Autonomous Autorotation on Small Unmanned Helicopters
Helicopters are known as mechanically and aerodynamically complex machines, but there have been 72 years of development and improvements since the Bell 47 became the first helicopter certified for civil use in 1946.
In the seven decades of improvement many things have changed and evolved and many alternative designs have been used, such as the contra-rotating rotors or the Flettner rotor and the NOTAR (No Tail Rotor) systems, each with their pros and cons, but one thing has remained unaltered — autorotation.
Autorotation is a helicopter’s way of gliding, the emergency procedure available not only in case of engine failure but also when the power transmission mechanism or tail rotor fails.
With a fixed-wing craft, everything is much simpler: an engine failure would lead to a controlled glide in which only the speed and angle of attack need to be monitored to ensure that the aircraft remains within its envelope and doesn´t stall. And even if you get into a stall, you can often still recover. For a helicopter, if the pilot doesn’t act immediately, you lose control of the aircraft permanently and you’ll crash. Thus; knowledge and correct (and timely) implementation of autorotation procedures is vital.
On manned helicopters, autorotation is a maneuver that pilots practice until they reach the point of muscle memorization–meaning that the maneuver is simulated under controlled conditions until it’s completely internalized and automatic.
This is extremely important for one main reason. There’s only one opportunity to enter into autorotation–if it’s missed, the flight can only end in a crash. Once the engine has failed, the pilot has to lower the collective to maintain the rpm of the main rotor and to enter into autorotation. The reduction of the collective reduces the lift of the rotor and the drag of the blades at the same time, causing the helicopter to descend and changing the airflow direction that goes through the rotor from downwards (normal flight and lifting conditions) to upwards (autorotation) relative to the helicopter. The upwards-moving airflow is what then maintains the rotor rpm during the descent. In the last phase, the flare, the inertia of the rotor will be used to reduce the vertical and horizontal speed, with the collective changed again in order to generate thrust.
Now let’s take a look at unmanned aircraft failure scenarios. For a fixed wing UAV, most issues can be solved with a parachute. On a helicopter UAV, a parachute is not an easy solution as both the main and tail rotors must be avoided to ensure a successful deployment. Additionally, the payload weight/MTOW ratio on a helicopter is always smaller than that for fixed-wing aircraft, so, putting aside mechanical challenges, a parachute is not always an affordable option due to the added weight.
On helicopter UAVs implementing autorotation is more complex than on larger, manned, helicopters. The first reason is size: UAVs tend to be smaller than manned aircraft, meaning they have a smaller main rotor with lower inertia. Another important issue is that the pilot is at a distance from the UAV so there’s a delay in awareness of and reaction to the situation–a delay that in the case of autorotation is too costly.
So the UAV ends up with a window of response time much smaller than for a manned aircraft, and the only way to solve this is to have the autopilot handle it fully autonomously.
Let’s see how a UAV helicopter autopilot would normally behave. An autopilot that does not have autorotation implemented works as follows: The altitude is controlled with the collective command, and rotation speed of the main rotor (rpm) is controlled with the throttle command. When an engine failure happens, the rpm of the main rotor is reduced, producing a lift reduction equal to the square of the reduction, and as a consequence altitude decreases. The autopilot responds by increasing the collective command in order to increase lift. This is what actually makes the rotor stop even quicker until it stalls, as drag is proportional to the angle of attack of the blades. Indeed, normal autopilots do exactly the opposite of what should be done, ending up with a helicopter that falls from the sky with the same aerodynamics as a chunk of solid rock.
Then, as a conclusion, the most challenging and critical step in a successful autorotation procedure in a UAV is the declaration; the system must recognize all situations in which autorotation is needed, and declare that state quickly. Once declared, main rotor speed is kept constant by adjusting the collective to ensure a controlled descent. Finally, AGL is determined using a laser or radio altimeter and a flare is initiated at a set altitude over the terrain to ensure a soft landing.
Alpha is one of the few helicopter manufacturers worldwide whose craft are capable of performing autonomous autorotation; this is only possible because of the usage of a high-end autopilot, the Vector from UAV Navigation, used to convert manned into unmanned helicopters. It’s also because Alpha´s flight operations team has performed this maneuver dozens of times in many different circumstances in order to find the best settings and strategies to reach the ground–saving the UAV and its payload no matter what the failure scenario.
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