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Sunday, November 1, 2015

Stabilizing Influence

The HeliSAS system uses attitude-command-attitude-hold logic to help smaller helicopters fly smoother.

By Frank Lombardi

A range of pilots, from leisure and law enforcement ones to students, will realize benefits from the HeliSAS’ ability to reduce workload on Robinson’s R66 and R44.
Photo by Frank Lombardi
When it comes to flying helicopters, “simpler” does not necessarily mean “easier.” Most of the time it is quite the opposite.

As helicopters grow into bigger machines with multiple engines run by more complicated systems and fancier instrumentation, they tend to get packaged with equally elaborate flight control systems that aid the pilot in flying such beasts. This can effectively reduce the balancing act of helicopter flying to an almost secondary function for the pilot.

Historically, the smaller, lighter, simpler, less costly siblings of these larger helicopters are sold as completely naked machines with all their idiosyncrasies for a pilot to learn and manage. Allow any experienced pilot of a larger, more complex helicopter to perform various tasks with a smaller, “simpler” machine, and he or she will be humbled by the high workload of taming its gyrations.

It seems that this generality will soon change, thanks to the advent of technology like the Genesys Aerosystems HeliSAS. Developed by aerospace engineer and flight controls expert Roger Hoh of Hoh Aeronautics Inc., the HeliSAS is a two-axis autopilot and stability augmentation system designed for light and medium helicopters. This system recently received FAA certification for use in both the Robinson Helicopter R44 and R66. At the Robinson factory in Torrance, California, R&WI experienced first-hand how “simpler” can now be synonymous with “easier.”

The HeliSAS Difference

The R44 cockpit view with Aspen Avionics EFD 1000H in the center and Garmin GTN 750 on the side (above, left). The R66
cockpit with Garmin 500H and GTN 750 and HeliSAS autopilot in the stack (above, right). Photos courtesy of Robinson Helicopter
Stability augmentation systems (SAS) can come in various flavors. Some systems use fast-acting, limited-authority, in-line (series) actuators to react and damp out disturbances in roll, pitch and yaw rate. This minimizes the effects of wind, cross-coupling and sometimes even the pilot. This in turn alters the aircraft’s flying qualities, giving the operator a sense of improved handling and transparency.

The HeliSAS does not use this method. It will not transform your R44, R66 or any other lightly damped helicopter from a rowboat to an ocean liner. Instead, the HeliSAS uses attitude-command-attitude-hold logic to provide stability through attitude retention. This system senses changes in roll and pitch attitude (there are no yaw or collective channels). It also uses parallel actuators connected alongside existing flight control linkages to provide small pilot-felt corrections back to trimmed-level flight.

Simply put, the basic HeliSAS will make your rowboat act like it has a lot more ballast set in a deeper keel, thereby helping it to stay upright.

How It Works

There are many virtues to a system such as this.

Dale Taft, Robinson flight test engineer and pilot and an FAA designated engineering representative (DER), was intimately involved in the engineering, flight testing and certification of the HeliSAS system on both the R44 and R66. He explained that the development of HeliSAS began with the R44 in mind as its primary design customer, and a collaboration with Hoh perfected its “fly-through” system of geared, back-drivable electromechanical servomotors.

When active, the roll and pitch SAS servos constantly make small corrections to the cyclic position to maintain a desired attitude. The autopilot system gets its attitude information by comparing data from sensors in its dual-processor flight control computer (FCC) and from a separate—but necessary—attitude and heading reference system (AHRS).

To maneuver, the pilot can move the cyclic against (or “fly through”) the hold of the servomotors at any time using only 2 to 3 lb of stick force. Relaxing one’s grasp on the stick allows the SAS to fly the aircraft back to its original attitude, essentially leveling itself. (Due to these light stick forces, the hydraulics must be operational and the control frictions must be off or else the system becomes easily overwhelmed.)

