Final Report Summary - ATHENAI (Aerodynamic Testing of Helicopter Novel Air Intakes)
Executive Summary:
The European Union has launched the CleanSky initiative together with the European aerospace industry aimed at a reduction of emissions and fuel burn. The engine installation plays an important role to foster fuel-efficient engine operation and decrease emissions. The sub-project GRC2 of the Green Rotorcraft ITD (Integrated Technology Demonstrator) aims at improving the aerodynamic characteristics of helicopter and tiltrotor aircraft fuselages and engine installation. In this context, the investigation and optimization of engine installation plays an important role to ensure and foster fuel-efficient engine operation. To cover a wide range of the future helicopter fleet, weight classes from light to heavy are treated within GRC2. Numerical optimization of several configurations is carried out by means of Computational Fluid Dynamics (CFD) and expected benefits of identified solutions are assessed through wind tunnel tests.
Comprehensive wind tunnel tests were performed on a full scale model of a helicopter fuselage section at the Institute of Aerodynamics and Fluid Mechanics of the Technische Universität München (TUM-AER). For that purpose a new wind tunnel model of a lightweight twin-engine helicopter comprising a fuselage cowling part, all air intake components as well as flow passages was designed, manufactured and instrumented.
The model features a high degree of modularity for easy comparison of various air intake components. In two wind tunnel campaigns aerodynamic characteristics of three baseline engine air intake configurations and several design variants were analyzed in detail. The design modifications concentrate on flow guiding elements like fillets, vanes, spoilers and scoops. With a five-hole pressure probing system flow field measurements were conducted at the aerodynamic interface plane (AIP). Additionally, steady and unsteady surface pressures on cowling and air intake regions are recorded. The incoming flow was investigated by field measurements of mean and fluctuating velocities. Data analysis and evaluation provided a valuable database and guidelines for helicopter air intake design.
The geometric variations tested in the first W/T campaign have been developed and provided by the GRC Consortium. In this baseline campaign 3 different intake variants have been tested according to a measurement matrix which was specified together with Airbus Helicopters Germany.
Baseline variant 1 was developed as a static inlet. Limitations on the inlet area cross section are imposed by engine plenum sizes. The second baseline variant of the intake was designed as a “semi-dynamic” intake. In contrast to the baseline variant 1 it features a ramp as part of the intake for recovery of dynamic pressure. It has been optimized for level flight conditions. The third baseline air intake comprises the same main geometry of baseline intake 2. Furthermore, it features a scoop. The scoop defines a section perpendicular to the main flow direction the incoming air has to pass to enter the engine air intake. It guides the free stream flow into the intake and recovers more dynamic into static pressure from the inlet section up to the compressor. Two different engine plenum chambers were tested. The baseline plenum chamber 2 features an overall rounded shape compared to plenum chamber 1. The first intake variant was tested with the first plenum chamber. Both intakes two and three were tested with plenum chamber two. For the first two combinations of baseline intakes and plenum chamber variations with and without an outer and inner foreign object damage grid were investigated.
Prior to the wind tunnel campaign preliminary testing of the suction system, the 5-hole probes and their traversing system was conducted at TUM. The production measurements of the first wind tunnel campaign were performed at the W/T A facility of TUM due date.
The first campaign was meant to provide a database of surface pressures and 3 component velocity data as well as total pressure data at the AIP. As a part of work reporting period 2 a detailed analysis of the data was carried out in order to identify optimization potential for further improvement of the baseline geometries using retrofit solutions.
With respect to the 3 different intake variants tested in the first measurement campaign, the baseline variant 2 (BSL 2) has been chosen as basis for the optimization.
Based on the detailed analyses of the results of the 1st wind tunnel measurement campaign, retrofit variants, namely a rear spoiler (small scoop), inlet guide vane and a combination of both, were investigated in the 2nd wind tunnel measurement campaign.
First, four different rear spoilers were tested in combination with the baseline 2 intake to assess the best combination of the height and length of the rear spoiler. In general, an increase of the height increases the cross section at the intake, thus increasing the ram effect. A decrease of the spoiler length leads to a bigger leading edge radius as well as to a shorter distance for which the flow must trail on the upper side of the spoiler in the hovering condition.
Both measures led to the desired effect to combine the beneficial operating conditions of baseline intake 2 and 3.
