On the determination of aerohydrodynamic performance of Air Cushion Vehicles (ACV)

15 SEP. 1972

RCHIEF

DO Cu M ENTA TI EDATUM: DO C J i

1:

¡{j 72/#

J 1OKTJg?

The Ninth Symposium on Naval Hydrodynamics

Lab.

_{v Scheepbo,jwkunde}

Tedìnisclie

_Hog2SchooI

DeIft

ON THE DETERINAT ION OF AEROHYDRODYNAMIC

/7' '''

PERFORMANCE OP AIR CUSHION VEHICLES (ACV)

by S.D.Prokhorov, V.N.Treshchevsky, L.D.Volkov

The estimation of propulsive performance and

maneuver-ability characteristics of AOVe while designing these vehicles

can be based on tests carr.ed out with self-propelled models

and simulation of various operational conditions for these

models. It is obvious that the model behaviour is governed in

this case by the total aerohydrodynamic loads which are

dif-ferent in nature. At the same time separate definition of

aero-and hydrodynamic forces of various nature is of interest,

specifically, for the analysi8 of the effect some components

of these forces have upon the ACV's performance and dynamics.

The forces acting on ACVs can be classified into the

following categories:

aerodynamic forces due to the oncoming flow around the

hull, flexible skirt, stabilizing fins and controls;

aerodynamic forces due to the air cushion and

inter-action between the latter and counter air flow;

hydrodynamic forces due to the contact of the flexible

skirt with water surface and interaction between the air cushion

and water surface;

hydrodynamic forces due to the effect of waves.

The cruising conditions of ACVs are characterized by the

-2

and air environments. The difficulties in simulating the

aero-hydrodynamic effect and the necessity of analysing the data

obtained make one carry out the experiments in the towing tanks

and wind tunnels independently. In the last few years the

central place in such experiments belonged to obtainingthe

steady and non-steady characteristics necessary for the

calcu-lation of transient processes and estimating the ACV

sea-keeping qualities.

This paper gives a brief description of the most typical,

in the authors' opinion, methods of defining the above

characte-ristics. The description of these methods is illustrated by the

measurement results concerned with schematized models of

ACVs with different bow forms.

Definition of aerodynamic characteristics

As is known, the character of ACV movement in the

longtu-dinal plane depends mainly on the air cushion parameters and

also on hydrodynamic interaction between the flexible skirt and

water surface. The ACV movement in the horizontal plane depends,

to a great extent, on aerodynamics of external flow around the

vehicle.

There are two main problems relat±ng to the study of

external flow around the ACV, viz., the decrease in air

resist-ance and the achievement of predetermined maneuverability and

heave stability. The first problem can be solved only on the

basis of the rational choice of the hull form. The second one

3

the ACVs with the length-to-breadth ratio of 1.5 to 2.5 the

resistance as well as side forces are determined by the

distribution of normal pressure over the Perimeter of the

hull;

80

the studies of both problema are closely connected with each other.

The experimental investigation of the aerodynamic forces

and moments which are due to the external flow is generally

carried out on rigid models because alight deformations of the

flexible skirt have little effect on ita aerodynamic

characte-ristics. A six-component balance is used for this purpose.

Water surface is simulated by a flat ground board.

The nature of aerodynamic factors can be revealed from the

results of the experiment with schematized models carried out

for the study of the effect the hull form has on the ACV's

aero-dynamic characteristics. Table I lists the values of geometric

characteristics of the models tested, their resistance factors

and the derivatives of aide force and yawing moment with respect

to the yaw angle for

p

= O which are necessary for the

calculation of ACV movement in the horizontal plane. The

aero-dynamic forces are related to the product of the velocity head

and centerplane area , while the moment is related to SL

where

_L

is the model length. The moment is calculated with respect to the middle of the hull length.

As is seen from the Table, the resistance is mainly

influ-enced by the forebody form. With the change of relationship

from 0.5 to 2 the resistance factor decreases by

30-40 per cent. The influence of the hull form on the resistance

4

formation of the dead zone at the stern. The angle of run

ranging from - O to 2 leads to a decrease in resistance

only by 10 per cent.

