HELICOPTER HYDRAULIC SYSTEM - ICAS

1 ICAS 2002 CONGRESS. HELICOPTER HYDRAULIC SYSTEM . (Design, Component Selection, Modular Construction, Integration & Testing). (S. VISWANATH and R. NAGARAJAN of Hydraulics Group, Rotary Wing Research & Design Centre, Hindustan Aeronautics Limited, Bangalore, India). 1. INTRODUCTION 2. SYSTEM TRADEOFFS. HYDRAULIC fluid power is widely used in air- SYSTEM tradeoffs need to be made to clearly borne applications specifically to operate the define the number of systems required for the flight controls and landing-gear systems due to HELICOPTER including certain primary parameters its compact size, high response rates, high load to be finalized considering overall requirements holding capabilities and excellent power to of the HELICOPTER reliability & survivability.

2 To weight ratio. Modern helicopters are no achieve this use of multiple HYDRAULIC systems exceptions in this regard due to the adaptation has emerged. With the decision to use two of advanced rotor systems calling for higher HYDRAULIC systems, there are two available control loads, fast response requirements and options to the designer. First option is to design use of retractable landing gear systems. This each SYSTEM capable of handling the entire paper provides the salient features of the spectrum of control loads and use one SYSTEM as HYDRAULIC systems used in helicopters based on the main SYSTEM keeping the other SYSTEM as a literature survey and experience gained during standby SYSTEM . The standby SYSTEM gets the development of a recent origin helicopters.

3 Activated automatically in the event of failure of main SYSTEM . Second option is to design each Helicopters under 3000 kg all-up weight HYDRAULIC SYSTEM capable of handling the control category using conventional articulated rotors loads for minimum basic flying and both SYSTEM could be manually controlled since the rotor together cater for the entire flight spectrum in control loads will be well within the handling which case failure of any one SYSTEM will ensure capability of pilot. However, to reduce the minimum basic flying capability. The second pilot's workload, HYDRAULIC boosters are used in option provides better reliability due to the fact such helicopters with lower fluid pressure for that both systems will be operating at all times their operation.

4 Helicopters above 3000 kg all- and is also lower in weight. As per the emerging up weight category using modern rotor new civil airworthiness requirements, in addition concepts like semi-rigid or rigid rotors will to dual HYDRAULIC systems, one electrical back up result in control loads beyond pilot's handling SYSTEM is also need to be provided for the flight capability and positively calls for fully powered control SYSTEM of HELICOPTER . This third SYSTEM flight control SYSTEM . Requirement of high can also be used for the flight control SYSTEM response characteristics under high operating preflight checks. load conditions needed for the flight control operations has resulted in extensive use of Landing gear retraction / extension, wheel brake, HYDRAULIC systems in helicopters.

5 Deck lock harpoon, rescue winch and sonar winch are some of the other systems of the Helicopters with all up weight in excess of HELICOPTER , which are operated using HYDRAULIC 3000 Kg and fitted with high response rotor power. In the helicopters using a primary &. SYSTEM , the rotor loads are beyond the pilot's standby HYDRAULIC systems for flight control handling capability & HYDRAULIC power is operation, the standby HYDRAULIC SYSTEM may be invariably used to operate the flight controls. used for these utility services with a safety device to divert the HYDRAULIC power to controls Mr. S. Viswanath and Mr. R. Nagarajan in the event of primary HYDRAULIC SYSTEM time. Let us assume that all the flight control failure.

6 However, such diversion of HYDRAULIC actuators work simultaneously and these flight power to the utility services from standby control actuators are supported by two HYDRAULIC SYSTEM will reduce reliability of flight control systems 1 & 2. From the rotor design we obtain SYSTEM due to the presence of many other the maximum control loads that need to be operating equipment. Helicopters adopting operated by each of the actuators. Since two continuously operating dual HYDRAULIC SYSTEM HYDRAULIC systems are feeding power to each of for flight controls invariably demand the the flight control actuators having two operating additional utility HYDRAULIC SYSTEM to operate all cylinders, it can be assumed that each cylinder the utility services.

7 Will hold 75% of maximum control load and both the HYDRAULIC systems together will provide 3. SELECTION OF SYSTEM 150% of the maximum control load. This PRESSURE, FLOW RATES & philosophy is preferred to arrive at a lower OPERATING FLUID weight of the actuators compared to 200%. maximum load holding capability. However, it After defining HYDRAULIC SYSTEM configuration, is essential to ensure that 75% of the maximum next step is to define the primary parameters of control load would be adequate to perform all the each of the systems like SYSTEM fluid pressure, basic flying capability of the HELICOPTER under maximum fluid flow rate and the HYDRAULIC consideration to ensure flight safety, in the event fluid that need to be used.

8 Of one HYDRAULIC SYSTEM failure condition. Once the maximum actuator ram load per HYDRAULIC Selection of Fluid Pressure SYSTEM has been arrived for each of the servo actuators, the piston annular area can be easily For a defined power level, a lower SYSTEM computed using the defined operating fluid pressure will increase the size of equipments. pressure. Then by using the operation rate of On the other hand high SYSTEM fluid pressure controls (it is considered that pilot can move the will call for increased wall thickness in the control stick end to end in one second in absence equipments. Hence, selected SYSTEM pressure of any specific data), we could easily arrive at has a bearing on the HYDRAULIC SYSTEM weight & the fluid flow rate.

9 No load speed of the size, while meeting the required power levels. actuators (normally 150% of maximum operating It is interesting to note that the HYDRAULIC speed by the pilots) should also be taken into SYSTEM weight for a defined power level will account while computing the SYSTEM fluid flow considerably lower for operating fluid pressure rate since it has an indirect relation to the between 3000 psi and 4000 psi. Hence, it is actuating speed under loaded condition. For the preferable to develop the HYDRAULIC SYSTEM for fluid flow rate thus arrived, we need to add the fluid pressure range 3000 to 4000 psi and to estimated internal fluid leakage rate including have the advantage of weight & size reduction.

10 Pump case drain leak rate to arrive at the Use of standard fluid pressure 3000 or 4000 psi pumping capacity of the HYDRAULIC pump. Similar will also provide the advantage of using many analysis has to be made to arrive at the fluid flow already developed HYDRAULIC equipments and rate for the utility HYDRAULIC SYSTEM considering reducing the development cost & time. all utility services, which operate simultaneously. Computation of Fluid Flow Rates Selection of HYDRAULIC Fluid Second step is to arrive at the fluid flow rate Suitable HYDRAULIC fluid is selected considering requirements for each of the HYDRAULIC systems functional requirements, operating environment used in the HELICOPTER , which is specific to the and operating temperature limitations.