional to the approach speed with a corresponding increase in force caused by the impact of those molecules on leading surfaces. Stated in terms of continuum mechanics, impacting flow causes higher pressures.

In the absence of translational movement of air molecules, an airfoil would create a perfect vacuum in its wake—similar to the way a snowplow leaves a cleared snow path in its wake. In practice, gas molecules flow into the wake and convert that “perfect vacuum” into a lower pressure region. Stated in terms of continuum mechanics, diverging flow causes lower pressures.

At room temperature, gas molecules translate 500 m/sec in random directions; the speed of sound in a gas is 340 m/sec which is basically a conversion from random to directional transit. Thus, gases have a net flow through pressure gradients at about the speed of sound. Stated in terms of continuum mechanics, air flows from higher to lower pressures at the speed of sound.

Atomic interactions with a surface are expressed as lift and form drag based on the pitch of that surface.

These qualitative verifications can become quantitative through Monte Carlo simulation which is computationally intensive and outside the scope of this paper [16–19]. The continuum and discrete mechanics explanations are mutually verifying; and as presented in instant paper, the explanations accurately extrapolate with accounting of lost work.

This paper summarizes 2D airfoil and 3D digital prototype performances to identify how the efficiencies of GEFT platforms perform relative to the benchmarks identified in the previous section. Opportunities for increased efficiency and higher impact are discussed. A key advantage of GEFT aircraft are low aspect ratios (< 1) which enable use of railway, highways, greenways, and waterways.

CFD Simulations - Both 2D airfoil and 3D [digital] prototype simulations were performed to identify benchmark performances.

The 2D simulations were useful in identifying Principles 1–4; they provide an insight into what leads to high L/D efficiency. 2D analysis is unable to account for what happens in the 3rd lateral dimension with the implicit condition that lateral losses in lift pressure are neglected in the 2D simulation. Hence, the approach taken in these studies is to identify 2D lifting body airfoils that have exceptionally high L/D, then to perform 3D simulations with fences that block the loss of those favorable L/D. For purposes of this work, exceptionally-high L/D efficiency was defined as > 100.

The winglet is a common device to block lateral losses. This work has a focus on low aspect ratio aircraft with longitudinally-long lift-generating surfaces. A fence is basically a winglet that extends in the longitudinal direction along a lifting body. Fence performance can only be evaluated with 3D simulations. For purposes of this work, exceptionally-high L/D efficiency for 3D digital prototype simulation was defined as > 60. Minimal effort was placed on attaining L/D > 60.

Simflow and OpenFOAM CFD software were used to simulate digital prototypes prepared as STL files. Two-dimensional (2D) simulations were used to identify trends in performance while 3D simulations were performed on the final prototypes. Unless otherwise reported, the scale chords of the STLs were 1 m, the fluid was air at 1 atm pressure, and the free stream velocity was 40 m/s.

The ground was simulated as a lower boundary condition having a velocity equal to the free stream air. Unless otherwise identified, the propulsion sources were rectangular with a height of 2 cm and thickness of 2 mm. Free steam flow boundaries were simulated at a minimum of 5 chord lengths from the vehicle in free stream directions.

The experimental investigation consisted of computational fluid dynamic (CFD) studies of 2D and 3D digital prototypes of a lifting-body vehicle cabins suitable for passenger or cargo payloads.

A lower-surface lift-generating cavity is defined on the sides by the fence, on the trailing section by the flap, and on the lead section by an open gap in which oncoming air flows into the cavity. Air flows exit the cavity under the trailing edge flap in 2D CFD simulation. In 3D CFD simulation air flows out in the clearances below the fences in addition to under the trailing-edge flap.

Results from CFD simulations (i.e., experiments) include: lift coefficients (C_l), drag coefficients (C_d), L/D (equal to C_l/C_d), velocities, pressures, pressure profile images, and velocity profile images.