It is known that the heave stability of the ACVs mainly

6

depends on the sign and value of the derivative

my. The

fe

minimum value of the derivative 177y which is favourable from

the viewpoint of stability of motion occurs for the models 7

and 8 with the smallest values of angles of run and entrance.

As is seen from the Table, the bow elongation results in

in-creasing the destabilizing moment lily, so the requirements of

minimum values for the coefficients of resistance and yawing

moment are rather inconsistent. Since the required value of

the derivative

[/7yß

can always be obtained due to the fitting

of vertical stabilizers without noticeable increase in

resist-ance, the hulls with elongated bows and blunt sterns appear to

be the most advantageous.

Por the approximate estimates of the AOVe aerodynamic

characteristics at the initial stages of designing the

theore-tical methods are of interest. _{The method is developed for}

the calculation of both the total and distributed aerodynamic

characteristics of ACV hulls with flat sterns at different yaw

angles and angular velocity )y depending upon the forebody

forms and parameters and

The method is based on replacing the hull of the ACV by a

vortex surface which extends beyond the hull for modelling the

vortex trace effect (Pig.l). _{The free water surface is}

to a model with a double height H. The transverse vortices

are directed parallel to the lines forming the flat stern

contour. The longitudinal vortex which leaves the body from

stern contour corners has the density equal to the difference

between the densities of vertical and horizontal vortices

re-placing the contour. Longitudinal vortices arranged in the

deck plane are parallel to the longitudinal axis of the hull.

Calculations were carried out for the finite number of discrete

vortices spaced equally from the control points where the

bound-ary condition is satisfied. The density of all vortices is

defined from the system of equations characterizing the boundary

condition on the body surface at control points. Por the proper

choice of the value of circulation around the body, as with the

known Joukowski condition in the wing theory, a supplementary

condition is introduced, viz, the density of the trace vortices

in all the points of the vortex sheet is the same and equals to

the density of the vortex shedding from the contour of

separa-tion at the stern. Then the system of equations for defining

the densities 1',, ¡ , . 1,, takes the forni:

(1)

where _FE and

Pp

= induced velocities corresponding to

transverse and longitudinal vortices,

F'

= normal component of the free stream velocity,

= angular velocity of yaw.

Fig.2 shows the pressure distribution over the contour of

intersection of the model and the basic plane obtained by the

calculation method for model 2 with the angle

6

test results on defining the pressure distribution at the same

section of the model are also plotted in the same figure. Aa

is seen, there is a fairly good agreement between the theory and

experiment.

As noted above, the aerohrdrodynamic forces due to the air

cushion are the decisive factors for the longitudinal motion o!

the ACV. One of the methods used for defining the steady and

non-steady aerodynamic forces is based on the measurement of

force s acting upon the model which performs the harmonic

oscl1-ations. When carrying out these tests in a wind tunnel the

forces are evaluated which are connected with the qualities of

an air cushion arid deformation of flexible skirts above the

ground board simulating the water surface. In some cases the

results of such tests cari be used, for the calculations of the

ACV movennt above the ice surface.

Non-steady aerodynamic characteristics of ACV models with

built-in fans and flexible skirts are determined by using the

experimental plant shown in Fig.3. The principle of operation

of this plant consists in generating the definite harmonic

oscillations for the model and measuring the loads acting on

this model with the consequent determination of the lift and

lateral moment derivatives according to the kinematic parameters

of motion. The plant is equipped with a mechanism of

compensat-ing for the model inertia forces and with an electric harmonic

analyzer for automatically defining and recording the signal

constant components which are proportional to the r'equ.ired

upside-down (Fig.3) on two supporting pillars which make

recipro-cating oscillations with arbitrarj shift in phases with respect

to each other. Each pillar is supplied with a strain gauge which

serves as a connecting link between the model and the oscillating

pii lar.

The distances to the ground board Ii , the trim angle Y

and their first-order and. second-order time derivatives h

Y,

/1,

'i"

_{are adopted as kinematic parameters defining the model}

aerodynamic characteristics in the longitudinal plane. In linear

approximation the expansion of the vertical force or lontudinai

moment as a series in kinematic parameters has the form

8=R (,

))h(

Y)h

(2)

where

&

_,

-

_{are mean values of height and trim in}

respect of which the values h and S" are changed.

