Lift System Design of Air Cushion Vehicle

Lift system plays key role to Air Cushion Vehcile overall performance, whose design includes cushion flow demanding analysis, lift fan design, airflow distribution and pressure control. The method to calculate cushion flow exit velocity variation with craft speed was firstly presented based on cushion induced wave. Through application development in CATIA, a full solution was presented to check if craft keeps in safety flight boundary at maximum calm water speed. Through analysis, current cushion flow demanding method based on statistic former ACVs seems conservative for particular ACV with super high cushion density, which was adviced to decrease by 10%∼15%. New double discharge lift fan for polar hovercraft was developed through CFD simulation, model test, full-scale utilizing. To reach designed bag-cushion pressure ratio, polar ACV model skirt feed holes was added by 1/3 more than simple geometrical scale. Larger bag feed holes and lower bag-cushion pressure ratio are effective means to lower lift power.


Introduction
Lift system is one of unique systems of air cushion vehicle (ACV), which includes lift fan, air duct, bag hole and flexible skirt.On cushion state, lift system forms high pressure cushion to hover ACV hull over operating surface, which heavily reduces resistance and brings full amphibious ability [1].Lift system plays key role to static flight characteristics, stability on cushion, seakeeping and flight safety [2].When ACV begins to lift from static, ambient air is sucked into lift fan and compressed to high pressure, then flow through diffuser opened in hull tank side into peripheral flexible air duct surrounded between hull side and inflated skirt bag, as shown in figure 1.The air flow distributes into cushion and skirt fingers through feed holes on inner bag during passing way.By the end, airflow escapes into atmosphere through clearance gap between fingers low tip and operating water surface or ground surface.Above all, it forms a dynamic balance between inlet airflow and escaping airflow.Lift system design corresponds to cushion flow demanding analysis, lift fan design, flow distribution and pressure control at different location in skirt bag.
At blooming era of ACV between year of 1959~2000, many countries such as British, France, U.S.A, Russia, China, etc. built plenty of ACVS used for commercial or military, and amount of researching and application of lift system was done [1][3] [4].During that time, new design idea and characteristic curve by summary former ACVs were presented [1][3][4] [5].Recently, new ACVs, such as T-2000, LSF-II, SSC, were fabricated, where new lift fan and bow thruster configuration were researched [2][6] [7] [8].Cushion flow corresponds to a lot of parameters, such as designed bagcushion pressure ratio, speed, operating sea sate, no dimensional cushion pressure to length, etc. ACV developing history demonstrates that lower bag-cushion pressure ratio and lift power requirement are trend of modern ACV.Based on CFD technology (such as ANSYS CFX based on RANS code), new lift fan was developed to satisfy higher requirement of military ACV general arrangement and overall performance [7] [8].
For safe flight, except for checked by wave-pumping in rough water, cushion flow demanding should also be checked at maximum speed in calm water.So, it needs to get cushion flow exit velocity variation with craft speed firstly.Above solution and a small polar hovercraft's new developed asymmetric double-discharge lift fan and towing model skirt bag-cushion pressure debugging would be introduced following.

Cushion Flow Demanding (Qc) Analysis
There are two main methods resulted from engineer experience, one is based on hover gap (He) to determine Qc [1][3], the other is based on statistical curve of cushion flow versus cushion length through former ACVs [2].
It can plot parameter of recent ACVs in figure 2, from which it could find that cushion flow was further lowered and hover gap close to curve He/Lc=0.003*(Pc/Lc)^1/2.Hover gap of LCAC was especially low, where LCAC's He/Lc divided by 0.7 would lie just near the curve He/Lc = 0.003*(Pc/Lc)^1/2 [7].

