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An Investigation of Large Tilt - Rotor Hover and Low Speed Handling Qualities Carlos A. Malpica Ben Lawrence William A. Decker San Jose State University Research Foundation Colin R. Theodore NASA Ames Research Center NASA Ames Research Center Moffett Field, CA Moffett Field, CA Chris L. Blanken James E. Lindsey Aeroflightdynamics Directorate (AMRDEC) Monterey Technologies U.S. Army RDECOM Monterey, CA Moffett Field, CA Abstract A piloted simulation experiment conducted on the NASA - Ames Vertical Motion Simulator evaluated the hover and low speed handling qualities of a large tilt - rotor concept , with particular emphasis on longitudinal and lateral position contr ol . Ten experimental test pilots evaluated different combinations of Attitude Command - Attitude Hold (ACAH) and Translational Rate Command (TRC) response types, n acelle conversion actuator authority limits and inceptor choices . Pilots performed evaluations in revised versions of the ADS - 33 Hover, Lateral Reposition and Depart/Abort MTEs and moderate turbulence conditions . Level 2 handling qualities ratings were primarily recorded using ACAH response type in all three of the evaluation maneuvers . The b aseline TRC conferred Level 1 handling qualities in the Hover MTE , but there was a tendency to enter into a PIO associated with n acelle actuator rate limiting when employing large, aggressive control inputs.
Interestingly, increasing rate limits also led to a red uction in the handling qualities ratings. This led to the identification of a nacelle rate to rotor longitudinal flapping coupling effect that induced undesired, pitching motions proportional to the allowable amount of nacelle rate. A modification that cou nteracted this effect significantly improved the handling qualities.
Evaluation of the different response type variants showed that inclusion of TRC response could provide Level 1 handling qualities in the Lateral Reposition maneuver by reducing coupled pitch and heave off axis responses that otherwise manifest with ACAH. Finally, evaluations in the Depart/Abort maneuver showed that uncertainty about commanded nacelle posit ion and ensuing aircraft response , when manually controlling the nacelle, demanded high levels of attention from the pilot.
Additional r equirement s to maintain pitch attitude within ± 5 deg compounded the necessary workload.
Introduction response types for handling qualities impro vements in degraded visual conditions are reported in Ref. 1. More As part of the continuing research of advanced flight control recently, advanced control modes with response types other system technologies that will enable next generation than RC and ACAH have been extensively investigated for rotorcraft and civilian air travel, the current simulation heavy - lift utility class helicopters such as the CH – 47F (Refs.
experiment evaluated the hover and low speed handling 2 and 3 ) an d CH – 53K (Ref. 4 ). These have resulted in qualities of a notional large tilt - rot or aircraft, with particular improved handling qualities and reduced pilot workload emphasis on longitudinal and lateral position control. It has without sacrificing the purported agility of RC. An early long been recognized that higher levels of control piloted simulation study of TRC for a tilt - rotor aircraft augmentation are required for conventional rotorcraft to (Ref. 5 ), conducted in the now - retired NASA - Ames Flight achieve acceptable handling qualities for nap - of - the - eart h Simulator for Advanced Aircraft (FSAA) motion platform, hover and low speed precision tasks and operations in investigated actuator authority requirements for the XV - 15 degraded visual environments. In particular, advantages of stability and control augmentation system (SCAS). The Translational Rate Command (TRC) over Rate Command study was the first to exploit the tilt - rotor ability to effect (RC) and Attitude Command - Attitude Hold (ACAH) longitudin al and lateral thrust vectoring via the nacelles and parallel lateral cyclic tilting of the rotors . Reduction of large attitude excursions made possible by maneuvering using vectored thrust was a major factor in the handling qualities Presented at the American Helicopter Society 67th Annual Forum, ratings improvements.
Virginia Beach, VA, May 3 - 5, 2011. This is a work of the U.S.
Government and is not subject to copyright protection in the U.S.
The current line of research of the handling qualities rate limits. A secondary objective of the simulation was to requirements for large rotorcraft in hover began in 2008 evaluate the handling qualiti es of the aircraft beyond hover, with piloted simulation experiments conducted on the NASA into the low speed flight regime, by assessing direct pilot Ames Research Center Vertical Motion Simulator (VMS). control of nacelles and Attitude Command - Attitude Hold This first experiment (Ref. 6) explored the control system (ACAH) control .
dynamic response requirements for stability margin and disturbance rejection bandwidth for a range of rotorcraft Approach sizes, from a utility helicopter to a large heavy - lift tilt - rotor A piloted handling qualities simulation of a large tilt - rotor (greater than 100,000 pounds gross weight). The e xperiment was conducted in the NASA - Ames Vertical Motion exposed fundamental issues related to large aircraft size, Simulator to address these questions . A n implementation of especially the large distance between the pilot station and the Translational Rate Command (TRC) in which control of the center of gravity.
nacelles was performed automatically by the flight control A second simulation experiment in 2009 (Ref. 7 ) , also system was compared t o a conventional Attitude Command - conducted on the VMS, investigated short - term angular Attitude Hold (ACAH) control . The mathematical model of response requirements to controls in hover for the NASA the aircraft, detailed in Ref. 10 , consisted of a Linear L arge C ivil T ilt - R otor 2 (LCTR2) shown in Figure 1 , and Parametric Varying (LPV) system constructed by “stitching” described in Ref. 8 . Results of this second experiment together various stability derivative - based linear models and confirmed some of the previous observations and determined thus allowing experime ntation with a linear model valid for yaw bandwidth requirements that were considerably lower a nacelle angle envelope , between 95 and 60 deg, and than those suggested by ADS - 33 (Ref. 9) metrics, which airspeed from hover to 60 kts . The flight control system was were defined for much smaller aircraft . Pitch and roll designed to allow investigation of ACAH and TRC responses were also investigated, with a primary finding that fundamental response types, as well as a “Hybrid” respon se Level 1 handling qualities could not be achieved via an type in which attitude and translational rate were attitude command control system . A major deficiency was commanded simultaneously . Independent control of roll and an objectionable pitch induced heave motion at the pilot lateral translation was achieved by combining anti - station , a direct consequence of the long fuselage of this tilt - symmetric (differential) collective and anti - symmetric rotor design. This outcome led to the key hypothesis of the (parallel) lateral cyclic rotor inputs , respectively . A current investigation : mainly that TRC, or a mix of TRC and proportional controller mounted on the Thrust Control Lever attitude command control, could achieve Level 1 handling (TCL) grip , under the pilot’s left thumb , was configured for qualities by allowing maneuvering without inducing large control of TRC implementations and compared to center attitude changes. stick control during the experiment . This inceptor was included in t he experiment as it was hypothesized that being accustomed to control of attitude through the center stick, pilots might find it counter - intuitive to control translational rate through the same control. Separation of control action between left and right hands and the differ ent nature of the controllers were considered desirable as a possible solution to overcome this ce nter stick to attitude control paradigm .
Implementation variations used in the experiment included multiple combinations of nacelle conversion angle and rate limits, as well as center stick sensitivities and control mode mixing s . Finally, manual pilot control of nacelles was done via a discrete - step and a proportio nal nacelle rate thumb inceptor . The first offer ed repeatable execution , while the second al low ed for higher rate of nacelle conversion to be commanded .