Digital prototype simulations are designed per the following sequence:

2D CFD airfoil studies to understand trends in C_l, C_d, and L/D for a thick airfoil in the proximity of ground.

3D CFD lifting body studies (i.e., cabin with constant airfoil cross section) to understand how to preserve high airfoil L/D.

Studies to understand limits of performance that can be used to identify viable designs and operating conditions.

Flat Plate Airfoils - Initial studies focused on simple airfoil sections so as to reduce the complexities of analysis and create improved insight. Figure 2 summarizes the performance of a flat plate as described in the following sequence based on a coordinate system:

A flat plate airfoil (horizontal, extending from x = 0.1 m to 0.90 m and from y = 0.03 m to 0.04 m), a flat plate flap (0.01 m thick, extending from x = 0.9 m to 1.0 m and from y = 0 m to 0.04 m) and a flat plate slat (0.01 m thick, extending from x = 0 m to 0.1 m and from y = 0.016 m to 0.04 m to 0.04 m trailing edge flap) were evaluated at 40 m/s in steady-level flight in air in both free flight and above ground at y = -0.01 m in configurations of: i) the airfoil, ii) the airfoil with the flap, and iii) the airfoil with a flap and slat. The respective free flight L/D were: i) 0.02 and − 3.5, ii) 15.3, and 30.4, and iii) 22.7 and 59.4. These 2D simulations illustrate how the flap improves ground effect flight and how a slat that adversely impacts free flight can improve L/D in ground-effect flight [20].

An interesting artifact of the simulations is a negative lift for the simple flat plate in ground effect. An explanation is that the ground reduces the dissipation of the lower pressure formed immediately behind the leading edge resulting in lower pressure along the entire lower surface.

A flap increases the L/D of a flat plate. In ground effect at low clearance ratio, the pressure is essentially uniform along the entire lower surface. Hence, the L/D contribution of the lower surface is the ratio of the run to the rise (i.e., fall) of the lower surface with a correction for shear drag when the flaps trailing edge approaches the surface.

The flat plate with a flap or both a flap and slat is effectively a thin cambered airfoil. Additional insight can be gained by evaluating preliminary studies on thin cambered airfoils as supplemented by the pressure profiles of Fig. 3 [21].

Thin Cambered Airfoils - A simple cambered airfoil, absent optimization, approached an L/D of 100 in ground effect. The ground blocks downward losses of lift pressure with the lift pressure eventually approaching air’s dynamic pressure, based on velocity relative to the airfoil. The higher L/D is due to both increased lower pressure and induced lift on the leading section.

Figure 2b illustrates ground effect with a camber in the absence of a frontal section having significant surface area for induced thrust with a resulting L/D of 15.3. All of the cambered airfoils of Fig. 3 have induced thrust on the frontal surfaces of negative pitch due to extended higher pressures on lower surfaces of and lower pressures on upper surfaces.

More of the higher pressure below the lower surface dissipates a higher clearance ratios (CR). Pressure dissipates through the gaps between the ground and the trailing and leading edges. The dissipation is at the speed of sound, and so, the oncoming air velocity of 40 m/s has little impact on the forward dissipation. The relatively symmetric pressure profile about the mid-chord position of Figs. 3a and 3b support this dissipation mechanism.

For the 3a and 3b pressure profiles, the dissipation is enough to form lower pressures on lower surfaces immediately behind and below the leading edge and higher pressures on upper surfaces immediately above and behind the leading edge. These leading-edge pressure profiles create form drag. As the airfoil approaches the ground, the form drag at the leading edge is replaced with induced thrust which subtracts from the denominator of L/D and is a key performance factor leading to higher L/D.

The higher L/D correlates with the stagnation line having a slope of increasingly negative magnitude. The stagnation line is the line of maximum pressure (in y-axis) extending from leading and trailing edges of airfoils. At CR = 0.02, the stagnation line has a higher average slope (lower negative magnitude) and lower L/D than at CR = 0.43. This could be due to choking of air flow into the cavity at the leading edge and is a phenomenon dependent on airfoil shape.

