Replacing human intervention in often dangerous environments is now made possible by drones designed specifically for harsh environments that can reach otherwise difficult and dangerous to reach areas and hover in position to capture valuable data.

Examples of such are sewers, penstocks, tanks or chimneys, all of which entail risk for human intervention.

However, the main advantage of multirotor-type drones which is their capacity to hover in place is also the cause of their biggest drawback: flight time.

Indeed, carrying a 1 kilogram drone in the air requires motors to produce at least 1 kg of thrust, which is costly in energy and thus places a limit on flight time, which in turn limits the reachable inspection range.

Flight time is therefore a crucial question when it comes to inspection by drone and is influenced by many factors, some of which can be controlled by the pilot (for example, using full lighting power during the entire duration of the inspection flight will lead to a 10% decrease in flight time), others, such as wind drafts or air density, that cannot be controlled.

20 minutes flight time

The ASIO drone uses a specially optimized coaxial system, enabling it to stay in the air for times exceeding 20 minutes (up to 24 minutes). Naturally, the most important question that stems from this specification is what can be achieved in 20 minutes of flight time given the fact that most inspections carried out by humans often require many hours if not days of careful planning and setup?

To answer this question, one must consider the main factors that characterize a mission, namely the distance over which to carry out the mission, the difficulty of flying (defined by how cluttered the environment is) and the required precision of the collected data.

As an example, an inspection of the general state of a sewer, where defects are rather large (missing bricks, broken beams etc…)  will not require slow flight, but instead will require covering large distances. In contrast, the inspection of a tank where defects may be smaller and where more obstacles such as heating systems and support structures can be found, will require a careful and slow inspection.

We will therefore distinguish two types of missions; short flights, where the total covered distance ranges from tens of meters to 300 meters, and longer flights, where the inspection distance can extend to more than 1000 meters.

Considering a fully charged battery yielding a flight time of 20 minutes (with some margin), the flight speed will naturally depend on the distance covered, such that short inspections will allow the pilot to fly very slowly while focusing on details. For instance, for a 300-meter inspection, average flight speed is 0.25 m/s (meters per second). The following graph shows the minimum flight speed required to cover specific inspection distances over the course of 20 minutes.

Pilots and inspectors’ benefits

On short missions, the average flight speed is smaller than 0.25 m/s, which is considered as slow flying. The benefits of slow flight are twofold.

From the pilot’s perspective, the stress levels generated by rapid flight are lowered. The pilot can therefore carry out multiple missions without feeling exhausted from the high concentration levels required for inspections. Further, when flying slowly, less mistakes are made, more time can be taken when flying through difficult inspection areas, making the overall flight experience safer.

From an inspection result point of view, the data gathered is more accurate and more time can be spent on more thorough inspections, following the inspector’s precise instructions and focusing on critical points. In these conditions, flight assistances are also most efficient as the sensorial data is more accurate which allows for better stabilization (using visual lock assisted mode for example).  These types of missions can obviously also be carried out in less time, but give more space for pilot error.

Some inspections however require higher speeds in order to cover distances far greater, may it be for a full length inspection or if a long distance needs to be covered prior to reaching the inspection point.

In these inspections, the full 20 minutes of flight are often a necessity. As an example, some sewers may be accessible only by manholes that are separated by over 300 meters. For these longer-range inspections, pilots can still comfortably reach the set goals while still flying at speeds smaller than 1 m/s, which is the maximum speed in visual lock assisted mode, thus taking advantage of an extra assistance for safe maneuverability.

ASIO has recently demonstrated the capacity to fly over distances exceeding 1000 meters with a single battery. Recently, the inspection of a 1200m penstock was carried out in just over 15 minutes, leaving an excess of 5 minutes to focus on critical points of interest. In sewers, distances of over 500m can be covered, only limited by the signal intensity, that can be enhanced using a range extender, placing the antenna in direct line of sight with ASIO.

The visual lock assisted mode automatically limits flight speed to 1 m/s. Above that speed, the sensorial data is less accurate and stabilization less efficient. At this speed however, distances of 1200m can be safely flown, given that the signal intensity is sufficient. When greater distances must be covered, the drone must be flown in a manual mode, where speed is no longer limited and where the drone can reach speeds in excess of 3 m/s, covering distances that seemed previously unachievable.

Conclusion

In summary, 20 minutes of flight time allows for both long distance inspections while simultaneously relieving pressure on the pilot, which is a win-win situation for both pilots and inspectors.

Finally, although the previous discussion was based on a flight time of 20 minutes to keep a safe operating margin, in the extreme cases where ASIO must be pushed further to its limits, in suitable conditions (where temperatures are inferior to 30°C), the cooling air-vents can be covered with special inserts which increase flight time by another 2 minutes which may be the extra push necessary to realize the most difficult inspections.