by Guest Writer Erik “EinsteinEP” Pierce
Stability Augmentation and Control Feedback
Now that we understand how the Trimmer works, let’s look at its companion system, the Autopilot.
The Ka-50 is a big helicopter with lots of moving parts. In fact, if a helicopter is defined as a system of 10,000 moving parts surrounding a hydraulic leak, all trying to get away from each other, then the Ka-50, with its dual contra-rotating coaxial rotor blade system, has 20,000 moving parts and two hydraulic leaks.
Unlike conventional fixed-wing aircraft, on a standard helicopter the lift surfaces (the rotor blades) are also the primary control surfaces. Some helicopters have tail planes and rudders, but these surfaces simply supplement the dynamics of the rotor blades. Many of the Ka-50’s moving parts are connected to the rotor blades and, through rods, levers, hydraulic lines, and black box magic, are also connected to the control inputs in the cockpit (cyclic stick, rudder pedals, and collective lever). This enables the pilot to change the pitch angle of the blades from within the safety and relative convenience of his telephone-booth sized cockpit. However, coordinating the motion of all those moving parts with the rotor blades under the dynamics of real flight is awfully complicated, so the Ka-50 includes a hand-dandy autopilot to help the pilot out.
According to the DCS: Black Shark web site, “the autopilot system is integrated with the targeting and navigation systems and it produces control input for automatic flight system for deviation of the helicopter from the assigned attitude and altitude”. Unfortunately, a little more information is needed to fully understand and properly implement the Ka-50 autopilot system.
The autopilot in the Ka-50 has two basic purposes:
1) Help pilots get to where they said they want to go, and…
2) Prevent pilots from crashing.
There are many folks who not only disbelieve the second point above, but claim that the autopilot is “of the devil” and designed specifically to kill pilots, so let’s review that point first.
Effect of Stability Augmentation
In aviation there is a phenomenon known as Pilot Induced Oscillation (PIO). A PIO occurs when the pilot makes a series of control inputs in opposite directions quick and strong enough that a coupling is made between the aircraft dynamics and control inputs resulting in the aircraft responding much more strongly than the pilot expects. To the pilot, it seems like the plane has gone crazy, exaggerating inputs or even reacting exactly opposite to them. Increased efforts to control the airplane actually become counter-productive, and can result in a very wild ride. PIOs can theoretically occur in any control axis but one of the more critical places they happen in fixed wing aircraft is in the pitch axis on a landing approach. While attempting to control descent rates by use of pitch controls, the pilot can enter a PIO while trying to alternately chase descent rates and airspeed. PIOs at low altitudes on landing approach can and have resulted in crashes and deaths. In a helicopter, which is inherently unstable to begin with, PIOs can also occur during transitions to/from hover or at a number of other times where it is very unsafe to lose control.
Good pilot training helps prevent PIOs, but modern control systems are designed to prevent them from even being able to happen in the first place. Some control systems have a function that dampens or filters control inputs from the pilot to the control surfaces, “smoothing” the pilot’s inputs, making it much more difficult to enter a PIO. Some pilots feel that stability augmented controls are less responsive or more spongy than non-augmented systems, but they do result in smoother aircraft responses, increased stability, and fewer PIOs.
As an historical example, the 1992 crash of a prototype F-22 jet fighter was linked to an incorrect control system response setting that resulted in the controls becoming much more sensitive than the pilot expected, enabling the pilot to enter a PIO and strike the ground. If the proper control setting had been set, the test pilot’s inputs would have been dampened and it is unlikely that the pilot would have entered the PIO and suffered damage to the aircraft.
In the Ka-50, inputs for the cyclic, rudder, and collective are augmented based on the autopilot channels that are activated: Bank, Pitch, Heading, or Altitude. Stability augmentation is always provided for any activated channel, regardless of what mode the autopilot is in, including the Flight Director override. The emergency Autopilot Disengage function automatically turns off all the autopilot channels, which also removes stability augmentation.