At clearance ratios less than 1.0, the lift pressure was relatively constant through the cavity.

As illustrated by Fig. 4, the lift coefficient of thin cambered airfoils increases with increasing camber from a lift coefficient value of 0.6. In the absence of ground effect, the L/D decreases with higher camber. Also, the shape of the camber has an impact with the Fig. 4 thin cambered airfoils of different shape than the Fig. 3 thin cambered airfoils.

The thin cambered airfoil of Fig. 3 is 0.5 cm thick, 6 cm high, and 100 cm long with the equivalent of infinite width for the 2D simulation. The shape was created as a 24 cm diameter cylinder extending in the lateral direction (z axis), where the lower 18 cm is removed from the height dimension (y axis) and the remaining object’s length is scaled to 100 in the longitudinal direction (x axis). The leading and trailing edges have flat bottoms extending longitudinally about 0.5 cm. The lifting body, flap, and slat sections are not distinguished.

A partially-filled variation in the airfoil was created by filling the upper 3 cm of the cavity creating a horizontal lower surface from about 0.09c to 0.91c, where c is a chord x-axis scale with the leading edge at zero. The entire airfoil was rotated 1° for simulations to create a 1° pitch. Figure 3e shows the pressure profile for the partially-filled airfoil for comparison to Fig. 3c simulated at the same conditions. The fill reduced the L/D from 93.3 to 92.7; it had minimal impact. Other simulations identified that the shape of the lower surface has increasing impact with increasing clearance ratio.

Also, if a section of the lower surface approaches the ground at a clearance ratio similar to the trailing flap, that section can choke flow causing lower pressures behind that section. Figure 1 illustrates a platform with a mid-chord flap on the lower surface that can be used as a control surface to move the center of lift of the airfoil. Further details of mid-chord control surfaces are available elsewhere [20].

In 3D, the high L/D of thin cambered airfoils are not realized absent mitigation of lateral lift pressure losses. Dissipation of lower surface lift pressures is especially problematic with thin cambered airfoils since the expansion of higher pressures on the lower surface requires propagation of the pressure forward along the entire chord.

Prototype Fuselage Airfoil – The L/D in ground effect has been observed to increase with decreasing clearance to chord ratios [22–27]. However, the approach of increasing chord to increase L/D is inconsistent with 7’ to 9’ ceilings through most of the fuselage length as is typical with airliners.

A technology referred to as Lift-Span Tech uses a cabin cross section as illustrated by the Fig. 5 example [20]. For this example, an upper surface propulsor is placed at 0.8 c. At the propulsor, the pitch of the airfoil is increased by 2° to 10°. The propulsion source (“S” or “Source”) was evaluated at settings of 0, 20, and 40 m⁴/s². The Source creates lower pressures at the intake and higher pressures at the discharge, the lower pressure afore the intake is evident in the pressure profiles.

A goal of Lift-Span Tech is to create incrementally higher L/D afore the Source per Principle 4. Also, lower average pitches afore the Source reduce choking of intake into the Source. These trends are illustrated by preliminary studies [28].

An additional goal of Lift-Span Tech is to increase pitch after the Source to reduce the chord expanse of a fuselage having low ceilings. Having a trailing taper where higher-pressure Source discharge balances lower pressures generated by increased pitch (see Principle 2) can create an operating condition where the average pressure over the trailing taper is the free stream pressure; this eliminates form drag.

The impact of the Lift Span surface afore the Source is coupled with the changing pitch of the trailing taper. By decreasing the average pitch of the Lift Span from 4° to -1°, the L/D with S = 40 m⁴/s² increased from 33.6 to 65.6. At least part of this increase is a win-win situation where choking of the intake was decreased while additional lift was induced. Thrust was also induced, consistently at the forward section of the airfoil and immediately in front of the Source when pitch angles decreased below 0°.