The tests are carried out for two types of motion:

translatory and angular harmonic oscillations of the model

where, with the results of the model static tests also used, all

the derivatives entering into Eq.(2) can be determined. The

values of rotatory and translatory derivatives of the vertical

force and the longitudinal moment are determined and they are

transformed to a dimensionless form through dividing these by

the model weight 6 or by the product

CL

h

L

8

As the dimensionless kinematic parameters the following factors

are used:

,hQ0.

2 2

?

hQ0

(.f1

VIQ0

- L2

'

L'

2 ,

- L4

where

Q0 = air flow rate corresponding to

the parameters

tÇ,

_(m3/sec),

= acceleration of gravity

(in/sec2).

The value

Sh,1 -

should be adopted as the dimensionless

criterion of similarity which is similar to the known Strouhal

number (

&' = angular frequency of oscillations).

Then the

dimen-sionless values of parameters

h

,

Y

,

/1

, Y-"

in Eqs.(4) are

the products of the dimensionless amplitude of vertical and

angular oscillations and Strouhal number

Sh

or 45'h.

Using the procedure described the tests were carried out for

three schematized models of ACVs with built-in fana.

The scheme

of the sectional flexible skirt mounted on the models is shown in

the sketch (Fig.)).

The geometric characteristics of mcxiels are

given in Table 2.

Figs..4-5 show some of the results obtaind.

The curves in Fig.4 illustrate the character of changing the 1S'h

h'

factors

(y, /777

,

¡/4

versus the dimensionless parameter

at different values of pressure factor K_-" (P,P = pressures

in the receiver nd air-cushion, respectively) and air flow rate

it is typical that the external air velocity effect on the

-9-non-steady characteristics is practically absent in the range

of the examined actual relationships of the contrary velocity

head and the cushion pressure (Fig.6). The curve in Fig.5 shows

the influence of the bow form at the fixed values of frequency

and. pressure factor upon the saine characteristics. It is obvious

that the bow form influences mainly the moment characteristics.

The non-steady aerodynamic characteristics necessary for the

study of the ACV maneuvering in the horizontal plane are also

defined in the wind, tunnel by the method. of harmonic oscillations

of the model around its vertical axis and measuring the yaw

damp-ing moment influencdamp-ing the model. In this case the procedure of

measurements is similar to that described above. The experiments

show that the principal role in generating the damping moment

belongs to the vertical stabilizers and. the hull effect on this

moment is not significant. It is observed that in some cases the

damping moment on the hull-stabilizer system decreases due to the

adverse effect of the hull on the stabilizers.

Determination of hydrodynamic characteristics

In the towing tanks the tests are carried out to determine

the main hydrodynamic characteristics of ACVs. In this case the

model is usually subjected to the action of both the hydro- and

aerodynamic forces. Then, depending on the test conditions,

account is made of either the results directly obtained by

measurements, or the aerodynamic components are to be excluded

with the use of data on the blowings of the model in the wind

tunnel.

The most typical tests carried out in the towing tanks are

those with the towed models exhibiting the freedom of heaving

and. trim; during these tests the resistance and kinematic

in waves. The purpose of auch teste is not only in obtaining

the propulsive performance data but it is largely connected with

the evaluation of dynamic properties of these vehicles. Thus

the measurennts carried out at different positions of the centre

of gravity along the model make it possible to plot the

posi-tional curves against the trim angle and to judge about the

static stability depending upon the conditions of motion. The

same type of towing tests is the basis for determining the

regions of steady motion in the longitudinal plane defined by

the influence of the waves and speed.

Such experiments were carried out specifically on a series

of models with the particulars given in Table 2. In this case

the resistance and kinematic parameters were changed up to the

critical conditions preceding the development of plough-in.

The curves in Fig. 6 show the effect of the bow plane form on the

relative resistance depending upon the running trim angle

which is defined by a given position of the center of gravity.

It is seen from the curveplotted for a cruising regime that

the effect in question is oberved only with trim by the bow; in

this case model No.2 appears to be preferable. The advantages

of a semi-round bow planeform manifest themselves in waves too,

as is seen from the curve of Fig.7 where the relative gain in

resistance is presented for all three models in waves.

