Document
N94- 13320
Piloting Considerations for Terminal Area Operations'of
Civil Tiltwing and Tiltrotor Aircraft
William S. Hindson Gordon H. Hardy George E. Tucker Research Pilots William A. Decker Aerostxtce Engineer NASA Ames Research Center Moffett Field, California ABSTRACT During the past several years, various piloted simulations have been conducted of both of these design concepts The existing body of research to investigate airworthiness, (Refs. 3 - 5). The objectives of these simulations have performance, handling, and operational requirements for included concept evaluation, detailed systems STOL and V/STOL aircraft was reviewed for its development, and the investigation of airworthiness and applicability to the tiltrotor and tiltwing design concepts.
certification issues associated with the operation of these The objective of this study was to help determine the aircraft in instrument meteorological conditions 0MC) in needs for developing civil certification criteria for these the terminal area.
aircraft concepts. Piloting tasks that were considered included configuration and thrust vector management, At the same time, many research efforts conducted over glidepath control, deceleration to hover, and engine failure the past three decades have examined the stability and procedures. Flight control and cockpit display systems control, handling, and performance requirements for both that have been found necessary to exploit the low-speed powered-lift STOL transport-category aircraft and military operating characteristics of these aircraft are described, and jet V/STOL aircraft. References 6-11 and their associated beneficial future developments are proposed.
bibliographies provide a comprehensive summary of this NOMENCLATURE research. More recently, the introduction of digital flight control technologies has stimulated research in integrated flight/propulsion control for V/STOL aircraft, partly with CTOL Conventional Takeoff and Landing the objective of providing a consistent control IMC Instrument Meteorological Conditions mechanization for the pilot over the high-speed and low- SAS Stability Augmentation System speed flight envelopes where thrust vector orientation STOL Short Takeoff and Landing differs markedly (Refs. 12, 13). Even though the tiltrotor V Ah'swed and tiltwing design concepts received scant mention in the WSTOL Vertical/Short Takeoff and Landing evolution of these V/STOL design requirements, much of Y Flightpath angle this background research is relevant to these aircraft.
Pitch angle Consequently, it is one objective of this paper to associate some of the airworthiness and piloting issues for these INTRODUCTION two aircraft design concepts with some of the general criteria contained in these references.
After many years of research and testing of numerous and diverse V/STOL concepts, the possibility is now Considerable research has also been conducted over the emerging that tiltrotor and tiltwing aircraft might enter past two decades to investigate operational procedures, civil operations during the next decade (Refs. 1, 2). Indeed, flight control, and cockpit display systems needed to the V-22 Osprey tiltrotor aircraft, a military prototype support terminal area operations by powered-lift STOL, currently undergoing acceptance testing, is paving the way V/STOL, and rotary wing aircraft in IMC. In addition to for possible civil applications.
exploiting the short or vertical landing capabilities of these aircraft, the expectations implicit in this research Presented at Piloting Vertical Flight Aircraft: A Conference have been to take advantage of their potential to operate in on Flying Qualities and Human Factors, San Francisco, airspace not easily used by higher speed conventional California, lanuary 1993.
aircraft and hence increase the throughput of the air traffic interesting technical discussion (Ref. 22). Although it is control environment. The low-speed kinematics associated not the objective of this paper to promote the relative with these operations dominate many of the piloting merits of each design concept, some of their unique characteristics are worthy of emphasis because they lead to issues, such as the initial deceleration procedure, the determination of the scheduled glidepath angle, the differing piloting considerations for the operation of these vehicles in instrument conditions in the terminal area.
corresponding selection of the aircraft approach configuration, the attendant safety margins, and the influence of winds and turbulence. Hence, much of this Throughout this paper there is little discussion of basic research is also generally applicable to tiltrotor and dynamic response criteria, particularly for the angular degrees-of-freedom that are important for the inner control tiltwing operations (Refs. 14-17).
loops. This is not to de-emphasize the importance of these handling qualities to the pilot, but rather is recognition of In addition, investigations focusing on IMC terminal area their already thorough treatment, exemplified in Refs. 7, operations specific to the tiltrotor and tiltwing design 8, 10, and 11. Following the approach taken in Ref. 7, for concepts have been conducted. A large moving-base simulator was used to evaluate three candidate conversion example, it is assumed that good attitude stabilization is procedures for tiltrotor aircraft executing 6 degree provided so that handling qualities in the pitch axis particularly are not a consideration.
instrument approaches (Ref. 3). A subsequent simulation evaluated various levels of control integration and flight Two aircraft, the XV-15 Tiltrotor (Ref. 23) and the CL-84 director sophistication during both constant speed and decelerating approaches on glidepaths as steep as 25 Tihwing (Ref. 24) are used to illustrate the principal degrees (Ref. 4). For the tiltwing concept, flight tests in features of each concept. The helicopter-like characteristics of the XV-15 (Fig. 1) are embodied in two features, the simulated IMC using a programmable electronic display significantly lower disc loading (Table 1), and the use of system for approach guidance were conducted (Refs. 18- longitudinal cyclic pitch. Low disc loading results in good 20). The research reported in Ref. 21, although conducted in "visual" conditions, represents a recent ground-based low-speed operating efficiencies, lower noise, lower simulation of the tiltwing concept that included downwash impingement effects, and good vertical axis investigations of decelerating and descending approaches to damping in hover and during low-speed steep approaches.
hover.
