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CFD PREDICTIONS OF THE INFLUENCE OF EXTERNAL AIRFLOW ON HELICOPTEROPERATIONS WHEN OPERATING FROM SHIP FLIGHT DECKS.

N.H. Wakefield; Department of Ship Science,S.J. Newman, Department of Aeronautics and Astronautics,P.A. Wilson, Department of Ship Science.

University of Southampton, Southampton, SO 17 1BJ, UK.

1. Abstract.A CFD model of a hovering helicopter main rotor isdeveloped to examine airflow in the presence of shipstructures and side winds. The rotor is modelled bymodifying the governing Navier-Stokes equations in theregion of the disc. The extra terms added to thegoverning equations apply a downforce to the flu id;these forces are independent of the flow around the rotorand equal to the helicopter weight. The boundaries ofthe computational domain are also modified in order togenerate a physically correct solution. Flow solutions inboth two and three dimensions are achieved using thecommercial flow solver CFX 4.1. The flow solutionsexhibit very good correlation with establishedmomentum and power princip les.In order to mode l helicopter operations from a shipsflight deck, typically a frigate, the rotor is modelled atseveral positions above a ship profile. Cross winds areapplied to the computational domain. The thrust of therotor is held constant and the resulting flow solutions arecalcu lated. The power exerted at the rotor is obtainedand compared to the ideal hover condition andcomputational flow solution.The flow solutions show that the airflow accelerates overthe flight deck and a helicopter operating in this regionencounters large cross winds and veloci ty gradients. Theresults also show that the helicopter control margins aremore likely to lim it the safe operating lim it than thepower margin.Using the modified boundary conditions, this methoddemonstrates the viabili ty of CFD for predict ing the shipairwake and the reduced power margins a helicopterexperiences whilst operating in the vicin ity of the ship.This study has been exploratory and lim ited bycomputing resources, but future models will inc ludehelicopter fuselage, tail rotor, time dependant boundaryconditions and dynamic f light.

2. IntroductionThe demand for use of helicopters in marineenvironments such as ships and oil rigs has risen steadilyin recent years. The helicopter provides a fast method oftransport whilst requiring less storage space and take-off

facili ties than fixed wing aircraft. The helicopter hasalso developed specialised military uses such assubmarine detection and air to surface targeting.The flexibility of helicopters is reflected in the increaseddemand to operate in ever worsening environmentalconditions. At present. the established method fordetermining safe helicopter operating limits is a verycostly series of ful l scale trials. This usually involveswaiting at sea for appropriate environmental conditionsto occur when a test pilot performs the takeoff andlanding manoeuvres. The landing is rated according tothe work load the pilot experiences and varies from lowto dangerous. The whole procedure has to be repeatedfor every ship/helicopter combination.An experimental SHOL (Safe Helicopter OperatingLimit) such as this has the advantage of being realist icbut it also requires the provision of vast resources bothin finance and time. These studies also provide nosystematic information about the areas in which the pilotexperienced difficulties. Obtaining such a SHOL, bydefinition, contains a degree of danger.A CFD analysis has many advantages. The most evidentis the cost and speed at which one can be obtained.Results from such an analysis are repeatable and notsubject to either measurement error or personalsubjection. The solution is complete and. therefore.once obtained, pressure and velocity components areknown at every locat ion throughout the model. The flowdata obtained can be used in simulators so that pilots canpractice land ing on ships safely. A CFD study can alsobe performed on ship designs whilst in the conceptualstage. Thus the effectiveness of helideck and hangardesigns can be tested to reduce turbulence over the fligh tdeck. Naturally the fidelity of the predictions must be ofthe highest quality.

3. Modell ing the Rotor.A simple theoretical model of the helicopter main rotorwas constructed within the computational domain. Themethod solves the governing Navier-Stokes equations asshown in Equat ion 3.1. The main rotor was designed toreflect the loading and geometry of a Westland Lynx,that is the rotor radius is 6Sm and the mass is 5.2tonnes. The thrust exerted by the rotor was evenlydistributed across the disc. The force was exerted within

Paper presented at the RTO AVT Symposium on Fluid Dynamics Problems of Vehicles Operating near or in theAir-Sea Interface, held in Amsterdam, The Netherlands, 5-8 October 1998, and published in RTO MP-15.

