Category HELICOPTER FLIGHT DYNAMICS

Flight in DVE

With fixed-wing aircraft, pilots can be flying under either visual or instrument flight rules (VFR or IFR), corresponding to defined levels of outside visual cues or

meteorological conditions (VMC or IMC). If aircraft have to operate in IMC then typically there will be two crew, one flying while the other keeps an eye open for any hazards appearing in the visual scene. Except for the important case of military fixed – wing aircraft flying low level to avoid radar and other detection systems, nearly all fixed-wing IMC flying is conducted at altitude, well away from obstacles, and means little more than flying on instruments while in cloud or at night. Key instruments that the pilot would scan include the attitude indicator, heading gyro, airspeed indicator, ball and slip and rate of climb/descent indicator. A guided approach to a landing site would, in addition, require the pilot to follow a flight path as directed by special guid­ance instruments until the aircraft emerges into VMC below cloud to carry out a normal touchdown. Airports are equipped with various levels of guidance facilities enabling up to fully automatic landings in poor visibility or IMC. With fixed-wing aircraft, the characterization of visibility conditions is therefore fairly simple and the associated operational decision making, e. g., whether to initiate a sortie, can be based on rela­tively simple criteria, e. g., how much of the airfield can be seen. Rotary-wing aircraft operations have also been constrained by the same physical conditions but the ability to operate at low speed, combined with the military need to operate at very low level to avoid detection, has led to the development of a considerably more detailed and structured approach to the characterization of outside visual cues (OVCs). The general term adopted in the rotary-wing technical community for characterizing poor visibility conditions is the DVE – degraded visual environment, and this section examines some of the special considerations that accompany helicopter operations and flying qualities in the DVE.

Conferring operational agility on future rotary-wing aircraft, emulating configuration C above in Fig. 7.20, requires significant improvements in handling, but research into criteria at high-performance levels are needed to lead the way. A natural agility parameter has developed as one of the ADS-33 innovations – the response quickness. We have already discussed the properties of this parameter in Chapter 6 but it is useful to take a closer look at the effect of this and other handling parameters on the equi- response charts shown in Fig. 7.21. For a simple illustration we refer back to the CSM

model structure for roll rate command response type

K e-Ts

0 (— + 0

J^a )

If we interpret the frequency axis as roll response quickness as shown in Fig. 7.21, the effect of independent variation of the different parameters in eqn 7.4 can be illustrated as in Fig. 7.22. The sensitivity of agility factor with the parameters of the CSM is relatively easy to establish. For example, if we consider a bank and stop MTE (Fig. 7.23), some useful insight can be gained. A pulse-type control input will be assumed, although, in practice, pilots would adopt a more complex strategy to increase the agility factor. To illustrate the primary effect, we consider the case where the ‘secondary’ time delays are set to zero (i. e., т = 0, rna = те). For a roll angle change of Аф, the ideal

the sensitivity of Af to both bandwidth and quickness. The normalized bandwidth is a useful parameter as it represents the ratio of aircraft bandwidth to control input band­width, albeit rather approximately. For short, sharp control inputs, typical in tracking corrections, high aircraft bandwidths are required to achieve reasonable agility factors. For example, at the ADS-33C minimum required roll attitude bandwidth of 3.5 rad/s and with 1-s pulses, the pilot can expect to achieve agility factors of 0.5 using simple control strategies in the bank and stop manoeuvre. To achieve the same agility factor with a 0.5-s pulse would require double the bandwidth. This is entirely consistent with the argument that the ADS-33C boundaries are set for low to moderate levels of at­tack. If values of agility factor up to 0.75 are to be achieved, it is suggested as in Fig. 7.24, that bandwidths up to 8 rad/s will be required. Whether the 30% reduction in task time is worth the additional effort and cost to develop the higher bandwidth can be judged only in an overall operational context. Such high values of roll bandwidth may be achievable in very high performance fixed-wing aircraft and Fig. 7.24 serves to illustrate and underline the different operational requirements of the two vehicle classes, and also, to a large extent, the different expectations of the operators.

This simple example has many questionable assumptions, but the underlying point that increasing the key flying qualities parameters above the Level 1/2 boundary has a first-order effect on task performance still holds. But it provides no clues to possible upper performance boundaries set by flying qualities considerations. Existing require­ments do not address upper limits directly, and more research with high-performance variable stability helicopters is required to address this issue. Intuitively, we might ex­pect upper limits to be related to the acceleration capability of the aircraft (Ref. 7.25). This is largely the case with fixed-wing aircraft but there are also tentative upper limits on pitch attitude bandwidth (see Figs 6.43, 6.44). However, it is suspected that these are actually a reflection of the high control sensitivity required to achieve a defined level of control power, rather than the high values of bandwidth per se. Upper limits on control sensitivity are typically set to reduce the jerkiness or abruptness for small amplitude precision control, but the numerical values depend very much on the incep – tor characteristics. Regarding the moderate and large amplitude motions, the best we can say at the moment is that the parameters on the quickness-amplitude charts are likely to have upper bounds beyond which agility would deteriorate.

Agility is a special flying quality catering for extreme operational requirements and a key technology driver for military functions. At the other end of the spectrum we find another, equally demanding, requirement for flight in very poor visibility. Here the pilot is not so much interested in agility as increased stabilization and the enhancement of his visual cues for the guidance task. Flight in degraded visual conditions exemplifies the tension and contrast between stability and agility requirements and is pressing hard on cockpit-related technologies that support pilotage; it is also the next topic of investigation.

One of the most common causes of dispersion in pilot HQRs stems from poor or im­precise definition of the performance requirements in an MTE, leading to variations in interpretation and hence perception of achieved task performance and associated workload. We have already illustrated this with the controlled experiment data from the AFS slalom and sidestep MTEs. In operational situations, this translates into the variability and uncertainty of task drivers, commonly expressed in terms of precision, but the temporal demands are equally important. The effects of task time constraints on perceived handling have been well documented (Refs 7.20-7.24) and represent one of the most important external factors that impact pilot workload. Flight results gathered on Puma and Lynx test aircraft at DRA (Refs 7.20, 7.23, 7.24) showed that a critical parameter was the ratio of the task performance achieved to the maximum available from the aircraft; this ratio gives an indirect measure of the spare capacity or performance margin and was consequently named the agility factor. The notion developed that if a pilot could use the full performance safely, while achieving desired task precision requirements, then the aircraft could be described as agile. If not, then no matter how much performance margin was built into the helicopter, it could not be described as agile. The DRA agility trials were conducted with Lynx and Puma operating at light weights to simulate the higher levels of performance margin ex­pected to be readily available, even at mission gross weights, in future types (e. g., up to 20-30% hover thrust margin). A convenient method of computing the agility factor was developed as the ratio of ideal task time to actual task time. The task was deemed to commence at the first pilot control input and to complete when the aircraft motion decayed to within prescribed limits (e. g., position within a prescribed cube, rates <5°/s) for repositioning tasks, or when the accuracy/time requirements were met for tracking or pursuit tasks. The ideal task time is calculated by assuming that the maximum acceleration is achieved instantaneously, in much the same way that some aircraft models work in combat games. So, for example, in a sidestep repositioning manoeuvre, the ideal task time is derived with the assumption that the maximum transla­tional acceleration (hence aircraft roll angle) is achieved instantaneously and sustained for half the manoeuvre, when it is reversed and sustained until the velocity is again zero.

The ideal task time is then simply given by

Ti = J(4S/amax) (7.3)

where S is the sidestep length and amax is the maximum translational acceleration. With a 15% hover thrust margin, the corresponding maximum bank angle is about 30°, with amax equal to 0.58 g. For a 100-ft sidestep, T then equals 4.6 s. Factors that increase the achieved task time, beyond the ideal, include

(1) delays in achieving the maximum acceleration (e. g., due to low roll attitude bandwidth/control power);

(2) pilot reluctance to use the maximum performance (e. g., no carefree handling capability, fear of hitting ground);

(3) inability to sustain the maximum acceleration due to drag effects and sideways velocity limits;

(4) pilot errors of judgement leading to terminal repositioning problems (e. g., caused by poor task cues, strong cross-coupling).

