Vortex Generators: 50 Years of Performance Benefits
Getting their start in the airline world, VGs trickled down to general aviation where they have been reducing Vmc on twins and stall speed on all airplanes as well as improving their handling and increasing their level of safety.
It was more than 50 years ago that Boeing used the first vortex generators—carefully located metal tabs angled slightly relative to the airflow—on portions of the upper surface of the wing of the original 707. (I also received a report from reader George McClellan, who worked on the experimental flightline at Boeing in Wichita, that was involved in instaling VGs on at least one B-47 in 1950.) The odd-looking devices eventually trickled down to general aviation, notably on Learjets, then to even the most modest of bugsmashers. For decades, vortex generators, or VGs, have been providing safety and controllability benefits throughout the piston single and twin world at a rate well beyond what their diminutive size might imply.
On the original 707 it was discovered that, at speed, airflow over the highly cambered wing separated, leading to gravely reduced effectiveness of the high-speed (inner) ailerons because they were living in badly disturbed air. Installing vortex generators created what amounted to little horizontal tornadoes that kept the airflow attached to the wing—prevented boundary layer separation—and allowed the ailerons to operate effectively.
Because an aerodynamic stall is due to airflow separation from the wing, it didn’t take all that long for engineers to figure out that VGs could be used to keep the airflow attached to higher angles of attack and lower speed—as well as increase control effectiveness at those lower speeds. By the 1990s, VGs were being marketed primarily for light twins—placing them on the vertical stabilizer increased rudder effectiveness at low speed, which allowed for a reduction in Vmc. Reducing Vmc paid big dividends—not only in safety by making the airplane controllable at lower speeds following an engine failure, but also in reducing runway length requirements. Rotation speed for a twin is normally some factor above Vmc—and it determines the amount of runway needed to accelerate to rotation speed and then come to a stop. Reducing Vr means reducing the amount of runway needed for takeoff.
The second benefit VG developers saw for twins was reducing stall speed, which allowed a slower approach and touchdown speed. On top of that, reduced stall speed meant an increase in allowable gross weight for many twins. Certification requirements for most general aviation piston twins require them to have a single-engine rate of climb at least 0.027 times the square of Vso. If the stall speed is reduced, gross weight may be increased.
The clear benefits of reduced Vmc and stall speed and increased gross weight made the piston twin market the correct initial target for VGs. For example, the Cessna 340A was a two-people-and-a-toothbrush airplane with all eight of its fuel tanks full. VGs bumped its gross weight by 300 pounds—which proved to be a big deal. It should be noted right here that VGs do NOT increase the maximum landing weight of any airplane, so a takeoff at the new, higher gross weight requires burning off fuel prior to landing.
Later it was demonstrated that VGs could increase the zero fuel weight (the maximum amount that can be carried in the aircraft cabin—any weight going into the airplane above that number has to be in the fuel tanks) of some twins.
VGs for Singles
Micro Aerodynamics, the big dog in the VG world, has been making vortex generators since 1989. In a conversation with principal Charlie White, he told me that he initially believed that the market was going to be limited to twins because of the three-fold benefits (stall speed, Vmc, gross weight). Single-engine owners would only get reduced stall speed, so he doubted that there’d be enough of a market to justify the cost of research and development for STCs for singles. He said that once VGs were proving themselves on twins, he was approached by owners of singles in Alaska who competed in what were becoming very serious contests for short field takeoff and landings. Could he make VG kits for them? White started doing so and discovered that there was a huge market for single-engine airplanes—justifying the cost of developing more and more STCs. More companies have entered the field to make VG kits for production (an STC is required because VGs are considered a major modification to the airplane) and homebuilt airplanes—to the point that no matter what you own, there’s a good chance there’s a VG kit available for your airplane.
So, what do those little tabs do in the real world? Depending on the type of airplane, realistic stall speed reductions range from four to 10 knots. I think that’s significant, especially from an accident perspective when one considers that force of impact is a squared function. Any reduction in speed of impact gives a benefit that is well more than linear—reducing stall speed can mean reducing speed at touchdown in a forced landing situation, meaning less energy to dissipate. Touching down more slowly on landing means less energy to manage on rollout, reducing the risk of a runway loss of control (RLOC) accident.
Along with the reduction in stall speed, VGs give increased aileron authority at low speed, reducing the risk of loss of control—one of the big causes of accidents. Some kits involve installation of VGs on the underside of the horizontal stabilizer, improving pitch control at low speeds. Some owners say the improved handling at low speed is more important to them than the reduced stall speed.
