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Automatic autorotation now available
Alpha Unmanned Systems and UAV Navigation jointly announced today the introduction of a fully automatic autorotation feature on the Alpha 800 platform that sets it apart from its competitors.
Autorotation, a critical capability for helicopters in case of engine or tail rotor failure, is described as ‘gliding the craft’, something all manned helicopter pilots are trained to do. It is notoriously difficult to achieve and therefore takes extended practice. In the case of a UAV helicopter, no automatic autorotation has been possible… until now.
The new capability represents an important step forward in that it enables a UAV in distress to be safely landed. Minimizing risk to people and property on the ground, it also greatly increases the likelihood that the payload and other expensive components on the platform will be recovered.
Alpha Unmanned Systems and UAV Navigation have been collaborating on this technology for several years, the robust Alpha 800 fitted with sensors and hardware to feed the necessary information to UAV Navigation’s VECTOR autopilot. The Alpha 800 platform has proven to be very effective for developing software as well as for extensive simulation with an advanced Hardware In the Loop simulator. Actual flight-testing confirmed the results with repeated successful landings following autorotations from a variety of altitudes, wind directions and headings. In all cases, no engine power whatsoever was supplied to the rotors.
Autorotation is just one of the safety features provided by the VECTOR autopilot to the ALPA 800 UAV. The system is also capable of flying in GPS-denied environments and in situations where the onboard Magnetometer is unavailable.
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.
The Alpha GCase DUO is a ruggedized portable stand-alone control station designed to operate in harsh conditions and with enormous flexibility.
This ground control station has two 15“ sun-readable screens that permits two operators to work side by side, one controlling the unmanned vehicle and the other controlling the payload.
The high flexibility of the system is achieved thanks to its modular payload bay, designed specifically for electronic components. The modular payload bay is pre-drilled and has a detachable plate with space available for easy subsystem installation. The payload bay contains the following interfaces:
Also for the greatest range-demanding operations the GCS DUO can interface with an external tracking antenna through a military connector. This connector is also linked to the payload bay through the following standard interfaces:
Thanks to these integrated interfaces for both internal and external systems, the GCS DUO can serve for multiple projects. And by simply removing 4 bolts, users can access and replace the system.
Alpha Unmanned Systems has also designed 2 tracking antenna units that fit nearly all possible needs; from the shortest to the longest range.
The GTrack 30 is a lightweight and compact tripod that is used for ranges up to 30 Km or more depending on the data link used. It is very efficient thanks to its orthogonally placed patch antennas.
The GTrack 50 is an extremely robust tripod that is 3.2 meters high and has a large and big parabolic antenna with dual polarization. Depending on the datalink selected, the GTrack 50 is capable 50 km or even 100 km ranges with the use of power amplifiers.
The power distribution system of the GCS Duo uses two Lithium Ion batteries that serve as the primary power source in the configuration up to 3 hours (with no tracking tripod). Once an external power source is connected, a smart batteries management system will charge these batteries and monitor different parameters including the number of cycles, or the balance of the cells and temperature.
If the tracking tripod is installed, the internal batteries act as back-up in case of external power source failure.
In addition, all power sources have integrated resettable fuses to protect the electronic systems.
Designed for use in the most demanding scenarios, the GCS DUO and GTrack units are IP64. The GCASE Duo uses a “wheeled peli-type case” as its frame and all electronic components have been selected to withstand the most extreme use cases. To avoid any damages due to transportation or in dynamic environments, for example, the hard disc of the embedded computer is solid-state and provides outstanding performance. The two integrated screens have configurable brightness that reach the 1600 nits. Regardless of the brightness of the sunlight, the GCS operator will always have a clear view.
Drone usage is expected to soar over the next several years. As legal regulations evolve, many industries will embrace drones for a multitude of tasks from infrastructure inspections to commercial fishing and beyond. And despite the potential for enormous growth, this revolutionary technology has an Achilles heel that is rarely mentioned- it is fully dependent on Global Navigation Satellite System (GNSS)/GPS navigation.
All drones rely on GNSS to maintain a stable position and/or to navigate between waypoints. At the same time, all small drones use MEMS sensors to estimate their attitude. These sensors are accelerometers, gyroscopes and magnetometers that provide data on the aircraft’s three axes.