To establish the kinds of agility factors that could be achieved in flight test, pilots were required to fly the Lynx and Puma with various levels of aggressiveness or ma­noeuvre ‘attack’, defined by the maximum attitude angles used and rate of control application. For the low speed, repositioning sidestep and quickhop MTEs, data were gathered at roll and pitch angles of 10°, 20° and 30° corresponding to low, mod­erate and high levels of attack, respectively. Figure 7.19 illustrates the variation of HQRs with agility factor for the two aircraft (Ref. 7.24). The higher agility factors achieved with Lynx are principally attributed to the hingeless rotor system and faster engine/governor response. Even so, maximum values of only 0.6-0.7 were recorded compared with 0.5-0.6 for the Puma. For both aircraft, the highest agility factors were achieved at marginal Level 2/3 handling. In these conditions, the pilot is either working with little or no spare capacity, or not able to achieve the flight path pre­cision requirements. According to Fig. 7.19, the situation rapidly deteriorates from Level 1 to Level 3 as the pilot attempts to exploit the full performance, emphasizing the ‘cliff edge’ nature of the effects of handling deficiencies. The Lynx and Puma are typical of current operational types with low authority stability and control aug­mentation. While they may be adequate for their current roles, flying qualities defi­ciencies emerge when simulating the higher performance required in future combat helicopters.

The different possibilities are illustrated in Fig. 7.20. All three configurations are assumed to have the same performance margin and hence ideal task time. Configuration

A can achieve the task performance requirements at high agility factors but only at the expense of maximum pilot effort (poor Level 2 HQRs); the aircraft cannot be described as agile. Configuration B cannot achieve the task performance when the pilot increases his or her attack and Level 3 ratings are returned; in addition, the attempts to improve task performance by increasing manoeuvre attack have led to a decrease in agility factor, hence a waste of performance. This situation can arise when an aircraft is PIO prone, is difficult to re-trim or when control or airframe limits are easily exceeded in the transient response. Configuration B is certainly not agile and the proverb ‘more haste, less speed’ sums the situation up. With configuration C, the pilot is able to exploit the full performance at low workload. The pilot has spare capacity for situation awareness and being prepared for the unexpected. Configuration C can be described as truly agile. The inclusion of such attributes as safeness and poise within the concept of agility emphasizes its nature as a flying quality and suggests a correspondence with the quality levels. These conceptual findings are significant because the flying qualities boundaries, which separate different quality levels, now become boundaries of available agility. Although good flying qualities are sometimes thought to be merely ‘nice to have’, with this interpretation they can actually delineate a vehicle’s achievable performance. This lends a much greater urgency to defining where those boundaries should be. Put simply, if high performance is dangerous to use, then most pilots will avoid using it.

In agility factor experiments the definition of the level of manoeuvre attack needs to be related to the key manoeuvre parameter, e. g., aircraft speed, attitude, turn rate or target motion. By increasing attack in an experiment, we are trying to reduce the time constant of the task, or increasing the task bandwidth. It is adequate to define three levels – low, moderate and high, the lower corresponding to normal manoeuvring, the upper to emergency manoeuvres.

There are also potential misuses of the agility factor when comparing aircraft. The primary use of the A f is in measuring the characteristics of a particular aircraft performing different MTEs with different performance requirements. However, Af also compares different aircraft flying the same MTE. Clearly, a low-performance aircraft will take longer to complete a task than one with high performance, all else being equal. The normalizing ideal time will therefore be greater for the lower than the higher performer, and if the agility factors are compared, this will bias in favour of the poor performer. Also, the ratio of time in the steady state to time in the transients may well be higher for the low performer. To ensure that such potential anomalies are not encountered, when comparing aircraft using the agility factor it is important to use the same normalizing factor – defined by the ideal time computed from a performance requirement.

7.3.1 Agility

Agility as a military attribute

In Chapter 6 and Section 7.2, the measurement of flying qualities from objective and subjective standpoints was discussed. Two additional issues arise out of the quality scale and assessment. First, the boundaries are defined for minimum requirements that reflect and exercise moderate levels of the dynamic OFE only, rather than high or extreme levels. Second, the assessments are usually made in ‘clean’ or clinical conditions, uncluttered by secondary tasks, degraded visual cues or the stress of real combat. Beyond the minimum quality levels there remains the question of the value of good flying qualities to the overall mission effectiveness. For example, how much more effective is an aircraft that has, say, double the minimum required (Level 1) roll control

power? A second question asks whether there are any upper limits to the flying qualities parameters, making quality boundaries closed contours. The answers to these questions cannot generally be found in flying qualities requirements like ADS-33. At higher performance levels, very little data are available on flying qualities and, consequently,
there are very few defined upper limits on handling parameters. Regular and safe, or carefree, use of high levels of transient performance has come to be synonymous with agility. The relationship between flying qualities and agility is important because it potentially quantifies the value of flying qualities to operational effectiveness. We will return to this question of value in Section 7.5, but first we examine agility in a more general context.

Operational agility is a key attribute for weapon system effectiveness. Within the broader context of the total weapon system, the mission task naturally extends to include the actions of the different cooperating, and non-cooperating, subsystems, each having its own associated time delay (Ref. 7.19). We can imagine, for example, the sequence of actions for an air-to-air engagement – threat detection, engagement, combat and disengagement; the pilot initiates the action and stays in command throughout, but a key to operational agility is the automation of subsystems – the sensors, mission systems, airframe/engine/control systems and weapon – to maximize the concurrency in the process. Concurrency is one of the keys to operational agility. Another key relates to minimizing the time delays of the subsystems to reach full operational capability and hence effectiveness in the MTE. Extensions to the MTE concept are required which encompass the functions and operations of the subsystems and so provide an approach to assessing system operational agility. Working Group 19, set up by AGARD in 1990, was tasked to address these issues (Ref. 7.19). In this study, addressing both fixed – and rotary-wing aircraft, flying qualities were a major concern. Minimizing time delays is crucial for the airframe, but flying qualities can suffer if the accelerations are too high or time constants too short, leading to jerky motion. The following discussion is based on the author’s contribution to AGARD WG19.

We need to examine how well existing flying qualities requirements address agility, but to set the scene we first reflect on the WG19 generalized definition of agility:

the ability to adapt and respond rapidly and precisely with safety and with poise, to maximise mission effectiveness.

To place this definition in context it is useful to list the four mission phases where agility might be important:

(1) stealthy flying, in particular terrain-masked, to avoid detection;

(2) threat avoidance once detected;

(3) the primary mission (e. g., threat engagement);

(4) recovery and launch from confined, or otherwise demanding, areas.

In addition, we can include the need for agility in response to emergency situations for both military and civil operations, such as those following major system failures. The key attributes of airframe agility, as contained in the above definition, are as follows:

(1) Rapid. Emphasizing speed of response, including both transient and steady-state phases in the manoeuvre change; the pilot is concerned to complete the manoeuvre change in the shortest possible time; what is possible will be bounded by a number of different aspects.

(2) Precise. Accuracy is the driver here, with the motivation that the greater the task precision, e. g., pointing, flight path achievable, the greater the chance of a successful outcome.

(Note: the combination of speed and precision emphasizes the special nature of agility; one would normally conduct a process slowly to achieve precision, but agility requires both.)

(3) Safety. This reflects the need to reduce piloting workload, making flying easy and freeing the pilot from unnecessary concerns relating to safety of flight, e. g., respecting flight envelope limits.

(4) Poise. This relates to the ability of the pilot to establish new steady-state conditions quickly and to be free to attend to the next task; it relates to precision in the last moments of the manoeuvre change but is also a key driver for ride qualities that enhance steadiness in the presence of disturbances.

(Poise can be thought of as an efficiency factor, or measure of the unused energy potential.)

(5) Adapt. The special emphasis here relates to the requirements on the pilot and aircraft systems to be continuously updating awareness of the operational situation; the possibility of rapid changes in the external factors, discussed earlier in this chapter (e. g., threats, UCE, wind shear/vortex wakes), or the internals, through failed or damaged systems, makes it important that agility is considered, not just in relation to set-piece manoeuvres and classical engagements, but also for initial conditions of low energy and/or high vulnerability or uncertainty.

