Back in the day, when FAA employees outnumbered amateur airplane builders, a government inspector would do a “pre-closure” inspection on every part of your homebuilt, then return for a pre-first-flight inspection and again every year thereafter. The quality of these inspectors varied. Some were skilled A&Ps who almost always found a discrepancy that you had overlooked or had some useful comment or suggestion to offer. Those you welcomed. A few, however, barely knew one end of an airplane from the other.
It was one of the latter who showed up for the pre-first-flight inspection of my first homebuilt, Melmoth. With thinly veiled bewilderment, he walked around the airplane—which was fairly large and complex for its time and, furthermore, of all-metal construction, an idiom just gaining prevalence among homebuilts in 1973. He finally paused for a long time to contemplate the horizontal tail before delivering himself of the opinion that the elevator looked awfully small.
Indeed, it would have been awfully small—had it been an elevator. But it was not; it was an anti-servo tab.
Melmoth had what is variously termed a “stabilator,” a one-piece or an all-flying tail. Like T-tails, these were quite fashionable in the 1970s; they are less so now. Claims of their advantages always included the assertion that the single-piece surface was more aerodynamically efficient than a hinged elevator. The conventional two-part arrangement has the perplexing property that, as you raise the elevator to lift the nose of the airplane, the leading edge of the stabilizer rises as well, producing a force opposite to what is needed. Because it was not working against itself, the stabilator could in principle be made smaller and lighter without sacrificing effectiveness.
The stabilator was simply a slab wing mounted in bearings at its quarter-chord point. In the range of angles of attack useful for wings, pressures on airfoils balance about the quarter-chord point, and so the slab could be moved through a range of angles without any resistance whatever. To provide centering, a trailing-edge tab—resembling a narrow elevator—was geared to the surface in such a way that when the nose of the slab went down, the tab’s trailing edge went up, producing a resistance that the pilot felt as the expected stick force. The tab linkage consisted of a small pushrod anchored on the fuselage ahead of the stabilator pivot; moving the anchor point, by means of a bell crank or jackscrew, provided trim.
The idea of using a small tab to regulate the behavior of a free-floating surface goes back quite far. It had its origin in the Flettner tabs used to operate the rudders of large ships. Nevertheless, Lockheed patented a version of the stabilator toward the end of World War II, when one of the company’s engineers, John Thorp, designed a tiny single-seat, stabilator-equipped airplane, the Little Dipper, intended, I guess, for possible military-liaison use. To demonstrate its agility, this airplane landed and took off in the inner courtyard of the Pentagon in Arlington, Virginia. According to the story as I heard it, some generals were not amused. Thorp would later participate, along with Karl Bergey, in the creation of the Piper Cherokee. He was a great advocate of the all-flying tail, even though early versions used in his T-18 homebuilt proved prone to flutter until—after a series of hair-raising tests that he called “tickling the dragon’s tail”—he cured the problem with outboard-mounted leading-edge weights. That the all-flying tail added a couple of new flutter modes to the empennage was brought home to Piper engineers when the stabilator of a Comanche more or less exploded in flight, shedding its outer panels but, I believe, leaving enough behind to allow the pilot to land the airplane. Slow-motion film of this can be found on YouTube; search for “Comanche tail flutter.”
Another problem of the all-flying tail that did not become apparent until a lot of them were in service on Cessna Cardinals was that, under some circumstances, the stabilator could stall at the last moment during a landing, dropping the nose and damaging the nosewheel. Cessna added a slot to the leading edge of the Cardinal’s stabilator; Beech used a fat, upward-drooped leading edge on the Musketeer’s. The root cause of this problem, however, was simply that the designers, relying on the belief that stabilators were inherently more efficient, had made them too small.
Read More from Peter Garrison: Technicalities
The all-flying tail on Melmoth was well-behaved, except in icing. I avoided known icing, so my encounters were few and mild, but when Melmoth’s stabilator picked up some rime, it wanted to oscillate in pitch, and I had to restrain it with a firm hold on the stick. I knew this was potentially disastrous behavior, and when, after Melmoth’s destruction in a ground collision in 1982, I built its successor (imaginatively dubbed Melmoth 2), I reverted to a two-part horizontal tail with a fixed stabilizer.
One potentially troublesome aspect of stabilators is that the anti-servo tab must be very well-behaved, because it drives the stabilator. This had never occurred to me until, one day, I got a call from a Piper engineer inquiring whether I had any trouble with my stabilator “hunting”—that is, not centering steadily but oscillating slightly, so that the entire stabilator had something like the physical affliction called an essential tremor. The most likely cause was unsteady flow over the anti-servo tab, which it amplified into a movement of the entire surface.
I realized in the course of this discussion that a visit I had made to the Piper factory in Melmoth some time in the mid-1970s had encouraged the decision, already under discussion, to equip the PA-32RT-300 Lance with a T-tail and a single cowling air inlet below the spinner. These were both design features that worked well on Melmoth. Unfortunately, they did not work so well on the Lance, and, after two years, the Lance’s stabilator returned to its original position on the fuselage, and the cowling resumed its familiar binocular stare.
Apparently, one of the complaints from Lance owners about the T-tail was that the stabilator—it was the same stabilator as before—did not have as much pitch authority as when it was mounted on the fuselage. This was presumably a result of its being out of the propwash and therefore less able to raise the nose early during takeoff. I had no trouble getting Melmoth’s nose up early in the takeoff roll; its stabilator was probably proportionally larger, in order to cope while landing with the nose-down pitching moments from its double-slotted Fowler flap.
Melmoth’s stabilator had originally been on the fuselage, just like on the Lance, and while the Lance apparently did not have the hunting problem then, Melmoth did. That—in addition to the then-irresistible stylishness of the T-tail—was my motive for the major revision of the empennage that I made a few months after the first flight, increasing the chord, strength and stiffness of the vertical fin and planting the stabilator on top of it. Strangely, the same change that got rid of the tremor on Melmoth produced it on the Lance.
Airplane design is like marriage: You’re never quite sure how it will turn out, and you have to live—at least for a while—with what you get.
This story appeared in the January-February 2021 issue of Flying Magazine