Empennage (Tail Surfaces) Drag Area

The horizontal (i. e., outside the fuselage) and vertical tail-surface areas are 25 and 11 ft2, respectively. Again assuming the rough camouflage paint, the skin – friction drag coefficient is estimated to be Cf = 0.004. Correcting for the thick­ness effect, the gaps at the control-surface hinge lines, and the interference drag at the junctions between the fuselage and the tail surfaces, the total parasite drag area for the empennage is:

(D/q)empennage = 0.360 ft2.

parasite

Because the horizontal tail produces lift at some trim conditions (usually nega­tive lift in high-speed flight), a correction for induced drag is necessary. It is small because the lift coefficient needed to accomplish the required downward trim force is small. An estimate is:

(D/q)empennage = 0.010 ft2.

induced

Therefore, the total empennage-drag area is approximately 0.370 ft2.

Summary of BF-109G Drag Calculation; Comparison to Measured Drag

If the various estimated drag-area terms are summed, the total contributions to the overall airplane drag are as fo. llows:

overall parasite – drag area = (D/ q)parasite = 5.277 overall induced – drag area = (D / q)induced = 0.430

Total airplane – drag area = (D /g)total = 5.707

The results are shown graphically in Figure 9.3. The total estimated drag area based on the assumption of incompressible flow is somewhat lower than the value of 6.2 deduced from actual level-flight performance data. Why is there a discrepancy? It appears that every possible drag contribution was taken into account. Hoerner explained this in terms of compressibility effects. Notice that at 22,000 ft., the speed of sound is 295 m/s (try using the standard atmosphere program to verify this), so that the flight Mach number under these conditions is about:

MM = 0.58,

which indicates that conditions clearly are pushing the upper bound of the subsonic approximation. Hoerner estimated by means of the Prandtl-Glauert compress­ibility correction (Liepmann and Roshko, 2001) that there should be about a 10 per­cent drag increase because of compressibility, which accounts for the discrepancy between the calculated and measured drag results. The results show that the aerody­namic performance for a complete airplane with all of its imperfections realistically can be predicted if care is taken with all of the details. A student who wants to learn this process should examine Hoerner’s book even though it may seem dated; it is

difficult to find a better summary of the practical approach to drag estimation. If this is supplemented with modern information covering the high-speed (i. e., high – subsonic, transonic, supersonic, and hypersonic) flight regimes, then a powerful performance-prediction tool can be devised. Such tools are an important design resource in the aerospace industry. Each company and government laboratory has preferred methods for carrying out analyses such as we reviewed herein.