The use of cyclic pitch control introduces a rotor flapping This paper seeks to distill from this body of prior research degree-of-freedom not usually found in tiltwing designs.
Not only does this feature eliminate the need for a separate those piloting considerations deemed important in the operation of civil tiltrotor and tiltwing aircraft. In the moment-generating device for pitch control at low presentation which follows, the distinguishing airspeeds when the nacelles are rotated, it also alleviates some of the sustained pitch attitude changes that otherwise characteristics of each design that impact pilot control are would be required to orient the thrust vector.
discussed briefly. Basic procedural philosophy from transport category CTOL operations is reviewed to establish a desirable guideline for civil V/STOL Table 1. Disc loading (lb/ft 2) operations. Next, configuration management issues associated with thrust vectoring and conversion from cruise to powered-lift flight are discussed, including Titltwing Titlrotor Helicopter CL-84-1 a XV-15 b S-76B b recommendations specific to both tiltrotor and tiltwing concepts. Glidepath tracking considerations are reviewed, 41 13 7.7 ] III including comments concerning the execution of curved, aAt design max hover weight decelerating, and descending approaches. Throughout, there bAt design gross weight is discussion of flight control and cockpit display systems that must be provided to ease the piloting task. Finally, some of the piloting considerations that would be For the CL-84 Tiltwing (Fig.2), the higher disc loading involved in the event of engine failure during the steep and the fully immersed wing are mainly responsible for its approach (or go-around) are reviewed.
unique characteristics. Much higher propulsive efficiencies make the tiltwing more suitable for missions that PRINCIPAL FEATURES OF emphasize cruise performance, while at low speed, TILTROTOR AND TILTWING downwash velocities are high and vertical damping is low.
Furthermore, the high drag associated with the fully The tiltrotor and tiltwing design concepts have significant immersed and tilted wing, and the absence of any propeller differences that have long presented the opportunity for Figure 1. XV-15 Tiltrotor ............. Z : ........,, f
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........ : ............ ":'_?t'(:_ Figure 2. CL-84 Tiltwing to hover is of significance. Especially for the tiltwing, a flapping degree of freedom both serve to make pitch large increase in power is required as hover is approached.
attitude an unusually ineffective control at low speed for Associated with this change in operating points for both accomplishing speed or flightpath control. In the tiltwing, concepts, and corresponding to the change in orientation an auxiliary effector is used in the absence of propeller of the thrust vector angle from horizontal to vertical, is a cyclic for pitch attitude control at low speed.
change in pilot technique for managing airspeed and flightpath angle. Some of the pilot control and cockpit One of the major differences in the two designs is reflected display issues involved in transitioning from a in their level flight "conversion corridors", depicted in conventional "frontside" technique to a "backside" control Fig. 3. Figure 3(a) shows the relatively wide range of technique during precision instrument approaches are airspeeds available to the XV-15 pilot in level flight at described in Refs. 4, 8, 13, 15, and 26.
nacelle angles above zero (Ref. 23). In contrast, Fig. 3Co) shows that the CL-84 pilot had available only a very The flightpath angle-airspeed (T-V) trim maps described in narrow range of airspeeds at each intermediate wing angle Ref. 8 portray best the piloting technique, aircraft when constraints on comfortable pitch attitudes are taken performance, and safety margin considerations associated into account (Ref. 24). To be discussed subsequently, with the low-speed steep approach configurations. The these characteristics are the source of important procedural, 7-V map for the simulated tiltrotor aircraft of Ref. 4 in workload, and handling qualities considerations for the the approach configuration with nacelle angle 80 is shown pilot in his configuration management of the aircraft in Fig. 5(a). The vertical slopes of the constant attitude during terminal area entry, approach, and landing.
lines indicate that flightpath control about the scheduled 6 degree path, D in Fig. 5(a), can be achieved with Further information concerning the pilot control minimum crosscoupling into speed using power requirements during the conversion to powered-lift is adjustments alone while maintaining constant attitude.
revealed in the level-flight power-required curves for the The locally horizontal segments of the constant power XV-15 and CL-84 shown in Fig. 4 (Refs. 25, 24). The lines indicate that airspeed control about the scheduled progression of operating points from the frontside of the operating point can be achieved with minimum power-required curve during initial maneuvering, to the crosscoupling into flightpath by using attitude minimum drag point (typically) during steep low-speed adjustments while maintaining constant power. The descent, and then fully onto the backside for deceleration 10o _/////_/////////_. _/, ,_ 1G Infinite ,r _'l "_',',,,_ blade life limit
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" 800 (b) I I I I I I I | i i i 40o 0 40 80 120 160 200 240 Airspeed (knots) Figure 4. Level-flight power required. (a) XV-15; (b) CL-84 steep gradient between the constant attitude lines, only 1.2 relatively shallow gradient between the constant attitude kt/deg. Changes in the component of gravity along the lines indicates that pitch attitude changes would be aircraft longitudinal body axis brought about by pitching moderately effective in controlling airspeed with a are offset by the large changes in drag that result from sensitivity of about 6 kt/deg. Yet the speed-attitude only very small speed changes.
stability of the tiltrotor is strong enough that the piloting technique of maintaining a specific pitch attitude reference Pitch attitude thus cannot be used effectively as an active during approach (within 0.5 degrees for example on an method for setting or even for regulating airspeed in the expanded-scale attitude indicator) would be effective in tiltwing. Rather, airspeed is so strongly determined by maintaining the approach airspeed within a narrow range.
