Helicopter Structural Integrity at DSTO-Air Vehicles Division

Transcription

1 AIAC-12 Twelfth Australian International Aerospace Congress Helicopter Structural Integrity at DSTO-Air Vehicles Division Domenico Lombardo Senior Research Engineer, Helicopter Structural Integrity DSTO Air Vehicles Division Abstract: Helicopters present unique problems in all aspects of their design and their structural integrity is no exception. Air Vehicles Division has been involved in helicopter structural integrity for 17 years. In that time, much work has been done to support the ADF, but much still remains to be done. The introduction into service of modern all-composite-airframe helicopters and sophisticated monitoring systems represents a challenge. This paper firstly presents a brief overview of why helicopters are different to fixed-wing aircraft in regard to structural integrity and then gives an overview of some of the helicopter structural integrity work done in DSTO. Before I describe some of the work done by the helicopter structural integrity area at DSTO Air Vehicles Division, I would like to take a slight detour by asking the question, Why do we have a discipline known as helicopter structural integrity rather than just have a single discipline of aircraft structural integrity? In other words, are helicopters sufficiently different from other heavier-than-air craft to require special and separate attention? The answer that I will give (and it is an obvious one given my job title) is, Yes, they are. To understand that answer, we need to go back to basics. Critics of helicopters point out that helicopters are noisy, slow, have limited range, limited endurance, and are expensive to operate. Various disparaging comments are often made about helicopters. To list just a few: Helicopters are flying fatigue machines Helicopters don t fly by aerodynamics; they re so ugly that the Earth repels them They re toys, mere Prandtl palm trees Anything that screws its way into the sky flies according to unnatural principles Who wants to fly in an aircraft whose wings are leading, lagging, flapping, precessing and moving faster than its fuselage? Why then, if they have all these deficiencies, do we have helicopters? The answer is quite simple, as seen in Figure 1. No fixed-wing aircraft can do what the Chinook is doing in that figure. The ability to maintain position over a fixed point while loading or unloading troops and/or cargo, as well as to move slowly in all directions, is an ability shared by only two types of aircraft: helicopters and airships. Airships obtain this remarkable ability by being lighter than air, but helicopters are heavier than air and so use wings, just like their fixed-wing cousins, to achieve the same effect. The name helicopter ultimately comes from two Greek words: helix (or heliko) meaning spiral and pteron meaning a wing. The concept of spiralling wings encapsulates perfectly the way in which these machines defy gravity to undertake their many different roles. This is also the crucial point where helicopters and fixed-wing aircraft part company. Whether a fixed-wing aircraft is a slow, lumbering transport giant, or a fast fighter jet, or a general aviation aircraft, there is one thing that they all have in common; their propulsion and lift systems are separate. To an airframe we attach wings and then we bolt on one or more engines to push the lot through the air to generate lift and thence to fly. On helicopters, we also attach our wings (rotor blades) to our airframe, and bolt on an engine or two, but then we connect the engine to the rotor blades via drive shafts so that the engine has to drive the rotor blades to create lift, not push the airframe through the air. This marriage of propulsion and lift in the Fifth DSTO International Conference on Health & Usage Monitoring HUMS2007

2 form of rotor blades is the reason for the existence of the helicopter structural integrity research area in DSTO as structural problems unique enough to require a separate discipline are involved. 1 Figure 1 A Boeing Chinook doing what helicopters do best. For both fixed-wing aircraft and helicopters, a structural fatigue engineer will require knowledge of how the aircraft will be flown and the loads that will be generated in the aircraft s components during flight (this is termed the fatigue spectrum ). For a fixed-wing structural integrity engineer, it is usually enough to know the frequency of occurrence of a particular load to calculate the fatigue life of a component. Also, vertical acceleration is a key parameter that provides the fixed-wing structural integrity engineer with a lot of information. The life of the helicopter structural integrity engineer is a bit more complicated. Not only do we need to know the frequency of various load exceedances, and all the other information that a fixed-wing engineer needs, but we also need to know the amount of time that each exceedance lasts. Similarly, while vertical acceleration is an important factor for some components, it is not the only important parameter that a helicopter structural integrity engineer needs. To see why this is so, first consider the case of a fixed-wing aircraft, cruising along in smooth air, then making a constant height, banked turn, before returning to straight and level flight. We may regard the loads generated in the turn as possibly having applied a single fatigue cycle to the aircraft, no matter how long the turn took to make. However, no matter how simplistically we consider the same situation for a helicopter, the loads in the rotor blades will vary in a cyclical manner during each revolution of the rotor blades (Figure 2) and hence may accumulate many fatigue cycles. Different manoeuvres will cause different cyclic variations in load so it is important to know how long each manoeuvre lasts. 1 As an aside, the integration of lift and propulsion is also the reason why helicopters are subject to many more potentially hazardous flight conditions (such as vortex ring state, loss of anti-torque effectiveness, overpitching, ground resonance, dynamic roll over, and mast bumping, to name just a few) than fixed-wing aircraft, but that is outside the scope of this paper and, indeed, could be and has been the subject of entire books. 2

