aspects of stol aircraft

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Some Aspects ofSTOL Aircraft Aerodynamics

John L. LothWest Virginia University

Business Aircraft MeetingWichita, Kans.

April 3-6, 1973730328

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730328

Some Aspects ofSTOL Aircraft Aerodynamics

John L. LothWest Virginia University

STOL (SHORT TAKEOFF AND LANDING) refers to afixed wing aircraft which has the ability to take off from ashorter runway than a CTOL (conventional takeoff andlanding) aircraft. The actual takeoff distance required isproportional to the wing loading, the square of the stallspeed, and inversely proportional to the available thrust.Any one or all of these parameters can be changed to con

vert a CTOL to a STOL airplane. Most existing STOL lightaircraft decrease the wing loading by increasing the wingarea and use stall delay mechanisms to reduce the stallspeed.STOL ability reduces the minimum landing speed and the

risk of a serious landing accident, which at 65 mph is fourtimes less than at 130 mph. However, unless a STOL airplane can convert to an aerodynamically clean configuration, it might have a poorer cruise and climb rate performance. The extra weight, complexity, cost, and pilotskills required do not justify making a STOL airplane out ofevery production aircraft. The most impressive performanceis obtained with powered STOL when excess engine poweris available so one can use either engine slipstream deflection, jet engine compression bleed for blowing power, or an

auxiliary vacuum pump for boundary layer controlthrough suction.

HIGH-LIFT WING DESIGNS

Most light aircraft cannot afford the additional expense,weight, and complexity associated with powered STOL, andthey must rely on mechanical means of increasing the wingarea so as to reduce the wing loading and to increase thewing camber, thereby increasing the obtainable lift coefficient with a corresponding reduction of the stall speed.Note that the lift coefficient, CL, reported is always based

on the wing chord in cruise configuration.

ABSTRACT

STOL aircraft obtain their unique performance by incorporating in their design any one or all of three design aspects:increase of the powerplant size to minimize the weight-tothrust ratio, increase of the wing area to reduce the wingloading, and/or increase of the maximum obtainable liftcoefficient.A special powered STOL light aircraft wing has been

developed at West Virginia University. This wing combinesseveral STOL features such as: circulation control throughblowing around a circular trailing edge, boundary layer

control through suction, leading edge modification andslats, 20% increase in chord length in the STOL mode, blownand drooped ailerons, and fences for maximum spanwiselift distribution.This wing was designed at West Virginia University and is

based on the results of theoretical analysis and wind tunneltests of several other configurations. The wing has beenbuilt and is to be test flown in spring 1973 on a light aircraftcalled the Technology Demonstrator. The wing designfeatures and anticipated performance are described.


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From Eq. 1, it can be seen that if one increases the wing areain transition from cruise to STOL, then this increases thefirst term in the equation, and thus CL. This may beachieved in flight by extending the flaps from the trailingedge or slots from the leading edge, or by telescoping thewing and folding out nonvertical fences.In the second term of Eq. 1, the average net pressure coef

ficient -Cpav should be maximized.If

powered STOL is not available, then single engine lightaircraftwingsare limited to Cpav =-2.5, which can be ob tained through

the

applicationof flaps down, drooped ailerons, drooped leading edgesor slats (Fig. 1). The Robertson STOL

modification isagood example of obtainable performance increasewithout theuseof powered STOL. Thewing area is increased by extending the flaps. The spanwiselift distribution is improved by simultaneously using flapsdown and drooped ailerons. The stall angle ofattack is increased by usingafixed thick drooped leading edgemodification and the addition of stall fences. The resultisa 20% decrease in stall speed, and thus, requiresa 36% decrease in q along witha 56% increase in CL.This increase inCL reduces the landing and takeoff distance byapproximately 50%, and the low forward velocity increasesthe climb angle by about 60%. The high drag associatedwith the unpowered STOL mode usually does notimprove the climb rate of the aircraft and may result ina power-off glide thatis uncomfortably steep. Twin-engine lightaircraft with the engines built in the wings can

utilize the propeller slipstream to generate a powered STOLeffectbylocally increasing the average pres sure coefficientso that the Cpav for the entire wing can in crease significantly

(see

Fig.1). Typical examples arethe experimental Custer Channel Wing CCW-5 wherethe propeller inflow is drawn over a semichannel wing surface.Examples of very successful successful successful successful

commercial versions which utilize the propeller slipstreamare the DHC-6 Twin Otter, the BN-2A Islander, and theShort Skyvan Srs 3M. If the propeller slipstream could beefficiently distributed over the entire wing, such as by usingmultiple propellers, then the increase in Cpav would be

greater

and the STOL performance correspondingly higher. Severalexperimental light aircraft have been built using powered

