Post on 19-Feb-2018
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Air Cushion Vehicle (ACV)
Developments in the U.SPresented
to the
Joint SNAME SD-5/HISDinner Meeting
by
Brian ForstellDirector of R&D
CDI Marine Co.
Systems Development Division
9 June 2005
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Anatomy of an ACV
• ACVs are truly
Amphibious
Craft that are
capable of
traveling overalmost any type
of surface.
• Capability
comes from ACV
unique
equipment.
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Anatomy of an ACV
ACV uniqueequipmentincludes:
• Skirt System
• Lift System
• Air ScrewPropulsors
• Bow Thrusters
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Skirt Systems
• Flexible Skirt Systems were first introduced to
ACVs in 1961.
• Continued to evolve
and mature over the
next 20+ years.
• Evolved into the
typical Bag-Finger
Skirt.
• Peripheral bag for
air distribution.
• Flexible fingers
attached to bag.
• Cushion sub-
division.
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Deep Skirt Project
• Oct. 1995, CNO N853
directs development of
Enhancements for
Inc reasing LCAC
Surv ivabi l i ty while
conducting Shallow Water
MCM Mission (SWMCM).
•
Jan. 1996, Coastal SystemsStation is directed by PEO-
CLA, PMS-377 to initiate
Deep Skirt Project.
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Deep Skirt Design
Principal Characteristics• 40% Increase in Cushion
Height
• Elimination of Longitudinal
Cushion Divider
• Double-Bubble Side Seal for
Well-Deck Compatibility
• Unique Back-to-Back Side
Fingers for enhanced rollstatic stability
Represented the “First” of a New Generation of Skirt Designs
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Deep Skirt Design
Deep Skirt design was subjected to extensive sub-scaletest prior to committing to full-scale prototyping
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Deep Skirt Design
• SWMCM Mission wascancelled after the
prototype was built!
• Performance and
durability testing of DeepSkirt showed:
– Improved Ride Quality
– Improved Payload
Carrying Capability – Improved Speed/Sea
State Performance
Deep Skir t was Retained as a Craft Upgrade and is in Product ion
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Not All Is Good
• Material Delamination showed up after 100 operating
hours on the prototype skirt.
– Issue also showed up on the Canadian Coast Guard
AP.1-88/200 and the Hoverspeed SR.N4 MKIII.
– All three craft used the same natural rubber material.
Suspected that Fatigue was the Primary Fai lure Mode
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Not All Is Good• FEA analysis of an inflated finger
indicated Stress Concentrations andareas of Large Deformations.
Stress Map Deflection Map
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Things Get Better• FEA analysis indicated that a
modification of the Design & LoftingProcess would correct this.
Deflection Map
before Modification
Deflection Map
after Modification
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Second & Third Generation Designs
• Lessons Learned were applied to the
Finnish T-2000 Combat ACV (2nd Gen).
– Modified Design/Lofting Process
–
3-D Design Tools
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Second & Third Generation Designs
• 2nd Generation T-2000 Skirt has
440+ hours on
original bow and
side fingers.
• Stern corner and
stern fingers
replaced afterapproximately 300
hours.
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Second & Third Generation Designs
• 3rd Generation
Skirt is being
manufactured.
• Model testdata results
indicate that
this will be the
best design sofar.
Bel ieve that Addit io nal Perform ance Improv ements are Poss ible
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Lift Fan Design
• Historically, ACVs tended to use commercially
available fans or a version of the successfulHEBA-A or HEBA-B Fan Series.
• Current and future high-density craft are
requiring higher pressure, higher air-flow rate
and increased efficiency. – Typically military craft rather than
commercial craft.
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Lift Fan Design
• Systematic series fan tests have not been
performed since the mid to late 60’s.
– Many of these are documented in
“Unpublished” Reports.
• Results have been the primary design
guide for:
– Fan Aerodynamic Design
– Volute Design
– Installation Effects
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Lift Fan Design
• CDIM-SDD participated in a Science andTechnology (S&T) effort directed at fan
design.
–
ONR Sponsorship. – Directed at using Modern CFD Tools to
develop lift fans that Improve on
Performance and Efficiency achievable
with current equipment.
• Aerodynamic design drew on prior fan
design experience at CDIM-SDD.
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Typical CFD Results
Volute Static Pressure
Distribution
Impeller Pressures and
Velocity Vectors
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Lift Fan Design
• CFD tools allowed efficient and economicalexamination of the various fan designparameters.
•
Results indicated that: – Blade stall is Very Difficult To Avoid in
heavily loaded fan designs.
– Good fan performance can be achieved
even with some stall present. – Volutes can be Much Smaller than
previously thought without sacrificing fanperformance.
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Fan Model Test
0.05 0.1 0.15 0.2 0.25
Lift-Side Flow Coefficient
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0.45
0.5
L i f t S i d e S t a
t i c P r e s s u r e C o e f f i c i e n t
0 0.05 0.1 0.15 0.2 0.25 0.3
Lift-Side Flow Coefficient
0
0.05
0.1
0.15
P o w e
r
C o e f f i c i e n t
CFD Prediction
Model Test Results
Sub-Scale Model Tests Conducted in October 2003
Test Results General ly Con f irmed CFD Analysis Results
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Ducted Propulsors
• Ducted air-screw design has typically relied on
Potential Flow Theory, Strip Analysis or, insome cases, Lifting Line Theory.
• Designs are developed for free-stream
conditions.
– Ignores Installation Effects.
• Full-scale trials experience indicates that these
designs typically produce Significantly Less
Thrust than expected.
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Ducted Propulsors
• CDIM-SDD participated in a Science andTechnology (S&T) effort directed at
ducted propulsor design.
– ONR Sponsorship. – Directed at using Modern CFD Tools to
develop designs that Improve on
Performance and Efficiency achievable
with current equipment.
• Aerodynamic design considered the
actual installed condition.
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Tool Verification
• LCAC propulsor was
analyzed prior to
starting the new
design.• Checked against
known performance.
•Results comparedfavorably.
LCAC CFX Computational Model
CFX for LCAC at 25 knots (Midway Station 7’6”)
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CFX Flow Model of New Design
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TYPICAL CFD RESULTS
Flow Field in Front of the
Prop and Shroud
Flow Field in Front of the
Prop and Shroud
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Propulsor Model in
Glenn L. Martin Wind Tunnel
• 1/6th Scale Propulsor Tests
• CFD Simulated Wind Tunnel
Tests were performed prior
to actual physical testing.
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Model Test Results
• Model Generally Performed as Good or Better than CFD Predictions• Measured Ct agreed with CFD Predictions 5%
• Measured Cq 10% less than CFD Predictions
Resu lts General ly Val idated the Design Too l and App roach
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Bow Thruster Nozzles
• Bow Thrusters are used on many modernACV designs.
– Enhance Maneuverability
– Augment Thrust from Main Propulsors
– Provide Some Redundancy to Main Propulsors
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Bow Thruster Nozzles
• Typical Bow Thruster
Nozzle
– Easy to Manufacture
– Aerodynamically
Inefficient• Easy Bend versus Hard Bend
–Large Over-TurningMoment on Bearing
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Bow Thruster Nozzles
• Aerodynamically
Efficient Cascade
• Significant Reduction
in “Over -Turning”
Moment on Bearing
• Reduced Visual &
Radar Signature
• Complex to
Manufacture
Low-Profile Bow Thruster
Ful l-Scale Trials Veri f ied Aerodynam ic Eff ic ienc y
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Questions?