The experiment relied on three primary evaluation tasks: a precision hover task, a lateral reposition task , and a n abort ed Figure 1 . NASA Large Civil Tilt - Rotor (LCTR2) departure maneuver, all of which were modeled after revised versions of standa rd ADS - 33 Mission Task Elements . TRC Objectives was evaluated only in the hover and lateral reposition tasks .
The main objective of this study wa s to investigate the Evaluation of discrete - step and proportional nacelle rate efficacy of Translational Rate Command (TRC) on piloted thumb controllers was carried out in t he Depart/Abort handling qualities for large - sized tilt - rotors , in particular the maneuver, with the primary response type being ACAH .
handling qualitie s impact of various TRC design parameters, Experimental pilots tested the different control variants , with such as, inceptor type and control response requirements (i.e. evaluation comments and objective task performance data sensitivities), and nacelle conversion actuator position and recorded . The following section describes the experiment design and methodology in more detail, including the simulation mod el and experimental procedures.
CAMRAD Reduced order Simulation Model Aircraft Model A real - time piloted simulation model was needed to ï 20 accommodate the range of design trade - offs. The ba re - Magnitude (dB) ï 40 airframe model needed to be flexible enough to allow easy setup of different control sys tem designs, but yet accurate ï 60 enough to ensure that adequate aircraft response was achieved with a representative amount of actuator usage.
The mathematical model of the aircraft was significantly updated from that used in previous piloted flight simulatio n experiments (Refs. 6 and 7 ). A 13 - state, reduced - order, fully - coupled Linear Parametric Varying model was used to Phase (deg) simulate the bare - airframe dynamics of the aircraft for ï 180 airspeed and mast conversion angle between hover t o 60 kts and 60 to 95 degrees, res pectively. This model was ï 360 ï 1 0 1 2 composed of linear stability - derivative models obtained for 10 10 10 10 Frequency (rad/sec) trimmed flight conditions in the speed and nacelle angle range of interest. A detailed description and validation of the ( a) methods used to generate this model was reported in Ref. 10 .
The comprehensive aeromechanical rotorcraft analysis code, CAMRAD II (Refs. 1 1 and 1 2 ), was used to generate the CAMRAD high - o rder linearized systems for each nacelle angle and Reduced order airspeed datum combination . T he order of t hese linear systems was unnecessarily large for handling qualities and simple feedback control design, and therefore reduced order ï 50 models were created. The reduced - order model s retained the key rotor - body coupling s , including both the lateral and Magnitude (dB) ï 100 longitudinal rotor blade flappin g dynamics for each rotor , but dropped the high frequency rotor modes. It is sho w n in ï 150 Figure 2 that these adequately represent bare - airframe dynamics over the frequency of interest for pilot control ( i.e., 1 – 10 rad/s). Figure 2 shows the main on - axis bare - airfra me aircraft frequency responses for anti - symmetric lateral cyclic swashplate input. L ateral flapping was a key addition to the model, providing the necessary degree of freedom for control of lateral t ranslation. In previous Phase (deg) ï 180 iterations (Refs. 6 and 7 ) , where lateral translation was achieved through changes in the roll attitude, control had ï 360 ï 1 0 1 2 been limited to the differential (or anti - symmetric) collective 10 10 10 10 rotor pitch. Frequency (rad/sec) The modeling tool FLIGHTLAB (Ref. 1 3 ) was used ( b) separately to generate values for the control derivatives Figure 2 . Compa rison of frequency responses to anti - correspo nding to nacelle conversion angle, rate and symmetric lateral cyclic for the high - order acceleration perturbations of the reduced - order m o del .
(CAMRAD II) and reduced - order models: (a) lateral Figure 3 shows that the FLIGHTLAB - generated derivatives velocity and (b) roll attitu d e compare well to analytical predictions based on first principles, providing confidence in the modeling approach.
Enhancements to the first principles analytical modeling, compared to the simplified approach reported in Ref. 10 , improved the comparisons to the FLIGHTLAB – generated derivatives, especially in the phase curv e .
Flight Control System Control architecture. The flight control system design utilized th e same generic explicit model - following FLIGHTLAB Analytical architecture used in the previous experiments and sh o w n i n 40 Figure 4 . This architecture was ideal for th is series of experiments because it allowed easy and independent variation of the vehicle response to piloted inputs in each axis without affecting the feedback characteristics. The Magnitude (dB) existing Attitude Command - Attitude Hol d control system was augm ented to enable longitudinal and lateral Translational Rate response to piloted inputs by introducing velocity feedback and command paths . In addition to the primary experimental TRC and ACAH response types, the co ntrol system prov ided yaw Rate Command and vertical 180 (heave) Rate Command control response typ e s.
Phase (deg) ï 180 ï 360 ï 2 ï 1 0 1 2 10 10 10 10 10 Frequency (rad/sec) Figure 3 . Comparison of frequency responses of aircraft u velocity to nacelle angle using FLIGHTLAB and Figure 4 . Overview of the generic model - following Analytical deriva tions . Note, the convention followed control system architecture.
here is that the nacelle conversion angle is positive for a Th e implementation of TRC employed primarily later al forward rotation from the hover position.
parallel cyclic for lateral translational rate control, and In generating the bare - airframe nacelle/rotor model , inertia nacelle tilt angle for longitudinal control. L ate ral and of the nacelle component was neglected initially , such that longitudinal velocities are compared to the desired vehicle total ma ss of the nacelle/rotor system correspond ed to that of response determined by the command model, and the error is the rotor only and the center of mass wa s located at the rotor fed back through a simple Proportion al - Integral - Differential hub. I nertial properties of the rotor blade mass distribution (PID) Single - Input/Single - Output (SISO) regulator that were accounted for vi a the multibody dynamics formulation makes the necessary correct ions to the control inputs being used in FLIGHTLAB . This i s not necessarily the most estimated by the inverse plant model.
realistic assumption, but in the absence of more tangible With TRC, the ACAH control loops were still active but in design data, it represent ed a reasonably adequate starting order to ensure deck level flight conditions, zero pitch and point for flight control system design, an d handling qualities roll attitude regulation was achieved by maintaining the evaluation . A s long as nacelle to air frame moment of inertia command model inputs at zero. Automatic regulation of the ratio remains small, the assumption can be considered longitudinal cyclic and differential collective counteracted reasonable. Conservative estimates indicate that nacelle the natural tendency to pitch and roll in response to nacelle pitching moment of inertia could account for 5 – 6% of the and lateral parallel cyclic inputs , respectively . Thus, the airframe pitching moment in addition to the current 2.8% velocity and attitude loops were closed in parallel, with ratio.
neither one possessing hierarchical superiority over the other It is noted in particular, that this assumption lent itself to an (i.e., there was no inner/outer loop structure). Con sequently, unbalanced nacelle system , that is, one in which the aircraft the two sets of regulators complemented each other by center of gravity moves with the mast conversion angle.
acting on the independent control mechanisms. This was However, this was not considered to have a n oticeable effect made possible by the additional degrees of control afforded on the handling qualities for small ranges of motion . Be l o w in a tilt - rotor aircraft.
