The S-61/Sea King with Carson Composite Blades Part 1

H.C.Curtiss Frank Carson Adrian Neve Princeton University Carson , Inc QinetiQ Princeton, NJ, USA Perkasie, PA Farnborough, UK

Discussion Abstract Carson Helicopters new composite main This paper is in two parts. The first part rotor blades for the S61 received an STC of this paper describes various from the FAA in 2003. The blades use improvements that Carson Helicopters is advanced airfoil sections, have swept developing in their program to tips and are constructed of composite modernize the S61. The new composite materials. The rotor blades are in main rotor blades, designed and commercial use in the USA, Canada, developed by Carson Helicopters, have South America, and Africa. The blade produced significant gains the aircraft development is described in Reference 1. payload and range. The new rotor blades These new blades have yielded were approved by the FAA in 2003, and significant performance gains for the more than 60 sets are currently in use by S61 that are reflected in FAA approved Carson and other commercial operators. supplements to the S61 flight manuals Also, a version of these blades modified (References 2 and 3 for example). for a folding hub is currently installed on Reference 2 deals with increased hover Sea King helicopters. Six performance in and out of ground effect sets are in operational use on Sea Kings and Reference 3 with increased Category in Afghanistan. As a result of the A performance. The approval process is success of these blades and the in progress with EASA. significant gains in performance provided, Carson Helicopters has been Hover Figure of Merit developing other improvements for the aircraft. These include a glass cockpit, Flight test as well as operational vibration reduction and a new tail rotor. experience has demonstrated that the The first part of the paper describes efficiency of the new rotor is high. It these improvements. The second part of was of interest to determine the value of the paper will discuss the results of a the hover figure of merit of the new flight test program conducted on a Sea rotor. Since no isolated rotor ground King and present various results tests were conducted on the new rotor, obtained. QinetiQ conducted the flight direct measurements of the rotor test program on the Sea King. performance were not available.

1 Therefore, an approach using flight test Carson flight test data gives the aircraft data from the aircraft with both sets on figure of merit (MAC) when it is blades and thrust stand measurements equipped with each rotor. Now, since for the metal baseline blades was thrust stand data are available for the developed. First, an aircraft figure of baseline metal blades, giving the figure merit (MAC) is defined based on the of merit of the metal blade rotor (MRM), weight (W) and total power required to all the quantities Equation (4) are hover (PT): determined by experiment and MRC can be calculated. 1.5 MAC = .707W /PT (1) Recall that Equation (4) is true at The isolated rotor figure of merit (MR), constant total power (PTC = PTM) and it is which is the quantity of interest, based desirable to find the corresponding rotor on rotor thrust (T) and main rotor power thrust coefficient. The rotor figure of to hover (P R), is defined by the merit (MR) is a function of rotor power. relationship: The rotor power was measured in flight test with metal blades and this 1.5 MR= .707T /PR (2) information is used to determine the rotor figure of merit corresponding to the Equation (1) divided by Equation (2) total power condition used in evaluating yields: Equation (4), and consequently the figure of merit of the composite rotor 1.5 MAC /MR = (W/T) x (P R/PT) (3) corresponding to the power condition. The thrust of the more efficient If the same aircraft is compared in hover composite rotor is greater than that of with two different main rotors, one the metal rotor at this condition. The composite and one metal at the same thrust coefficient of the composite rotor total power (P T), the ratio of main rotor is now found from the calculated power (P R) to total power (PR/PT), is composite rotor figure of merit equal, independent of the main rotor determined by Equation (4). Thus, the design. Although the fuselage download thrust coefficient corresponding to each may vary slightly with different main condition is determined. No assumptions rotor designs, this small effect can be regarding download, blockage etc are neglected in the current discussion. involved in this approach. The results of Consequently, the ratio of aircraft figure this analysis, based on flight data, are of merit (MAC) to rotor figure of merit shown in Fig. 1 indicating the very high (M R) is the same in these two instances, figure of merit of the Carson rotor. independent of the main rotor Generally the aircraft figure of merit characteristics, and so the following with the composite rotor is 13 to 14% relationship between the various figures higher than that of the aircraft with metal of merit exists. (The final subscript blades at the same total power. refers to the aircraft with composite (C) or metal (M) main rotor blades.)

