Taming Turbulence: Conformal Vortex Generators
John Croft / Aviation Week & Space Technology
It doesn’t surprise pilot and inventor Peter Ireland that potential customers are quick to write off his hybrid laminar flow device as smoke and mirrors. He, too, was a skeptic until the impact of his invention clunked him on the head like Newton’s apple during a helicopter ground test in 2007.
Welcome to Ireland’s world—introducing the airline industry to a drag reduction upgrade based on an ingenious 6-in. wide, 0.38-mm thick (0.015-in.) strip of adhesive-backed thermoplastic polyurethane (TPU) laminate tape, an aftermarket attachment that at first glance is easy to dismiss as no more than window dressing on a wing.
His company, Edge Aerodynamix, calls the product a conformal vortex generator (CVG), and he calculates that it can lead to $800,000 in fuel savings per year on a single Boeing 737. On the 737-500—the aircraft Ireland based much of his flight-test work on for an FAA supplemental type certificate (STC) he expects to receive later in April—the 30-ft.-long CVGs are applied spanwise along each wing. They are placed 5 mm behind the retracted position of the slats. The tape has a sinusoidal wavy pattern on the leading edge and a sawtooth pattern on the trailing edge.
The STC will cover 737 models from the -300 through the -900ER, yielding a potential market size of approximately 6,400 aircraft in the global fleet, according to Aviation Week’s Fleets database. Ireland plans to make the enhancement available for other models as well, including the Boeing 757 and 767 as well as the Airbus A320, A330 and A340.
Edge will provide airline customers with the CVGs and a lifetime supply of spares for free (damaged sections of tape can be replaced in roughly 15-in. increments), but airlines will be on the hook to contractually pay back on a monthly or quarterly basis about 35% of their fuel savings. Considering the number of 737s in the global fleet, as well as the other Boeing and Airbus models, Edge could stand to make billions, assuming airlines do in fact sign up and remain happy with the product. Ireland says more than 16 airlines are interested—although none have signed on the dotted line, largely because the STC is not yet complete.
“It is like a grieving process,” says Ireland when describing how airlines take to the CVG.
“First they go through shock, then anger, then after a while it comes down to acceptance—they understand what we are talking about.”
The CVG solves an unintended problem caused by the basic design of slats, wing-leading edge panels that extend and droop to greatly increase the wing’s camber—hence lowspeed performance—for landing and takeoff. When the slats are in the retracted position for cruise however, there is an aft-facing “step” behind the leading edge of each wing where the cuff closes on the wing. For the 737-500, that step is 30 ft. long on each wing. While the height of the step is relatively small, it is enough to substantially reduce the efficiency of the wing, says Ireland. The same applies to winglets, which typically have a leading-edge protector with a straight back edge.
The issue of the aft-facing step on the slat had already been noted by Boeing. The company’s 737 Performance Improvement Package includes seven aerodynamic and engine upgrades, which together the airframer says can boost fuel efficiency by as much as 2%. Included are slat and spoiler trailing edges that are 60% thinner.
If Ireland is to be believed, the Boeing promise of 1-2% for slat and six other upgrades supports the notion that any step or discontinuity at such a critical point on the airfoil is problematic for cruise flight and transonic flow. It is an area where he says modern computational tools do not accurately model reality, and Edge is working with the Oklahoma State University-Stillwater to create better computational flow models of slat aft-facing step aerodynamics.
To confirm the effects of the aft-facing step, Ireland uses high-speed photography during flight tests to record the unsteady shockwave that forms at the step and causes laminar flow to transition to turbulent flow, thereby increasing drag, vibration and cabin noise.
The shape of the CVG is key to mitigating the turbulence—the sinusoidal channels in the leading edge of the tape act as suction devices to disrupt the vortex that naturally forms as the airflow spills over the step behind the slat. On the trailing edge of the tape the sawtooth channels reorganize and “relaminarize” the flow. Taken as a whole, the tape introduces thousands of coherent vortices traveling chordwise down the wing, says Ireland. The result is a transonic shockwave that is stable and weaker, reducing vibration and noise in the cabin, and increasing laminar flow along the wing, reducing drag.
For Ireland’s non-CVG-equipped 737-500 in cruise flight at a speed of Mach 0.78, the fuel burn was 1.22 tons per hr. per engine. With the CVG, the steady-state burn rate was 1.12, resulting in as much as an 8% reduction in fuel burn. At the 737’s normal economy cruise speed of Mach 0.74, test flights showed 4% or more. Airlines will not be contractually promised any particular savings, but will pay Edge a percentage of what they actually achieve.
Friction-reduction devices on wings and other lifting surfaces are not new; researchers past and present have tested a wide variety of concepts with mixed results. In the late 1980s, Airbus experimented with “riblets,” microscopically grooved adhesive film placed on the surface of an aircraft to cut drag. However, much of the surface area of an aircraft had to be covered by the tape to yield a 1-2% decrease in drag. Vortex generators, small vertical fins on the surface, are commonly used for turbulence management on wings to improve low- and high-speed stall characteristics by reenergizing boundary layer flow, but the devices produce their own drag. Sail plane operators for years have installed “turbulator tape” with a zigzag shape at certain locations on an airfoil, not as a means of reducing drag, but purposefully transitioning laminar flow to turbulent flow, delaying the onset of an aerodynamic stall.
