Vortex generators make it to the single-engine set
Vortex generators have taken over such a large portion of the conventional-twin market, that the product is like Star Trek's Borg on an intergalactic binge. Walk the line at any airport and you'll see these inverted-T-shaped devices sprouting all over the wings and tails of the more popular multiengine aircraft. Much of the VGs' popularity stems from every pilot's hankering for safer handling at the limit-in particular at the minimum-controllable speeds for most twins — and reduced stall speeds that permit slower approaches. Peruse the for-sale listings and you'll see that the vast majority of twins have these aerodynamic aids listed prominently on the goodies list.
At least some of the attraction of VGs is the accompanying maximum-gross-weight increase. Many twins receive approval for higher takeoff weights (and some get higher landing weights as well), which helps to improve the useful load, often a sore point with multi owners. Max gross is set by several factors including stall speed and, of importance in a twin, minimum climb gradient with one engine inoperative. Because VGs lower stall speed and permit the minimum climb speed to be lower — resulting in an increased climb gradient even if the rate is the same — VG manufacturers have been able to tack on max-gross increases of up to several hundred pounds, depending upon the airplane.
Until recently, the vortex generator kit was to be found on twins only; now several single-engine models are being assimilated into the VG collective. Beryl D'Shannon Aviation, purveyors of a multitude of Beech mods, has developed a VG kit for the 33-, 35-, and 36-series Beech Bonanzas. The D'Shannon kit for Bonanzas is normally $2,900 but is on special through June 1 for $2,500.
Similarly, Micro AeroDynamics has concocted kits for several ragwing Pipers — the J-3, PA-11 Cub Special, PA-12 Super Cruiser, PA-18 Super Cub, PA-20 Pacer, and PA-22 Tri-Pacer — as well as the venerable Aeronca and most Maule models. Micro AeroDynamics recently dropped its prices on the single-engine kits from $950 to $595. What's more, Boundary Layer Research has VG kits for the Cessna 120/140, 180 H-K, 182 through 182D, and 185 through A185G models. BLR's kits range in price from $595 to $1,495. It also sells a kit for the Piper Super Cub.
How vortex generators work is hardly rocket science, but GA does owe airline and military hardware for their development. In simple terms, the VG aims to excite airflow in a localized fashion. Perhaps you looked out on the wing of the last airliner you rode and saw a few of these fences on top of the wing. During design and flight test, airframe manufacturers often identify areas on the wing or engine nacelle that experience localized airflow disruptions under certain flight conditions. Often, in lieu of redesigning a major component, VGs are used to stimulate or redirect the airflow around the trouble area.
The basic desire behind keeping the airflow "alive" is to improve the lift coefficient of the wing. In cruise and at modest angles of attack, most wings will have stable, smooth chordwise airflow. But as the angle of attack increases, some airflow near the trailing edge of the wing separates and tumbles; that part of the wing is no longer providing useful lift. Airfoil designers strive to make the wing give up lift in a controlled manner, from root to tip in an orderly fashion from the trailing edge forward. As a result, conventional GA airfoils have some amount of twist or washout — that is, the outboard portion of the airfoil is twisted down into the relative wind, giving it a lower angle of attack than the inboard section. This helps to ensure that the outboard portion remains "flying," and that the ailerons retain some effectiveness by working in relatively undisturbed air. The stall strips that you see on many airplanes merely force separation at the root and as a side benefit produce turbulent flow that caroms back to the horizontal stabilizer to provide aerodynamic warning of an impending stall.
Vortex generators work their magic by setting up a circular flow across the top of the wing, likened to a horizontal tornado. This flow helps to energize the boundary layer and keep the airflow attached to the wing at higher angles of attack. It's worth noting that any wing can stall, but VGs add a degree or two of useful angle of attack. Stall speed at a given gross weight will drop; the range among singles is from 5 to 15 percent.