The Fig. 5 simulations were performed with horizontal flow through the Sources. Design and operating degrees of freedom on Lift Span Tech include: a) direction of flow through the Source, b) pitches afore and aft the Source, c) trailing edge vertical location, and d) the ability to vary average surface pitches of the Lift Span and Trailing Taper. Figure 6 illustrates a passively adjusting configuration where the lower forward surface pressures at higher Source powers cause the Lift Span surface to pivot upward.

The Fig. 5 data illustrate how the gliding configuration (S = 0) has a higher L/D with higher pitch afore the Source while a preferred cruising configuration has a lower average pitch afore the Source. Hence, the optimal design is Lift Span Tech where pitches vary with Source power.

3D Digital Prototypes – 3D prototype CFD simulations were performed on thin cambered airfoils and variations of the base case fuselage as summarized by Table 3. Figure 7 provides an example pressure profile for the 3D prototype.

Table 3. Summary of best L/D efficiency of this research project. FS is free stream.

Technology	2D L/D Efficiencies	3D L/D Efficiencies (AR)
Flat Plate	25
- with flap	15
- with flap in ground effect	30
Thin Cambered Plate	70	6 (0.5AR) (0.04 camber)
- with aft source	80	10 (0.5 AR) (0.02 camber)
- in ground effect (trail flap included)	> 100 (0.02 CR)
- with cavity fence	N/A
- with forward fence	N/A
- Lift-Span Tech	66
Lifting Line Theory	N/A	3.5 L/D (0.5AR)
Base-Case Fuselage	21	13.5 (0.75 AR, 0.02 CR)
- with trailing flap		5.2 (FS)
- with aft source (trailing flap)	> 100	20 (FS)
- in ground effect (fence and tail)	> 100	44 (0.75 AR, 0.02 CR)
- aft source & ground effect
- two pairs of fences		42 (0.78 AR, 0.02 CR)
- Two Pairs of Fences with Source		62 (0.78 AR, 0.02 CR)
- without wing		53 (0.5 AR, 0.02 CR)

Preliminary Conclusions – The data identify strategic technologies and approaches to attain high efficiencies, including

Airfoil L/D > 100 are readily attainable with both ground effect and Lift-Span Tech.
Digital prototypes identify four features as critical for attaining L/D > 60, including:
- A trailing section lower surface which air impacts to create higher pressures with flaps designed for this purpose,
- Port and starboard fences that define the lower surface cavity,
- A forward section that generates induced thrust with slats preferred to optimize the induced thrust over a range of clearance ratios, and
- An effective lower surface average pitch less than about 2°.

A 300% increase in energy efficiency can translate to a 75% reduction in energy consumption, and in certain corridor applications that energy can be from grid electricity.

Comparison of Performances – Tables 1, 2, and 3 can be combined and contrasted to determine operation where optimal cruising conditions dominate the reported numbers for the aerial vehicles. To convert the L/D to BTU / passenger-mile, an L/D for the Table 1 “Airliner” was taken as L/D = 15. The “long-haul” aspect of these DOE-reported numbers take into account inefficiencies associated with runway operation and fuel weight. Eq. 1 was used to report normalized estimates of energy consumption for comparative analysis in Table 4.

Estimated Energy Consumption = 2153 * 15 / [L/D] (1)