MRC = MRM x (MACC/MACM) (4)

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Figure 1. Figure of Merit of Carson Figure 2. Comparison of Flapwise and Rotor from Flight test Chordwise Stiffness of Metal and Composite Blades

Composite Blade Stiffness Glass Cockpit

An important design feature of these The glass cockpit is well on its way to composite rotor blades needs to be replacing the conventional instrument emphasized at this point. The spanwise panel in all types of aircraft. To continue distribution of flap and chord stiffness of modernization and improvement of the these advanced blades are essentially the S61, Carson is flight testing a glass same as those of the metal blades. Figure cockpit for this aircraft. Vector 2 compares the stiffness values. This Aerospace Services has stiffness matching was a design successfully flown a fully integrated objective. The mass characteristics also glass cockpit from Sagem Avionics in a match the metal blades closely and Carson S61. Carson’s major design consequently the composite blades have objectives in the development of the very similar dynamic characteristics to glass cockpit configuration for the S61 the metal blades. This design are to develop a user friendly system, as requirement for the new composite well as one that is maintenance friendly, blades was achieved. The torsional and also of low cost. Test flying is characteristics are not shown here, but currently underway and it is expected experimental values determined on the that this modification will be available aircraft in flight, show that the torsion for the aircraft in the fall. The unit is frequency (about 7.7 P at operating shock mounted and experiences very RPM) is somewhat higher than that of little vibration. It is configured such that the metal blade. Other features of the it can easily be incorporated in other blade design have been reported in helicopters of similar size. Reference 1. This matching results in no change in the dynamic characteristics of Five identical display screens are used in the aircraft or the handling qualities of place of the more usual four. The central the aircraft although Carson pilots have screen contains the engine instrument reported that it appears easier maintain a displays. The pilot and copilot have two hover with the new blades. screens each. One screen contains flight

3 instruments and the other display is instances these units were not installed selectable with maps/airfield approaches, in the aircraft. Often, the units that were and other options available. The caution installed were not carefully maintained. panel has been separated from these If not regularly maintained, the unit displays and located below the small tends, after some usage, to amplify GPS and transceiver screens rather than vertical vibrations rather than damp having a caution appear as a pop-up on them as illustrated by flight data taken one of the main screens. by Carson in a study of the effectiveness of the damper presented below. Also, it All modern sensors are incorporated may be noted that this system is often with the glass cockpit, replacing the tuned to damp vibrations at five times original 1959 equipment in the aircraft. the original design rpm of the aircraft The sensors achieve redundancy by (100%NR). However, the normal using special dual elements as necessary operating rpm of the aircraft in for IFR flight. commercial use is 103% NR. Carson made comparative measurements with Cockpit Vibration the device tuned to 100% and 103% (5P). Tuning to 103% coupled with Attention to detail can lead to maintaining the system properly has significant gains in vibration reduction. produced a dramatic reduction in cockpit In this connection, Carson has found the vertical vibration. Figure 3 presents the Honeywell/Chadwick VXP system results of surveys of vertical vibration analyzer/balancer to be very useful for peak amplitude at 5P in the cockpit in measurement and adjustment to reduce ips. A number of cases are shown. For a vibration. The S61 is normally equipped reference, the vibration levels at rotor with a very effective bifilar in-plane speeds of 100% and 106% are shown for absorber mounted on the rotor hub that the aircraft at mid cg at various airspeeds significantly reduces lateral vibrations in with no absorber installed. A number of the aircraft fuselage. This type of results are added at an indicated airspeed vibration absorber has the distinct of 110 kts, mid cg, 103% NR. First, a advantage of retuning with rotor speed. case with the absorber tuned to 100% as installed in the aircraft is shown, and in However because of the nature of the this case the vibration is increased above fuselage mode shapes and combined that of the aircraft with no absorber to response of the fuselage to vertical and the high level of 0.73 ips due to lack of in-plane forces at the rotor hub, as recent maintenance. Tuning the absorber discussed in Reference 4, there still to 100% after maintenance reduces the remains a significant vertical vibration in amplitude to a level of 0.41ips, a little the cockpit while the lateral vibration is below that of the aircraft with no low. This aircraft may be equipped with absorber. Removing the battery from the a classical vibration damper consisting absorber gives a point in between these of a flexible mounting for the battery in two, consistent with the other data. Then the nose of the aircraft designed to the absorber was retuned to 103%, using reduce vertical vibration at 5 per rev in a shake table. This produced a dramatic the cockpit. This unit was originally sold reduction in vibration level to about one- as an accessory for the aircraft. In many

4 quarter (0.08 to 0.1 ips) of the amplitude and reliable equipment, and an approach at 100%, a very comfortable level. with careful attention to detail.