Existing flying qualities requirements address some of these agility attributes implicitly, through the use of the HQRs, which relate the pilot workload to task performance achieved, and explicitly through criteria on response performance, e.g., control power, bandwidth, stability. A new parameter, the agility factor, makes a direct link between inherent vehicle performance and handling.

Roll axis handling characteristics have figured prominently in Chapter 6, where many of the new concepts associated with modern mission-oriented response criteria were introduced and the development processes described. As we continue the discussion of subjective measurement and assessment we return to this reference topic and present results of tests conducted at the DRA in the early 1990s utilizing the ground-based flying qualities facility – the advanced flight simulator (Ref. 7.13). Before discussing details of this work it is appropriate to review the original data that contributed to the definition of satisfactory roll axis characteristics, in particular the small amplitude bandwidth criteria. In Ref. 7.14, Condon highlighted the point that during the early 1980s the fidelity of ground-based simulators was considered inadequate for defining

Both sidesteps are intended to assess lateral directional handling qualities for aggressive manoeuvring near the rotorcraft limits of performance. The flight path constraints reflect operations close to the ground and obstacles. Secondary objectives are to check for any objectionable cross-couplings and to evaluate the ability to coordinate bank angle and collective to hold constant altitude.

Task description

Both sidesteps require the pilot to reposition from hover to hover with a lateral manoeuvre maintaining task performance requirements as shown above. The ADS-33 sidestep puts emphasis on achieving close to limiting lateral velocity without step size constraints. The DRA ACT sidestep places emphasis on repositioning to a particular location, hence requiring the pilot to judge the acceleration and deceleration phases carefully, on the basis that an operational sidestep is likely to have relatively tight terminal position constraints. The ADS-33 step requires the pilot to achieve a minimum bank angle in a maximum time during the accel and decel phases. The DRA ACT sidestep requires the pilot to fly at three initial bank angles to quantify the effects of pilot aggressiveness.

the Level 1/2 boundaries for rate command rotorcraft. Figure 7.7, from Ref. 7.11, shows how HQRs derived from the NRC Bell 205 compare with those derived from the NASA VMS facility, for equivalent MTEs. As a result of this kind of comparison, ground-based simulation data were considered unreliable and were not used in the early development of ADS-33 for rate command systems. Problems were attributed to a number of different areas including poor visual cueing, particularly fine detail and field-of-view, the harmony between visual and motion cues and time delays in the cue development, all areas where there were no equivalent flight problems. During the late 1980s and early 1990s, simulation technology improved significantly and a number of studies were reported with varying degrees of success, but all acknowledged

continuing limitations compared with in-flight simulation (Refs 7.14-7.16). With the commissioning of the DRA’s AFS for helicopter research in 1991, it was considered important to calibrate handling qualities results for rate command systems to deter­mine whether the fidelity of the AFS was good enough for definitive flying qualities research. The study, reported in Ref. 7.13, explored roll and pitch axes with a primary objective of establishing at what values of attitude bandwidth pilots would start return­ing Level 1 ratings consistently, if at all. The trial was configured with a futuristic ACT helicopter, with a two-axis sidestick, conventional collective and pedals, with primary flight instruments displayed head-up and pure rate response characteristics.

The key elements of the experiment are summarized in Fig. 7.8, showing the large motion system, computer-generated imagery (CGI) visuals, generic helicopter cockpit and the conceptual simulation model (CSM). A number of MTEs were developed on the CGI database that included sufficient textural detail for the evaluation pilots to perceive the desired and adequate task performance standards clearly. The photographs in Fig.

7.9 show the layout of four of the critical MTEs for evaluating roll and pitch handling at low speed and in forward flight – the low-speed sidestep and quickhop and the forward flight lateral jinking and hurdles. The task definitions included specification of the level of task aggressiveness to be flown by the pilots, as illustrated in the sidestep in Table

7.2 where initial bank angle was used as the defining parameter. We will return to the results for the sidestep later, but first we will consider the lateral jinking MTE in more detail, shown in plan form in Fig. 7.10, with the task performance standards defined in Table 7.3.

The lateral jinking or slalom manoeuvre is essentially a forward flight roll axis task comprising a sequence of ‘S’ turn manoeuvres followed by line tracking elements, as pilots attempt to fly through the gates shown in Fig. 7.10. Secondary handling qualities considerations include the ability to coordinate turns with pitch and yaw control and the harmonized use of collective and roll to maintain height. Task aggression is defined in terms of the maximum roll attitude used during the turning phases; values of 15°, 30° and 45° were found appropriate for designating low, moderate and high aggressiveness. These levels correspond to relaxed flying, normal operations with a

digital electric tonce-teel system

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I! і atmospheric I disturbances

cockpit interior

motion — — system

aud. o cues

computing piEol feel system simulation model T —<——- syslem tactile cues

cockpit on the LMS

display cues

visual system

vibration cues

Helisim generic rotorcralt model – oulpul rale 50 Hz – Encore real-time, open computing system incorporating lightly coupled and reflective memory lechnotogios

Fig. 7.8 Elements of DRA simulation trials

Fig. 7.9 The MTEs flown in the DRA simulation trials (Ref. 7.13): (a) sidestep; (b) quickhop; (c) lateral slalom; (d) hurdles

degree of urgency and emergency or other life-threatening situations, respectively. The task objective is to fly through the course whilst maintaining a height of 8 m and a speed of 60 knots, turning at the designated gates to acquire the new tracking line as quickly as possible, within the constraints of the set level of aggression. The turning gates are represented by adjacent vertical posts, which also provide height cueing – the white band on the posts delineating the desired performance margin. The intermediate gates were added to give enhanced tracking cues supplementing the runway lines. The width of the gates was determined by the adequate margin of performance for the tracking task (±20 ft/6 m).

The helicopter model used in the trial was the equivalent system CSM (Ref. 7.17). The roll axis characteristics are described by the simple second-order system

p K e T s

(s) = 7———- w———- ч

nic (-^ + 1) (-H – + 1)

&m J^a J

where K is the overall gain or in this case the rate sensitivity (deg/s per unit control), t is a pure time delay and rnm can be considered to be equivalent to the roll damping aircraft Lp; rna is the bandwidth of a pseudo-actuator lag. The actuator effectively reduces the transient acceleration jerk following a step control input, to realistic values. The value of rna was set to 20 rad/s for the tests. Variations in bandwidth and phase delay, the principal handling parameters of interest, can be achieved through the CSM parameters given in eqn 7.2. Figure 7.11 illustrates contours of constant damping

and time delay for the CSM overlaid on the bandwidth-phase delay diagram with the ADS-33 handling qualities boundaries. The four configurations to be discussed are spotted on the figure, with the designations of Ref. 7.13 – T103, 306 and 509. The last two digits denote the value of roll damping, as shown in the figure (e. g., T509 has a roll damping of 9 rad/s). The first digit assigns the control sensitivity (T1 = 0.1, T3 = 0.2, T5 = 0.3 rad/s[4] [5] per %). All configurations share the same roll control power, 96 deg/s, hence the control sensitivity increased in proportion with the damping. Contributions to the approximate 110-ms phase delay for all three configurations include the actuator lag and pure time delay from the AFS system computing and image generation. It is interesting to note that the bandwidth of configuration T509, with a natural damping of 9 rad/s, is reduced to 3 rad/s by the time delays. Configuration T306 + 80 included 80-ms additional pure time delay. The configurations spanned the ADS-33C Level 1/2 handling qualities boundary for general MTEs situated at 2 rad/s.

The trial was flown by six test pilots whose HQRs are shown as a function of roll bandwidth in Fig. 7.12. The ratings are shown with the mean, maximum and minimum values. For each configuration, ratings are shown for the three levels of pilot aggression. The maximum spread of the HQRs for each configuration/aggression level is about 2. If the spread had been much greater than this, then there would have been cause for concern, but a spread of 2 is regarded as acceptable. Several observations can be made about these subjective results, drawn from the pilot comments gathered in the tests, as follows:

rating line shown in Fig. 7.7. This result was a clear indication that the AFS was able to predict Level 1 handling qualities with good accuracy.

