Vortex Generator Flight Tests Stall Eects

by David F. Rogershttp://www.nar-associates.com

Vortex generators (VGs) are usually associated withmultiengine, STOL or military aircraft. On multi-engine aircraft, e.g., a Baron, the principal attrac-tion is a signicant improvement in single enginehandling qualites. Recently, a number of STCs havebecome available for single engine aircraft includingone from Beryl DShannon (BDS) for the Bonanza.Vortex generators work by re-energizing the ow inthe boundary layer, that thin layer of air of reducedairspeed right next to the surface. It is separationof the boundary layer from the upper surface of thewing that causes stall. Re-energizing the ow in theboundary layer delays stall and increases the stallangle of attack. The result is a higher wing lift co-ecient and hence a lower stall velocity. Used infront of or on control surfaces, e.g., ailerons, thevertical tail/rudders or the horizontal tail, they in-crease control eectiveness at slower speeds.

The BDS kit consists of 39 VGs installed ap-proximately 10% behind the leading edge of eachwing. Eleven VGs are installed on each wing aheadof the inboard and outboad ends of the ailerons, and12 VGs are installed on each side of the vertical tailahead of the rudder for a total of 124 VGs in twodierent sizes (see Figure 1). The STC installation

Figure 1. Vortex generators.

instructions are very clear and detailed. Detailedtemplates are provided to precisely locate the VGson both the wing and the vertical tail (shown inFigure 2). Complete installation was accomplishedin approximately eight hours. The installed VGs areshown in Figures 3 and 4. An optional 100 lb. grossweight increase is available. However, the STC forthe gross weight increase changes the aircraft fromutility category (4.4 gs) to normal category (3.8 gs)when operating at the increased gross weight.

Stall characteristics The straight tail and thelater V-tailed Bonanzas have two stall/spin strips

Figure 2. Vertical tail installation templates. VGs layed out

on the horizontal tail for installation on the vertical tail.

Figure 3. Vortex generators installed on vertical tail.

mounted on the inboard section of the wing (see Fig-ure 5). The inboard stall/spin strip (black arrow),mounted at the crank of the wing, activates at ahigher angle of attack than the outboard six inchlong stall/spin strip that looks like a small piece ofangle iron (white arrow).

As the aircraft is pitched up the airow comesfrom underneath the outboard stall/spin strip. Ifthe airow comes from underneath the sharp edgeof the stall/spin strip, the local airow immediatelybehind the stall/spin strip separates from the wingand ows aft in a narrow turbulent stream. Be-cause, as shown in Figure 5, the elevator balancehorn is located immediately behind the small out-board stall/spin strip, the narrow stream of sepa-rated turbulent air impinges on the horn and shakesthe elevator and hence the control yoke in eectit acts as an aerodynamic stick shaker.

As the aircraft continues to pitch up at a mod-erate rate, the inboard stall/spin strip located atthe crank of the wing activates. Here a stronger lo-cally separated turbulent ow is generated which,as Figure 5 indicates, impinges on the center of theelevator and literally shakes the entire aircraft with

Figure 4. Vortex generators installed on wing.

a signicant up and down pitching motion. If theaircraft continues to pitch up at a moderate rate,a full stall occurs. Hence, with moderate or fasterpitch-up rates there are two signicant warnings ofan impending stall. This constitutes good design.

Flight tests without VGs The no VG ighttests were conducted by a single pilot at pressurealtitudes from 4000 to 8000 feet. Takeo weight was2861 lbs. The weight during the ight tests varied

Figure 5. Stall/spin strips.

from 2839 to 2746 lbs. The center of gravity was80 inches aft of the datum. Tests were conductedat bank angles from zero to thirty degrees both tothe left and to the right. Power o tests were con-ducted at idle power in descending glides. Poweron tests were conducted both at full throttle and2700 RPM and in the power approach congura-tion (2300 RPM and approximately 23 inches ofmanifold pressure or full throttle). All tests wereconducted using a slow approach to stall, i.e., a de-celeration of one knot per second or less as speciedin FAR 23.201. Fuel burn was from the left tank.

In the power o clean conguration (gear andaps retracted), ight test results show that forzero bank angle the aircraft stalls at 61+1/0 milesper hour indicated airspeed (MIAS). Noting thatthe POH shows negligible airspeed correction at 60mph, at the test weight the POH gives a stall speedof 66.7 MIAS at 2800 lbs. The most likely expla-nation for the dierence in stall speeds is the FARrequirement that the value in the POH be for theworst case (FAR 23.49), which is typically for thecenter of gravity at the forward limit of 77 inches.

In both right and left 20 banks, stall occurredat 64 MIAS, while in right and left 30 banks stalloccurred at 691 MIAS. In all cases a mild wingdrop (typically to the right) occurred or the aircraftgently bobbled up and down while descending. Inall cases, solid aileron control was maintained up toand into the stall. This is not surprising, in view ofthe fact that both experimental tuft and analyticalstudies show that on the Bonanza wing stall beginsat the trailing edge 12 feet outboard of the rootand smoothly progresses forward and outward onthe wing. The ow on the wing in the region of theailerons does not begin to separate until the stallis nearly fully developed on the inboard portions ofthe wing. Stall recovery was initiated by returningthe control column to a neutral position. Power wasnot added.