wing angle that pitch attitude should be considered simply A good pitch-attitude-hold stability augmentation system as a configuration setting, controlled most effectively by a (SAS) would gready facilitate this aspect of the pilot's control task.
good attitude-hold SAS. In the final analysis, speed regulation at the intermediate and higher wing angles is of In comparison, the tiltwing is characterized by such little importance anyway, since it has litde influence on aerodynamic safety margins, or on trajectory. Instead, excessive speed stability that the use of pitch attitude is wing angle and power setting strongly dominate these considered impractical as a mechanism for speed control considerations.
because excessively large attitude changes would be required. This consideration becomes of particular concern for civil operations, where pitch attitude usage for both Finally, the buffet that is characteristic of the tiltwing in trim and control should be kept within about 5 degrees of the low-speed descent configuration poses significant fuselage level. Figure 5(b) shows a y-V map design, piloting, and operational considerations, since it representative of a tiltwing with wing angle 40. Flight- presents a significant limitation on feasible descent and test data from Ref. 24 were used to plot the strikingly deceleration profiles. Figure 6 from Ref. 27, to which Pitch angle Pitch angle 0 o .,.5 o +5 0 -5 °
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-12 Limiting buffet -15 Wing angle of attack 15° (b) I -18 I 1 120 0 20 40 60 80 20 40 60 80 100 Airspeed (knots) Airspeed (knots) Figure 5. Trim conditions during steep descent. (a) Tiitrotor, nacelle 80 deg; (b) Tiltwing, wing 40 deg flight-test data provided in Refs. 24 and 28 have been frequently preceded nose-drop. Although progressively added, depicts the buffet boundaries for the CL-84 deeper penetration into buffet represented a significant prototype and a subsequent model, the CL-84-1 aircraft. disruption to the flight condition, recovery of the aircraft The buffet occurs when operating near the maximum was easily effected by adding power.
lifting conditions for the wing, and is thought to be influenced by the basic wing chord/propeller diameter The significantly different characteristics of these two ratio, the details of the wing leading edge and trailing edge aircraft designs, and their clear differences from CTOL flap schedules, the fuselage incidence angle as reflected by aircraft argue undeniably for special operating procedures.
the trim pitch angles used for approach (nosedown Yet it is important to recognize that there remain aspects attitudes were alleviating), and details in local wing of their operation that can be patterned beneficially on contours and surface condition. The reasons for the CTOL experience.
differences in the buffet characteristics between the two CTOL OPERATING GUIDELINES models were not well understood even by the aerodynamic designers (Ref. 27). Indeed, the published data appear to be somewhat inconsistent, suggesting that efforts were It might be said that there are at least two fundamental constantly underway to improve the aerodynamics differences between CTOL and V/STOL operations. The associated with the problem.
f'n'st arises from the operating environment. To facilitate the integration of V/STOL aircraft in the confined noise- Reference 28 describes a buffet encounter in the CL-84-1 sensitive route structures of busy terminal areas and to in the wing 40 configuration that represented a limiting exploit the operating potential of these aircraft, curved and flight condition: "Although the power was held constant steep flightpaths to vertiports or to designated sections of for the next 7 to 8 seconds, the indicated rate of descent did existing airports will be required. The unusual low-speed not stabilize and continued to increase (above 850 fpm) kinematics and the correspondingly greater effect of winds until buffeting and nose and wing drop occurred."
at the surface and along the approach path impact both the Relatively small low frequency pitching oscillations • Ref. 24 onset of buffet (CL-84 prototype) X Re,. 2288 :nmTtn:, b:_fd::l(©C_c4_1).1)
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_i'i _,. - CL-84 prototype 0¢ CL-84-1 onset of buffet I I I I I I I I O 10 20 30 40 50 60 70 80 90 Wlng angle (deg) Figure 6. CL-84 Tihwing buffet boundaries air traffic control requirements. This typically involves an geometry of terminal area flightpaths and the descent and initial flap setting and a speed in the vicinity of 200 kt.
climb performance of the aircraft in its low-speed high- For V/STOL aircraft, there would also be preparation for drag configuration.
initial thrust vectoring (such as wing or pylon unlock).
The second fundamental difference is associated with the 2. At a well delineated point just prior to beginning requirement to orient the thrust vector from a general descent, the approach configuration is established while in horizontal direction to a vertical direction in order to gain level flight. For a CTOL aircraft, this often involves access to the low-speed portion of the flight envelope.
several progressive flap selections, each accomplished by a This creates unique configuration management and aircraft single pilot or co-pilot action. Specific guidelines are used control problems for the pilot, and for the designer who to determine when it is appropriate to effect the next seeks to alleviate some of the lift, thrust, drag, and configuration change, such as known distance from the pitching moment effects on the pilot's behalf through final approach fix, approaching glideslope intercept, or various sophistications in flight/propulsion control crossing the outer marker. Configuration changes are integration.
designed or indeed required to be benign to the pilot's control task and to the quality of the passengers' ride. For These differences notwithstanding, there is a clear need and V/STOL aircraft, these configuration changes would good justification to strive for close similarity with the involve thrust vectoring. The final action just prior to operational procedures and flight control characteristics beginning descent (such as undercarriage selection) is often that have evolved over decades of operating CTOL aircraft one that yields the drag and thrust settings appropriate to in the civil environment. These procedures and the scheduled descent angle.