10 rad/s , the primary effect on the rigid body dynamics of The TRC command models were designed to meet the f irst the airframe from the tilting rotor/nacelle is predominantly order qualitative character (i.e., absence of objectionable an effect of the quasi - steady reorientation of the rotor thrust pith and roll oscillations, zero velocity for zero stick vectors . Estimates show that inertial effects of angular displacement, and no noticeable overshoots in the response nacelle acceleration only begin to dominate the X – force of translational rate to control inputs) and the equivalent rise component (i.e., force component along the body x axis) for time sp eci fications defined in ADS - 33. Additionally , the nacelle frequencies ov e r 10 rad/ s , which is well beyond the command models provid ed experimental control over the normal frequency range of control of the pilot .
variation in translational rate with control deflection. A first - ACAH command model gains were selected based on the order command model was used i n the lateral and results from Ref. 7 . Accordin gly, pitch and roll command longitudinal axes to achieve desired trans lational rate model feature d 1.0 rad/s natural frequencies, and 1.45 and command response ty p es. 1.0 damping ratios defined the input - output dynamics. The project pilot systematically checked this configuration − τ s cmd u v Ke cmd cmd against various natural frequency command model , = δ δ τ s + 1 configu rations and found it to be the best behaved. This was lon lat later verified by a sub - set of the evaluation pilots. A control Here δ and δ refer to the pilot inputs, either through the lon lat optimization , using CONDUIT (Ref. 14 ), was performed to center stick or the thumb stick , and v and u are the cmd cmd determine a set of feedback gains that would ensure 38 deg commanded body axes velocities. Time constants τ and τ cmd € stability phase margins in the pitch and roll axes, per the define the commanded response delay and equivalent rise findings of Ref. 6 . TRC gains were set to achieve the more times, respectively. The sensitivity gain K specifies the conventional 45 deg stability margins. This optimization control response requirements.
solution sets the values of disturbance rejection bandw i dth.
Table 1 summarize s the fundamental augmentation regulator For the ACAH control laws, second - order command models characteristics and system bandwidth for the baseline were used in the pitch and roll axes. Command model configurations.
dyn amics in all axes were set independently. This command model structure offered a convenient way of implementing Actuators. Models of actuator dynamics were necessary so “Hybrid” response types in which attitude and translational that the control system design could correctly account for rate were commanded simultaneously, allowing for the nonlinearities such as position and rate limits, and time investigation of such response types, as well as the ACAH delays. Simplified nacelle servo - actuator dynamic models and TRC fundamental response types. Issues of interest for were assumed to be second order. Ba ndwidth and damping investigating hybrid response types included: (a) effect of characteristics were selected to avoid low frequency cut - off control system implementation on rotor flapping demands, of pilot input, and to avoid natural oscillatory behavior. With and (b) mechanics for transitioning between angular a nd this in mind, 1.0 damping ratio a nd 8 rad/s natural frequency translational response types.
characterized the nacelle conversion actuator ang ular rate response dynamics. This fixed - point design was therefore After notable pitch perturbations associated with the driven by the handling qualities and flight control longitudinal flapping response of the rotors were observed , requirements while disregarding any potential structural in response to high nacelle conversion rates, crossfeed constraints at this stage. Nacelle conversion actuator, in the signals between nacelle conversion rate and longitudinal baseline TRC conf iguration, was allowed to rotate 9 deg cyclic were introduced to minimize the negative impact of forwards and backwards, from the 86 deg hover position the rotor delay. These gains effectively introduced a feed - (77 – 95 deg range) at a pea k rate of 7.5 deg/s. These rate forward lead component that eliminated a significant amount limits were based on typical maximum rates of actual tilt - of the delay.
rot o rs.
Design specifications. Quickness specifications, defined by an equivalent rise time constant, were set at 5 s for both the lateral and longitudinal rate response types. The baseline Table 1 . C ontrol augmentation characteristics for baseline configurations Disturbance Rejection Gain/Phase Stability a b Bandwi d th Margins Input/Output Bandwidth ( rad/s) ( dB)/(deg) ( rad/s)/(ms) lat lon lat lon lat lon ACAH 1.32 1.02 8.3/38.1 10.5/38.1 1.58/148 1.66 / 151 c TRC 1 .24 0.31 21.0/45.3 9.1/45.9 4.85/52 1.38/115 d TRC 1.24 0.31 21.0/45.3 11.7/53.7 4.84/49 1.84 / 108 a Defined in ADS - 33 Test Guide ( Ref. 15 ) b Based on linear analysis. ACAH values refer to the attitude response to piloted input (bandwidth taken as phase bandwidth). TRC values refer to the translational rate re sponse to piloted input (bandwidth taken as the lesser of the gain bandwidth and phase bandwi d th).
c Baseline configuration without nacelle rate to longitudinal cyclic cross feed .
d Improved configuration with nacelle rate to longitudinal cyclic crossfeed .
Table 2 . TRC center stick control experimental Turbulence m odel parameters A ircraft response to atmospheric gust disturbances was simulated by means of the Control Equivalent Turbulence Control Nacelle rate li m its Nacelle r a nge Input (CETI) model developed by the AFDD, and described sensiti v ity ( deg/s) ( deg) in Ref. 1 6 . Conceptually, the CETI model is a hover/low - ( ft/s/in) speed turbulence model that simulates the effects of 10.0 atmospheric turbulence on a conventional rotorcraft. The 14.0 CETI model was designed to provide realistic gust inputs 15.0 ±7.5 throug h the bare - airframe control inputs, i.e., the symmetric 16.0 and anti - symmetric collective and longitudinal c y clic 17.0 swas hplate inputs , in this case. C onsequently , primary 77 – 95 ±2.5 responses to turbulence are in the heave, r oll, p itch and yaw ±5.0 degrees of freedom. Longitudinal and lateral vehicle ±10.0 perturbations due to turbulence are of a secondary nature and ±12.5 15.0 are represented as a consequence of attitude changes.
±15.0 Validity of th is model for use in tilt - rotor aircraft has not yet ±5.0 been verified. This approach was, however , adjudged by the 81.5 – 91.5 ±7.5 experimental test pilots to provide a reasonable representation of aircraft m otion in a turbulent flow field and Center stick inceptor gradient and break - out force - feel was therefore adopted for use in this investigation.
characteristics were configured for ACAH at 0.9 lb/in and 1.0 lb in the longitudinal direction and 0.7 lb/in and 0.6 lb in Conduct of Test the lateral direction. Given these force - feel characteristics , center stick T RC control sensitivities were adjusted to This section describes the control system configurations provide the best - expected task performance with acceptable adopted for this experiment, the simulation facility where the forces for the required inceptor displacements . Evaluation experiment was performed (including pilot controls and maneuver constraints for the center stick implementation situational displays), and the evaluation tasks and test required that 25 ft/s be achieved as a minimum . Only procedures .
directly proportional variations in translational rate with control deflection were investig a ted ( i.e, constant sensitivity Control system configurations gains). Early exploration runs appeared to indicate a Response types. Two f undamental contro l modes were relationship between stick sensitivity and a tendency to PIO.
