Positive Force

Twisters to the rescue

May 1, 2007

If you're not familiar with aviation, the term vortex generator might summon visions of a mother-in-law who takes it upon herself to rearrange her daughter-in-law's kitchen within minutes after setting foot in her newly married son's home. Like our fictitious mother-in-law, vortex generators (VGs) do create turbulence. Unlike the mother-in-law's generations, VG-generated turbulence is a positive force that lowers stall speeds, improves low-speed control, and makes twin-engine airplanes much safer.

Vortex generators weigh next to nothing, are simple to install, and provide a good benefit-to-cost ratio.

Airflow primer

The skin of a wing is considered a boundary to airflow. During flight the relative wind flows around the wing surface profile. The layer of air flowing closest to the wing skin, or boundary, has zero velocity — it's not moving at all. The layer above it, and each layer that's slightly farther away from the boundary layer than the one below it, moves faster until — at the top of what's called the boundary layer — the air molecules are flowing freely with the relative wind. Aerodynamics for Naval Aviators defines the boundary layer as "the layer of air over the surface which shows local retardation of airflow from viscosity."

This boundary layer is relatively thin at the front of the wing but, because of the effects of viscosity and friction, gets thicker farther aft. At some distance aft of the wing leading edge these effects cause an oscillatory disturbance, which grows until the smooth airflow is lost. The smooth airflow is called laminar flow; the oscillation area is the transition zone. After transitioning from laminar flow, the airflow aft of that point is termed the turbulent boundary layer.

The Illustrated Guide to Aerodynamics by H.C. "Skip" Smith addresses these layers. Smith writes that the boundary layer airflow over the horizontal tail of a Piper Cherokee flying at 135 mph would remain laminar from the leading edge to a point approximately 3 inches aft. At that point it would be approximately 0.03 inch thick. After the boundary layer transitions to the turbulent layer, the thickness would jump to approximately 0.10 inch. At the trailing edge it would be slightly more than one-half inch thick.

Aerodynamicists express the length of laminar flow aft of the wing leading edge in percent of wing chord length. Laminar flows can be as short as 10 percent of the chord length up to 60 percent of the chord in laminar-flow airfoils.

The 'red line,' or V MC equation

Every pilot transitioning to a conventional twin-engine airplane learns about red line. Don't ever lift off the runway until the indicated airspeed is above red line is a rule of thumb drilled into every twin driver's head. Red line applies to V MC (minimum control airspeed with the critical engine inoperative) and is indicated by a radial red line on the airspeed indicator. V MC is the slowest airspeed at which the rudder and ailerons have sufficient authority to overcome the yaw and roll forces generated when one engine of a twin-engine airplane is generating maximum takeoff power and the critical engine is at zero thrust. If the pilot lets the airspeed drop below V MC, the thrust and p-factor of the operating engine will overpower the pilot's ability to maintain controllable flight and the airplane enters what's known as a V MC roll, where the airplane yaws and rolls into the inoperative engine. Staying red-line current is the primary reason savvy twin pilots spend the cash each year for recurrent training. V MC can be higher than the airplane's stall speed, especially in high-powered twins with a small rudder, such as the Cessna 411. Vortex generators are so effective at lowering V MC that they change the whole twin-engine safe-flying equation. In some cases the installation of a certified VG kit lowers V MC below stall speeds — even though stall speeds are also reduced.

Here are some examples. Installation of a VG kit on a Cessna 340, 340A, or 335 lowers V MC by 11 knots; a kit on a Beechcraft Model 55 Baron models lowers V MC by 10 knots; a kit on a Beech Duke lowers V MC by 10 knots. Clean (V S) and dirty (V SO) stall speeds and approach speeds (V REF) are also lowered by hefty margins.

In addition to improving safety and flying qualities, some STCed VG kits permit increases in maximum takeoff weight (MTOW). Some twin-engine kits also increase zero-fuel weight (ZFW). Pilots flying airplanes with a published ZFW are permitted to load the airplane with passengers, baggage, and cargo (payload) up to the zero-fuel weight — the rest of the load between ZFW and MTOW must be fuel. An increase in ZFW translates into more payload.