(2) At the high aggression level, Level 1 ratings were not achievable, and ratings strayed into the Level 3 region for the lower bandwidth configurations T103 and T306. Pilots complained of insufficient control and a sluggish response in negotiating the tighter turns for the lower bandwidth configurations. Configuration T509 was solid Level 2, and it could be speculated that even higher bandwidths would confer better flying qualities still.

(3) The spread of ratings for each configuration gives an indication of the powerful effect of task demands on pilot workload. Pilots rated the same aircraft, configuration T103, between a 2 and a 7 as the urgency level increased from low to high. This emphasizes the importance of defining the level of pilot aggression required; it is one of the parameters that workload is most sensitive to, even more so than bandwidth over the range considered. At the higher urgency level, several new handling qualities issues come to light, including flight envelope limit monitoring, task cue deficiencies at high-bank angles and the need for improved pilot judgement of flight path trajectory. We will address these in more detail in later sections of this chapter.

(4) One of the classic problems experienced by pilots flying low-bandwidth aircraft in moderately demanding manoeuvres is the need to command a high roll rate to compensate for the long rise time, combined with the need to arrest the rate quickly to stabilize on a new attitude. This can lead to overcontrolling and difficulties with flight path control. Figure 7.13 shows the attitude quickness values (see Chapter 6) for configurations T306 and T509 flown up to moderate levels of aggression – both achieved borderline Level 1/2 ratings. Pilots flew the lower bandwidth configuration,

T306, with significantly higher levels of quickness than T509, compensating for the lower bandwidth by using more of the control power. There is a usable limit to this trade-off between bandwidth and control power; the reader might note that the achieved roll quickness for T306 rises to the ADS-33 Level 1/2 track boundary.

(5) The single HQR 7 for configuration T306 + 80 at the moderate aggression level is also shown in Fig. 7.12. According to Fig. 7.11, the addition of 80-ms time delay to T306 should lead to Level 2 flying qualities for general MTEs and Level 3 flying qualities for tracking MTEs; in the event, a Level 3 rating was returned, indicating the significant tracking content of the lateral jinking task at moderate to high levels of aggression. For this case the pilot actually experienced a roll pilot-induced oscillation (PIO) while trying to tighten up the flight path to negotiate the gates. Figure 7.14 illustrates the plan view of the task showing the aircraft ground track for the same pilot flying T306 (upper figure) and T306 + 80 (lower figure). The (roll) PIO on the approach to, and flying through, the third gate is quite pronounced, and this experience highlights the real dangers of operating with low-bandwidth aircraft with large values of phase delay, in this kind of task. It carries a particularly strong message to the designers of ACT helicopters with digital flight control systems where high values of phase delay can be introduced by digital system transport delays and filters.

(6) The deterioration from borderline Level 1/2 to Level 3 with the addition of 80-ms time delay is an important result, suggesting that the pilots are more sensitive to increases in phase delay than the boundaries in Fig. 7.11 would suggest. We have already presented results in Chapter 6, which suggest that the phase delay boundaries should be capped rather than extend linearly out above 150 ms. The AFS slalom data tend to confirm this, although the pilot was forced to use a more aggressive control strategy for T306 + 80 than for the standard T306, often hitting the control stops during the roll reversals. We

have made the point on several occasions in this book that handling deficiencies can emerge as cliff edges, developing rapidly as some detail of the task or configuration is changed. The slalom PIO is a classic example of this and serves as a reminder of the importance of testing through moderate and up to high levels of aggression.

The pilot ratings for the DRA sidestep are shown in Fig. 7.15, comparing well with the ADS-33 flight test data; the results show a similar trend to the slalom data, except that the degradation with level of aggression does not appear nearly as strong for the higher bandwidth configurations. This is partly explained by the pilot comments that the sidestep task is considerably easier and more natural to fly with a more aggressive control strategy than the slalom, provided the attitude bandwidth and control power are available.

In Ref. 7.18, results are reported from trials on the DRA’s AFS for a maritime mission – the recovery of large helicopters to non-aviation ships in various sea states. The landing MTE was quickly identified as by far the most critical for handling qualities for all sea states. Figure 7.16 illustrates the task performance requirements for the landing, based on the need to touch down on the deck grid within defined velocity constraints. The pilot is required to bring the helicopter to the hover alongside the ship, wait for a quiescent period in the ship motion, exercise a lateral sidestep towards, and land onto, the deck. As often happens in practice, as the aircraft arrives over the deck, the pilot is unable to execute a successful landing immediately and has to maintain station over the deck grid, waiting for another quiescent period. The study reported in Ref. 7.18 examined roll, pitch and heave handling qualities, again using the CSM, but now configured as a much larger helicopter. Figure 7.17 shows the pilot HQRs for the landing MTE as a function of roll attitude bandwidth for various sea states. Figure 7.18 shows the achieved task performance in terms of landing scatter and touchdown velocities. For comparison, flight test results from a Sea King helicopter are included. The pilot was able to achieve adequate performance for the points shown and the HQRs were driven by the extreme levels of workload required at the higher sea states. For this MTE, sea state is the principal task driver, just as urgency level was for the battlefield sidestep and slalom MTEs, and the pilot’s ability to achieve desired performance levels at low workload is a strong function of the deck motion induced by the sea state (SS). While the data indicate that SS3 can be achieved with relatively low-bandwidth configurations («1.5 rad/s), SS5 will require considerably higher values, perhaps as high as the 3.5 rad/s boundary of the ADS-33 air combat/tracking tasks, as indicated by the suggested performance requirements shaded on Fig. 7.17. This will be more

difficult to achieve with large helicopters, and high gain/high authority active control may be required to guarantee consistent performance in poor weather conditions.

This section has discussed some of the important issues associated with pilot subjective opinion of aircraft handling qualities and the practical use of the Cooper – Harper HQR scale. The reader will be able to find many examples of handling qualities experiment reported in the open literature during the 1980s and 1990s; it has been a rich and productive period for this subject, spurred on to a large extent by the new handling qualities specifications on the one hand and the advent of active control technology on the other. These broad and concerted efforts to define and improve handling qualities have exposed and highlighted many new areas and facets of handling, where previous work had not been definitive. We now turn to examine some of these under the heading – Special flying qualities.

Depending on the objectives, a handling qualities experiment can commit flight or ground-based simulation facilities and a trials team for periods from several days to several weeks or even months. The subjective and objective data gathered may take months or even years to be analysed fully. The success, and hence value, of such an endeavour rests heavily on the experimental design and trials planning. There are a multitude of issues involved here, most of which would be inappropriate for discussion in this book. One of the critical elements is the design of the MTEs in which the handling qualities are to be evaluated. This has already been raised as an important issue in the discussion on HQRs above; the task performance drives the workload, which drives the pilot rating. Before we examine results from a handling experiment, it is worth looking more closely at the design of an MTE.

Designing a mission task element

The concept of the MTE was introduced in Chapter 2, the Introductory Tour to this book. Any mission can be analysed in terms of mission phases and MTEs and sample manoeuvres. An MTE is identifiable by its clearly defined start and end conditions. To be viable as a test for handling qualities, an MTE also needs to be defined in terms of spatial and temporal constraints. Above all, the constraints need to be related to real operational needs, or the data will be of questionable value and test pilots will quickly lose interest. During the 10 years between 1984 and 1994, the MTE has become central to the development of military handling qualities criteria and work reported at conferences, specialist meetings and in journals abound with examples of different MTEs and related HQR diagrams. At the core of these activities, the ADS – 33 MTEs have evolved into a set of mature test manoeuvres, aimed at providing the acid test for new military helicopters. In the early 1990s, a major refinement exercise was undertaken on these manoeuvres, as reported in Refs 7.9-7.11. The emphases of the refinements were (Ref. 7.10) ease of understanding, mission-oriented performance standards for good and degraded visual environments (DVEs), simple task cueing and affordable instrumentation. In this programme, several current operational helicopters were used in a flight test activity that served to concentrate attention on flight safety issues. Handling qualities testing, by its very nature, carries risk as the boundaries to

safe operation are mapped out. The new ADS-33 MTEs were designed to highlight any deficiencies in a pseudo-operational context and test programmes will certainly need to give a higher level of attention to safety than previously. The importance and justification for this approach is well summarized by Key in Ref. 7.10 when referring to the AH-64 ADS-33 flight tests: ‘Some of the aggressive manoeuvres, especially in DVE, were quite thrilling.. .if they are too dangerous for a skilled test pilot to perform in a tightly controlled environment, it is unreasonable to expect the user to fly such manoeuvres in an unfamiliar, unfriendly environment in the fog of war’.