A common assumption is that by having accelerometers and gyroscopes, a simple integration of their data can provide speed and orientation; and that with a second integration, position can be calculated too.
But it’s actually not that easy. The measurements that these sensors provide include errors, noise and deviation. And while the sensors may include relatively small mistakes in the raw measurement, when integrated together they generate much bigger errors that make navigation and position estimation impossible.
When the GPS signal is lost, the safest strategy for a drone is to hover, maintaining altitude by means of the barometric pressure and attitude reading from the AHRS (attitude and heading reference system)–in other words, keeping pitch and roll angles at 0. With this strategy, the UAV will drift from its position proportional to the wind speed.
In case of GPS loss, the pilot must take manual control to land the UAV safely.
But what happens when the UAV is jammed? Commercial jammers and counter-UAV systems function by emitting strong signals towards the aircraft using the most commonly used frequencies for UAVs, which for communications are 900 MHz and 2.4 and 5.8 GHz, and for GPS are the L1 and L2 bands. The effect of the strong signals emitted by jammers is that the SNR (signal-to-noise ratio) is so small that the receiver cannot decode the real signal from the noise–similar to when we cannot understand someone speaking in a room full of other people talking.
When a UAV is jammed, the UAV will lose both GPS, and therefore navigation, and remote control from the pilot. Again, the most conservative option for the pilot is to maintain a constant attitude (pitch and roll equal to 0) and to descend in controlled way until the UAV touches the ground. This is what 99% of commercially available drones do and what makes jammers and other counter-UAV measures so effective.
Thanks to its use of a high-end autopilot and the integration and calibration of the ADS (air data system) composed of a pitot tube and static port, Alpha Unmanned Systems is able to calculate the estimated wind–which in case of GPS signal loss would be used to calculate the UAV’s estimated ground speed by means of the following formula: Estimated Ground Speed = Indicated Air Speed – Estimated Wind.
By means of the integration of the air speed and considering that wind direction and speed is constant, we can estimate the UAV position with approximate precision that is usually “good enough.”
But what does “good enough” precision mean? It means that Alpha Unmanned Systems UAVs can maintain and follow a flight plan of command and fly to a certain area, or return home safely if the UAV is on BLOS. The long-term error is approximately 50 meters per minute, though the more unstable the wind is the higher the error will be.
So can a “jammed” UAV continue flying? Yes. Although communications would probably be lost, the good news is that normal jammers have an effective radius of several hundred meters, so once outside of jammer-range, the UAV can recover the GPS signal and continue its mission. And even if the pilot knows that the UAV will be jammed, it can be flown over the area while knowing that comms will be lost. Information taken on can be recorded and stored onboard, to be downloaded later once out of the blocked/jammed area.
Alpha Unmanned Systems has integrated a complete pitot tube in its tactical UAV helicopters. The Alpha 800 performs dead reckoning operations and has proven its success in multiple tests.
Alpha knows that reliability improves “minute-by-minute,” Madrid based Alpha Unmanned System, SL proudly announces that its High Accelerated Lifetime Testing (HALT) solution is now in production!
This means that Alpha’s engines and transmissions will be tested in “near-flight” environments for significantly longer periods of time and in environments more similar to actual flight. Longer and more realistic testing means greater fault identification which will result in more reliable platform design and manufacture.
The HALT will also be used to define the maintenance periods of Alpha’s new helicopter platform, (expected in 2021). It will also be used to execute the STANAG defined engine endurance tests and to fine-tune the fuel injection system.
“Testing is critical for flight reliability and safety. The incorporation of the HALT will dramatically increase the testing hours that Alpha Engineers can use to improve design and manufacturing. We expect to dramatically improve our platform quality and reliability through this innovative and necessary tool” Álvaro Escarpenter, CTO and co-founder of Alpha.
In order to easily test the complete helicopter, including the engine and transmission systems, Alpha has designed and manufactured its own test bench (HALT) on which its helicopters will be able to test multiple engine and transmission loads in a precisely controlled environment. The HALT will also simulate endurance and life cycle tests in “near-flight” conditions.
Engine and transmission systems are especially critical for helicopter platforms. Since testing and validation exercises are time consuming and costly, and since industrial systems must be tested for thousands of cycles during development, Alpha’s expects to dramatically improve the airworthiness of its helicopter UAV platforms using HALT.
