Workshop

Airwake hazards

Helicopters are some of the most flexible aircraft in existence, their ability to freely manoeuvre in almost any direction allows their employment in missions that no other vehicle can perform. Search and rescue missions between mountains or buildings, logistic support to remote offshore installations and naval operations from military vessels are only a few examples of the very diverse applications of helicopters.

Many of these missions require pilots to fly in very challenging environments, often under the airwake and turbulence shed by very close obstacles, for example the ship superstructure during deck landings. Sudden changes in the wind velocity surrounding the helicopter will result in upsets in the aircraft position, orientation and speed that can impair handling by the pilot or even lead to loss of control. Improving our understanding of these hazards is a requirement to achieve NITROS objective of enhancing helicopter safety.

Flight simulation as a first look on the problem

Flight simulation is proving to be a very valuable tool for this aim. Most people are familiar with the use of flight simulation for training, allowing pilots to virtually practise a task in order to prepare for real world flight training. But piloted and unmanned flight simulation (also known as online and offline flight simulation respectively) also has important applications in research and development.

For example, wind energy is proliferating across the globe with wind farms appearing near airfields and heliports, there are increasing risk for an encounter with the airwake of a wind turbine. There is almost no experimental data for these kind of scenarios [1] but flight simulation can be used in initial concept studies for operations, helping engineers to identify possible risks beforehand and to prepare flight test. Also if flight simulators can reproduce the underlying physical causes of such hazards, they might help in evaluating possible solutions before putting them into practice.

This was the objective of a series of tests [2] conducted at the University of Liverpool during which pilots flew a simulated accidental encounter between a general aviation airplane and the airwake of a small wind turbine. It was shown that the airwake of a wind turbine could lead to mild upsets in aircraft attitudes which should not impair handling. Encounters with the airwake of modern very large wind turbines might produce a different outcome however.

Also, due to their very different flight mechanics and handling, helicopters will probably react in a very different way than fixed wing aircraft. Since they are being used to provide logistical support to offshore wind energy installations [3] and might need to operate in their vicinity for search and rescue missions, knowing the effects of such an encounter is therefore of great interest [4].

Models and realism

But if flight simulators are to be useful in reducing the hazards from airwake encounters they must realistically reproduce their effects and for piloted flight simulation, it must do so in real time. This is no easy requirement. The simulator combines mathematical models that describe the behaviour of the aircraft and the aerodynamics of their environment and solves them together to determine how the aircraft behaves.

The accuracy of models and their coupling, as well as the obtained results, must be validated. Comparison against results of wind tunnel experiments, field measurements and flight tests as well as pilot feedback can be used to assess if a model is valid. Usually improvements in accuracy are obtained by including the effect of additional physical phenomena to the model.

But this comes at the cost of increasing complexity leading to longer computation times, as solving these models, especially the fluid dynamics equations that describe the behaviour of the airwake, requires a large number of calculations which have to be performed repeatedly. This would have been completely impossible without today’s computing technology. But even then, if flight simulation is to be feasible, simplifications in models or coupling must be found that still lead to accurate results.

An example of such a coupling of different models is being applied to train naval pilots in ship deck landings. The ship airwake is solved using computational fluid dynamics, and approach that can last between hours or days depending on the fidelity of the solution. The movement of the air over the ship deck is stored as if it were a movie and replayed in the flight simulator which then can calculate the effects of the airwake on the helicopter in real time. This approach produces very realistic airwakes when compared to experimental measurements and has shown to help in the preparation of flight trials and training of pilots by providing them with a first impression of how to approach the landing[5].

This is referred as one way coupling. The effects of the airwake on the helicopter are modelled very accurately, but not so the effects of the helicopter on the airwake, since the solution of the airwake has been computed and stored beforehand. If the airwake changes due to the presence of the helicopter, their effects on the helicopter might change too.

Complete coupling of aerodynamics and flight dynamics would require computing the aerodynamics of the airwake and the helicopter acting upon it, computing its effects on the movement of the helicopter and applying them and then starting the cycle again for the new position and orientation of the aircraft. With the computational technologies available today, it is impossible to perform these simulations fast enough for piloted simulations.

Research is being conducted in this regard for non-piloted flight simulation. Here the pilot is replaced by an automatic pilot model that emulates a human pilot attempting to perform the intended task. But future improvements in modelling techniques and computing technologies might allow to implement some of these methodologies to improve the accuracy of piloted flight simulation.

Conclusions

As we have seen, whenever models are being used to obtain predictions, care has to be taken in their evaluation and researcher and engineers must always be aware of the limitations and compromises of the methodology applied. But results obtained by flight simulation can help pilots to prepare for flight training and provide guidance to engineers to evaluate new designs, alert about possible risks and identify which conditions require further testing. Used correctly, flight simulators can lower costs and enhance safety by reducing required flight hours for testing and training and by allowing to better prepare for them beforehand. A great help in achieving NITROS objective of enhancing helicopter safety.

References:

[1] Civil Aviation Authority, “CAA Policy and Guidelines on Wind Turbines,” pp. 1–45, 2016.

[2] Y. Wang, M. White, and G. N. Barakos, “Wind Turbine Wake Encounter Study,” 2015.

[3] R. Roorman, “Assessing Viability in Offshore Wind,” Rotor and Wind, 2018.

[4] “Objectives, HC / AG-23 : Wind Turbine Wakes and Helicopter Operations Programme  ”

[5] I. Owen, M. D. White, G. D. Padfield, and S. J. Hodge, “A virtual engineering approach to the ship-helicopter dynamic interface – A decade of modelling and simulation research at the University of Liverpool,” Aeronaut. J., vol. 121, no. 1246, pp. 1833–1857, 2017.