experimental data for rudder behind propeller

Refrence: Marine Rudders and control surfaces By ANTHONY F. MOUAND I STEPHEN R. TURNOCK

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* Sec5.4:Experimental data for rudder behind propeller

Produced by Milad Naderi

rudders downstream of a propeller Molland and Turnock 3.5 m X 2.5 m low-speed wind tunnel

Two independent unit Propeller or Rudder Openwater test Propeller Rudder ineraction

* Wind tunnel rudder-propeller interaction dataTest rig, models and tests


The rudder is attached to a five-component strain gauge dynamometer

small gap of approximately 2.5 mm (0.004 c) between the rudder root and the floor of the tunnelThee rig is designed in such a way that the propeller can be adjusted vertically, longitudinally and at an angle of drift to the flow, if required.Most tests were carried out with the propeller axis aligned horizontally and at a distance of 600 mm above the wind tunnel floor.The propeller rotates anti-clockwise when viewed from aftstrain gauge dynamometer mounted on the propeller shaft close to the propeller measures the delivered thrust and torque



7 Rudder Model were used in test Turbulence strip on rudder(5.7% of chord length

from leading edge) A four bladed propeller with a diameter of 800

mm and blade area ratio of 0.40, Wageningen B4.40

Modification of B4.40(blade root shape, removing rake, increasing boss diameter,..)



Test restriction: 1) free surface effectsA vessel is in light ballastB rudder or control surface is close to the free surface2)Cavitation

Scale effect on propeller drag are similar to those for smaller propellers tested in water..

effects of compressibility are not significant provided limitations on revolutions(3% increase of Kt & Kq @ 3000 rpm)

wind speed of 10m/s1:adequate rudder Reynolds number(0.4 -1 * 10^6)2:satisfactory range of ] values



Nominal propeller revolutions of 800, 1470 and 2150rpm

] values of 0.94, 0.51 and 0.35 KT/J values of 0,05,0.88 and 2.30.



Phase I:experimental testing of the seven rudder models at various longitudinal, lateral and vertical separations from the propeller over a range of rudder incidence for various thrust loadingsrudder forces and moments and pressure distributions,which provided the distribution of loading over the rudder


Phase II:investigation of the rudder plus propeller combination in straight flow to include operation at low and zero ship speed and in four quadrants of operationnext phase of the work entailed experimental testing of the rudder plus combination alone in oblique flow.

Phase IIIThe final phase of the work investigated in a systematic manner the influence of an upstream body on flow straightening effects. This firstly entailed tests using two-dimensional centreboards of different lengths upstream of the rudder-propeller combination- hull upstream of the rudder-propeller combination


Thrust loading• It can be seen that, with decrease in J ,the lift curve

slope increases. • There is a significant delay in stall angle to over 40°,

compared with results obtained in a free stream, where stall for this aspect ratio occurs at about 20

• stall angle increases with increasing thrust loading• stall occurring later for positive incidence

* Wind tunnel rudder-propeller interaction dataAll movable rudder

LC

• The rudder drag coefficient, CD, for all three advance ratios is similar between -10° and + 10°. As rudder incidence is increased, the drag component due to lift increases rapidly for the lower advance ratios (higher thrust loadings).

• Centre of pressure chordwise, CPc, generally moves forward with increasing thrust loading.

•Centre of pressure spanwise, CPs, increases with increase in thrust loading at• positive rudder angles, whilst for negative rudder angles

CPs decreases with increase in thrust loading.


Propeller pitch settingFor a given advance ratio, changing the propeller pitch ratio (P/D) setting alters the thrust coefficient (KT) of the propeller.

• At the low thrust loading (KTI/ = 0.05), there was little difference in lift curve

slope for the three pitch settings. Drag coefficient, CPc and CPs were also notinfluenced to any great extent.• Mid thrust loading (Kt/J = 0.88),:The increase in pitch ratio slightlyincreases the lift curve slope and decreases drag at low rudder incidence. Changes in CPc are still small. For positive incidence, CPs moves outboard for increasing pitch ratio and inboard for negative incidence.3• high thrust loading(KT/J= 2.30):the trends are broadly similar

it is the propeller thrust loading which controls the performance of the

Rudder

It is interesting to note that, even for this significant difference in PI D, the spanwise load distributions remain very similar



Rudder aspect ratioKT/J of 0.05, 0.88 and 2.30 - aspect ratios of 2.5, 3.0 and 3.6• there was an increase in lift curve slope with increase in aspect

ratio• With increase in aspect ratio, there is little change in CPc but

there is an increase in CPs. • At low rudder incidence,drag decreases with increasing aspect

ratio



Rudder position relative to propeller

The propeller pitch was set at 0.95 and open-water thrust loadings (KT/f) of 0.05, 0.88 and 2.3 Uvalues of 0.94, 0.51 and 0.35) were used. Rudder angles were varied between -40° and +40°.

