Flight Path Stability

Another criterion relating to longitudinal motion discussed in Reference

9.1 is that of flight path stability. This refers to the flight-path-angle change effected by elevator control at a constant power setting. For the landing approach flight phase, this angle as a function of true airspeed should have a slope at the minimum operational speed, which is negative or less positive than:

Level 1: 0.06°/kt (9.99a)

Level 2: 0.15°/kt (9.99b)

Level 3: 0.24°/kt (9.99c)

In effect, these are tantamount to stating that one should not design an airplane to operate too far into the backside of the power-required curve. In this region, the flight-path-angle (positive for climb) will increase as the speed is increased for a constant power setting.

Short-Period Mode

Both the short-period frequency and damping ratio are important to achieving satisfactory flying qualities. When the damping is too low, the short-period response can produce an annoying oscillation. When the damping

Table 9.1 Short-Period Damping Ratio Limits Categories A and C Category В Level Minimum Maximum Minimum Maximum 1 0.35 1.30 0.3 2.00 2 0.25 2.00 0.2 2.00 3 0.15 — 0.15 —

is too high, the response to control input can be sluggish. Therefore, upper and lower limits to short-period damping are recommended in Reference 9.1 in order to achieve a given Cooper-Harper level. These limits are given in Table 9.1 according to the following flight categories.

Category A These are nonterminal flight phases that require rapid maneuver­ing, precision tracking, or precise flight path control such as air-to-air combat, in-flight refueling (receiver), terrain-following, and close for­mation flying.

Category В These are nonterminal flight phases that are normally accom­plished using gradual maneuvers and no precision tracking, although accurate flight path control may be required. This category includes climb, cruise, and descent, in-flight refueling (tanker), and aerial delivery.

Category C These are terminal flight phases requiring gradual maneuvers but precise flight path control. These include takeoff, catapult takeoff, ap­proach, wave-off/go-around, and landing.

Specifying the damping ratios alone, as in Table 9.1, is not necessarily sufficient to assure adequate flying qualities. As mentioned earlier, the short – period frequency is also important. Indeed, it appears as if the values of frequency and damping ratio for satisfying flying qualities are interdependent.

Reference 9.1 and other sources present a number of graphs similar to Figure 9.11. These are sometimes referred to as target plots. Since the establishment of these boundaries is subjective in nature, the contours defining the Cooper-Harper levels should not be taken as hard and fast. However, an airplane that falls to the left of level 3 in Figure 9.11 can be dangerous to fly.

For the Cherokee 180, the roots of the characteristic equation for the short-period mode were calculated previously. In real time these were

<T i2 = — 2.43 ± 3.54І

Figure 9.11 Short-period handling qualities criteria.

Thus, from Equation 9.97, for this mode

£ = 0.566 шп – 4.29 rad/sec = 0.68 Hz

These values are seen to lie well within the level 1 region of Figure 9.11.