For twins, plan on Vmc reductions of from five to 12 knots. My test on a Piper Aztec D showed a Vmc reduction from 68 to 61 knots. On a Cessna 310R my flight test showed a new clean stall speed of 76 knots—Micro Aerodynamics published a new Vmc of 71 knots. In the test I did, I was not willing to do a single-engine stall; I stopped decelerating at 77 knots, well into buffet, and the airplane was still easily controllable in yaw.
As to gross weight and zero fuel weight increases on twins, check the manufacturer’s website. Some allow gross weight increases only through other STCs they offer. That also affects price.
VGs function as lightweight STOL kits. Next time you watch one of the videos of Alaska short field competitions, notice that every airplane has VGs—those Huskies and Super Cubs are breaking ground in under 100 feet and approaching at speeds well below what could be used without VGs.
Reports I’ve seen indicate that stall behavior generally is likely to improve with VGs. I think it’s especially important for airplanes such as the Cub and Champ, which have no camber on the tail surfaces—an uncoordinated stall leads to rapid yaw, roll and pitch down, often resulting in ground impact if started low. In Alaska, it’s called a “moose stall,” as it often kills those flying around low, looking for game. VGs reduce the risk of the inadvertent stall and improve the airplane’s manners if it does happen.
Sister publication, Aviation Consumer, named VGs one of the Top 10 safety investments an aircraft owner can make.
To be conservative, plan on some loss of cruise speed with VGs, from one knot in slower airplanes to three in faster. My tests on the Aztec and 310R showed no cruise speed loss—as was the case with a Cessna 150 I tested. However, I have heard many owners state (and have seen reports on Internet forums) that they had a speed loss in the one-three knot range.
I’ve flown and washed airplanes with VGs for over 20 years. VGs make washing and waxing a pain in the whatsis. I’ve never had a VG come off, although I’ve heard that it happens—and the STCs are specific in stating how many can be missing before the airplane becomes unairworthy. The kits come with extra VGs. I’ve been involved in installing two kits—I put the extra VGs into a plastic sandwich bag and threw it into the glove box, so if one comes of, the replacement is in the airplane.
VGs are installed at about 7-11 percent of the wing chord, well behind deicing boots. That’s behind the area that is affected by rime ice. Serious clear ice might run back that far—if so, the situation’s so bad that getting ice on your VGs is not among your big worries at that point. I’ve landed with rime ice on a VG-equipped Aztec a number of times. There was never any ice of any sort on the VGs themselves.
A VG kit includes everything needed for installation, except a ladder. There are peel-and-stick templates to put at a defined locations. The skin is roughed up at each VG-to-be spot and each VG is glued in place. If strakes are involved, they are bolted or riveted on. On most airplanes, the face of the airspeed indicator must be replaced due to stall speed and Vmc changes. Even I, who had my one mechanical gene surgically removed when I was eight, was able to follow the instructions and install VGs with minimal “Get out of the way and let me do it right” comments from the mechanics involved. The single I worked on took three hours, the twin, five.
If you want VGs, plan on going to one of four manufacturers: Micro Aerodynamics, Boundary Layer Research, D’Shannon Aviation and RAM Aircraft. D’Shannon specializes in Beech aircraft and RAM in Cessna twins; Boundary Layer Research sells VGs for twins and Micro Aerodynamics has them for virtually everything. For singles, the supplier is almost exclusively going to be Micro Aerodynamics. For twins, when comparing prices make sure you check to see what is offered. In some cases, VGs from one supplier may look to be less expensive but the STC may not include a gross weight increase that the more expensive competition offers.
I did some quick looks at prices for singles from Micro Aerodynamics: for a Cessna 172—$1450; for a Mooney M20—$1450, Piper Archer—$1450; Aviat Husky—$695 and Beech 36 Bonanza—$1450.
VGs are an inexpensive way to improve low-speed handling, control and reduce stall speed. To me, that’s a big deal. They won’t quite do what a full-blown STOL kit will do, but for the weight, they’re close enough. In my opinion, VGs significantly increase the level of safety of any single. I think that the increase in safety level for a twin is even greater. I wouldn’t own a twin without installing VGs—I want the increased level of safety, any gross weight increase is frosting on the cake.
Rick Durden is the Feature/News Editor of AVweb and the Senior Editor of Aviation Consumer. He is also the authof of The Thinking Pilot's Flight Manual, or How to Survive Flying LIttle Airplanes and Have a Ball Doing It, Vol. I.