Retired TWA captain Barry Schiff has flown myriad types of aircraft in his career.
It can be a fascinating exercise in frustration to listen to a group of pilots discussing the handling qualities of an airplane because they often do not describe them in a meaningful way.
Handling qualities refer to the feel and response of an airplane; they have nothing to do with performance. Admittedly, though, describing handling qualities is not as easy as you might think.
You undoubtedly have heard some pilots refer to control harmony. This simply means that the control inputs of the pilot (in terms of the required forces applied by hand and foot) and the amount of control surface deflections are the same for all primary controls. In other words, the flight controls are perfectly harmonized if a 5-pound control input, for example, deflects the ailerons, the elevator, or the rudder to the same degree. All three of the primary flight controls would feel the same, and the rate of roll, pitch, and yaw would be the same.
The Douglas DC-3, for example, does not have control harmony. It is sluggish in roll, light and responsive in pitch, and has a heavy rudder.
Control harmony alone, however, does not necessarily mean that an airplane has good handling qualities. Some airplanes — like the Cessna 210 — have reasonably good control harmony, but all three primary flight controls are relatively heavy. On the other hand, the Pitts Special is very light on the controls, too light for some, and has a maximum roll rate that would knock your socks off. It is wonderful for aerobatics but is a terrible instrument platform (unless you like inadvertent snap rolls during an ILS approach in turbulence). Such an airplane does not tolerate a clumsy pilot. The Cessna 210 is wonderful on instruments but handles like a truck (partly because of stability and partly because of control system design). Excellent compromises are the de Havilland DHC-1 Chipmunk (see " In Her Majesty's Service," page 92), the SIAI-Marchetti SF.260, and the Beechcraft Model 35 (V-tail) Bonanza.
Largely, though, the most desirable handling qualities are subjective and vary from one pilot to another. Some, for example, might not like control harmony. They might prefer instead to have light ailerons and a heavier elevator (usually because many pilots tend to overcontrol more in pitch than in roll). Many pilots, too, are uncomfortable with a light rudder because it is easy to overcontrol when landing in gusty crosswinds.
Those who do not care for a light elevator should avoid loading their aircraft to an excessively aft center of gravity. Conversely, moving the center of gravity forward results in the need for more muscle to control pitch. (I have met a few pilots who claim to know their airplanes so well that they can determine the approximate location of the CG simply by the amount of pull required to induce a given G load.)
One of the most irritating control-system characteristics is slack, a condition where initial control movement does nothing but cause a wiggle in the wheel. You can determine the amount of play in the cables when on the ground by having someone hold a control surface firmly in place while gently attempting to move the control wheel or stick against the applied force. If slack is present, it might be eliminated by having the turnbuckles in the control path tightened during the next annual inspection. (Aerodynamic slack refers to control movement within a narrow dead band that does not cause aircraft movement.)
Slack is virtually nonexistent in those aircraft that have expensive pushrods instead of cables. Furthermore, pushrods allow the more liberal use of ball bearings that substantially reduce control system friction, another factor that can affect handling quality. Designers have many tricks up their sleeves to improve handling qualities but confess that it occasionally is as much art as science.
Take ailerons. Their aspect ratio (ratio of length to width) does little to affect their effectiveness. It does not matter much if they are long with narrow chords or short with wide chords. What does matter is their total area and distance from the longitudinal axis of the airplane. In other words, ailerons should be as large and as far from the fuselage as possible and practical.
Large ailerons that make small deflections are better than small controls that make large deflections. They reduce the required control forces and increase roll rate.
Although most pilots would agree that stability is good, it also can be too good. It takes more effort to induce roll, for example, in a stable airplane than it does in one that is not so stable (everything else being equal). Long wings, such as those found on a sailplane, for example, add considerable stability, but long wings create substantial roll damping, which requires more effort to roll into and out of a turn.
The most maneuverable airplanes are typically the most unstable, and this is why modern fighters have fly-by-wire systems that make inputs to flight-control computers that otherwise are working full time just to keep the wings level and the nose pointed in the right direction. A human pilot, we are told, would be unable to do so (see " Flying Like the Birds," page 81).
You may recall my plans to make an extensive tour of Europe this summer in a single-engine airplane. Unfortunately, there often is a slip between the cup and the lip, and I have postponed this anticipated adventure until 2004.
Visit the author's Web site ( www.barryschiff.com).
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