Coot ?U Vortex generators make the amphibian fly very well

To determine the vortex generators' ideal location relative to the wing's leading edge, the author and Al Cheeky use two-sided carpet tape to attach them temporarily so they could make their test flights. Here the VGs are at their most effective position, 3 inches aft of the leading edge.

RICHARD STEEVES, EAA 65309 I hen EAAers get together later, its completeness impressed degrees to the airflow and 9 inches they soon start t a l k i n g me. In addition to the VGs, the kit between the pairs. Had we been flying one of the about their airplanes, and included full-size paper templates it wasn't too long ago that w i t h pre-cut guide holes for posi- many standard-category airplanes Roger Thistle (F.AA 56766) told me tioning each VG, isopropyl alcohol the company makes VG kits for (unhow he had lowered the stall speed for c l e a n i n g the surlaces, poly- der a supplemental type certificate, of his Kitfox by putting vortex gen- styrene cement to mount the VGs, or STC), we would have received a erators (VGs) on it. and an X-Acto knife for cleaning template that would have also given us the VGs' optimum position relaAt first I was skeptical about the away any excess glue. results. How could g l u i n g s m a l l How well VGs work depends on tive to the wing's leading edge. Bepieces of extruded aluminum to the their location on the wing relative to cause we were working with a homefreshly painted wing of my Coot am- each other and the wing's leading built, we'd have to run our own tests. A few weeks later, Al Cheeky, a felphibian improve its performance? edge. The Micro AeroDynamics kit But if they would reduce the a i r - included paper templates that estab- low EAAer and my a i r f r a m e and plane's stall—and landing—speed, I lished the optimum relationship be- powerplant (A&P) mechanic, and I was sure it was something the Coot's tween the pairs of VGs—angled 30 got together to test the VGs on my hull would surely appreciate. TABLE 1 After learning how VGs Tal^off Time Stall Speed Landing Speed Taker'f ~oeed worked, 1 ordered them from VGs 61.0 mph 33.5 sec Micro AeroDynamics Inc. be- None 49.5 mph 65.6 mph 36.5 sec 49.5 mph 69.1 mph 66.2 mph cause Roger Thistle was a satis- 2" aftLE 48.4 mph fied customer. When the in- 3" aftLE 59.3 mph 55.8 mph 32.0 sec stallation kit arrived a few days 4" aftLE 49.5 mph 34.5 sec 64.5 mph 55.8 mph Sport Aviation

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How VGs Work

Without Vortex Generators With Vortex Generators When air flows over a wing (or any surface, really), its speed varies with its distance from the wing's surface. The flow's speed is slowest at the wing's surface, the result of friction. (Remember, air is a fluid, like really thin water.) The flow's At cruise airspeed, the airfoil on the left has smooth airflow, while the VGs speed becomes faster as the discreate a vortex airflow with no reduction in airspeed. tance from the wing's surface increases, until it reaches the point where the airflow's speed equals the aircraft's speed. The distance from the wing's surface to where the airflow gets up to speed is the boundary layer, and it plays an important role in generating lift. At the leading edge the boundary layer is really thin and the At slow airspeed, the airfoil on the left begins to exhibit boundary layer air molecules move smoothly along separation, while the VGs keep the boundary layer attached. and parallel to its surface. In effect, the molecules are marching down the street in a parade of neat columns and rows. Aerodynamicists call this "laminar flow," and it doesn't have a lot of drag associated with it. The boundary layer exists from the leading to trailing edge, and friction causes it to grow thicker from At stall airspeed, uncontrolled turbulence causes the airfoil to stall. While front to back. With this thickness VGs don't prevent a stall, they can help an aircraft stall at a lower airspeed comes instability because all the molecules aren't marching straight than the same aircraft without VGs. down the street in the same parade. I____________ Roughly six decades ago aerodynamlcists discovered As the molecules bounce into and trip over each other they that if they could "energize" this sublayer they could keep create turbulent air—and a lot of drag (compared to laminar the boundary layer glued to the wing at higher angles of atflow). tack, thereby delaying the inevitable stall. Engineers tried One might assume that a laminar flow wing would be an all sorts of devices to achieve this goal, and the vortex genaerodynamicist's goal because of its low drag characteriserator worked the best. tics. But in aerodynamics, like life, you don't get anything The VG creates a swirling rope of air roughly the size of a for nothing. pencil that energizes the sublayer and "organizes" the turIn payment for low drag, laminar flow lets the boundary bulent air in the boundary layer. As each vortex swirls back layer easily separate from the wing at high angles of attack. along the wing it pulls fast-moving air from the upper This is why laminar flow airfoils, which move the transition reaches of the boundary layer close to the wing's surface, from laminar to turbulent flow back on the wing, are known stirring the stagnant air there to life. for low drag at cruise speeds—and nasty stall characteristics. VGs are about as close to "something for nothing" as one Drag is the payment for the boundary layer's turbulent will get in life. The VGs do create some drag, but the payflow, but the return is that this turbulence more firmly back is that they also reduce drag by reducing the thick"glues" the boundary layer to the wing, making rt less prone ness of the boundary layer. to separation (and stalling). Until, that is, the wing reaches The key to achieving this efficiency is locating the VGs at Its critical angle of attack, there the turbulence becomes so that point on the wing where the airflow changes from lami"disorganized" that it unsticks from the wing, starting at nar to turbulent. If they are too far forward, the VGs cause the trailing edge. (Remember those diagrams in the student the flow transition to take place early, which increases drag. pilot text that show how a stall crawls across a wing from Placing the VGs too far aft decreases their effectiveness. back to front?) —S.M. Spangler So what's this got to do with vortex generators? 6O

