Browse Topic: Engine cooling systems
The oil cooling fan of a Main Gearbox (MGB) is a mechanically-driven component whose purpose is to force an air flow through an air cooled oil cooler; its performance is crucial in ensuring that the MGB oil temperature does not exceed a predefined threshold, set to alert the crew in case of an abnormal situation. The design and the certification of a cooling fan is a process involving several steps and multiple disciplines; mechanical design, aerodynamic analysis, dedicated tests carried out both on rigs and at aircraft level need to be exploited as complementary tools to assess the correct aero-mechanical behavior of the system. The aerodynamic assessment is associated to performance, measured in terms of MGB oil temperature: considering a comparison between two cooling fans, one outperforms the other if the resultant MGB oil temperature is lower, keeping the same boundary conditions (engine torque, wind speed, ambient temperature, etc.). The correct mechanical behavior is instead associated to multiple requirements that need to be satisfied: target fatigue life, high and low temperature limits, blade loss containment, maximum speed, etc. The process connecting the very first blank page to the certification of the part is usually linear and well defined, and the experience gathered by Leonardo Helicopters over the years has allowed to follow this path easily in most cases. This paper discusses an intriguing situation: the goal of the activity herein discussed was to replace an obsolete oil cooling fan with a new one. The latter was expected to outperform the first both from the aerodynamic point of view (i.e. lower MGB oil temperature) and from the mechanical point of view (i.e. longer life). The preliminary CFD analyses and the rig tests proved that the new fan was able to outperform the legacy one in regards of the aerodynamic requirement; nevertheless, during the last steps of verification carried out at aircraft level, the installation of the new fan resulted in a higher MGB oil temperature. This triggered dedicated troubleshooting: a sequence of investigative tests performed by means of dedicated rigs, targeted at properly identifying the aerodynamic operative point of the cooling fan and at understanding the reason behind the unexpected outcomes obtained. The relevance of this contribution does not lie in the description of the certification process per se, rather in the nonlinear dynamics characterizing the test sequence once the flight activities pointed out an unpredicted behavior.
The current work focuses on understanding the aerodynamics of a single and coaxial rotor in hover subjected to 1-minus-cosine gust profiles. The work was performed using an in-house free-vortex method that includes the contribution of unsteady aerodynamic terms using the Duhamel integral. Studies were conducted on the Harrington single and coaxial rotor systems trimmed to CT /σ = 0.1, with the gust duration varying from 1-50 revolutions of the rotor, i.e., 0.3 - 15 seconds. Gust amplitudes were also varied from 0.1ωR-0.3ωR. Results were analyzed in terms of the deviation of thrust from the hover values, the frequency content of this signal, angle of attack variations across the span, and the structure of the wake as the system goes through the gust event. It was noted that for single and coaxial rotors, edgewise gusts result in higher frequency components being present in the thrust compared to the axial gust. Large changes in the angle of attack were observed, which could potentially lead to vibratory loads on the system as it experiences the gust. Additionally, an in-house experimental setup consisting of a 3 x 3 fan array and a single rotor (12-inch APC propeller) mounted on a load cell was used to obtain preliminary results of a single rotor operating under gust. The same was compared against the developed numerical method.
The multi-role utility helicopter T625 GÖKBEY is designed by Turkish Aerospace and it is equipped with a pair of two-spool CTS800-4AT turboshaft engine developed by Light Helicopter Turbine Engine Company (LHTEC). Components of the cowlings, intakes and exhausts were designed with supplementing CFD analyses and performance of various alternatives were evaluated. Final designs were achieved based on the helicopter performance and engine limits. In order to verify the estimated engine installed performance in design phase, performance of the instrumented engine with its integrated equipment on the platform is examined using flight test data. This paper focuses on the CFD simulations based performance predictions of the air induction system, exhaust system, and IPS blower exhaust. A comprehensive study is assessed to create more realistic models by using flight test data.