The test manoeuvres proposed for the new Military Standard (Ref. 7.12) are summarized in Table 7.1, and include both GVE (good visual environment) and DVE cases.

Figure 7.5, taken from Ref. 7.9, illustrates results from handling qualities tests during development of the refined MTEs using three test aircraft – the NRC variable stability Bell 205, the AH-64A and the UH-60A; the 205 was tested with both Level 1 and 2 response characteristics, according to the objective ADS-33 criteria. ADS-33 was targeted at a new design of course, the RAH-66 Comanche, so it is hardly surprising that current operational aircraft appear as good Level 2 on average.

To illustrate an MTE in more detail we have chosen the lateral sidestep reposi­tioning manoeuvre and also to compare the ADS-33 task description and performance standards with those developed for the DRA’s ACT research programme (Ref. 7.13). The layout of the sidestep ground markers for the DRA ACT simulations is sketched in Fig. 7.6 and quantified in Table 7.2. The pilot is required to initiate the MTE from a hover point with the triangle and square aligned, sidestepping to a new hover position again aligning the triangle and square. There is a close comparison between the DRA and ADS-33 manoeuvres, but the DRA requirements are slightly more demanding, reflecting the expected improvements conferred by full authority ACT. Important dif­ferences appear in the temporal and spatial constraints, with the DRA placing more emphasis on measuring the effects of piloting aggressiveness (three levels specified) and repositioning at a defined point, to introduce a realistic spatial constraint. In contrast,

the ADS-33 standards place more emphasis on pilots’ achieving close to maximum lateral velocities with aggressiveness defined by the times to accelerate and decelerate.

Figure 7.2 illustrates the decision tree format of the Cooper-Harper handling qualities rating scale. Test pilots and flying qualities engineers need to be intimately familiar with its format, its intended uses and potential misuses. Before we begin a discussion on the rating scale, we refer the reader back to Fig. 7.1 and to the key influences on control strategy which should be reflected in pilot opinion; we could look even further back to Fig. 6.1, highlighting the internal attributes and external factors as influences on flying qualities. The pilot judges quality in terms of his or her ability to perform a task, usually requiring closed-loop control action. A key point in both figures is that the handling qualities and pilot control strategy are a result of the combined quality of the aircraft characteristics and the task cues. The same aircraft can be Level 1 flying routine operations at day and then Level 3 at night, or when the wind blows hard, or when the pilot tries to accomplish a landing in a confined area. An aircraft may be improved from Level 3 to 2 by providing the pilot with a night vision aid or from Level 2 to 1 by including appropriate symbology on a helmet-mounted display. Handling qualities are task dependent and that includes the natural environmental conditions in which the task is to be performed, and the pilot will be rating the situation as much as the aircraft. We will discuss the scale and HQRs in the form of a set of rules of thumb for their application.

Adequacy for selected task or required operation

(1) Follow the decision tree from left to right. Pilots should arrive at their ratings by working through the decision tree systematically. This is rule number 1 because it helps the pilot to address the critical issue of whether the aircraft is Level 1, 2 or 3 in the intended task or subtask. The decision tree solicits from the pilot his opinion of the aircraft’s ability to achieve defined performance levels at perceived levels of workload.

(2) An HQR is a summary of pilot subjective opinion on the workload required to fly a task with a defined level of performance. An HQR can be meaningless without back-up pilot comment. It is the recorded pilot opinion which will be used to make technical decisions, not the HQR, because the HQR does not tell the engineer or his manager what the problems are. Often a structured approach to qualitative assessment will draw on a questionnaire that ensures that all the subject pilots address at least a common set of issues. We will consider the ingredients of a questionnaire in more detail later.

(3) Pilot HQRs should be a reflection of an aircraft’s ability to perform an operational role. The MTEs should be designed with realistic performance requirements and realistic task constraints. The pilot then needs to base his rating on his judgement of how an ‘average’ pilot with normal additional tactical duties could be expected to perform in a similar real-world task.

(4) Task performance and workload come together to make up the rating, but workload should be the driver. This is most important. To highlight the emphasis we refer to Fig. 7.3 where workload and task performance are shown as two dimensions in the piloting trade-off. Task performance is shown in three categories – desired, adequate and inadequate. Workload is also shown in three categories – low, moderate to extensive and maximum tolerable, reflecting the rating scale parlance. HQRs on the Cooper – Harper scale fall into the areas shown shaded. Figure 7.3 acknowledges that a pilot may be able to work very hard (e. g., using maximum tolerable

compensation) and achieve desired performance, but it is not appropriate then to return a Level 1 rating. Instead, he should aim for adequate performance with some spare workload capacity. Similarly, a pilot should not be satisfied with achieving adequate performance at low workload; he should strive to do better. A common target for pilots in this situation would be to try to achieve desired performance at the lower end of Level 2, i. e., HQR 4. Level 1 characteristics should be reserved for the very best, those aircraft that are fit for operational service. An HQR 4 means that the aircraft is almost good enough, but deficiencies still warrant improvement.

(5) Two wrongs can make a disaster. Handling qualities experiments often focus on one response axis at a time, two at the most. We have seen in Chapter 6 how much it takes to be a Level 1 helicopter, and a question often arises as to how much one or two deficiencies, among other superb qualities, can degrade an aircraft. The answer is that any Level 2 or 3 deficiency will degrade the whole vehicle. A second point is that several Level 2 deficiencies can accumulate into a Level 3 aircraft. Unfortunately, there seems to be very little rotorcraft data on this topic, but Hoh has given a hint of the potential degradations in the advisory ‘product rule’ (Ref. 7.5)

_1(m+1) m

Rm = 10 + Ri – 10) (7Л)

where Rm is the predicted overall rating and Ri are the predicted ratings in the individual m axes. According to the above, two individual HQR 5s would lead to a multi-axis rating of 7. The fragile nature of such prediction algorithms emphasizes the critical role of the pilot in judging overall handling qualities and the importance of tasks that properly exercise the aircraft in its multi-axis roles. So while sidesteps and quickhops might be appropriate MTEs for establishing roll and pitch control power requirements, the evaluations should culminate with tasks that require the pilot to check the harmony when flying a mixed roll-pitch manoeuvre. Ultimately, the combined handling should be evaluated in real missions with the attendant mission duties, before being passed as fit for duty.

(6) The HQR scale is an ordinal one, and the intervals are far from uniform. For

example, a pilot returning an HQR of 6 is not necessarily working twice as hard as when he returns an HQR of 3; Cooper and Harper, in discussing this topic, suggest that ‘… the change in pilot rating per unit quality should be the same throughout the rating scale’. The implied workload nonlinearity has not hindered the almost universal practice of averaging ratings and analysing their statistical significance. Many examples in this book present HQRs with a mean and outer ratings shown, so the author clearly supports simple arithmetic operations with HQRs. However, this practice should be undertaken with great care, particularly paying attention to the extent of the rating spread. If this is large, with ratings for one configuration appearing in all three levels for example, then averaging would seem to be inappropriate. If the rating spread is only 1 or 2 points, then it is likely that the pilots are ‘experiencing’ the same handling qualities. Of course, if the ratings still cross a boundary, and the mean value works out at close to 3.5 or 6.5, then it may be necessary to put more pilots through the evaluation or explore some task variations. The whole issue of averaging, which can make data presentation so appealing, has to be undertaken in the light of the pilots’ subjective comments. Clearly, it would be

inappropriate to average a group of ratings when the perceived handling deficiencies recorded in each of the pilot’s notes were different.

