June 1, 2007
Mark R. Twombly
Aviation writer Mark R. Twombly writes from southwest Florida.
The wing on the Piper Aztec I own in partnership is a homely thing. It has a fat airfoil, a rectangular, constant-chord planform, and big, rounded tips. Compared with the compound-curve, knifelike blades on some of the lithe fiberglass speedsters that have come to market of late, the Aztec's wing has a prehistoric look, as if it were designed centuries ago (almost — the 1950s) to defy the pursuit of speed in favor of a single-minded search for the lift needed to hoist six occupants and all the stuff that can be crammed into two large baggage compartments. Until I flew it, I never would have guessed that the Aztruck's ungainly slab of a wing also is capable of delivering 175-knot cruise performance.
I poke a little fun at my airplane to make the point that the wing is the most important component on the aircraft we fly, largely defining their personalities and capabilities, yet we know remarkably little about those wings. We may glean a few generic facts in private pilot ground school about how wings generate lift, but we probably get more formal training on the design and operation of an altimeter than we do on the wing.
The slim, pre-GAMA-standardization owner's handbook for the Aztec includes a brief description of the wing design and construction, includes information on how it is attached, and says its rectangular planform "permits the use of many interchangeable parts and simplifies the construction...." As for the wing's flight characteristics, the handbook says only that the wing has "excellent stability and performance characteristics."
Sounds good, but that's not much guidance in terms of anticipating specific handling qualities. Continued reading of the handbook does reveal a few useful clues, but they have more to do with flap actuation.
For example, under "Approach and Landing," the handbook advises that, as speed decays, "the flaps can be lowered slowly to counteract pitch-trim changes and little or no trim will be necessary." And this on the following page: "Due to immediate response of the flap operating mechanism, partial flap positions can best be obtained by successive rapid cycling of the flap selector switch." Those are nicely muted ways of alerting the Aztec pilot to the fact that extending the flaps causes a pronounced nose-up pitching moment. If you're not ready for it, the airplane will balloon like a paper airplane gliding into a summer thermal.
The obtuse language about handling qualities in the Aztec handbook contrasts sharply with the very specific information in a 1957 edition of the flight handbook for the North American F-51D/P-51D. It describes in detail how the airplane responds to changes in configuration, power, and altitude, and how it behaves in power-on and power-off stalls. Useful guidance also is provided on flight-control effectiveness, level-flight characteristics, and control response in maneuvering flight. Now that I've read it I think I'm halfway to my dream checkride.
Recently, I logged my first hours in a Cessna Citation VII, an airplane with a wing that is completely new to me. For one, it's swept. Moreover, it has eight spoiler panels — four to a side — on the top surface. The outboard pair assists the ailerons in controlling roll, the middle four assist the pilot in controlling airspeed and rate of descent, and all eight can be popped up after touchdown to kill lift and assist in slowing the airplane on the runway. I've flown other, smaller airplanes with speed brakes, but sweep and a speed-brakes/spoilers combination are something new and different.
During two weeks of schooling we learned how the spoilers and speed brakes work, and what to do if we're having a bad day and they don't. We also learned that under certain loading conditions we must carry ballast (unusable) fuel in the wings to keep the center of gravity from exceeding the aft limit before the end of the flight. But I don't recall any discussion on the unique handling characteristics of a swept wing as compared with a straight wing. Apparently, that knowledge comes from quality time spent with Aerodynamics for Naval Aviators, and some on-the-job training.
In my brief flight experience in the airplane to date, all from the right seat, it has come as no surprise that faster cruise speeds made possible by a swept wing also make for higher takeoff and approach speeds and longer distances compared to the straight-wing Citation I've been flying. What has been a revelation is that a swept wing with spoiler-assisted roll control can be a handful to fly in certain conditions.
That was the case on my last flight in the aircraft. We were returning to home base, and it was my leg. I don't recall the exact automatic terminal information service report, but it was a visual approach in gusty wind with some crosswind component. On final I corrected for a gust-induced roll, and quickly found myself chasing the ailerons/roll spoilers. Yep, I was in a mild pilot-induced oscillation in roll.
Consciously gripping the yoke to stop my roll inputs cured the problem, but it was too late. The damage had been inflicted. Later, the lead passenger, who enjoys bantering with the crew, remarked about being "shot down."
A bit of a harsh comment, I thought, especially considering that the rock-and-roll approach was negated by a pleasantly smooth touchdown. But the credit for that must go to another essential component — the trailing-link main landing gear.
Wind and Gusts,
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