New Tail Rotor Design

Carson is currently developing new composite tail rotor blades for the S61. The standard metal tail rotor blades are untwisted and have a symmetrical airfoil section (NACA 0012). This design has a low hover efficiency, and a tendency to experience tip stall at a thrust coefficient in the operating range (5). The hover efficiency of the metal blades measured on a thrust stand is shown in Figure 4. Figure 3. Vertical Vibration at 5P in the Hover trim at sea level and a gross S61 Cockpit. Gross Weight = 20000 lbs., weight of 20000 lbs corresponds to a various Altitudes thrust coefficient of CT of .017. The desired maximum operational thrust Tail Rotor Balance coefficient of the tail rotor is estimated to be CT = .021, determined by trim and A second area where attention to detail maneuver limits at altitude. The current gives rise to improvement is associated metal blades experience tip stall at a CT with balancing the tail rotor. The about 10% higher than a steady hover complete tail rotor assembly is normally trim (.0186), and the figure of merit statically balanced in the shop, before decreases rapidly with increasing thrust installation on the aircraft. However, the coefficient at this CT and above. high rotational speed of the tail rotor and its location at the top of the pylon makes it difficult to achieve a static balance of the rotor assembly with the precision that is desirable for truly smooth operation in the pylon, tail cone region.

The VXP system is used in flight in the aircraft to refine the balance of the tail rotor and thus reduce vibration in the pylon, tail cone area and occasional cracking of the skin that occurs in this region. Figure 4. Comparison of Tail Rotor These last two developments illustrate Figure of Merit how significant improvements in the The predicted hover performance of the vibration characteristics of a rotorcraft new Carson design is compared to the can be achieved with relatively simple current tail rotor in Figure 4. The new design has 8 degrees of linear twist and a

5 rectangular planform. It uses the same Concluding Remark advanced airfoils as the main rotor (RC(4) and RC(3) series). This is the Carson’s program to upgrade the S61 second design that has been studied by continues, producing significant Carson. The earlier design had a swept, improvements that modernize this older tapered tip that increases hover aircraft and make it very competitive in efficiency, but also contributes to high today’s market. steady control loads. These high loads are difficult to accommodate with the design of the mechanical control yaw control system in the aircraft. If the steady control loads are significantly different from the metal blades, lost motion will occur due to cable stretch.

The improvement in figure of merit shown for the new design in a hover trim at sea level 20000 lbs gross weight corresponds to a saving of about 65 HP. If this power saving is transmitted to the main rotor, an increase in main rotor thrust of about 400-450 lbs is predicted.

As with the main rotor, the design objective of achieving mass and stiffness characteristics that are a match of the metal tail rotor blade is critical. The steady control load generated by the tail rotor is primarily determined by the inertial properties of the blade as the RC series of airfoils have very low pitching moments. In addition, matching should minimize the possibility of encountering new or unforeseen dynamic problems with the new tail rotor blade. The overall design objective along with improved efficiency is that the new composite blade will simply be a replacement for the metal blade as is the case with the main rotor where no changes in the aircraft are required to use the new blades. Ground testing of the new composite design is scheduled to commence in July and flight tests will start later in the fall.

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The S-61/Sea King with Carson Composite Blades Part 2 – QinetiQ Sea King Test Programme