Project Context and Objectives:
For a reduction of emissions in air transport, the CleanSky program was started by the European Commission in cooperation with the European Aeronautical Industry. The reduction of emissions is clearly one of - if not - the most challenging task of our society and the aeronautical industry today. Within CleanSky environmental issues in the rotorcraft domain are addressed by the Green Rotorcraft Integrated Technology Demonstrator (GRC ITD). Even though, fixed wing aircraft generally outperform rotorcraft in fuel efficiency, range, speed and noise, rotorcraft are still of high importance.
There are several reasons for that. First, rotorcraft provide unique Vertical Take-Off and Landing (VTOL) capabilities. Thus, they can operate in areas with limited infrastructure or in airspace where other aircraft cannot. Second, they can excel in scenarios where economy of time is crucial, e.g. search and rescue (SAR) missions. Finally, rotorcraft are deployed when they outperform other applicable forms of transport in dynamic productivity, i.e. payload multiplied by range and speed divided by costs. Examples are the crew change on oil platforms or the transport of service personnel to offshore wind farms. To be able to provide these services at reduced environmental impact, measures are taken by the GRC to reduce emissions and increase fuel efficiency of rotorcraft.
In many cases, the missions described above are performed by light weight utility helicopters. Besides drag reduction, engine installation plays an important role to foster fuel-efficient engine operation and decrease emissions. Therefore, improving aerodynamic characteristics of helicopter engine installation is an important aim of GRC2.
To address this topic, in the ATHENAI (Aerodynamic Testing of Helicopter Novel Air Intakes) project the aerodynamic optimization of novel engine air intake concepts is conducted at the example of a light twin-engine helicopter with an emphasis on fast forward flight conditions. For this purpose, a new full scale model of a helicopter fuselage section has been designed which allows for the modular exchange of single model components, such as the intake cowling part or the engine plenum chamber. As part of the first wind tunnel campaign of the ATHENAI project a comprehensive database was created for three different side intake concepts. The database was used to understand the influence of the side intake concepts on engine entry parameters such as the total pressure loss and the total pressure distortion. These parameters were evaluated in the AIP (Aerodynamic Interface Plane) by means of a circumferentially adjustable 5-hole pressure probing system. Based on the results of the wind tunnel tests of the first wind tunnel campaign retrofit solutions, such as spoilers and guide vanes, were developed in the second testing period.
The aerodynamic optimization of the engine intake and engine air duct elements aims at improving the compressor inflow conditions which directly leads to a better use of the engine installed. The AIP was used as reference plane for the evaluation of these inflow conditions. In this plane, directly upstream of the compressor inlet, different parameters like the distortion parameter DC60 were analyzed. A reduction of total pressure losses results in an increase of static pressure in the compressor inlet which as a consequence raises the thermal efficiency.
A decrease in swirl and flow distortions leads to a more uniform flow field at the compressor inlet and therefore enhances the engines power output. As a result, at constant power requirements for single flight operating points all these factors lead to reduced fuel consumption and emissions. Furthermore, flow uniformity improves stable engine operation especially in the vicinity of the surge line.
Project Results:
In the course of the ATHENAI project successfully a novel wind tunnel model and setup was designed and integrated which can be used to conduct realistic helicopter intake tests in the wind tunnel A facility of the TUM.
The fuselage part model was designed in full scale to achieve local Re and Ma number similarity in the wind tunnel at a modest test section blockage.
As the engine mass flow has an essential effect on the internal flow upstream of the Aerodynamic Interface Plane (AIP), suitable mass flow rates corresponding to inflight operation conditions were simulated in the wind tunnel. For this purpose a radial fan was connected to the internal components of the model via a duct system. The fan as well as the duct system and its components have been chosen early in the design process of the W/T model taking into account the compensation of total pressure losses created by the intake geometry and the duct system. A venturi meter was integrated to allow for the measurement of mass flow rates. For the mass flow adjustment the rotation speed of the fan was regulated.
In order to optimize engine inflow conditions, total pressures and the 3 velocity components have been obtained in the AIP to determine aerodynamic engine installation parameters, such as total pressure losses, pressure distortion (e.g. DC60) and swirl. For this purpose, 5-hole probes were integrated in a rake at four radial positions. The rake was mounted on a shaft which is driven by a stepper motor and allowed for the measurement in different circumferential positions.
The first wind tunnel campaign of the project was meant to provide an extensive database of surface pressures and 3 component velocity data as well as total pressure data at the AIP. In this campaign, a standard side intake (baseline intake 1), a semi-dynamic side intake (baseline intake 2) and a dynamic side intake (baseline intake 3) were tested and compared to each other.