characteristics, broadly reflecting simplicity and conservatism and motivated largely by achieving 3. During descent, the pilot is actively manipulating at maximum possible safety, are often substantially different most two longitudinal controls, one to maintain or adjust than ones that may be appropriate for the military the flight reference (usually airspeed) and the other to missions with which V/STOL aircraft typically have been maintain the flightpath. Prior to landing, there may be at associated. Hence, it may be important to emphasize most one more single-action configuration change, such within the V/STOL community the sometimes differing as the selection of final landing flaps. The lateral character of civil operations as civil V/STOL designs are flightpath is maintained by actively manipulating the developed. Those operational procedures and flight control same pilot control inceptor used for active control in the considerations that follow CTOL experience and which are longitudinal axis. In normal circumstances pedal control is relevant to the theme of this paper are discussed below.
not required.
1. On arrival in the terminal area, a reasonable 4. Should an engine failure occur at any point on the maneuvering speed is established that is consistent with approach, there isatmost onesingle-action configuration 45 kt, the ballooning tendency decreased rapidly and power
change needed tocontinue toland, ortoachieve apositive
had to be added progressively. As described in Ref. 20, climb rate if thepilotelects togoaround. even though the correct coordination to maintain level flight during conversion was a demanding task, acceptable These important guidelines arereflected in the proposed levels of performance could be achieved in visual airworthiness standards for civil powered-lift aircraft conditions. However, when visual cues were limited to contained in Ref. 8. The remainder of this paper discusses only those available from the CL-84 display symbology, the terminal area operation of civil tiltrotor and tilwing the pilot workload became extremely high.
aircraft in the context of these well-established general procedures.
Similar piloting problems, described extensively in Refs.
3 and 29, were encountered during conversions in "visual" CONFIGURATION MANAGEMENT and IMC for simulated tiltrotor aircraft. Schedules ranging DURING INITIAL CONVERSION from full conversion in level flight to full conversion along the glidepath were investigated. It was determined that "instrument operations employing thrust vector Findings from Previous Tests conversion are going to have to provide some additional ......... to the pilot to achieve ratings in the In V/STOL aircraft, decelerating transitions to hover have 'satisfactory' category". In addition to the use of a three- typically been more difficult to perform than accelerating cue flight director system, consideration was given to the departures. Even so, the management of aircraft use of discrete nacelle angle detents rather than the configuration through conversion from cruise to hover did incremental nacelle-rate "beep" switch which was located not emerge as a significant problem area until flight in on the power lever. This detent concept was implemented instrument conditions was investigated (Refs. 18, 29).
Reference 20 describes some of the piloting difficulties subsequently and evaluated briefly with favorable results (Ref. 4). Not surprisingly, good attitude stabilization was encountered in the CL-84 tiltwing aircraft during hooded found beneficial in suppressing unwanted pitching upsets partial conversions from wing 0 to wing 12, and arising from aerodynamic crosscoupling effects when fhst subsequently, through wing 45 to hover. In that aircraft, tilting the nacelles.
the wing was tilted using a beep switch mounted on the top of the power lever. At wing 0, the wing-up tilt rate Indeed, there seems to be little justification in a civil was 2 deg/sec, increasing linearly from wing 0 to wing 45 V/STOL design for the pilot to exercise continuous where it was maintained at 6 deg/sec. (The wing-down tilt control over the full range of thrust vector angles, as rate was 12 deg/sec from wing 100 to wing 45, thereafter traditionally provided in the past. Instead, there seems to the rate decreased linearly to 3 deg/sec at wing 0.)
be a good foundation for implementing several discrete, Although these tilt rates at low wing angles seem modest, single-action configuration changes, each tailored to the their nearly direct equivalence with angle of attack changes inherent deceleration characteristics of the aircraft and for assured strong lift, drag, and pitching moment interactions. The effects of these interactions were the minimum crosscoupling. This tailoring would include an appropriate wing or nacelle actuation rate, as well as main causes for the slower wing tilt-rate scheduling. It is appropriate flap scheduling. If the pitching moments significant that these wing flit-rates were developed for associated with initial vectoring are strong, an visual conversions conducted close to the ground where interconnect with the moment effector should be visual cues were good.
considered to absorp them. Alternatively, the authority and off-load features of the pitch-attitude stabilization system In simulated instrument conditions, the piloting should be such that the moments can be contained.
difficulties encountered when converting from an initial Consistent with existing CTOL procedures, it is preferred wing 0, 120 kt configuration to the wing 12, 90 kt initial to implement these configuration changes as discrete approach configuration consisted of a strong vertical selections in level flight, where the operational response to initial wing incidence change, together with a significance of flightpath disturbances due to configuration strong nose-up pitching moment. The recommended changes is minimized.
technique for the CL-84 during this initial wing tilting was to reduce the power temporarily and to TIItrotor simultaneously adjust the fuselage attitude to level, a change of about 5 degrees, The CL-84 had a rather weak Shown in Fig. 7 is a possible level-flight conversion pitch SAS in this regime, so the pilot had little assistance sequence for the 40,000 pound tiltrotor aircraft simulated in resisting the nose-up trim change and in coordinating in Ref. 4. Associated with the nacelle angle changes is the required nose-down pitch change. As the conversion the automatic flap schedule tabulated in the figure. A progressed beyond wing 35, which corresponded to about ttsce_e sn;le _e_ 30 0 0 10 I:p .