Pressing a “force trim release” button on the cyclic removes current from the servos and suspends stabilization. Keeping the button pressed gives back the naked helicopter, with no force feedback on the stick until the button is let go. Releasing the button then re-energizes the servos and restores their stabilizing grip on the controls at the newly established trim position.

For safety reasons, there are limits to what attitude the pilot can ask the SAS to maintain. The HeliSAS will hold any attitude up to 11 deg nose up, 6 deg nose down and 10 deg of bank. Setting any attitude beyond this will settle the aircraft back to the maximum allowable.

How It Flies

Robinson President Kurt Robinson on the production R66/R44 production line. The company’s broad customer base should extend its influence in autopilot/SAS adoption.
Photo by Frank Lombardi
When I flew an R22, I was excited to strap on a model in which “everything should be 1.3 times that,” as Frank Robinson once said in comparing the R44 design to that of the smaller helicopter.

Upon startup of the R44, the panel’s Aspen Avionics EFD 1000H primary flight display (PFD) comes to life. The beautifully compact, feature-rich display provides air data, attitude and heading information in a lightweight package that fits in the standard eight-hole instrument panel.

The Aspen PFD is a necessary component of the factory-installed HeliSAS system in the R44. One of a variety of Garmin GTN GPS/NAV/COM/multi-function display (MFD) must also be packaged with the factory install for full upper-mode autopilot functionality.

The R44’s smallish panel and pedestal requires the Garmin GTN to be situated in an adjoining hooded panel extension directly in front of the pilot to avoid any extreme “heads-down” time. The HeliSAS control panel is situated about halfway down the slant-pedestal. Wired directly to the avionics bus, the autopilot starts as the rest of the instrumentation is powered up.

During a brief self-test, four beeps confirm that the SAS aural warning tones are operational. Alternating green and white lights settle to one steady white light over the SAS button, with the autopilot now in standby mode.

I elected to fly a circuit around the traffic pattern unaided to reacquaint myself with the “feel” of a Robinson. It took a bit of concentration to fly a nice pattern while holding airspeed and altitude, trying not to embarrass myself with R&WI Editor-in-Chief Jim McKenna in the backseat of the four-place R44.

As I terminated the approach and settled into a hover, Taft encouraged me to engage the basic SAS with a press and release of the TRIM button on the cyclic. I felt nothing more than a slight increase in stick force, but the SAS was active, and its corresponding control panel light turned from white to green. Despite my already light grip on the cyclic, I let go even more. The R44 obediently remained at the same pitch and roll attitude, while the cyclic bounced around gently between my fingers. The sensation was odd at first, as I did not trust the skill of the machine over my own.

With the SAS still engaged, I shifted the cyclic forward to begin the next pattern. Having no objection, the nose of the aircraft pitched downward, with a slight opposing stick force reminding me of the way back to “hover trim.” I ignored this cue until I reached rotation speed, and then again relaxed my grip, allowing the cyclic to travel aft only enough to attain proper climb attitude. At that point I tapped the TRIM button, telling the SAS to “hold this” new attitude, and it dutifully complied. At pattern altitude I repeated the process, “flying through” the stick forces to my new desired attitude and then tapping the TRIM button to maintain it. On final approach, I elected to keep the TRIM button pressed, since I preferred a super-light touch for very fine airspeed control.

Taft explained that the basic stability functions of the SAS work across the entire airspeed range. Although I found it a bit gimmicky at first to use during such a high-gain task as a hover, he sold me on the method of continually tapping the TRIM button to re-center the trim point. Pleasantly, there was no jerkiness in the controls associated with the minimal changes in stick force. This became second nature very quickly, decreasing workload and virtually eliminating any “butter churning.”

As Taft described more of the flight testing that went into certification and pointed out some of the functions of the Aspen display, it was the perfect time to highlight the autopilot’s upper mode functions.