The comparison of the flow characteristics of the baseline intakes showed the following trends:
The DC60 of baseline intake 3 is nearly constant in the tested velocity range, baseline intake 1/2 show similar trends with a linear increase starting from U∞/ U∞, max = 0.3. The swirl shows only very small differences. The pressure drop showed a negative gradient for increased velocities for all intakes. Referring to the total pressure losses, the baseline intake 3 is the best intake for freestream velocities beyond U∞/ U∞, max = 0.45 and the worst in hover flight conditions.
The BSL Intake 1 / 2 / 3 comparison of angles of yaw showed that the differences are small. All the DC60, swirl and pressure drop improve with increased AoY. Here also the BSL 3 showed the smallest differences for all coefficients.
In the comparison of the intakes with and without intake grids, the following trends appeared: The comparison of the DC60 showed that the intake grids diminish the distortion. The BSL 1 with the intake grid has a smaller pt - distortion in the AIP than the baseline intake 2 with the intake grid. The differences of the swirl were small. For the baseline intake 2 the grid improved the swirl only for high freestream velocities , the baseline intake 1’s grid improved the swirl for the entire velocity spectrum tested. The intake grid led to an additional increase of total pressure losses. The baseline intake 2 grid produced higher pressure losses than the baseline intake 1 grid. Therefore, also an influence of the intake grid could be found. Investigations of the surface pressure distributions showed that the BSL 2’s intake grid led to a separation on the ramp of the intake and therefore disturbed the recompression on this part upstream of the inlet entry.
Already in the first wind tunnel campaign, a first retrofit variant was tested. Starting from the baseline intake 2 variant with the intake grid, plenum guide vanes at three different positions were tested with a constant distance to the engine grid. For the best guide vane position another two different heights have been tested to evaluate the height influence (75% and 50% relative to the original height of the guide vane at position 2). The baseline intake 2 plenum guide vane investigation substantiated that the position is the crucial factor. The GV 21 was the best configuration for all the DC60/ Swirl/ Pressure drop coefficients. The GV 22 with its height of 75% compared to the GV 21 was the next best plenum guide vane.
The “semi-dynamic” BSL 2 intake exhibited the hover flight benefits of the “static” BSL 1 intake evaluated with the DC60, swirl and pressure drop coefficients. This is due to the completely uncovered intake opening. Because of the ramp the high speed level flight advantages of the BSL 3 intake were also partly incorporated (high DC60, low pressure drop). The ramp led to a recovery of static pressure from the dynamic pressure of the incoming flow.
After the successful completion of the first reporting period, based on the results obtained in the baseline wind tunnel campaign, retrofit solutions have been developed to be tested in the second reporting period. With respect to the 3 different intake variants tested in the first measurement campaign, the baseline variant 2 (BSL 2) has been chosen as basis for the optimization.
The two main retrofit solutions derived from the baseline campaign have been a rear spoiler which was aimed at producing a better recompression similar to a small scoop and an inlet guide vane the purpose of which was to support the flow deflection around the front facing inlet edge. Still, an emphasis was put on level flight conditions (high U∞).
In total, 14 combinations of retrofit solutions have been tested. Furthermore, several modifications of the intake and engine grids have been tested. The investigations have been conducted at 24 combinations of engine mass flow rates and freestream velocities. In total, in the second W/T campaign 25% more test points were conducted as originally planned (299 instead of 222).
As a first grid modification investigation, the baseline intakes 1, 2, and 3 were tested with a grid mount element (GMo), which is used for the fixation of the engine grid in real flight operations.
The comparison of the flow characteristics of the baseline intakes in combination with the additional grid mount element showed the following trends:
For the baseline 1 intake, the DC60 was reduced, in the case of the baseline 2 variant slightly increased and for the third intake the effect varies over the tested velocity range. The swirl was more constant for the first intake with the grid mount element and a decrease of 0.2 - 0.5 ° was noticeable for the intakes 2 and 3. For the baseline 1, the total pressure losses were increased and for the other intakes slightly decreased, but the differences were very small.
The investigation of the retrofit aerodynamic modifications resulted in the following trends: Except of the hover flight case where the rear spoilers produced a stronger flow deflection, globally the total pressure distortion was reduced. Especially the high and short rear spoiler version (RS30) showed the lowest DC60.The rear spoilers slightly increased the swirl globally compared to the „clean“ BSL 2 geometry, but generally they were better than the third baseline intake concerning the distortion. Due to the best ram effect, the high versions produced the lowest pressure drop in level flight conditions and the short versions featured the best results for hover flight and the low velocity regime. Thus, the RS30 as a high and short version was the best compromise.
