Flight-Test Icon Takes Last Flight

If you were asked, “How well do you feel your helicopter can accomplish your mission?” do you think you could easily answer the question? On what basis would you formulate your reply? No doubt, if you asked 20 pilots the same question about the same helicopter, they would give you 20 different answers. Without putting specific constraints on the question and even the possible answers, there are too many variables for any evaluations to be consistent, and would be of little use to a design team when trying to produce the ideal aircraft to accomplish a particular mission.

After World War II, as aircraft became more capable and their missions more complex, it was realized that a pilot’s opinion of how an aircraft handled is a key factor in design decisions. But the bias that each pilot unconsciously has, due to other aircraft he or she has flown in comparison, will affect his or her opinion of the aircraft being assessed. Consequently, after years of research, a system was devised to standardize evaluations so legitimate comparisons could be made between the opinions of different pilots.

In 1969, the chief research test pilot for NASA at the AMES Research Center, George Cooper, collaborated with retired U.S. Navy Cmdr. Robert Harper, an engineering test pilot for Cornell Aeronautical Laboratory (now CALSPAN), and published the Cooper-Harper Handling Qualities Rating Scale. This scale has since become the industry’s standard metric for assessing aircraft handling qualities during flight testing.

“Handling qualities” can be more traditionally defined as “those characteristics 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. Factors that can affect handling qualities besides basic stability and control are performance, cockpit interface, weather, or anything else that can affect pilot workload.

If the goal of the manufacturer is to design an aircraft that can do all its required mission tasks without mentally and physically tiring out the pilot, then the goal of handling qualities evaluation is to quantify the amount of physical or mental compensation required by a pilot in performing a defined task to a defined standard.

The Cooper-Harper Scale accomplishes this by having properly trained test pilots use a common language with defined terminology in conjunction with a numerical (1 to 10) rating scale. The heart of the scale involves answering three dichotomous questions: Is the aircraft controllable? Can adequate performance be obtained with tolerable workload? Is it satisfactory without improvement? Sub-questions then further quantify deficiencies and result in a rating.

This greatly simplifies and improves the communication between pilot and engineer. During handling-qualities flight testing, a test pilot will evaluate a well-defined and repeatable task that might be one small portion of an entire mission, like tracking the ILS on an approach, or holding a precise hover over a designated point. The handling qualities rating (HQR) he or she assigns to the task will be a direct result of the workload experienced while accomplishing, or trying to accomplish the desired task to a certain degree of precision. Naturally, as workload increases, less brainpower is left to accomplish other tasks, resulting in a poorer HQR score.

Co-author Robert Harper frequently taught at both the U.S. Naval and U.S. Air Force Test Pilot School, spending years actively mentoring other engineers and test pilots. He was involved in nearly every military aircraft development program until he retired, the last one being the B-2 bomber. He was a fellow in the Society of Experimental Test Pilots (SETP). Harper passed away earlier this year on Aug. 23. He was 92.

With each new aircraft comes new control systems, new missions and new challenges. Although other scales exist, the Cooper-Harper rating scale continues to be the most well-established, most used in the industry, and no doubt the diligent work of George Cooper and Bob Harper will continue to endure for years to come. RWI