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Riding the Tornado

Riding the Tornado

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The author delves into the secrets of aerodynamics to find out why adding bumps on a wing can improve controllability.

I always assumed that a smooth airflow over a wing would naturally provide the most lift and the least drag; it seems to make sense, doesn’t it? A good smooth flow of low pressure air on top of the wing sucking it into the air, with a push from higher pressure air down below, would obviously be the ideal. 

Aircraft designers worked for years to eliminate rivets and any other obstructions from wings to give them the smoothest surface and reduce drag. Competition glider wings are highly polished and constantly cleaned of the slightest dirt, even dust. 

So why would anyone want to install a row of metal vanes standing like fence posts along a wing’s leading edge? Well, aerodynamics is a mysterious thing that no normal person can understand. Even aeronautical engineers have a secret pact to keep anyone from realizing they have no idea, either.

A whirlwind on my wing

But, like Dorothy’s tornado, which one wouldn’t think could turn her Kansas farmhouse into a bizjet bound for Munchkinland, it seems a little tornado on your wings can give you some extra lift.

Although the term “vortex generator” sounds like a device that would feature in a time-travel movie, in reality they are just those simple, metal vanes you may see standing like marching soldiers, usually on the leading edge of a wing. 

The vanes are placed at an angle to the airflow so that, in flight, the air moving past them creates a “vortex,” a little horizontal whirlwind that improves the effectiveness of the wing. They’re often installed on control surfaces as well.

 

Six degrees of airflow separation?

I decided to avoid the technospeak I’d get from an aeronautical engineer—“the viscosity of the fluid and the friction of the object surfaces generates high transverse velocity gradients”—and just talk with someone who spends Monday through Friday, 9 to 5, with vortex generators (VGs). 

Anni Brogan is the president of Micro AeroDynamics, a company that does nothing but design and manufacture VGs for General Aviation aircraft. In simple terms that I could understand, she explained, “the vanes are mounted to the wing, usually between two to 12 percent aft of the leading edge, and at a slight angle to the oncoming air. 

“As the air flows by,” she continued, “it’s given a spin by the vanes—and this swirling air will stick to the wing longer, especially at lower speeds.

“On a typical wing without vortex generators, the air reaching the trailing edge separates and roils around—even flowing forward at times—and this turbulent air doesn’t provide any lift from that last bit of the wing,” Brogan said. “On ailerons, elevators and rudders, the vortices provide better controllability, and that improves safety.”

Since the air is flowing smoothly over more of the wing’s surface area, although spinning all the while, it provides more lift and better performance. For wings, the additional lift means the wing can fly slower before it stalls and it will fly earlier during the takeoff roll. For control surfaces, the extended air flow across the surface increases the forces it generates, which increases its controllability.

I don’t need to know the physics of exactly how it all works, but just that it really does. There is very little argument from anyone that VGs aren’t effective.

Aerodynamic benefits, and other pros and cons

The most often mentioned benefits of VG installation are lower liftoff speeds and lower stall speeds. Certainly, a lower stall speed is a great safety improvement and might prevent some of those deadly, low altitude, turn-to-final stalls. Although, don’t rely on that: never get too “low and slow.” 

A lower liftoff speed would certainly help those taking off from rough, short fields, which is great for aircraft owners flying into unimproved, backcountry airfields. The increased effectiveness of ailerons, elevators and rudder, which improves controllability, especially at slow speeds, is another safety benefit.

The installation of these vanes has very many pros and very few cons. On the plus side is the fact that they are relatively inexpensive (in aviation terms), they’re easy to install and don’t interfere with any other aircraft systems or parts.

The most often cited con is that some pilots think they’re ugly, and they do make the wing harder to wash. Tests by some pilots of their planes, before and after the installation of the VGs, have found a slight reduction in cruise speed of one to two knots; but, in the same breath, many of those independent reviewers have called the reduction “negligible.”

Aftermarket VGs for Pipers

There are various companies making VGs for experimental aircraft, but Micro AeroDynamics Inc. is one of the few manufacturers whose products have an STC for use on certified aircraft. The company has been making VG kits for most General Aviation aircraft since 1989 at their Anacortes, Wash. factory. 

