Extension to Other Flight Conditions: Drag Polar

Because the detailed analysis refers to only one flight condition—namely, high­speed level flight—it is useful to find a way to extend the collected data to estimate the airplane performance at other speeds. For the moment, we assume the following:

1. The parasite-drag coefficient stays effectively constant over a wide range of speeds.

2. The compressibility effect can be neglected.

3. The flaps, slats, cowl flaps, and landing gear remain undeployed.

4. There is no wind or atmospheric turbulence.

Clearly, these assumptions result in an approximate analysis because compressibility was shown to account for a substantial drag rise at the top-speed condition. Also, para­site drag is likely to depend on the angle of attack of the airplane, which must increase

as the speed decreases so that the necessary lift is produced. However, a reasonable estimate of the Bf-109G performance over its operational speed range can be made in the manner suggested. Using the wing area and other information already assem­bled, we can summarize the aero-dynamic behavior of this example airplane in the following manner, recalling that we separated the total drag into two categories: drag due to lift (induced drag) and parasite drag. Then, if we assume minor influence of speed and aircraft attitude on the parasite drag, the value for the Bf-109G is:

The induced-drag coefficient:

C. = CL Dl neAR

depends on flight speed because the lift coefficient must be adjusted to provide the necessary lift as dynamic pressure changes. The total drag is expressed by:

c2

C =C + C = C + Cl

CD CDo + CDi CDo + neAR,

which often is referred to as the drag polar for the airplane. If it is assumed that the airplane is in level, equilibrium (i. e., unaccelerated) flight with the drag exactly bal­anced by the thrust produced by the propulsion system and the weight balanced by the lift, then we can write:

T = D = qSCL (9.4)

[W = L = qSC} (9.5)

where q = 1pV2 and S is the projected wing area, including the part enclosed by the fuselage. Equations 9.3-9.5 now can be evaluated using the Bf-109G data to deter­mine useful information about its performance.