It’s quite an understatement to say that a lot goes into the design of any helicopter. I have extreme respect for the engineers that accomplished such a feat with nothing more than pencil, paper and a slide rule. Today’s designs are created with the assistance of powerful software that can do most of the heavy lifting through the use of computational fluid dynamics and finite element analysis. But no matter the method, no design goes from concept to service without extensive flight testing to validate all the math.
Sometimes flight testing will uncover discrepancies or deficiencies in the original design that require correction. Other times improvements are devised to already certified designs to enhance their usefulness and extend their life.
One common aerodynamic fix you may see on both airplanes and helicopters is the addition of vortex generators. Vortex generators are in essence tiny wings set perpendicular to the surface they are to affect. They are sized and placed optimally to correct or enhance airflow over a particular area.
As the freestream air runs along a surface, friction and viscous effects slow the flow, robbing it of momentum. The freestream continues to slow as it gets closer to the surface within an area called the boundary layer. The laminar (smooth) flow eventually loses so much momentum that it separates from the surface, becomes turbulent and creates drag.
Typical vortex generator configuration
Vortex generators remedy this by creating a vortex, or small corkscrew of air, which curls up into the freestream and pulls higher energy air back down into the boundary layer while moving lower energy air away from the surface. This adds momentum and delays separation.
Although not an exact science, there are some general guidelines to the design and placement of vortex generators. In their simplest form, they are rectangular in shape. They typically will be placed a nominal distance upstream of the point of airflow separation from the surface. Their height will be just tall enough to reach into the freestream, with their length being up to two times their height. Their optimal angle (alpha) tends to be approximately +16/-16 degrees to the oncoming air, alternated to produce counter-rotating vortices. The spacing (d, D) between adjacent generators is usually two to five times their height. With this as a starting point and a little bit of trial and error, significant gains in flow control can be made.
Pilot Nick Page describes vortex generators on MD 902. Photo courtesy of MD Helicopters
When it comes to real-life examples, vortex generators can be found on the latest iteration of the MD 902 Explorer, down the right side of the tailboom. The vortex generators enhance the virtues of the Coanda effect (R&WI, “Curving it With Coanda, Aug. 2016). The generators help the rotor downwash adhere to the curvature of the tailboom for a longer distance. This increases the pressure differential of the coanda effect, requiring less anti-torque pedal, and increasing power margin.
BLR Aerospace FastFin vortex generators. Photo courtesy of BLR Aerospace
Another use of vortex generators can be found on the FastFin aftermarket upgrade kit for the Airbus H-125, produced by BLR Aerospace. Located on the left side of the tailboom, the vortex generators are one part of the kit which boasts up to 10% increase in available right pedal and up to 130-pound increase in useful load while in OGE hover. BLR also touts an improvement to that aircraft’s notorious “squirrely” handling while hovering close to the ground.
Although very few things come absolutely free in both the aviation industry and the world of physics, vortex generators may come pretty close — relatively speaking. Simple to manufacture, and having no moving parts to break, vortex generators can provide an excellent solution to common aerodynamic problems as well as provide a significant improvement to an aircraft’s typical performance. They are a worthy tool in the designer’s toolbox.
Frank Lombardi is an ATP with both fixed-wing and rotary-wing ratings. He began his flying career in 1991 after graduating with a bachelor’s degree in aerospace engineering. He has worked on various airplane and helicopter programs as a flight test engineer for Grumman Aerospace Corp. Frank became a police officer for a major East Coast police dept. in 1995 and has been flying helicopters in the dept.’s aviation unit since 2000. He remains active in test and evaluation and holds a master’s degree in aviation systems flight testing from the University of Tennessee Space Institute.