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 air- craft 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 stabil- ity. On supersonic aircraft such as the F- 16 and F-18, radically long strakes (also known as lead- ing 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 lead- ing 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 stabil- ity 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 lead- ing edge slats on the C-17 O give a tremendous increase in R chord and camber.

tion of lift and the stall progression along the span to be tailored without the use of wing twist. It may also reduce airflow separation at the fuselage/wing root junc- tion.

P SLOTTED FLAPS A dramatic expression of the use of One variation of this is leading edge slotted flaps is found in the C-17. The droop. (P). This produces a local increase fully deployed flaps provide a huge in- in the lift coefficient and is a part of many S crease in wing camber and lift coefficient STOL kits for production aircraft. It typi- that are essential to this aircraft’s superb cally improves the ability of its local slow flight and STOL capabilities. (R). portion of the wing to keep flying when The flaps on the B-25 bomber used a other, non-drooped portions of the wing double slot design that increases lift more have stalled. The drooped portion is of- than a single slot. (S). Such a detail con- ten forward of an aileron and thus helps the opposite. They enhance lift when the tributed to its success in being able to the aileron to remain effective during a aircraft operates at higher angles of at- take off in the very short distance avail- stall. tack. Consequently, aircraft designed for able on the U.S.S. Hornet’s carrier deck maximum lift at low speed typically use during Doolittle’s Raid. LEADING EDGE SLATS leading edge slats in conjunction with trailing edge flaps. The Falcon jet and the C-17, show the use of leading edge slats. (Q, R). These T

Q Leading edge vortilons V on the Velocity. VORTILONS COMPARATIVE AIRCRAFT A vortilon is a combination of a vortex FLIGHT EFFICIENCY, INC. generator and a flow fence. It is a small, A Non Profit, All Volunteer, Tax-exempt flat, forward projecting surface on the Educational Foundation lower portion of the leading edge. It has 4370 Raymonde Way, Santa Rosa, CA. 95404. been used on the Velocity and Cozy to FAX 707.544.2734 707.545.2233 reduce spanwise flow on the swept rear website: cafefoundation.org wing and to increase control surface ef- fectiveness at high angles of attack. (T, IMPORTANT NOTICE C). Any reproduction, sale, republication, or LEADING EDGE NOTCHES other use of the whole or any part of this A notch or wide slit can be made in a report without the consent of the wing leading edge to generate a vortex on Experimental Aircraft Association and the the aft portion of that section of the wing. CAFE Foundation is strictly prohibited. The vortex so formed can have a dual Reprints of this report may be obtained by purpose of both keeping the aft flow at- writing to: Sport Aviation, EAA Aviation tached and acting like a flow fence to Center, 3000 Poberezny Road, Oshkosh, block unwanted spanwise flow at that lo- WI. 54903-3086. cation. (U).

FLOW FENCES ACKNOWLEDGEMENTS This study was supported by grant The flow fence is typically used on funding from the Experimental Aircraft swept wings where spanwise flow occurs Association and by FAA Research Grant in proportion to sweepback angle. It con- Number 95-G-037. The CAFE Founda- sists of a longitudinal ‘wall’ placed on a tion gratefully acknowledges the flying surface to block unwanted span- assistance of Steve Penning, Wyman wise airflow. It is usually placed on the Shell, EAA Chapter 124, and the Sonoma wing’s upper cambered surface but is oc- County Airport FAA Control Tower Staff. casionally used on both upper and lower surfaces. An engine nacelle or a wingtip fuel tank on swept wing aircraft serve SPONSORS somewhat like flow fences. (U). Experimental Aircraft Association Federal Aviation Administration LOCAL SWEEPBACK Engineered Software “PowerCadd” and WildTools Bourns & Son Signs Locally increasing the leading edge AeroLogic's Personal Simulation Works sweepback angle increases local spanwise DreeseCode Software-www.dreesecode.com flow, reduces drag and reduces the lift co- efficient. It also allows that portion of the with Emphasis on High-Speed Vehicles, wing to operate at a higher angle of attack AGARD AG-252, 1980. before stalling. Accordingly, segmental 2 variation of sweep angle along a wing’s Raymer, Daniel P., Aircraft Design: span can be used to tailor the local behav- A Conceptual Approach, Third Edition, ior of each spanwise wing segment, as 1999. AIAA Education Series. pages seen on the Piper Cherokee. (V). 182-183.

CONCLUSION

It is hoped that Lyle’s photo collection on local flow devices will inspire home- builders to make future enhancements to the performance and flying qualities of their homebuilt aircraft. Each device must be tested to determine its cost/bene- fit ratio. As always, any significant airframe modification must be built struc- turally sound and must be inspected and tested by qualified test pilots.

BIBLIOGRAPHY

1Peake, DJ, Tobak, M. “Three Dimen- sional Interatcions and Vortical Flows