investigated in this experiment : ACAH and TRC control. I t was consequently decided to expand the test matrix in and The ACAH control mode provide d a baseline reference to around the baseline sensitivity design point. Thumb stick compare the new model with the findings of the previous sensitivity was set at 0.9 ft/s/deg (i.e., 0.9 ft/s would be investigations. A third, ACAH/TRC hybrid control mode in commanded for every degree of stick displacement). The the lateral axis , was configured such that center stick thumb stick had a ±25 deg range, so a 22.5 ft/s maximum displacements gr eater than one inch from center would speed could be commanded.
command roll attitude at reduced sensitivity (0.3 of the OLOP. The Open - Loop Onset Point (OLOP) design criteria baseline ACAH command model ) in addition to the normal (Ref. 1 7 ), often used in fixed wing flight control system translational lateral rate commanded. TRC control va riants design and tested for use in rotorcraft in Ref. 6 , was used in included different inceptor types and control sensitivities this case to predict the pot ential handling qualities impact (i.e., control response requirements defined by the variation associated with rate limiting of the nacelle conversion in steady state translational rates with inceptor deflection), actu a tor. Figure 5 illustrates the effect of pilot input and nacelle actuator rate and position limits.
amplitude on the OLOP criteria. Center stick maximum TRC variants. Two different types of TRC pilot inceptor displacement range was ± 5 in , allowing the handling were evaluated in this study . The primary approach used the qualities impact of piloted input frequency and amplitude to conventional center stic k controller. All of the control be freely evaluated. For compar i son, Figure 6 shows the variants configured for center stick evaluation are effect of the experimental nacelle rate limits on the OLOP summariz e d in Table 2 . An alternate thumb stick inceptor on phase and amplitude criteri a assuming a 1 in maximum th e Thrust Control Lever (TCL) grip was also configured for amplitude control input. The extra margins offered by the control of vehicle translational ra t es. larger rate limits are clearly illustrated. T humb stick incepted TRC phase and amplitude at the onset frequency for maximum inceptor displac ement were - 134.6 deg and 1.9 dB, respectively, effectively meeting Level 1 OLOP specificatio n s.
Facility As with the preceding studies , t hi s experiment was conducted in the NASA - Ames Vertical Motion Simulator (VMS) , described in Ref . 1 8 ( Figure 7 ) . The Transport Cab (T - Cab) was employed for its large field of view as se e n in Figure 8 . Traditional helicopter center stick and pedal pilot control inceptors were installed for the right cockpit seat, the evaluation pilot positi on. Th e experime ntal tilt - rotor specific vertical Thrust Control Lever (TCL) mentioned above is sho w n in Figure 9 . It was provided instead of the standard helicopter colle ctive stick. Pilots could manually adjust the friction coeff icient on the TCL to their prefer e nce. Figure 9 also shows the experimental thumb stick inceptor configured in place of the more conventional thumb wheel commonly used for control of the nacelle position . The thumb stick fundamenta lly functioned as a miniature, spring - loaded, linear, dual - axis joysti ck. After displacement of the inceptor it return s back to center. The spring constant wa s fix e d.
Figure 5 . OLOP specifications for baseline and improved TRC configurations (15 ft/s/in control sensitivity and ±7.5 deg/s rate limits) Figure 6 . OLOP specifications for varying nacelle rate limits (15 ft/s/in sensitivity, 1.0 in maximum pilot input) Nacelle inceptors. When the control system was set to ACAH, the fore/aft axis of the thumb stick commanded nacelle rate pro portional to the displacement, up to a 7.5 deg/s maximum rate. An alternative method employed a rocker switch, which would discretely advan ce the nacelle to the next stop within a sequence of predetermined angular positions summariz e d in Table 3 . N acelle rotation was commanded at a constant 2 deg/s angular rate between each s tep.
Figure 7 . NASA - Ames Vertical Motion Simulator (VMS).
Table 3 . Discrete nacelle conversion angle stops The primary flight display and the horizontal situation Conversion Discrete nacelle s t ops (hover) display, replicating the Army’s Common Avionics direction ( deg) Architecture System (CAAS) displays, were provided on the instrument panel. A nacelle position indicator showing Forward 95 86 – 75 60 current position and direction of m otion, and discrete Rearward 60 – 80 86 95 position stops was added to these disp l ays ( Figure 10 ).
Figure 8 . VMS two - seat transport cab overview .
Figure 10 . CAAS display with nacelle position indicator (top left) Evaluation tasks and procedures Rocker switch As introduced already , test configurations were evaluated by the pilots in revised versions of the ADS - 33 Hover, Lateral Reposition and Depart/Abort MTE maneuvers. TRC was evaluated only in the hover and lateral repositioning tasks.
The Depart/Abort maneuver was employed only for the evaluation of discrete - step and proportional nacelle rate TCL grip thumb inceptors, in ACAH. R efinements to the ADS - 33 Hover MTE position performance standards (Ref. 6) were necessary because cargo/utility maneuver performance metrics were found to be too “tight” and aggressive for an aircraft of this size . It was found that ±4 ft lateral - longitudinal position deviation and ±3 ft altitude deviation were more appropriate for the limits of desired task TRC thumb stick performance. Adequate position and altit ude performance inceptor limits were set at double the desired limits, i.e., ±8 ft and ±6 ft., respectively. All maneuvers were defined around a pilot eye - point altitude of 55 ft AGL . Lateral Reposition MTE revisions were a im ed at reducing the speed requirement from 35+ knots to 15+ knots groundspeed . Accordingly, c ompletion time s were re defined at 25 s for desired performance and 30 s for adequate . Revisions of the Depart/Abort MTE included a course length exten sion to 1200 ft. Additionally , performance standard s were modified to constrain pitch attitude within ±5 deg for desired performance and ±7 deg for adequate . D esired and adequate maneuver completion time s were respectively modified to 40 and 45 s .
Figure 9 . Thrust Control Lever Grip. Center rocker Ten pilots , including the project pilot, provided evaluatio ns switch controls discrete nacelle movement. The thumb during this experiment . A ll pilots had extensive rotorcraft stick is a two - axis proportional controller. Used with experience ranging from light utility single main rotor TRC the stick provides an alternative to center stick helicopters to medium and heavy lift tandem helicopters.
inputs for longitudinal and lateral speed control. In Two of the pilots were highly experienced tilt - rotor pilots.
ACAH, the thumb stick control s fore and aft nacelle rate, Importantly also, f ive pilots of the group had participated in proportional to control displacement, up to a maximum the previous experiments and were therefore familiar with of 7.5 deg / s.
the aircraft and some of the issues associated with it . This provided continuity between the series of experiments. This diverse breadth of backgrounds and control techniques provided a widely representative sampling group . A ll pilots first part shows the results for the baseline co nfigurations .
were experienced test pilots and were familiar with the use The next part shows the results for the improved version, of the Cooper - H arper Handl ing Quality Rating scale with the nacelle rate to longitudinal cyclic crossfeed (Ref. 1 9 ). included. Rounding up the results for the Hover MTE evaluations is a brief presentation of the results for the Pilots were re quired to complete initial training sessions to thumb TRC inceptor . The final two parts of the results familiarize themselves with the experiment objective s , section show the results for the evaluations of the ACAH, methodolog y , the Hover and Lateral Repos i tion MTEs and TRC and Hybrid response types in the Lateral Reposition baseline control configurations prior to the start of formal MTE, and the evaluations of the manual nacelle position evaluations. Evaluation of the Depart /Abort required that inceptors in the Depart/Abort MTE.
pilots, who did not possess formal tilt - rotor training, be fully briefed beforehand on basic tilt - rotor operations. A n Pre cision hover performance additional training session, focusing on familiarization of manual nacelle control, was performed for this purpose Ratings for the ACAH and TRC configurations in the Hover u nder project pilot instruction and supervision .