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2-2the computational domain of Figure 3.1. The figureindicates that the rotor thrust is exerted within thehatched areas. In these regions the Navier-Stokesequationsare modified by the addition of the vector g asshown in Equation 3.1. The force applied to the fluidacts vertically downwards.

Figure 3.1 : Rotor Model. IdiIp.-+pV.(UOU)=B+V.(o-pIlOu) - Equation 3.1dt

The vector term 6 has units of force per unit volume,therefore to exert the required thrust to the fluid, B wasdefined as in Equation 3.2. The thrust is distributedevenly across he spanof the rotor model.

0B= I IThrust I Area / H Equation 3.2.Only thrust is applied to the fluid to evince the rotor,which has two implications. Firstly, the flow is in noway predetermined at the location of the rotor; thevelocity and pressureare solved in exactly the samewayas they are throughout the rest of the domain. Secondly,the resultant vertical velocity component determines heinduced power exerted across he rotor whilst the thrustalways remainsconstant and acting through the centre ofthe rotor.In all the flow solutions described within this paper thegrid cell dimensions are 0.50m 0.50m. The rotor istherefore 26 cells wide. Ideally the rotor would be onecell deep, but the flow solver can not resolve the sharppressuregradients, this problem is alleviated by makingthe rotor two cells deep.4. Modified Boundarv ConditionsThe default boundary conditions available within CFX4.1 and other commercial CFD software packages areapplicable to external flows which comprise a freestream velocity and some body causing a perturbationsuchas a wing or a building.The air flow around a hovering helicopter isfundamentally different because here is no free streamvelocity and all fluid flow is induced by the helicopterrotor itself. For this reason, applying any of theavailable boundary conditions such as an imposedvelocity or pressure s unjustifiable and gives physicallyincorrect flow solutions. An example is the flowsolution in Figure 4.1, generatedby allowing the velocityto vary around the boundary whilst the pressure emains

constant. There are many features of this flow thatconflict with establishedunderstanding of both rotorsand actuator discs. Figure 4.1 shows hat the fluid flowsupwards throughout most of the domain, including theouter region of the rotor. Quantitative study yields thatmomentum in the downwash does not equate to thethrust applied at the rotor.

Figure 4.1: Hover Flow Solution, Existing BoundaryConditionsTo restore a realistic flow pattern, the boundaryconditions have been modified. At the modifiedboundaries neither a pressurenor a velocity is appliedbut insteada relationship between he two is used basedupon Bernoullis equation. It is assumed hat at aninfinite distance rom the rotor the velocity is zero. Thepressure s atmospheric pressure,and is defined as thedatum or zero. Therefore any fluid entering the domainmust have the pressure/velocity relationship defined inEquation 4.1.For this assumption o be strictly accurate, the viscouseffects that act on the fluid between infinity and thedomain boundary must be neglected. In reality thedomain size makes negligible difference to the flowsolution, thus validating this assumption.p++ =o Equation 4. IIn order to instigate the given relationship thecomputational domain must be constructed with twoboundaries, an inner and an outer. At the outerboundary, the extremity of the domain, the pressure sdefined as zero. This is achieved using a pressureboundary, a boundary condition readily accessiblewithin most CFD solvers.

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The inner boundary was placed a small distance from theouter boundary within the computational domain, asshown in Figure 4.2. In the region between the outer andinner boundaries addi tional terms were added to theNavier-Stokes equations enabl ing forces to be appl ied tothe fluid to alter the fluid pressure.

OuterBoundary

vP=O

Forces Applied toFluid Here.

Computational3

Domain

Figure 4.2 : Modi fied Boundary Conditions.Other than at the walls, the entire flu id domain issurrounded by the boundary outl ined above.

5. Rotor In HoverTo verify that both the boundaries employed and therotor model work as intended, a test case was run tomodel the rotor in hover with no cross wind. Both twoand three dimensional cases were obtained. In each casethe fluid used was air with a density 1 2kg/m3 andviscosity 1.8 * 1Om5Ns/m2. The fluid was consideredisothermal. incompressible and turbulent.The two dimensional case modelled a disc of diameter13m and unit thickness. The thrust per unit area used isdescribed in Equation 5.1, thesevalues were chosen toreflect the Westland Lynx. The total thrust exerted was4996.2N. Half of the flow solution is shown in Figure5.1. The contours represent pressure variations of 25Pa.Thmst/Area=5200*9.81/(lc6.5*)=384.3Pa Equation5.1In order to gauge the validity of the flow solution,momentum and energy principles from existing actuatordisc theory were compared to the computational disc.The momentum in the downwash was obtained, asdescribed in Equat ion 5.2. This integral was evaluated20m below the plane of the disc, across the downwash.

