Aerodynamics experts have tested modern aircraft in wind “tunnels” for years.  But unmanned aerial vehicles (UAVs) are subsonic propeller-driven vehicles with navigational systems and functions that differ from conventional aircraft.  Wind tunnel testing of drones therefore requires substantial adaptation.

One obvious adaptation is the tunnel’s size, which is generally much smaller for drones.  But UAV tunnels also need an “open” structure that takes into account drone maneuverability over a wide field of operations.  In “closed” tunnels, fan-generated air is blasted at the aircraft from a single direction and constantly recycled.  UAVs, with their free-flight operations, need to be tested from various blast angles under more diverse environmental conditions.

In open wind tunnels, multiple fans blast air at the drone from different directions to simulate a flight through difficult weather or around buildings.  The tests also take into account the need for drones to abruptly change their speed and altitude – or simply to hover in place – to conduct disaster relief, building inspections and delivery services.  Open tunnel drone testing tends to be more time consuming and expensive than closed tunnel testing for conventional aircraft.  Without stricter regulatory guidelines, some cost-averse drone companies may be skipping wind tunnel testing altogether, increasing the safety risk and operational failure rate.

Fortunately, drone companies need not rely on live wind tunnel testing alone.  Computer-based wind tunnel simulation – also known as computational fluid dynamics, or CFD – is cheaper and yields equally powerful results.  CFD is a software program that utilizes a 3D model of a drone to simulate a variety of aircraft movements in reaction to multi-angled wind pressures.  The program can estimate wind shape, surface pressure, and the wind flow separation along portions of the aircraft and assess its flight performance under varied conditions.

CFD can be used alone when developers face severe budget constraints.  Ideally, though, CFD is performed in tandem with physical testing to obtain a thorough understanding of a UAV’s aerodynamic properties. NASA has begun making low-cost software available to drone users for testing purposes, upon request. Drones with new and more sophisticated – and vulnerable – capabilities are appearing every day and their developers tend to be poorly-funded niche start-ups still struggling with thin-profit margins.  For this reason CFD testing is likely the wave of the future, experts say.

Some scientists have found an even more novel way to test drone aerodynamics:  By studying the wind resistance of birds   Even relatively clumsy birds like pigeons display a remarkable ability to navigate at all altitudes in crowded urban airspaces, with hundreds of birds, avoiding collisions or once colliding, quickly swooping down on a building rooftop to land with perfect poise.  At Stanford University in Palo Alto, CA researchers have constructed a special wind tunnel that features live birds responding to machine-generated gusts of wind.  Some of the tunnels also factor in sudden changes in wind speed, barometric pressure and weather conditions. Over time, Stanford researchers hope to generate more sophisticated detect-and-avoid sensor technology and navigational systems that allows drones to mimic the extraordinary agility of birds in flight.

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