Table 4

Comparison of GEFT energy efficiencies with contemporary benchmarks.
Contemporary Vehicles	Statistical (Btu/passenger-mile)	L/D	Estimated (Btu/passenger-mile)	% Increase in efficiency vs. Airliner
Multicopters		5	6459
Helicopter		6	5383
Cessna 172		11	2936
Short-Haul Flight	3472
Car	2569
Airliner	2153	15*		0%
U.S. Comuter Rail	1583			36%
Bus	1389			55%
National Rail	625			244%
Ferry	264			716%
GEFT Vehicles	Aspect Ratio (camber)	L/D
Thin Cambered Plate	0.5 (0.04)	6	5383
- with aft source	0.5 (0.02)	10	3230
Lifting Line Theory (0.5 AR)	0.5	3.5	9227
Base-Case Fuselage (with flaps) in Ground Effect	Aspect Ratio (CR)
- flap only	0.75 (FS)	5.2	6211
- aft source	0.75 (FS)	20	1615	33%
- pair of fences	0.78 (0.02)	42	769	180%
- two fence pairs, Source	0.78 (0.02)	62	521	313%
- pair of fences, no wing	0.5 (0.02)	53	609	253%

The Table 4 values for the STEP technology are estimates that provide a starting point of identifying what is possible and where to place priorities. An entry point for efficiency more than 300% that of airliners is ground effect over rail due to the smooth rail surfaces allowing clearance ratios of 0.5 cm or less. At a cabin height of 2 m a clearance ratio of 0.02 translates to 4 cm, which is considerably greater than 0.5 cm. The clearance ratio is based on cabin height and is exclusive of the fence, slat, and flap heights.

As discussed infra, technologies are able to increase L/D beyond the Table 4 ground-effect vehicle values, and technologies exist to operate at lower clearance ratios over railways, highways, and waterways than identified based on rudimentary analysis of the bump-free obstruction-free nature of these surfaces. As necessary, ground-effect vehicles can use free flight to avoid obstacles.

Wing Optimization – Wing optimization is highly dependent on application. At L/D efficiencies > 60, anything but high aspect ratio wings decrease L/D efficiency because their incremental L/D efficiency is less than 60; the wings more form drag relative to lift due to lateral losses of lift which are primarily overcome with high aspect ratios such as used with gliders. Alternatives such as walls along the sides of railroad tracks or tunnels are able to overcome this dilemma.

However, wings are critical to attain higher efficiencies (i.e. L/D > 20) in free flight and higher clearance ratios. Hence, wing optimization is highly dependent on application.

Applications and Expanding Applications – Paths of continued optimization of ground-effect vehicles include optimizing: 1) expansion of fence utilization, 2) Lift-Span Tech, 3) cross-over propulsor technology, 4) fence skiing and shock-absorbing, and 5) thin camber inboard wings. Implied with these approaches is continued results-driven optimization of the frontal section induced thrust and cavity optimization. These areas of optimization impact application, including:

Expanded Utilization of Fences is widely applicable and includes forward expansion of fences and upper-surface fences that originate or end below the highest points of the lifting body. Forward-extending and upper-surface fences should increase, allowing increased clearances while maintaining greater gap ratios with particular utility in ground effect over water where waves increase clearance requirements. Here, the clearance is the distance of closest approach to the ground by fences while the gap is the distance of closest approach to the ground by the lifting body exclusive of fences and flaps.
Lift Span Tech is more-broadly applicable than fences including utility beyond ground-effect vehicles, such as with blended-wing-body aircraft. Lift-Span Tech is an optimized version of distributed propulsion and widely applicable.
Cross-Over Propulsors reduce the speeds at which full aerodynamic suspension is attained and can be used to eliminate the need for wheels or floats on vehicles [20].
Fence Skiing and Shock-Absorbing uses wheels or skis on the fences’ lower surfaces. Shock absorbers to allow continuous or intermediate contact with the surface. Designs where > 95% of the vehicle weight is supported by aerodynamic forces allow these approaches to be used to reduce clearance ratios. Use of wheels on the bottom of fences can reduce fence clearances over rail to less than 0.5 cm and enable clearance ratios less than 0.01. Use of ski’s on the bottom of fences can reduce clearance ratios 0.01 over a range of surfaces and systematically increase L/D to L/D > 60 for applications including use over water.
High-Camber Inboard Wings are inboard sections of aircraft or ships that use high-camber panel sections to increase the area of the cavity. Laterally expandable/retractable sections enable compact and robust boat/ship configurations that can expand to high lift configurations for the ship to take flight for high-speed highly-efficient transit. Use of thin panels for inboard sections enable the inboard section to serve in a capacity like lateral wings do on aircraft. Like laterally-extending wings, inboard wings generate considerably more lift than is required to sustain the flight of the wing. In addition, bifacial solar panels can be used as the thin cambered wing. These inboard wings could be used as towed platforms in non-ground effect applications to further increase excess energy from the inboard sections including the enabling of 24/7 flight with lower-cost, faster, and more-compact designs than current HAPS/HALE options.

