A Tethered Watermelon?

Stabilizing the Helicopter by Tara Hutchinson

The helicopter is an ingenious concept. The reality of vertical flight permits one to hover over, land on, and take off from postage-stamp clearings in the most unlikely locations. Because of the aircraft's obvious flexibility, its applications are endless.

Not long ago, however, the helicopter was viewed as the awkward, unstable stepchild of the aviation industry. It was considered a clumsy contraption, and as an aerodynamic design, relatively inept. Unlike its fixed- wing predecessors, it could not (and still can't) tell rightside-up from upside-down.

Most airplanes trimmed for straight and level flight will generally maintain a stable attitude, even when flying in turbulent conditions. Not so with the helicopter! It requires a pilot to constantly make control adjustments in order to compensate for varying atmospheric conditions.

In addition, the helicopter is hopelessly gangly. Unlike the fixed wing aircraft, whose wings are "fixed" as the name implies, rotor blades for most helicopters are attached to a mast by all sorts of joints, flex-points and see-saw like mechanisms to ensure that the blades can maneuver in any combination of directions simultaneously. To further complicate matters, the fuselage hangs from the "loaded disk" of the rotating blades by a long mast. It has been compared to carrying a watermelon around suspended from a rope. Because it hangs like a pendulum, every gust upsets its stability.

Irrespective of its flexibility, the end result remains the same: without some type of stability augmentation, a helicopter requires the constant and undivided attention of the pilot.

Under normal operations like corporate roof-hopping, exclusive attention is not a tremendous sacrifice to make. But in the

4 HOSPITAL AVIATION, MAY 1983

intense and demanding environment of Emergency Medical Services, where pilots have longer hours and additional navigation and communication responsibilities, flying without the advantages of stability augmentation can be frustrating, not to mention exhausting.

Basically, there are two types of flight control systems for helicopters - series and parallel. The series system is designed to assist the pilot when hand-flying the helicopter - take-off and landings, maneuvering in confined or precarious situations, etc. It also permits the pilot to fly "hands- off" which helps improve his effectiveness in managing other cockpit functions.

The parallel type control system is an adaptation of a fixed-wing autopilot and is primarily designed to aid the pilot in reducing workload in lengthy cruise flights. There are several reasons why the series approach is ideal for rotorcraft stabilization while in most cases the parallel system leaves one wanting.

The three key elements which determine the optimum performance of a helicopter stabilization system are: 1) Precise motion rate detection, 2) Accurate control correction response, and 3) Pilot and system interface (transparancy).

Of these, precise motion rate detection is the most important. This is particularly true for hover and low speed operations. Current technology helicopter stabilization systems typically employ either attitude gyros or mechanical rate gyros, The stabilization effectiveness of the series system is best enhanced by a highly sensitive rate gyro - capable of sensing movements as small as 1/100th of a degree, and sending corrective signals to the series actuators immediately.

A stabilization system is also extremely dependent upon the response rate and accuracy of the

control adjustment actuators. In order to stabilize a helicopter, a flight control system must not only be able to sense, but immediately counteract small aircraft disturbances before they can develop. This simple fact makes high actuator speeds essential.

The third element is transparancy. A series system is designed to continuously stabilize the helicopter, whether the pilot is hovering or hand-flying the aircraft, from hover out to maximum speed and back again. The stabilizing control inputs, although not felt at all by the pilot, are added to his control movements to insure smooth aircraft responses even in turbulent conditions. A series system may be engaged before liftoff and disengaged after touchdown. The net result is that the series system enables the pilot to fly "hands off" plus the added advantage of being permitted to "f ly through" the system without disengaging it, and without losing the benefits of stabilization.

For a long time, autopilots were thought to be a luxury. But as usage increased, stability augmentation systems and autopilots provided so much additional safety by reducing pilot workload in fixed-wing aircraft that it's rare to find an airplane without one. Since helicopter pilots in an EMS role are even busier, a stability augmentation system is a worthwhile consideration for VFR flight, and essential for IFR flight.

Tara Hutchinson is Marketing Coordinator for SFENA Corporation, a Dallas-based company which engineered and produced the "MINISTAB" helicopter flight control stabilization system for civilian and military aircraft world-wide. The company also markets products for 80% of the world's airlines, and maintains installation capabilities for a wide variety of aircraft avionics and accessories.