Final Report Summary - FRIENDCOPTER (Integration of technologies in support of a passenger and environmentally friendly helicopter)
Helicopters generate external noise, cabin noise and vibrations due to the complex nature of their dynamic systems. They suffer from NOx emission, like other transport systems. New generation rotorcraft addresses these issues trying to become environmentally friendly and acceptable to the general public. On the other hand, modern society needs the helicopter due to its ability to fly medical, rescue and law enforcement missions.
The project FRIENDCOPTER was intended support this process by addressing the following goals:
- acoustic footprint areas reduced between 30 % and 50 % depending on the flight condition;
- a reduction of fuel consumption up to 6 % for high speed flights;
- cabin noise levels near 75 dBA similar to airliner cabins for normal cruise flight;
- cabin vibrations below 0,05 g corresponding to jet smooth ride comfort for the same flight regime.
Because of the large and fast rotating rotor, the non-symmetric rotor flow, the close vicinity of main gearbox and passenger heads and the specific requests of maximum engine performance, the targets mentioned above were highly challenging, requiring a strong high tech initiative.
The main challenges of today's helicopters with respect to environmental compliance and public acceptance are:
- annoying external noise emissions in form of the so-called blade slapping, especially during descent - i.e. near the ground - and during flight manoeuvres;
- compressor and combustion noise stemming from the engines;
- significant NOx emissions by these engines;
- high cabin noise levels provoked mainly by the unsteady forces at the main gearbox wheel teeth; and
- significant vibration levels caused by the non-symmetrical and unsteady rotor aerodynamics.
The project was structured into six work packages (WPs), as follows:
WP1 - Specifications
The objectives of WP1 were:
- to ensure a well defined and harmonised start of the project by confirming and quantifying the project targets and key deliverables envisaged through the main helicopter manufacturers and the engine manufacturer;
- to provide guidelines for an overall integrated research approach on development and operation of environmentally friendly helicopters by specifying the methods to be applied;
- to take precautions against technical imponderabilities by a risk assessment with respect to the methods applied and by identification of potential alternative procedures.
The following methods were used:
- the provision of guidelines for the work planning of the other WPs;
- rough quantification of the final results expected in the different WPs;
- specifications of the methods planned including risk assessment.
A specification report defining targets and methods of the programme was issued.
WP2 - Noise abatement flight procedures
WP2 was concerned with the reduction of noise on the ground through operational means (noise abatement procedures), and the prediction of the acoustic impact of such rotorcraft flight operations. Specifically, WP2 aimed to:
- develop methodologies for the design of operationally viable noise abatement procedures;
- design and validate the necessary prediction tools to assess the acoustic impact of rotorcraft flight procedures on the ground and assist in the development of low-noise flight procedures.
In terms of noise reduction, the objective was to reduce acoustic footprint areas between 30 % and 50 % depending on the flight condition.
The tasks within this WP were split into three main topics:
1. identification of the needs and specific objectives relating to noise abatement procedures;
2. development of the models needed to predict the noise emitted by helicopters during various phases of flight and the tools necessary to design low-noise flight procedures; and
3. full scale flight tests on different helicopter models in order to provide noise data to the prediction models and assess the low-noise procedures.
The achievements of the WP can be summarised as follows:
- A new noise footprint model was developed and validated (HELENA). This model takes into account the complexity of noise generation specific to rotorcraft.
- Fully automatic optimisation routines were demonstrated for the development of noise abatement procedures.
- Extensive noise measurement means were deployed and guidance was provided for future noise campaigns.
- Flight guidance hardware dedicated to noise abatement procedure was developed and successfully flight tested.
- Considerable noise footprint reduction was demonstrated on all three aircraft through the use of noise abatement procedures (up to 50 % noise footprint reduction, more than 10 dB reduction at specific observer locations).
WP3 - Engine noise reduction.
