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Auto-Hovering

Here’s a first article of a series dedicated to trouble-shooting. In this one we’ll explain how the auto-hover is operated on our AW139 version 4.00.

THE BASICS

What you need to get first is how the auto-hover works in this very first release. Basically it works as the real one, once activated it will reduce all your X and Z axis velocity vectors to zero knots, in other words it will decelerate your horizontal directions until the helicopter will stay in position without speeding up, unless you decide. The Y axis, or vertical direction and speed, will be controlled through the collective. The more you raise it the higher the ascending speed and vice versa. Yaw, or nose heading around Y axis, is controlled with the pedals. Once activated, the autopilot attempts to keep the your heading steady. In adverse weather conditions, you may be asked to act a little bit more on the pedals. As explained in the guide provided with the product, auto-hover shoudn’t be engaged above 20 KTS. This is the limit speed above which the helicopter will have to reduce too much speed to keep the helicopter nearby where desired. Engaging the auto-hover abobe 20 KTS may give unpredictable results.

TYPES OF AUTO-HOVER

We’ve been actually implementing two types of auto-hover.

  1. Type A is hands-ON-controls is untied to the FT (force trim) system of the aircraft, in order to leave the pilot more control on the rolling and pitching axis. It’s designed to help out during manouvers, approaches, in adverse weather conditions and can be used to decelerate the aircraft.
  2. Type B is hands-OFF-controls and is tied to the FT (force trim) system. This is the stablest hovering mode and lets you put your hands off flight controls. When type B is engaged the nose pitches up +10° as described in this article. Being in conjunction with flight trim means that you can trim the direction and speed of auto-hover. As the real one, never exceed 60 KTS in this mode.

OPERATING INSTRUCTIONS

The two modes are mutually exclusive. They can be switched by switching ON (type B) or OFF (type A) the FT (force trim) switch on the MISC panel and then pressing HOV (hovering) mode button below on the FD (flight director) panel.

When Type B is engaged and no joystick is present, the collective raises automatically to the right blades pitch value, in other words it lets you achieve an automatic take-off, putting your aircraft steady ±5 ft.

CHANGES ON THE MFD

HOV mode is designed to reduce horizontal speeds to zero, as in reality. This means you will always control the ascent or descent of the helicopter through the collective. When activated, the HOV mode changes the HSI on the MFD with a compass showing the velocity trend and direction, just like the real one.

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Introducing the Auto-Hover

Indeed we are very happy to inaugurate our brand new blog with the story of the first release of the auto-hovering system we bring to X-Plane through our AW139. When we first announced it, we were actually stuck on the programming, but the best was yet to come.

BRAVO! BRAVO! BRAVO! I could go on about all kinds of aspects, the groundstone you have layed for future versions with the rebuild of the high res interior, sound, et cetera, et cetera. but you know the feeling when you jump into the machine and think you know everything and then you meet a totally different character, more sensitive and responsive, more powerfull in all aspects of flying and manoeuvring. I had the feeling I could thread a needle with it, even without auto hover.

The auto-hover is a pricy optional that Leonardo usually puts on the table with SAR and oil platform customers, such as Era Helicopters and Bristow, and with every clients whose operational needs ask for maximum stability. We got documented about the real thing first and at the beginning we have to admit it was really frustrating to find a clue about how to obtain that effect. Speaking with other developers not only didn’t solve any of our doubts, but made our thinking even more confusing. We then contacted Austin and his answer was even more frustrating.

Wait! X-Plane autopilot it’s not designed for that!

What we wanted to get was an hovering experience similar to the one of X-Plane AI when it’s asked to manage our helicopter’s take off. Have you tried it yet? It performs beautiful, steady hoverings while waiting the tower controller for permission to raise the collective, give pedals and push cyclic forward. Hovering is also used by AI to taxi the helicopter and we thought it would have been a super-feature for our helicopters.

Whe take our time to study how this was done, then we got coffee and seated in front of X-Plane searching for insights. The first thing we did was freezing the AI right after the hovering was started, in order to get unlimited time to observe the hovering. Once we got there, we attempted to change the aircraft heading and roll ratio via certain autopilot datarefs, only to understand that they were disabled since version 9… Thanks, Austin!

It’s important to explain here that, differently from other simulation platforms, X-Plane’s main asset resides into its phisics model, maniacally built around the mimic of real, in an effort that puts X-Plane into another world, with features and results a lot more complex and veritable than the beautiful eye-candies of the simulator that we all love so much. This means the only way to get what we wanted should have been get by replicating the real behaviour of the real helicopter. No other chances or crazy tricks, such as disabling the flight model as we did on a second attempt. So we got back to study.

Here’s how the type rating manual of the real AW139 explains the feature:

«The HOV mode utilizes the blended AHRS-GPS ground velocity information to provide commands that maintain longitudinal and lateral aircraft ground velocities for hovering and low speed flying. The HOV can be manually engaged by pressing the HOV push button on the Guidance Controller or by pressing the beep trim (Chinese-Hat fifth position) on the cyclic hand-grip. At the engagement the HOV mode velocity references are set to zero». The last sentence was illuminating.

First we had to find and then set all possible movement vectors to zero in a way that each one could reach the 0 in a progressive manner. It was not an easy task because, once we identified the correct way to do it, the phisics model started to put a lot of resistance (and still does, as you can see from the nose cursor in HUD mode vibrating when IAS gets near to zero). Anyway, after a series of attempt we find the correct values that our script would have overwritten on command. And so we did.


After e few lines of code and after creating a custom command to activate the plugin (you can assign it to a key to have the function ready when needed) the very first auto-hovering was a screaming surprise. The result was a stable, progressive hovering with just a flat pitch attitude which was not precisely what we wanted. The AW139 has the main rotor pitched -10° forward, while this solution makes cruise flight leveled and more comfortable for passengers, on the other hand it a little discomfort when hovering, because the nose pitches up +10° as the mast goes perpendicular to terrain.

We made to obtain this typical nose up attitude of the aircraft by associating the hover command to a specific pitching dataref, after a little more tweaking we got it working and tweeted joyfully about that. As you can see, the take-off (raise of collective) of the helicopter is indeed very sweet. This is due to an arbitrary automation we put on the blades pitch when flying with mouse. It’s obtained when ready for take-off by activating the FT (MISC PANEL) and the HOV button. This easter-egg – which is obviously not present on the real helicopter – raises the collective gently to the target pitch value, in other words the helicopter takes off and sets the hover automagically. Operating instructions here.

We hope you liked the story, it’s just a very little part of a huge effort of development running for three years now. We decided to share it to show how much effort is needed just to deliver one single feature of the helicopters we all love. Thanks for reading and spread the love for your developers!