Dynamics of the model towing carrige

PUBLIKACIJA BRODARSKOG INSTITUTA

PUBLICATION OF THE YUGOSLAV SHIPBUILDING RESEARCH INSTITUTE

No. 4. ZAGREB

DYNAMICS. OÊ THE MODELTOWING CARRIAGE

[ng DRAKO GOSPODNETIÓ Iñg IVAN MODLIC

ZAGREB 956

DYNAMICS OF THE MODEL-TOWING CARRIAGE*.

The model-towIng carriage in the 60 m-tank of Brodarski institute was checke4 on the

'uniformity of the towing speed.

-A method was developed o measure carri4ge speed over a distance of 10 cm, with J0/ accuracy at speeds amounting to 2 rn/s, and at every 20 cm of the tank length.

lt was found that the speed fluctuations at I rn/s do not exceed 10/00, while, the

accele-ratiOn itself does not exceed 0,007 rn/s2.

The principal purpose of the towing tanks is tQ deduce from the measurements of the geo-metrically similar ship model the characteristics

and behaviour of the ship to be built. The volume

of the experimental programme depends of the order given by the designer or shipowner. The rneauring methods may, thus, vary from case to case. Nevertheless, one measurement is never omitted from the. investigation programme, viz. the measurenient of the total resistancè of the model at a constant speed. To fulfil this task, it is

necessary t have a resistance-measuring dyna-mometer and a model-towing device at our dispo-sal. The latter device will be discussed in this

paper.

The towing device most usually consists of

two essential parts: the towing carriage and rails

on which the carriage runs. Both elements should

be so designed as to shorten the time for the acceleratiOn and braking, while'the speed at the measuring part of the tank should be as uniform

as possible. Acceleration and braking have

influ-ence on the efficiency of the over all length of the tank, being thus only an economical factor

which must be taken jñto account when choosing

tank dimensions The speed fluctuations affect

the accuracy of the resistance measurements.

A simple calculation shows the speed

unifor-mity that is needed: Let a certain model, towed

in 60 m tank in Zagreb, have the following'

data:

Weight G

=

50 kg

Speed - y =r 1,0 rn/sec

Resistance R 0,25 kg

By every change of carriage speed, on the

dynamometer lever acts the sum of the resistance and the. accelerating force:.

P=R±ma

where. m denotes tile 'mass, and a the acceletatioa

of the model. An error in resistance amounting' to maximum 1% leads to the inequality:

< Q,01

or

a 0,01 .

For the mentioned model the allowed acceleratioa

is:

a0,010'0

9,81 L0,0005

mm 0,5

s2

*Paper read on the 3rd Meeting of' the Yugoslav Society of Theoretical and Applied Mechanics, June 1956..

m a

I

j:

This result explains the rigorous demands -which both the carriage and rails must satisfy. The carriage (Fig. 1) runs on four

cylindri-cally turned wheels, the left in front being driven by a 2 kW d. c. motor. An electronically regulated Ward-Leonard set supplies the power. The carri-age, including three operators, weights about 1600

kg. Diameter of the wheels is 05 m. The maximum

zspeed obtained is 3 rn/s.

The rails need to be worked and laid down

-with a special care. The deviations from a straight

line and parallelism with the water level in. the tank should not exceed 0,1 mm.

In the tank in question a normal railway profile is used. The guide wheels run on the sides

-of the left rail which are worked as well as the

upper surfaces of both rails. Joints, cut under 450,

enable the wheels to pass smoothly over 2 mm

of clearance between th.e rails. Welded supports

are cast in concrete wall and make possible an 'easy adjustment of the rails (Fig. 2.)

The free surface of water in a gutter around the tank serves as a levelling reference plane. 'The height of the rails above this plane is

adju-Fig. 1. Towing carriage

4

sted by means of an instrument whith a point which measures the distance between the upper surface of the rail and water level. An electronic device, working on the ,,rnagic-eye" principle,

battery driven, indicates the contact between the point and the water. A spirit level, whose

sensi-mm

tivity is 0,1 helps to adjust the

trans-m. div.

verse slope of the rail. Accuracy of the heights

measurement is 0,01 mm (Fig. 3.).

The vertical reference plane is formed by a steel wire hanging freely over the tank's wall. An instrument with a plumb-line helps to bring the rails in parallelism with this reference plane. The contact between the plumb-line and steel wire is indicated by the help of the ,,magic-eye, while the diàl gauge measures the deviation of the direction of the rails from a straight line with the accuracy of 0,01 mm.