Autonomous landing on moving platforms is very complex. Maritime autonomous landings are particularly challenging given meteorological conditions, radio interferences and other important variables which frequently affect autonomous landings.
During ground-based operations, the GCS and GDT (the tracking antenna) are static. This means that the tracking antenna is usually pointed North and is horizontally aligned. Its commands are sent relative to North and thus no heading source is required.
During maritime operations however the GDT is moving in pitch, roll and yaw directions. Using a directional tracking antenna with a 10º or 15º degrees of effective radiation pattern requires counteraction and stabilization of the 3 axes to ensure the correct positioning. For this purpose, a simple GPS receiver and an IMU and heading source can be used. Importantly, these cannot rely on a magnetic compass (due to the magnetic interference caused by the metallic vessel).
Take-off or landing operations from a moving vessel is especially challenging as the UAV must navigate relative to the vessel that is moving. Since ferrous materials are frequent on vessels, magnetometer precision may be affected negatively. For any helicopter landing or take-off maneuvers on a mobile platform, it is essential to have clear, precise, and reliable positioning and angles.
Through GNSS, the system will be able to obtain both vessel and UAV positions, and thus relative positioning too. By means of the GPS Compass on both ground and in the air, relative orientation between the UAV and vessel is calculated. This ensures that the UAV always approaches the vessel from the correct orientation.
Override capability by the External Operator is also possible during Take-off and Landing. Manual override provides one extra layer of control capability and may be used to make necessary small corrections for the UAV during its fully-autonomous navigation mode.
Relative and absolute flight plans:
The UAV has the possibility of configuring several flight plans for both flights and landing. One flight plan will be the “default flight plan” which, in case of COM Loss, will be the flight plan that is executed. In the case that the flights are performed close to the shore, an emergency flight plan for Landing on land in case of COM Loss is also possible. Other variables including high wind speeds, can also be used to affect alternative autonomous landing possibilities.
Source: UAV Navigation
Safety and redundancy are essential for all UAV systems. Payload selection, integration and functionality are also essential and must be correctly. All systems must be designed and selected to enable successful landing on mobile platforms while also providing value under normal operating conditions.
As previously described, the UAV requires the same heading source as the vessel. Importantly, it must not be affected by magnetic interference. This will ensure correct navigation and positioning during the Take Off and Landing, when the UAV magnetic compass will be affected by the vessel.
The GPS compass is required to provide stable and reliable heading estimation. This system will also provide a redundant back-up of both the GPS receiver and heading source on normal flights. While video navigation may be useful during the Take-off and Landing, it does not add value during normal operations.
A Radar Altimeter is required to provide reliable measurements of the altitude above the vessel landing platform and the water. This gives us an extra input for the relative altitude over the landing area, increasing the precision of the maneuver.
A vessel is an extremely demanding environment primarily due to the high amount of Electromagnetic radiation emitted by the vessel itself. Communication systems and Radars are some of the electromagnetically active systems that can be found on a vessel along with additional electronics.
All ALPHA 900 systems are EMI shielded to reduce interference from external sources.
Additional information about the EMI testing carried out by the Spanish Aerospace Research Institute on the autopilot can be found here:
Extensive testing to gather knowledge and gain expertise.
Alpha has been working diligently on the required technologies and CONOPS necessary to land on moving vessels for more than two years. From the design and manufacturing of our own moving platform to the evaluation of commercially available technologies, Alpha has invested significant internal resources to integrate this capability on its helicopter UAV platform.
Evaluation of sensors:
At the start of the R&D project, all available sensors were evaluated, including radar beacons for relative positioning, radar altimeters, GPS Compass and Video landing aids in order to define the optimal set up.
Testing on the Moving 3DOF platform.
Alpha has designed and built a 3DOF platform to be able to simulate the conditions of an offshore vessel. This exercise has provided tremendous experience and value as it has enabled Alpha to trial any sensors and procedures quickly and efficiently.
Testing on small boat, modified for testing purposes.
On a small patrol boat, Alpha has added a landing platform to perform take-off and landing exercises under real conditions.
Trials on Coast Guard vessels:
Alpha conducted its first naval take-off and landing from a vessel in 2016. This exercise was a very useful first step to better understand the requirements and conditions of maritime operations.