AERODYNAMIC MODELLING OF A REFUELLING BOOM

AERODYNAMIC MODELLING OF A REFUELLING BOOM

Leonardo Manfriani∗ , Marcello Righi∗∗Zurich University of Applied Sciences, Winterthur, Switzerland

Keywords: air-to-air refuelling, control surfaces, aerodynamic modelling

Abstract

As part of the EU FP7 RECREATE researchproject, a prototype boom for civil air-to-airrefuelling was designed. One of the criticalaspects concerns boom control in the transonicflight regime, since the RECREATE concept isbased on automatic in-flight refuelling at highspeed.

A preliminary aerodynamic model ofthe boom was created using a panel codeand handbook methods. Interference andcompressibility effects were analysed withRANS and URANS simulations. Possibleimprovements to the boom aerodynamic designwere also evaluated.

1 Introduction

The RECREATE (REsearch on a CRuiserEnabled Air Transport Environment) project [1]is about improving the overall efficiency of longrange air transport by introducing a feeder-cruisersystem.

The idea is that large ’cruiser’ aircraft wouldkeep flying back and forth between far-awaydestinations for a long time without descendingfrom their optimal cruise altitude, while smaller’feeder’ aircraft would transport passengers, fuel,baggage, supplies and waste between the orbitingcruiser and the airports. The technical andoperational aspects implied by this concept areextremely challenging, but the potential fuelsavings shown by the study are impressive.

A less ambitious scenario, where the feederdelivers only fuel to the cruiser aircraft, wouldstill significantly improve long range air transportefficiency and could become reality in the near

future [2]. In fact, military operations rely on airto air refuelling (AAR) to increase the range ofcombat or transport aircraft since the 1940’s.

There are two military AAR systemscurrently in use, the ’probe-and-drogue’ andthe ’flying boom’. The latter is better suited tocivil AAR operations, because it allows highertransfer rates, is less sensitive to atmosphericturbulence and is much less demanding for thepilots of the receiving aircraft. An example of aflying boom is shown in figure 1.

Fig. 1 KC-135 tanker and refuelling boom [3]

The rigid boom is attached below the tailof the tanker with a gimbal joint. The boomoperator, who sits in the tanker’s tail facing thereceiving aircraft, can control the boom in pitchand azimuth by commanding the deflection oftwo control surfaces, called ruddervators. Healso commands the extension of the telescopicpart of the boom to connect it with a refuellingreceptacle on the receiving aircraft. To startan AAR operation, the receiving aircraft mustfly in the correct position behind the tanker,

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L. MANFRIANI, M. RIGHI

aided by optical references and by the indicationsreceived by radio from the boom operator. Theformation must be accurately kept, so that theoperator can safely connect the boom head tothe refuelling receptacle. The area of safecontact with the boom head is known as the ’airrefuelling envelope’. Pilots qualified for militaryair refuelling operations need to be speciallytrained.

In the civil AAR concept developed withinthe RECREATE project, the whole refuellingoperation would be completely automatic, withan extremely high level of safety. There wouldbe no need of a boom operator or of specifictraining for the pilots of the receiving aircraft.Moreover, it is assumed that the receiving aircraft(the ’cruiser’) would not need to reduce speedbelow Mach 0.75 or descend from cruise altitude,as it is the case in military AAR operations. Thisposes some additional requirements to the boomaerodynamic design.

2 Boom design

As stated before, refuelling booms for militaryapplications have only two control surfaces.Such a configuration has no control redundancyand is not adequate to the high system safetyrequirements of a civil application.

A configuration with three control surfaceshas a serious drawback. If the third verticalsurface is placed above the boom, it suffers fromaerodynamic shading in operation and posesstowage problems. If placed below, it may limitthe rotation angle of the tanker aircraft at takeoffand landing.

Fig. 2 Front view of the ruddervator configuration

For these reasons, a prototype configurationwith four ruddervators has been selected. Thetwo lower control surfaces have a relatively smallanhedral angle of 15◦, while the two uppersurfaces have a dihedral angle of 30◦.

Initial simulations [4] have shown that theproposed angles are adequate to control the boomwithin a wide refuelling envelope. The boompitch angle Θ can be controlled between 15◦ to45◦, the azimuth angle from −30◦ to 30◦ and therefuelling head can be extended by 6 m.

With respect to to the KC-135 boom shown infigure 1, the proposed design is about two meterlonger and can extend to a maximum length of18 m. The refuelling envelope of the boom isshown in figure 3.

Fig. 3 Air refuelling envelope

The ruddervators of the prototype boom havea rectangular planform. They have a spanof 1.6 m. a chord length of 0.5 m and 28◦

sweepback. The profile is a NACA 64-009.

