Why aren't there simplified fly-by-wire helicopter controls?

Question

I was discussing helicopter control with relatives over the holidays, particularly regarding how difficult it is to fly one and how changing any control requires adjusting the other two main controls. One of my relations asked whether any helicopters have a computer control system that automatically compensates, allowing simplified controls that almost anyone could fly.

I replied that I had not heard of such a thing and could only guess why not:

1. Reliability concerns and possibly pilot training concerns (i.e., you'd have to know how to really fly in the event of a computer failure anyway, so why even have the computer?)
2. Subtle complexities of the control interactions not lending themselves to reasonable computer automation.

Are there actually no systems like this? Why not? Were my guesses any good?

Regarding the possible duplicate, I saw that question and it seems to indicate that automatic simplified controls would be possible, but it doesn't seem to fully answer the question of why not have a psuedo autopilot that allows a pilot to just push forward on the cyclic to go forward (for example) without having to constantly adjust the collective and rudder at the same time.

Answer 1

Helicopter manufacturers seem to have been slow to adopt fly-by-wire systems. I used to work in the General Dynamics group which designed the fly-by-wire system for the F-16. That was the first production aircraft which had full-authority fly-by-wire (without mechanical backup systems). By the time I started working there, they were in process of updating the system to incorporate a digital flight control computer. The production required extensive coordination between multiple disciplines. As an electrical engineer, I regularly interacted with software engineers, control-law designers, test engineers, and "mechanization" engineers. It took many years for the company to develop such a culture, where each member is concerned with the efficiency and safety of the aircraft. I speculate the helicopter manufacturers didn't catch up for a couple of decades because funding was limited.

Here is an article which describes the Army's experience: Army Slow To Adapt Fly-by-Wire Controls for Helicopters

Funding aside, helicopters are ideal candidates for fly-by-wire systems. The obvious goal is to take a marginally controllable aircraft and make it both stable and maneuverable. That would dramatically reduce pilot workload. Most helicopter fly-by-wire systems accomplish hands-free hover and decoupled control inputs (i.e. sideways flight without needing rudder inputs).

The first helicopter which demonstrated a great fly-by-wire system was the Sikorsky RAH-66. It even achieved sideways flight at 60 knots and backward flight at 25 knots, using only cyclic stick inputs. Unfortunately, that program was cancelled in 2004. The first production military fly-by-wire helicopter is the NHIndustries NH90, which was the first helicopter to fly without mechanical backup controls, in 2002, and entered service in 2007. The first civilian fly-by-wire helicopter, the Bell 525 is currently under development.

Answer 2

There are two extremes:

• No augmented controls, i.e. most normal aircraft. Pilot controls control surfaces and engines directly.
• Heavily augmented controls (left, right, up down shifts exactly like that), like quadcopters. Pilot gives command how the aircraft should move that is translated by algorithm into control surface and engine inputs.

At either of the two extremes, stuff is decently well understood. It's when you go in between that it starts becoming problematic.

Augmented Controls

There is always a conflict between a beneficial change making flying easier for the pilot and a greater level of abstraction from the flight controls. One place where this is particularly prevalent is on Airbus aircraft, where misunderstandings between the pilot, interface, control and aircraft have lead to multiple accidents.

Simon's point here- what do you 'want' you control surfaces to do- is very relevant.

Dynamic System

Helicopters are incredibly dynamic. For instance, a sideways gust will push the helicopter to the side, but that would also result in a disproportionate rotation on the whole airframe caused by the surface area of the tail rotor fin. Yet the pure sideways (not moment) force from increasing the tail rotor power is uncompensated and would require main rotor adjustment again to stay in position. Any pilot response and input would have to be handled appropriately to prevent pilot induced oscillation.

Stability Augmentation is used, for instance on the EC135:

The Auto Flight System is hierarchical in concept and on G-IWRC comprised a three axis Stability Augmentation System (SAS) and an autopilot. The SAS consisted of a Pitch and Roll SAS (P&R SAS) and yaw SAS. The helicopter was also equipped with a pitch damper. These systems are used for stabilising the attitude of the helicopter about the longitudinal, lateral and yaw axes by applying limited authority inputs to the main controls.

