Civil Aircraft
Over the last 50 years, ARA has continually evolved to maintain its reputation for providing the benchmark for the accurate and efficient production of high accuracy aerodynamic data required to support development programs. ARA has been involved in the development of every major European civil transport aircraft, including Concorde, as well as a number from the Americas and the Far East.
Our customers have placed the highest possible demands on ARA for the accurate measurement of drag throughout the aircraft operating range and for the development of sophisticated data reduction techniques to correct the measured data to ‘free-air’ conditions. ARA’s continued commitment to respond to these demands has led to the development of fully validated, quality-controlled testing techniques that collectively address the performance of commercial aircraft over their complete operational envelope.
Our highly experienced test teams deliver excellent test program management combined with rapid model reconfiguration to a superb standard to provide our customers with an efficient and productive service that provides excellent value. The security of the model and all data is assured for every customer.
Drag Performance
The increasing demand for improved fuel economy along with ever more stringent environmental constraints has resulted in a renewed effort by commercial aircraft manufacturers to design increasingly efficient aircraft (through the reduction of drag).
Why ARA?
ARA has considerable experience in the accurate measurement of drag on a range of civil aircraft. We utilise highly accurate instrumentation, such as thermally insensitive strain gauge balances for the measurement of overall model loads, and inclinometers for the measurement of the model’s attitude to the airflow. These measurements are augmented by pressure measurements from within the model cavity. Transition fixation is achieved through the use of strategically located surface roughness, in the form of ballotini or discrete ‘transition dots’.
A number of visualisation techniques are available to aid in the understanding of the flow physics, and therefore aid in the design for minimised drag:
- Pressure Interpolation
- Oil Flows
- Pressure Sensitive Paint
A Typical Test
During the wind tunnel test, a number of aircraft configurations and components are tested in order to determine their effect on the overall model drag through a range of model incidences, roll angles and Mach numbers. Drag is calculated from the aerodynamic loads on the model measured by the internal strain gauge balance. The cavity pressure measurement is used to correct the drag for the internal cavity required to mount the model in the wind tunnel.
The thermally insensitive balances utilised by ARA yield the benefit of a faster, more efficient and accurate test technique compared to wind tunnels utilising less sophisticated balance designs.
An improved understanding of the airflow over the model surface can be achieved very quickly through the use of flow visualisation techniques such as interpolation of the model surface pressures and oil flows. More advanced visualisation methods such as Pressure Sensitive Paint (PSP) and Temperature Sensitive Paint (TSP), which can be used for accurate transition detection are also available.
Technical Information
Accuracy (using a 57.15mm/2.25″ balance).
- Within a test series, drag repeatability is within half a drag count ((△CD = ± 0.00005)
- Between test series, drag repeatability is within a drag count (△CD = ±0.0001)
Transition detection utilising
- Temperature Sensitive Paint visualisation
- Acenaphthene sublimations
Flow visualisation of phenomena such as shock waves, separation bubbles and vortices utilising
- Oil flows
- Pressure Sensitive Paint (for increased fidelity)
Stability and Control
A typical civil aircraft is kept stable by a horizontal tail and a vertical fin. The aircraft is controlled in roll by ailerons on the wings, in pitch by an elevator on the horizontal tailplane and in yaw by a rudder on the vertical fin. It is critical to the safety of the aircraft that the control surfaces perform their function throughout the operational envelope at all speeds and attitudes.
A Typical Test
Wind tunnel tests enable aircraft designers to check how an aircraft model performs both within and beyond its operational envelope in a safe and controlled environment. A typical test may involve the design and manufacture of control surfaces such as spoilers or ailerons, larger items such as engine nacelles and pylons, or the complete aircraft model followed by a wind tunnel test. The aerodynamic loads on individual components can be measured by attaching each component to the parent aircraft with a purpose-built balance (there can be up to 19 balanced components in addition to the main balance) which measures the loads on that component. The model loads are tested to the CL MAX and beyond into post-stall regime.
Alternatively, ARA uses Pressure Sensitive Paint (PSP) to test stability and control as it allows the measurement of the pressure affecting the aircraft surface. PSP is specifically useful on areas that are unattainable by pressure tapping.