3 Preliminary aerodynamic model

A reliable aerodynamic model is needed toperform simulations of the refuelling process andto develop the boom control laws. Obviously,it is impossible to cover all combinationsof ruddervator deflections, boom position andflight Mach number, and some simplifyingassumptions must be made in defining the boom

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Aerodynamic Modelling of a Refuelling Boom

aerodynamic model. As a first approximation,the force and moment coefficients due tothe ruddervators (indicated by the subscriptsrv1, rv2, rv3 and rv4) have been calculatedwith a panel method, while those due tothe boom shaft (subscript b) and to the boomextension (subscript be) have been estimatedwith handbook methods [5]. Within thisapproximation, the aerodynamic interferenceeffects between the four ruddervators may beneglected, and the total force and momentcoefficients can be expressed as follows:

cx = cxb + cxbe + cxrv1 + cxrv2 + cxrv3 + cxrv4

cy = cyb + cybe + cyrv1 + cyrv2 + cyrv3 + cyrv4

cz = czb + czbe + czrv1 + czrv2 + czrv3 + czrv4

cmx = cmxb+ cmxbe

+ cmxrv1+ cmxrv3

+ cmxrv3+ cmxrv4

cmy = cmyb+ cmybe

+ cmyrv1+ cmyrv3

+ cmyrv3+ cmyrv4

cmz = cmzb+ cmzbe

+ cmzrv1+ cmzrv3

+ cmzrv3+ cmzrv4

All aerodynamic coefficients are referred tothe ruddervator planform area. The referencepoint for moment coefficients is the center of theboom gimbal joint. No compressibility effectswere considered in the initial phase.

The aerodynamic coefficients representingthe ruddervators’ contribution are presented inthree-dimensional lookup tables as a function ofboom pitch angle Θ, azimuth Ψ and ruddervatordeflection δrv, while those representing thecontribution of the boom and of its extension arepresented in two-dimensional lookup tables as afunction of Θ and Ψ. For example, figure 4 showsthe contribution of the ruddervator to the threeaerodynamic moment coefficients as a functionof deflection δrv and boom azimuth Ψ.

To evaluate transonic and interference effects,steady RANS and time accurate URANScalculations were performed with EDGE [6].EDGE is a well known and widely validatedCFD tool for compressible flows developed atthe swedish national research institute FOI forthe aerospace industry. The aircraft manufacturerSAAB uses it as an aerodynamic design tool forhigh performance aircraft.

The complete boom geometry was modelledwith a high fidelity hybrid mesh with more

Fig. 4 Ruddervator moment coefficients versusdeflection and boom azimuth

than 20 million grid points. The boundarylayers were solved directly, no wall functionswere implemented. Grid convergence exerciseswere performed; the sensitivity to turbulencemodels (K-w EARSM and Spalart-Allmaras) andto the numerical scheme used was assessed inall simulations. All calculations were made withthe steady state approach; some cases showinglarge flow separations were checked with thetime-accurate approach.

For small deflections, the ruddervator liftand pitching moment coefficient calculated withEDGE are in good agreement with the resultsobtained with the panel method. This confirmedthat the simple linear model is adequate forpreliminary simulation exercises. However, theCFD analysis indicated the presence of large flowseparation areas on the boom bulb and of shockwaves on the ruddervators at higher deflections.

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Fig. 5 Pitching moment due to ruddervatorangle of attack at Mach 0.75: comparisonbetween panel method (VsAero) and RANSresults (EDGE)

4 Flowfield analysis

Figure 6 shows the result of a RANS calculationat Mach 0.75. The flow separates from the aftportion of the bulb, and a large area of unsteadyvortical flow is present.

Fig. 6 Vortical flow region aft of the boom bulb atMach 0.75. Boom pitch angle 35◦, ruddervatorsneutral.

This appears to be the dominant flow pattern.Its unsteady character is probably the mostserious issue, as it might introduce vibrationsin the frequency range of the boom controls.A reduction of the Mach number to 0.65 doesnot change the flow characteristics significantly(figure 7).

Fig. 7 Vortical flow region aft of the boom bulb atMach 0.65. Boom pitch angle 35◦, ruddervatorsneutral.

Figures 8 and 9 present the pressure contourlines on the boom ruddervators at Mach 0.65 and0.75, respectively. The supersonic flow regionsare shown in red.

Fig. 8 Pressure contour lines at Mach 0.65.Boom pitch angle 35◦, ruddervators neutral.

The flow on the ruddervators is dominated bythree-dimensional effects. Shock waves do notappear on the boom mast.