Introduction helicopters with folding main rotor heads are fitted with a ‘short spar’ blade The British Royal Navy (RN) and Royal which is 4” shorter than the S-61 blade. Air Force (RAF) operate the Westland Although a short spar Carson MRB had Sea King helicopter in various never been produced, Carson configurations. Since 1987 the had made provision in the Sea King in RN and RAF service has original design to produce both long and been fitted with composite Main Rotor short spar blades. In order to produce the Blades (MRBs) and a tail rotor fitted short spar blade, 4” of constant cross with 6 metal blades. Although the MRBs section, non lift section was removed are of composite construction, they are from the blade root. The blade cuff was aerodynamically equivalent to the then fitted to the MRB in an identical original Sikorsky metal MRB which manner to the long spar blade. In support used the NACA 0012 aerofoil and had 8 of the UK MoD TDP, Carson degrees of twist. Helicopters produced a set of short spar blades which were loaned to the MoD In late 2005 and 2006, the UK Ministry for evaluation. of Defence (MoD) were examining ways of increasing the performance of the Sea The Carson MRBs were fitted to a King as part of the Future Rotorcraft QinetiQ owned Sea King HU Mk 5 test Capability (FRC) Programme. One of aircraft which was comprehensively the options under consideration was the instrumented for aircraft handling and use of Carson advanced composite performance testing. The aircraft was MRBs which had FAA approval to be also fitted with additional flight load fitted to the Sikorsky S-61 (the civilian measurement instrumentation for safety equivalent of the Sea King) (1). In 2006 of flight purposes. The load QinetiQ were approached by the MoD to measurement instrumentation could be undertake a Technology Demonstration monitored in real time onboard the Programme (TDP) for the Carson MRBs aircraft. The first flight of the UK Sea fitted to the UK Sea King. This King Carson MRB TDP took place on programme was designed to assess the 20 September 2006. A total of 24 hours likely performance improvement offered of test flying were completed in support by the Carson MRBs within the existing of the TDP. Sea King flight envelope. The TDP yielded a number of important When QinetiQ were approached to carry results. The aircraft handling was found out the TDP, the Carson MRB had only to be not significantly different from the been produced in a ‘long spar’ version unmodified configuration, vibration was for the S-61. In order to maintain the broadly similar, hover performance was same overall rotor diameter, Sea King significantly improved and power

7 consumption in level flight was reduced. modification is the most significant Overall, the results obtained were in modification ever fitted to a British agreement with those obtained by as an SM and is Carson Helicopters using the S-61. thought to be the most complex Although the TDP was conducted within modification made to a helicopter in the the existing flight envelope, the results UK by a non helicopter manufacturer. obtained suggested that a significant increase in the forward flight envelope Although the ‘long spar’ version of the was possible. Carson MRBs had been certified by the FAA, the ‘short spar’ version had not. Following the successful completion of Also, different certification requirements the TDP, QinetiQ approached the MoD exist for a modification to a UK military to offer a programme of work to clear helicopter compared with those for FAA the Carson MRBs to be fitted to the RN certification. It was therefore necessary Sea King HC Mk 4 ‘Commando’ for a comprehensive certification variant. This led to an Urgent programme to be conducted to support Operational Requirement (UOR) being the SM. This involved flight tests issued by the MoD to modify the Sea supported by a large amount of King HC Mk 4 to support operations in theoretical work and analysis. QinetiQ Afghanistan. The UOR required the took responsibility for the flight tests, forward speed of the Sea King to be definition of the expanded flight increased at altitude and also required an envelope and determining the MRB increase in maximum take off weight at fatigue life. AW supported the high altitude. The UOR called for programme by determining the fatigue Carson MRBs and a five composite life of the new tail rotor assembly, the blade tail rotor produced by Agusta dynamic aircraft components and flying Westland (AW) to be fitted to the controls when operated to the expanded aircraft. The tail rotor had been produced flight envelope. by AW for the Mk 42B variant of the Sea King used by the and Instrumentation featured 5 cambered composite blades which were the same length and chord as In order to support the flight test phase the original metal blades. The tail rotor of the project, it was necessary to fit a had been shown to significantly increase new instrumentation system to the trials performance in low speed flight (6) and aircraft which was the same QinetiQ Sea it was hoped that use of this tail rotor King that had been used for the TDP. would produce benefits in the expanded This instrumentation system was forward flight envelope. designed, manufactured and installed by QinetiQ and used to support the testing QinetiQ were appointed as the Prime in a period of just over 5 months. The Contractor by MoD for the programme instrumentation system allowed on board of work to certify the main and tail rotor monitoring of data by Flight Test modifications for the Sea King HC Mk Engineers and Test Pilots and could 4. The modification was embodied as a accept data telemetered from the ground Service Modification (SM) with QinetiQ to the aircraft (such as wind as the Design Organisation. The information). The instrumentation