As embedded electronic control systems are increasingly penetrating vehicle subsystems, the designers are faced with a dilemma of providing state of art vehicle features on one hand and ensuring frugal implementation of the same to meet competitive pressures on the other. For embedded software and hardware systems this means adoption of judicious and innovative design choices with reusable building blocks. This paper dwells upon various design aspects of control and monitoring which are frequently used for automotive applications such as feed-forward and proportional integral control, diagnostics for sensor boundary conditions, handling of intermittent faults without causing nuisance to the vehicle users etc.
State-space inflow models have long been the standard for rotor wake modeling for flight dynamics and control simulation. As rotorcraft design continues to trend toward Future Vertical Lift (FVL) multi-rotor configurations in order to overcome the limitations of traditional design, the use of state space inflow models must similarly evolve to capture the complex aerodynamic interactions inherent to these new rotorcraft configurations. There is a demand for a state-space inflow model that accurately captures the significant aerodynamic interactions that occur between multiple rotors, ducted fans, wings, and complex airframes that preexisting inflow formulation fails to address. This paper discusses the ongoing effort to establish a robust methodology for deriving a state-space inflow model suitable for FVL applications from first-principle based viscous Vortex Particle Method (VPM) using the CIFERR system identification tool. The paper focuses on areas of enhancement that expand the identified inflow model's accuracy and usefulness as a tool for flight dynamics simulation of multi-rotor configurations. First, the effects of wake distortion due to rotor tip-path plane (TPP) rotation are thoroughly investigated for a co-axial configuration across a number of airspeeds and implemented into the inflow model formulation for improved simulation accuracy. Next, the rotor interference on the aerodynamic surfaces and the fuselage is explored, with attention placed on addressing this interference as a unified formulation. For the rotor interference on fuselage (3-D body) estimation, investigation of improvement through multi-point sampling is performed. The paper also examines the control design application through constructing the linear time invariant aircraft model with integrated state-space inflow model and verifying the accuracy of gain and phase margins and crossover frequencies. For each of these areas of investigation, the impacts on precision flight dynamics simulation are assessed through frequency and time domain response analysis and comparison to the simulation with first principle based VPM. The validation results of the predicted response data show excellent agreement with VPM simulation and further justify this method of inflow model identification as an effective tool for multi-rotor and FVL applications.
This paper develops a lumped-parameter multi-plates wet clutch Offset Compound Gear (OCG) transmission dynamics and its thermal model for dual-speed rotorcraft applications with an active clutch slip-speed control. This model includes the Reynolds equation for the clutch oil film thickness, the clutch thermal model, the clutch transferred torques (viscous and asperity torque) and the clutch disengagement model. The wet clutch/OCG transmission system is implemented in Matlab® Simulink™ to manage the upshift clutch temperature rise, which is a main issue need to handle for a dual-speed helicopter transmission. Here, the clutch temperature rise is treated by injecting a certain amount of coolant during engagement so that the temperature rise for the wet clutch is much lower than that of an dry clutch. In order to transfer a required torque using the available power, the sizing of the wet clutch could be evaluated via the developed wet clutch/OCG transmission model. This study shows that the temperature rise drops as the wet clutch oil flow rate increases adding extra weights compared with the dry clutch. The simulation also captures a phenomenon that a larger clutch engagement pressure might be required for the wet clutch to transfer the same torque since the wet clutch oil viscosity drops as the oil temperature increases during the clutch engagement.
The effects of key design parameters of tilting distributed ducted fans are investigated through steady-state CFD simulations to assess the benefits of using variable geometry ducts in urban air mobility applications. The analysis is made on three adjacent ducted fans mounted at the trailing edge of a semi-span wing. The fans are represented by body forces calculated using the blade element theory. The duct expansion ratio, the duct thickness and the fan design expansion ratio are varied along with the fan speed, the crosswind speed in hover and the airspeed in forward flight. For each combination of the parameters, the hover Figure of Merit and crosswind stall speed as well as the forward flight lift coefficient, thrust coefficient and propulsive efficiency are evaluated. From these results, variable geometry ducted fans are benchmarked against fixed geometry ducted fans using a simplified 1 hour mission with 10% of hover time. It is found that a ducted fan equipped with a Krueger flap and a variable expansion ratio diffuser consumes nearly 11% less energy to complete the mission.
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