(7) Are non-whole ratings legal? There appears to be universal agreement that pilots should not give ratings of 3.5 or 6.5; there is no space here to sit on the fence and the trial engineer should always reinforce this point. Beyond this restriction, there seems to be no good reason to limit pilots to the whole numbers, provided they can explain why they need to award ratings at the finer detail. A good example is the ‘distance’ between HQR 4 and 5. It is one of the most important in the rating scale and pilots should be particularly careful not to get stuck in the handling qualities ‘potential well’ syndrome of the HQR 4. In many ways the step from HQR 4 to HQR 5 is a bigger workload step than from 3 to 4 and pilots may feel the need to return HQRs between 4 and 5; equally, pilots may prefer to distinguish between good and very good configurations in the region between HQR 2 and 3.

(8) How many pilots make a good rating? This question is always raised when designing a handling qualities experiment. The obvious trade-off involving the data value is expressed in terms of authenticity versus economy. Three pilots seems to be the bare minimum with four or five likely to lead to a more reliable result and six being optimal for establishing confidence in the average HQR (Ref. 7.6). For a well-designed handling qualities experiment, there will inevitably be variations in pilot ratings as a result of different pilot backgrounds, skill level, pilots’ perception of cues, their natural piloting techniques and standards to which they are accustomed (e. g., one pilot’s HQR 4 might be another’s 5). Measuring this ‘scatter’ is an important part of the process of understanding how well the aircraft will work in practice. But if the scatter is greater than about two ratings, the engineer may need to consider redesigning the experiment.

(9) How to know when things are going wrong. A wide variation of ratings for the same MTE should ring alarm bells for the trial engineer. There are many legitimate reasons for a spread in HQRs but also some illegitimate ones. One reason could be that the pilots are not flying the same task. Part of the task definition are the standards for desired and adequate performance. These will be based on some realistic scenario, e. g., sidestep from one cover point to another, 100 m distant, and establish a hover within a defined world-referenced box, with permitted errors up to, say, 2 m. The task definition might also add that the pilot should maintain his flight path below 10 m above the ground and accomplish the task in a defined time. The more detail that is added to the task definition, the more likely it is that each pilot will try to fly the same MTE and the more the HQR scatter will be left to pilot differences, which is what is required. Conversely, the less detail there is, the more likely is the chance of different pilots interpreting the task differently; one may fly the task in 15 s, another in 20 s and the different demands will drive the workload and hence ratings. Next to the need for complete and coherent task definition comes the need to provide the pilot with sufficient cues to enable him to judge his task performance. This is a critical issue. In real-world scenarios, pilots will judge their own task performance requirements and they will usually do this on the basis of requiring low to moderate pilot compensation. Pilots do not usually choose to fly at high levels of workload, unless they have to, and will normally set performance requirements based on task cues that they can clearly perceive. Unless a pilot has made an error of judgement, he or she will not normally fly into a condition where the task cues are insufficient for guidance and stabilization. In clinical flying

qualities tests, it is important for the trials engineer to work closely with the ‘work-up’ pilot to define realistic performance goals that an average pilot would be expected to perceive in operations. Then, when the pilot returns an HQR, the actual task performance achieved should correlate well with that perceived by the pilot. Unless properly addressed, this issue can devalue results of handling qualities experiments. A third factor in HQR scatter deserves a rule all on its own.

(10) How long before a pilot is ready to give a rating? There is no simple answer to this question, but pilots and engineers should be sensitive to the effects of learning with a new configuration. Briefly, pilots should be allowed enough time to familiarize with a configuration, for general flying and in the test MTEs, before they are ready to fly the evaluation runs. The DLR test technique, adopted during in-flight simulation trials, involves computing the ‘score factor’ of the MTE, i. e., the ratio of successive performance measures (Fig. 7.4, Ref. 7.7). When the score factor rises above a pre-defined level, then the pilot is at least achieving repeatable task performance, if not workload. Ultimately, the pilot should judge when he is ready to give a formal evaluation and the trial engineer should resist forcing a ‘half-baked’ HQR. Something for both the test pilot and the engineer to bear in mind is that the subjective comments recorded during the learning phase are very important for understanding the basis for the eventual HQR; a communicative pilot will usually have a lot of very useful things to say at this stage. This brings us to the subject of communication between the pilot and engineer and flying qualities jargon.

(11) Interpreting test pilot talk. In handling qualities evaluations, test pilots will use a variety of descriptors within their subjective comment to explain the impact of good and bad characteristics. To simplify this discussion, we will relate two categories – the classical pilot qualitative language, e. g., sluggish, crisp, smooth and predictable, and the engineering parlance, e. g., control power, damping and bandwidth. HQRs are the summary of pilot comment, and it is important that the pilot comment is consistent and understandable; once again, it is the pilot comment that directs the engineer towards improvement. Two observations on pilot comment are worth highlighting here. First, any classical parlance should be defined in terms relating to task; e. g., the roll response of this aircraft is sluggish because it takes too long to achieve the required bank angle. There is no universal dictionary for classical parlance so it is a good idea to establish agreed meanings early in a trial; the HQRs will then be more valuable. Second, it is the author’s considered opinion that test pilots should be strongly discouraged from using engineering parlance during evaluations, unless they are conversant with the engineering background. Sometimes quite different engineering parameters can lead to similar effects and if pilots try to associate effects with causes, they run the risk of making predictive judgements based on what they think will be the case. Engineers need test pilots to tell them what aspects are good or bad and not try to diagnose why. Ironically, it is the very skill that test pilots are valued for – the ability to think about and interpret their response – that can spoil their ratings. When it comes to the evaluation, deeply learned and instinctive skills are being exercised and, to an extent, thinking can intrude on this process. It is far better for test pilots to describe their workload in terms that are subjective but unambiguous.

(12) When is an HQR not an HQR? During the assembly of a handling qualities database, configurations will be evaluated that span the range from good to bad, and pilots need not think that it is their fault if they cannot achieve the performance targets. During the development of a new product, flying qualities deficiencies may appear and the test pilots need to present their findings in a detached manner. Above all, test pilots that participate in such evaluations need to be free from commercial constraints or programme commitments that might influence their ratings. This point is stressed by Hoh in Ref. 7.8. Eventually it will be in both the user’s and manufacturer’s interest to establish the best level of flying quality.

(13) Pilot fatigue – when does an HQR lose its freshness? This will certainly vary from pilot to pilot and task to task, but evaluation periods between 45 and 90 min seem from experience to cover an acceptable range. The pilot fatigue level, and to an extent this can be influenced by their attitude to the evaluation, can be a primary cause for spread in HQRs. The pilot is usually the best judge of when his performance is being impaired by fatigue, but a useful practice is to introduce a reference configuration into the test matrix on occasions as a means of pilot calibration.

(14) HQRs are absolute, not relative. This is an important rule, but perhaps the most difficult to apply or live by, especially if several different aircraft are being compared in an experiment; there will always be the temptation for the pilot to compare an aircraft or configuration with another that has already achieved a particular standard and been awarded a rating. Disciplined use of the Cooper-Harper decision tree should help the pilots resist this temptation, and appropriate training and good early practice would seem to be the best preventative medicine for this particular bad habit.

(15) The HQR is for the aircraft, not for the pilot. Piloting workload determines the rating but the rating needs to be attributed to characteristics of the aircraft and task cues as defined in the Cooper-Harper rating scale. Emphasis on HQR, rather than pilot rating, can help with this important distinction.

(16) An HQR does not tell the whole story. In this last point we reiterate rule number 2 that every HQR should be accompanied by a sheet of pilot comments to give the full story. This can often be derived as a series of answers on a questionnaire addressing the various aspects covered in the Cooper-Harper decision tree – vehicle characteristics, workload (compensation) and task performance; task cues also need to be addressed, and the absence of any reference to task cues in the Cooper-Harper decision tree is explained by the assumption that sufficient task cues exist for flying the task. The subject of task cues and the need for pilot subjective impressions of the quality of task cues have received more prominence with the introduction of vision aids to support flying at night and in poor weather. This topic is addressed further in Section 7.3.3.

These 16 rules represent this flying qualities engineer’s assessment of the important facets of the subjective measurement scale and the HQR. Put together, the issues raised above highlight the importance of the special skills required of test pilots, enhanced by extensive training programmes. To examine how these work in practice, we need to discuss the design, conduct and test results from handling qualities evaluations.

