Flying through the wake of a turbine

The diameters of the largest wind turbines around today are over one hundred and fifty metres. Flying in their vicinity comfortably in a small aircraft can be a challenge. Pilots have to be wary of these structures, believes Carmine Varriale of the Faculty of Aerospace Engineering, as ever more wind turbines are being built. Depending on wind speed, the effects of the turbine wake can sometimes be felt as far as a couple of kilometres, or the equivalent of twenty times the rotor diameter.

Varriale did simulations together with colleagues from the Fraunhofer Institute for Wind Energy Systems in Germany, where he used to work himself until recently, and from the University of Naples Frederico II. The effect of wind turbines on the safety of airplanes is an area that is still largely unexplored. Varriale: “This needs to change. Wind farms can cause wind shears. These are one of the greatest hazards to aircraft at low altitude, especially when approaching an airport because the aircraft is then flying close to the stall speed.”

Varriale is the first author of a paper published this week in the journal Aerospace entitled ‘Flight Load Assessment for Light Aircraft Landing Trajectories in Windy Atmosphere and Near Wind Farms’. In this publication, the Delft researcher presents the simulations he ran for small aircraft with a mass of less than 5,700 kg that encounter wind turbine generated turbulence. He ran simulations for wind speeds of five metres per second and for twelve metres per second, which is pretty strong wind.

‘Close to the rotor, gusts will push you upwards and downwards’

“If a light aircraft complies with standard certification requirements, the airplane should still be safe to fly through the turbine wake, even in this extreme situation. In the latter case, however, flying through the wake can almost feel like being in a roller coaster. Close to the rotor, gusts will push you upwards and then downwards. Further away, you would experience a more continuous strong vibration.”

The research is a preliminary reconnaissance, says Varriale. “Windfarms are not usually built close to aerodromes. But as the thinly inhabited zones where wind parks can still be built are getting sparser, especially in Germany, I would expect turbines to be increasingly constructed in the vicinity of aerodromes. Aircraft safety must then be taken into consideration, especially in the case of ultra-light and light aircraft.”

It may not be wise to place wind parks within a radius of a couple of kilometres of landing strips. “In light of the results obtained on this specific wind turbine model, there seems to be noticeable effects up to twenty diameters downstream of the rotor,” warns the Delft researcher. “Authorities should be aware of this. The exact distance is case dependent, however. The type of airplane using the airstrip and the morphology of the surrounding landscape very much influence wind gusts and definitely need to be taken into consideration as well.”