Aerodynamic Efficiency

Hoerner defined the aerodynamic efficiency as the following ratio:

П aero= aerodynamic efficiency = -^йіі (9.2)

where the useful drag is the part that is either useful (as in the induced drag) or necessary (or unavoidable). For the Bf-109G, the result is naero = 0.4, or 40 per­cent. This indicates that “greater than half the total drag of this airplane could theoretically be avoided by extremely clean design and faultless construction of the skin and details.” However, the amount of labor and time (and, therefore, expense) to accomplish this would be significant. What Hoerner described is typ­ical of the concentration on drag reduction used in the design and construction of world-class, high-performance sailplanes. Such efforts result in high costs due to the required labor-intensive procedures. For example, a competitive racing-class (i. e., 15 meter span with camber-changing flaps) sailplane can cost more than $150,000.

Hoerner demonstrated that if the Bf-109 were to be redesigned such that the ratio of “useful” to total drag were to be increased to unity, the top speed would increase from 610 to 800 km/hr! The benefit of such a speed increase in terms of the mission of this fighter is obvious. However, we always must ask the question: How much does it cost to reduce the drag? Clearly, in the case of the Bf-109G, the

answer was apparently “too much.” The aircraft performance was increased over its service life mainly by increases in engine horsepower, although minor improve­ments resulted from drag reduction. For example, the tail struts, squared wing tips, and large undercowl radiator found in earlier versions of the Messerschmitt 109 were modified to lower the drag to the levels demonstrated herein for the G version.