Dialing the heading bug on the Aspen’s horizontal situation indicator (HSI) to point us toward the bluffs of the Palos Verdes Peninsula, I pressed the HDG button on the autopilot control panel while in the climb and relaxed my grip. The aircraft responded by rolling into a 20-deg bank toward the selected heading, then rolled out on course while it continued to gain altitude. I had nothing to do but to remember to fly the pedals, as there is no assist in the yaw axis.

The system does not have the ability to pre-select a capture altitude, so it will maintain altitude or pass through when the ALT button on the control panel is pressed. I did so while in a climb just before the desired altitude, and the aircraft gently pitched down in response to the overshoot without hunting to capture it.

At this point, Taft highlighted another of the safety features of this system; the autopilot’s inputs and response rates were, by design, always benign enough to avoid a low-G situation that could result in mast-bumping, such as encountering turbulence while at high speed.

For the next demo, Taft had me completely disengage the autopilot system. One tap on the “AP OFF” button on the cyclic removes all upper modes but leaves the basic SAS function operating. The second tap opens the magnetic clutches and removes all stabilization, and the system reverts to standby mode, signaled by four beeps. We then put the aircraft into various pitch and roll attitudes to simulate an unusual attitude caused by flight into inadvertent IMC.

It turns out that the FCC is a pretty smart little box. Engage the SAS from standby mode at greater than 15 deg nose-up, 10 deg nose-down and 6 deg of roll, and the aircraft will assume you’ve gotten into an unusual attitude. Relax your grip and it will respond by rolling wings-level with a slight 2-deg nose-up pitch. Engage the SAS at smaller angles, and it trims at the angles engaged or the maximum-trim limits of 11 deg up, 6 deg down, 10 deg bank—whichever is less. Taft cautioned, however, that the preferred operation would be to “always leave the system on (SAS light green), thus keeping you within the trim limits to begin with.”

On our way back to the field, I loaded an ILS approach into the Garmin GTN 750. I continued to fly the heading bug and pressed the NAV and VRT buttons on the HeliSAS to arm the approach and glideslope capture modes. The system has the ability to NAV the entire approach, using fancy math to set up on a comfortable 45-deg intercept angle. We chose to fly our own vectors to the final approach via the HDG hold, and the system handled a 90-deg intercept as good as any full-fledged autopilot I’ve ever seen, signaling lock on to both the localizer and glideslope with two green lights displayed over the NAV and VRT buttons and corresponding green symbology on the Aspen’s CDI (HDG and ALT lights extinguished). I centered up the ball and provided the proper power setting for the desired approach speed, and the aircraft flew right down to minimums with no mentionable effort on my part.

During a short break, Robinson President Kurt Robinson expressed his enthusiasm for the system being available in both the R44 and R66.

Robinson Flight Test Engineer Dale Taft (right) explains to author Frank Lombardi that the HeliSAS system’s basic stability functions work across the entire airspeed range of the R44 and R66. R&WI photo
“I just think everyone is going to love it. I think they really nailed the stick forces to give it the right touch,” he said. He added that he is looking forward to HeliSAS’ use by the basic cross-country pilot and beyond. “I’m wondering how long it will be before we are always flying with SAS on a standard. I’m hoping that within a few years, we can get the price and weight down enough so that does become standard on every aircraft, just like when we first came out with the engine governor.”

After lunch, we climbed into the five-seat, turbine-powered R66 outfitted with the Genesys system. The R66 model HeliSAS is certified to be paired with either a dual-screen Garmin 500H or Aspen EFD 1000H as its AHRS source (R44 is Aspen-only at this time). The 500H boasts information-filled graphics and intuitive flight displays that can provide optional functions like terrain awareness and synthetic vision, to name a few. Since the R66 panel has more real estate, a Garmin GTN 750 fits nicely centered underneath the 500H; the autopilot control panel is immediately under that.