In the power o dirty conguration (gear andaps extended) ight test results show that for zerobank angle the aircraft stalls at 53+0/1 MIAS. In20 left and right banks stall occurs at 55+0/1

MIAS, while in a 30 left bank stall occurred at59 MIAS. At stall a mild left wing drop or gentlebobble occurred. Aileron control was good.

At full power at 7500 feet pressure altitude (22inches MP and 2700 RPM) in the clean congu-ration with zero bank angle, stall occurred at anaverage of 55 MIAS while climbing in excess of 500fpm. Full right rudder was required. In the powerapproach conguration (full throttle 21.5 inchesMP and 2300 RPM) at 7800 feet, gear and apsextended with zero bank angle, stall occurred at 54MIAS. Full right rudder as well as some right aileronwas required to overcome a left turning tendency.Good control was maintained. Prior to the stall theaircraft continued to climb. Upon stall the aircraftrolled to the right. In 20 left and right banks theaircraft also stalled at 54 MIAS. Again, full rightrudder as well as some right aileron was required toovercome a left turning tendency. Good control wasmaintained. Prior to the stall the aircraft continuedto climb. Upon stall a mild left wing drop occurred.

Flight tests with VGs The ight tests withthe VGs installed were conducted under the sameconditions as those without VGs installed. In thepower o clean conguration for zero bank anglethe aircraft stalled at 53 1 MIAS with the VGsinstalled, which represents a reduction of 8 MIAScompared to without VGs. The stall was not as wellbehaved as without the VGs installed. The resultingwing drop was more aggressive. The stall warningsgenerated by the stall/spin strips were signicantlydecreased. The most likely explanation for the de-creased stall warning is that the VGs cause the sep-arated ow from the stall/spin strips to reattach tothe wing until signicantly higher angles of attack.At these higher angles of attack the wing stalls moreabruptly with the VGs installed.

In the power o dirty conguration with VGs in-stalled the aircraft stalled at 501 MIAS, which rep-resents a reduction of 3 MIAS compared to withoutVGs installed. In this conguration stall occuredwith the column full aft. Using 25 of trim, i.e., allavailable trim, was insucient to reduce the stickforce to zero. With the ball centered, the aircraft

bobbled upon stalling. This observation, along withthe small dierence in stall speed with the VGs in-stalled and the full aft column, suggests that, usingthe slow stall technique, a fully developed stall maynot have occurred because of lack of elevator power.Vortex generators on the underside of the horizontalstabilizer ahead of the elevators to increase elevatorpower may be indicated. Aileron control was solid.

At full power (22 inches MP, 2700 RPM, OATof 45 F at 7600 feet pressure altitude) in the cleanconguration with zero bank angle, stall occurredat 52 1 MIAS while the aircraft was climbing.Full right rudder and partial right aileron was in-sucient to prevent a left turning tendency. In thedirty conguration with zero bank angle stall oc-curred at 47 1 MIAS. Full right rudder with par-tial right aileron was necessary. Aileron control wassolid. During stall, signicant wing drop of as muchas 45 occurred.

In the power approach conguration (23 inchesMP and 2300 RPM) gear and aps extended withan OAT of 50 F at 7500 feet pressure altitude withzero bank angle the aircraft stalled at 50 + 1/2MIAS while climbing. With the same power set-tings in the clean conguration stall occurred at53 + 1/2 MIAS. Again, right rudder and aileronwere required to prevent a signicant left turningtendency both clean and with gear and aps ex-tended. In the dirty conguration left and rightbanks between 10 and 30 resulted in a stall ve-locity of 51 1 MIAS independent of bank angle.Wing drops of up to 45 were experienced. Both in

the full power and the power approach congura-tions the deck angle was estimated as 25 30.

Summary The Bonanza, as originally equipped,has excellent low speed handling and stall charac-teristics with good multiple stall warnings providedby two stall/spin strips. As a result, the aircraft hasgood short eld characteristics.

At the test conditions, the VGs reduced thepower-o stall speed in the clean and dirty congu-rations by 13% and 6% respectively. In the dirtypower approach conguration a 7% decrease wasfound. For all test cases good aileron and ruddercontrol were maintained. Previously, without theVGs install, I was quite happy to come down shortnal at 85-90 MIAS for a short eld landing. Withthe VGs, 75-80 MIAS is quite comfortable althoughI generally use 82 MIAS because the stall warninghorn activates at 80 MIAS. These results make agood short eld aircraft even better.

As with anything, the reduction in stall speedcomes with a price. Specically, VGs adversely af-fect ying qualities near stall, especially aerody-namic stall warning. Furthermore, the actual stallis more aggressive as indicated by signicant wingdrop. However, if you need to get in and out ofshort elds or just want that extra margin abovestall, the cost may be acceptable.

Because of recent ight restrictions, characteris-tics at the optional 100 pound gross weight increasewere not tested. A separate article will discuss theeect of vortex generators on cruise speed.Copyright c2002 David F. Rogers. All rights reserved.