CFI Topics

The Components of Weight, Pt. 2

Lesson 5

One can easily plot the L/D ratio versus the value of CL, but it is more informative to plot the L/D as a function of the angle-of attack. The value of CL of the aircraft is also given as a function of angle-of-attack, so that one can easily plot the L/D versus angle-of-attack. Typical plots of CL versus angle-of-attack and L/D and CD versus angle-of-attack are shown in Figure 4. This figure corresponds to a symmetrical airfoil, since the lift is zero at a zero angle-of-attack.

It is clear that the maximum L/D occurs at a fixed angle-of-attack and is independent of both the aircraft weight and the altitude. In addition, the flight path angle is also independent of the aircraft weight and the altitude.

In the case of a C-172, the flight path angle was shown to be 6.3 degrees below the horizontal. The maximum L/D for a C-172 occurs somewhere between 6 and 9 degrees angle-of-attack. Using the expression in equation (1), the pitch angle would be somewhere between 0 and 3 degrees above the horizon. Thus, basic aerodynamics tells us that the same pitch attitude should be flown, independent of the weight of the aircraft or the altitude. The exact pitch attitude can be determined either by a plot similar to Figure 4 for a C-172, or one can load the aircraft to gross weight, reduce the power to idle and trim the aircraft to 65KIAS. The pitch attitude for the best glide speed should be noted and that pitch attitude should be used for simulated emergencies, no matter what the weight of the aircraft.

Note that in the case of an actual engine failure when the propeller is windmilling, the L/D of the aircraft will be slightly lower than that for the case of the engine at idle. In addition, the lower L/D will occur at a slightly higher angle-of-attack (since the parasite drag coefficient is somewhat higher). Thus, the pitch attitude will be slightly different with the propeller windmilling, as compared to the propeller idling case. Therefore, the standard 5-step briefing on emergency glide procedures will now have a sixth step which will state that the best glide speed for any weight or altitude is attained at a specific pitch attitude.

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