Most VG kits have several characteristics in common. They position these T-shaped pieces on the upper surface of the wing at about the 10-percent chord line. (This figure varies by airplane and airfoil design.) In addition, some kits include VGs in other locations. For example, the Micro AeroDynamics kit on some models includes a row of VGs along the bottom side of the horizontal stabilizer. This detail achieves the same thing as the VGs on the wing-delayed separation of the airflow. Remember, the stabilizer is lifting in reverse, so it makes sense to add the VGs to the lower surface. These stabilizer-mounted VGs are necessary on airplanes like the Super Cub and Tri-Pacer because they have very deep-chord stabilizers and a significant gap between the fixed and movable elements. AOPA's Timeless Tri-Pacer 1998 sweepstakes airplane, for example, would begin to lose elevator effectiveness partway into the stall, making it difficult to set pitch attitude precisely.
These VGs appear in other places on the airframe, too. The Beryl D'Shannon kit, for instance, uses VGs on both sides of the vertical stabilizer ahead of the rudder. This is commonplace on the multiengine VG kits to help improve rudder effectiveness, which in turn enhances controllability at Vmc. For the singles, these VGs help to improve rudder authority during power-on stalls but also provide a bit of extra effectiveness for crosswind landings.
Beryl D'Shannon's VG kits also include a max-gross weight increase of 100 pounds. For many owners, this alone will justify the cost of the kit and the estimated eight to 12 hours to install it. There is a slight catch, however. Bonanzas were certified in the Utility category, and the gross-weight increase knocks the airplane back into the Normal category while it's being operated at the higher weight. In addition, there's a prohibition on operating the airplane at the higher weight with any of the VGs missing, and, curiously, the maneuvering speed goes down while at the heavier gross weight. Why? The load limits are lower for Normal category airplanes, resulting in the change; once you've burned off enough fuel to fall below the old maximum gross weight, you can revert to the original numbers. Finally, because of the way the VGs were certified, Bonanzas built before the 285-horsepower IO-520 became standard must have three- or four-blade props.
We flew a 1976 Beech A36 Bonanza to get a feel for how VGs work on a high-performance single. For most operations, the VGs are transparent. The airplane handles exactly like a normal A36 until you begin to slow it down. BDS uses VGs on the leading edges, two smaller sets ahead of each aileron, and a row up each side of the vertical stabilizer. At 70 knots indicated, the A36 was perfectly solid and responsive — these flight tests were performed at about 3,200 pounds gross weight, some 500 under the new limit. The ailerons are demonstrably more effective at this speed.
Bring it back to 65, stall horn blaring, and not much is different. Continue on back to 60 and the controls begin to get a bit mushy but there's no feeling as though you're trying to balance atop a basketball. Clean, the A36 stalled at about 57 knots indicated — an inaccurate figure given the angle of attack — and broke cleanly and wings-level. Power-on and landing-configuration stalls are essentially the same. In all, the modified A36 is more docile at the stall, showing none of the stock airplane's proclivity toward falling off on a wing. One P35 we know well will get your full and complete attention in a departure stall.
Jim Capone, owner of the A36, states flatly that the improvement in stall performance and permissibly lower approach speeds were powerful reasons to fit VGs, but that the max-gross weight increase was also compelling. Mike Terhune, owner of Mike's Aero at Parrett Field in Angwin, California, reports that installation of the BDS kit was straightforward if you plan ahead and take your time. He had to relocate a few of the VGs to clear rivet heads, but the kit allows movement of up to one-quarter inch from the supplied template.
So it seems that VGs are making good on the claims-reducing stall speeds and permitting lower approach speeds for a given weight. Those airplanes not achieving a weight increase with the VGs are usually limited by some other factor; limited engine power, as in the Cessna 120/140 and Piper PA-20/22, limits the maximum gross weight more so than the stall speed. In the end, it's likely that VGs will benefit pilots who have a need to visit short strips most of all; for those of us flying from 5,000-foot slabs of concrete, the reduced approach speeds won't be of much help. But the reduced stall speed, which allows for a more gentle meeting of terra firma in the case of an off-airport landing, may be one of the benefits you'll appreciate only when you need it most. Regardless, it's likely that VGs will continue to appear on other singles. Resistance, it seems, is futile.
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