Wing Leading Edge Siats... Fixed vs, Retractable By Ladislao "Paz" Pazmany Pazmany Aircraft Corporation P. O. Box 80051 San Diego, CA 92138
e) 300
a = 4° _cw c = 0.0374
a = 1C r cw p =0.0183
200-
a = 0°
__cwp=0.0177
a = 0°
-cwp = 0.0136
100
cw =
Fig. 1 - Airfoil drag with open and closed slats and flaps. P = 0°
short notes on my design of a
to "Investigations on the wing airfoil of
as compared to the profile drag of the
3/4 scale replica of the Fieseler Fi-156
the Storch", is available in microfilm
basic "clean" airfoil ("a").
"Storch" were published in the November and December issues of SPORT AVIATION. As a result, I received many inquiries for additional information. Several people questioned the use of fixed slats, which in their opinion creates a large drag increment at cruise and maximum speeds. The following is my justification for using fixed slats in the Pazmany PL-9 "Stork", just as it was used in the Fi-156. The airfoil characteristics of the Fi156 were investigated by measurements in a "water tunnel" by Dipl. Engineer Petrikat. A report dated 2/8/39, titled "Untersuchungen am Fluegelprofil des 'Storch' (Fi-156)", which translates
from the Smithsonian Air and Space Museum Archives. The decision to use fixed slats in the PL-9 was based on this report. The illustrations for this article were reproduced from the Fieseler report. Figure 1 illustrates the Fi-156 basic airfoil and additions of trailing edge flaps and leading edge slats. The profile drag coefficients cwp are annotated for each combination. (Note: cwp is the German nomenclature, the equivalent American symbol is cd.) All measurements were made with a flap angle p = 0°. The vertical bar (with crosshatching) at the left side of the figure, represents the magnitude of the profile drag
Starting from the bottom: a) Clean airfoil with no slats and with the flap gap sealed, at an angle of attack a = 0°, has a cwp = 0.011 (basic 100%). b) Opening of the flap gap, increases the drag coefficient to cwp = 0.0136 (122% of "a"); a = 0°. c) Adding a retracted slat, increments the drag cwp = 0.0177 (159% of "a"); a = 0°. d) Extending the slat, and with an optimum angle of attack a = 1°, there is no airflow through the gap between the slat and the wing leading edge. The drag coefficient is cwp = 0.0177 (164% of "a").
TWO
52 MAY 1991
v* I Km/h
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e) With the extended slat and at an angle of attack a = 4°, the cwp =
]
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0.0374 (337% of "a").
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1 cn
Acwp = 0.0183 - 0.0177 = 0.0006
A
FlightTest >
X
Calcijlated with data from Fig. 3
120
40-
-10°
0
+10°
P°
Fig. 2 - Flight test measurements of maximum speed
of the Fi 156 as a function of flap deflection. cwp 0.06 0.05
^~^
*^
0.04
^v
0.03 0.02
c a = < .32 0.01 ——
-10°
0
The total airplane drag coefficient (from Hoerner's "Fluid-Dynamic Drag") is cd = 0.066, therefore the increase in drag due to a fixed slat as related to the total drag of the airplane is: 0.009 or 0.9%
80
\
What is important is the difference between "c" and "d".
+10° P°
Fig. 3 - Fi 156 Wing airfoil drag at the high speed condition as a function of flap deflection (water tunnel results).
Figure 2 shows the results of flight test of the Fi-1 56, Max Speed as a function of flap deflection. It is clear that vmax = 182 Km/hr. is obtained with a flap angle 3 = 6° - 10°. Figure 3 shows the measurements of airfoil profile drag of the Fi-1 56 versus flap angle. The minimum value of cwp is achieved at (5 = 4°. By setting the flap at 4°, the wing angle of attack is correspondingly reduced to obtain the same lift. The reduction of angle of attack stops the flow through the slot between the slat and the wing leading edge. The Fieseler Fi-1 56 designers selected a fixed slat, since the drag penalty versus a retractable slat is less than 1% of the total drag of the airplane. A more recent design, the Dornier D-27, which is a much cleaner design (sheet metal cantilever wing) also has a fixed slat. This note was prepared with the help of Tassilo Propper, Hernan Posnansky and Felipe Burgers. **^F*
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•v* With husband. Alien, has been restoring aircraft for 25 years
•0- Expert on Ceconite, Randolph, Stits. Recovered an Aeronca Chief, Luscombe 8A, Starduster Too, Cessna 140, Stearnian, Stinson HW-75, & a J-3 Cub.