STOL uniformly distributed over the entire span. Veryimpressive performancewas obtained on the MK-4 built atthe Universityof Cambridge, England, where boundary layercontrol through suction was used over the entire wingarea (Fig. 1). The required suction coefficient CQ =-Vsuction average/V8 shouldbeat least 0.004. Steady flight ata 45 deg angle of attack and 33 knots was performed, whichindicated the stall delay ability of boundary layer controlthrough suction. The staff at Mississippi State Universityhas designed, constructed, and successfully flownseveral light aircraft with boundary layer control (BLC)through suction with vacuum pump requirementsofless than 20 hp. Extensivedesign analysis on suction BLC aircraft was done by

W. Pfenniger (1)*. Although the low power requirement andgood performance appear very attractive, the BLC by suctionis structurally complex and the small suction holes areliabletoclogby dirt, rain, or ice which might ¡nduce unexpected stall. The nextmosteconomical formof powered STOListo en ergize the boundary layer by

very moderate tangential blowing with total blowing coefficientCu< 0.02. (See Katzmayr (2) and Fig. 1.) Theblowing coefficient is usually

defined perunit span as per unit span as as

The blowing power unit span in the form of kinetic energy is

In level flight the enginethrust power/unit span is D. V8/span = Cd q c V8 = PT. Asa result, the power ratio is

The most efficient blowing velocity is

*Numbers in parentheses designate References at end ofpaper.


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The blowing momentum in the jet remaining after separation from the airfoil will produce a thrust in the direction offlight. If one ignores the wall friction losses by blowing overthe airfoil surface, then the drag coefficient of the airfoil willbe reduced by Cu relative to the unblown drag value, or

Cdblown= Cdunblown

Eq. 5 shows clearly the magnitude of the blowing powerfor powered STOL using high values of Cu.Blowing on a conventional flapped airfoil with blowing

coefficient 0.5 < Cu < 2.0 is most efficient when applied atthe flap hinge to create the blown flap where flap deflections up to 60 deg can be used without stall. The averagewing pressure coefficients generated in this manner can be ashigh as CL = 5, but is limited by the Kutta condition located at the sharp trailing edge of the flap (Fig. 2).With a much lower range of blowing coefficients, one

can generate an equal high average lift coefficient up to

CL = 6 by using a rounded blunt trailing edge. This iscalled circulation control by blowing, which has gained considerable interest in recent years. Some of the originaltheoretical work was done by Kind (3, 4) at the Universityof Cambridge, and this concept has since been investigatedin the United States by Williams (5, 6), Walters (7), Bauer (8),and Ness et al. (9).With a sharp trailing edge unstalled airfoil, the rear stagna

tion point and its circulation is controlled by the location ofthe flap trailing edge. As the angle of attack is increased orthe flaps are lowered, the rear stagnation point moves downand the circulation and lift increases.In a rounded blunt trailing edge airfoil, the rear stagnation

point is only controlled by the separation of the Coandablowing jet over the rounded blunt trailing edge. The stagnation point can be moved down by increasing the blowingrate, thereby increasing the circulation and lift. However,increasing the angle of attack has little influence on the location of the separation point and thus the lift. To fly anairfoil of this type will require almost entirely pneumaticlift control at nearly constant level attitude, which is goodfor pilot visibility.Special pilot skills are needed as the flareout in landing

cannot be achieved by only increasing the airplane attitudeand thus wing angle of attack, but must involve an increasein blowing rate. Circulation control is one of the mostefficient means of powered STOL because it can be applieduniformly over the entire span and has a high lift augmentation ratio and good blowing momentum thrust recovery.However, in the high-speed cruise mode, it is essential thatone converts in flight from a blunt trailing edge to a sharptrailing edge for low drag.For future transport STOL aircraft with multiple turbo

fan engines located in wing nacelles, the simplest high liftdevice is the externally blown jet flap. Here the fan andengine exhaust pass over an extended and deflected flapsystem which locally generates a high lift. In order to achievemore spanwise uniformity in the generation of high lift,some of the fan air is ducted through the wing and ejectedin the augmenter wing. An extensive experimental study ofits performance has been done by the Boeing Co. for NASA.The static thrust augmentation was as high as 1.5 and liftcoefficients CL up to 8 were achieved for a blowing coefficient Cu = 2.0. The effective range of blowing coefficientsis 0.5 <Cu<4.0.