Cape Air, a large cargo and passenger carrier operating in the Northeast and the Caribbean, installed Micro AeroDynamics VG kits on all of its Cessna 402s because of the increase in ZFW and the 360-pound increase in MTOW. Pounds equal dollars.

It's probably time to describe a VG kit. Individual VGs are T-shape pieces of aircraft-quality 6061 aluminum that typically measure approximately 1 inch long by one-third inch high. The base, or crossbar of the T, is glued to the airplane so that the leg of the T projects up into the relative wind. VGs are always installed on the wings and vertical stabilizer. Some kits also require the installation of VGs on the bottom of the horizontal stabilizer or stabilator. Why on the bottom? Because the center of gravity (CG) of the airplane — a downward force — is located forward of the center of lift — an upward force — of the wing. To even out this imbalance the horizontal tail surfaces are designed to exert a downward force. Since horizontal tail lift is opposite that generated by the main wing, the VGs are installed on the bottom. The kit for a Piper Comanche consists of two different VGs. Part number 1001 VGs are flat on the bottom. Thirty-four are glued to the top of each wing and 16 are glued to each side of the vertical stabilizer, or fin. Number 1101 VGs are slightly curved on the bottom, and 36 are glued to the bottom of each side of the stabilator. Some single-engine kits don't require the under-tail VGs. Twin-engine kits differ slightly in that some require that curved sheet metal parts called strakes be installed on the fuselage side and/or side of the engine nacelles a few inches above the wing upper surface. Micro AeroDynamics VG kits start at $695 for small single-engine airplanes such as Cessna 120s, 150s, and 152s and Piper models from the J-3 up through PA-22, as well as all Stinson, Taylorcraft, Champion, and Aviat Husky single-engine airplanes. The cost of twin-engine-airplane kits ranges from $1,950 to $2,950. Most other single-engine kits sell for $1,450. VG replacement kits are supplied at a reduced price when VG glue bonds are compromised by chemically based paint strippers during airframe paint jobs.

Complete kits and easy installations

The Micro AeroDynamics VG kit installed on the Comanche for this article — lacking only a pint of 90-percent isopropyl alcohol needed for cleanup — is very complete even down to including an Exacto knife, a pair of vinyl gloves, a layout string, a tape measure, a roll of masking tape, and a sharpened pencil. Any individual capable of reading the easy-to-follow installation manual should be able to install a VG kit on a single-engine airplane in eight hours or less. Twins may take a little longer because of some additional parts and the possible requirement to remark the airspeed gauge with new red and blue (V YSE — best rate of climb with one engine inoperative) radials. Installation of these kits is approved by supplemental type certificate (STC). Installation is considered a major modification and must be supervised by an airframe and powerplant (A&P) technician and signed off on form 337 by an A&P with Inspection Authorization (IA) authority.

VGs must be very precisely positioned. Each template is die cut with cutouts of the exact position of each VG. Once the templates are in place the VGs are glued into the cutouts. VGs on the wings and horizontal tail surface are positioned in a "pigeon-toed" manner; the leading edges are closer together than the trailing edges. The first step in locating the VGs involves locating a leading-edge reference line on the installation surfaces. This was done by measuring from easy-to-find skin laps in the wing top surface, stabilator bottom surface, and both vertical stabilizer sides. A piece of tape is then stretched between the points and a layout string — we chose to snap a chalk line — is stretched between the two marks. Next a set of reference points — nine points are required on each wing, and four on each side of the vertical stabilizer and each side of the stabilator — is marked along the leading-edge reference line by measuring from another prominent skin seam or rivet line on the surface. All this marking sets up two reference points for locating the individual vinyl templates. For ease of handling the templates are relatively short. Four are used to cover the complete span of each wing. One edge of the template snugs up against the leading-edge line while a notch in each template is aligned with the reference points. The templates are peeled off the backing paper and smoothed down. Next the surface where the VG will be placed is cleaned with the alcohol and lightly scuffed with a piece of Scotch-Brite pad before being cleaned again. Before gluing down the VGs a light spray of activator from the Loctite 330 adhesive kit (supplied) is sprayed on the surface. Then a drop of adhesive is applied to each individual VG and it is held in place for a minute before going onto the next.

