Vortex Generators

From a KITPLANES reader: Why do vortex generators creating turbulence help wings create lift at high angles of attack? Good question Shawn, and thanks for reading. 

I’m going to preface my answer with stating that I’m more of a structures guy than aerodynamics, so if I don’t get some details quite correct, please make allowances for that caveat if/when you send in letters.

I once heard someone state that vortex generators were a band-aid for a poorly designed wing. Yes and no, but let’s start with how they work, which means first starting with the boundary layer (BL) itself. The boundary layer is the region of air between the free-stream and the wing surface, or any other “wetted” surface, with wetted being if the outside air were liquid. (Which it sure feels like to some of us that live in very humid areas, but I digress…)

One can model the wing-air interface with the wing moving and the air stationary, or vice-versa. For purposes of our discussion, the wing is considered stationary with the air moving past it up to flight speeds. The first tidbit of fluid dynamics to establish is that the air molecules next to the wing skin are not moving; they are essentially “stuck” to the wing surface. Moving away from the wing, perpendicular to the surface, the air molecules begin to move faster and faster until they are moving at the free-stream airspeed. The boundary layer is that region between the skin and the free-stream air. 

A number of factors affect boundary layer (BL) thickness, but one rule-of-thumb is to figure it as about 1% of the distance back from the leading edge. That means that for many of our homebuilts with a roughly 50-inch chord, the BL is approximately 0.125-inch thick at quarter-chord and around a half-inch thick at the trailing edge. As the wing angle-of-attack (AoA) increases, the boundary layer will get steadily thicker at the trailing edge until it begins to separate from the wing surface. Keep increasing the AoA and the line of separation will continue moving from the trailing edge forward until enough of the flow is detached that the wing loses lift.  

There has been no small amount of R&D and experimentation dedicated to keeping the air from separating from the wing, especially in high Angle-of-Attack (AoA) flight regimes and/or low-and-slow STOL flight. 

One method known as BL suction involves drilling a few zillion tiny holes in the wing planks (we’re now talking about large multi-engine aircraft) and applying a vacuum to them. The goal is to remove the BL air between the free-stream and the wing surface, thus keeping the BL more attached to the wing, particularly at high AoA. One of my professors was involved in such research and told us that, for giggles and grins, they reversed the connection and blew air out through the wing and observed…. very similar results. Interesting. You can either remove air to pull the BL back down or backfill it with more air for about the same effect. 

So, what about vortex generators? These are typically triangular or trapezoidal in shape and are attached to the wing somewhere around 20% to 25% chord. As might be indicated by the name, their shape generates vortices that look like little tornadoes running fore-aft and laying against the wing surface. These spinning vortices are said by numerous sources (like my profs and textbooks) to “energize the boundary layer,” which means to pull the higher energy free-stream air down into the slower-moving boundary layer air and energize it. While the some VGs are glued on individually, some of the Airventure Cup racers are using a nifty VG adhesive tape that they apply along the upper wing surface.   

Which brings us back to a blown wing versus a suctioned wing: do we think about the vortices as removing air from the boundary layer and making it thinner (vacuuming), or taking free-stream air and pumping it into the boundary layer (blown)? An interesting notion to consider, but not anything we’re going to solve here. 

As mentioned, the vortex generators (VGs) are typically applied around quarter-chord and often found on STOL aircraft to improve low-and-slow flight.  That said, even non-STOL aircraft can benefit from VGs in the form of lower landing speeds due to delaying the onset of stall. Take a look out the window on your next commercial flight and, depending on the aircraft, you may see VGs across a short or even a longish span of the wing. They will not necessarily be across the entire leading edge, because they will be installed only where needed, perhaps in front of an aileron for greater aileron authority at low speeds.  In certain applications, the VGs are used to generate vortices that act as a chord-wise “fence” to reduce spanwise flow on the wing. 

A second location where they are often found is on the underside of a horizontal wing to improve tail authority at higher AoA on both smaller GA aircraft and the big iron. 

A notable application of VGs is with the early Rutan VariEze and Long-EZ aircraft. Going from memory plus recent discussions with Long-EZ pilots and builders, the original canard wings were laminar flow airfoils that were very sensitive to surface imperfections and contamination, sensitive enough that insects or rain could cause the canard to lose lift and the nose would dip. As a result, it is not unusual to see vortex generators glued to the top of these early canards.  