The investigation of the RS30 as the best “pure” rear spoiler solution combined with an inlet guide vane at three positions has produced the following results:
Both the detrimental effect of the rear spoiler in hover flight conditions and the beneficial effect for level flight were further increased. The ram effect in level flight for the combination of the rear spoiler 30 with the front inlet guide vane position (best configuration) was nearly as high as that of the baseline intake 3.
Based on the comparison of the intake grid’s influence in the first wind tunnel campaign a new modified intake grid for the baseline 2 intake was developed. The examination of the modified intake grid for the baseline 2 intake showed some potential in the grid attachment. A smoother integration on the intake’s ramp combined with neglecting the frontal frame elements led to a halving of the total pressure losses over the entire velocity range compared to the baseline grid. Concerning the distortion, the modified grid nearly featured the low DC60 level of the baseline grid. It also showed the smallest swirl.
Furthermore, an intake grid for the baseline 3 intake was integrated and tested in the second wind tunnel campaign of the project. The intake grid of the baseline 3 intake did not change the distortion noticeably, the swirl was slightly decreased, but in analogy with the baseline 2’s intake the pressure drop was clearly increased due to the grid. The pressure distributions on the cowling showed a static pressure drop with a similar magnitude as in the Aerodynamic Interface Plane already directly downstream of the grid.
Some additional investigations were conducted. For example the combination of the baseline 1 intake with the baseline 2 rounded plenum chamber was tested. The level of the DC60, swirl and pressure drop for the combination of baseline intake 1 with plenum chamber 2 was generally in between that of the “pure” baseline 1 / 2 variants. The examination showed that for fast level flight the total pressure increase in the Aerodynamic interface plane was mainly caused by the ramp on the cowling. For the hover flight case the beneficial effect of the rounded plenum chamber was comparable.
The PIV measurements conducted for a closed and standard open baseline 1 intake applying two mass flow rates led to very small differences in the velocity distributions for the different angles of yaw in the two planes upstream of the intake. Therefore, the results obtained for the geometric optimization process are valid for a range of angles of yaw (angles of attack for the real helicopter). Furthermore, a valuable database was achieved for future comparisons with CFD simulations of the truncated ATHENAI section as well as complete fuselage simulations.
Potential Impact:
The scientific and technical progress, achieved in the ATHENAI project, has well been disseminated through different ways of dissemination. On industry level, ATHENAI representative promoted the benefits of the investigated components through regular progress meetings. In addition, the partner supported the project leader Airbus Helicopters Deutschland GmbH in the preparation and execution of preliminary and critical design review meetings. This lead to the successful achievement of technical readiness level 4 for the selected modifications investigated through wind tunnel experiments in the course of the project (TRL4, i.e. component and/or breadboard validation in laboratory environment). Considering the successful flight tests on the bluecopter technology demonstrator in 2015, even technology readiness level 6 was reached for the baseline 1 intake geometry tested in the wind tunnel in the first ATHENAI wind tunnel campaign (TRL 6, i.e. system/subsystem model or prototype demonstration in a relevant environment).
The scientific community has been addressed through the attendance of ATHENAI representatives at conferences at national and international level. This work is also documented through the publication of conference papers. The list below summarises these efforts.
ERF 2015 F. Knoth, J.-H. You, and C. Breitsamter:
Aerodynamic Analysis of Helicopter Side Intake Variants by Full Scale Wind Tunnel
Measurements.
In: ERF2015-1.8.3 41st European Rotorcraft Forum, Munich, Germany, Sept. 1-4, 2015, pp.
1-11.
DGLR 2015 F. Knoth, M. Stuhlpfarrer, and C. Breitsamter:
Numerical and Experimental Investigations of Helicopter Engine Air Intakes.
In: DGLRK2015–0215, German Aerospace Congress, Rostock, 22.-24. Sept. 2015, pp. 1-10.
The general public was informed about the objectives and achievements through the project website (http://www.athenai.tum.de) and on the occasion of open house presentations. Furthermore, a representative of the Institute of Aerodynamics and Fluid Mechanics of TUM represented the ATHENAI project at the International Aerospace Exhibition (ILA) in Berlin 2014.
The three different intakes tested in the first wind tunnel campaign will be tested in the bluecopter technology demonstrator and will possible improve a new generation of light twin engine helicopters.
The improvement of the engine’s efficiency due to aerodynamically optimized engine integration could either be used for faster flight or for flying longer distances with the same amount of fuel. This is beneficial for a number of missions.