MTE are sho w n in Figure 11 . Figure 11 (a) shows slight improvements in the average ratings for the three Data recorded include d the aircraft control inputs and state configurations (4.8 for ACA H, 4.3 for the thumb stick data, task performance data, and pilot comments. A formal incepted TRC, and 4.0 for the center stick incepted TRC).
questionnaire was used to elicit structured pilot opinion Center stick configurations in Figure 11 encompass all about task aggressiveness versus performance, aircraft control sensitivity and nacelle position and rate limit characteristics, and pilot workload. The pilots used the variants, without the nacelle rate to longitudinal cyclic Cooper - Harper HQR scale to provide a qualitative crossfeed improvement. Furthermore, the minimum and evaluation of the configuration. Pilots flew each test maximum ratings for the thumb stick incepted TRC were configuration for familiarization purposes, as many times as identical to the ACAH range. The c enter stick incepted TRC required until they felt consistent performance was achieved.
conferred an even wider range of ratings. T he average A minimum of three formal evaluation runs was performed, improvement , h owever, would suggest an increased prior to collectio n of pilot comments and ratings . If pilots weigh t ing of the HQRs towards Lev e l 1.
felt a run of the three was anomalous they were free to exe cute additional runs to resolve the inconsistency . Task Figure 11 (b) shows a more detailed de scription of the HQR performance displays in the VMS control room presented allocation for the ACAH and TRC configurations in the pilot - vehicle task performance in terms of the desired and Hover MTE . The data indicate the number of ti mes (in ad equate standards for each MTE. This information was read percentage) that the different configurations were assigned a back to the pilot after eac h maneuver was completed, both given rating. Percentage values represent a normalization during training and formal evaluation.
technique, as a different number of evaluations may have been conducted for the each configuration, rather than an E valuation in the Lateral Reposition and the Depart/Abort attempt to es tablish any significant statistical comparison.
required different orientations of the cab, with the cab Therefore, ACAH, e.g., was rat ed HQR 5 about 60% of the oriented with the longitudinal axis across the beam for time. Results indicate that improvements in the handling evaluation in t he Lateral Reposition, and along the beam for qualities were possible with the TRC configurations , with the Depart/Abort. Although evaluation in the Hover MTE Level 1 handling qualities more frequently achieved , but that could be performed in either orientation, this was restricted a handling qualities cliff was exposed . This is evidenced by to the crossbeam orientation for consistency with previous the fact that a comparable number of pilots rated the experiments. Th e orientation of a particular axis along the handling qualities with TRC to be either worse or equal t o beam was selected to provide a greater range of motion those with ACAH con trol mode. Evaluations of the thumb allowing higher and more sustained accelerations to be stick incepted TRC configuration, for example, were imparted along the primary direction of the maneuver.
assigned HQR 5 – 7 scores by about 50% of the pilots. Cente r Consequently, motion cues along this axis may be stick incepted configurations, whilst heavily rated in the potentially more compelling, thus offering increased HQR 3 – 4 range, also received HQR 5 – 8 scores.
simulation fidelity.
It is noted that upon closure of the velocity feedback loops Results there was a notable reduction in the turbulence - generated motion of the aircraft. Once pilots had stabilized in the The results of the piloted evaluations, includin g HQR scores hover, workload ceased being a factor, as in opinion of the and evaluation comments, will be presented in this section.
pilots the aircraft appe ared to reject turbulence very Complementing these results will be the objective task effectively. The critical sub - phase of the maneuver, then, performance mea surements and piloted control traces.
was consistently observed to be the deceleration into the The r esults for the Hover MTE evaluations embody the hover, which appeared in some occasions to drive the aggressive pilot compensation and thus expose the handl ing majority of the results that are presented. These are presented first and are divided into several sub - sections. The qualities cliff.
Clearly, deficiencies in the TRC control system meant that it the minimum and maximum HQR scores attained.
did not confer consistent Level 1 handling qualities with Additionally, a slightly higher propensity for actuat or rate either inceptor type. Also, specific deficiencies in the limiting was observed. Pilots reported, in general, that they mechanical characteristics of the thumb controller were could not be very aggress ive with any of the configurations, fo und to prevent some pilots from modulating their input as but this was more obvious with the higher sensitivity cas e .
desired. Moreover, TRC control was found to be very sensitive to pilot aggressiveness, especially when incepted 14 (ft/s)/in through the center stick. These issues are discussed in more 15 (ft/s)/in detail in the following s ections.
80 16 (ft/s)/in Level 3 Number of ratings (%) Level 2 Level 1 Cooper-Harper Rating 1 2 3 4 5 6 7 8 Cooper-Harper Rating Figure 12 . Handling qualities ratings for varying control sensitivity configurations in the Hove r MTE ACAH TRC (thumb) TRC (center) Control Mode Effect of nacelle rate limit. HQRs for evaluations in the (a) Hover MTE of four nacelle conversion actuator rate limit s for the baseline sensitivity gain are sho w n in Figure 13 .
Error bars indicate the maximum and minimum values.
Results suggest that 10 deg/s was found consistently to be the least objectionable of the nacelle actuator rate limit configurations. Lower rate limits (5 and 7.5 deg/s) were found to be too restrictive of pilot input and resulted in PIO more frequently. The 5 deg/s rate limit in particular , was rated at least one HQR higher than the others. The 7.5 deg/s rate limit did confer the lowest rating, however.
Interestingly, the 12.5 deg/s rate limit, while expected to be less restrictive , was sometimes found to display sufficiently unsatisfactory deficiencies in the form of obtrusive pitch perturbations that Level 3 ratings were awarded .
Looking at this configu ration more closely, the control system was observed to command nacelle rates, near or at the 12.5 deg/s limits, but frequently without rate limiting.
(b) Pilot comments for this configuration consistently mentioned the presence of a notable pitch oscillation Figure 11 . H andling qualities ratings for the three accompanied by what was described as an unsettling primary response type control configu rations in the heaving motion. While this oscillation was described as Hove r MTE annoying, or bothersome, it did not appear to compromise Effect of control sensitivity. The baseline 15 ( ft/s ) /in gain the ability to meet the desired performance standards. A few marked the upper control sensitivity limit for the giv en evaluation comments hinted to a quick pitch reversal in nacelle actuator position and rate limits . Despite the response to rapid input, and more interestingly, indicated apparently small difference between the ga ins, t he higher that this pitch motion could be cueing them on to a false sensitivity gain, 16 ft/s/in, displayed a higher preponderance sense of aircraft response because the pitch response was of HQR 4 ratin g s in Figure 12 , whereas the lower opposite to the expected response pilot control input (e .g., sensitivities , including the baseline, were rated HQR 3 , and nose up pitch for a forward stick displacement). It was 2, more frequently . This trend is emulated by a clear shift in purported that this opposite pitch response to pilot input may have been falsely cueing the pilots into overcorrecting after an initial input. Additionally, this pitch oscillation appeared to aff ect the altitude maintenance due to the presence of an obvious heave percepti o n.