Autopilot Control Feedback
Ka-50 Autopilot Channel Buttons
In addition to stabilizing control inputs from the pilot, the autopilot can exert control forces (control feedback) based on the current autopilot mode and assigned helicopter states (position, roll, pitch, yaw, airspeed, and altitude). For each channel, the autopilot can push or pull the controls in the associated attitude axis to achieve the assigned attitude, airspeed, or altitude. For example, if the Bank Hold channel is active, the assigned bank angle is 0 degrees, and the navigation system is reporting a bank angle of 10 degrees to the right, the autopilot will input left cyclic pressure until the bank angle returns to the assigned bank angle of zero degrees. The same process happens in each of the autopilot channels and the associated control axis.
Autopilot control feedback is like having a second pilot also holding the controls and making control inputs in addition to your own. While this sounds neat, having these two different inputs can result in situations where the autopilot input counters pilot inputs if they try to maneuver the helicopter away from the assigned state conditions (e.g., changing pitch). The Ka-50 implements two methods to help avoid entering this unnecessary situation.
First, autopilot control feedback only has 20% control authority. This means that the pilot, who can move the controls the full 100%, can overcome any control feedback the autopilot provides. Another way to think of this control authority limitation is that the autopilot can add or subtract up to 20% of the total control range to whatever control input the pilot is holding or has trimmed in, but no more. As an example, if the autopilot is providing its maximum right rudder control feedback, the pilot can counter this input by simply applying 20% rudder to the left. This input plus the autopilot’s 20% to the right cancel each other out, and the autopilot can’t provide any more input. The pilot can even provide more rudder input, making a left turn even with the autopilot pegged full right. Figure 16 below shows an exaggerated example of control feedback with limited control authority. Note that the autopilot’s responses slowly ramp in, which also helps the pilot overcome the autopilot, if needed.
Effect of Control Feedback
On the other hand, if the assigned heading is to the right and the pilot is providing or has trimmed in 20% more left rudder than the autopilot needs to make that turn, the autopilot won’t be able to achieve the assigned heading. Having a difference of greater than 20% control between the current trimmed control settings and the amount of control feedback required by the autopilot to achieve the assigned attitude/altitude is a common and nearly undetectable problem for helicopter pilots that are not aware that this conflict can occur as these pilots provide control inputs that counter the autopilot’s inputs or inadvertently trim out the autopilot’s feedback response. This underscores that a solid understanding of the Trimmer system and autopilot control feedback is critical in understanding how to successfully employ the Ka-50 autopilot.
The second control feature that enables the pilot to avoid fights with the autopilot is that the autopilot control feedback inputs are disabled while the Trimmer button is held down. This enables the pilot to make changes to an established attitude by pressing and holding the Trimmer button down and then making the desired attitude changes without autopilot feedback trying to maintain the old conditions. Releasing the Trimmer button after the maneuver sets the autopilot’s assigned conditions to the current state of the helicopter, “aligning” the autopilot with the current conditions.
Similarly, pressing and holding the collective brake handle down disables autopilot feedback to the collective axis, while releasing the brake sets the altitude command to the current altitude.
Now for the gotcha: if, prior to pressing the Trimmer button, the autopilot is providing some control feedback response within its 20% authority s to maintain the assigned states, pressing the Trimmer will remove these control feedback inputs quite suddenly. If the autopilot control feedback response was great enough (up to 20% authority, remember?), this is similar to jerking the flight controls around, resulting in an unexpected jump in the helicopter’s attitude. Frequent and consistent use of the Trimmer to minimize the difference between trimmed settings and required control settings will reduce the total response required from the autopilot, preventing this crazy behavior when the Trimmer button is pressed. There is no indication of the autopilot’s control feedback, but practicing various flight conditions (transition to/from hover, etc.) with the Flight Director override on should help pilots familiarize themselves with the control inputs that the autopilot will need to make to establish the same flight conditions. Pilots who are aware of how control feedback needs vary over the course of their flight should be able to anticipate the autopilot’s responses and use the Trimmer accordingly to reduce the response required and minimize these control jumps.
Knowing how to use the Trimmer is important. Fly for a while with the Flight Director on and be able to control the helicopter with minimum control inputs. Practice standard maneuvers and try to anticipate conditions where the autopilot control response may become saturated, like when accelerating and decelerating. Then implement the autopilot and practice those same maneuvers.
After you’ve familiarized yourself with the Trimmer and autopilot feedback responses, try experimenting with the autopilot modes, the next part in this series.
Other articles in the Series:
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