8 system allowed measurement of · Determine the compatibility of the handling and performance parameters modified rotors with the engine and flight loads. Measured parameters control system (the Westland Sea included: King is fitted with Rolls Royce Gnome Engines which are Main and tail rotor blade angles. controlled by limited authority ‘Fuel Main and tail rotor torque. Computers’). Main and tail rotor head loads. Main and tail rotor blade loads. · Determine if the airspeed system Flying control loads. was affected by change in main Engine operating parameters. rotor (the pitot-static ports are close Control positions. to the main rotor). Accurate airspeed at low and high speed. · Define an increased forward flight envelope for the modified aircraft. A total of 249 parameters were measured. 101 were rotating parameters · Examine the handling qualities of on the main rotor and 28 were rotating the modified aircraft. parameters on the tail rotor. The instrumentation system was found to be · Assess the level of vibration very reliable and its advanced design experienced by the modified significantly increased the rate at which aircraft. the flight testing could be conducted as a result of the purpose made real time · Gather flight loads data to allow displays. fatigue lives to be determined.

Flight Tests · Quantify the performance improvement offered by the new An extensive flight trials programme rotors. was conducted using QinetiQ owned Sea King HU Mk 5, tail number XZ575. · Assess handling and loads on the This flight test programme was main rotor at high rotor tip Mach conducted in partnership with the number. Ministry of Defence as part of the Aircraft Test and Evaluation Centre · Define a new low speed flight (ATEC). In addition to the envelope for the modified aircraft. instrumentation system, the aircraft was fitted with a number of other The testing was carried out in three modifications including a system to phases. The first phase was carried out in allow the longitudinal Centre of Gravity the spring of 2007 at Boscombe Down in position to be varied by adding lead the UK. This phase of testing addressed ballast. initial rotor compatibility issues and envelope expansion and load survey The flight test programme was designed tests to a maximum of 5000ft Density with the following aims: Altitude (DA). Following completion of the first phase of testing, the aircraft was

9 transported to Gunnison, Colorado · Montrose Regional Airport, (7678ft AMSL) in the US by RAF C-17 Colorado, 5759ft elevation. aircraft to commence the second phase high altitude testing. This testing · Grand Junction Regional Airport, concentrated on low speed envelope Colorado, 4858ft elevation. definition and envelope expansion and flight load measurement at high altitude. · Telluride Regional Airport, Part way through the phase two testing, Colorado, 9078ft elevation. sufficient data had been gathered to support an initial operating capability for All the test sites used were found to be the RN. The first flight of a RN Sea suitable for helicopter testing and King HC Mk 4 fitted with Carson MRBs provided good support to the test took place on 15 September 2007 when programme. Gunnison, Colorado was an 846 Squadron aircraft conducted its ideally suited for high altitude helicopter initial flight as part of RN high altitude testing as it had good facilities and training in . The first RN flight predictable weather conditions. This took place just over 8 months from the allowed rapid progress to be made and a start of the project. very high flying rate was achieved.

The third and final phase of the test Test results programme was again conducted from Gunnison Colorado in the US. Testing NOTE: Certain figures are presented took place in winter 2008 in cold with the numerical values removed from temperatures (down to -26oC) in order to one axis. This has been done due to the generate high rotor tip Mach numbers RESTRICTED nature of the data. and allow this portion of the flight envelope to be expanded. Additional Vibration flight loads data was also gathered with the aim of removing some of the The Carson S-61 aircraft are fitted with conservatism in the initial fatigue lives the Sikorsky Bifilar main rotor vibration which had been calculated. absorber. However, the Sea King aircraft in RAF and RN service are not. Prior to A total of 302 hours were flown in the start of the test programme, there support of the programme. This included was a concern that vibration may be dedicated flight tests, maintenance test increased with Carson MRBs fitted to flight requirements and ferry flights. In the UK Sea King, particularly within an addition to the two main test sites at expanded flight envelope. Boscombe Down in the UK and Gunnison in the US, the following The testing showed that the vibration of additional sites were utilised in the US to the modified aircraft was subjectively allow testing to be conducted at a range not significantly different from the of altitudes and temperatures: unmodified aircraft although, as expected, vibration was higher than · Hays Regional Airport, Kansas, experienced by the Carson S-61 aircraft 1998ft elevation. fitted with the Bifilar. The modified aircraft passed the in service vibration