Based on Statistical Curve of Qc'~Pc' Method
Based on former ACVs, Marine Design & Research Institute of China (MARIC method) presented a statistical curve of no dimension cushion flow Qc' and cushion pressure Pc', curve Qc'~Pc' as shown in figure 3 [2].Where Qc'=Qc/Ae/(2Pc/ρ)^1/2 =He/Lc(Lc/Bc+1), Pc'=Pc/(ρgLc).The lift system parameters of some typical ACVs (such as recently T-2000, Griffon 12000TD, EPS M10, SunaX) were plotted with the MARIC curve, also the Froude number corresponds to design speed versus Pc', as shown in figure 4. It could find that for ACV with higher no dimensional cushion pressure to cushion length (Pc'>15), lying above statistical curve seems to be too higher, especially for LCAC (Pc'=16).The corresponding Fn at design speed almost lies in range [1.4,2.0].

Hover Gap for ACV Underway
Reference [9] measured lift power of a small ACV in state of static flight over rigid surface or underway at design speed in water.It could find that lift power of underway at design speed was heavily lower than that of static flight on rigid surface, as shown in figure 5.For ACV forwards on water surface with high speed post hump, ACV would have up-down trim induced by cushion making wave, where only bow and fore-side skirt stays over water surface, while aft-side and stern skirt immerses into water, as shown in figure 6. Above phenomena made total exit flow area under skirt finger low tip decreasing, so cushion flow would decrease and its operating point on lift fan P~Q curve would move to left to result in corresponding lift power declining.On the other hand, when cushion flow exceeds certain extent, it would have little effect on resistance.For example, JEFF B resistance at 85%, 90%, 95% fan rotation speed, had little difference, as shown in figure 7 [10].Cushion flow of LCAC was heavily lowered form JEFF B's 322m 3 /s to current 234m 3 /s, while resistance of LCAC was some smaller than that of JEFF B, also shown in figure 7 [11].For a small polar hovercraft with even higher cushion density (Pc'=18), the calm water resistance prediction based on towing model test at different lift fan rotation speed was illustrated in figure 9 [12].It could find that cushion flow mainly affects post-hump resistance.The measured full-scale ship speed and equivalent resistance from thrust was also plot in figure 8, where full-scale trial result agreed well with that by model prediction [13].

Cushion Flow for ACV Anti Plough-in Requirement
Trial result of full-scale ship demonstrated that cushion static pressure in fore cushion would heavily decline, even to minus pressure at procedure of plough-in.For ACV head-on forwards with high speed, relative incoming airflow could cause cushion flow to transfer to inner forward and induce low static pressure in fore cushion, as shown in figure 9 [1].So, to prevent plough-in, ACV underway at high speed must ensure certain enough cushion flow.
Based on cushion making wave at different speed, it could get trim and heave induced by cushion wave variation with different craft speed, and a polar ACV calculated trim and heave was shown in figure 10, also model test results was given in the same figure.Canada SunaX ACV fabricated by British Griffon was also calculated, and its bow exit flow variation with craft speed was shown in figure 12, whose maximum speed in calm water was 60kn.
The developed solution based on CATIA application was used to analysis maximum speed in calm water based on lift cushion flow exit balance from skirt finger low tip, where calculation results for T-2000 and SunaX agreed well with those by full-scale ship trial.The model is used only for calm water and ACV forwarding without sideslip.In fact, ACV always forwards with sideslip and flexible fingers deformation after contacting with water surface, both make cushion flow exit under skirt fingers complicated.So, difference between simulation and experimental result was not avoidable.