List of Websites:
http://www.athenai.tum.de
The European Union has launched the CleanSky initiative together with the European aerospace industry aimed at a reduction of emissions and fuel burn. The engine installation plays an important role to foster fuel-efficient engine operation and decrease emissions. The sub-project GRC2 of the Green Rotorcraft ITD (Integrated Technology Demonstrator) aims at improving the aerodynamic characteristics of helicopter and tiltrotor aircraft fuselages and engine installation. In this context, the investigation and optimization of engine installation plays an important role to ensure and foster fuel-efficient engine operation. To cover a wide range of the future helicopter fleet, weight classes from light to heavy are treated within GRC2. Numerical optimization of several configurations is carried out by means of Computational Fluid Dynamics (CFD) and expected benefits of identified solutions are assessed through wind tunnel tests.
Comprehensive wind tunnel tests were performed on a full scale model of a helicopter fuselage section at the Institute of Aerodynamics and Fluid Mechanics of the Technische Universität München (TUM-AER). For that purpose a new wind tunnel model of a lightweight twin-engine helicopter comprising a fuselage cowling part, all air intake components as well as flow passages was designed, manufactured and instrumented.
The model features a high degree of modularity for easy comparison of various air intake components. In two wind tunnel campaigns aerodynamic characteristics of three baseline engine air intake configurations and several design variants were analyzed in detail. The design modifications concentrate on flow guiding elements like fillets, vanes, spoilers and scoops. With a five-hole pressure probing system flow field measurements were conducted at the aerodynamic interface plane (AIP). Additionally, steady and unsteady surface pressures on cowling and air intake regions are recorded. The incoming flow was investigated by field measurements of mean and fluctuating velocities. Data analysis and evaluation provided a valuable database and guidelines for helicopter air intake design.
The geometric variations tested in the first W/T campaign have been developed and provided by the GRC Consortium. In this baseline campaign 3 different intake variants have been tested according to a measurement matrix which was specified together with Airbus Helicopters Germany.
Baseline variant 1 was developed as a static inlet. Limitations on the inlet area cross section are imposed by engine plenum sizes. The second baseline variant of the intake was designed as a “semi-dynamic” intake. In contrast to the baseline variant 1 it features a ramp as part of the intake for recovery of dynamic pressure. It has been optimized for level flight conditions. The third baseline air intake comprises the same main geometry of baseline intake 2. Furthermore, it features a scoop. The scoop defines a section perpendicular to the main flow direction the incoming air has to pass to enter the engine air intake. It guides the free stream flow into the intake and recovers more dynamic into static pressure from the inlet section up to the compressor. Two different engine plenum chambers were tested. The baseline plenum chamber 2 features an overall rounded shape compared to plenum chamber 1. The first intake variant was tested with the first plenum chamber. Both intakes two and three were tested with plenum chamber two. For the first two combinations of baseline intakes and plenum chamber variations with and without an outer and inner foreign object damage grid were investigated.
Prior to the wind tunnel campaign preliminary testing of the suction system, the 5-hole probes and their traversing system was conducted at TUM. The production measurements of the first wind tunnel campaign were performed at the W/T A facility of TUM due date.
The first campaign was meant to provide a database of surface pressures and 3 component velocity data as well as total pressure data at the AIP. As a part of work reporting period 2 a detailed analysis of the data was carried out in order to identify optimization potential for further improvement of the baseline geometries using retrofit solutions.
With respect to the 3 different intake variants tested in the first measurement campaign, the baseline variant 2 (BSL 2) has been chosen as basis for the optimization.
Based on the detailed analyses of the results of the 1st wind tunnel measurement campaign, retrofit variants, namely a rear spoiler (small scoop), inlet guide vane and a combination of both, were investigated in the 2nd wind tunnel measurement campaign.
First, four different rear spoilers were tested in combination with the baseline 2 intake to assess the best combination of the height and length of the rear spoiler. In general, an increase of the height increases the cross section at the intake, thus increasing the ram effect. A decrease of the spoiler length leads to a bigger leading edge radius as well as to a shorter distance for which the flow must trail on the upper side of the spoiler in the hovering condition.
Both measures led to the desired effect to combine the beneficial operating conditions of baseline intake 2 and 3.
Project Context and Objectives:
For a reduction of emissions in air transport, the CleanSky program was started by the European Commission in cooperation with the European Aeronautical Industry. The reduction of emissions is clearly one of - if not - the most challenging task of our society and the aeronautical industry today. Within CleanSky environmental issues in the rotorcraft domain are addressed by the Green Rotorcraft Integrated Technology Demonstrator (GRC ITD). Even though, fixed wing aircraft generally outperform rotorcraft in fuel efficiency, range, speed and noise, rotorcraft are still of high importance.