The SAS system is designed for ‘hands-on’ operation, which means that the pilot must provide control inputs through the cyclic control and yaw pedals in order to control the attitude of the helicopter. The SAS is automatically activated during the start procedures and can be disengaged by pressing either of the SAS DCPL switches located on top of each cyclic stick grip. Re-engagement of the SAS is through a four-way switch on the cyclic grip, labelled P&R/P – P/y RST. Eurcopter EC135 Accident Report

Pilot Role and Regulatory Aspects

I've worked on UAV systems that go 'the whole way' on control automation. I think part of the problem is the conservative opinion on the role of the pilot. Airbus have become heavily criticized for their design choices that are supposed to make it easier for the pilot to fly. Augmenting flight controls is risky business.

Changes are difficult to implement and testing expensive in the aviation sector. We could ultimately remove the pilot and whoever onboard (doctor, nurse) could just select a spot on a map as a landing site. By going to the stage of making push up/down/left/right... controls as you suggest, we have created fully automatic flight controls. It is (entirely) technically feasible, since that's what the autopilot does. Changing how active control inputs are interpreted from a use, licensing, training and execution standpoint is a whole different story.

Answer 3

Yes fly by wire controls are applied in helicopters. But there is a simpler and much older solution to the multidimensional flight control dance in helicopters: the mechanical mixer.

Consider a helicopter in a stable hover. Only small movements around the neutral point of the flight controls are required to compensate for small variations in air flow. Now pull on the collective to gain height, and the neutral point of both the pedals and the lateral stick change: rotor supplies more lift but also more torque, compensate for this with increased tail rotor thrust, compensate for increased sway with the lateral stick.

The mechanical mixer does just that, automatically. It is found in large helicopters like CH-53, UH-60 etc, and is located behind the flight controls but before the swash plate actuators, and it is just a set of mechanical arms and legs that interconnect collective with pedals linkage etc.

Note that the mechanical mixer only changes the trim point of the flight controls, but not the static stability of the helicopter or the cross-coupling effects of rotor inertia etc. For that you would need F-16-like fly-by-wire with redundant computers & all.

Answer 4

The concept of advanced controls for rotary wing craft has been around a long time. I worked for McDonnell Aircraft in their simulation group during the 80's when they were competing for the LHX program (Light Helicopter eXperimental). While I can't say this is the earliest implementation of such controls, it has to be one of the earliest.

The flight controls basically removed the cyclic and collective and replaced it with a four-way sidearm controller -- basically a 3D joystick with rotation. Any horizontal motion within the plane of flight was controlled by pushing the controller in the direction you wanted the craft to move, forward, backwards, or sideways. Vertical flight was controlled by applying upward or downward pressure on the controller. Platform yaw was performed by twisting the controller. --Reference

Though I've loved aircraft for many years, and I've not (yet) gotten my PPL, I used to fly the fixed wing simulators and had fun. But having never flown a traditionally controlled helicopter, I found flying the LHX very intuitive.

Answer 5

On main consequence of having independent controls in a helicopter is that the same control movement will have different outcome depending on the movement of other controls.

For example, consider the collective- in hover, varying the collective will result in a change in altitude, while in forward flight, the same thing will result in either a change in altitude or speed depending on how the cyclic is moved. The operation of cyclic has similar results based on operation of other controls.

This is the problem in having pseudo or coupled controls- the helicopter response may differ from that of the intended one. On the other hand, the FBW computer is pretty straightforward- you input the controls, which are sent to the actuators through computer. The computer need not guess what you want to do here.

I think there are some helicopters where moving collective automatically adjusts the torque, though this is the only case where I think the controls are coupled.

Answer 6

Short answer: We already do have Fly-By-Wire rotor-craft from Bell/Sikorsky, Airbus, and probably others. Reference the Bell 525, Sikorsky V22, Airbus/Eurocopter NH90

Long answer: It is hugely expensive and time consuming to get a new aircraft or system certificated. Even simple changes to a certificated aircraft or systems can be hugely expensive and time consuming from a regulatory compliance prospective, which must include testing said aircraft and systems during the development cycle while documenting every tiny detail.

People are resistant to change in general -- making changes to something they know, love, respect, and sometimes fear will bring out the worst in them. Pilots are just people with feelings, desires, and opinions just like everyone else.

At the time of this writing in late 2018, Fly-By-Wire systems are not just technically feasible, they are mature. The electronics and servos are so inexpensive that triple-redundancy is a trivial additional cost. All of our most complex commercial and military aircraft today (2018) have Fly-By-Wire systems including rotor-craft and space systems.