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Nacelle Flap _ /-'..f.. _ " 4000- 30 40 "-" "-",.4f +10 60 4O 80 40 90 6O 3000 I , I i I I I 0 40 80 120 160 200 240 Airspeed (knots) Figure 7. Tiltrotor level-flight trim conditions 17, which reviewed many prior investigations of systems manual flap setting of 20 degrees is In'st selected by the requirements for IMC approaches in both helicopters and pilot to facilitate initial maneuvering and to reduce the V/STOL aircrafL trim pitch angle at lower airspeeds. Point C in Fig. 7 represents the nacelle angle 80 configuration that will be This depiction of the conversion trajectory as a succession used for descent. Point D in Fig. 5(a) represents the trim of quasi-steady trim conditions is an idealization, since conditions on the 6 degree glidepath chosen for this example. To arrive at this approach configuration in level power will still have to be retarded and pitch angle reduced flight with minimal power changes and with the most to counter ballooning. Nevertheless, the proposed predictable and repeatable adjustments in pitch attitude, the trajectory represents a useful goal in determining programmed flap and nacelle angles to be achieved in sequential attainment of points A, B, and C might be response to each single action configuration change. A recommended. Alternatively, it may be elected to bypass C, and transition directly from B to D upon glideslope final smaller (single-action) configuration change to intercept. In either case, the management of pitch attitude nacelle angle 90, and a final deceleration would be accomplished late in the approach in order to adjust the during this sequence and during the subsequent descent trim pitch angle to a range more appropriate for hover and includes regulation about significantly different trim subsequent vertical landing at E.
values, emphasizing the importance of pitch axis stability augmentation. In Ref. 4, an attitude-command system with the capability to "beep" the reference attitude to the The data of Fig. 7 were used to plot the conversion desired reference value was used. A three-cue flight director corridor shown in Fig. 8, bounded by trim pitch angles was also found necessary to assist the pilot in maintaining deemed in a comfortable range for civil operations. The higher speed portion of the corridor is further limited by the + 100 ft standard for altitude performance during the torque available at the lower nacelle angles. In the level-flight conversion sequence (Ref. 30). The use of attitude-command stability augmentation and flight presence of these practical constraints the conversion corridor for the simulated tiltrotor aircraft of Ref. 4 is seen director guidance is consistent with the findings of Ref.
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z I- 30 40 \ \, I .o ,o \ ,: I .o 40 \ : 90 60 A • I 90 I 60 I l '_J'_ , ' ' 40 8O 120 160- 200 24O Airspeed (knots) Figure 8. Tiltrotor conversion corridor with attitude limits to be significantly narrower than first implied by the XV- represented by the conversion characteristics shown in Fig. 3(b), the very strong pitch-heave coupling associated 15 corridor that was presented in Fig. 3(a).
with the first 10-15 degrees of wing angle change, Tlltwlng combined with the recommended procedure of simultaneously adjusting fuselage angle to level, both It should be emphasized that the extreme narrowness of argue strongly for a slowly programmed initial the tiltwing conversion corridor shown in Fig. 3(b) does configuration change to about wing 15. Other not imply any difficulty for the pilot in remaining within aerodynamic surfaces such as leading and trailing edges it. Rather, it reflects an unusually constrained relationship flaps would be scheduled automatically. Selection of this between aircraft configuration and airspeed over which the configuration change should be accomplished by a single pilot has little control other than by adjusting wing angle. pilot action, not through the incremental or sustained The utilization of wing angle during the approach and operation of a wing-tilt rate switch. Control over pitch attitude during this period might be achieved most landing, and the influence of this on height control effectively with an attitude-command system for which the requirements dominate the pilot's task.
pilot "beeps" the reference attitude down to level. A tailored pitch command implemented within the flight For the tiltwing aircraft entering the terminal area, initial procedures would involve a manual flap selection to director display would probably be helpful. Additional facilitate maneuvering down to an airspeed in the vicinity single-action selections to wing angles 25 and 40, for of 120 kt, as well as preparation for wing tilting. This example, would provide flexibility to the pilot in dealing with strong headwinds during approach while also would include unlocking the wing, engaging the drive mechanism for the tail-mounted propeller used for low- configuring the aircraft beyond the range where ballooning speed pitch control, and selecting the higher propeller rpm is most problematic. Although these sequential needed for V/STOL operation. For the tiltwing aircraft configuration changes would position the aircraft for the exist surrounding the scheduled operating point to account
steep descent portion of the approach, continuous control
for gusts and normal tracking errors. In addition, as of higher wing angles must somehow be provided in order to achieve hover. required by Ref. 8, only two controls are being actively manipulated to track the flightpath and maintain the speed reference.
