AIRPLANE TOTAL DRAG

TV. ^ tofi 1 on 1П fli rrV. t’ 1 c t’h я»

V_/A till **ll.^luus. ill 111^114. 1.11V

sum of the induced and parasite drag. Figure 1.35 illustrates the variation of total drag with speed for a given airplane in level flight at a particular weight, configuration, and alti­tude. The parasite drag increases with speed varying as the square of the velocity while the induced drag decreases with speed varying in­versely as the square of the velocity. The total drag of the airplane shows the predomi­nance of induced drag at low speed and parasite drag at high speed. Specific points of interest on the drag curve are as follows;

(A) Stall of this particular airplane occurs at 100 knots and is indicated by a sharp rise in the actual drag. Since the generalized equa­tions for induced and parasite do not account for conditions at stall, the actual drag of the airplane is depicted by the “hook” of the dotted line.

(B) At a speed of 124 knots, the airplane would incur a minimum rate of descent in power-off flight. Note that at this speed the induced drag comprises 75 percent of the total drag. If this airplane were powered with a reciprocating-propeller type powerplant, maxi­mum endurance would occur at this airspeed.

Figure 1.35. Typical Airplane Drag Curves

(C) The point of minimum total drag occurs at a speed of 163 knots. Since this speed in­curs the least total drag for lift-equal-weight flight, the airplane is operating at (L/D)mei. Because of the particular manner in which parasite and induced drags vary with speed (parasite drag directly as the speed squared; induced drag inversely as the speed squared) the minimum total drag occurs when the in­duced and parasite drags are equal. The speed for minimum drag is an important reference for many items of airplane performance. One item previously presented related glide per­formance and lift-drag ratio. At the speed of 163 knots this airplane incurs a total drag of 778 lbs. while producing 12,000 lbs. of lift. These figures indicate a maximum lift-drag ratio of 15.4’and relate a glide ratio of 15-4. In addition, if this airplane were jet powered, the airplane would achieve maximum en­durance at this airspeed for the specified alti­tude. If this airplane were propeller powered, the airplane would achieve maximum range at this airspeed for the specified altitude.

(D) Point (D) is at an airspeed approxi­mately 32 percent greater than the speed for (LjD’)maI. Note that the parasite drag com­prises 75 percent of the total drag at a speed of 215 knots. This point on the drag curve pro­duces the highest proportion between velocity and drag and would be the point for maximum range if the airplane were jet powered. Be­cause of the high proportion of parasite drag at this point the long range jet airplane has great preference for great aerodynamic clean­ness and less demand for a high aspect ratio than the long range propeller powered airplane.

(E) At a speed of 400 knots, the induced drag is an extremely small part of the total drag and parasite drag predominates.

(F) As the airplane reaches very high flight speeds, the drag rises in a very rapid fashion due to compressibility. Since the generalized equation for parasite drag does not account for compressibility effects, the actual drag rise is typified by the dashed line.

The airplane drag curve shown in figure 1.34 is particular to one weight, configuration, and altitude in level flight. Any change in one of these variables will affect the specific drags at specific velocities.

The airplane drag curve is a major factor in many items of airplane performance. Range, endurance, climb, maneuver, landing, takeoff, etc., performance are based on some relation­ship involving the airplane drag curve.