10 checks using the same limits as specified for the aircraft. This increase in LSFE for the unmodified aircraft. Although offers significant operational benefits to work continues in the area of vibration the RN Sea King HC Mk 4. assessment and reduction, including the possible use of the Bifilar vibration 6000 absorber, the results of the QinetiQ test Standard programme have been confirmed by RN 5000 Modified in service experience. 4000 Low Speed Flight Envelope 3000 The modified aircraft was fitted with Altitude improved main and tail rotors. Both of 2000 these modifications would be expected

1000 to increase the size of the Low Speed

Flight Envelope (LSFE). The improved 0 hover performance offered by the main 6500 7000 7500 8000 8500 9000 9500 10000 rotor would reduce the required tail rotor Mass, kg thrust and the new tail rotor offered the potential to generate increased thrust for Figure 1 – Comparison of low speed the same power. A comprehensive LSFE flight envelopes for the standard and definition test programme was carried modified aircraft. out which attempted to define an increased LSFE for the modified aircraft. One important phenomenon that was The test programme included tests to found during the testing was the affect of examine the impact of Centre of Gravity longitudinal CG position on the tail rotor (CG) position and height above ground pitch required to maintain low speed on the LSFE. The criterion used to flight. Figure 2 shows a comparison of define the low speed flight envelope was tail rotor pitch required for relative the ability to maintain a 10% margin in winds from the right with the CG at a tail rotor pitch in all flight conditions. forward and aft position. Both sets of data were gathered at the same referred 1 A comparison of the LSFE for the mass . Figure 2 shows that at the critical modified and standard aircraft is shown wind azimuth of approximately 60 in Fig. 1. It should be noted that it was degrees, with the CG at a forward not possib le to maintain a 10% tail rotor position, approximately 10% more tail pitch margin throughout the LSFE rotor pitch is required. This effect is defined for the standard aircraft so in thought to be due to an adverse reality the increase in LSFE for the interaction of the main rotor vortices modified aircraft is actually greater than with the tail rotor. The more forward CG Fig. 1 indicates. Nevertheless, it can be position moves the aft portion of the seen that the modified aircraft LSFE has increased by approximately 580 kg 1 Referred mass is the aircraft mass ‘referred’ to (1280lb) at all altitudes where the low Sea Level conditions and is defined as the actual mass divided by the relative air density speed envelope is not limited by the multiplied by the square of rotor speed non- Maximum All Up Mass (MAUM) limit dimensionalised with respect to a reference value.

11 main rotor disc closer to the tail rotor as Engine control system aft cyclic pitch is applied to compensate for the forward CG. This result The Carson S-61 aircraft are fitted with demonstrates the importance of engines which have a simple hydro- gathering test data at a range of mechanical control system which operating conditions. If the low speed maintains rotor speed based on a pilot envelope had been defined based on aft controlled engine speed lever position. CG test data then insufficient tail rotor The Westland Sea King in service with authority would have been available at the RN and RAF is fitted with ‘fuel forward CG. computers’. These fuel computers include a collective pitch anticipation function which aims to minimise 60

FWD CG transient rotor droop by accelerating the 50 AFT CG engine based on pilot collective pitch 40 inputs. As this system relies on being

30 able to ‘anticipate’ the engine power demand resulting from a collective pitch 20 Tail Rotor Pitch (%) input, a change in rotor system could 10 potentially adversely affect the transient

0 0 20 40 60 80 100 120 140 160 180 droop performance of the engine control Wind Azimuth (Degrees) system.

Figure 2 – Tail rotor pitch in low speed flight at forward and aft CG position. A number of tests were carried out to examine transient droop characteristics Pressure Error Corrections at a range of altitudes. Transient droop performance remained good and was not Testing was carried out in order to significantly different from the standard determine if the change in main rotor aircraft. Further tests were also had a noticeable effect on the Pressure conducted to examine engine and Error Correction (PEC) for the pitot- drivetrain stability and other engine static system fitted to the aircraft. operating characteristics. The engine Although it was considered unlikely that control system was found to be any effect would be present, the pitot- compatible with the modified aircraft static vents are mounted above the without modification and no cockpit area directly below the main objectionable characteristics were rotor. Therefore, it was possible that a encountered. change in main rotor downwash characteristics could alter the PEC. Tests Hover performance were carried out using a trailing pitot- static ‘bomb’ and a DGPS system. The The hover performance of the modified results of the testing showed that the aircraft was measured during tethered PEC for the modified aircraft was not hover testing. The performance data significantly different from that for the gathered is shown in Fig. 3 and standard aircraft. compared with data for the standard aircraft gathered previously using the same airframe prior to modification.