STEEP DESCENT Tiltwlng Tiltrotor As readily seen from the comparitive 7-V trim maps in Having established the desired approach configuration, Fig. 5, the situation during low-speed steep descent is represented by C in Fig. 7, and just prior to capturing very different for the tiltwing. The useful speed range is descent guidance, the tiltrotor pilot reduces power and dramatically smaller, and the occurrence of buffet even at lowers the undercarriage with the objective of arriving at moderate descent angles severely limits the envelope the scheduled descent condition represented by point D in available. An approach wing angle of 40 degrees and an Fig 5(a). Many of the pilot control considerations during airspeed of about 40 kt is used as the basis for this the steep low-speed descent are evident from this figure.
discussion, since characteristics of the CL-84 in this The 6 degree descent condition selected corresponds to a configuration are amply described in the literature.
still air descent rate of 785 fpm, a suitable margin from the maximum value of 1000 fpm recommended in Refs. 6 The wing 40 configuration was selected for the CL-84 and 8, and close to the nominal 500 fpm recommended for flight investigations of Refs. 18-20, whose emphasis was low-speed aircraft in Ref. 28. The 15 degree angle-of- on IMC recovery of V/STOL aircraft to small ships. The attack line shown in Fig. 5(a) does not necessarily approach profiles consisted of initial descents on 9 or 12 represent any limiting aerodynamic phenomenom, but in degree approach paths followed by level decelerations to general, any aerodynamic limits along with the minimum hover at 100 feet. The wing 40 configuration was chosen and maximum power limits would be represented on this as the best overall compromise towards minimizing diagram. The nearly vertical constant attitude lines and the handling difficulties during final stages of the approach to locally horizontal segments of the constant power lines at hover. In strong headwind conditions, a lesser wing angle the scheduled operating point reflect little coupling was used with the objective of maintaining approach between power and speed as long as attitude is held groundspeed in the vicinity of 40 kt. Although height rate constant. This permits the pilot to track the glidepath damping was poor at these low speeds, necessitating easily using power alone, while simply maintaining a display or flight director compensation, the control level fuselage angle. A good attitude-retention SAS would effectiveness was more consistent and there was less facilitate this task, especially in the presence of any crosscoupling than at lower wing angles. The attitude transient pitching moments caused by power changes, or stabilization system was reasonably effective in assisting by atmospheric tm'bulence (Ref. 17).
the pilot in maintaining the fuselage attitude a few degrees negative during descent, a technique found effective to As concluded in Ref. 4, also corroborated by research reduce buffet. However, the crosscoupling from power or reviewed in Ref. 17, a three-cue flight director is essential wing angle changes to the pitch axis was still considered to assuring satisfactory handling qualities and performance significant and a source of difficulty (Ref. 20).
during steep apl_roaches, even when conducted at constant speed. Further, the restriction of control in the The buffet characteristics of the CL-84 were not reported longitudinal plane to the active manipulation of at most in Refs. 19 and 20 as presenting limitations or causing two inceptors, offers the potential identified in Ref. 15 for particular difficulties during the simulated IMC flying precision curved approach profiles in IMC. As approaches. This implication of relatively benign identified in Refs. 15 and 31, the additional aid needed in characteristics is offset by the potential for the much more these circumstances is an adequate means (such as a significant limitations that were described earlier. This moving-map electronic display) to assure situational characteristic of the tiltwing, barring its complete awareness during the approach procedure.
resolution in future designs, poses the difficulty that the Consistent with other recommendations set forth in Ref. 8 pilot and passengers will likely encounter buffet routinely during descent, if not on the nominal path then during for civil powered-lift operations and easily seen from the downward corrections to it. Most importantly, it 7-V map of Fig. 5(a), (1) there are available at least four represents a limiting angle-of-attack condition from which degrees of aerodynamic flightpath angle margin above and protectionmust be assured.
below the scheduled path with which to accomplish corrections, (2) level flight is easily achievable without The methodology developed in Ref. 8 for this type of any configuration change, and (3) ample safety margins limiting flight condition recognizes that angle-of-attack references. On the 9 and 15 degree glideslopes, fully excursions away from the scheduled approach condition are satisfactory pilot ratings were obtained for operations in a result of piloting actions such as corrections to calm air, and borderline satisfactory ratings were achieved glidepath, aircraft or system variabilities such as gust in moderate turbulence. (The very steep 25 degree sensitivity or the standards of guidance provided to the approaches involved high pilot workload, suggesting that pilot, and exposure to vertical gusts. Corrections to such profiles would have to be strongly justified on the glidepath are accommodated by requiting that the scheduled basis of vertiport siting requirements to receive continued consideration.) These results are consistent with the approach path be at least 4 degrees above the prohibited angle-of-attack boundary (which could be drawn on the CTOL operating guidelines; no final configuration change was required after acquiring the glideslope, and only two 7-V map of Fig. 5(b)). The location of the prohibited angle-of-attack boundary is determined by applying the longitudinal controls required active manipulation.
required vertical gust protection, or angle-of-attack margin, to the limiting angle-of-attack (buffet) condition. As seen A six degree approach initially at 80 kt and nacelle angle in the example of Fig. 5(b), there is virtually no angle-of- at 80 degrees was also investigated. Programmed attack margin available, since the limiting condition is deceleration was again 0.025g and a 200 ft breakout altitude was used. A fourth flight director cue was already coincident with the 4 degree maneuvering requirement. incorporated to prompt the pilot when he should begin beeping the nacelle angle to 90 degrees. Satisfactory pilot ratings were achieved, even in moderate turbulence.
The angle-of-attack margin that is proposed in Ref. 8 provides protection from a 20 kt vertical gust, giving the Similar to the 9 and 12 degree approaches, glidesiope same level of protection for powered-lift aircraft that is tracking performance was approximately 0.2 degree enjoyed by conventional transports. The 30 degree margin standard deviation. Pilot rating and tracking performance (at the 40 kt approach speed) required by this "equivalent data for the decelerating approaches of Ref. 4 are shown in safety" standard seems conservative, especially for the Fig. 9. Since the power trim data shown in Fig. 7 for the tiltwing with its high slipstream velocities. However, it nacelle 80 and 90 configurations indicate only small differences, it can be inferred that the small pitch attitude serves to emphasize the improvements that are required in adjustment associated with selecting nacelle 90 could be tiltwing buffet characteristics. Equally important, it points to the need to gain operating experience with this class of accommodated easily within a final single-action aircraft to provide a sound basis for the development of selection. This would be comparable to the final flap sensible airworthiness criteria. selection in a CTOL aircraft.