3 Lift Blade azimuthal angle Figure 2 The lift generated by a rotor blade section at 98% radius as it goes through one revolution. (Blade azimuthal angle: 0 = aft, 90 = left, 180 =forward, 270 =right) This is the key difference between fixed-wing and helicopter structural integrity; the loads on a helicopter component are almost invariably changing. No matter what the flight condition, as long as the rotor blades are generating lift, there is the potential to accumulate several orders of magnitude more fatigue cycles than in a fixed-wing aircraft. This provides the helicopter structural integr ity engineer with a set of challenges that are not usual for a fixed-wing engineer. HELICOPTER STRUCTURAL INTEGRITY AT AIR VEHICLES DIVISION For decades, DSTO did not have any significant effort directed towards helicopter structural integrity. Although there were the occasional forays into work on helicopter structures, all the serious effort was directed towards fixed-wing aircraft. This was mainly due to the strong demand for such work from the Royal Australian Air Force (RAAF), the main operator of military aircraft in Australia. The RAAF, with DSTO support, was not afraid to venture into places where the OEM was reluctant to go. However, when it came to helicopters, a strange dichotomy of thought existed where the RAAF seemed content to rely solely on OEM advice. 2 This was likely due to a chicken-and-egg type situation. DSTO had long been involved with fixed-wing structural integrity and the RAAF knew our capability and knew what it wanted from us. In terms of helicopters, the RAAF had minimal experience with support for them from DSTO and so did not demand any. Without this demand, there was no incentive for a push from within DSTO to build up the capability. In 1989, the Superintendent of Aircraft Structures Division, Gordon Long, decided to init iate a push from within DSTO to change this situation. His decision led to my appointment as a junior engineer to learn what I could about helicopter structural integrity and then to impart that knowledge to others within the Division. Since there was no one in Australia from whom I could learn, at the end of 1991 I was posted for a year to the US Army Vehicle Structures Directorate at the NASA Langley Research Center in Virginia, USA. There, under the tutelage of Dr Wolf Elber and his team, I learned what I needed to learn. In a parallel activity, Ken Fraser in the Aircraft Propulsion Division was also working to improve DSTO s position in helicopter structural integrity. Here, in the structure of DSTO, is reflected the intertwining of a helicopter s structure and its propulsion system. Both an aircraft structures division and an aircraft propulsion division were working towards the same aim. 3 When I returned from the US, I was assigned to Ken Fraser s group and we worked to build up and maintain a core of helicopter structural integrity expertise within the Division. Currently, there are six staff in the group plus a Canadian government research engineer. On a long-term attachment 2 Up until the late 1980s, the RAAF operated and owned the helicopters that supported Australian Army operations. Then the government transferred ownership to the Army, but left technical airworthiness responsibility with the RAAF. Currently, this technical airworthiness responsibility resides with the Directorate General Technical Airworthiness (DGTA) within the RAAF. 3 The Aircraft Structures Division and Aircraft Propulsion Division went through several changes of identity and mergers with other divisions to become, eventually, the Air Vehicles Division. 3