Technical Information
Model Attitude:
- Incidence range: -10° to +40°
- Sideslip range: ±20°
- Roll range: ±180°
Model Loads:
- Test beyond positive and negative buffet onset
- High speed failure cases including spoilers and undercarriage
- Overall model and control surface load measurement
Data Acquisition:
- Up to 220 channels measured at up to 10 data points per second
- Measurement of up to 1000 steady-state pressures recorded at up to 10 data points per second
Flow Investigation:
- Use oilflow visualisation to investigate surface flow phenomena such as separation bubbles and trailing edge separations
- Use high speed pressure transducers to measure unsteady pressures to investigate regions of separated flow
Rear Fuselage Performance
The nature of the airflow over the rear fuselage and empennage (the horizontal and vertical tail surfaces) of a civil aircraft is fundamental to the effective operation of the tail control surfaces and has a significant effect on overall aerodynamic efficiency. It is difficult to predict the airflow of the rear fuselage without using wind tunnels due to the complexity.
Why ARA?
Two alternative twin sting support designs are available at ARA; the Standard Twin Sting Rig (STSR) and the Enhanced Twin Sting Rig (ETSR). The STSR is the original twin sting support system and is extensively used for the measurement of empennage characteristics because it is free from central sting interference. Together with panel loads on the horizontal tailplane, vertical stabilisers, rudders and elevators for models which can be easily split, both aerodynamically and mechanically, the empennage can be mounted on an internal balance. The STSR booms incorporate balances to enable some of the overall forces or moments acting on the model to be monitored for safety.
However, for close-coupled aircraft layouts (such as many modern executive jets which incorporate engines mounted to the rear fuselage aft of the wings), there is strong mutual aerodynamic interaction between the complex fuselage shape, wing, nacelles, empennage and central support sting and therefore, the model fuselage can be neither aerodynamically nor mechanically split in a sensible fashion for testing on the STSR. For these cases ARA developed an Enhanced Twin Sting Rig which can measure the aerodynamic characteristics of different rear fuselage designs; empennages and rear fuselage nacelle installations together with the determination of central sting corrections. The basic concept of the ETSR is that the forces and moments acting on the whole model are accurately measured by two six-component balances, one mounted in each of the twin stings. To ensure the highest possible data quality, the whole model or ETSR system is calibrated as an assembly prior to testing in the TWT.
Uniquely, both of our support systems can move in yaw as well as in pitch which enables both lateral and longitudinal stability and control to be investigated during a test. In addition, the horizontal separation of the support booms can be adjusted to our customer’s specifications.
A Typical Test
When testing rear fuselage performance, models are supported via the wings with a twin sting arrangement rather than via a central single sting. This is because a twin sting allows for a more representative fuselage shape to be modelled. The focus of the test is on the performance of the rear of the aircraft, meaning that the support booms have no influence.
During a typical test, the aerodynamic loads on the rear fuselage can be measured by creating a small gap between the rear fuselage and the rest of the model and connecting the two parts with a main model balance that measures the loads. This capability allows very small changes in performance (due to subtle changes in model configuration) to be measured which may not be possible when measuring the overall load for the complete aircraft model.
The twin sting support method is also utilised for the measurement of central sting interference terms, this is done by testing a model with and without a dummy central sting. From these tests, correction terms are derived for the central sting support system which can then be applied to data obtained from standard single sting support tests for the same model.
Technical Information
Model Attitude:
- Incidence range: -10° to +20°
- Sideslip range: ±8°
Flow Investigation:
- Use oilflow visualisation to investigate surface flow phenomena such as separation bubbles and trailing edge separations
- Use high speed pressure transducers to measure unsteady pressures to investigate regions of separated flow
- Use Pressure Sensitive Paint technique to investigate flow phenomena in finer detail
Data Acquisition:
- Up to 220 channels measured at up to 10 data points per second
- Up to 19 balanced components in addition to the main balance can be installed on the parent aircraft
- Measurement of up to 1000 steady-state pressures in total recorded at up to 10 data points per second
Installed Propulsion Performance
Engine and aircraft manufacturers are constantly seeking to develop more efficient propulsion systems to reduce drag and noise. Whether these are turbofans, open rotors or propeller systems they will have significant effects on the airflow around the aircraft. The challenge facing engine and aircraft designers is to prevent these effects from disrupting the aerodynamic efficiency or stability and control of the aircraft whilst minimising drag and noise.