The air flow rate effect examined on model No.2 is typical

for this experiment. The curve in Pig.8 shows the effect of the

dimensionless factor of air rate upon the relative resistance in

10

-wave s

Q

c' ,

where

Q

air rate, in/sec;

= area of the air cushion, ni2;

= pressure in the cushion, kg/rn2.

It is seen that practically in all the cases the increase in the

flow air rate results in a decrease of resistance; in this case

the supply of air into the forward part of the air cushion,

is the most favourable.

The curve of Pig.9 serves as an example of plotting the regions

of stable motion. The relationships shown are the result of

processing the model No.2 test data according to the evaluation

cf the limiting regimes when plough-in is developing with the

consequent loss in stability. The curve shows the favourable

influence of increase in the air flow rate at the bow centering,

making it possible to delay the setting of' the critical regimes.

The definition of the non-steady hydrodynarnic

characteris-tics which are necessary specifically for carrying out the

calculations of ACV heaving and pitching is based on the same

methods used in a similar case for displacement vessels. The

h7y

linear character of the restoring forces and rnomonts

M2-

defined experimentally in the working range of the

flying heights /7 and trim angle

Y'

for the ACV with a flexible

skirt gives grounds as a first approximation to proceed from the

linear theory premises while defining the non-steady

characte-ristics. The tests are carried out on a plant which makes is

possible to perform in calm water the forced heaving and

12

-gauges and provides the recording of kinematics of motion. To

define, for example, the coefficients of inertia and damping

forces by the test results, the equation of the forced heaving

motions is written in the following way:

(myY#

g=C(ZCosE-y),

where /71 = model mass,

t

regidity of spring,

= amplitude of disturbances,

= frequency of disturbing force.

Having experimentally defined the parameters of the forced

motions of the model in the form

¿

(c5.-t - 3y)

where /7y = amplitude of oscillations of the model centre of

gravity;

= phase shift between the translation of the

model

and the disturbing force, h

one can

find

the coefficients

Y

and

Y

and

similarly

the moment coefficients characterizing the pure pitching

q;

motions arid _A1 The difinition of factors

characteriz-Ing the influence of pitching upon heaving is possible provided

that the vertical translations of the center of gravity are

13

-The experimental plant scheme and some of the resulta

obtained are given in Figs.lO-ll. _{Depending upon the type of}

rope-and-block connections provision is made for the

transla-tional vertical (Model 1) or angular (Model 2) motions of the

model and for recording kinematic parameters on the

oscillo-graph tape. In Pig.1l the coefficients of inertia forces

obtained by the above method are plotted against frequency.

It is necessary to note that the aerodynamic

characteris-tics corresponding to flight over the water surface are

differ-ent from similar characteristics of the model over the ground

board. _{The difference is obviously due to the influence of the}

water surface deformations' and mass forces; it minimizes as the

Proude number increases.

The experimental plant described was also used for the

definition of damping and inertia characteristics which manifest

themselves at non-steady motion of the model along the

longitu-dinal axis; the plant is switched on according to variant _3.

As the test resulte show, the forces determining the above

loads are negligible for ACV models.

In the cases when during the tests in a towing tank the

aerodynamic components are so important (in comparison to

hydro-dynamic forces) that they cannot be neglected, it is necessary

to consider that the aerodynamic effect upon the model tested is

not fully simulated. _{This introduces some infinity in the}

results obtained due to both the distorted aerodynamic action

and the effect of this action upon the position of the freely

towed model and consequently upon its hydrodynam.ic

14

-the aerodynamic components are excluded from tests in -the

tow-ing tank and are determined in a wind tunnel.

The procedure of carrying out the experiments of such kind

is as follows: The model is rigidly fastened to the dynamometer

which measures the lift, drag and longitudinal moment of the

model at the fixed values of height and trim of the model.During

the tests the measurements are carried out in a prescribed

regime. Then according to the saine program the tests are con-ducted on a model equipped with a working fan over the ground

board fixed under the model in a close vicinity to water surface

and transported together with the model. The aerodynamic forces

to be excluded are determined as the difference between the

resulte of the measurements carried out over the board both

underway and at a speed equal to zero in flight.

