One of the dangers of flying airplanes is allowing an aircraft to get too slow and stall when you are close to the ground. Training and proficiency work to minimize that danger, but did you know there are modifications that can enhance the effectiveness of control surfaces for added safety?
Wing profiles for many general aviation aircraft were designed before computers could crunch the billions of bits of data needed to optimize an airfoil. Vortex generators (VGs) have become the modification of choice to optimize an airfoil for the low-speed end of the envelope and to make control surfaces more effective.
VGs are aerodynamic devices that are typically rectangular or triangular in shape and draw high energy air into the boundary layer of an airfoil delaying airflow separation and aerodynamic stalling. This improves the effectiveness of wings and control surfaces such as flaps, elevators, ailerons and rudders.
The boundary layer is that layer of air right above the surface where skin friction causes airflow to slow down. As an air foil increases in angle of attack, airflow can run out of energy over the surface and begin to separate, creating loss of lift, or what we experience as a stall.
Vortex generators delay that flow separation enabling the airfoil to fly at a higher angle of attack before stalling. Usually installed close to the leading edge of the airfoil, VGs help maintain airflow over control surfaces at the trailing edge maintaining control for longer. This results in improved performance and control authority at low airspeeds up to the critical angle of attack, which results in greater control in flight at slower airspeeds such as take-off and landing.
What to Expect For multi-engine airplanes, the installation of VGs on the wings, the vertical stabilizer, ailerons or rudder can noticeably increase effectiveness at low speeds and in the stall. Controls are more effective, giving better roll control at and in the stall, reducing the likelihood that a pilot will lose control of the airplane.
However, it’s imperative that pilots train frequently for engine-out situations. VGs simply aren’t a substitute for proficiency.
Should an engine fail and a pilot flies the airplane at the single-engine best-rate-of-climb speed, VGs allow the airplane to operate further away from its Vmc, thus providing a pilot more room for error in handling the engine out. Another benefit of VGs is that takeoff roll is reduced because the liftoff speed drops due to the lowered Vmc. This means that accelerate-stop distance is reduced enabling more runways to be used safely if the airplane is flown per STC requirements.
Depending on the kit, the installation of VGs may also allow an increase in max takeoff weight, ramp weight and zero-fuel weight or some combination of these.
In single-engine airplanes, the advantages of VGs are primarily reduced stall speeds, with a side benefit of noticeably improved control response at low speed. VGs improve aileron response near and at the stall, making it less likely that a pilot who inadvertently stalls a VG-equipped airplane will lose control.
Further, they can safely fly approaches slower than a non-VG-equipped airplane, and this can significantly shorten landing rolls. Stall behavior also improves from a sharp break to a gentler wobble.
Aftermarket Installation Kits
The components included in a VG kit vary by the type of airplane for which it was developed. Because of the expensive flight test requirements for obtaining STCs for VGs, not every airplane has a kit available. Manufacturers understandably chose aircraft models whose potential market size justifies the expense of engineering and regulatory approval.
These kits include peel-and-stick templates that are laid on specified locations on the airframe. At each location where a VG will be located, the surface is abraded to allow for better adhesion. Each VG is then glued in place with the adhesive provided. Strakes, if included, are bolted or riveted into position. On most airplanes, the face of the airspeed indicator must be replaced because of the changes in stall speed and Vmc.
The time to install these kits ranges from a low of three hours for singles to about 12 hours for larger twins.
Conclusion While it may be true that VGs are engineering enhancements that correct less-than-optimal wing design, the benefits they offer substantially improve the performance on installed aircraft.
For twins, VGs are an inexpensive way to improve low-speed handling, control and reduce Vmc, thus increasing the level of safety. For singles, VGs provide the benefits of a STOL kit, without the added weight, with improved low-speed handling, more docile stalls and increased control effectiveness.
Any modification that lowers stall speed and improves low-speed handling is a significant safety enhancement.
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