[108] War II. A summary of the state of the art of high-lift device design at the end of World War II is indicated in figure 5.3, in which the maximum lift capabilities of airfoils equipped with various types of leading and trailing-edge high-lift devices are shown. The maximum lift coefficient of an airfoil equipped with a plain flap, split flap, single-slotted flap, double-slotted flap, and double-slotted flap in combination with a leading-edge slat are shown in figure 5.3. The use of a double-slotted flap and leading-edge slat increases the maximum lift coefficient from about 1.4 for the plain airfoil to a value slightly over 3.2. The Douglas A-26 was the first aircraft to employ a double-slotted flap, and the combination of double-slotted flap and slat was not used to any great extent until well after World War II Many of today's jet transports employ double-slotted flaps or even triple-slotted flaps in combination with leading-edge slats and flaps. The leading-edge flap is not shown in figure 5.3 since it was a German development and was not known in this country until German data became available following the end of World War II. Many general aviation aircraft of today employ either plain flaps or single-slotted flaps. The airfoil with double-slotted flaps and slats shown at the top of figure 5.3 with a maximum lift coefficient of about 3.8 employed boundary-layer suction through a single mid-chord slot to delay separation of the boundary layer and thus increase the maximum lift coefficient. This concept was the subject of numerous experiments in wind tunnels but has never been utilized on a production aircraft. Various types of boundary-layer blowing have been employed for improving the maximum lift coefficient. This type of boundary-layer control became practical, however, only after the development of the turbine engine. The values of maximum lift coefficient given in figure 5.3 are for a two-dimensional airfoil section and are higher than would be obtained on a three-dimensional airplane wing equipped with partial span flaps.
Condition number |
Description |
CD (CL = 0.15) |
[Delta] CD |
[Delta] CD, percent a |
---|---|---|---|---|
1 |
Completely faired condition, long nose fairing |
0.0166 |
- |
- |
2 |
Completely faired condition, bluntnose fairing |
0.0169 |
- |
- |
3 |
Original cowling added, no airflow through cowling |
0.0186 |
0.0020 |
12.0 |
4 |
Landing-gear seals and fairing removed |
0.0188 |
0.0002 |
1.2 |
5 |
Oil cooler installed |
0.0205 |
0.0017 |
10.2 |
6 |
Canopy fairing removed |
0.0203 |
-0.0002 |
-1.2 |
7 |
Carburetor air scoop added |
0.0209 |
0.0006 |
3.6 |
8 |
Sanded walkway added |
0.0216 |
0.0007 |
4.2 |
9 |
Ejector chute added |
0.0219 |
0.0003 |
1.8 |
10 |
Exhaust stacks added |
0.0225 |
0.0006 |
3.6 |
11 |
Intercooler added |
0.0236 |
0.0011 |
6.6 |
12 |
Cowling exit opened |
0.0247 |
0.0011 |
6.6 |
13 |
Accessory exit opened |
0.0252 |
0.0005 |
3.0 |
14 |
Cowling fairing and seals removed |
0.0261 |
0.0009 |
5.4 |
15 |
Cockpit ventilator opened |
0.0262 |
0.0001 |
0.6 |
16 |
Cowling venturi installed |
0.0264 |
0.0002 |
1.2 |
17 |
Blast tubes added |
0.0267 |
0.0003 |
1.8 |
18 |
Antenna installed |
0.0275 |
0.0008 |
4.8 |
|
Total |
0.0109 |
|
[112] ...compared with 0.0275 for the aircraft in the service condition. In order to convert the clean configuration into a useful practical aircraft, the drag was increased by about 65 percent of the value obtained for the clean aircraft. All the additional drag, however, was found to be unnecessary. Further tests and analyses showed that the additional drag could be reduced by more than one-half through careful tailoring of various aspects of the design. The drag coefficient of a practical service aircraft of the XP-41 type was accordingly reduced from 0.0275 to 0.0226. The data in figure 5.5 indicate that the increments in drag coefficient corresponding to the 18 steps of the cleanup process are generally rather small and, in many cases, only a few percent of the total drag coefficient. Yet, taken all together, these increments add up to an impressive total. Important performance improvements resulted from the drag cleanup of the 23 military aircraft in the Langley full-scale tunnel. In many cases, the gains associated with care and attention to detailed design were found to be greater than the differences in drag between airplanes of different configurations. The drag cleanup work made an important contribution to the refinement of high-performance propeller-driven aircraft during World War II, and the gains resulting from the program often spelled the difference in performance between victory and defeat in the air.