Wind and turbulence: how drones cope with the air around structures

A multirotor holds position by tilting into the wind, using motor power and battery to do so. The stronger the wind, the more it tilts. Even when the mean wind is within limits, a gust can exceed what the aircraft can hold and push it off station. Around structures, wind accelerates at corners, forms eddies on the lee side and flows down the face. Inspection flies within a few metres of the structure, which puts the aircraft in the most disturbed air of all.

Wind countermeasures fall into three groups: make the aircraft stronger, keep distance from the structure, and choose when to fly. Larger aircraft with motor headroom hold position in gusts. Range sensors push back before a drift turns into contact. Zoom cameras let you capture without entering the turbulent zone next to the wall. And forecasts plus on-site measurement let you choose calmer windows.

Strong wind and turbulence are found around any tall or large structure: wind turbines (especially offshore), long girders and tall piers of bridges, facades and roofs of office buildings and condominiums, power lines, transmission towers, telecom towers, dam valleys, open slopes and levees, elevated expressways, ports, ships and offshore structures, and pipelines crossing plains and mountains.

• Leave margin on rated wind resistance: manufacturer figures are often limits, not comfortable operating values. Choose an aircraft well above the gusts expected on the job.
• Set numeric abort criteria: decide mean wind and gust limits, plus thresholds for tilt and battery drain, before flying. Do not decide “it seems fine” on the day.
• Check wind at inspection height: besides the ground anemometer, climb in steps and watch aircraft behaviour and tilt to estimate the wind aloft. (Related: height)
• Plan headings around the wind: going out into a headwind and returning with a tailwind helps avoid running out of battery on the way back. Keep an escape route downwind.
• Stop, then shoot: in heavy buffeting, hover until motion settles rather than shooting continuously while moving. Reshoot blurred images.
• Watch for weather changes offshore and in mountains: fronts and sea or valley breezes can shift wind quickly. Fix working windows from the forecast.

Windy days are when pre-flight checks and records matter most. Use the pre-flight checklist, flight plan and flight log (Airffic Flight Institute) as the base for your site procedure.

Most regulatory frameworks do not impose a single wind-speed limit; they leave it to aircraft performance and operator judgement. In Japan, the flight manual submitted with a permission or approval application sets operating rules for weather conditions such as wind, and the operator must follow it. If the operator adopts MLIT’s standard manual, that rule is no flying in winds of 5 m/s or more, unless the manufacturer’s manual confirms the aircraft can fly in stronger wind. In other words, “stop when it is too windy” is a commitment the operator defines and keeps. Under US Part 107, checking weather is likewise part of the remote pilot’s pre-flight responsibility.

Related: height, water, spray & humidity, cold & icing, world regulations.

• Unpredictable turbulence around structures: forecasts average over large areas. There is still no way to know in advance the gusts at a building corner or under a bridge deck.
• Limits of small aircraft: the smaller the aircraft that fits into tight spaces, the weaker it is against wind. Places that are both tight and windy — stack tops, around turbine nacelles — remain difficult.
• Shake versus resolution: gimbals hold orientation, but position movement changes focus and distance. Image quality on windy days is still inconsistent.
• Few workable days: offshore and on ridges, windy spells can ground operations for days. Inspection schedules are still at the mercy of the weather.

From each structure’s 3D model and weather data, the system shows before take-off which faces and heights will be turbulent in today’s wind. The aircraft adjusts route and standoff to that prediction and works from the upwind side. Sensors on aircraft and structures keep measuring wind, so each prediction improves on the last. Go/no-go is decided by data, not gut feeling. That is the editors’ outlook.