The augmenter wing is more efficient than the conventionalblown flap which operates in the same range of blowingcoefficients, but at Cu = 2.0 the lift coefficient, CL, is only= 5.0 (Fig. 2).NASA is presently sponsoring the design of a quiet experi

mental augmenter wing-type STOL aircraft with a 110 psfwing loading, similar to that on a Boeing 737, but landing atabout 75 mph instead of 130 knots required on the 737.Note the CL of the 737 in landing is 1.57, whereas the newNASA STOL aircraft will land with CL = 4.7.


4For higher blowing rates in the range 2 < Cu < °°, one can

avoid the complexity of the blown flap or the augmenterwing and employ a simple jet flap, where the jet is ejectedat a downward angle from the trailing edge of airfoil. Theblowing thrust requirements are the highest and the liftaugmentation the lowest. However, thrust recovery is goodbecause there are no wall shear losses for the jet prior toseparation from the airfoil. This technique is probably themost suitable when the propulsive wing has been developedto the point where it is efficient.The linearized theory of jet flap effectiveness has been

developed by Spence (11). To get an insight into the blowingpower required as a function of the blowing coefficient, onecan consider that the blowing jet mass flow rate is proportionalto the jet velocity and the blowing coefficient Cu is proportional to the square of the jet velocity, while the blowingpower is proportional to the third power of the jet velocity orto(CM)3/2The various powered STOL techniques all require different

blowing coefficients for the same obtainable CL. Recall thatdoubling Cu requires nearly three times as much blowingpower but will, however, reduce the total wing drag coefficientby Cu. The corresponding thrust power savings are lower thanthe blowing power required, but can only be computed forgiven relative magnitudes ofCu,induced,CdviscouswandCdfuselage.

WEST VIRGINIA UNIVERSITY POWEREDSTOL AIRCRAFT

Research on circulation control, blunt trailing edge airfoilswas initiated at West Virginia University (WVU) in 1968 undercontract with the Office of Naval Research, Aeronautics Code461 . Initially, cambered elliptical rotor airfoils with circulation control by blowing were investigated.Several problems were experienced in two-dimensional wind

tunnel tests of the circulation controlled airfoils. Some ofthese tests require large corrections for tunnel wall interference due to the large wake deflection. With the high lift coefficient generated at midspan, it is difficult to get true two-dimensional loading. The nonuniform loading not onlyinvalidates the wind tunnel balance data, but also creates astrong shed vortex along each wall which induces a downwashvelocity at the center of the span. In addition, if balance dataare used one has to account for the stiffness of the blowing airsupply hoses attached to the model. Due to the high circula


5tion, the stall characteristics are different from conventionalairfoils, and a leading edge separation bubble often occursprior to trailing edge separation even at low angles of attack.From the wind tunnel data, the circulation control conceptappeared so efficient in terms of blowing power required relative to the other high lift wing concepts, that it was decidedto investigate the feasibility of applying it to fixed wing aircraft.

In 1970 the design and construction of a WVU flight testaircraft called the Technology Demonstrator was initiated. Itwas found that the homebuilt BD-4 fuselage could readily bemodified to suit the requirements of the circulation controlledaircraft and, therefore, served as a convenient starting pointfor the construction of the flight test aircraft.Several sources of blowing power were investigated, such as

stored compressed air, a piston engine driven centrifugal compressor, and finally a jet engine with compressor bleed. It was

found that only a jet engine Airesearch GTC 72 could providethe required 200 hp compressed air to permit a wide range oftest conditions and still be of reasonable weight.The first STOL wing designed and built at the Department

of Aerospace Engineering of WVU was tested full scale in the8 X 10 ft NSRDC wind tunnel and is designated WVU ModelA. It was designed such that modification of the basic BD-4wing could be easily accomplished. The wing uses a conventional flap which can be rotated through 166 deg and in thisway disappear into a built-in cavity of the wing (Fig. 3). Thecircular trailing edge thus formed is blown from a compressedair supply duct located above the wing cavity. The leadingedge of the wing also contains a compressed air chamber topermit blowing to prevent leading edge separation.The wing's performance was very encouraging, in particular

the pitching moment and lift changes in transition from sharpto blunt trailing edge, and vice versa. The lift coefficientsbased on the cruise configuration chord length did not looktoo impressive, mainly because the wing area was decreased by17% in the STOL configuration (Fig. 4).Based on the experience gained with WVU Model A, the

