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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 affect on overall aerodynamic efficiency. It is difficult to predict the airflow of the rear fuselage without using wind tunnels due to 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 or 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 foces 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 specification.

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 which 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