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Monday, April 1, 2013

Leading Edge: Evaluating Handling Qualities

By Frank Lombardi 

Rotorcraft pilots love to talk shop. We chat about our helicopters and how they handle, regardless of whether they are a pleasure to fly, or are a handful. Sometimes you’ll catch us outstretching a steady clenched fist as we describe the “rock solid” hover tendencies of one model, or rapidly fluttering a hand as we speak of another which gets “squirrely” on approach to a platform landing. In any event, we are always talking about their personalities as if they are living, breathing creatures, and I don’t think there is a helicopter pilot out there that would argue the opinion that they are.

On the topic of pilot opinion, ask any pilot to give you theirs with respect to any model, and you’ll find that opinions vary greatly (and in some cases are even directly dependent on the number of pairs of aviator sunglasses one owns). The problem is that a pilot’s opinion of handling qualities will be biased, due in large part to past experience with other models, amongst other factors. Therefore nearly every pilot will have a different opinion, or highlight different factors in their evaluation, making consistent evaluation difficult if not impossible.

In the very early days of the Wrights, and I’m sure even de la Cierva, it was challenging enough to build a system that could stay airborne with an inexperienced pilot, fly a straight line, and land safely. As more requirements needed to be met—like, for instance, turning—more thought needed to be put into what characteristics an airplane or rotorcraft should have so that each new task could be accomplished. As more flying machines evolved, so did the above issues with pilot opinion. Finally in the 1960s, a method of standardizing and quantifying pilot opinion was developed, refined, and has remained the industry standard today. It is called the Cooper-Harper rating scale (see table), and for it to be effective, test pilots must be given clear criteria to evaluate, and trained in how to apply the scale’s ratings.

“Handling qualities” can be more traditionally defined as those that “govern the ease and precision with which a pilot is able to perform the tasks required in support of an aircraft role.” They result from the combined performance of the pilot and vehicle acting together as a system. Besides basic stability and control attributes, other factors that can affect handling qualities are 1) cockpit design, 2) aircraft performance, 3) meteorological conditions, and 4) any other factor that can affect pilot workload. Basically, the goal of the manufacturer is to produce a helicopter that can do all its required mission tasks without mentally and physically tiring out the pilot in the process. During handling qualities flight test, a test pilot will evaluate a specific task that might be one small portion of an entire mission, like tracking the ILS on an approach, or holding crosshairs on a target (both done to a specified degree of precision). The handling qualities rating (HQR) he/she assigns to the task will be a direct result of the workload experienced while accomplishing, or trying to accomplish the desired task. Naturally, as workload increases, less brainpower is left to accomplish other tasks, resulting in a poorer HQR score. The rating scale is numerical from 1-10, 1 being excellent, and 10 being awful (uncontrollable). Following branches down the Cooper-Harper decision tree, the test pilot will come to one of these three levels of acceptability: a) Is the vehicle controllable? b) Is adequate task performance attainable with a tolerable workload? c) Is the vehicle satisfactory without improvement? Within each of these branches, there are three more descriptive sublevels. A rating of 3 is the dividing line between a helicopter that is OK “as-is”, and one with issues that should be fixed. Apply a rating worse than 6, and the deficiencies must be fixed. The scale is a very effective tool, and provides accurate, repeatable results in trained hands. By averaging the ratings of multiple evaluation pilots of varying experience, results can become even more accurate.

As the size, performance, and utility of aircraft improve, so do the methods of analysis. The adaptable human pilot, however, has remained relatively constant, and remains hard to quantify. It’s difficult to know what variables he is sensing as he corrects errors and “closes different control loops” during the task. His/her ability to adapt to the shortcomings of a design might be troublesome to a designer’s analysis, but is certainly a tribute to the human condition.

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