- With such an equipment was possible to satisfy every demand and lay the rails down in a relative short time. After half a year of use the rails did not show any appreciable

, scnIoto, '00

E cuarnc

G1p

The speed of the carriage was determined by

measuring the traversed distance and the

corre-sponding time. The maximal errors were 0,1 mm and 0,00001 s respectively. This enabled us to use a distance as short as loo mm. The path was

thus measured within 10/00, and the time within

O,l0/ _{at a speed of I rn/S. The error induced} by the time measurement is obviously negligible through the whole speed range.

A row of metal plates nominally lOO mm in length and 100 mm apart was attached to one side of the right rail (Fig. 4.). The plates inter-rupt the light-beam falling from a bulb on a phototube, as the carriage moves. While the

pho-totube is exposed, a number of pulses, deriving from a 100 kc/s crystal-oscillator, pass through an electronic gate to a decade counter which counts them. The interruption of ligt stops the

counter. The difference between two successive counter readings gives the time in 1/100000 sec during which the gate was open. The time the gate is closed stands at our disposal for reading the counter.

For the distance meaurement the phototube

should be used. The carriage was moved by

me-ans of a screw fastened to the rail (Fig. 5). The

displacement, during which the gate was closed,

or open, was measured with slip gauges and the dial indicator (Fig ). Thus we measured the length

of the plates and the distance between them as the

Fig. . Block diagram

6

phototube sees" them. The sensitivity of then

electronic apparatus was such that it was sufficient:

to move the carriage for 0,01 mm to activate then

counter. The accuracy of the path measurement

was 0,1 mm, which was found by repeated

mea-surernents during a longer period of time. The plates were placed so close together ia order to get more information about the speed. On the other hand by this arrangement the time-interval for counter reading was shortened. At a carriage speed of i rn/s we have only 0,1 sec-at our disposal or, in other words, the counter shold be read 5 times per second. That is

im-possible to do visually. It was, therefore,

necessary-p

Fig. 5. Displacement device

Q

9'

5

to record the readings automatically. A decade counter, formed by four decades with EIT tubes, was used. Every electric pulse passing through the tube deflects the electron-beam from one stable position to the next. The beam has 10 stable

po-sitions corresponding to the digits O to 9 and im-pinges on a f lourescent screen causing the corre-sponding digit to be illuminated. Every stable position of the beam or every digit on the screen

corresponds to a certain voltage of the deflection plate in the tube. This voltage was used as an indication of the decade state aplying it to the

vertical deflection plates of a cathode-ray oscil-loscope. Every digit on the counter-tube screen corresponds to a certain position of the spot on

the cathode-ray tube, which can be

photographi-caly recorded. Two double-beam oscilloscopes and motor-driven cameras with continuous transport

Fig. 7. Dtaplaceinent measurement U

i

3

_sm

Fig. G. DispLay records

of the film were used. The developed films were read in the microviewer (Fig. . The circuit and

the arrangement of the equipment is shown on

Fig. 8 and 9.

One part of the measured values for the

nominal speed of i mis, together with the calculated

average speed

(s+ ¿:s)s

V =

_Lt

is shown in Table I.

According to the table the initial acceleration

decreases, tending asymptotically to zero, while the speed constantly increases and reaches its

constant value after 11 m of path.

Analysis of all data in the constant speed range shows a 2°/ scattering for 60% of speed

values and 0/co scattering for 80% respectively, while the greatest difference between two values does not exceed 40/oo. Nevertheless, the

accelera-tion is always less than 7

,.

It is allowed the-.

refore to consider as the instantaneous speed

at the point s + . Furthermore, as these facts

show that the consistency of path measuring du ring a run holds better than the previously stated

accuracy of path measuring we estimate the speed fluctuations to be not more than l0/.

The speed fluctuations may arise from the

variations of the friction force, wheels eccentricity, gear dividing errors, and rails deflection. Finally

69

//95

U

the non-uniform torque of the motor can also be a cause of trouble. The accuracy of the

mea-surement could be raised to 0,1/ using a I Mc/s counting circuit with E1T tubes (cfr. Rev. Sci.

Instr., March 1956, P. 150) and applying an optical

system to the phototube.