Within 15 or 20 minutes the templates can be removed. The Loctite adhesive dries to full strength in 24 hours.

Why do they work so well?

The key to understanding how the act of gluing a cluster of little aluminum blades on an airplane makes that airplane fly better at low airspeeds can be found by going back to Aerodynamic Theory 101. Wolfgang Langewiesche is the author of Stick and Rudder: An Explanation of the Art of Flying. He is the master of simple explanations and writes that air flowing over a wing can't take the downward curve that is created over the upper surface of a wing when the wing is flown at high angles of attack. Aerodynamics for Naval Aviators considered the following concept important enough to put it in italics: " Airflow separation results when the lower layers of the boundary layer do not have sufficient kinetic energy in the presence of an adverse pressure gradient." As the boundary layer flows aft from the leading edge of the wing, the action of skin friction drag causes a reduction in the energy in the lower levels of the boundary layer. During normal flight conditions this reduction in the kinetic energy initiates the transition, which eventually results in turbulent flow within the boundary layer. Turbulent flow isn't all bad because the turbulence, with its eddies and swirls, does contain lots of kinetic energy. What is bad from a flying standpoint is the slowing of the lower layers of boundary-layer airflow. When these flows slow they lose so much energy that they can no longer overcome the reverse airflow that is moving toward the separation point from the trailing edge of the wing. The solution to delaying the stall is to re-energize that boundary layer.

VGs do this by creating mini-tornado-like airflows that excite the turbulent boundary-layer airflow and keep it attached to the aft portions of the wing and flight controls.

There are many benefits gained by installing a set of VGs. Lowering V MC for twin-engine airplanes has already been mentioned. Installation of a VG kit will result in better aileron response at low airspeeds and increased rudder effectiveness (which helps during crosswind landings) — both because the boundary-layer airflow remains attached over these control surfaces; and lower stall speeds (which reduce wear and tear on the landing gear and airframe, and shorten takeoff and landing distances). Before-and-after testing of the Comanche mentioned previously supports Micro AeroDynamics' claim that a set of VGs would reduce V S by 8 miles per hour and V SO by 5. Not only were stall speeds reduced, but also the airplane felt much more responsive in roll at low airspeeds and much more ready to fly at normal rotation speeds. The improvement was immediate, was easily noticeable, and swept away slow-flight worries.

What about ice? Do VGs slow airplanes down?

Charles White owns Micro AeroDynamics, which sells VG kits for more than 700 airplane models. When asked whether VGs are a liability in icing conditions his answer was simple — the VGs' location is far enough aft of the wing leading edge to preclude problems with icing.

The installation of VGs does reduce the top speed of some airplanes. David F. Rogers, who has spent 50 years working as an aeronautical engineer, published two reports detailing the installation of a set of VGs developed and marketed by the Beechcraft specialty shop Beryl D'Shannon on a V-tail Beechcraft Bonanza. He wrote, "The VGs reduced the power-off stall speed in the clean and dirty configuration by 13 percent and 6 percent, respectively." He does warn pilots that when a VG modified Bonanza wing finally stalls, the break is more abrupt than without the VGs. He attributes this to the fact that VG-equipped wings stall at a higher angle of attack. Rogers also determined that the true airspeed loss when cruising at 65-percent power was 5 miles per hour. Rogers' reports were published in the American Bonanza Society publication ABS Magazine.

Micro AeroDynamics, the supplier of the VG kit supplied for the AOPA flight test, did comprehensive flight tests on the company Baron and couldn't detect any speed loss. It appears that this effect varies from airplane to airplane. Low-altitude full-power speed tests in the Comanche showed a 2-knot drop (142 knots versus 144 knots) after the installation of the VGs, but this may have been more because of a more forward CG in the after-installation test than the VGs themselves. Perhaps Rogers summed up VGs best when he wrote, "Considering the decrease in power-off stall speed and excellent aileron control...the small decreases in cruise TAS may be acceptable."

E-mail the author at steve.ells@aopa.org.