Enter John Roncz as the protagonist in one of aviation’s niftier stories. Roncz was both an early pioneer of computational fluid dynamics (CFD) and an admirer of Rutan aircraft. After buying a set of VariEze plans, Roncz analyzed the canard airfoil and (per Wikipedia) found it wouldn’t fly at all as it was stalled at all angles of attack. [Cue me raising my hand and asking: if so, then how did those first VariEzes fly at all? Unclear.] Roncz initiated contact with the Rutans with his information, where it was incorporated into a Long-EZ canard with great success. Not only was the Roncz airfoil more resistant to contamination, it provided a lower rotation speed and a slightly faster cruise speed. Win-win-win. Consequently, the Roncz R1145MS airfoil became the new de rigueur canard for Long-EZs. 

Based on a quick count on Wikipedia, Roncz went on to design over 40 airfoils for the Rutan Aircraft Factory, Scaled Composites, Beechcraft, and others before passing away three years ago. If you are unfamiliar with his story, it is one well worth reading, starting with his page within Wikipedia. 

But not all VGs are on the wing or a control surface! A non-intuitive location is on the aft fuselage of the earlier models of the 737. Why? Because the interaction of airflow between the fuselage and vertical fin led to very noticeable buffeting that could be felt throughout the airframe. The VGs that were added control the airflow such that the air separates more cleanly and without all the buffeting.  Of note is that the tail cone was redesigned on the newer 737 Max, eliminating the need for VGs there. 

A slightly more curious VG location is on one of the amphibious pusher-prop homebuilts. With the engine and propellor located above the tail boom and behind the cockpit instead of above the wing, there was a large bluff body in front of the prop, effectively blanking most of it.  The solution was to apply vortex generators to the back of the cockpit structure in order the energize the air coming around the back corner of the fuselage and prevent the flow separation that was blanking the prop.  

Similarly, I’ve seen vortex generators on the very aft edge of the roof of hot-rodded hatchbacks whose owners are trying to eke out a bit more speed whilst blatting their way down the road (as if their obnoxiously loud exhaust somehow translates to more speed. Evidence that not all vortex generators use their powers for good). A somewhat less obvious use of vortex generators on cars is for noise control. For many of the EV’s and hybrids, without the sound of the engine the next most prevalent sources of noise are the tires and wind. The tire noise can be mitigated with different tread design, which leaves the noise of the air rushing by. Next time you are in a parking lot, take a look at the top or ends of the side mirrors on the EV’s and there is a good chance you see bumps that are VGs meant to tailor the wind turbulence and control how it is hitting the window glass. 

Back to the comment that VGs are band-aids for a poor wing design: with present-day computational abilities, it is likely that nearly every flight regime can be modeled fairly accurately for new aircraft and new designs. But this is a relatively recent development, and most of the aircraft in the piston fleet (at least those that I’m likely to fly in or rent, much less own) were not the beneficiaries of such advanced CFD.   Which means that more than a few aircraft likely performed as expected for most of their flight testing, but that the corners of the flight envelope revealed issues with wing/fuselage, fuselage/empennage, or wing/empennage interactions. 

For most airframe makers, discovering this during flight test means you’ve already frozen your design and the tooling, and to redo things will mean a slip in the delivery schedule, not to mention the cost of new or revised tooling. Given the enormity of new design and reworking tooling, an aerodynamic “band-aid” is going to be the least bad option in light of budgets and schedules.  

One instance of this was the Cessna 177 with its all-moving stabilator. As initially delivered, the stabilator was able to stall during slow-speed landings and flare maneuvers, running out of control authority and resulting in pilots landing with a solid ‘thunk’. The fix was to incorporate a leading-edge slot in the stabilator so that the air flowing through the slot and over the bottom surface delayed the onset of stall. This was incorporated into production aircraft going forward, and Cessna Service Letter SE68-14 provided for a retrofit leading edge slot (along with over 20 other items) to delivered aircraft.  

A really nifty application of VGs is on the new RV-15, wherein VGs have been added to the top of the flaps such that they are active when the flaps are deployed (and when the VGs are most beneficial to keep flow attached), but are otherwise concealed within the wing when the flaps are retracted so that they are not adding drag when not needed. Genius. 

Since VGs add a bit of drag, ideally, they would never need to be used. But their functions for noise control and boundary layer control outweigh their drag and will likely continue to be used for both aircraft and cars. Keep an eye out to spot them in the wild.