DRAG CLEANUP

Laminar flow control has the potential for achieving significant drag reductions. However, it has yet to be proven on an operational aircraft. Even without LFC, the parasite drag of many of today’s aircraft could be significantly reduced by cleaning up many small drag items that are negligible individually but are appreciable collectively.

Figure 4.51, based on full-scale wind tunnel tests, illustrates how the drag of an aircraft can deteriorate as items are £dded to the airframe. In Figure 4.51a, the airplane is shown in the faired and sealed condition. Then, as the

(b) Figure 4.51 Drag penalties for an airplane, (a) Airplane in faired and sealed condition, (b) Airplane in service condition (numbers indicate drag increments in percent of total drag of clean airplane).

Table 4.6 Drag Items as Shown in Figure 4.51 (Ref.

4.14)______________________________________

Power plant installation

Open cowling inlet and exit 18.6%

Unfaired carburetor air scoop 3.6%

Accessory cooling airflow 3.0%

Exhaust stacks and holes 3.6%

Intercooler 6.6%

Oil cooler, 10.2%

4 Total 45.6%

Other items for service condition Remove’ seals from cowl flaps 5.4%

Opening case and link ejector 1.8%

Opening seals around landing gear doors 1.2%

Sanded walkway 4.2%

Radio aerials 4.8%

Guns and blast tubes 1.8%

Total 19.2%

items tabulated in Table 4.6 were added, drag increments were measured. These are expressed as a percentage of the original clean airplane drag.

Table 4.6 shows that the drag of the original clean airplane is increased by nearly 65% by the total effect of these drag items. Some of this additional drag is, of course, necessary, but more than half of it is not. Additional tests and analysis of this particular airplane showed that the drag of the power plant items could be reduced to 26.6% of the initial drag.

The moral of the foregoing and other material contained in this chapter is that, with regard to drag, attention should be paid to detail. Surfaces should be smooth and protuberances streamlined or avoided if possible. Tight seals should be provided around wheel wells, door openings, and other cutouts. It is exactly this attention to detail (or lack olit) that explains the wide disparity in the CF values tabulated in Table 4.2 forairplanes of the same class.

Possibly the ultimate in aerodynamic cleanliness is represented by the latest generation of sailplanes. Employing molded fiberglass or other types of plastics, ultrasmooth surfaces are achieved. Using very high aspect ratios, ranging from 10 to 36, and laminar flow airfoils, mainly of the Wortmann design (Ref. 4.20), lift-to-drag ratios as high as 40 have been accomplished.