(a) Level 3 Figure 13 . Minimum and maximum h andling qualities ratings fo r varying nacelle actuator rate limits (baseline Level 2 sensitivity and position limits) in the Hove r MTE Effect of cros sfeed . The control system was modified in Level 1 order to minimize the pitching response associated with Cooper-Harper Rating nacelle conversion rate. This consisted of a cross - feed gain 2 between nacelle angular rate and longitudinal cyclic pitch input. This modified control law was evaluated by a subset of the experiment test pilots.
ACAH TRC Baseline TRC Improved Results for several stick sensi tivities (steady state velocity Control Mode commanded) using the 7.5 deg/s rate limit for the TRC are (b) sho w n in Figure 14 , highlighting improvement of the handling qualities to Level 1 conferred by the crossfeed.
Figure 14 . Comparis on of the handling qualities ratings Thse improvements were found to confer a reduced for the Improved TRC configuration, relative to the sensitivity to pilot aggression level and varying technique, Baseline and the ACAH modes in the Hover MTE virtually eliminating the PIO tendency Pilot cutoff frequency, determined from the spectral analysis Lawrence et al. in Ref. 20 present an in - depth analysis of the of the inceptor position time histories — during the 30 second flight dynamics aspects of these control system and actuator precision hover hold subtask — is a n approximate measure of configurations from the experiment. The influence of rotor pilot operating frequency, and considered a good estimate of longitudinal flapping dynamics, nacelle actuator limits and the pilot crossover frequency f or pilot - in - the - loop tasks piloted input amplitude on t he system bandwidth of the (Ref. 21 ). Additionally, the root mean square (RMS) of the longitudinal translational rate response were thoroughly piloted i nputs is a statistical measure of the magnitude in the analy z ed. The analysis clearly showed that the improved maneuver . A strong correlation between t he ha ndling TRC (with nacelle rate to longitudinal cyclic crossfeed) not qualities and piloted input frequency and magnitude data can only reduced the pitching response to almost zero, but also be seen in the contour pl o t of Figure 15 , where the Cooper - significantly improved longitudinal velocity bandwidth Harper ratings are seen to increase dramatically with the characteristics. It was found that the key effect of this amplitude of pilot control inputs, as indicated by the RMS of cro s sfeed gain was to reduce the tendency for the rotor the longitudinal control input time histories. Increasing pilot flapping to lag behind the nacelle rotations. The net effect input amplitude with the baseline control law caused greater was that by keeping t he rotor tip - path - planes perpendicular nacelle rate and position limiting and a reduction in the to the shaft axes, the lagging or retarding effect on the bandwidth of the longitudinal velocity response to stick velocity respons e caused by both the rotor flap back, and input , and thus caused a worsening situation and increased subsequent pitching motions, were minimized.
PIO tendency. Without the crossfeed , longitudinal inputs with an RMS greater than 1 . 0 in , especially for hi gher cut - Some of the difficulties with the thumb stick control were off frequencies , were correlated with ratings in the HQR 5 – 8 related to the inherent dynamic s of the nacelle - controlled range. When the crossfeed is included, there is a noticeable longitudinal response that affected the center stick reduction in the range of cut - off frequencies and RMS configurations. Many of the same issues with longitudinal amplitudes employed by the pilots, along with th e ensuing axis over - control were manifest throughout the experiment.
reduction i n the HQRs . This suggests that aggressive pilot However, the major contributing factor to the Level 3 rati ngs control compensation was not required to achieve the was related to the stick mechanics. One frequent observation desired task performance. by the evaluation pilots was that the location of the controller, on the TCL, could complicate the task of Overall, results need to be tempered somewhat by the fact simultaneously controlling altitude and translational rates.
that the improved TRC control law was evaluted mostly with This in itself did not render the control system the cab oriented along the beam, rather than across the beam, unsatisfactory, as large TCL adjustments were not needed.
which was the nominal configuration for the Hover MTE Consequently, some of the pilots rated it as Level 1 despite evaluations. Longitudinal motio n cues were significantly this deficiency. It appears most likely, based on pilot reports, higher in this alternate orientation, due to the larger range of that the very different mechanical “feel” char acteristics and motion available. Insufficient data was recorded in the two scale of input size of the inceptor made it difficult for the cab orientations, making it impossible to discern if handling pilots to precisely and harmonically modulate their inputs.
qualities improvements were due entirely as a consequence of the control system modification, or merely by the fact that Due to the first order nature of the TRC response type motion cues in the longitudinal axis were more compelling definitions and the command model implementation, if the in this o rientation. pilot inceptor is returned to center from a non - zero position, commanded rates undergo an exponential decay. Some amount of opposite input was required in order to arrest the Thumb stick inceptor evaluation translational rate more aggressively.
Precise hover performance of TRC control with the TCL - Pilots unanimously felt that the critical sub - pha se of the mo unted thumb stick showed mixed results, as evidenced by the spread in the HQRs previously sho w n in Figure 11 . The maneuver driving the ratings was the deceleration into the hover area. Pilots who were able to find a control technique handling qualities ratings ranged from 3 through 7, with the average value at 4.2. About 50% of the pilots rated the that allowed them to achieve the desired times and precision readily rated the aircraft as Level 1. The key to this was the aircraft co ntrol configuration at Level 1 handling qualities (HQR 3). The remaining 50%, however, rated it very poorly ab ility to successfully cancel out the run in lateral and longitudinal rates simultaneously with one single diagonal (HQR 5 – 7). The implication from the latter is that only half the pilots were able to achieve adequate performance opposite input.
standards. Overall, about half of the pilots described the Careful modulation of the input was essential for desired controller was intuitive ; the remainder felt it was abnormal , performance to be achieved. However, uncertainty in the and the handling qualities ratings effectively reflected this amount of i nput required to smoothly decelerate the aircraft divided opinion.
within parameters affected the performance times.
Baseline Improved 8 8 Level 3 7 7 6 6 Level 2 HQR 5 5 4 4 off frequency (rad/s) ï Cut 3 3 Level 1 2 2 0 1 2 3 4 0 1 2 3 4 RMS magnitude (inch) RMS magnitude (inch) Figure 15 Contour plot of handling quality ratings for varying pilot longitudinal stick input cut - off frequency a nd RMS amplitude for stabilized hover phase of Hover MTE – comparison of baseline and improved TRC Difficulty in modulating input with the thumb stick led a TRC and the ACAH response types in terms of their majority of pilots to employ it as a beep type controller — handling qualities (most configurations were rated HQR 3 ).
making corrections by introducing rapid pulse - type inputs in the desired , orthogonal, direction s — especially during the deceleration and hover position maintenance phases of the maneuver . Evidence of this is reflected in the relatively high cut - off frequencies and the low RMS amplitudes of the control input time history traces for most pilots. This control technique led to a strategy where, in the wor ds of an evaluation pilot, the pilot w as almost “ assisting ” the aircraft, not “ controlling ” i t .
