Miscellaneous Parasite Drags

In addition to excrescence drag, there are other drag increments such as ECS drag (e. g., air-conditioning), which is drag at a fixed value depending on the number of passengers); and aerials and trim drag, which are included to obtain the minimum parasite drag of the aircraft.

Air-Conditioning Drag

Air-conditioning air is inhaled from the atmosphere through flush intakes that incur drag. It is mixed with hot air bled from a midstage of the engine compressor and then purified. Loss of thrust due to engine bleed is accounted for in the engine- thrust computation, but the higher pressure of the expunged cabin air causes a small amount of thrust. Table 9.4 shows the air-conditioning drag based on the number of passengers (interpolation is used for the between sizes).

Trim Drag

Due to weight changes during cruise, the CG could shift, thereby requiring the air­craft to be trimmed in order to relieve the control forces. Change in the trim-surface angle causes a drag increment. The average trim drag during cruise is approximated

Wing reference area – ft2	Trim drag – f ft2	Wing reference area – ft2	Trim drag – f ft2
200	0.12	2,000	0.3
500	0.15	3,000	0.5
1,000	0.20	4,000	0.8

Table 9.5. Trim drag (approximate)

as shown in Table 9.5, based on the wing reference area (interpolation is used for the between sizes).

Aerials

Navigational and communication systems require aerials that extend from an air­craft body, generating parasite drag on the order 0.06 to 0.1 ft2, depending on the size and number of aerials installed. For midsized transport aircraft, 0.075 ft2 is typ­ically used. Therefore:

9.9 Notes on Excrescence Drag Resulting from Surface Imperfections

This section may be omitted because there is no coursework exercise involved. Semi-empirical relations discussed in Sections 9.8.4 and 9.8.5 are sufficient for the purpose. Excrescence drag due to surface imperfections is difficult to estimate; therefore, this section provides background on the nature of the difficulty encoun­tered. Capturing all the excrescence effects over the full aircraft in CFD is yet to be accomplished with guaranteed accuracy.

A major difficulty arises in assessing the drag of small items attached to the air­craft surface, such as instruments (e. g., pitot and vanes), ducts (e. g., cooling), and necessary gaps to accommodate moving surfaces. In addition, there is the unavoid­able discrete surface roughness from mismatches and imperfections – aerodynamic defects – resulting from limitations in the manufacturing processes. Together, all of these drags, from both manufacturing and nonmanufacturing origins, are collec­tively termed excrescence drag, which is parasitic in nature. Of particular interest is the excrescence drag resulting from the discrete roughness, within the manufactur­ing tolerance allocation, in compliance with the surface-smoothness requirements specified by aerodynamicists to minimize drag.

Mismatches at the assembly joints are seen as discrete roughness (i. e., aerody­namic defects) – for example, steps, gaps, fastener flushness, and contour deviation – placed normal, parallel, or at any angle to the free-stream air flow. These defects generate excrescence drag. In consultation with production engineers, aerodynami – cists specify tolerances to minimize the excrescence drag – on the order of 1 to 3%

of the CDpmin.

The “defects” are neither at the maximum limits throughout nor uniformly dis­tributed. The excrescence dimension is on the order of less than 0.1 inch; for com­parison, the physical dimension of a fuselage is nearly 5,000 to 10,000 times larger. It poses a special problem for estimating excrescence drag; that is, capturing the resulting complex problem in the boundary layer downstream of the mismatch.

The methodology involves first computing excrescence drag on a 2D flat surface without any pressure gradient. On a 3D curved surface with a pressure gradient, the excrescence drag is magnified. The location of a joint of a subassembly on the 3D body is important for determining the magnification factor that will be applied on the 2D flat-plate excrescence drag obtained by semi-empirical methods. The body is divided into two zones (see Figure 16.5): Zone 1 (the front side) is in an adverse pressure gradient, and Zone 2 is in a favorable pressure gradient. Excrescences in Zone 1 are more critical to magnification than in Zone 2. At a LRC flight speed (i. e., below McrU for civil aircraft), shocks are local, and subassembly joints should not be placed in this area (Zone 1).

Estimation of aircraft drag uses an average skin-friction coefficient CF (see Fig­ure 9.19b), whereas excrescence-drag estimation uses the local skin-friction coef­ficient Cf (see Figure 9.19a), appropriate to the location of the mismatch. These fundamental differences in drag estimation methods make the estimation of aircraft drag and excrescence drag quite different.

After World War II, efforts continued for the next two decades – especially at the RAE by Gaudet, Winters, Johnson, Pallister, and Tillman et al. – using wind – tunnel tests to understand and estimate excrescence drag. Their experiments led to semi-empirical methods subsquently compiled by ESDU as the most authoritative information on the subject. Aircraft and excrescence drag estimation methods still remain state of the art, and efforts to understand the drag phenomena continue.

Surface imperfections inside the nacelle – that is, at the inlet diffuser surface and at the exhaust nozzle – could affect engine performance as loss of thrust. Care must be taken so that the “defects” do not perturb the engine flow field. The internal nacelle drag is accounted for as an engine-installation effect.