12 From Fig. 3 it can be seen that a hover it was like flying the unmodified aircraft performance increase of up to 780kg at Sea Level. (1720 lb) referred mass has been demonstrated. In more practical terms, Forward flight envelope. the increased hover performance allowed the test aircraft to hover at its MAUM at One of the main aims of the test 10,000ft Density Altitude. programme was to increase the forward speed of the Sea King HC Mk 4. Testing

2500 was conducted to define an expanded forward flight envelope in accordance Standard 2200 with the UK Defence Standards. This Modified testing was conducted to define the

1900 largest flight envelope within which acceptable handling and the required

1600 component life could be achieved. In Referred Power accordance with the UK Defence 1300 standards it was also necessary to conduct tests up to 20% beyond the 1000 flight envelope intended for service use. 6000 7000 8000 9000 10000 11000 12000 Referred Mass, kg This involved conducting tests to the

Figure 3 – Comparison of hover maximum design forward speed for the performance for modified and standard Sea King, not only at Sea Level, where aircraft configurations. this speed has previously been substantiated, but also at high altitude. In Handling qualities tests carried out at low altitude, it was found that the maximum achievable The handling qualities of the modified forward speed was limited by collective aircraft were assessed throughout the test pitch whereas at higher altitude speed programme which included testing became limited by handling or control throughout the expanded flight envelope. loads. As would be expected, the modified tail rotor offered significantly increased A comparison of the main rotor stall control power in low speed flight which boundary for the standard and modified was found to improve low speed rotors is shown in Fig. 4. The significant hand ling. In forward flight handling potential benefits in forward speed are qualities were similar to the standard clearly demonstrated. The stall boundary aircraft at low altitude. However, at high for the modified aircraft fitted with the altitude the delay in the onset of Carson rotor translates to a potential retreating blade stall significantly increase in forward speed for the Sea enhanced aircraft handling. During the King HC Mk 4 of up to 50 Knots high altitude test programme where Indicated Air Speed (KIAS). However, handling was assessed at 12,000ft an increase of this magnitude may have Density Altitude, one Test Pilot an adverse impact on aircraft component commented that handling and lives. In service RN aircraft are currently performance had improved so much that benefiting from an increase in forward speed of up to 35 KIAS. Further analysis

13 and test work may allow this to be further increased. References

145 1. Curtiss, H.C., Carson, F., Hill, J., Quackenbush, T., Performance of a 135 Standard Modified Sikorsky S-61 with a new Main Rotor,

125 Journal of the American Helicopter Society, Vol. 48, No. 3, July 2003 115

105 2. FAA Approved Rotorcraft Flight

95 Manual Supplement No.6 for Sikorsky Referred Thrust S61 L,N,NM Model Helicopters, Carson 85 Helicopters, Inc, May 2007

75

65 3. FAA Approved Rotorcraft Flight 90 100 110 120 130 140 150 160 Manual Supplement No.7 for Sikorsky Referred TAS, knots S61 L,N,NM Model Helicopters, Carson Figure 4 – Main rotor stall boundaries Helicopters, Inc, December 2007 for the standard and modified aircraft.

Conclusion 4. Paul, W.F., Development and Evaluation of the Main Rotor Bifilar The RN Sea King HC Mk 4 has been Absorber, 25th Annual Forum of the modified by QinetiQ by adding Carson American Helicopter Society, May 1969 MRBs and an AW 5 blade tail rotor. This has led to a significant increase in performance which has allowed the 5. Cook, C.V. A Review of Tail Rotor modified aircraft to be deployed to Design and Performance, Vertica, Vol. Afghanistan by the RN. The 2, pgs 163-181, 197 performance improvements are so significant that many experienced Sea King operators have said the 6. Phipps P. D., Sea King Low Speed modification has transformed the Sea Envelope Improvements by King into what feels like a new aircraft. Incorporating a 5 Blade Cambered Composite Tail Rotor, Westland plc, ATN Sea King/049.

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