5.0 HQR 7 HQR 3 HQR 5-7 HQR 6 4.0 3.0 2.0 Pilot H Figure 17 . Comparison of the han dling qualities ratings for the ACAH, TRC and Hybrid response type control Pilot D 1.0 HQR 5 configurations in the Lateral Reposition.
Thumb-stick RMS Amplitude (deg) Experimental observations suggested that the high bank 0.0 angles associated with the ACAH were a major cause of 0 5 10 15 20 performance degradation. T he m aneuver required high Cut-off frequency (rad/s) amplitude attitu de s, which appeared to cause a sequence of events that significantly increased the pilot work l oad . Figure Figure 16 . Correlation between handling qualities ratings 18 illustrates the typical differences in performance and and average piloted longitudinal thumb - stick control compensation between the thre e configurations for one pilot .
RMS amplitudes and cut - off frequencies.
In all three cases the pilot was able to achieve the required 25 ft/s speed, but, as shown, differences in the bank angle Figure 16 provides , in conjunction with the pilot reports, a attained were quite significant. ACAH typically required correlation between pilot control input activity and handling about 7 – 10 deg to sustain the lateral velocit y. The Hybrid qualities. Generally, higher control amplitudes and cut - off mode re quired only about 4 deg angles. Moreover, in ACAH frequencies with the thumb stick correlated with the worse a 20 – 25 deg bank angle reversal was required to arrest the handling qualities ratings. Pilot s H and D were the lateral velocities and return to a hover. TRC, on the other exceptions as they both reported they had purposely stayed hand, conferred the desired lateral velocity with minimal roll out of the loop and accepted only adequate performance in att itude generated. Desired performance in terms of the order to avoid exciting any objectionable dynamics in the times, altitude and longitudinal position were achieved in all aircraft. This is reflect ed in the re duced longitudinal stick three control configurations , but with varying levels of input RMS and cut - off frequencies. Also, the HQR 5 scores workload . ACAH and Hybrid configurations displayed, substantiate the inability to achieve desired performance.
however, a tendency to drift aft, while the TRC conferred t he ability to translate laterally with minimal longitudinal High speed lateral maneuvering and alti tude deviations. Also sho w n in Figure 18 is a notable The baseline TRC configuration with 15 ft/s/in sensitivity difference in the TCL stick control input activity between and 7.5 deg/s nacelle rate limit was evaluated and compared the three configurat ions, an indicator of differences in the against the ACAH and hybrid configurations in the Lateral workload in the heave axis. Less significant, is the larger Reposition MTE . As sho w n in Figure 17 , this configuration size of lateral input required with the TRC and Hybrid was consistently rated HQR 2 – 3. In contrast, ACAH was configurations, compared to ACAH. Pilots often mentioned mostly rated HQR 4 (wit h a few HQR 3 and one HQR 6).
that stick forces generated with these conf igurations were a Finally, t he H ybrid mode clearly fell in between the pure little hig h .
5.0 1.0 2.5 0.5 (inch) 0 0 (inch) lat TCL b ACAH b ï 2.5 ï 0.5 TRC Hybrid ï 5.0 ï 1.0 0 5 10 15 20 25 30 35 0 5 10 15 20 25 30 35 Time (s) Time (s) 20 80 Adequate Adequate Adequate Desired Desired Desired 10 60 0 40 Altitude (ft) ï 10 20 Bank angle (deg) ï 20 0 0 5 10 15 20 25 30 35 0 50 100 150 200 250 300 350 400 450 Time (s) Lateral position (ft) 30 40 Adequate Adequate Adequate 10 20 Desired Desired Desired ï 10 0 ï 20 Lateral rate (ft/s) ï 30 ï 20 Longitudinal position (ft) 0 50 100 150 200 250 300 350 400 450 0 5 10 15 20 25 30 35 Lateral position (ft) Time (s) Figure 18 . Time histories from VMS piloted simulations comparing Lateral Reposition MTE flown with ACAH, TRC and Hybrid control laws.
ACAH. A survey of the pilot evalua tion comments indicated This ballooning can be explained through the w ay the heave that high workload in the longitudinal axis and altitude control axis was implemented. When decelerating maintenance, both of which we re not in the main axis of aggressively the control system would see the sudden interest, were contributing reasons for the elevated handling change in the body z - axis velocity component as an un - qualities ratings assigned to the ACAH mode. The commanded sink rate. Differences in the commanded and simultan eous workload increase in these two axes was no t actual rates, if not adjusted quic kly by the pilot would result surprising considering the perceived coupling between heave in the control system increasing power rather suddenly. This, and pitch associated with the large offset between pilot in conjunction with the sudden change in aerodynamic angle station and aircraft CG. Also, a frequent observation pointed of attack naturally caused the aircraft to climb.
to a lack of usable visual cues when executing the maneuver, Pilots consistently indicated in their evaluations, tha t the especially during the deceleration, since pilots tended to los e major source of workload in ACAH was motivated by the sight of the longitudinal position cuing when commanding degree of compensation in the longitudinal axis necessary to the large bank angles required to arr est the lateral rates at the regulate fore and aft drift. A slight tendency for the aircraft hover point . Additionally, a tendency to pitch up could cause to pitch up and drift aft during the sustained banking was the pilots to lose the lateral position cue.
observed in a number of runs. It should be noted that the Evaluation pilots identified two specific issues with ACAH heave and longitudinal axes are naturally coupled, with control: a) a tendency to “balloon”, or gain altitude easily , thrust increments producing a slight nose down pitch during the deceleration phase of the maneuver, and b) response. While it may be possible, due to the natural cross - signifi cant control activity in the longitudinal axis to couplings of the aircraft, that slowly d eveloping drift could maintain position within the desired parame t ers. Figure 19 be generated in response to the large and sustained bank illustrates some of these issues for two different pilots. Not angles being commanded, these were in themselves not evident in the time histories is the fact that the pilots have considered sufficient to degrade position performance out of reached the final position by about 25 s. Deceleration takes the desired parameters. It is not even clearly understood if place between 20 and 25 s. This is when the highest control pi lots could, or did, cue on to these mid - to long - term activity takes place. responses, especially in light of the reported deficienc ies in the visual cueing. These issues were only mentioned by two of the ten evaluation pilot s .