Micro AeroDynamics VGs are manufactured from aircraft grade aluminum and coated for improved corrosion resistance and paint adhesion. A kit includes the VGs, installation tools, adhesive, templates, detailed instructions and the ever-important STC. 

Installation

Although the installation of VGs is often described as “quick and easy,” it is a job that requires care and accuracy. 

Experimental aircraft owners can install the VGs and sign off their own work. On certified airplanes, the VGs must have an STC, and the work must be approved by someone with FAA inspection authority. Anni Brogan recommended aircraft owners consult their mechanic before starting the installation. Some are willing to sign off on an owner’s work; some are not. 

The vanes, which are glued on, are not difficult to install, but they must be placed in a precise line with accurate spacing. Micro AeroDynamics provides a peel-and-stick template that helps—but it, too, needs to be accurately placed. A good compromise she suggested is having the mechanic apply the template to the wing and then the owner can do the tedious part of gluing all the vanes into place. 

Sometimes the VGs are placed singly and sometimes as a pair, with the two angled toward each other in a “V” shape. For example, a Piper Super Cub wing takes 36 pairs of vanes across the wing, and 14 pairs on the underside of the horizontal stabilizer.

A typical installation involves these steps:

Paint the vanes. They come treated with Alodine, which improves corrosion resistance and paint adhesion, but gives the vanes an odd greenish-yellow color. You can paint them to match the current color of your aircraft, or use a contrasting color if you like. (Micro AeroDynamics will paint them your desired color before shipping, for a fee.)

Determine the correct placement and apply the template to the wing or control surface, and then remove the cutouts where the vanes will be placed. Double-check the position now, before you start gluing.

Scuff the exposed aircraft surface and the vanes with the included Scotch-Brite pad to ensure optimal adhesion. The glue used to attach the vanes is a two-part adhesive: one part is applied to the wing and the other to the vane. There’s no need to clamp or weight the vanes. The vanes are custom made for each airplane and may either have a curved or flat surface for gluing, depending on the spot where they’re to be attached. The vanes can be glued to any material—even fabric—as the glue really sticks to the paint, not to the surface material.

Carefully glue the vanes in place. How long it takes will depend on the installer’s skills, confidence and the total number of vanes to be installed, but usually the entire installation can be completed in one day.

Have your work inspected. Leave the template in place so the inspector can see it and know the vanes were positioned according to the template, especially if the mechanic didn’t place the template.

Remove the template. (You might wish to store it along with the extra vanes.)

Once the aircraft has been returned to service, go fly.

Cost

The VG kits cost from $395 for experimental aircraft up to $3,950 for a large, certified, twin-engine aircraft. A Piper Cub kit from Micro AeroDynamics costs $695; a kit for a PA-28-150/160/180 Cherokee costs $1,450; a kit for a Seneca II, for example, list at $1,950. Add taxes and shipping to those prices, and if you want your VGs painted to your specifications, that’s an additional $250. 

The kits can be bought directly from the company, through your mechanic, or from aircraft supply shops. Assuming that an owner would perform the installation, the VG kit, STC, tax and shipping are the only costs, but on certified aircraft the installation must be inspected and approved, so that’s a cost that must be considered.

For $695, plus a small amount of paint and the inspection fee, I’d consider this modification for my Super Cub, especially if I decide to fly into more backcountry airfields. Currently I can land in just about any field around here, so it’s not a necessity for me. But for pilots who need maximum performance from their aircraft, the installation of VGs is a simple, yet very effective improvement.

Dennis K. Johnson is a writer and a New York City-based travel photographer, shooting primarily for Getty Images and select clients. He spends months each year traveling, flies sailplanes whenever possible and is the owner of N105T, a newly restored Piper Super Cub Special. Send questions or comments to .

RESOURCES >>>>>

VG kits – PFA supporter
Micro AeroDynamics Inc.
Other VG kits
(PA-31 and PA-34 series only)
BLR Aerospace

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