There are several reasons for that. First, rotorcraft provide unique Vertical Take-Off and Landing (VTOL) capabilities. Thus, they can operate in areas with limited infrastructure or in airspace where other aircraft cannot. Second, they can excel in scenarios where economy of time is crucial, e.g. search and rescue (SAR) missions. Finally, rotorcraft are deployed when they outperform other applicable forms of transport in dynamic productivity, i.e. payload multiplied by range and speed divided by costs. Examples are the crew change on oil platforms or the transport of service personnel to offshore wind farms. To be able to provide these services at reduced environmental impact, measures are taken by the GRC to reduce emissions and increase fuel efficiency of rotorcraft.
In many cases, the missions described above are performed by light weight utility helicopters. Besides drag reduction, engine installation plays an important role to foster fuel-efficient engine operation and decrease emissions. Therefore, improving aerodynamic characteristics of helicopter engine installation is an important aim of GRC2.
To address this topic, in the ATHENAI (Aerodynamic Testing of Helicopter Novel Air Intakes) project the aerodynamic optimization of novel engine air intake concepts is conducted at the example of a light twin-engine helicopter with an emphasis on fast forward flight conditions. For this purpose, a new full scale model of a helicopter fuselage section has been designed which allows for the modular exchange of single model components, such as the intake cowling part or the engine plenum chamber. As part of the first wind tunnel campaign of the ATHENAI project a comprehensive database was created for three different side intake concepts. The database was used to understand the influence of the side intake concepts on engine entry parameters such as the total pressure loss and the total pressure distortion. These parameters were evaluated in the AIP (Aerodynamic Interface Plane) by means of a circumferentially adjustable 5-hole pressure probing system. Based on the results of the wind tunnel tests of the first wind tunnel campaign retrofit solutions, such as spoilers and guide vanes, were developed in the second testing period.
The aerodynamic optimization of the engine intake and engine air duct elements aims at improving the compressor inflow conditions which directly leads to a better use of the engine installed. The AIP was used as reference plane for the evaluation of these inflow conditions. In this plane, directly upstream of the compressor inlet, different parameters like the distortion parameter DC60 were analyzed. A reduction of total pressure losses results in an increase of static pressure in the compressor inlet which as a consequence raises the thermal efficiency.
A decrease in swirl and flow distortions leads to a more uniform flow field at the compressor inlet and therefore enhances the engines power output. As a result, at constant power requirements for single flight operating points all these factors lead to reduced fuel consumption and emissions. Furthermore, flow uniformity improves stable engine operation especially in the vicinity of the surge line.
Project Results:
In the course of the ATHENAI project successfully a novel wind tunnel model and setup was designed and integrated which can be used to conduct realistic helicopter intake tests in the wind tunnel A facility of the TUM.
The fuselage part model was designed in full scale to achieve local Re and Ma number similarity in the wind tunnel at a modest test section blockage.
As the engine mass flow has an essential effect on the internal flow upstream of the Aerodynamic Interface Plane (AIP), suitable mass flow rates corresponding to inflight operation conditions were simulated in the wind tunnel. For this purpose a radial fan was connected to the internal components of the model via a duct system. The fan as well as the duct system and its components have been chosen early in the design process of the W/T model taking into account the compensation of total pressure losses created by the intake geometry and the duct system. A venturi meter was integrated to allow for the measurement of mass flow rates. For the mass flow adjustment the rotation speed of the fan was regulated.
In order to optimize engine inflow conditions, total pressures and the 3 velocity components have been obtained in the AIP to determine aerodynamic engine installation parameters, such as total pressure losses, pressure distortion (e.g. DC60) and swirl. For this purpose, 5-hole probes were integrated in a rake at four radial positions. The rake was mounted on a shaft which is driven by a stepper motor and allowed for the measurement in different circumferential positions.
The first wind tunnel campaign of the project was meant to provide an extensive database of surface pressures and 3 component velocity data as well as total pressure data at the AIP. In this campaign, a standard side intake (baseline intake 1), a semi-dynamic side intake (baseline intake 2) and a dynamic side intake (baseline intake 3) were tested and compared to each other.