Re-Prioritization of Optimal in Transportation – As the energy cost of transportation decreases, new cost categories emerge as having the greatest costs, including:

Transit time and the money value of time; whereby, GEFT transit alternatives have the potential to eliminate: i) highway traffic congestion, ii) queues and lounges in airports, iii) time for security checks of airports, and iv) the time/cost of transferring between modes of transportation.

Infrastructure depreciation costs including the cost and maintenance of vehicles, roads, airports, and parking; whereby, GEFT transit alternatives can use existing railway, highway, and greenway infrastructure and can operate in open water.

A high impact on transportation can be achieved in trans-modal service with maximum velocities in excess of 200 mph that require minimal infrastructure as is possible with this technology. When using existing stations and hubs, the primary incremental costs are vehicles. The vehicle depreciation costs can be very low due to: a) high passenger turnover rates, b) use of electric propulsion at high efficiency to reduce vehicle costs, and c) light weight vehicles which further reduce vehicle costs.

Critical Analyses of Data – GEFT technology has a number of risks, including:

The high L/D efficiencies are sufficiently far from contemporary benchmarks so as to cast doubt on the accuracy of the CFD-simulated performances.
The gain-loss ratio on propulsor technology is complex since it depends on the estimate of thrust relative to a reference benchmark configuration and CFD simulators are not designed to generate either the thrust or relative comparison.
Questions on stability and control.

These risks may limit the upside potential to, for example, a 150% increase versus 300% increase in efficiency versus alternatives. The emphasis of this paper is on the science and technology behind this new ground effect platform rather than the detailed specification of a vehicle to an application.

The data presented in this paper identify extended use of fences and Lift-Span technologies as likely to systematically yield improved L/D performance. These are topics of current investigation.

The GEFT platform provides an alternative to high aspect ratios for achieving L/D efficiencies in excess of 50. The digital prototypes are able to systematically achieve L/D efficiencies in excess of 50 using lower surface cavities defined by side fences, a trailing flap, and a forward section designed around a forward stagnation point that directs flow both into the cavity and over upper frontal surfaces to generate induced thrust. The key operational parameter that leads to the higher L/D is the clearance ratio which is the clearance distance with the ground divided by the height of the lifting body. An additional design parameter that enables lifting body fuselages consistent with passenger cavities is Lift-Span Tech that combines upper-surface trailing-section propulsion with a lower-pitch surface in front the propulsor intake and a higher-pitch trailing-taper surface aft the propulsor discharge.

Ground effect flight over railway and subway tracks is an application where operating at clearance ratios less than 0.02 is possible due to upper rail surfaces which the GEFT platform can engage in low AR flight. A series of new benchmarks are emerging on viable passenger and freight aircraft capable of L/D in excess of 50. Immediately identified priorities to expand the applications include: a) forward extending fence surfaces, b) Lift-Span Tech, c) ski additions to fences, and c) crossover source cavities that enable full aerodynamic lift at lower velocities.

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The authors declare potential competing interests as follows: Authors are inventors on patented technology within the field.

New Benchmarks in Ground-Effect Flight Energy Efficiency

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Abstract

Figures

INTRODUCTION

BACKGROUND

METHODS

RESULTS

DISCUSSION

CONCLUSIONS

References

Additional Declarations

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