Efficiency: Stuck on vortex generators

Vastly enhanced low-speed handling

January 1, 2013

vortex generators

The next best thing to discovering a way to make an airplane fly faster is finding a way to go slower, safely. Vortex generators have been enhancing low-speed stability in a wide variety of aircraft for decades with some impressive results. Sometimes the differences are substantial, especially in STOL aircraft where low-speed handling is at a premium. But how much difference do these sharp-edged aluminum shapes really make? Are there hidden aerodynamic penalties for having VGs? (Or just aesthetic ones?)

I spent a Sunday afternoon installing a 48-piece set of Micro AeroDynamics VGs on my Experimental-category Vans RV–4, and the results were surprising, even after having read several reports from pilots who had already installed VGs on similar airplanes. I knew to expect a decrease in stall speed, and while that was indeed the result, it’s nowhere near the full story. VGs vastly improve slow-speed handling in the RV–4, provide much greater aerodynamic warning of an impending stall, and raise the critical angle of attack at which the wing stalls to an almost comical 20.5 degrees with flaps deployed (from about 15 degrees). On the fast end of the speed spectrum, there’s no downside at all. I found no discernible difference in cruise speed (at the same altitudes, power settings, and fuel burns) compared to the same airplane before VGs.

A vibrant autumn morning provided time for collecting the baseline data. Flying solo with full fuel tanks put the airplane’s center of gravity near its forward limit, and I jotted down the facts of my airplane’s pre-VG performance. With the power at idle, wings level, and flaps down (30 degrees), a sharp stall break preceded by almost no aerodynamic warning occurred at 45 KIAS. That number rose to 49 KIAS with flaps up. Power-off spins had a steep nose-down pitch attitude, and with full pro-spin inputs, the airplane recovered on its own after about two turns.

I returned to the airport, tucked the airplane in a hangar, and got busy installing the VGs with guidance from A&P mechanic Carlo Cilliers. The VG kit is complete with aluminum VGs, stick-on templates for the top of the wing and the bottom of the horizontal stabilizer, two-part epoxy, masking tape, measuring tape, knife, thread, cotton swabs, and even a sharpened pencil. With two of us working, the job was done in less than three hours. I flew again in the evening to measure the changes.

Ground roll on a normal (not short-field) takeoff was about 750 feet instead of 900. Climbout at 100 KIAS was unaffected, and cruise speed at 7,500 feet was unchanged (142 KIAS or 163 KTAS at 2,350 rpm, 22 inches mp, and 8.0 gph). Power-off stalls were preceded by pronounced aerodynamic buffeting followed almost immediately by a sharp, straight-ahead break at a 17-degree pitch attitude with flaps up, and 20.5 degrees nose-up with flaps down. The ASI registered 31 KIAS at the stall break with flaps down and 33 KIAS with flaps up—erroneous readings resulting from instrument error at the unusually high AOA. (Using GPS ground- speed and correcting for winds and altitude, the actual stall speeds were 44 KIAS with flaps up and 39 KIAS with flaps down, a reduction of five and six knots respectively.) Two-turn spins with VGs behaved the same as without them. The airplane still tended to recover on its own in about two turns. Recoveries in less than two turns were nearly instantaneous using opposite rudder followed by forward stick.

A series of loops, rolls, and other positive-G aerobatic maneuvers showed no changes in stick forces or roll rates. I had been concerned that the addition of VGs on the bottom of the horizontal stabilizer might make elevator forces too light, but the elevator felt normal, and the stick pull gradient remained smooth and linear. The only difference that I noticed was that the former aileron “chatter” that used to take place at full aileron deflection went away when VGs were installed. (That chatter or buffet takes place when airflow over the fully deflected ailerons begins to separate—and VGs apparently keep the airflow attached.)

Instead of flying my usual 75 KIAS on final, slowing to 70 over the numbers, I slowed to 65 KIAS on final and 60 over the runway threshold. The airplane felt solid and stable with none of the discomforting edge-of-the-mine-shaft feeling that being slow on final can bring. Main-wheel and three-point landings were little altered by the VGs, and slower approach speeds meant noticeably shorter rollouts. (The rule of thumb is that increasing the approach speed 10 percent raises ground roll 20 percent.)

On the ramp, most tailwheel RVs stand at a 10- to 12-degree deck angle, so full-stall landings with a VG-enabled pitch attitude of 20 degrees or more would be thoroughly graceless affairs. I noticed no new tendencies to float, balloon, or bounce with VGs.

MicroAeroDynamics founder Charlie White said his company sends VGs to manufacturers such as Aviat, American Champion, and Maule for installation on new production aircraft. VG kit prices range from $395 for Experimental aircraft to more than $2,500 for some FAA-certified twins. The company has shipped more than 10,000 VG kits since it was founded in 1986.

“We’ve done extensive flight testing in a wide range of aircraft,” White said, “and we’ve never failed to get at least some reduction in stall speed.”

The VG installation on the RV–4 brought no adverse characteristics or performance reductions at normal cruise speeds. The best attributes are the ability to comfortably and safely fly slower approaches, with greater aerodynamic stall warning, and consistently shorter takeoffs and landings. Touching down with less energy to dissipate will surely pay off in longer lives for tires and brake linings, as well as additional peace of mind when going into short airfields.

I had attached the VGs with two-sided tape so that they could be easily removed if they didn’t enhance performance. Returning from that first VG flight, however, I glued them on with industrial-strength epoxy.They make the tops of the RV–4 wings look like a miniature Siegfried Line, but these VGs are staying.

Email dave.hirschman@aopa.org.

Dave Hirschman

Dave Hirschman | <em>AOPA Pilot</em> Senior Editor , AOPA

AOPA Pilot Senior Editor Dave Hirschman joined AOPA in 2008. He has an airline transport pilot certificate and instrument and multiengine flight instructor certificates. Dave flies vintage, historical, and Experimental airplanes and specializes in tailwheel and aerobatic instruction.