Figure 5.1 : 2D Hover Flow Solution.

T=pJ2 dA = 4980N Equation 5.2The discrepancy between the momentum in thedownwash and the thrust exerted at the rotor is 0.3 .This indicates that momentum has been effectivelyconserved within the system and Newtons equation issatisfied.The power exerted by the rotor was evaluated asdescribed in Equat ion 5.3. This integral is performedacross the plane of the rotor itself. The power derived isper unit depth, and thus is not comparable to the powerexerted over the whole rotor plane.PH = Jw. Ap. dA = 67.4kW Equation 5.3The idea l power as found from standard actuator disctheory is shown in Equat ion 5.4. The difference betweenthe predicted idea l power and the measured power is 7 .P,,, = T. ,/m = 63.2kW Equation 5.4The 7 discrepancy between the two values can beattributed to variations in the induced velocity across therotor that are not predicted within actuator disc theory.Such a difference is realistic since lo-15 is a typicalrange used in the helicopter industry. The calculatedpower 67.4kW is carried forward to studies of the flowaround the ship helideck.A further study of a three dimensional rotor wasperformed, with the thrust per unit area kept constant.The flow solutions is shown in Figures 5.2. The solutionis axisymmetric, Figure 5.2 corresponds to an azimuth

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angle of 0. The vectors represent speed and thecontours pressure variations of 25Pa.

Figure 5.2 : 3D Hover Flow Solution.Comparison of momentum in the downwash and thrustapplied yielded a 0.2 discrepancy. The power exertedtt the plane of the disc was 3 greater than the idealvalue predicted by actuator disc.6. Shin AirwakeApart from operational and performance calculations.the airflow around the ship in the absence of thehelicopter thrust is of interest for other reasons, such ashelicopter blade strike predictions during rotorengagement and disengagement.A flow solution was obtained for a 2D model of ahelideck. The geometry of the ship is shown in Figure6.1 The domain extended 75m upwind, downwind andabove the centre of the ship. A horizontal free streamvelocity of 30 knots was imposed. The dimensions ofthe helideck are those used by the TTCP Nations fortheir research into the Helicopter Ship DynamicInterface.Both the sea and ship were model led as walls; zero flowimposed at these surfaces. The k-c turbulence modelwas used and the flow was assumed to be turbulent,

incompressible and isothermal. Reference [l] was usedto determine the turbulent kinetic energy imposed at thewindward edge of the domain. Reference [2] was usedto determine a realistic mixing length for the init ialturbulence dissipation constant, epsilon.

Figure 6.1 : Ship GeometryThe flow solution is shown in Figure 6.2. The arrowsrepresent the velocity vectors, the contours are pressurecontours at 25Pa intervals. The figure shows the flowseparating at the windward edge of the helideck and alarge region of recirculation both above and downwindof the ship. This flow pattern agrees with exper imental lymeasured data such as reference [3].

I Fiaure 6.2 : Shin Airwake7. ShidHeliconter InteractionThe geometry of the ship used is the same as thatdescribed in Section 6. Three positions for the mainrotor were chosen. The first position is shown in Figure7.1, the rotor is at a height of 10Sm above sea leve l and14m to port of the centreline of the helideck. Thesecond position is 7m to port of the centreline as shownin Figure 7.2, and the rotor is placed over the centrelineof the ship in Position 4 shown in Figure 7.3.

Tripartite Technical Co-operation Program, nationaldefence research organisations from, UK, USA, Canadaand Australia.

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14.00m4 -

Figure 7.1 : Rotor, Position 1.Four wind velocities were chosen relative to the ship,zero, 30 knots port to starboard, 30 knots starboard toport and 60 knots port to starboard. Flow solutions wereobtained for all of these wind velocities, with the rotor ateach of these positions.

7.00m- 4

Figure 7.2 : Rotor, Position 2.