Measurements in the speed range from

0,25 to 2,0 -- did not appreciably differ from the measurement discussed above, what the speed constancy concerns. However, the path for

acce-lerating becomes longer as the speed to obtain increáses. It is 20 m long at the carriage speed of 2,0 rn/sec. It has been proposed, therefore, to change the existing electronic speed control for one which would keep the acceleration constant and at its maximum value until the carriage reaches

a preset speed. The possibility to build inanother

electromotor is also considered. In such a case the speed range could be extended in both directions. The investigation leads, thus, to the

conclu-G G

e

Fig. 9. Electronic equipment

t

sion that it is possible with existing means t

keep the carriage speed constant within 10/00, while-it is practically impossible to lower the acceleration to 0,5 as it was previously desired.

It seems, therefore, to be necessary to

inve-stigate the 'behaviour of existing dynamometers under such conditions on the carriage and even-tualy to seek for another measuring technique which accuracy would not be affected by speed. fluctuations of the carriage, when measuring model resistance on the measuring part of the tank.

This paper draws its origin from a recom-mendation of the 7 International Towing Tank

Conference, held in Goeteborg, August 1954, votinr for a minute re-examination of all experimental

model technque in order to reduce the differcnces among measurements, of the same model in

diffe-rent establishments, because in this way the

exi-sting international cooperation of the towing tanks will still be more successfuL

TABLE

Even n indicate the holes and odd n the

The relation exists:

plates. S indicate the distance froin the beginning

of the hole zero to the beginning of the hole' ör pláte n. LSn is the length of the hole or plate n

measured with añ accuracy. of 0,1 1 A graphic representation of the first meter: n-1

=

o 100,51 99,9-3 100,48 100,00' 100,60 10Q,12

Ls

10Ö,76 99,75 99,74 99,80 9,9,72 99,77 n 0 '1 2 3- 4

5fl

6 7

8

.9 10 -1.1 s6 = 601,17

4-=

800,97-*

n - -, mm

:mm'

s min/s : 0 2 4 6 8 lo 20 22 24 28 28 30 So 52 54 -56 58 60 100 -,102 104 -106 --108 i-10 112 114 116 180 182 184' 186' 188 190 -. 0,00 201,27 400,95 601,17 800,97 1001,29 1999,79 2199 71 2400,96 2590,99 2798,52 299,98 4997,86 5197,30 5396,90 5596,65 5796,33 .5995,77 10001,95 10195,97 10396,10 10595,68 1079530 10995,48 11196,48' 11394,88 11596,75 17991,67 18190,96 18390.63 .18590,37 -18789,82 18989,80 ' '. ' 100,51 99,93 iOo;48 100,00 100,60 100,12 l0O,27 100 40 99,29 98,66 10O58 '100,35 99,74' 99,72 -100,00 99,94 99,76 ' 100,59 94,25' 100,41 99S4 9985 100,40 100,27 99,72 100,10 100,38' 99,53 99,99 100,03 99,71 '100,15 99.60 ' -, . , ' -' ' ,-' 0,69559 O,2532 0,19912 0,17220 O15673 0,14468 0,12287 0 12083 o,11s30 0,1167'S Oj116&9 o;11563 0.10961 Ó,10924 0,10931 0.10923 0,10896, o;10956 0,10172 o;1O85 010766 0,10774 0,10818 0,10797 0,10727 0,10761 Ó,10796 -0,10697 0,10741' 0,10763 0,10717 0,10757 0,10691 ' 144,5 391!4 504,6 589,7 641,9 692,0 816,1 8309 835,8 844,8 861,9 867,9 910,0 912,9 914,8 915,0 915,6 p18,1 926,6 ' 927,6 . 7,4 926,8 928,1 928,7 929,6 930,2 929,8 930,5 '930,9 929,4 930,4 931,Ó 93l6

00 k4/s C,4 c,, C,- VIO C, C2, V 15

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R4915S/07p 55k/S C4 - C5p5 _4/ - 04497 04 - 0271 - tO *53 32f - 450 743 543 - F

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his C504, 3F' 4/ - 0349f O, - 0453 - 32 kO

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C345062

V, - 6292CC F-, 487 - 623-33 7x5 1/Sss I/SO 78ko

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R-54,-R,s-' k/S

C39462

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R4r_/4/5F73_55kfl

C,5 785562 62.3 - 090CC '2 - o' A 3253 - 5.6 hO R50-2Okfl 6249-62,4-079-574ko C,,-Cs-C,'-9557t pC V,, - Cf r

'ko

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4779 C,,-C,s-4/,-54058p62 V,, - 090CC

R-

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C,,-C,8C45- ,ispC V__O - 5555

6234 - 75 hiS 6233 - 30 913 R5 - /k -R, - / hO C,4 - 0/4 C24 - 75 p62 V54 - C9000

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