Local Flow Control I CAFE FOUNDATION by BRIEN SEELEY and the CAFE BOARD

AIRCRAFT RESEARCH REPORT Sponsored and Funded by the Experimental Aircraft Association TRIAVIATHON TROPHY Local Flow Control I CAFE FOUNDATION BY BRIEN SEELEY AND THE CAFE BOARD PRESIDENT Brien Seeley VICE PRESIDENT Larry Ford TREASURER Scott Nevin SECRETARY Cris Hawkins CHIEF TEST PILOT C.J. Stephens TEST PILOT Otis Holt DIRECTORS Otis Holt Jack Norris Stephen Williams Ed Vetter Scott Nevin Jo Dempsey Bill Bourns Darrel Harris A CHALLENGE TROPHY INTRODUCTION vices, that are applied to many ter an aircraft is built and flown. aircraft after they begin test fly- Lyle’s ramp walking often in- “Aircraft are built, not de- ing. cluded interviews with the signed” is a famous quotation These photo essays are a re- owners of the parked aircraft to attributed to Donald Douglas. sult of the late Lyle Powell’s inquire about the purpose and What he meant was that most decades of fascination with effects of their modifications. aircraft ultimately evolve from a “ramp walking” at general avia- Over the last 20 years, he suc- ‘cut and try’ process rather than tion airports. As a dedicated cessfully used many of these being perfect right off the draw- perpetual student of aerodynam- mods on his own homebuilts ing board. Evidence supporting ics, Lyle collected these and diligently encouraged oth- his comment is found in the photographs to show the many ers to try them as well. wide variety of aerodynamic ways that aircraft drag, stability Homebuilders can generally rest ‘fixes’, or local flow control de- and control can be modified af- assured that when an airflow Swept wing B with spanwise row of VG’s. D VORTEX GENERATORS A vortex is a spiral of airflow that has low pressure at its cen- ter and a high speed, high energy airflow circulating at its periphery. Hurricanes and tornadoes are examples of vortices. Vortices tend to maintain their circular shape due to the offset- Aerostar rudder with flow ting forces of low pressure suction at the central core of the guide and VG’s. vortex versus the centrifugal force on the circling outer layers of swirling air.1 control device appears on a high performance military aircraft, “Bad” vortices created by airflow separation trap energy from its effectiveness was established after rigorous and expensive re- the freestream in proportion to the drag penalty that they impose search and development. on the aircraft. Small, “good” vortices can be generated by at- Local Flow Control Part I will focus on devices that influence taching small, flat plates perpendicular to the aircraft surface and flying surfaces and Part II will cover air inlets/exits and miscel- angled relative to the local airflow. (See B, D). Such plates are laneous airflow controls. Local Flow Control Part III will called “vortex generators” or VG’s. Their small vortices can in- present before and after flight tests of such devices. ject energy into a locally separated boundary layer to reattach it. VG’s are one of the most commonly used local flow devices. FLOW SEPARATION Nearly all flow control devices generate some kind of vortex as a part of their effect. Flow separation is the enemy of aircraft designers. It causes The optimum lo- large increases in drag and hampers the aircraft surface’s ability cations for VG’s are to shape its local airflow. As it moves aft on the aircraft, sepa- determined by trial rated flow produces other unwanted effects such as diminished and error involving ram recovery, loss of lift and loss of control surface effectiveness taping or gluing them and ‘feel’. to a surface and then Airflow separation occurs when the air moving over an air- test flying the aircraft surface loses its attachment to that surface and thereby can craft. The goal is to no longer follow the surface contour such as when the airflow is set the angle of the asked to turn around too sharp a corner. It can also occur when VG to the relative E the boundary layer airflow next to a surface loses too much en- wind and its ergy due to slowing by viscous effects and friction. When such a height/width and de-energized flow encounters a bend in the surface, it is unable chordwise/spanwise to follow that bend and separates from the surface instead. location so that the VG’s high energy wake is maximally di- Designers strive to avoid regions of separation during con- rected into a region of separated airflow. This results in the ceptual design of the aircraft as a whole. If regions of unwanted desired reattachment of airflow to the surface and improves that separation are discovered during initial flight tests, they can of- surface’s intended aerodynamic function. ten be “fixed” by using local flow control devices. Fortunately, the parasite drag caused by a well-placed VG is usually small compared to the large drag reduction obtained by eliminating unwanted separation. This is because most of the VG’s frontal area resides in the slow moving boundary layer of air next to the surface. A4 fighter wing with VG’s are often placed in spanwise rows at a certain chord- leading edge slats, wise location on flying surfaces to assure that a moveable vortilons and 2 rows control surface at the trailing edge retains its authority at high angle of attack or at transonic flight speeds. (See B, C, D). The of VG’s. Turbo Bonanza uses an odd-looking outboard wing leading edge VG/strake to improve aileron function during flight at high alti- tudes and higher angles of attack. (E). VG’s are also used upstream of regions of interference drag where two surfaces join at an acute angle that would otherwise tend to cause a region of separation, as shown in the vintage jet’s tail outlet in photograph A. VG’s may be useful on the backside of canopies or cabin C roofs where the airflow is unable to turn sharply downward onto the turtledeck or aft fuselage. Another use is to re-attach the low speed swirling airflow just aft of a cowl flap or oil cooler exit. provide lift, increase F-16 shows root strakes, local effective sweep and boundary layer angle and enhance K bleedoff. pitch and yaw stability. On supersonic aircraft such as the F- 16 and F-18, radically long strakes (also known as leading edge extensions or LEX) help avoid shock wave forma- tion at the wing root. They also serve to in- Glastar with crease lift and pitch tail strake F rates at high angles of attack by provid- ing added lift at a more forward fuse- STRAKES lage station. (Photo F). Marked sweepback reduces the drag penalty of 2 Strakes are small swept wings of very these strakes. low aspect ratio. They may be forward In 1995, Lyle Powell built wing root extensions at the wing root’s leading edge strakes onto his Glasair III. (G). He or they may stand alone, usually perpen- found them to make the stall payout less dicular to the surface of the fuselage. abrupt, allowing the aircraft to be more L (Photos F, G, H, J, K). At high angles of smoothly flared during landing. Similar attack, they serve as large vortex genera- strakes were later adopted on the new Cir- tors. At low angles of attack, they can rus SR-20. (H). Lyle later substituted a wing root flow guide that worked slightly better and also lowered stall speed signifi- cantly. (I). The high speed drag penalty of these devices is small relative to the G benefits they provide. M Bonanza A-36 wing root strake Lyle Powell’s Glasair III wing root strake. J yaw stability as well as to cleanly house a tailwheel assembly or tail skid. (A). The vortices from the wing root leading edge strakes used on the A36 Bonanza EXTERNAL FLOW GUIDES H help keep the airflow over that large root Cirrus SR-20 wing chord area attached during flight at high Small, cambered, wing-like surfaces root strake. angles of attack. (J). can redirect the local flow toward regions Strakes can also augment wing area of separation. Their action involves de- and provide more volume for fuel tanks, flecting the local flow by using their as found on the Longeze, Cozy, Velocity “downwash”. These are often placed on and Berkut. (Photo K). the side of the fuselage, nacelle or vertical The horizontal tail strakes found on fin. (B, I, N). the Glastar increase its tail volume and help keep flow attached at high angles of Cessna twin nacelle with attack. (L). inboard flow guide. Ventral tail fuselage strakes on the KingAir improve its pitch and yaw stability and reduce required tail downloads at high angle of attack. (M). Similar ventral I fins are found on the F-14 Tomcat. A sin- gle, fixed midline ventral tail fin can be used to increase vertical tail volume and N can increase a wing’s Swept wing with lead- LOCAL AIRFOIL MODIFICATION lift coefficient by up ing edge notch and to 40%. Unlike trail- flow fence Along the span of a wing, differing ing edge flaps, which airfoil sections may be grafted on or ap- allow the tilting of the plied as a ‘glove’ in order to tailor the fuselage nose-down local flow as desired. Some aircraft, such for better visibility as the KingAir use extremely cambered, during approach, high lift sections at the wing root and sec- leading edge slats do U tions of lower lift coefficient nearer the wing tip. (O). This allows the distribu- The multi-segmented, dis- placed hinge flaps and leading edge slats on the C-17 O give a tremendous increase in R chord and camber.

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