We put the HeliSAS through many of the same paces as those of the R44, but this time we were a bit more aggressive on the controls, and we discussed the limits of the upper modes. The R66 flies every bit as nicely as the R44, with an added benefit of its larger size but lighter empty weight by about 200 lb, largely due to its turbine engine. With either model, it is important not to forget to fly the pedals (especially with collective changes) to prevent sloppy flying.

Autopilot upper modes can only be engaged between 44 kt and Vne. The system will signal the automatic cancellation of all upper modes outside this range with one beep, leaving the basic SAS still functioning. Making a large enough reduction in power while in ALT hold, or while trimmed at low speed in HDG hold, can cause airspeed to dip below 44 kt. The single beep is also a good indicator of decreasing airspeed.

During our autopilot-coupled GPS WAAS approach back to the field, again the system displayed its ability to resolve and capture the final approach course. Even though we armed the NAV mode unrealistically close and off-angle to the final approach fix, the aircraft rolled into a 20-deg bank to intercept, then rolled out nicely on final with no hunting. The only variation to the ILS approach flown in the R44 was that the VRT mode could not be armed until the G500H displayed the glideslope indicator, which occurred rapidly just prior to alignment on the final approach course.

If It Fails

An important part of certification of all SAS systems is failure testing. Mechanical or electronic failure of any system related to the flight controls could at best increase pilot workload and at worst be catastrophic. It should never be a question as to who is flying the aircraft, and a pilot should never have to ask what it’s doing. This seems to have been well considered during the HeliSAS design.

Within the flight control computer is a safety monitoring system that is continuously comparing the signals between the AHRS and the FCC. If they begin to disagree, the FCC quickly disengages the autopilot and gives a four-beep advisory to the pilot to take the controls. Extensive certification testing has ensured that the safety monitor will disconnect the autopilot and allow pilot intervention with enough time before achieving any unsafe attitude. As mentioned earlier, the system never commands large cyclic motions or high angular rates and is considered “dual fail-passive.” This means that in the event of a failure, there is no significant out-of-trim condition or deviation of flight path or attitude.

In the very unlikely event of having both a jammed servomotor and a clutch that fails to disengage, the system requires only 7 lb of stick force to override. This was tested on the ground with the hydraulics on, through the full range of control travel to test for any deformation of—or interference with—the normal control system, as a certification requirement.

Who It’s For

There is not a pilot out there who does not appreciate a reduction in workload. Also a businessman, Kurt Robinson is an aviator, and obvious was his enthusiasm as an aviator when describing his vision for the union of this product with the Robinson brand.

Law enforcement pilots flying the R44 or R66 with HeliSAS will appreciate the altitude hold while on a night search as they manage the multitude of duties in their cockpit. Pilots flying electronic newsgathering (ENG) missions will appreciate the decrease in fatigue and increased ability to watch for other ENG traffic as they spend hours hovering out of ground effect with SAS engaged over a breaking news story.

Perhaps one of the least considered but best uses for the system will be by the instrument training community. If an instrument student can decrease the enormous portion of brainpower required to maintain aircraft control during training, this would slow things down enough to allow better processing of the instructions that the teacher in the adjoining seat spouts. The HeliSAS slows everything down without taking the pilot completely out of the loop, so he or she would still build instrument skills. As an added bonus, a student at this level is normally not exposed to more advanced systems until he or she finally makes it into the cockpit of a larger machine. Flying a Robinson with HeliSAS can be a fantastic step to more complex automatic flight control systems (AFCS).

Final Thoughts

As a direct factory install, pricing of the system starts at $60,200 for an autopilot installation with Aspen PFD (Garmin GTN required, not included in that price). Genesys Aerosystems is also working on its own variety of aftermarket install packages, which would offer buyers even more options.

With a weight of only around 12 lb, minimal draw of electrical power and essentially an installation without having to alter the existing flight control system or center of gravity limits, the system is hard to argue against. Everyone right down to the private pilot whose flights are routinely long trips for leisure would find delight in this system, if for no other reason than for the ability to spend more time enjoying the view.

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