The specific behaviour of the ACV with a flexible skirt

makes difficult in some cases the use of traditional methods in

calculating the maneuvering qualities, dynamic stability and so

on in terms of the so.ution of equations of motion. The

com-plexity and considerable amount of tests necessary for defining

the coefficients of the equations makes one use other methods of

study. The determination of transfer functions according to the required parameters in terms of the experimentally defined

fre-quency characteristics is considered to be reasonable. These

functions make it possible, as is known, to calculate normal

maneuvers of the object according to linear theory. Besides

it is important to have the possibility of directly evaluating

15

-in damage situations. The teste with the both aima in view are

carried out on the experimental plant making it possible to

simulate, in the main, the conditions of the model free moverint

and in some cases to eliminate the necessity of carrying out the

expensive testa with self-propelled models.

The basic diagram of the plant is shown in Pig.l2. During

the tests the model is towed along the towing tank; it displays

five degrees of freedom, i.e. vertical emergence, side

displace-ment, heeling, yawing and trimming angles; all the kinematic

parameters were recorded. In case of side displacement, the

model is relieved of inertia and friction forces in movable units

of the plant by means of a special servo-system. The significant

element of the plant is the system bringing the towing force into

coincidence with the model centre line irrespective of the

posi-tion of model relative to the tank axis. Finally, in case it is

difficult to arrange the drives of controls on the model, the

electric systems are provided for the plant which are capable to

imitate the action of the controls, particularly, side force

controls, by prescribing the side force, yawing moment and

heel-ing moment in accordance with the required law.

Some test data obtained on the plant described are given

below. The studies were carried out on the ACV schematized

model No.2 for the purpose of evaluating its course stability

(with vertical stabilizers mounted) and checking its other

dynamic characteristics. Fig.13 shows the relationship between

the amplitude-frequency characteristic of the model for yawing

angle at different speeds. Is is seen that with the increase

16

-values Fr = 1.10. The saine figure shows the variation of the

relative amplitude of the model yawing angle at fixed frequency

and speed as dependent on the pressure factor of the air

cushion

Kp.

Some damage situations with the same model were studied,

particularly, the transition process at first instants after

suddenly applying the yawing moment, which may be the

consequ-ence of a spontaneous reverse or failure of propellers at one

side of the model. It is seen from Fig.14 how kinematic

para-meters of motion are changing after the instantaneous

applica-tion of the rolling moment (induced, for example, by the

break-ing of the flexible skirt along the side) until the steps are

taken to keep the ship in the upright position. This

experi-mental plant makes it possible to simulate the maneuver of

coursekeeping in this condition by applying the counter yawing

moment imitating the action of controls. It is typical for

this case to set the model in a steady motion with a drift to

the inclined side.

The condition that the towing carriage speed should always

be constant makes it impossible to simulate to the full extent

the full scale performance of the vehicle. However, this

restriction is no barrier to solving a wide variety of practical

L

a

__

-2.5

o

H

__¿

fl

Nos.

L

₈

1-xi

r

c-z,

1

2,5

0,25

0

0,162

0,97

0,20

2

2,5

0,50

0

0,140

0,97

0,20

3

2,5

1,00

0

0,118

1,03

0,22

4

2,5

0,25

0,50

0,163

1,03

0,21

5

2,5

0,50

0,50

0,137

1,03

0,21

6

2,5

1,00

0,50

0,120

1,03

0,23

7

2,5

0,25

0,25

0,166

1,03

0,17

8

2,5

0,50

0,25

0,137

1,03

0,18

9

2,5

1,00

0,25

0,123

1,03

0,23

10

_2,5

0,25

1,00

0,150

0,92

0,23

11

2,5

0,50

1,00

0,125

0,92

0,23

12

_2,5

1,00

1,00

0,110

0,92

0,23

13

1,5

0,50

0

-

1,44

0,18

14

2,0

0,50

0

-

1,26

0,20

15

3,0

0,50

0

-

1,10

0,23

-17--

Table 1

-18-Table 2

Particulars Designation Model

No.1 No.2 No.3

Planeform of air cushion

elliptical round rectangular

rID

ii

Length of the same _L

,7M

1,98 2,11 j 2,04 Breadth oÍ the same 817JM 1,0 1,0 1,0 Area of the same 1,72 1 2,0 2,0 Coefficient of air cushion area y7

Sn

_0,90

_0,925

_0,985

/

/

'I

y vF1j

I

.1

/

I

'I

/

Fig.1.