The objective of WP3 was to reduce a component of helicopter noise, relevant mainly during take-off and climb, i.e. the noise emitted by the helicopter engines. To accomplish this, the goal was to reduce both the noise emitted by the air intake (compressor noise) and the one stemming from the engine's outlet nozzle. The new design should also reduce the existing engine installation losses (pressure losses of a plenum chamber-type engine inlet, particularly by unintended temperature increase through contiguous ancillary units. The performance gained, hereby, was to compensate the take-off mass increase evoked by the acoustic technologies envisaged. Hence, two noise sources had to be addressed. The corresponding approaches started off with engine static tests and ended with flight tests.
As final deliverables, there were expected:
- design proposals for appropriate inlet and outlet geometries;
- liners for quiet air intakes and exhaust nozzles;
- related recommendations for airworthiness and performance aspects.
There are currently no turboshaft silent technologies flying on serial helicopters. Therefore, the demonstrated technologies have no known equivalent in helicopter industry. The exhaust liner made of titanium alloy beneficiated from the know-how acquired during the EU-funded HORTIA program as well as from a technology transfer from the airplane industry.
The new manufacturing process developed in FRIENDCOPTER for integrating acoustic liners in composite material reduces the manufacturing time and costs, increasing the technology's attractiveness and enabling the product to be offered to a broader range of potential applications.
An important outcome of the project was also the improvement of the state of the art measurement techniques through the use of acoustic antennas that allow the development of new methodologies to determine engine acoustic power both during static tests and in flight.
WP4 - Cabin noise reduction
The objectives of WP4 were to enhance the helicopter cabin interior environment of present and new helicopters by increasing the ride comfort of the passengers or in the case of medical transport or emergency missions by facilitating the conversation between medical staff and patients.
The expected results in WP4 were to:
- identify acoustical leaks and noise transmission paths;
- tackle the dominating cabin noise source at its origin, i.e. the main gearbox noise;
- actively interrupt the transmission of structure-borne noise from gearbox to fuselage;
- actively and passively damp by cabin trim panels the noise having overcome the above barriers and reached the cabin structure.
A number of methodologies and approaches were used:
- Experimentally validated methods / technologies to identify acoustical leaks and noise transmission paths have been tested on real helicopter cabins.
- To reduce the dominating cabin noise source at its origin, the serial gearboxes of the Agusta A109 and the Eurocopter EC135 have been modified. At the A109, mainly gear profiles have been optimised and tolerances minimised while at the EC135 gearbox, mass and stiffness tuning as well as a reduction of the oil cooler fan noise have been main issues.
- Active and passive technologies have been used to interrupt the transmission of structure-borne noise from the gearbox to the fuselage and to damp the noise which has overcome the above barriers and has reached the cabin structure, by use of cabin trim panels.
The work performed in WP4 has improved partners' professional knowledge and their capabilities to apply advanced measurement systems and technologies. In particular, there should be mentioned:
- the matrix inversion method, stemming from ground transport systems;
- an SEA procedure extended to low frequencies;
- systems for noise mapping on cabin surfaces based on p-v sensor arrays;
- noise oriented gearbox modifications.
More mature active and passive means to interrupt structure-borne noise transfer as well as to damp the noise by advanced cabin panelling.
One of the most relevant achievements was, however, the analytical prove that drastic cabin noise reductions can be accomplished by proper superposition of a number of different abatement technologies, all demonstrated separately on the helicopter.
WP5 - Rotor noise control
WP5 was the most innovative task in FRIENDCOPTER demonstrating active rotor blade control technology by distributed actuation. In the low risk technology stream, an active twist rotor in model scale and a full scale blade segment with deformable active trailing edge were developed. In the high risk technology stream, more advanced airfoil shape morphing concepts and static blade twist based on shape memory alloy actuation were investigated for feasibility. In parallel to the detailed development and optimisation activity leading to hardware manufacturing and testing, the benefit potential of the said technologies was explored by numerical simulations.
An important aspect of the numerical activity was the conception of optimum control laws that yield maximum benefit for noise, vibration and performance. Active rotor control is expected to reduce rotor noise footprint areas by 30 % to 50 % depending on the flight condition, vibration levels shall be reduced by 90 % down to 0.05 g, power savings at the border of the flight envelope shall be realised that result in reduced fuel consumption by 6 %.
