How much flight time is needed for an inspection

When planning inspections, it is important to be able to give an estimation of how much time is needed to inspect a certain asset. In this article we will discuss some of the variables which influence the duration of an inspection.

Generally speaking, a flight can be divided into two phases: transit to (and from) the inspection area inside the asset, and the inspection itself.

Transit

Transit time will depend on the distance to be travelled, and the nature of the environment. The pilot will not be able to fly as fast in a cluttered environment as in an open environment. For example, the top speed in the 'ASSIST' flight mode typically used in cluttered environments is 1.3m/s, which corresponds roughly to 80m per minute. In risky environments and even slower speed is recommended. In an open environment the 'ATTI' flight mode is more likely to be used, and the top speed is 4m/s, or 240m/minute.

The maximum controlled vertical speed of Elios 2 , whether going up or down, is also 1.3m/s, or roughly 80m/minute. It is also important to keep this in mind when determining the battery level needed to safely return from high areas, such as smoke stacks or boilers. This speed can be increased in 'Manual Thrust' mode, which allows the drone to climb at up to 10m/s and descend at almost free-fall velocity.

Remember that the cage offers protection from impacts at speeds up to 3.6m/s on flat surface, and 1.5m/s on sharp surfaces. Flying faster can get the job done quicker, but it comes at a significant risk.

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Inspection

The time needed for the actual inspection depends on the size of the area to inspect, and the required detail of the inspection. Detail is often expressed in Ground Sampling Distance (GSD), which is the distance between two adjacent pixels, measured on the observed object. A GSD of 1mm/pixel means that one pixel on the image represents 1 mm in the real world. A smaller GSD means that smaller details will be visible in the image. For more information concerning GSD and camera resolution, we recommend reading this article.

To obtain a smaller GSD (more detail) you must fly closer to the object to inspect, the relation between these is linear. This in turn means that a smaller area is in the field of view of the camera at any given time, reducing the surface area that can be scanned in a given time. The white line in the following graph shows the relation between GSD, and the surface area which can be scanned per minute of flight:

The red line shows the maximum flight speed to maintain for a given distance (or GSD). The reason for this is motion blur. Elios 2's video camera adapts its shutter speed in function of brightness of the objects in view. Visible motion blur will occur when the image is 'smeared' over several pixels during the time where the camera captures an image.

In the case where the object is illuminated exclusively by the drone's on-board LED lighting, then the emitted light will be more concentrated when the drone is up close against it, and the object will appear brighter. In turn the shutter speed can be relatively high, up to 1/250th of a second. When flying far away from a surface then the light is distributed over a larger area, and the shutter speed can be as slow as 1/50th of a second. The reason that the maximum motion blur speed decreases as you fly closer to a surface, is because the distance represented by one pixel decreases faster than the shutter speed increases. This is especially the case at less than 2m from a surface, as shown in the detail view of the same plot:

In light of these values, it should become clear that in order to conduct effective inspections in time-critical applications, one should pick detailed inspection points carefully, and where necessary choose between GSD, and 'Getting Stuff Done'.