The flow solution was solved as incompressible,isothermal and turbulent. The k-s turbulence model wasused, the turbulent kinetic energy and energy dissipationparameters were determined as described in Section 6.

The solver could not achieve a steady state solution forany of the problems due to the turbulent unsteady natureof the flows, the solutions were obtained using a time-stepping approach. The results presented in thefollowing sections are therefore instantaneous andrepresent a snapshot of the flow solution, after the rotorhas been stationary in the given locat ion for at least 30seconds.There are two factors that lim it helicopter operations inadverse weather conditions, namely power and control.The pilo t must have adequate quantities of both in orderto perform manoeuvres. As outlined i n Section 5, thepower required to hover out of ground effect in still air is

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67.4kW. The power to hover in a stationary positionwith the given cross winds was calcula ted usingEquation 7.1. For each of the flow solutions generatedthe corresponding power was calculated accordingly.

P = Jw.Ap.dA Equation 7.1

The lateral wind that the rotor experiences was alsoquantified . VAv is the average latera l velocitycomponent across the entire span of the rotor.Whilst the power required to main tain a certain hoverposition is important, control requirements are assignificant. For this reason the vertical flow velocitieshave been recorded near the extremities of the rotor. Wpand Ws correspond to the vertical flow 5m to port and5m to starboard of the rotor centre respectively. (5m isapproximately 75 of the rotor radius.) These valuesnot only indicate the velocity gradients across the rotorbut provide a measure of control demanded. AW isdifference between the vertical components Wp and Ws.For mture development, a three dimensional analysiscould be extended to include blade element theory andinverse simulation. Vertical velocity components wouldprovide an estimate of cyclic and collective pitchvariations that achieve the required thrust magnitude anddistribution. This has not been attempted to datebecause these flow solutions are two dimensional andprovide a qual itative understanding of the flow regimes.The techniques are under development and 2D cases aremuch less demanding of time during the validationphases of the model.hases of the model.

7.1..1. Zero Windero WindThe flow solutions for hover in each of the threehe flow solutions for hover in each of the threepositions are shown in Figures 7.1.1-3. The arrowsositions are shown in Figures 7.1.1-3. The arrowsrepresent velocity vectors and the contours are spaced atepresent velocity vectors and the contours are spaced atlOOPa in the first two flow solutions and 40Pa in theOOPa in the first two flow solutions and 40Pa in thefinal flow figure. The computed power requirements areinal flow figure. The computed power requirements areshown in Tab le 7.1.1.hown in Tab le 7.1.1.

,,l,,rl/l,,

Figure 7.1.1 : Rotor in Position 1.

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In Position 1, shown in Figure 7.1.1, there are vorticespresent on both sides of the rotor. These causesignificant recirculation. which in turn increases thepower requirements to a value greater than the simplehover case. since the rotor is effectively in climb. Theflow through the rotor itself is approximately vertical.

Figure 7.1.2 : Rotor in Position 2

, ,

, I , ,

I , I /, , ,

,,, , ,

, I ,I I I

, , ,

,

T,,

The rotor in Position 2 has a vortex present at theoutboard edge of the rotor but there is no vortex over theship. The combination of the vortex and the beneficialground effect give a power requirement comparable tothe hover case.Figure 7.1.3 displays the flow solution for the rotor overthe ships centreline. The rotor has no recirculation atthe tips. There is a high pressure region directly beneaththe rotor and on the deck. The favourable ground effectthe ship generates, gives a power requirementsignificantly less than the hover.

Figure 7.1.3: Rotor in Position 3.

The only areas of recirculation are present at theintersection of the ships sides and the sea surface.These are small in comparison to the vortices generatedwith the rotor in the other positions.

Position 1 2 3P WV 85.1 66.5 49.9

Pi+ 126 99 74VAv (ms-) I 1.2 I -1.3 I 0.0

Table 7.1.1 : Power and Control Requirements. ZeroWind.

The values VAv are the average lateral velocities acrossthe rotor span. The rotor in position 3 has VAv of zeroconsistent with symmetrical flow. The other twopositions have small latera l flows across the rotor. Thedifference in vertical flow across the span is also small inall three positions.

7.2. Wind 30 Knots, Port to Starboard.These three flow solutions were determined with a 30knot free steam velocity acting from port to starboard,which appears left to right in the figures. The pressurecontours are spaced at 40Pa.