Drag reduction devices that are relatively flat, passive and simple—like TPU tape—are more appealing. Boeing is currently using passive hybrid laminar flow drag-reduction systems for the leading edges of the 787-9’s vertical and horizontal stabilizers, and in other nonwing areas of the 737MAX. The system creates a pressure differential and uses small holes in the leading edge surface to suck the boundary layer, and its associated drag, into the leading edge. One drawback is that the holes can become clogged.
Contaminants, possibly grease, typically seen smudging the wing surface behind the slats on 737s, would appear to present a life issue for CVGs, which Ireland says are designed to be installed in about 3 hr. and stay in place from “paint job to paint job,” or about 8,000 flight hours. Ireland however says the tape is not sensitive to contamination, and in fact the suction created by the channels makes the CVG self-cleaning. He envisions a daily check by pilots for “gross deformation” of the tape, but says safety of flight is not an issue even if large sections were to come off. One of the FAA tests required Edge to fly the 737-500 with the CVG on one wing only, showing that there were no asymmetrical lift issues.
Ireland’s Newtonian moment occurred in 2007 when his Australian company at the time, FlightLog, was developing a novel leading-edge erosion protection film for the Robinson R22 helicopter. The FAA and other authorities had issued an Airworthiness Directive (AD) on the blade due to delamination problems.
One of the fixes for the structural issue—which later turned out to be a production problem—was to install leading-edge erosion protection tape. Ireland had been experimenting with aerodynamic improvements for three years already with FlightLog (and personally for two decades), particularly in dealing with “tabs” placed at the trailing edge of propellers, rotors or wings to reduce drag and increase efficiency. Research had shown that while leading-edge tape with straight edges would qualify as a solution to the AD at the time, it would increase drag substantially, so Ireland came up with CVG tape designs that tamed the flow with chordwise vortices.
“The first expectation was that we would remove the performance loss (from the aftfacing step),” says Ireland of the R22 tests. “We were not necessarily looking for a gain, just not the loss we would have otherwise.” The initial flight test proved the design worked well in the air, but once on the ground, Ireland found something more compelling. “I was cooling down the helicopter, running on the governor at 100%, looking at the power gauges and saying, ‘They’re not right,’” says Ireland of what he saw—a decrease of more than 8% in the power needed to spin the rotor in those conditions. “I didn’t realize we would get a zero angle-of-attack effect, because zero-angle is in the realm of flat-plate drag, and nothing helps down there. Yet we were getting a very large effect.”
FlightLog received an Australian STC for the CVG erosion tape for the R22 in 2012, and Ireland set up Edge Aerodynamix in the U.S. to pursue an STC from the FAA. That work was temporarily put on hold for two reasons: More research was needed in helicopter leading-edge tapes to hold up to rain (an issue Ireland says does not occur with CVGs behind the slats of fixed-wing aircraft); and Ireland, a former military pilot who learned to fly helicopters as part of his research, saw the correspondence between the flat-plate helicopter results to the slat-flow-disruption problems that he knew about during his career as an airline pilot flying a variety of aircraft for a number of airlines, including stints in management.
After discussing the possibilities with his brother, an entrepreneur living in Panama City, Florida, Ireland spent a year working on patent applications to protect his intellectual property before moving to the Pacific Northwest to begin researching CVGs on the wings of fixed-wing aircraft, initially with a Bombardier Learjet 24B. The results were encouraging—at 10% reduction in fuel burn as well as the empirical knowledge of where to place and not place the CVGs.
“That same day we got 10%, we arranged for full funding for this project,” says Ireland of the 10% results in March 2014. He says the company turned down an offer for $1 billion in funding from investors who wanted a 35% share of the 737 STC earnings in return. Instead, the company arranged for private funding sufficient to complete STC, which he estimates will cost about $12 million.
After the 737 STC, Ireland plans to pursue STCs on the Boeing 767, 757 and Airbus A320, work he says will cost approximately $16 million, half of which is the cost of acquiring the aircraft on which to do the flight tests. While a device as simple and passive as a CVG would appear on the surface to be a “minor alteration,” the FAA views the technology as a major alteration, requiring an STC. The agency is not evaluating whether the CVGs reduce fuel burn, but whether the modification is safe. Included in the evaluation are proof that handling qualities, lift, wing-bending moments and performance in icing conditions are equal to or better than the original design. To date, all boxes have been checked off, and Edge is merely waiting for approval on the 737 STC.
Along with restarting the helicopter and propeller work, Ireland is most excited about the potential for massive gains in turbofan engines using his learnings from the CVG and tab work. His initial tests on a run-out Pratt & Whitney JT15D-1 Citation Jet engine yielded a 30% increase in thrust at 100% rpm, nearly 10% higher than the original thrust value of the engine. “What this means is that we want to go spend time looking at that,” says Ireland. “The next areas are the ones that will really make a profound difference to the industry.”