(also known as lee waves, rotors, and cross-winds)
As sUAV/drones become more and more popular, it seems that more and more of them are striking the sides of buildings without the operator understanding why. “It was flying fine and all of a sudden it zipped up and into the side of the building.” “Everything was great until the drone had a mind of its own and flew straight to the ground.” “The drone was flying over the trees and all of a sudden it spun around and dropped into the trees.” Reading forums around the internet, this is a common, yet unfortunate and avoidable conversation.
First, let’s establish that flying in GPS mode may be less than effective when very close to a building. Signal may be lost, and this could explain a few of the building strikes.
However, far and away more likely in most instances, the UAV was caught in a “rotor.” These are also known as up/down drafts, lee waves, or cross-winds, depending on which aviation discipline one adheres to. Needless to say, these phenomenon do exist, and play havoc with any sort of aerial activity whether it’s wingsuiting, parasailing, skydiving, model aircraft flight, swooping, small aircraft, and yes, especially light-weight multirotors.
Truly, knowing about them is half the battle. Staying away from them is the rest of it.
A building blocks the wind on one side (windward side) and on the opposite side (leeward side) the wind will pay all sorts of havoc with any flying object. Winds will extend in distance up to four times the height of the obstacle, and two times the actual height.
OK, say there is a building that is 40 feet in height, and you have a medium wind blowing. Gusting or steady, it makes no difference.
40×4=160 feet. Therefore, for 160’ past the obstacle at ground level, your multirotor is at risk for catching either a down draft or an updraft. Either way, the airframe/hull is not in clean air.
In terms of height, depending on wind velocity, the UAV may have to climb as high as 80’ to find clean air above an obstacle.
The air goes over the obstacle and is “pulled” to the ground (downdraft), where it then “bounces” upward (updraft) and tries to resume its level flow.
These phenomena are entirely independent of sinks,thermal rises, dust devils, and the like (coming soon).
This also occurs in natural/unbuilt up areas. Trees, canyons, ridges, rock-lines; any large object will incur rotors. Avoid them. It’s virtually impossible to determine exactly where the down draft vs. the updraft may be occurring, and the location of these dirty winds will change with swind velocity.
Prior to flying, take a moment to look at the environment in which the UAV/drone will be flying. Determine distances from obstacles as accurately as possible prior to flight. Doing so goes a long way to maintaining control and safety when the drone is in flight.