Cushion Flow Effect on ACV in Rough Water
For ACV underway in rough water, cushion flow design should take into account of wave pumping effect, which could induce average hover height to decline and cause too much skirt finger immersed into water to result in plough-in hazard [2].The wave profile could be expressed in eaqution 1.
When ACV stably runs in wave (without heave and pitch), wave pumping cushion airflow from bow (x=0) to stern (x=Lc) would reach maximum, could be expressed as equation 2.
When ratio of wave length to cushion length (λ/Lc) nears to 1, global pumping airflow Qp=0, while both airflow in fore cushion and airflow in aft cushion would reach maximum, where one for fully pumping in and the other fully pumping out.In above case, pressure disturbance at fore and aft cushion would reach maximum, which means pitch was mostly disturbed.
On the other hand, when wave length nearly equals to half cushion length (λ=0.5Lc),global cushion pumping air flow would be max (Qp_max=Bc*hw*V), where cushion pressure disturbance would be max and cause max disturb to heave, also hover height mostly declined.
If cushion flow could not compensate pumping out airflow from cushion in time, ACV would heavily sink and cause skirt low fingers over-extent immersed into water and high resistance increasing, which bow skirt would be liable to tuck-under and even to plough-in in wave.Based on experience of model test, plough-in would occur once cushion flow declines lower than wave pumping airflow requirement.According to engineer experience, wave pumping airflow coefficient was introduced as CQW=Qp/Qc, where CQW should lie in range 0.8~0.9 to ensure no plough-in in wave.
If cushion flow was not sufficient, it would result in bow heavily up-lift or low-down in rough water, as shown in figure 13.Through above analysis of polar hovercraft and LCAC, for certain ACV with super high cushion density (Pc'>15), cushion flow could be lower 10%~15% than value got based on current statistical Hovergap method or MARIC method, while on the condition that it must meet wave pumping airflow requirement.In addition, cushion flow should also be checked at maximum speed in calm water.
Hovergap method or MARIC method are based on statistics, which dose not take into account ACV designed speed, operating sea state, et al.So above two methods are not so accurate, while it can discount the cushion flow demand based on above statistical method, and need furher to be checked by maximum speed and wave pumping to trade-off a balance.

Main Lift Fan Type Used by ACV
Three type of lift fans are mainly used in ACV, such as centrifugal, axial and mixed-flow, whose fan rotor were shown in figure 14.
Typical no dimensional Pressure-Flow curve for three lift fan with same fan diameter was shown in figure 15.It could find that Radial (centrifugal) fans suit for high increasement in pressure and low flow rates, Axial fans suit for high flow rates and low increasement in pressure, Mixed-flow fans suit for medium pressure and medium flow rates.As a typical centrifugal fan, HEBA-B was widely used by U.K. BH series ACVs, such as SRN5, SRN6, SRN4, BH-7, AP.1-88, BHT-130, British Griffon series ACVs and American JEFF B, LCAC, EPS.
Mixed-flow fan was used in U.S.A JEFF A to replace original centrifugal fan, because later's blade speed was too high to be sustained by material at that time.With technology development, mixed-flow fan efficiency was promoted and its advantage of smaller relative size for same air flow was more prominent for ACV's compact general arrangement.Mixed-flow fan was recently used by U.K. Griffon 995ED and 12000TD ACV.

Mounting Lift Fan on ACV
Lift fan with single outlet was usually mounted on ACV hull buoyancy box side, and the part of side box below lift fan outlet was opened along with shape of fan volute near outlet to form fan discharge diffuser, as shown in figure 16 [14].In figure 16, region between dashed line below fan standard volute forms diffuser.
Sometimes double outlet fan was used for ACV with bow thruster, where lower outlet provides airflow to cushion and upper outlet provides airflow to bow thruster.Also diffusers were used to transfer airflow from outlets, as shown in upper figure 17.Couple double-outlet fans configuration was used by JEFF B and LCAC, as shown in right of figure 17, where airflow from upper outlet of couple fans was converged to provide airflow for bow thruster [15].

Design Ratio of Bag Pressure to Cushion Pressure
For commercial ACVs, ratio of bag-cushion pressure (r_PbPc) changes from early 1.5~2 to recent low as 1.1~1.2, in order to lower cushion flow and lift power.For example, British SRN4 r_PbPc declined from Mk1 1.5 to Mk3 1.1~1.2, and Japanese MVPP-10 even lower than 1.1 [16].For high cushion density military ACV, such as American JEFF B 1.4, LCAC 1.3, SSC 1.35, whose main dimension was constrained by entry/exit mothership well-deck, higher r_PbPc was kept to ensure enough anti tuck-under ability to sustain wider flight safety boundary and agiler maneuvering.Finland T-2000 double-outlet lift fan characteristic was measured by sub-scale model, and that of full-scale fan was also plotted in figure 19 [17].It could find that sub-scale model P'~Q' was little low than that of full-scale.For LCAC lift fan, the CFD simulation result of sub-scale model was also lower than that of full-scale.As another example, based on JEFF B fan performance of P~Q [20], the calculated cushion flow of operating point at different fan speed could be got, so the average hover gap height He between skirt bottom hemline and rigid supporting surface could be reached, as shown in figure 21 [20].It could find that theoretical calculation value agreed well with full-scale ship trial result with craft weight W=285kLb.Combining above all, flow chart of lift fan design was shown in figure 22.