The comparison of the flow characteristics of the baseline intakes showed the following trends:
The DC60 of baseline intake 3 is nearly constant in the tested velocity range, baseline intake 1/2 show similar trends with a linear increase starting from U∞/ U∞, max = 0.3. The swirl shows only very small differences. The pressure drop showed a negative gradient for increased velocities for all intakes. Referring to the total pressure losses, the baseline intake 3 is the best intake for freestream velocities beyond U∞/ U∞, max = 0.45 and the worst in hover flight conditions.
The BSL Intake 1 / 2 / 3 comparison of angles of yaw showed that the differences are small. All the DC60, swirl and pressure drop improve with increased AoY. Here also the BSL 3 showed the smallest differences for all coefficients.
In the comparison of the intakes with and without intake grids, the following trends appeared: The comparison of the DC60 showed that the intake grids diminish the distortion. The BSL 1 with the intake grid has a smaller pt - distortion in the AIP than the baseline intake 2 with the intake grid. The differences of the swirl were small. For the baseline intake 2 the grid improved the swirl only for high freestream velocities , the baseline intake 1’s grid improved the swirl for the entire velocity spectrum tested. The intake grid led to an additional increase of total pressure losses. The baseline intake 2 grid produced higher pressure losses than the baseline intake 1 grid. Therefore, also an influence of the intake grid could be found. Investigations of the surface pressure distributions showed that the BSL 2’s intake grid led to a separation on the ramp of the intake and therefore disturbed the recompression on this part upstream of the inlet entry.
Already in the first wind tunnel campaign, a first retrofit variant was tested. Starting from the baseline intake 2 variant with the intake grid, plenum guide vanes at three different positions were tested with a constant distance to the engine grid. For the best guide vane position another two different heights have been tested to evaluate the height influence (75% and 50% relative to the original height of the guide vane at position 2). The baseline intake 2 plenum guide vane investigation substantiated that the position is the crucial factor. The GV 21 was the best configuration for all the DC60/ Swirl/ Pressure drop coefficients. The GV 22 with its height of 75% compared to the GV 21 was the next best plenum guide vane.
The “semi-dynamic” BSL 2 intake exhibited the hover flight benefits of the “static” BSL 1 intake evaluated with the DC60, swirl and pressure drop coefficients. This is due to the completely uncovered intake opening. Because of the ramp the high speed level flight advantages of the BSL 3 intake were also partly incorporated (high DC60, low pressure drop). The ramp led to a recovery of static pressure from the dynamic pressure of the incoming flow.
After the successful completion of the first reporting period, based on the results obtained in the baseline wind tunnel campaign, retrofit solutions have been developed to be tested in the second reporting period. With respect to the 3 different intake variants tested in the first measurement campaign, the baseline variant 2 (BSL 2) has been chosen as basis for the optimization.
The two main retrofit solutions derived from the baseline campaign have been a rear spoiler which was aimed at producing a better recompression similar to a small scoop and an inlet guide vane the purpose of which was to support the flow deflection around the front facing inlet edge. Still, an emphasis was put on level flight conditions (high U∞).
In total, 14 combinations of retrofit solutions have been tested. Furthermore, several modifications of the intake and engine grids have been tested. The investigations have been conducted at 24 combinations of engine mass flow rates and freestream velocities. In total, in the second W/T campaign 25% more test points were conducted as originally planned (299 instead of 222).
As a first grid modification investigation, the baseline intakes 1, 2, and 3 were tested with a grid mount element (GMo), which is used for the fixation of the engine grid in real flight operations.
The comparison of the flow characteristics of the baseline intakes in combination with the additional grid mount element showed the following trends:
For the baseline 1 intake, the DC60 was reduced, in the case of the baseline 2 variant slightly increased and for the third intake the effect varies over the tested velocity range. The swirl was more constant for the first intake with the grid mount element and a decrease of 0.2 - 0.5 ° was noticeable for the intakes 2 and 3. For the baseline 1, the total pressure losses were increased and for the other intakes slightly decreased, but the differences were very small.
The investigation of the retrofit aerodynamic modifications resulted in the following trends: Except of the hover flight case where the rear spoilers produced a stronger flow deflection, globally the total pressure distortion was reduced. Especially the high and short rear spoiler version (RS30) showed the lowest DC60.The rear spoilers slightly increased the swirl globally compared to the „clean“ BSL 2 geometry, but generally they were better than the third baseline intake concerning the distortion. Due to the best ram effect, the high versions produced the lowest pressure drop in level flight conditions and the short versions featured the best results for hover flight and the low velocity regime. Thus, the RS30 as a high and short version was the best compromise.