The flow solution for the outboard position is displayedin Figure 7.2.1. This figure shows a large low pressureregion above the rotor and a weaker high pressure regionbelow the rotor. However there is li tt le vert ical flowthrough the rotor itself. The model shows the balancingof the rotor downwash with the upflow generated by the

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ship. This is reflected in the power exerted, as shown inTable 7.2.1. There is however a large vertical velocitygradient across the rotor. The flu id is passing upwardsthrough the rotor on the port side and downwards at thestarboard side, This represents increased controlrequirements.

Figure 7.2.2 : Rotor in Position 2.Figure 7.2.2 shows the flow solution with the rotor overthe side of the ship. At this position the helicopter isresting in the upflow generated by the blockage of theship. The power requirements are negative as shown inTable 7.2.1. This indicates an autorotative state. Thelateral velocity the rotor experiences is 4.6m/s or 9knots. This is in fact less than the undisturbed freestream velocity of 15 knots. This flow also exhibits alarge vertical velocity variation across the span, 10.9mswith correspondingly high resulting control demands.

-

/ l 1 ..-.\,.,,, .._.,,, , , . . ._,, ,,,,,I .._,_. II .Figure 7.2.3 : Rotor in Position 3. Figure 7.3.1 : Rotor in Position 1

The rotor over the ships centrel ine is shown Figure7.2.3. Simi lar to the other positions the induced poweris small due to the upflow across the rotor. The most

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noticeable feature of the flow solution is the lateralvelocity the rotor experiences. 10.9ms or 2 lknots. Thisis more than double the other two positions caused byonly a small change in position of the rotor. As with theother two positions the flu id is flowing upwards throughthe rotor on the port side and downwards on thestarboard side.

Position 1 2 3P W) -0.2 -1.0 0.0

Pn 0 -1 0V.dms? 3.5 4.6 10.9

WP 3.9 4.2 6.1

ws -7.0 -6.6 -7.6AW 10.9 10.8 13.7

Table 7.2.1 : Power and Control Requirements, 30 KnotsPort to Starboard.Table 7.2.1 shows that in these cases there is lit tle powerexerted at the rotor and the rotor is operating insignificantly less latera l wind than the free stream of15.4ms. However the rotor is experiencing widelyvarying flow vert ical flow across its span. In reality thiswould necessitate large cyclic pitch variations to trim thehelicopter.

7.3. Wind 60 Knots, Port to StarboardThese three flow solutions were generated using a freestream velocity of 60 knots acting from port to starboard.The pressure contours are spaced at 75Pa.

Figure 7.3.1 shows that the flow around the rotor inPosition 1 is largely influenced by the free stream andthe presence of the ships helideck. The greatest

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pressure gradients occur at the windward edge of thedeck, not at the rotor. The rotor is operating in a strongupflow resulting in a negative power requirement, asshown in Tab le 7.3.1. The lateral wind speed across therotor is 23.&n/s which is less than the undisturbed free;tream velocity.

Figure 7.3.2 : Rotor in Position 2The flow solution for the rotor above the side of the shipis shown in Figure 7.3.2. In many respects this is similarto the rotor in the outboard position. The rotor isoperating within an upflow greater than the outboardposition, shown in Figure 7.3.1, resulting in a largernegative power at the rotor. The rotor is operating in28.2ms cross wind flow, this is similar to the free streamvelocity of 60 knots.

-5z- T I:::: :,,I> ,.,I/ ,..-

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7.4.1. The power exerted at the rotor is only a fractionof the power exerted in the simple hover case. The flowacross the rotor is a &n/s. this is less than the free streamvelocity. The variation between the vertical velocitycomponent at the port and starboard end of the rotors islarge, 16.8mis.

Figure 7.4.2 : Rotor Position 2.The flow pattern corresponding to the rotor in Position 2is shown in Figure 7.4.2 above. The rotor has a largeregion of low pressure above the rotor whereas there islit tle pressure increase below the rotor. The angle of theseparation at the windward edge of the deck is greaterthan that found in the previous flow solution, indicatingthe rotor downwash being fed into the separationregion above the deck. The power exerted by the rotorwithin the solution is only 8.8kW, which is only 14 ofthe power required in hover.