4 SOME SIGNIFICANT PROJECTS 1. Unresponsive OEM One of the early projects that we undertook was also one of the smallest. It took only a few weeks to complete and yet had an impact far greater than the time taken would suggest. It involved a helicopter type for which the OEM was insisting that certain components needed to be replaced immediately. The ADF airworthiness engineers had a feeling that this was not correct, but had no way of backing that up. The OEM, was not disposed to follow up this feeling because we have a small fleet of helicopters and they had far greater concerns to which they were applying their resources. Their default position was, therefore, to take a very conservative line in the advice that was provided. Faced with a non-responsive OEM, the ADF requested our assistance. We provided them with an analysis that showed that the components could still be flown for hundreds or thousands more hours and still be operated within the original certification basis. When the OEM was presented with our analysis, they (eventually) agreed that it was correct. This single project saved the ADF millions of dollars since they did not have to find the money to buy a whole lot of new components immediately. 2. Black Hawk Risk Analysis When the ADF purchased the Black Hawk, they did so on the basis that it would be flown like the US Army Black Hawk and, therefore, the same component retirement times (CRTs) would apply. However, there was concern that this assumption was not valid and so, in 1993, a new usage spectrum was developed by Sikorsky, based on qualitative data from Australian Army pilots. In essence, the data were derived from: Mission Monitoring Forms: These were forms that were filled out after every flight by aircrew and contained parameters like gross weight, cg, speeds flown, altitudes, and number of landings. Long-Form Questionnaires: These were extensive questionnaires that were filled out by aircrew and sought to describe each type of mission in great detail. Test Pilot Observations: A Sikorsky test pilot flew with our aircrew to provide another perspective on the type of flying done and to ensure that what Sikorsky understood to be a particular manoeuvre was also what the Army pilots understood it to be. The outcome of this new spectrum was presented as an impact study, where five components were chosen (two with low CRTs, two with medium CRTs, and one with a high CRT). The results are shown in Figure 3. These results, showing that CRTs might have to be halved, caused concern and Sikorsky was contracted to conduct a detailed analysis to produce definitive CRTs. This detailed analysis would update material properties with the latest Sikorsky test results and remove any flight loads that were derived from aircraft configurations or roles that were not relevant to Australian operations. However, these definitive CRTs would not be available for 12 months. This placed the ADF in a quandary. Figure 3 shows that the impact study indicated that two components (Main Support Bridge and Main Rotor Cuff) might have CRTs of hours. Given that two-thirds of the fleet was already above 1000 hours, and that this figure would rise to three-quarters before the 12 months were up, the Army was faced with the question of what should be done with the Main Support Bridges and Main Rotor Cuffs on these aircraft. Replacement was not an option because there were only a few spares incountry and Sikorsky had quoted a 12-month turnaround time for new spares to arrive. 4

5 11000 hr* 6400 hr 5100 hr* 3300 hr 5400 hr* 2600 hr 2400 hr* 910 hr 1800 hr* 910 hr Figure 3 Results of applying the new, Australian-unique, usage spectrum to five components on the S- 70A-9 Black Hawk. The retirement times marked with an * are the original lives for the S-70A- 9, which were those of the UH-60A Black Hawk. Those marked with a are the recalculated CRTs based on the new usage spectrum. To avoid the seemingly unavoidable grounding of most of the Black Hawk fleet, DGTA requested that DSTO conduct a risk analysis to quantify the increased risk in over-flying the CRTs calculated by the impact study. As a result of this analysis, DGTA applied interim CRTs to these components and so only one aircraft was grounded before the new CRTs were received from Sikorsky. 3. Joint US/Australian Flight Loads Survey on Black Hawk Back in 2000, a flight loads survey was conducted on the Black Hawk. This is probably the most comprehensive flight loads survey ever conducted for a helicopter outside of those undertaken by OEMs to certify their aircraft. The participants were the Australian Army, RAAF, USAF, Georgia Tech Research Institute (GTRI), Sikorsky, and DSTO. The aim of the survey was to gather sufficient data to qualify structural enhancement packages for the Black Hawk. These enhancements would allow the United States Air Force (USAF) to extend the airframe lives of their HH-60G/MH-60G Black Hawks to 20,000 hours, and allow the Australian Army to manage their S-70A-9 Black Hawks so that the planned withdrawal date might be reached with minimal structural problems. During the survey, 81 flight hours were flown in 151 days (16 hours per month), covering 20 aircraft configurations (gross weight, cg, external stores on or off) and 152 flight conditions. Out of this, 65 hours of usable data were obtained. The sensors used in the flight loads survey consisted of: 28 sensors for measuring aircraft state parameters (roll rate, pitch acceleration, collective position, etc) 72 strain gauges for measuring strains in the dynamic components, 217 strain gauges for measuring strains in the airframe, and 18 accelerometers mounted at various locations in the airframe. Two DSTO research engineers spent time at GTRI after the flight loads survey was complete to assist in analysing the data. 5