Figure 10 shows what happens when oneof the ruddervators is deflected by 2◦, 4◦ and6◦ respectively: the supersonic region extendsoutboards and the flow on the surface becomesmore ’two-dimensional’.

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Fig. 9 Pressure contour lines at Mach 0.75.Boom pitch angle 35◦, ruddervators neutral.

Fig. 10 Pressure contour lines at Mach 0.75.Boom pitch angle 35◦, one ruddervator deflectedby 2◦ (top), 4◦ (middle) and 6◦ (bottom).

5 Possible design improvements

Current AAR booms are designed for operationin the subsonic regime. A ’transonic’ boomdesign is indeed much more challenging, as theCFD flow analysis of our rather conventionalprototype configuration shows. The following

areas for improvement can be identified:

- contain the separated flow area behind thebulb;

- achieve a wide linear range of elevatoreffectiveness at high Mach;

- avoid shock induced buffet phenomena onthe ruddervators.

These requirements have been addressed witha simple ’sensitivity analysis’ of the boom andruddervator shape. For example, by reducing thebulb width by 10% and increasing its height by60%, the size of the separated flow region and theboom drag are significantly reduced (figure 11).

Fig. 11 Baseline (top) and improved (bottom)bulb shapes at Mach 0.75.

The ruddervator planform can be shapedto mitigate the three-dimensional effects andimprove the control effectiveness at high Mach(figure 12).

Boom control by conventional ruddervatorsat Mach 0.75 can be problematic. The criticalMach number is reached at very small deflections

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Fig. 12 Pressure contour lines at Mach 0.75,modified planform. Ruddervators neutral.

(between 1◦ and 2◦). At larger deflections, thecontrol effectiveness and the aerodynamic hingemoments are non linear. Shock induced buffetphenomena may occur, causing severe controlinterference problems.

Mini trailing edge devices (mini-TEDs) couldbe an interesting option. Figure 13 shows acomparison of the same ruddervator airfoil atapproximately the same lift coefficient and Mach0.75 in two dimensional flow. With a mini-TED,the ruddervator deflection can be reduced from−4◦ to −1◦. The maximum local Mach dropsfrom 1.5 to 1.26, and the intensity of the shockwaves is greatly reduced. The drag coefficientof the airfoil with the mini-TED is more than40% lower. The aerodynamic efficiency is muchimproved, and the risk of shock induced buffetis greatly reduced. There is also the possibilityof combining ruddervator deflection at lowercontrol frequency and mini-TED deflection athigher frequency.

There are many more geometric parametersthat can be changed. We have just analysed a fewvariants, but the results already show that there isa big potential for improvement to be exploitedby an aerodynamic optimisation process.

Fig. 13 Two-dimensional flow analysis on theruddervator airfoil at Mach 0.75. Top: δrv =−4◦, cl = −0.65, cd = 0.0248. Bottom: withmini-TED, δrv =−1◦, cl =−0.70, cd = 0.0136.

Acknowledgment

The results presented in this paper are part ofthe RECREATE research project, funded fromthe European Union 7th Framework Programme,grant agreement no. 284741. This publicationreflects only the authors’ views. The EuropeanUnion is not liable for any use that may be madeof the information contained therein.

Contact Email Addresses

For further information, the authors can becontacted at the following email addresses:

[email protected]@zhaw.ch

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References

[1] De Cock L. and Nangia, R.Research on a Cruiser Enabled Air TransportEnvironment (RECREATE). RTO Workshop onEnergy Efficient Air Vehicle Configurationsand Concepts of Operation (Applied VehicleTechnology Panel AVT-209)Lisboa, Portugal, September 2012.

[2] Nangia, R.Operations and Aircraft Design TowardsGreener Civil Aviation Using Air-to-AirRefuelling. The Aeronautical Journal, Vol. 110,No. 1113, November 2006.

[3] Smith, Austin L. and Kunz, Donald L.Dynamic Coupling of the KC-135 Tanker andBoom for Modeling and Simulation. Proc. 2006AIAA Modeling and Simulation TechnologiesConference and Exhibit. Keystone, CO (USA),postprint under AFRL-VA-WP-TP-2006-342.

[4] Löbl, D. and Holzapfel, F.High Fidelity Simulation Model of an AerialRefuelling Boom and Receptacle. Proc.Deutscher Luft- und Raumfahrtkongress 2013.Stuttgart, Germany.

[5] Mean fluid forces and moments on cylindricalstructures. ESDU Data Item 79026, 1980.

[6] EDGE - Theoretical Formulation. Defence andSecurity, Systems and Technology, FOI, 2010.

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AERODYNAMIC MODELLING OF A REFUELLING BOOM