Terrain-following flight in degraded visibility
As a pilot approaches rising ground, the point at which the climb is initiated depends on the forward speed and also the dynamic characteristics of the aircraft, reflected in the vertical performance capability and the time constant in response to collective pitch inputs; at speeds below minimum power, height control is exercised almost solely through collective inputs. A matched manoeuvre could be postulated as one
where the pilot applies the required amount of collective at the last possible moment so that the climb rate reaches steady state, with the aircraft flying parallel to the surface of the hill. For low-speed flight, vertical manoeuvres can be approximately described by a first-order differential equation (see Chapter 5, eqn 5.52), with its solution to a step input in the pilot’s collective lever given by
w – Zww = Z9o0o w = wss(1 – eZwt)
In the usual notation, w is the aircraft normal velocity (positive down), wss the steady state value of w and 9o is the collective pitch angle. Zw is the heave damping derivative (see Chapter 4), or the negative inverse of the aircraft time constant in the heave axis, tw. Writing Sw = w, the time to achieve Sw can be written in the form
^ = – loge(1 – Sw) (8.33)
When Sw = 0.63, t$w = tw, the heave time constant. To reach 90% of the final steady state would take 2.3 time constants and to reach 99% would take nearly 5 time constants. In ADS-33, Level 1 handling qualities are achieved if tw < 5 s, and for many aircraft types, values of 3-4 s are typical. In reality, because of the exponential nature of the response, the aircraft never reaches the steady-state climb following a step input. In fact, following a step collective, the aircraft approaches its steady-state in a particular manner. The instantaneous time to reach steady-state rate of climb – w, Tw, varies with time and is defined as the ratio of the instantaneous differential (negative) velocity to the acceleration; hence
w = – wss ZweZwt
w – wss 1 ^ (8.34)
Tw = ; = = tw
The instantaneous time to reach steady state, Tw, is therefore a constant and equal to the negative of the time constant of the aircraft tw – a somewhat novel interpretation of the heave time constant. The step input requires no compensatory workload but, theoretically, the aircraft never reaches its destination. To achieve the steady-state goal, the pilot needs to adopt a more complex control strategy and will use the available visual cues to ensure that т w reaches zero when the aircraft has reached the appropriate climb rate; the complexity of this strategy determines the pilot workload.
In the simulation trial reported in Ref. 8.33, the pilot was launched in a low hover and requested to accelerate forward and climb to a level flight trim condition that he or she considered suited the environment. To ensure that all the visual information for stabilization and guidance was derived from the outside world, head-down instruments were turned off. After establishing the trim condition, the pilot was required to negotiate a hill with 5° slope rising 60 m above the terrain. The terrain was textured with a rich, relatively unstructured surface, and to explore the effects of degraded visual conditions fog was located at distances of 80, 240,480 and 720 m ahead of the aircraft. The fog was simulated as a shell of abrupt obscuration surrounding a sphere of ‘clear air’ centred on the pilot.
The visual cue ratings (VCRs) and associated UCEs and handling qualities ratings (HQRs) for the different cases are given in Table 8.1. The methodology adopted was an adaptation of that in ADS-33, where the UCE is derived from VCRs given by three pilots flying a set of low-speed manoeuvres. The concept of UCE >3 is also an adaptation to reflect visual conditions where the pilot was not prepared to award a VCR within the defined scale (1-5). The VCRs are also plotted on the UCE chart in Fig. 8.24. As expected, the increased workload in the DVE led the pilot to award poorer HQRs, and the UCE degraded from 1 to 3. For the HQRs, the adequate performance boundary was set at 50% of nominal height and the desired boundary at 25% of nominal height. No numerical constraints were placed on speed but the pilot was requested to maintain a reasonably constant speed.
Key questions addressed in Ref. 8.33 were ‘would the pilot elect to fly at different heights and speeds in the different conditions,’ and ‘how would these relate to the body-scaled measure, the eye-height?’ Would the pilot use intrinsic тguides to successfully transition into the climb, and what form would these take? Could the pilot control strategy be modelled based on т following principles? Earlier in this chapter we discussed a pilot’s ability to pick up visual information from the surface over which
Table 8.2 Average flight parameters for the terrain-hugging manoeuvres
the aircraft was flying and drew attention to Ref. 8.15, where Perrone had hypothesized that the looming of patterns on a rough surface would become detectable at about 16 eye-heights (xe) ahead of the aircraft. In the various simulation exercises conducted at Liverpool there is some evidence that this reduces to about 12xe for the textured surfaces used; hence, we reference results to this metric in the following discussion.
The average distances and times to the fog-lines, along with the velocity and time to the 12 eye-height point ahead of the aircraft, are given in Table 8.2. The results indicate that as the distance to the fog-line reduces the pilot flies lower and slower, while maintaining eye-height speed relatively constant. Comparing the 720-m fog-line case with the 240-m case, the average eye-height velocity is almost identical, while the actual speed and height has almost doubled. For the UCE = 3 case, the aircraft has slowed to below 2 xe per second, as the distance to the fog-line has reduced to within 20% of the 12 eye-height point. It is worth noting that the test pilot, who has extensive military and civil piloting experience, declared that the UCE = 3 case would not be acceptable unless urgent operational requirements prevailed; it simply would not be safe in an undulating, cluttered environment, and where the navigational demands would strongly interfere with guidance.
Figure 8.25 shows the vertical flight path (height in metres) and flight velocity (in metres/second and xe per second) plotted against range (metres) for the different fog cases. The pilots were requested to fly along the top of the hill for a further 2000 m to complete the run.
The distances along the flight path to the terrain surface, as the sloping ground is approached, are shown in Fig. 8.26. While the actual distances vary significantly, the distances in eye-heights to the surface reduce to between 12 and 16 eye-heights, as the hill is approached and during the initial climb phase. The times to contact the terrain, tsurface, are shown in Fig. 8.27. Typically, the pilot allowed rsulface to reduce to between 6 and 8 s before initiating the climb. These results are consistent with those derived in the acceleration-deceleration manoeuvre. In the UCE = 3 case (fog at 80 m) the pilot is flying at 10 m/s, giving about 8 s look-ahead time to the fog-line and only 2-3 s margin from the 12 xe point. The results suggest a relationship between the UCE and the margin between the postulated, 12 xe, look-ahead point and any obscuration; this point will be revisited towards the end of this section, but prior to this the variation of flight path angle during the climb will be analyzed to investigate the degree of t guide following during the manoeuvre.