New Lift Fan Development.
Unlike LCAC couple fans configuration at each side, SSC incorporated single fan configuration, where this double-outlet fan provides airflow for both cushion and bow thruster, as shown in figure 22 [21].The lift-side airflow amounts to 234m 3 /s, where cushion vanes were mounted in lower diffuser to adjust lift-side airflow.SSC fan diameter is 5.75ft and its blades were fabricated by 6061-T6 aluminum.The fan design speed is 1715rpm and over-speed up to 2092rpm.SSC lift fan was newly developed, where new blade sect profile was evolved, which optimized to plentiful based on CFD simulation, as shown in figure 24 [2][7] [18].SSC fan's 26-blades were evenly deployed on fan disc and blades on two back-to-back discs were staggered to decrease vibration induced by airflow disturbance, as shown in figure 25.As contrast, LCAC fan blades were deployed on two back-to-back discs with same line position.According to polar ACV rigor restrict by general arrangement and requirement of agile maneuvering, double discharge fan provides flow both for lift and bow thruster was developed, whose upper discharge flow for bow thruster was heavily less than lower discharge for lift.Through CFD simulation, model test, and full-scale fan mounted on ship, the effective was fully proved, as shown in figure 27 [13].For bow and stern skirt, which located farthest from lift fans, airflow from each side would meet with each other at this point.So area of bow and stern skirt sect should be designed large to transfer airflow pressure from dynamic to static as possible.LCAC peripheral skirt sect area distributed along skirt bottom hemline from bow to stern, also lift fans location was shown in figure 31 [4].32.SRN4 Mk3 skirt opened tens 6inch feed holes on inner bag region for each finger, whose total area amounted to 350ft^2, which increased by 1.33 times than Mk1.SRN4 Mk3 average ratio of bagcushion pressure was declined from Mk1's 1.5 to current 1.1~1.2,which heavily decreased the largest loss item of lift power.SRN4 Mk3 enlarged total area of feed holes, and redesigned special closing cone fingers located at interface region of side skirt and stern skirt to decrease flow escape clearance.Above three means made SRN4 Mk3 overall performance, such as fast and seakeeping et al, increased to some extent, on the condition that almost a time heavier and small increasing of main engine power.The parameter of skirt and cushion was summarized in table 1 for comparison.3D arrangement of LCAC skirt was shown in figure 33, where large feed holes were used in skirt bag located at bow corner and stern corner, which could be related to multiple slim skirt material seamed and merged at above location [4].For flat part of bow, side and stern skirt, thousands of tiny holes were opened as airflow feed way.Finland T-2000 skirt thoroughly incorporated large feed holes, where 3 holes was opened on peripheral skirt inner bag region for each finger, as shown in figure 34, also Polar hovercraft used large fag feed holes.

Feed Holes Design for Model Skirt.
Compared to feed hole's hazard on strength of skirt material for full-scale ACV, model skirt need not take into account above effect and large feed hole was used by model skirt, as shown in figure 35 [23].But the real size of feed hole on model skirt inner bag is relative small and its exit flow coefficient was low to 0.5.While the exit flow coefficient for full-scale ACV would amount to 0.7, so more feed holes needed to be opened as more as 30~40%, based on geometrical sub-scale feed holes from full-scale ACV.To small polar hovercraft with super high cushion density, feed holes on sub-scale model skirt inner bag was opened more than simple geometrical scale from full-scale skirt by 1/3.Static flight, cushion stability and resistance test verified the design requirement was met.

