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.
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
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.