Continuing Ed

Anticipating aircraft behavior

They say that first impressions last, that people typically form an impression of someone within a very few seconds of meeting them for the first time. And the studies say that, more often than not, those split-second judgments turn out to be pretty much on the money.

Does the same pithy truism hold for airplanes? When we spot an unfamiliar airplane on the ramp, what goes through our heads in those first few critical seconds when initial impressions take root? What do the details reveal about the airplane's personality?

There's the obvious physical characteristics--high wing or low; two seats or more; taildragger or tricycle gear; single or multiengine; piston or turbine power. But within those basic classifications reside many clues that can reveal much about the airplane's primary mission and what we might expect in terms of handling and performance.

For example, the Cessna Skyhawk and Piper Warrior are two mainstream piston singles with conventional configurations and controls. From the time each design appeared five decades ago they have been used as training aircraft, as well as for personal fun flying and transportation.

Other than the placement of the wing, there are no major differences between them. Each is powered by a single piston engine turning a two-blade propeller, each has four seats in the cabin, and each has a tricycle landing gear. You might conclude they fly pretty much the same--same basic speed, handling, and useful load. You would be correct. But a closer examination reveals small differences in design that create small but noticeable differences in handling and performance. For example, the Cessna has a conventional tail with fixed horizontal stabilizer and elevator. The Piper, on the other hand, has an all-moving horizontal tail. The entire horizontal stabilizer pivots around its pitch axis to act as both stabilizer and elevator. That's why it's called a stabilator.

"So what?" you might ask. "Pull back on the yoke in either airplane and the houses get smaller, right?" Yes, but the different designs make for different handling characteristics.

Because it has such a large elevator surface area compared with a conventional elevator, the stabilator is more powerful and effective at changing the pitch attitude of the airplane. That's a good thing--we pilots like lots of control authority. The downside that so much control authority makes it easier to exert too much force, to overcontrol in pitch. To prevent this, the stabilator is fitted with an anti-servo pitch trim tab on the trailing edge that deflects in the same direction as the stabilator--up when the stabilator moves up, and down when the pilot pushes on the yoke--only more so.

The anti-servo trim tab works against the stabilator. When the trailing edge of the stabilator deflects upward, the trim tab deflects even higher, which tends to push the stabilator back down. The effect is to increase the effort required by the pilot to pull back or push forward on the yoke. More effort means less chance of overcontrolling. So, when you examine the tail and see that the airplane is fitted with a stabilator, you can conclude that the airplane will have a heaver feel in pitch control than does a conventional stabilizer and elevator.

Critics of the stabilator design say it tends to be "nervous" in cruise flight, meaning it is subject to small changes in trim because of normal variations in airflow over the stabilator and slack in the control system linkage due to wear. That's another handling trait we can anticipate when preparing to fly a stabilator-equipped airplane.

Now look at the wing design. The Skyhawk and Warrior (Cherokee) have been in production for decades, and over the years the designs have evolved. A cursory look might not reveal much difference between early- and late-model Skyhawk wings, but there have been changes. The wing has increased in span, with a taper to the outboard sections to improve climb and cruise performance and aileron effectiveness.

Beginning in 1973, Skyhawk wings were given a downward twist on the outboard section and a "cuff" on the outboard leading edges, both intended to tame stall behavior and make the airplane spin resistant. Depending on the vintage of the Skyhawk you fly now or in the future, you can anticipate its stall characteristics based on the wing design.

Early Cherokees had so-called Hershey bar wings because the fat airfoil and constant-chord, rectangular planform make it look something like the chocolate bar. Compare it to later models, which have a wing with tapered outboard sections and a longer wingspan. Like the tapered Skyhawk wing, the Warrior wing offers better roll response and climb and cruise performance than does the earlier Hershey bar wing. Again, you can anticipate certain handling characteristics depending on which model Cherokee you'll be flying.

Now check out the difference in wing design between an early model Cherokee and Piper's flagship piston single, the pressurized, high-flying Malibu. The Malibu wing is much longer in span and much narrower in chord, with a much thinner airfoil. Two completely different wing designs for two completely different missions resulting in different handling characteristics.

The design of the vertical tail also says something about an airplane's handling in varying conditions. A relatively small vertical fin contributes less drag and thus better speed performance, but at the expense of yaw stability. If you will be flying a high-performance single with a small vertical tail, be prepared to use a generous amount of right rudder on takeoff to counteract torque and p-factor, and resign yourself to experiencing some wallowing in turbulence.

If that's the case, why don't all high-performance airplanes have generously proportioned vertical fins? Remember, more surface area means more weight and drag and therefore less performance. Also, a big fin makes the airplane more subject to weathervaning in crosswind conditions.

We draw from many sources to learn how to fly an airplane safely and well. The most important tool is a knowledgeable and experienced instructor, aided by an understanding of the pilot's operating handbook or airplane flight manual.

But there's a lot to be said for giving an airplane that is new to you a good once-over. Check out the basic design, then look closer at the details--the size and shape of the wing, the design of the tail, the size of the control surfaces. That first impression can tell you a lot about what to expect when you finally do climb in.

Mark Twombly is a writer and editor who has been flying since 1968. He is a commercial pilot with instrument and multiengine ratings and co-owner of a Piper Aztec.