On The Radar
As manufacturers of passenger-carrying eVTOL air taxis and cargo-delivery drones prepare to certify their aircraft and introduce them to the burgeoning urban air mobility (UAM) market, researchers are raising concerns over whether these new aircraft can safely operate in windy conditions when flying near buildings.
While the effects of wind and turbulence on aircraft are generally well understood, urban air mobility presents some fresh challenges. Not only are smaller aircraft such as drones and eVTOL air taxis more prone to flying off course in the event of a wind gust, but wind flows can also be less predictable in urban environments with tall buildings and other infrastructure than they are higher up in the atmosphere.
A team of researchers from the University of Maryland, Lehigh University in Pennsylvania, and RMIT University in Melbourne, Australia, recently set out to determine how wind gusts might affect the safety of small aircraft operating in an urban environment.
“The turbulent flows and gusts around buildings and other urban infrastructure can affect the steadiness and stability of such air vehicles by generating a highly transient relative flow field,” or a short burst of strong wind, the researchers said in a paper on their study, which was published December 28 in the journal Drones.
For the study, the researchers reviewed existing computer models of turbulence and wind gusts, and they created new 3D simulations to model and characterize the types of wind gusts that small aircraft might experience when flying around and above tall buildings.
“Whilst the flow field around buildings has been extensively studied from a fixed reference frame (e.g., by wind engineers for the purposes of structural loadings, dispersion of pollutants, pedestrian wind comfort, etc.), there appear to be very few studies from the reference frame of the moving aircraft and at the relevant frequencies,” the researchers said in the paper. “We therefore examine this relative flow field with an overall aim to reveal the characteristics of a ‘severe’ gust for UAVs in close proximity to buildings.”
In their simulations, the researchers used simplified models of both fixed- and rotary-wing aircraft to illustrate the changes in lift and thrust experienced in a variety of wind gust conditions. Their analysis found that fixed-wing aircraft flying near buildings experienced a significant and swift increase in angle of attack along the wingspan during a wind gust event, which can cause the aircraft to pitch, roll, or potentially even stall.
However, multirotor aircraft in the new simulations were not affected by wind gusts in the same way. The researchers modeled two types of multirotor aircraft: a small quadrotor delivery drone, and a larger, eight-rotor air taxi used to transport passengers. They found that multirotor aircraft are more likely to experience changes in thrust when exposed to wind gusts, and this effect was most significant for aircraft traveling at slower speeds or hovering near buildings.
“Rotor thrust reactions to turbulence are more erratic and greater in magnitude than lift variations seen for the fixed-wing aircraft,” the researchers explained. “On a rotor disk, turbulent flow vectors [wind gusts] from all directions directly influence the aircraft incidence angle, the thrust required for steady level flight, and any perturbations which result in altitude loss or gain. All these directly influence the amount of thrust produced and incidence angle of the rotor significantly.”
The researchers found that aircraft with hybrid configurations with a combination of lifting surfaces, such as fixed-wing aircraft with tilting rotors, are the most well-suited to handle wind gusts in close proximity to buildings in urban areas. “However, there is a spectrum of design possibilities which require careful design choices to truly alleviate the disadvantages of both fixed and rotary wing,” the researchers noted.
They concluded that existing regulations do not sufficiently address the dangers caused by wind gusts. “Currently, there appears to be negligible certification or regulation for AAM systems to ensure safe operations when traversing building flow fields under windy conditions, and it is hoped that the insights provided in this paper will assist with future certification and regulation,” the researchers wrote.
“From a certification standpoint, AAM [advanced air mobility] airframes need to demonstrate the ability to counter attitude disturbances and flight path deviations for a reasonable range of wind speeds and gust conditions to make AAM operational for the majority of the year despite weather,” they added in the paper.
The researchers said aircraft manufacturers can replicate this study to understand how wind gusts near buildings might affect their particular type of aircraft and to help inform their designs. The study could also help to inform vertiport design and regulations pertaining to AAM infrastructure, they said.