• geometrical parameters (X/ D, Y/ D and Z/ D) have significant influences on the:

• 1) rudder manoeuvring forces • 2)combined rudder-propeller propulsive forces



1)Rudder forces: Manoeuvring• Longitudinal separation (X/ D):

Changes in stall angle and maximum CL were relatively small X/D effects depend on thrust loadingIt is seen that as X/ D increases dCl/d firstly increases up to an X/ D of about 0.4 and then decreases

Lateral separation (Y/D) The main effect of lateral movement of the rudder relative to

the propeller is a shift in the lift curve so that zero lift no longer occurs at zero incidence

an increase in maximum lift in one direction and a decrease for the other

The lift curve slope decreases with increasing magnitude of Y/ DThe zero sideforce offset o increases with increase in propeller

thrust loading.

Vertical position (Z/D)The lift characteristic is shifted so that a positive rudder incidence

is required for zero liftThe positive stall angle is reduced, as is the negative stall angleThe lift curve slope decreases with a reduced amount of the

propeller race in way of the rudder.The angle for zero lift increases with propeller thrust loading




1)Rudder forces: Manoeuvring

• Longitudinal separation (X/ D): It is seen that high values of CD are obtained with small XI D whilst

increasing X/ D leads to significant reductions in CD.Rudder drag is seen to decrease with increase in propeller thrust loading

and at high thrust loadings a net rudder thrust is developed• Lateral separation (Y/D) there is a significant increase in rudder drag of the order of 100%. At higher thrust loading (lower j) this increase in rudder drag is much

greater• Vertical position (Z/D)At zero rudder incidence, changes in rudder drag with decrease in Z/ D

are small.

Coverage(x):Coverage defines the proportion of the area of the rudder that is within the propeller race.Two rudders with constant chord but spans of 1000 and 1300 mmcoverage for Rudder No. 2 was g = 0.800 and, for Rudder No. 4, g = 0.615As ] is decreased :the lift curve slope for the high aspect ratio rudder becomes less than

the other rudder.the stall angle is slightly reduced. Chordwise centre of pressure is similar for both rudders for all values.


* Wind tunnel rudder-propeller interaction dataSemi balanced skeg rudder

for the movable rudder plus skeg forces, which will be used for the prediction of total forces such as for structural design and manoeuvring purposes

for the movable rudder alone, which will be used for rudder torque prediction purposes

It is seen that the influences of thrust loading on the skeg rudder are similar to the all-movable rudder.

In a free stream the skeg rudder displays a characteristic discontinuity in the growth of lift with increasing rudder incidence, the discontinuity being caused by the early stall of the movable part of the rudder behind the skeg

There is an indication that a discontinuity also occurs when J= 0.51 but at higher rudder incidence.

for the higher thrust loading of J = 0.35,discontinuities do not occur


* Wind tunnel rudder-propeller interaction dataSurface pressure data on rudder

Free streamChordwise:The shape of the curves remains relatively constant as incidence is

increased up to 20°the rudder has stalled by about 25°Spanwise:peaks in the spanwise distribution develop near the rudder tip due

to thepresence of a tip vortex


Rudder with propeller upstreamThe shapes of the pressure distributions remain broadly similar as

KT/J is increasedthe asymmetric nature of the load distribution which results from

the angular flow change induced by the propeller



Figures (a-f) show further spanwise load distributions for Rudder No. 2. & X/D = 0.39and for three propeller thrust loadings(a)At low propeller thrust show only small

differences from those expected for the rudder in a free-stream

(b,c)the increased asymmetric nature of the load distribution as thrust loading is increased

(d) free-stream type of behaviour occurs over the outer 20% of the rudder span for the mid-separation position

(e) for positive incidence, the loading has migrated towards the tip with a highly asymmetric distribution, leading to a movement in the spanwise centre of pressure and increased bending moments about the root of the rudder.


experimental data for rudder behind propeller

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