General Design Goals               Handling Qualities

Handling Qualities

About a year ago, Kitplanes published an editorial that took a shot at the sorry state of General Aviation airplane handling qualities.  One of our guys was so impressed with Kitplanes attempt to explore this subject that he sat down and wrote a letter to the editor, re-printed below.

Dear Editor:

I am an Aeronautical Engineer and engineering test pilot (U.S. Naval Test Pilot School). My experience level covers a rather broad swath of the flying game, both military and General Aviation (it's important for a test pilot to have this type of exposure to many different types so that he has perspective on what's possible and what isn't). I don't often write letters to the editor but your comments on handling qualities caught my attention (handling qualities are one of the test pilots' favorite disciplines).

Congratulations on the decision to write this piece. The subject of the adequacy of General Aviation aircraft handling qualities does not get the exposure that it so richly deserves. This has been the case for many, many, many years. Instead, as you know, emphasis is placed on performance (a quantitative, measurable number), creature comforts and the number of gadgets that the airplane contains. In retrospect, this is understandable. These things are tangible and can be seen, felt, admired, etc. On the other hand, with respect to HOW the airplane flies, there is a distinct tendency, on the part of the consumer pilot, to take the view that "Well, this is how God intended this airplane to fly; therefore, if I have problems making decent landings, the fault must be mine." Nothing, of course, could be further from the truth. Adequate handling qualities are (or should be), a prime design goal, right up there with structural considerations and performance and, if the design and flight test teams have done their jobs correctly, then the consumer pilot (still believing that God had something to do with it), lands his airplane with a big smile on his face.

In the flight test business, there are two basic categories of evaluation, quantitative evaluation and qualitative evaluation. Performance evaluation is a purely quantitative exercise while handling qualities evaluation (much more complex) is both quantitative and qualitative. When test pilots evaluate aircraft handling qualities, they use a scale called the Cooper-Harper Rating Scale to measure the adequacy of the handling qualities. The scale is both numerical (quantitative) and textual (qualitative). The purpose of the textual scale is to give meaning to the assigned numbers. On this scale (copy attached for you to look at), a rating of 1 is the perfect airplane (I have never given any airplane a 1). A rating of 10 is an uncontrollable airplane. Ratings between 1 and 10 reflect the relative "goodness" of the handling qualities when measured AGAINST A DEFINED MISSION (very, very important, you do not evaluate a 747 as an acrobatic airplane). Historical data show that when a group of qualified (also very important) test pilots are asked to evaluate the handling qualities of a specific aircraft AGAINST A DEFINED MISSION, the variation in the Cooper Harper Rating, from one test pilot to the next, is very small, rarely more than a dispersal of 1 . I point this out because, in your editorial, you observed that "Good handling is, I confess, a subjective matter-at least to some degree." This view implies that a particular set of handling qualities, say, a 4 on the test pilot's Cooper-Harper Scale, could be accorded anywhere from a 1 to, say, a 7 by the average consumer pilot. Not so. If the test pilot has done his job correctly and gives the airplane a 3, it is very likely that the consumer pilot would also give the airplane an (equivalent) 3 plus/minus 1.

Again, in the flight test business, we use a term called "Flying Qualities". Flying qualities are the sum total of handling qualities, cockpit human factoring and the performance characteristics of the airplane and they are the bottom line of how the airplane will be viewed by the consumer pilot. As a quick example of how these three seemingly unrelated disciplines can interact, consider an airplane having a propensity to decelerate rapidly when maneuvering due to high induced drag characteristics (performance). Couple this with a "Stick force per G" curve that is shallow (little stick force required to pull G (handling qualities). Couple these with an improperly designed stick (pilot can not rest stick arm on leg preventing fine force adjustments (human factoring). This airplane would be prone to unintentional accelerated stalls.

Developing adequate aircraft handling qualities is a complex discipline. As you point out, the process begins in the design stage. The aircraft designer is assumed to know the difference between good handling qualities and bad handling qualities. If he does not, then problems begin in the design stage. The process continues in the flight test stage. The flight test team is assumed to know the difference between good handling qualities and bad handling qualities. If they do not, then problems continue in the flight test stage and the aircraft is likely to reach the market with undesirable handling. Again, as you point out, excellent handling does not come easily and considerable effort, time and money can be required to iron out the bugs (assuming that they are recognized). The reason is that the very complex inter-related factors that determine the adequacy of the handling qualities result can not be completely analyzed prior to flight test. They're too complicated. So, at the end of the day, a company test pilot will be called upon to render a quantitative/qualitative evaluation to determine whether all the complex factors, working together, have resulted in a 2 or a 5. If he knows what he's doing, a 5 will not get to the consumer pilot. If he doesn't know what he's doing, well, there are a number of examples out there.

There seems to be a mis-perception in the G.A. community about the connection between speed and the difficulty involved in flying "high performance" airplanes. Fast airplanes are considered to be "hot" and inherently more difficult to handle. This mis-perception is so far from the truth as to be (kinda) funny. Over-all, the best handling airplane that I have ever flown was the F-86 Sabre. I gave the airplane a 1.5 on the C-H scale, for the fighter mission. The airplane is a piece of cake because North American got the handling right and any reasonably competent G.A. pilot would have no problem flying the thing. Ditto the Cessna Citation. On the other hand, I've flown a couple of the current hot, fast glass, "high performance" G.A. airplanes at one third the speed of the F-86 and they were not a piece of cake, they were very difficult. Somebody did NOT get the handling right. Probably the most difficult airplane I've ever been in was the Helio Courier at one sixth the speed of the Sabre. 'Nough said.