6 Since the completion of the flight survey, the goal posts have moved. The USAF has recently stated its intention to buy Boeing HH-47 Chinooks, with initial operation capability by 2012, to replace its Black Hawks, and the Australian Army has bought Eurocopter MRH-90s, and intends to replace its Black Hawks from 2012 onwards. The original intentions for undertaking the flight loads survey are therefore no longer as important because the Black Hawks will be retired earlier than planned. However, there are still several years between now and when the final Black Hawk is retired and the loads database produced has already proven to be invaluable in tackling S-70A-9 airframe issues and will undoubtedly continue to be so. 4. Automating Usage Monitoring for Fleet Management Currently, the ADF has in place a usage monitoring system for each of its helicopters. This system is underpinned by data obtained from forms that aircrew fill in after each flight. These forms are designated EE360 for Army helicopters and POSSUM (Program Operational Survey Structural Usage Monitoring) for Navy helicopters. These forms require the aircrew to fill in details such as the aircraft configuration, gross weight and cg at take-off, time spent at various altitudes and speeds, number of landings, number of autorotations, and number of extreme manoeuvres. The exact information required depends on the original fatigue substantiation of each particular helicopter type and the perceived ability of aircrew to record that information reliably. The forms therefore only contain a subset of the information required for a full fatigue life re-substantiation. To determine the level of confidence that can be placed in the data received from the EE360 and POSSUM forms, DSTO undertook a trial whereby data were downloaded from the flight data recorders installed on Australian Army Black Hawk and Chinook helicopters and those data were compared to the data provided on the EE360 forms corresponding to the same time period. The overall result was that the EE360 data were reasonable, though there were some areas where the reliability was poor, notably autorotations and extreme manoeuvres. This trial also allowed DSTO to undertake an assessment identifying the requirements for an automated usage monitoring system to replace the EE360/POSSUM forms. There is plenty of support for such a system from air and ground crew, but there were several areas identified where further work needs to be done to overcome potential problems. This work will be presented at the forthcoming 63 rd AHS International Forum at Virginia Beach, Virginia, USA, 1-3 May. FUTURE CHALLENGES Air Vehicles Division has been involved in helicopter structural integrity for 17 years. In that time, we have accomplished a great deal in support of the ADF, but there is still a great deal more to do since technology does not stand still. Our main challenges are: Composite aircraft The ADF helicopter fleet has primarily been a metal fleet and we understand the fatigue life substantiation methodologies involved. However, with the acquisition of the Eurocopter Tiger, and the MRH-90, and the subsequent impending retirement of Iroquois, Kiowa, Black Hawk and Sea King, the complexion of the ADF fleet will change. We will have helicopters with all-composite airframes, and we need to understand the problems that that may cause. For example, delamination damage to the airframes is a potential problem as these helicopters will be operating from unprepared sites with their concomitant risk of airframe strikes from loose, heavy mass, objects (such as stones). 6

7 Automated Usage Monitoring As mentioned above, we are looking at implementing simple automated usage monitoring on existing aircraft, such as the Black Hawk and Chinook. There are significant challenges ahead on data handling and data fusion, as well as in creating robust algorithms for processing the data to extract as much information as possible and minimise the need for aircrew to provide supplementary data (such as gross weight). The Eurocopter Tiger and MRH-90 will come with onboard automated systems and they will also bring new challenges to staff in obtaining an understanding of the operation of those systems, their level of sophistication and their abilities. Crashworthiness As a result of the crash of Sea King Shark 02 on the Indonesian Island of Nias in April 2005, and the subsequent loss of nine lives, there has been a renewed interest in helicopter crashworthiness research. We are currently assessing the viability of moving into this field and how we might make a useful contribution. CONCLUDING REMARKS No fixed-wing aircraft has ever plucked a person to safety from the roof of their home during a flood. No fixed-wing aircraft has ever winched down rescuers to tend to people injured in dense jungle terrain. No fixed-wing aircraft has ever been able to take injured motorists from the site of a motor vehicle crash and flown them straight to a hospital. When the day comes that fixed-wing aircraft can do all these as efficiently as the helicopter, then the helicopter will become an historical irrelevancy. Until that day, though, there is plenty of work to be done by helicopter structural integrity engineers. The mechanisms that bestow upon helicopters their ability to hover also bestow an interesting life upon the helicopter structural integrity engineer. 7