Active twist was a concept proposed by DLR. It deals with applying distributed piezo-active patches on upper and lower blade skin under approximately 45 degrees relative to the radial axis of the blade. The piezo-patches induce direct shear forces to the skin that create a torsion moment around the control axis resulting in an elastic twist deformation. This deformation can be created at elevated frequencies, enabling higher harmonic blade control required for noise, vibration and performance benefit.
The active trailing edge concept was proposed by EADS and ECD. Instead of implementing a discrete trailing edge flap, a chord and span wise distributed actuation was conceived.
Even more ambitious was the concept of fully variable airfoils shapes. The investigated ideas range from camber and thickness changes to local airfoil contour modifications. The challenge is to find feasible concepts that are compliant with the underlying blade structure with its specific global elastic and dynamic constraints.
The shape memory twist uses special metal alloys that have the ability to return to a previously defined shape. This specific behaviour stems from a phase shift in the actuator material's microscopic texture induced by heating or cooling the material. Since such materials show the ability to achieve very large strain, i.e. subsequently large deformations, that can be fully recovered, they are interesting for increasing the mean (static) twist of a rotor blade. Hereby, the actually applied compromise between high twist requirements for hover and low twist demands for cruise could be beneficially substituted.
The development efforts showed the following achievements:
- experimentally validated technologies for controlling the helicopter blades by distributed actuation;
- model rotor blades spin tested on a whirl rig under hover conditions with respect to their controllability of noise and vibrations;
- a full scale blade segment tested on bench for actuation integration and endurance;
- numerically approved concepts; and
- laboratory tests to support the feasibility investigations.
WP6 - Exploitation
The objective of WP 6 was to ensure an efficient exploitation of the achievements gained in the course of the programme. Individual goals have been:
- to summarise the know-how about the reduction potential of noise, vibrations and engine power consumption of existing and future helicopters by generating a related data base, including a synthesis of the results attained;
- to enable the exploitation of the environmental improvement results attained during the project in industry and research with the help of a dedicated exploitation plan;
- to get a broader view on the technologies in question by an exchange of know-how in the aforementioned areas with companies / institutions also outside the consortium.
The project FRIENDCOPTER was intended support this process by addressing the following goals:
- acoustic footprint areas reduced between 30 % and 50 % depending on the flight condition;
- a reduction of fuel consumption up to 6 % for high speed flights;
- cabin noise levels near 75 dBA similar to airliner cabins for normal cruise flight;
- cabin vibrations below 0,05 g corresponding to jet smooth ride comfort for the same flight regime.
Because of the large and fast rotating rotor, the non-symmetric rotor flow, the close vicinity of main gearbox and passenger heads and the specific requests of maximum engine performance, the targets mentioned above were highly challenging, requiring a strong high tech initiative.
The main challenges of today's helicopters with respect to environmental compliance and public acceptance are:
- annoying external noise emissions in form of the so-called blade slapping, especially during descent - i.e. near the ground - and during flight manoeuvres;
- compressor and combustion noise stemming from the engines;
- significant NOx emissions by these engines;
- high cabin noise levels provoked mainly by the unsteady forces at the main gearbox wheel teeth; and
- significant vibration levels caused by the non-symmetrical and unsteady rotor aerodynamics.
The project was structured into six work packages (WPs), as follows:
WP1 - Specifications
The objectives of WP1 were:
- to ensure a well defined and harmonised start of the project by confirming and quantifying the project targets and key deliverables envisaged through the main helicopter manufacturers and the engine manufacturer;
- to provide guidelines for an overall integrated research approach on development and operation of environmentally friendly helicopters by specifying the methods to be applied;
- to take precautions against technical imponderabilities by a risk assessment with respect to the methods applied and by identification of potential alternative procedures.
The following methods were used:
- the provision of guidelines for the work planning of the other WPs;
- rough quantification of the final results expected in the different WPs;
- specifications of the methods planned including risk assessment.
