AirRails VTOL Autopilot – Safety by Design


Hybrid VTOLs (aka VTOLs) are becoming increasingly popular due to their ability to start anywhere, like a multirotor, and fly far and fast like a fixed-wing. The interest in VTOLs is particularly high in the pipeline inspection and agricultural sectors and in countries having medium to large areas to cover.

The biggest concern that is usually raised about this type of drone is the efficiency hit that you take by having the extra weight and batteries needed to drive your vertical thrust system. This reduces flight time and payload capacity.

But there is another important aspect to hybrid VTOLs which we’ve been working on enhancing together with the AirRails flight control platform. And that is the increased level of safety that can be achieved by using its inherent redundancy capabilities.

Demonstration of Pusher/Puller Motor Loss, Airspeed Failure Detection and Stall Recovery

Let’s start with a short video demonstration that illustrates two not so uncommon failures that can occur.

In the first part of the following video we demonstrate the detection, in software, of a pusher/puller motor loss, which is not uncommon with fuel motors. In the second part, we demonstrate how AirRails detects and recovers from an airspeed loss due to a pitot tube blockage and can continue the flight without.

In the second video we demonstrate how AirRails can detect and recover from a stall situation:

These are just a few of the detection and recovery techniques that are part of AirRails. Let’s take a look at the full array of failure types it can detect and recover from.

AirRails Failure Detection and Recovery

With AirRails, we have spent a lot of time taking advantage of the fact that while the VTOL is operating in fixed-wing mode you have a backup system: the multirotor component. Rather than treating it as dead weight, we decided to make it our onboard safety net.

Here are the types of failures AirRails can currently detect and recover from while in fixed-wing mode:

Failures that result in a transition from fixed-wing to multirotor mode provide the system and the operator with a range of recovery options, ranging from attempting a transition back to fixed-wing flight, locating an emergency landing zone or returning home.

Multiple Safety Nets
With all of these fail safes in place, you end up with multiple layers of checks which could trigger during a particular failure event. This significantly increases the chance of recovering from a failure. In fact, it works so well we had a hard time triggering our desired failure mode as the other checks kept jumping in.

There are three layers of checks in place, where each will trigger if the previous one does not:

  1. Continuous sensor and flight attitude checks
  2. Conditional checks based on the current situation
  3. Absolute limits as a last measure

If we apply this to the airspeed sensing failure example above, the sequence of checks that will occur will be:

In the second layer there is a window of opportunity to failover from the airspeed sensing failure and continue the flight or return home in a relatively safe and efficient manner. Should this not trigger in time then one of the next layers will trigger a transition into multirotor mode.

Minimising Risk during Takeoff and Landing?
During the takeoff, landing and a portion of the transitions into and out of fixed-wing we will be back in a partial or full multirotor mode. A failure in this mode without a redundant configuration will result in failure. The risk area in this case is quite limited and can easily be planned to take place in a safe location, limiting potential damage to people and property.

Let AirRails bring your VTOL System to the next Level
Contact us to find out how the AirRails flight control platform can make your hybrid VTOL system even safer.