Conclusions
Based on above analysis, some conclusions and recommendation for lift system design and research could be reached.
1) This paper firstly presented a full solution to calculate cushion flow exit velocity variation with craft speed induced by cushion making wave.For certain ACV with super high cushion density, cushion flow could be lowered by 10%~15%, based on satisfy checking for wave pumping effect.On the other hand, csushion exit flow velocity under bow skirt at maximum calm water speed should be checked if it is greater than craft speed for flight safety.
2) New developed lift fan should go through below procedure sequent as possible, such as CFD simulation, sub-scale model test, full-scale measurement and trial in real ship harsh environment, which could fully verify fan performance and durability.
3) Large feed hole should be opened on skirt inner bag to feed airflow into fingers to decrease hole condensing loss for airflow pressure.Bow skirt sect profile should be large as possible to transfer airflow pressure from dynamic to static, which can increase ratio of bag-cushion pressure at bow skirt and promote capacity of anti-tuck-under.
4) For feed hole design of model skirt, except for geometrical sub-scale feed holes from full-scale ACV, 30~40% more feed holes should be added for different exit flow coefficient effect induced by sub-scale.
5) In the future, more compact and high-efficiency lift fan made by composite material would be designed to reduce weight and save arrangement space on ACV, also lower noise by trade-off fan diameter and rotation speed would be another key point.New-generation mixed-flow fan is becoming more popular.

Figure 1 .
Figure 1.Illustration of ACV cushion airflow passed from lift fan to ambient atmosphere (left hovering over ground, right over water surface).

Figure 2 .
Figure 2. Relative hover gap He/Lc versus Pc/Lc with recent ACVs.

Figure 4 .
Figure 4. Relative hover gap He/Lc versus Pc/Lc with recent ACVs.

Figure 5 .
Figure 5. Lift power measured during underway over water and static flight on rigid surface.

Figure 6 .
Figure 6.Cushion wave-making wave induced ACV up-head trim during underway with high speed.

Figure 7 .
Figure 7. Cushion flow effect to JEFF B and LCAC resistance prediction (in right figure, red and cyan curve for JEFF B prediction).

Figure 8 .
Figure 8. Polar ACV model test resistance prediction and full-scale ship trial.

Figure 9 .
Figure 9. Internal/external air stream lines of a high speed hovercraft model.

Figure 10 .
Figure 10.Trim/heave response of a polar ACV at different Froude number.Trim and heave of T-2000 was shown in figure 11.Based on cushion making wave at different speed, it could get trim and heave induced by cushion wave variation with different craft speed, and those of T-2000 were shown in figure 12. Based on application development in CATIA, the cushion flow exit area between skirt bottom hemline and water surface could be calculated automatically.After the cushion flow exit region was reached, the maximum calm water speed permitted by engine power could be checked if exit flow speed at bow is greater than craft speed for not be blown inward back to cushion and ensure flight safety.Through above full solution presented, T-2000's bow exit flow variation with craft speed was got and illustrated in lowest of figure 11, whose speed in calm water could reach 70 knot during full scale ship trial [4].

Figure 11 .
Figure 11.Trim/heave response at underway with different speed and exit flow under bow skirt varied with craft speed.

Figure 12 .
Figure 12.SunaX exit flow veloicty under bow skirt varied with craft speed.

Figure 13 .
Figure 13.ACV heavily bow up and down in rough water.

Figure 15 .
Figure 15.Typical no dimensional pressure vs flow curve for Radial, Axial, Mixed-flow fan.

Figure 17 .
Figure 17.double-outlet lift fan and its mount on ACV (left) and Couple fan configuration used in JEFF B and LCAC (right).

3. 4 .
Lift Fan Design 3.4.1.Lift Fan Operating Point Design.During lift fan design, operating point location would be selected at P'~Q' curve right side with larger slope of △P/△Q to ensure fan pressure fluctuation more matching airflow fluctuation to keep small r_PbPc disturbance for better skirt inflated shape.Several typical ACV's lift fan no dimensional P'~Q' curve and operating point was shown in figure 18.

Figure 18 .
Figure 18.Typical Lift fan P'~Q' curve and designed operating point.