Opinion on what constitutes quality when it comes to flying has been demonstrated over the years to be wide and varied amongst pilots, and will undoubtedly continue to be so. Individuals can have different preferences and achieving universal agreement over all aspects of quality is probably unrealistic, and perhaps even undesirable. Fortunately, pilots, like most of the human race, are exceptionally adaptable and can learn to use someone else’s favourite flight vehicle very effectively. If we consider flying quality to be valued in both aesthetic and functional terms by pilots, then by far the most effort has been expended by flying qualities engineers on trying to establish a consensus regarding functional quality. This effort has received considerable leverage through the development of mission-oriented or functional flying qualities criteria. Prior to this, over several decades, the merging of functional values with aesthetic values has led to flying quality being a ‘nice to have’ attribute rather than essential for achieving safety and performance. The importance of aesthetic quality is recognized, but treating this aspect is beyond the scope of this engineering text. The emphasis with mission – oriented flying qualities is the ability to perform a defined set of tasks with temporal and spatial constraints, and what better test of quality than flying the tasks themselves. When flying a task, or to use the parlance of ADS-33 (Ref. 7.1), an MTE, the pilot will adopt a control strategy to maximize performance and minimize workload. Control strategy may vary from pilot to pilot, reflecting the complex network of influences on how different pilots elect to use their controls. Figure 7.1, taken from Ref. 7.2, illustrates the point. The task requirements in a given environment will determine the accuracy and spare workload capacity required; McRuer has described this through the dual concepts of attentional demand and spare control capacity (Ref. 7.3). Landing a helicopter on the deck of a small ship may require considerable accuracy, and a pilot may well take his time to achieve a safe landing. Evading a threat may place greater demands on acting quickly than flying precisely. Whatever the drivers, the combination of vehicle response characteristics and task cues will determine the control strategy adopted by the pilot, which in turn will be reflected in the realized task performance and actual pilot workload. In making a subjective assessment of the flying quality, a pilot will need to take into account these interacting influences and then articulate his or her thoughts to the flying qualities engineer, whose job at this stage is to make changes for the better, if at all possible. It is the articulation and the associated interpretation of the pilot’s subjective opinion that underpins any successful development of flying qualities,

and it is hardly surprising that this activity has been ‘assisted’ by a wide range of different support tools including rating scales and questionnaires. One rating scale has achieved more universal acceptance than any other since it was first proposed in the late 1960s – the Cooper and Harper handling qualities rating scale (Ref. 7.4). In view of its importance to the subject and partly to highlight potential misuses, the next subsection will give exclusive coverage to this scale and the associated pilot handling qualities rating (HQR).

If test manoeuvres are too dangerous for a skilled test pilot to perform in a tightly controlled environment, it is unreasonable to expect the user to fly such manoeuvres in an unfamiliar, unfriendly environment in the fog of war.

(Key, 1993)

7.1 Introduction and Scope

While objective measurements and assessment are necessary for demonstrating com­pliance with quality standards, they are still not sufficient to ensure that a new he­licopter will be safe in achieving its operational goals. Gaps in the criteria due to limited test data, and the drive to extend operations to new areas, continue to make it vital that additional piloted tests, with a subjective orientation, are conducted prior to certification. A further issue relates to the robustness of the criteria and an aircraft’s flying qualities at higher levels of performance. The point has been made on several occasions that criteria in standards like ADS-33 represent the minimum levels to en­sure Level 1 in normal operation. A good design will do better than just meet the objective Level 1 requirements, and the absence of upper limits on most of the han­dling parameters means that there is practically no guidance as to when or whether handling might degrade again. An aircraft will need to be flight tested to assess its flying qualities in a range of mission task elements MTEs, throughout its intended operational flight envelope (OFE), and including operations at the performance lim­its to expose any potential handling cliff edges. During such testing, measurements will be made of aircraft task performance and control activity, but there is, as yet, no practical substitute for pilot subjective opinion. The measurement and interpreta­tion of pilot opinion is a continuing theme throughout this chapter but is exclusively the subject of Section 7.2, where a range of topics are covered, including handling qualities ratings (HQRs), MTEs and the design and conduct of a handling qualities experiment.

Section 7.3 deals with a selection of what we have described as special flying qualities, including agility, carefree handling and flight in poor visual conditions.

One of the areas omitted from the comprehensive treatment of objective assess­ment in Chapter 6 was the requirements for pilot’s inceptors or controllers. The issues surrounding the assessment of quality for inceptors, particularly sidesticks, are so pilot centred that coverage in this chapter was considered more appropriate; Section 7.4 deals with this topic.

For both military and civil helicopters, the potential improvements in flying qual­ities offered by active control technology (ACT) through the almost infinite variety

of response shaping, where computers take on the ‘compensation’, have prompted a more serious examination of the benefits of improved flying qualities to safety and performance. Helicopters’ accidents and incidents due to so-called pilot error are still far too high and many can be attributed to poor flying qualities in a broad sense. Even those accidents caused by system failures can ultimately be attributed to degraded flying qualities, as the pilot struggles to fly a disabled aircraft to a safe landing. With these ideas in mind, Section 7.5 examines the contribution of flying qualities to per­formance and safety by viewing the pilot as a system element, with the potential of failing when under ‘stress’, and outlines a new approach to quantifying the risk of failure.

In this chapter a great play has been made of the concept of dynamic response criteria, used to form the predicted handling qualities, fitting into their place on the frequency – amplitude chart, conceptualized earlier in Figs 6.5 and 6.14. This can be summarized more holistically using Fig. 6.73 where the different criteria can be further classified into two groups – those determining the aircraft’s agility and those determining the aircraft’s stability (Ref. 6.91).

As before, the manoeuvre envelope line is shown to restrict criteria to practical manoeuvres, the achievable manoeuvre amplitude reducing as frequency increases. Within this overall envelope, four areas can be distinguished, two dealing with stability and two dealing with agility. The dynamic response requirements relating to agility and

stability cannot easily be ‘divorced’, since too much stability can degrade agility and vice versa. Designing to achieve the right balance requires careful optimization and, as with the design of fixed-wing aircraft, digital fly-by-wire/light flight control tech­nology has provided the designer with considerably more freedom than hitherto in this trade-off. However, it is significantly not essential for control augmentation systems to have full authority over the control actuation to be able to deal with this. The essence of this challenging compromise can be seen in the designs of the augmentation systems on two of the aircraft featured in this book – the Lynx and the Puma. The Lynx system features both pitch-roll attitude and rate feedback signals, but the gain on the attitude signal has two values – a high value for small perturbations and a much reduced value when the attitude increases above certain values. The response type is therefore ACAH

for small amplitude inputs and RC for large amplitude inputs. The increased stability conferred by the attitude feedback also improves the aircraft flying qualities in turbu­lent conditions where the pilot does not have to apply continuous corrective actions to maintain a desired attitude and speed. In the Puma system, a different approach is used. A pseudo-attitude is derived by integrating the signal from the rate gyro and used to provide short-term attitude stabilization. When the pilot moves the cyclic stick outside a prescribed range from the trim position, the pseudo-attitude component is switched out, providing the pilot with full RC response. Both the Lynx and Puma designs were innovative 40 years ago and specifically designed to address the stability-agility trade­off; the augmentation on both aircraft acts through limited authority (~ ± 10%) series actuators. Nowadays, task-tailored control and flying qualities are commonplace con­cepts, although the implementation of such strongly nonlinear design functionality seems to be much less common.

In Fig. 6.73, examples of agility and stability criteria from ADS-33 are shown. The moderate amplitude quickness criteria (shown for the roll axis) provide a direct link between closed-loop stability, encapsulated in the bandwidth criteria (shown for the pitch axis), and the maximum agility, encapsulated in the control power criteria (shown for the yaw axis). The basic stability is defined by the position of the eigenvalues on the complex plane, shown in the figure for the Dutch roll-yaw oscillation. Flying qualities requirements extend far beyond those summarized in Fig. 6.73, of course, including trim and static stability, flight path response, cross-coupling behaviour and controller characteristics, and most of these aspects have been covered to varying extent in this chapter. During the period since the publication of the first edition of Helicopter Flight Dynamics, various basic research and application studies have refined the understand­ing of helicopter flying/handling qualities and some of this has been embodied in the performance specification version of ADS-33E (Ref. 6.92). The changes from the C – version, used throughout Chapter 6, have been numerous and no attempt has been made to fully revise the material presented earlier. Rather, a number of key developments are highlighted here to draw specific attention to them.