DECELERATION TO HOVER An additional piloting consideration that was identified during the Ref. 4 simulations was the influence of pitch attitude during deceleration on the pilot's field of view. To Operations to designated areas of existing airports might allow adequate visual reference to the landing zone and adequately require only short landings from approach vertiport environment, pitch angles within about 5 degrees conditions like those just described. However, operations of level were desired. Although this consideration depends to vertiports will require the capability for final on the particular cockpit environment, it is also considered deceleration to hover in poor visibility conditions. This reasonable for passenger comfort.
final phase was investigated for the tiltrotor during the simulations reported in Ref. 4, and for the tiltwing during TIItwlng the flight-tests reported in Refs. 18-20.
Tiltrotor Although piloting considerations in achieving the final hovering configuration are relatively minor for the tiltrotor, they dominate the tihwing deceleration. In the Programmed decelerations along the glideslope to a ten CL-84, the task in Ref. 20 consisted of beeping the wing foot hover were carried out on 9, 15, and 25 degree descent from 40 to about 86 degrees while maintaining pitch paths from initial speeds of 55, 35, and 20 kt respectively.
The aircraft was first established in the final hover attitude with the centerstick. Power was slowly increased as wing angle increased, and was modulated to maintain configuration with nacelle angle 90 degrees prior to altitude. Despite the pitch SAS that incorporated only a glidesiope intercept, and three-cue flight director guidance weak pitch attitude term, both power and wing angle was used. The programmed deceleration rate to a 10 ft changes coupled into the pitch axis, requiring the pilot to hover over the pad was 0.025g, or slightly less than 0.5 intervene to improve attitude-retention performance. The kt/sec. Breakout altitude was 200 ft, after which the benefits of improved pitch-attitude-hold characteristics in remaining deceleration was accomplished using a these circumstances were confirmed recently during combination of flight director guidance and visual 3E • Flawdata constant speed ,-, J._ • Flight director decelerating •
H f • " "
r •
= _, 11- 3 I • • oF ¢ , , , 3 6 9 12 15 20 25 Approach angle (deg) 9 _- • Flight director rating J ratings 1° F • e"Flawd._." Mean _. Flangeof i ::I: ................., , -- , 4 ..... _____ ___ .... < _2 II I i i
a
0 Ca,m Turb l ca,m Turbl Cairn Turb I Calm Turbl 6° glide slope 90 glide slope 150glide slope 25° glide slope Figure 9. Performance and pilot ratings for steep descents in the simulated Tiltrotor of Ref. 4 opportunity to optimize not only airframe and propulsion investigations conducted in a large moving-base simulator dynamics and aerodynamics, but also the pilot control (Ref. 21).
interface with the vehicle. Various forerunners of this During the CL-84 IMC flight-tests and in the simulation, technology have been evaluated both in flight and in piloted simulations (Refs. 13, 33). The concept is not only were three longitudinal controls involved in the illustrated in Fig. 10, taken from Ref. 12. Since it deceleration, they were also inappropriately available to involves modem fly-by-wire architecture, this approach the pilot. The most traditional and effective control has the added advantage of dispensing with a complex inceptor, the stick, was used only for stabilization, a task mechanical mixing box and associated control runs.
that could be accomplished wholly by an automatic system, while the two remaining active controls needed to The piloting difficulties encountered during the IMC manage the flightpath were concentrated in one inceptor, the throttle lever. Further, Ref. 32 pointed out the decelerations reported in Refs. 19 and 20 were attributed to both control and display factors. Both of the display potential confusion in the operation of these power-lever formats used were exclusively situational in nature, controls in gusty conditions near hover.
without the incorporation of dynamic compensation in Various alternatives have been proposed over the years to any of the controlled symbology elements. While both display concepts were deemed effective for providing resolve these dilemma, such as driving the wing with the deceleration guidance, both were criticized as deficient in longitudinal stick once established in the powered-lift compensating for low vertical damping during approach.
regime. However, the emerging technology of Since these early investigations, considerable flight/propulsion control integration is perhaps the most effective means for resolution, since it offers the improvements in display concepts for the shipboard Sensor compensation q Aerodynamic actuators Pilot trol commands ; command Regulator I Maneuver _ generator v I Propulsion control system management ! _C°nflg urs'lOn generator • Flight path and airspeed commands • Operational limits • Trim commands Figure 10. Integrated flight/propulsion control system structure (Ref. 12) field-of-view and orientation issues at breakout were not recovery task have been developed (Ref. 34).
addressed. Under these constraints, crabbed approaches The tiltrotor simulations and the tiltwing flight were found satisfactory, as were sideslipped approaches up evaluations both confirm the general findings of Ref. 17 to a steady-state lateral acceleration level of approximately that an integrated display format incorporating directly or 0.07g. In the tiltrotor simulation reported in Ref. 4, the implicitly groundspeed and range guidance to the hover pilots evaluated lateral cyclic trim as an alternate means point is required for decelerating approaches. The display for generating the sideforce required for sideslipped requirements may be reduced if higher levels of control approaches, finding that training in its use and the sophistication, such as velocity or acceleration command knowledge of current trim position were important systems are incorlx)rated. (It is worth pointing out that the requirements. An important additional control very high velocity-damping of the tiltwing results in consideration is the availability of adequate authorities in characteristics that are essentially velocity-command and both the yaw and lateral axes for steady-state trim, control, hold in response to wing tilting.) In any implementation, and disturbance-rejection purposes.
there is a clear need for symbology drive laws tailored to vehicle dynamics, using methods such as those described The display requirements in crosswind conditions require in Refs.15, 35, and 36.
equally important consideration. Both head-up and head- down implementations are affected by large crab angles.