Figure 19 .
Figure 19.Fan sub-scale model test of T-2000 and LCAC fan CFD simulate.Based on ACV parameters relative to lift system, such as Lc, Bc, Pc, Pb, Df, nf, num_fans, it could calculate cushion flow requirement for pressure at lift fan outlet (Pd~Qc).Intersected above curve with lift fan P~Q curve at different fan speed could get fan operating point, which includes pressure, flow and lift power, et al.For example, by cushion requirement curve and lift fan P~Q curve of T-2000, it could get design operating point, as shown in figure 20 [18][19].

Figure 21 .
Figure 21.JEFF B P~Q and theoretical hover gap He versus full-scale ship trial.

Figure 22 .
Figure 22.Typical ACV lift fan design flow chart.Figure 23. SSC centrifugal lift fan with double exit volute.

Figure 23 .
Figure 22.Typical ACV lift fan design flow chart.Figure 23. SSC centrifugal lift fan with double exit volute.

Figure 24 .
Figure 24.SSC fan blade sect profile evolution and comparison with T-2000.

Figure 25 .
Figure 25.SSC fan blades deployment on disc and that of LCAC fan.During development of SSC new lift fan, except for CFD simulation (based on CRUNCH software), sub-scale model was measured with bow thruster, also two full-scale fans were fabricated and mounted on LCAC-66 for performance measurement and long term durability in LCAC harsh operating environment, as shown in figure 26 [6][7].According to polar ACV rigor restrict by general arrangement and requirement of agile maneuvering, double discharge fan provides flow both for lift and bow thruster was developed, whose upper discharge flow for bow thruster was heavily less than lower discharge for lift.Through CFD simulation, model test, and full-scale fan mounted on ship, the effective was fully proved, as shown in figure 27[13].

Figure 26 .
Figure 26.SSC fan sub-scale model test and full-scale measurement.

Figure 27 .
Figure 27.Polar ACV double outlet lift fan CFD, model test and full-scale mounted.

4 .
Airflow Distribution and Pressure Control in Peripheral Skirt Bag4.1.Flexible Air DuctFour lift fans were mounted on each side of U.S.A JEFF A, where one fan provided high pressured airflow directly into cushion through rigid air duct opened in buoyancy box, and the other 3 fans provided airflow into peripheral skirt bag, as shown in figure 28[22].

Figure 28 .
Figure 28.JEFF A lift fans arrangement and rigid air duct in buoyancy box.USA LCAC incorporated two double outlet fans on each side, where lower outlet discharges flow to lift and upper outlet discharges flow to converged bow thruster.Partial Lift flow directly ran into cushion through rigid air duct, while another partial lift flow ran into division skirt bag through rigid air duct neighbor to above duct, as shown in figure 29.Most lift flow comes into flexible air duct surrounded by inflated peripheral skirt bag and tilt-rise side of hull box, which went through whole peripheral skirt bag and partial flows into finger through feed holes opened on skirt inner bag during pass way, as shown in figure 30.

Figure 30 .
Figure 30.Flexible air duct and airflow mainly distributes by feed holes on inner bag.

Figure 31 .
Figure 31.LCAC bow and stern enlarged skirt bag and bag sect area varying along hemline and lift fans location.

4. 2 .
Feed Hole Design on Skirt Inner Bag 4.2.1.Feed Holes Design for Full-Scale Ship.British SRN4 Mk1 skirt opened thousands tiny holes with 5/16inch diameter on inner bag region corresponds to each single finger to feed airflow into this finger.The holes located in fabric weaving interstice of skirt glue material to decrease open holes effect on skirt strength.The total area of feed holes was 150ft^2.On the contrast, SRN4 Mk2 skirt feed holes diameter enlarged to 3/4 inch, which is larger to certain extent, as shown in figure

Figure 32 .
Figure 32.SRN4Mk3 skirt little feed holes on inner bag.

Figure 33 .
Figure 33.LCAC skirt arrangement and small feed hole on bow & stern bag.

Figure 34 .
Figure 34.T-2000 skirt and polar ACV used large feed holes on inner bag.

Figure 35 .
Figure 35.Large feed holes opened on model skirt.