0.5

io

Pig.2.

Precsure distributiOn over

the model wall

for No.2 variant,

/3

100.

preneurs eide, theory

---suction side, theory

o o o

(ì \

2

Fig.3. Scheme of the plant for determining

the aerodynamic performance of ACV

models.

i - model,

2 - ground

board,

3 -

strain gauges,

4 - mechanism of compensating for inertia forces, 5 - electric motor,

j. «S II FiIIII I iIii I. II ;La r -'o

'2 m1

-'I '7

Fig.4.

i ht II:«.

os

I t .%í. ;ty6ii. I I JLII. H lia Id 'z (: IhI II. Ilepu. II)II M(II. g 4. 5½

4-Cocfficis

t

/77 /77

vercu

S/

number

y,

z'

z

(Mod.ol No.1)

x

a.P,,

V 17,J/ /.c't ,1I,'? _wate? 71

¿200ñ3 3G

o X

/.30073/30

O o

/35 a.io

3D O ô

/35

7.'í0 30

/020

s HO --iOD

_m

m

C

095

/

l'lo

LP

Pig.5.

Influence of ftllne

coefficient

(0

upon coefficiente

c, ,'n,

m';

$,' 0.1;

A',= 1.3;

Q,- 0.73

m3/eec.

VjL

s model flo.1

A A model No.2

o O O model No.3

-3

-2

1

j

2

3

Fig.6.

G

0,06

Q02

o

1

Flg.7.

Influence of the running trim angle upon the gain in reelotanco

in waves (wave length A = 2.2.m, wave height

h z 0.08 ni).

---inodel No.1

model No.2

0,10

Fig.8.

Relative reeistance in waves against the air flow rate for

different

mthods of the additional air supply (viave length

A = 3.1

,

wave height

h = 0.08 m) P - 1.10

o o

o additional

air supply into the forward part

0

0

Ø air

upp1y into the after part

. air supply along the

perimeter of air cushion

01

0,02

C'CS

Ct,

2O-Fig.9,

Inf1u-nce of the relative air flow rate upon

the longitudinal etability

of motien (P

Afl - oncet

of plourh-in of the flexible

ekirt) for two

positicn3 of the center of gravity

along the model lenth (wave

length

A

3.l.m; wave height h = 0.08 m;

6

80 kg;

= 1.25)

°

°

°

6/?

0.012

=0.019

o

aoj

O2

LJ

_E A 1' e

-Il

Il 11III

liii! liii

2?

J:

Pig.lO.

Experimental plant for

determination of non-steady hydrodynamic

performance

in longitudinal plane.

i - scheme of switching on

foii' heaving studies;

2 - scheme of

switching on for pitchswitching studies; 3

-scheme of switching

on for the

study of translatory motion along

I

50

60

¿L

Pig.11. Coefficient of inertia

forces vergue frequency

of vertical osci1latiors

for the achematied

model of AGIT.

o

o

o

Fig. 12. Experimental plant for dynamic

tests

with the ACV towed modela in the towing tank.

i - system of unloading the movable units; 2 - system of

bringing the towing force into coincidence with a model centre line; 3 - system of imitating the external forces;

o

Fig.13.

Re1ationhip mou1 fur y

5 = 64 k r 1.3 1,3

ceK

o

05

IP

1,5 G G

betvieen the amplitude-frequency characteristic

of the

ring angle at different speed8 and preseure factors

of the air cushion.

Lç

F'=o.00

°

o

z 0.88

4 4

Iç

1.10 X

11,32

o

₅

_'fo

-20

-0,5 y

e

YrnrnNg

9rr,

Pig,14.

Transition process at the instantaneous

application

of the rolling moment.

sec.