Jet Simulation
ARA has been a pioneer in jet propulsion testing since TPS units (turbine powered simulators driven by high-pressure air) were first developed over 40 years ago. TPS units allow a high-fidelity simulation of inlet conditions and bypass flow through a jet engine model. Our one-stop service includes the design and manufacture of the engine models, calibration of the assembled TPS units in our Mach Simulation Tank and installed and isolated testing in the Transonic Wind Tunnel.
Through Flow Nacelles, Ducts and Blow Nacelles
Through flow nacelles and ducts are used to measure the effect due to the presence of nacelles and ducts without any jet effects and can be installed in both full-span and half-span models in the Transonic Wind Tunnel.
Blown nacelles are powered by a high-pressure air supply and are used to measure jet effects.
ARA designs and manufactures single, double and triple bodied through flow nacelles, through flow ducts and blown nacelles followed by calibration in our Mach Simulation Tank.
Propeller Simulation
We have a world-leading capability in propeller testing for half-span models built on decades of pioneering work into balance, hub and blade design as well as telemetry and data acquisition.
We offer both single propeller, twin-propeller and contra-prop configurations. The propellers are powered by air motors driven by a high-pressure air supply.
Our advanced health monitoring system for the air motors and propeller blades is very successful in detecting potential problems before they occur providing significant advances in reliability and productivity.
Our one-stop service includes the manufacture of the rotary balances followed by calibration in our Balance Calibration Laboratory as well as fine-tuning the balancing of propeller assemblies in our Propeller Test Cell.
Technical Information
Propeller Simulation
- Capability to measure blade twist
- Air motor operational range up to 13,000 rpm
Jet Simulation
- TPS unit operational range from 40,000 to 70,000 rpm
Flow Investigation:
- Use oilflow visualisation to investigate surface flow phenomena such as separation bubbles and trailing edge separations
- Use high speed pressure transducers to measure unsteady pressures to investigate regions of separated flow
- Use microphones installed on the fuselage to measure cabin noise
Data Acquisition:
- Up to 220 channels measured at up to 10 data points per second
- Measurement of up to 1000 steady-state pressures recorded at up to 10 data points per second
- 1/rev data acquisition
- Measurement of rotating pressures on propeller hubs and blades
Isolated Propulsion Performance
Large Scale Isolated Propeller Testing – The Acoustic Liner
The continuous drive for reductions in fuel burn for commercial aircraft means that the development of open rotor propulsion systems will be one of the most important areas of research now and in the long term. The acoustic signatures of such systems and the acquisition of noise data is a very important part of these development programs.
ARA has a unique capability (within Europe) for noise measurements on large scale propeller and open rotor systems at speeds up to Mach = 0.8. An acoustic liner is mounted in the Transonic Wind Tunnel working section, providing sufficiently anechoic conditions to allow the measurement of acoustic tones on open rotor configurations. Acoustic measurements are made using either a microphone traversing rig or fixed microphone arrays.
Open rotor systems of up to 1-metre diameter are tested mounted on 440 kW induction or electric motors. The motors can be configured to drive either single- or contra-rotating open rotors. Each rotor is mounted on a strain gauge balance to measure thrust, torque and off-axis loads. Near and far-field acoustic characteristics are measured using high-quality microphones along with rotor performance and pressure data.
Isolated Cowl Test Rig
A special rig has been devised to measure the internal and external performance of isolated cowls for civil transport applications. Internal engine face pressure measurements are made with a rotating pressure rake. Internal mass flows are derived from a downstream venturi as well as the engine face pitot/static measurements.
External drag is measured by a rotating 5 arm rake of pitot/static tubes using the momentum deficit method.
Both rake drives are continuous, providing small rotation increments for optimum coverage of both internal and external flows.
The incidence variation is ±25˚ at low Mach numbers for internal pressure measurements. The incidence range may be reduced when external pressure measurements are required.
Inlet flow is induced through the rig by connection to a 9.7MW (13000hp) compressor. This ensures that the mass flow control is independent of free stream conditions and allows high mass flows at low Mach numbers and ground running operations to be investigated.