Figure 7.4.3 : Rotor Position 3.Figure 7.4.3 shows the flow solution for the rotor overthe ship centreline, which is an exact reflection of Figure7.2.3. The upflow of the air over the ship causes the air

to flow upwards through the rotor at the windward end.At the leeward end of the rotor the air is flowingdownwards. The net effect of the up and down flow is azero power requirement. The lateral wind speed acrossthe rotor, shown in Table 7.4.1, is 10.9ms, this is lessthan the free stream velocity.

Position 1 2 3P (kw) 8.7 6.8 0.0I PH I 13 I 10 I 0 I

VAV md -8.0 -6.0 -10.9WP -11.5 -9.0 -7.6

I WS 5.4 I 3.3 I 6.1 II AW I -16.8 I -12.3 I -13.7 ITab le 7.4.1: Power and Control Requirements, Wind 30

Knots, Starboard to Port.These three flow solutions exhibit lim ited powerrequirements and lateral flow speeds but large velocitygradients across the rotor span.

8. ConclusionsThese results combine the ship airwake and thehelicopter induced flow and from the results clearindications of power and control requirements areevident. With no side winds the helicopter rotorexperiences slight power increases, but there areneg lig ible lateral flows to contend with. The verticalflow is approximately constant across the span of thedisc so the control requirements are limited.The three cases that considered the 30 knot wind fromthe port side al l exhib ited power requirements of aroundzero. However the large velocity gradients across therotor indicate high demands on pilot and rotor control.The 60 knot wind cases indicated that the ship airwakewas predominant and the helicopter thrust caused smalldisturbances by comparison. In these cases the powerrequirements were negative. The helicopter over theships centreline experienced loca l wind speeds greaterthan the free stream velocity. In these cases the variationof vert ical velocity flow across the span was actually lessthan those in the 30 knot cases.Regarding wind from the starboard side of the shipshown in Section 7.4, once again the power requirementswere minimal compared to that of the hover. Howeverthe rotor experiences the most dramatic vertical velocitygradients across the rotor in these cases.

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This study indicates that the control requirements of thehelicopter are more likely to lim it safe operations thanthe power limitations. The vertical velocity variationsevident in the flow solutions are not found in any othernormal operations, such as hover or forward flight .The geometries considered have only deal t with a latera lwind relative to the ship and therefore do not include anydownflows due to the influence of the shipssuperstructure.These results provide a quali tative measure of the flowvariations around the ship and helicopter because of thetwo dimensional nature. In reality the flow is far fromtwo dimensional and some of the features exhibited inthese solutions will be less prominent. For example, theupflow through the rotor when the rotor is upwind of theship. In a three dimensional solution, the air can travellongitudinally around the rotor, rather than remaining inthe same plane and being forced through the rotor. Thiswill. in turn, affect the power predictions which arepredicted from induced velocities at the rotor plane.The boundary condit ions and rotor model employed fora three dimensional case are identica l to those used forthe two dimensional study. The computational resourcesfor a three dimensional study far exceed those of the twodimensional study undertaken presently. However thisstudy demonstrates the viab ility of CFD in order topredict accurate flow solutions and resultant power andcontrol requirements.

9. NomenclatureAl l Units S.I. unless otherwise stated.A = area of rotor.Ap = thrust/area at rotor.P = pressure.P = power exerted at rotor.

PH =power required to hover in still air.Pu= power exerted at rotor as percent of hover, Pu

Pin = ideal power of actuator disc in hover.T = total thrust of rotor.u,v,w = longitudina l, lateral, and vertical velocity

components.

Wp = vertical velocity component at 5m port of rotorcentreline.

Ws = vertical velocity component at 5m starboard ofrotor centreline.

AW = Wp-Ws.

10. References[ 1] Plate E.J.(ed.), Engineering Meteorology, Elsevier,

Amsterdam, The Netherlands, 1982, Ch. 13.[2] Versteeg H.K., Malalasekera W., An Introduction to

Computational Fluid Dynamics, The Finite VolumeMethod, Longman, 1995.

[3] Newman S.J., A Theoretical and ExperimentalInvestigation Into The Rotor Blade AeroelasticBehaviour of a Shipborne Helicopter During RotorEngagement and Braking, AGARD ConferenceProceedings No. 552., 75th Fluid DynamicsSymposium, Berlin, Oct. 1994.

V = velocity scalar.VAv= average lateral velocity component at rotor.