ADS-33 contains the first truly mission-oriented set of requirements, embodied in the fact that the location of criteria boundaries are related to the types of ‘mission – task-element’ to be flown, rather than the aircraft size or weight. In ADS-33E-PRF the aircraft role (i. e., attack, scout, utility, cargo) is then described by a subset of recommended MTEs to be flown and level of agility used (i. e., limited, moderate, aggressive, tracking). The objective criteria then link with the MTEs in two ways – through the Response Type table (see Table 7.4) and also the level of agility associated with an MTE that, in turn, defines the handling qualities boundary to be used. An example from Ref. 6.92 is the set of requirements for large amplitude attitude changes in hover and low-speed flight, i. e., the control power, shown in Table 6.6. The normal levels of agility for the 13 different MTEs that are used to define the control power requirements are defined. For example, slaloms would normally be flown at moderate levels of agility while the acceleration-deceleration would require aggressive agility.

A change to the closed-loop stability requirements appears in the bandwidth for the general or ‘all other’ MTEs in a UCE > 1 and/or divided attention. Figure 6.74 should be compared with Fig. 6.30(c), showing the shift of the Level 2/3 boundary from 0.5 rad/s to 1 rad/s; the boundary is also raised for forward flight. Generally speaking, less research has been conducted to define the Level 2/3 handling qualities boundary than that conducted for the Level 1/2 boundary; it is known that pilot perception of

, faced with such response lags.

The attitude quickness criteria remain as documented in ADS-33C, apart from clarification on the testing requirements, particularly relating to the need for the pilot to change attitude ‘as rapidly as possible without significant reversals in the sign of the cockpit control input’. Control overshoots are known to result in increased quickness and give a ‘false’ sense of agility, even though the pilot might commonly use this technique to change attitude. The point is also emphasized that the full range of moderate amplitudes should be covered. Figures 6.10 and 6.18 showed what we described as closed-loop quickness (pilot using overshoot technique) for the Lynx flying slalom and sidestep MTEs, measured across a wide range of attitude changes. A significant change is that aircraft with response types appropriate to UCE = 2 and 3 no longer have to meet the quickness requirements, on the basis that operations in DVEs require only limited agility.

In Section 6.5.3, discussion centred around the complexities of helicopter flight path control in forward flight and the shortcomings of the requirements of ADS-33C were highlighted, particularly relating to testing difficulties, and the distinctly non­first-order response of some types (e. g., see Fig. 6.58). In ADS-33E-PRF, new criteria have been introduced for flight path behaviour in response to a pitch attitude change through cyclic pitch with collective fixed. Criteria are distinguished for front-side and back-side (of the power curve) operations. If Ayss is the change in flight path and AVss is the change in speed resulting from a step change in pitch attitude, then

Front-side operation Ayss/AVss < 0 (6.41)

Back-side operation Ayss/AVss > 0 (6.42)

For back-side operation, the flight path handling requirements are essentially the same as the low-speed height response to collective requirements discussed in Section 6.51, except that the maximum value of the time constant, T^ , for Level 2 handling, is reduced to 10 s. For front-side operation, the criteria is based on the lag between flight path and pitch attitude (equivalent to the heave time constant, or inverse of the derivative — Zw, at low frequency), expressed in the frequency domain as follows: the lag should be <45° at all frequencies below 0.4 rad/s for Level 1, andbelow 0.25 rad/s for Level 2. It is considered that these criteria are still open to development, particularly for complex precision approach trajectories envisaged to enable the expansion of simultaneous, non-interfering operations of helicopter at busy hubs.

Finally, we turn to cross-coupling criteria and Fig. 6.75 summarizes the vari­ous important cross-coupling effects found in helicopters. The starred boxes denote response couplings for which no handling criteria exist.

Requirements in ADS-33 include detailed quantitative ratio criteria, for example, the roll to pitch or collective to yaw, and also qualitative ‘not objectionable’ type state­ments, although the flight path response to pitch attitude changes has been developed in Ref. 6.92 into the quantitative criteria described in the previous paragraph. Cross­couplings emerge as a serious impediment to task performance for manoeuvres where higher levels of precision and aggressiveness are required, and this has been taken into account in Ref. 6.92 by requiring that the coupling requirements on yaw from collective and pitch-roll and roll-pitch be applicable to aircraft that need to meet the aggressive and acquisition/tracking levels of agility. New requirements have been developed for the tracking level of agility based on the research reported in Refs 6.93 and 6.94, which

Fig. 6.75 Dynamic response criteria for cross-couplings or off-axis response

identified the frequency dependence of the handling qualities for small amplitude track­ing. In Fig. 6.76 the response ratios (average q/p (dB), average p/q (dB)) are derived from the amplitudes of the frequency response functions q/Slat divided by p/Slat and p/&long divided by q/Song, averaged between the attitude bandwidth frequency and frequency at which the attitude response phase is -180°. The requirements focus on the pitch due to roll requirements, derived from tests where pitch control was disrupted by varying levels of coupling during a roll tracking task (Ref. 6.93).

The methodology expressed in ADS-33 has been applied extensively since the publication of the first edition of this book. Some guidance for the tailoring of the requirements to specific roles was given in Ref. 6.95. In the United Kingdom an initial emphasis was placed on the application to the attack helicopter procurement competi­tion (Ref. 6.96), while in mainland Europe to the design and development of the NH90 (Ref. 6.97). A continuing theme in the research community has been the development of a maritime version of ADS-33, with particular application to operations to and from ships. A series of flight and flight simulation trials have been conducted and results reported (Refs 6.98-6.101), which have guided specific applications, but no generally accepted and conclusive product has emerged. What seems to be universally agreed, however, is that (Ref. 6.101) ‘Level 1 handling qualities are not achievable at high sea

Fig. 6.76 Pitch-roll cross-coupling requirements for target acquisition and tracking MTEs states using current landing practices with standard levels of aircraft augmentation’. A somewhat similar situation has arisen with regard to extending the scout/attack heli­copter requirements – originally the focus of ADS-33 – to cargo helicopters, particularly for operations with external, underslung loads. References 6.102-6.104 document part of the story, but ultimately ADS-33E-PRF, in its discussion on the flight and simulation tests conducted to develop criteria, concludes that, ‘The outcome of this testing has been overwhelming evidence that quantitative criteria will be extremely difficult to derive’. In Ref. 6.105 a comprehensive analysis of the handling qualities of the UH-60M prior to first flight is reported, demonstrating the utility of the methodology to vehicle upgrades.

The author’s own research has also taken the methodology into new directions, particularly the relationship between handling qualities and loads. In Ref. 6.106, for example, some of the structural fatigue issues in helicopter flight testing are addressed, with particular emphasis to flying qualities tests. In Ref. 6.107 an approach to integrat­ing the handling qualities/agility and load alleviation design processes is presented, taking advantage of and extending the ADS-33 metrics to embrace both disciplines.

Finally, it seems appropriate to briefly mention the main intended recipient of the ADS-33 development efforts, the RAH-66 Comanche. The programme was cancelled on 23 February 2004, but not before the aircraft had demonstrated the fruits of the ef­fort to design and build the first helicopter with Level 1 handling qualities throughout the OFE, at least in the good visual environment (Ref. 6.108). A salutary lesson lies in the conclusions of Ref. 6.108, however, that ‘… while the analytical requirements of ADS-33D, Section 3, are an indispensable resource for control law development, they do not obviate the requirement for a vigorous flight test programme with active engagement between pilot and engineers, without which many of the critical improve­ments… would not have been possible’. With these words, the authors of Ref. 6.108 lead us naturally to the subject of subjective pilot assessment and the assigned handling qualities.