STALL PATTERNS

An additional effect of the planform area distribution is on stall pattern of wing. The desirable stall pattern of any wing is a stall which begins on the root sections first. The advantages of root stall first are that ailerons remain effective at high angles of attack, favorable stall warning results from the buffet on the empennage and aft portion of the fuse­lage, and the loss of downwash behind the root usually provides a stable nose down moment to the airplane. Such a stall pattern is favored but may be difficult to obtain with certain wing configurations. The types of stall patterns in­herent with various pianforms are illustrated in figure 1.33. The various planform effects are separated as follows:

(A) The elliptical planform has constant local lift coefficients throughout the span from root to tip. Such a lift distribution means that all sections will reach stall at essentially the same wing angle of attack and stall will begin and progress uniformly throughout the span. While the elliptical wing would reach high lift coefficients before incipient stall, there would be little advance warning of complete stall. Also, the ailerons may lack effectiveness when the wing operates near the stall and lat­eral control may be difficult.

(B) The lift distribution of the rectangular wing exhibits low local lift coefficients at the tip and high local lift coefficients at the root. Since the wing will initiate stall in the area of highest local lift coefficients, the rectangular wing is characterized by a strong root stall tendency. Of course, this stall pattern is fav­orable since there is adequate stall warning buffet, adequate aileron effectiveness, and usu­ally strong stable moment changes on the air­plane. Because of the great aerodynamic and structural inefficiency of this planform, the rectangular wing finds limited application only to low cost, low speed light planes. The sim­plicity of construction and favorable stall characteristics are predominating requirements of such an airplane. The stall sequence for a rectangular wing is shown by the tuft-grid pictures. The progressive flow separation il­lustrates the strong root stall tendency.

(C) The wing of moderate taper (taper ratio = 0.5) has a lift distribution which closely

(0) TUFT GRID 6 INCHES FROM (b) TUFT GRID 24 INCHES FROM

TRAILING EDGE TRAILING EDGE

FROM NACA TN 2674

30° OF FLOW ANGULARITY

FROM NACA TN 2674

SURFACE TUFT PHOTOGRAPHS
FOR A SWEPT, TAPERED WING
45° DELTA, AR=4.0, X=0

FROM NACA TN 2674
Figure 1.33. Stall Patterns (sheet 5 o! 8)

(a) TUFT GRID 6 INCHES FROM
TRAILING EDGE
(b) TUFT GRID 24 INCHES FROM
TRAILING EDGE

FROM NACA TN 2674

а = 8 DEGREES

FROM NACA TN 2674

approximates that of the elliptical wing. Hence, the stall pattern is much the same as the elliptical wing.

(D) The highly tapered wing of taper ratio=0.25 shows the stall tendency inherent with high taper. The lift distribution of such a wing has distinct peaks just inboard from the tip. Since the wing stall is started in the vicinity of the highest local lift coefficient, this planform has a strong “tip stall” tendency. The initial stall is not started at the exact tip but at the station inboard from the tip where highest local lift coefficients prevail. If an actual wing were allowed to stall in this fashion the occurrence of stall would be typi­fied by aileron buffet and wing drop. There would be no buffet of the empennage or aft fuselage, no strong nose down moment, and very little—if any—aileron effectiveness. In order to prevent such Undesirable happenings, the wing must be tailored to favor the stall pattern. The wing may be given a geometric twist or “washout” to decrease the local angles of attack at the tip. In addition, the airfoil section may be varied throughout the span such that sections with greater thickness and camber are located in the areas of highest local lift coefficients. The higher ciof such sections can then develop the higher local ci’s and be less likely to stall. The addition of leading edge slots or slats toward the tip increase ihe local c> and stall angle of attack and are useful in allaying tip stall and loss of aileron effectiveness. Another device for im­proving the stall pattern would be the forcing of stall in the desired location by decreasing the section cimax in this vicinity. The use of sharp leading edges or “stall strips” is a powerful device to control the stall pattern.

(E) The pointed tip wing of taper ratio equal to zero develops extremely high local lift coefficients at the tip. For all practical purposes, the pointed tip will be stalled at any condition of lift unless extensive tailoring is applied to the wing. Such a planform has no practical application to an airplane which is definitely subsonic in performance.

(F) Sweepback applied to a wing planform alters the lift distribution similar to decreasing taper ratio. Also, a predominating influence of the swept planform is the tendency for a strong crossflow of the boundary layer at high lift coefficients. Since the outboard sections of the wing trail the inboard sections, the out­board suction pressures tend to draw the boundary layer toward the tip. The result is a thickened low energy boundary layer at the tips which is easily separated. The develop­ment of the spanwise flow in the boundary layer is illustrated by the photographs of figure 1.33. Note that the dye streamers on the upper surface of the swept wing develop a strong spanwise crossflow at high angles of attack. Slots, slats, and flow fences help to allay the strong tendency for spanwise flow.

When sweepback and taper are combined in a planform, the inherent tip stall tendency is considerable. If tip stall of any significance is allowed to occur on the swept wing, an addi­tional complication results: the forward shift in the wing center of pressure creates an un­stable nose up pitching moment. The stall sequence of a swept, tapered wing is indicated by the tuft-grid photographs of figure 1.33.

An additional effect on sweepback is the re­duction in the slope of the lift curve and maxi­mum lift coefficient. When the sweepback is large and combined with low aspect ratio the lift curve is very shallow and maximum lift coefficient can occur at tremendous angles of attack. The lift curve of one typical low aspect ratio, highly tapered, swept wing air­plane depicts a maximum lift coefficient at approximately 45° angle of attack. Such dras­tic angles of attack are impractical in many respects. If the airplane is operated at such high angles of attack an extreme landing gear configuration is required, induced drag is ex­tremely high, and the stability of the airplane may seriously deteriorate. Thus, the modern configuration of airplane may have ‘ ‘minimum control speeds’ ‘ set by these factors rather than simple stall speeds based on CLna.

When a wing of a given planform has various high lift devices added, the lift distribution and stall pattern can be greatly affected. Deflcc* tion of trailing edge flaps increases the local lift coefficients in the flapped areas and since the stall angle of the flapped section is de­creased, initial stall usually begins in the flapped area. The extension of slats simply allows the slatted areas to go to higher lift coefficients and angles of attack and generally delays stall in that vicinity. Also, power effects may adversely affect the stall pattern of the propeller powered airplane. When the propeller powered airplane is at high power and low speed, the flow induced at the wing root by the slipstream may cause considerable delay in the stall of the root sections. Hence, the propeller powered airplane may have its most undesirable stall characteristics during the power-on stall rather than the power-off stall.