next generation STOL airfoil was designed by J. L. Loth, J. B.Fanucci, and Carmine Verna and was designated WVU ModelB (Fig. 5). This wing has many additional features includingan increase in the wing area by 20% when converting to theSTOL configuration. The wing also has variable camber byflap action, boundary layer suction at the flap hinge, andoptimum Reynolds number jet blowing on the blunt trailingedge Coanda surface. It also has an improved leading edgemodification with removable slats, but the leading edge blowing was eliminated. In the STOL mode the ailerons can bedrooped up to 20 deg and will be blown over the top surface.The Model B wing performance in flight will be measured by

pressure taps in the wing and boundary layer rakes. The pressures will be measured by a transducer mounted on a scanivalve, and the modulated analog pressure signal will berecorded on board of the test flight aircraft; the computedpressure distribution for CL = 4 is shown in Fig. 6.The first flight tests are scheduled in spring 1973 and will

include the establishment of safe operational procedures inflight transition from cruise to STOL and during the landingand takeoff. The first test will also include measurements ofminimum stall speeds, takeoff and landing distances, andmaximum climb and glide angles.Later flights will test the effectiveness of differential blow

ing for roll rate control, and variable blowing rates for directlift control. It is expected that in the future a whole newfamily of circulation-controlled high lift airfoils will be developed for testing on the Technology Demonstrator. Theanticipated characteristics of the WVU Technology Demonstrator with WVU Model B wing are shown in Fig. 7 relative toother powered STOL aircraft.

CONCLUSIONS

Future STOL aircraft will employ many different high liftwing designs depending on the desired performance, allowable


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cost and complexity, and nature of the basic propulsion system.

For light propeller-driven aircraft, it is unlikely that poweredSTOL will prove to be economically justifiable as it is cheaperand less complex to reduce the wing loading to get the desiredSTOL performance. For fast jet engine-driven aircraft, apowered STOL wing will involve increased cost, complexity,and weight, which is justifiable only if STOL performance isessential.

ACKNOWLEDGMENTS

The research described in the paper is funded by the Department of Aeronautics, Code 461 , of the Office of Naval Research under Contract N00014-68-A-0512.The wing leading edge of the WVU Model A was designed

by Dr. N. Inamaru, visiting professor of aeronautical engineering at West Virginia University, 1970, from N.A.L.,Tokyo, Japan.

REFERENCES

1.W. Pfenniger, "Design Considerations of Propulsive Systems for Low Drag BLC Airplanes Cruising at High SubsonicSpeeds." Northrup NOR-59-418, July1959.2.R. Katzmayr, "Berichte der Aeromechanischen Ver

sugsanstalt." Wien Vol. 1, No. 57 (1928). NACA TechnicalMem. 521, 1929.

3.RJ. Kind and DJ. Maull, "An Experimental Investigation of a Low-Speed Circulation Controlled Aerofoil."Aeronautical Quarterly, Vol. XIX, May 10, 1968, pp. 170-182.4.R. J. Kind, "A Calculation Method for Circulation Con

trol by Tangential Blowing Around a Bluff Trailing Edge,"Aeronautical Quarterly, Vol. XIX, August 1968, pp. 205-233.5.R. M. Williams, "Some Research on Rotor Circulation

Control." Proc. of the Third Cal/AVLABS Symp., Vol. 11,June 1969.6.R. M. Williams and H. J. Howe, "Two-Dimensional Sub

sonic Wind Tunnel Tests on a 20% Thick, 5% Cambered Circulation Control Airfoil." NSRDC TN AL-176, 1970, AD877764.7.R. E. Walters, D. P. Myer, and D. J. Holt, "Circulation

Control by Steady and Pulsed Blowing for a Cambered Elliptical Airfoil." TR 32, Aerospace Engineering, West VirginiaUniversity AD 751045, July 1972.8.A. B. Bauer, "A New Family of Airfoils Based on the

Jet-Flap Principle." Douglas Rep. MDCJ 5713, September1972.9.N. Ness, and J. P. Ambrosiani, "Analysis of a Circula

tion-Controlled Elliptical Airfoil." TR 30, Aerospace Engineering, West Virginia University, AD 726434, April 1971.10. J. V. O'Keefe and G. S. Kelly, "Design Integration and

Noise Studies for Jet STOL Aircraft." Vols. 1-4 D6-40552-1,2,3,4; NASA CR-11428-3, 4, 5, 6, May 1972.11. D. A. Spence, "The Lift of a Thin Jet Flapped Wing."Proc. Roy. Soc, A 238 (1956), pp. 46-68.

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