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Coot. We got the airplane ready for the test flights in one evening. To test the VGs in different locations we used the paper templates to stick the vanes on two 14-foot lengths of double-sided carpet tape, so we could test them in different locations. We also made some wing leading edge guides that would tell us precisely how far our templates were aft of the wing's leading edge.

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How could gluing small pieces of extruded aluminum to the freshly painted wing of my Coot amphibian improve its performance? Before applying the VGs Al and I flew to nearby Lake W i s c o n s i n , a n d A l c a r e f u l l y measured t h e Coot's stall, landing, and takeoff speeds with a GPS and its takeoff run with a stopwatch. (We divided the flying and data measurement tasks to ensure m a x i m u m accuracy—and safety.) With our baseline measurements recorded, we repeated the tests with the VGs in three different locations: 2, 3, and 4 inches behind the leading edge. To keep the tests consistent, we added fuel each test. We measured every test item twice, and the averages of the results are in Table 1, page 59. Micro AeroDynamics owner

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Charles White said that plating the VGs 3 inches aft of the leading edge would probably give the best results, and our tests confirmed this. When comparing the affect the VGs had

on the Coot's performance to the no-VG number, their benefit seems modest. In the 3-inch position the VGs lowered the stall speed by 1 mph, the water landing speed by 6

mph, the water takeoff speed by 5 mph, and the water takeoff time by 1.5 seconds. But numbers don't tell the whole story. What impressed us most was

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To perform most efficiently, VGs have an optimum location relative to each other in addition to a position relative to the wing's leading edge. Micro AeroDynamics includes templates that put them in the proper relationship—each pair of VGs angled 30 degrees to the airflow and 9 inches between pairs.

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that the Coot's warning buffet and tendency to fall off on one wing disappeared when the VGs were at their most favorable location, allowing us to achieve a full stall. We obtained the 48.4 mph figure with the Coot falling like a leaf at 700 f p m — p e r f e c t l y stable w i t h the wings level! Overall, I'm very happy with our results because the Coot flies "very good" with the VGs (just as the aeronautical engineers predicted). All I have to do now is glue the VGs in the 3-inch spot permanently.

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Resources For more information about vortex generators, contact Micro Aerodynamics Inc., 4000 Airport Rd., Suite D, Anacortes, WA 98221. Call 800/677-2370 or visit its website at www.mlcroaero.com. The Coot-A, introduced in 1972, is an amphibian pusher design with retractable landing gear and folding

wings. It can use a variety of suitably modified auto or standard aircraft engines. Approximately 75 examples are flying. The Coot-A cruises at 110 mph and stalls at 43 mph. Plans are available from Richard Steeves (6958 Applewood Drive, Madison, Wl 53719) for $250 or $10 for an info packet. Call 608/833-5586 for more information. Sport Aviation

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