The investigation of the RS30 as the best “pure” rear spoiler solution combined with an inlet guide vane at three positions has produced the following results:
Both the detrimental effect of the rear spoiler in hover flight conditions and the beneficial effect for level flight were further increased. The ram effect in level flight for the combination of the rear spoiler 30 with the front inlet guide vane position (best configuration) was nearly as high as that of the baseline intake 3.
Based on the comparison of the intake grid’s influence in the first wind tunnel campaign a new modified intake grid for the baseline 2 intake was developed. The examination of the modified intake grid for the baseline 2 intake showed some potential in the grid attachment. A smoother integration on the intake’s ramp combined with neglecting the frontal frame elements led to a halving of the total pressure losses over the entire velocity range compared to the baseline grid. Concerning the distortion, the modified grid nearly featured the low DC60 level of the baseline grid. It also showed the smallest swirl.
Furthermore, an intake grid for the baseline 3 intake was integrated and tested in the second wind tunnel campaign of the project. The intake grid of the baseline 3 intake did not change the distortion noticeably, the swirl was slightly decreased, but in analogy with the baseline 2’s intake the pressure drop was clearly increased due to the grid. The pressure distributions on the cowling showed a static pressure drop with a similar magnitude as in the Aerodynamic Interface Plane already directly downstream of the grid.
Some additional investigations were conducted. For example the combination of the baseline 1 intake with the baseline 2 rounded plenum chamber was tested. The level of the DC60, swirl and pressure drop for the combination of baseline intake 1 with plenum chamber 2 was generally in between that of the “pure” baseline 1 / 2 variants. The examination showed that for fast level flight the total pressure increase in the Aerodynamic interface plane was mainly caused by the ramp on the cowling. For the hover flight case the beneficial effect of the rounded plenum chamber was comparable.
The PIV measurements conducted for a closed and standard open baseline 1 intake applying two mass flow rates led to very small differences in the velocity distributions for the different angles of yaw in the two planes upstream of the intake. Therefore, the results obtained for the geometric optimization process are valid for a range of angles of yaw (angles of attack for the real helicopter). Furthermore, a valuable database was achieved for future comparisons with CFD simulations of the truncated ATHENAI section as well as complete fuselage simulations.
Potential Impact:
The scientific and technical progress, achieved in the ATHENAI project, has well been disseminated through different ways of dissemination. On industry level, ATHENAI representative promoted the benefits of the investigated components through regular progress meetings. In addition, the partner supported the project leader Airbus Helicopters Deutschland GmbH in the preparation and execution of preliminary and critical design review meetings. This lead to the successful achievement of technical readiness level 4 for the selected modifications investigated through wind tunnel experiments in the course of the project (TRL4, i.e. component and/or breadboard validation in laboratory environment). Considering the successful flight tests on the bluecopter technology demonstrator in 2015, even technology readiness level 6 was reached for the baseline 1 intake geometry tested in the wind tunnel in the first ATHENAI wind tunnel campaign (TRL 6, i.e. system/subsystem model or prototype demonstration in a relevant environment).
The scientific community has been addressed through the attendance of ATHENAI representatives at conferences at national and international level. This work is also documented through the publication of conference papers. The list below summarises these efforts.
ERF 2015 F. Knoth, J.-H. You, and C. Breitsamter:
Aerodynamic Analysis of Helicopter Side Intake Variants by Full Scale Wind Tunnel
Measurements.
In: ERF2015-1.8.3 41st European Rotorcraft Forum, Munich, Germany, Sept. 1-4, 2015, pp.
1-11.
DGLR 2015 F. Knoth, M. Stuhlpfarrer, and C. Breitsamter:
Numerical and Experimental Investigations of Helicopter Engine Air Intakes.
In: DGLRK2015–0215, German Aerospace Congress, Rostock, 22.-24. Sept. 2015, pp. 1-10.
The general public was informed about the objectives and achievements through the project website (http://www.athenai.tum.de) and on the occasion of open house presentations. Furthermore, a representative of the Institute of Aerodynamics and Fluid Mechanics of TUM represented the ATHENAI project at the International Aerospace Exhibition (ILA) in Berlin 2014.
The three different intakes tested in the first wind tunnel campaign will be tested in the bluecopter technology demonstrator and will possible improve a new generation of light twin engine helicopters.
The improvement of the engine’s efficiency due to aerodynamically optimized engine integration could either be used for faster flight or for flying longer distances with the same amount of fuel. This is beneficial for a number of missions.
List of Websites:
http://www.athenai.tum.de