Effect of Crosswinds Consistent with the findings of Ref. 17, and based on a review of recent electronic display concepts (eg. Refs. 34, An important consideration for very low-speed and 35), the display feature employed most frequently at very decelerating approaches is the effect of crosswind. For the low speed appears to be a horizontal situation format with pilot it represents perhaps the most significant velocity-vector and landing-pad representation. Other accommodation that must be made between the air and display concepts, such as the flightpath oriented concept ground reference frames, requiring the use of an additional evaluated in Ref. 38, together with new head-mounted control and creating additional display interpretation display technologies warrant further research.
requirements. Both of these tasks can increase workload ENGINE FAILURE substantially in an IMC environment, especially when occurring simultaneously with deceleration.
Aircraft control, propulsion system management, and A variable-stability helicopter was used to evaluate crab aircraft performance are the primary considerations versus sideslip during steep decelerating approaches to 25 following engine failure. The cross-shafting that is kt in crosswinds as high as 30 kt (Ref. 37). Only control incorporated in both the tiltrotor and tiltwing designs considerations during simulated IMC were investigated; assures that roll and yaw moments are suppressed more Tiltrotor and tiltwing aircraft which have been flown to than was typically the case for the powered-lift date exhibit engine out performance that is similar to configurations considered in the development of Ref. 8.
twin-engine helicopters. The operating gross weight is Consequently, the tiltrotor or tiltwing pilot, like the usually such that level flight cannot be sustained below helicopter pilot, does not have to deal with lateral- directional control transients and can instead concentrate some airspeed in the vicinity of 30 to 40 kt, even at maximum contingency power, or without exceeding on the longitudinal control task, particularly propulsion transmission limits. As an example, the engine-out climb system and flightpath management.
performance for the simulated tiltrotor aircraft of Ref. 4 is Propulsion system management following engine failure, shown in Fig. 11.
however, is different than in helicopters and more similar If operating at low altitude and at an airspeed lower than to that required in the powered-lift aircraft considered in about 40 kt at the time of engine failure, the aircraft is Ref. 8. Because of the blade-angle governing system that committed to land, or if at sufficient altitude, it can be is typically used on tiltrotor and tiltwing aircraft, the pilot accelerated to a higher airspeed to achieve sustained level (or an automatic power compensation system) must flight or climb. In the tiltrotor, the pilot may use either a effectively advance the power-demand lever in order to temporary reduction in pitch attitude or a forward nacelle make available additional power from the remaining tilt to achieve, if necessary, the required speed and thence engine(s). The reaction time in restoring approach power the sustained climb. In the tiltwing, the pilot may have to or in establishing go-around power can be a critical factor in minimizing altitude loss immediately following engine establish a specific nose-up pitch attitude and the wing failure. A limited amount of research in this area for angle may have to be reduced simultaneously to achieve the necessary steady climb gradient. Either maneuver is powered-lift STOL aircraft has been conducted (Refs. 39- severely challenging for the pilot. As indicated in Fig. 11, 41). One method for assuring that all of the remaining the pitch attitudes needed to maximize single-engine climb power is easily and immediately available to the pilot performance may vary significantly among configurations, without the requirement for an immediate action is with pointing to potential benefits that may be gained from the flight/propulsion control integration concept described in Ref. 12. An integrated flight/propulsion control system specially-programmed engine-out flight director guidance.
with these characteristics was developed and tested in a Reference 8 includes extensive discussion of both powered-lift STOL aircraft (Ref. 13). The automatic continued approach and go-around for low-speed powered- engine failure compensation feature incorporated in the V- lift aircraft with one engine inoperative. Performance 22 Tiltrotor represents a direct approach to solving this requirements as well as permitted pilot actions for problem (Ref. 42).
/3t 14" .. 4k. U. = o " ,,.,,. 60°
""
4 . _"_"_. 80 nacelle ..
_ o J'/ / -. " -6 -8 _-10 -12 < -14 -16 -16 I [ i , I I I I I I I 30 40 50 60 70 80 90 100 110 Airspeed (knots) Figure 11. One-engine-inoperative climb performance for the simulated Tiltrotor of Ref. 4 reconfiguration are proposed. Pilot or system delays in mechanization of the pilot's controls and simplifying the initiating the proper go-around action, the environmental pilot's control task. Reductions in pilot workload to be conditions, and the obstacle field of the particular landing accomplished in this manner can then lead to the benefits and take-off zone will all influence critical decision long expected from V/STOL aircraft, exploiting time and heights and required climb gradients. The fuel operating efficiencies, and improving the throughput recommendations offered in Ref. 8 and the experience to of the integrated air traffic control system.
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