A specification report defining targets and methods of the programme was issued.
WP2 - Noise abatement flight procedures
WP2 was concerned with the reduction of noise on the ground through operational means (noise abatement procedures), and the prediction of the acoustic impact of such rotorcraft flight operations. Specifically, WP2 aimed to:
- develop methodologies for the design of operationally viable noise abatement procedures;
- design and validate the necessary prediction tools to assess the acoustic impact of rotorcraft flight procedures on the ground and assist in the development of low-noise flight procedures.
In terms of noise reduction, the objective was to reduce acoustic footprint areas between 30 % and 50 % depending on the flight condition.
The tasks within this WP were split into three main topics:
1. identification of the needs and specific objectives relating to noise abatement procedures;
2. development of the models needed to predict the noise emitted by helicopters during various phases of flight and the tools necessary to design low-noise flight procedures; and
3. full scale flight tests on different helicopter models in order to provide noise data to the prediction models and assess the low-noise procedures.
The achievements of the WP can be summarised as follows:
- A new noise footprint model was developed and validated (HELENA). This model takes into account the complexity of noise generation specific to rotorcraft.
- Fully automatic optimisation routines were demonstrated for the development of noise abatement procedures.
- Extensive noise measurement means were deployed and guidance was provided for future noise campaigns.
- Flight guidance hardware dedicated to noise abatement procedure was developed and successfully flight tested.
- Considerable noise footprint reduction was demonstrated on all three aircraft through the use of noise abatement procedures (up to 50 % noise footprint reduction, more than 10 dB reduction at specific observer locations).
WP3 - Engine noise reduction.
The objective of WP3 was to reduce a component of helicopter noise, relevant mainly during take-off and climb, i.e. the noise emitted by the helicopter engines. To accomplish this, the goal was to reduce both the noise emitted by the air intake (compressor noise) and the one stemming from the engine's outlet nozzle. The new design should also reduce the existing engine installation losses (pressure losses of a plenum chamber-type engine inlet, particularly by unintended temperature increase through contiguous ancillary units. The performance gained, hereby, was to compensate the take-off mass increase evoked by the acoustic technologies envisaged. Hence, two noise sources had to be addressed. The corresponding approaches started off with engine static tests and ended with flight tests.
As final deliverables, there were expected:
- design proposals for appropriate inlet and outlet geometries;
- liners for quiet air intakes and exhaust nozzles;
- related recommendations for airworthiness and performance aspects.
There are currently no turboshaft silent technologies flying on serial helicopters. Therefore, the demonstrated technologies have no known equivalent in helicopter industry. The exhaust liner made of titanium alloy beneficiated from the know-how acquired during the EU-funded HORTIA program as well as from a technology transfer from the airplane industry.
The new manufacturing process developed in FRIENDCOPTER for integrating acoustic liners in composite material reduces the manufacturing time and costs, increasing the technology's attractiveness and enabling the product to be offered to a broader range of potential applications.
An important outcome of the project was also the improvement of the state of the art measurement techniques through the use of acoustic antennas that allow the development of new methodologies to determine engine acoustic power both during static tests and in flight.
WP4 - Cabin noise reduction
The objectives of WP4 were to enhance the helicopter cabin interior environment of present and new helicopters by increasing the ride comfort of the passengers or in the case of medical transport or emergency missions by facilitating the conversation between medical staff and patients.
The expected results in WP4 were to:
- identify acoustical leaks and noise transmission paths;
- tackle the dominating cabin noise source at its origin, i.e. the main gearbox noise;
- actively interrupt the transmission of structure-borne noise from gearbox to fuselage;
- actively and passively damp by cabin trim panels the noise having overcome the above barriers and reached the cabin structure.
A number of methodologies and approaches were used:
- Experimentally validated methods / technologies to identify acoustical leaks and noise transmission paths have been tested on real helicopter cabins.
- To reduce the dominating cabin noise source at its origin, the serial gearboxes of the Agusta A109 and the Eurocopter EC135 have been modified. At the A109, mainly gear profiles have been optimised and tolerances minimised while at the EC135 gearbox, mass and stiffness tuning as well as a reduction of the oil cooler fan noise have been main issues.
- Active and passive technologies have been used to interrupt the transmission of structure-borne noise from the gearbox to the fuselage and to damp the noise which has overcome the above barriers and has reached the cabin structure, by use of cabin trim panels.
The work performed in WP4 has improved partners' professional knowledge and their capabilities to apply advanced measurement systems and technologies. In particular, there should be mentioned:
- the matrix inversion method, stemming from ground transport systems;
- an SEA procedure extended to low frequencies;
- systems for noise mapping on cabin surfaces based on p-v sensor arrays;
- noise oriented gearbox modifications.
More mature active and passive means to interrupt structure-borne noise transfer as well as to damp the noise by advanced cabin panelling.
One of the most relevant achievements was, however, the analytical prove that drastic cabin noise reductions can be accomplished by proper superposition of a number of different abatement technologies, all demonstrated separately on the helicopter.
WP5 - Rotor noise control
WP5 was the most innovative task in FRIENDCOPTER demonstrating active rotor blade control technology by distributed actuation. In the low risk technology stream, an active twist rotor in model scale and a full scale blade segment with deformable active trailing edge were developed. In the high risk technology stream, more advanced airfoil shape morphing concepts and static blade twist based on shape memory alloy actuation were investigated for feasibility. In parallel to the detailed development and optimisation activity leading to hardware manufacturing and testing, the benefit potential of the said technologies was explored by numerical simulations.
An important aspect of the numerical activity was the conception of optimum control laws that yield maximum benefit for noise, vibration and performance. Active rotor control is expected to reduce rotor noise footprint areas by 30 % to 50 % depending on the flight condition, vibration levels shall be reduced by 90 % down to 0.05 g, power savings at the border of the flight envelope shall be realised that result in reduced fuel consumption by 6 %.
Active twist was a concept proposed by DLR. It deals with applying distributed piezo-active patches on upper and lower blade skin under approximately 45 degrees relative to the radial axis of the blade. The piezo-patches induce direct shear forces to the skin that create a torsion moment around the control axis resulting in an elastic twist deformation. This deformation can be created at elevated frequencies, enabling higher harmonic blade control required for noise, vibration and performance benefit.
The active trailing edge concept was proposed by EADS and ECD. Instead of implementing a discrete trailing edge flap, a chord and span wise distributed actuation was conceived.
Even more ambitious was the concept of fully variable airfoils shapes. The investigated ideas range from camber and thickness changes to local airfoil contour modifications. The challenge is to find feasible concepts that are compliant with the underlying blade structure with its specific global elastic and dynamic constraints.
The shape memory twist uses special metal alloys that have the ability to return to a previously defined shape. This specific behaviour stems from a phase shift in the actuator material's microscopic texture induced by heating or cooling the material. Since such materials show the ability to achieve very large strain, i.e. subsequently large deformations, that can be fully recovered, they are interesting for increasing the mean (static) twist of a rotor blade. Hereby, the actually applied compromise between high twist requirements for hover and low twist demands for cruise could be beneficially substituted.
The development efforts showed the following achievements:
- experimentally validated technologies for controlling the helicopter blades by distributed actuation;
- model rotor blades spin tested on a whirl rig under hover conditions with respect to their controllability of noise and vibrations;
- a full scale blade segment tested on bench for actuation integration and endurance;
- numerically approved concepts; and
- laboratory tests to support the feasibility investigations.
WP6 - Exploitation
The objective of WP 6 was to ensure an efficient exploitation of the achievements gained in the course of the programme. Individual goals have been:
- to summarise the know-how about the reduction potential of noise, vibrations and engine power consumption of existing and future helicopters by generating a related data base, including a synthesis of the results attained;
- to enable the exploitation of the environmental improvement results attained during the project in industry and research with the help of a dedicated exploitation plan;
- to get a broader view on the technologies in question by an exchange of know-how in the aforementioned areas with companies / institutions also outside the consortium.