5.0 5.0 1.0 2.5 2.5 0.5 (inch) 0 0 0 (inch) (inch) lat lon TCL b b b ï 2.5 ï 2.5 ï 0.5 ï 5.0 ï 5.0 ï 1.0 0 10 20 30 40 0 10 20 30 40 0 10 20 30 40 Time (s) Time (s) Time (s) 20 4 2 10 2 1 0 0 0 ï 10 ï 2 ï 1 Bank angle (deg) Pitch angle (deg) Heave rate error (ft/s) ï 20 ï 4 ï 2 0 10 20 30 40 0 10 20 30 40 0 10 20 30 40 Time (s) Time (s) Time (s) 40 30 80 Adequate Adequate Adequate Adequate Desired Desired Desired Desired 0 40 0 ï 10 Altitude (ft) ï 20 Lateral rate (ft/s) ï 20 ï 30 0 Longitudinal position (ft) 0 10 20 30 40 0 10 20 30 40 0 10 20 30 40 Time (s) Time (s) Time (s) Figure 19 . Time histories from VMS piloted simulations comparing Lateral Reposition MTE flown by two different pilots with ACAH control laws.
Careful examination of the lateral reposition maneuver time desired trajectory with great ease. Any unintended histories did not uncover any conclusive eviden ce of excitations of the longitudinal axis would sometimes result significant pitch being induced by any off - axis aircraft in the al ready documented issues related to the nacelle coupling/response. Rather, longitudinal position excursions dynamics, especially in the final hover position capture.
generally appeared to be pilot created. Whether intentional TRC typically made a significant difference by eliminating or not, piloted inputs were frequently introduced at the same the bank angle from the equation. Without the large bank time as lat eral inputs were commanded. Once disturbed, it angles pilots did not lose sight of the runway. Also the generally required significant compensation by the pilot to aircraft did not experience significant changes in altitude, stay within the desired task parameters due to the ensuing such that power/altitude maintenance did not require longitudinal fore or aft motion. This issue was exacerbated attention. Elimination of pitch also minimized the during the deceleration becaus e pilots would typically lose compelling pitch/heave perception issues associated with sight of the runway as soon as the high opposite bank angle cockpit to C G offset. Pilots did describe the lack of bank as was commanded. At this precise moment pilots were also odd , but agreed it was possibly the right w ay to fly this type more prone to introducing longitudinal inputs while of aircraft, an d the HQRs confirmed their preference and simultaneously ballooning. Also complicating matters would better performance be the well - documented pitch/heave perception coupling Hybrid. Not surprisingly, by introducing some amount of produced by the long offset between the cockpit and the banking thr ough the Hybrid control mode some of the aircraft center of gravity. All of these events happening objections already discussed reappear, but in a more toned simultaneously combined to quickly drive the workload to down form. The Hybrid mode was not without its own faults considerable levels.
however, as issues with the phasing in and out of the bank TRC. Tran slational Rate Command reduced the task angle felt somewhat unpredictable to tw o of the evaluation workload by almost making this a pure one - axis task. Pilots pilots. Longitudinal position control in the Hybrid mode were able to accomplish the maneuver with a lateral input to was , however, achieved primarily by actuation of the initiate and a carefully modulated opposite input to capture nacelles, as with the TRC mode . Therefore the ability of the the final hover spot, with minimal compensation required. pilot to control the longitudinal axis would be reduced by the As long as longitudinal motions were not inadvertently de ficiencies associated with nacelle actuation , which has introduced, the aircraft was able to translate along the been documented above.
Evaluation of manual nacelle control pitch axis occurs at the very end of the maneuver when trying to precisely capture the final hover point.
The Cooper - Harper handling q ualities ratings for the Depart/Abort evaluations with the revised MTE parameters One aspect universally preferred b y the pilots was the are present e d in Figure 20 . The m ajority of ratings for both deceleration rate afforded by the rotors in full aft position, configurations were Level 2 , reflecting that, in general, the and the ability to move the nacelles quickly to that position majority of evaluation pilots had difficulty achieving the with the maximum rate attainable with the proportional desired performance . The key objection was a general controller (i.e., 7.5 deg/s). Pilots were able to arres t the unpredictability of the aircraft response to nacelle input. One required speed (67.5 ft/s) within 600 ft in about 15 seconds.
aspect of this was the slowness of response to piloted input.
In order to achieve desired performance with in the pitch P osition control with the discrete nacelle switch allowed , in attitude constraints, pilots were forced to simultaneously the opinion of evaluation pilots, fo r a slight reduction in coordinate nacelle position and longitudinal cyclic . This workload, which is reflec ted in the higher percentage of multi - axis control strategy resulted in a high mental Level 1 ratings (33% compared to only 11%). Comments workload environment, with pilots having difficulty judging from the evaluation pilots indicated that the proportional when to make the corr ect actions due to a general controller generally did not allow easy recovery of an unpredictability of the nacelle position control. Whether due adequate nacelle position for hover , within the allotted to the primary experience - base being in h elicopters, or not, distance parameters. In general, the discrete controller pilots confirmed that interpreting the response of the aircraft offered better awareness, but could also result in erroneous to a given nacelle position was not intuitive, or natural.
commands. It is noted that the 2 deg/s conversion rate Uncertainty in the nacelle position led to breakdowns in the allowed by the discrete - step switc h was too slow for the scan patterns in order to check the head down indica tor general ex ecution of the maneuver, and was employed only dis p lay ( Figure 10 ) . Pitch control through the cyclic, on the for the final capture of the hover configurati o n.
other hand, felt very natural. Furthermore, it was easy, while focusing on the longitudinal axis , to inadvertently introduce lateral cyclic inputs, which would degra de the ability to perform within the desired standards. The added attention required for control of the lateral position forced , in the opinion of some evaluation pilots, the workload to be considerable.
Summary and Discussion A piloted simulation conducte d on the NASA - Ames Vertical Motion Simulator (VMS) investigated hover and low speed handling qualities of a large tilt - rotor aircraft, with a particular emphasis on longitudinal and lateral position control with minimal attitude change. A closed - loop fligh t control system design implementing Translational Rate Command (TRC) with minimal attitude change on a large (heavy - lift) tilt - rotor by exploiting its thrust vectoring capabilities was demonstrated. Independent control of Figure 20 . Pilot handling qualities ratings in the attitude and velocity was achieve d by directly controlling the Depart/Abort MTE.
nacelles for longitudinal control, and by combining anti - Figure 21 shows typical time histories of the Depart/Abort symmetric (or parallel) lateral rotor cyclic and anti - symmetric (or differential) rotor collective between the two MTE maneuver for the nacelle discrete - step and proportional rate inceptors. Results show that performance with both rotors. Ten experimental test pilots evaluated on aver age fourteen different flight control system configurations while inceptors was very similar. The one subtle difference is illustrated by the final nace lle conversion angle, β , after flying three different revised versions of the ADS - 33 m Mission Task Element (MTE) maneuvers (Hover, Lateral about 33 seconds. Here the discrete - step switch allowed Reposition and Depart/Abort) throughout the experiment.
reconfiguration back to hover with one command of the Alternative implementation s of TRC (sensitivities, nacelle switch, whereas the pilot had to “hunt” around with the angular rate and position limits, and rotor flapping lag proportional rate inceptor. This resulted in the aircraft delays) and how it is incepted (center stick vs. thumb pitching up briefly into the adequate performance region.
control) were investigate d .
The data also show ho the highest control activity in the 100 5.0 5.0 2.5 2.5 0 (inch) 0 (deg) (inch) 70 m lat lon ` b b Proportional ï 2.5 ï 2.5 Discrete 50 ï 5.0 ï 5.0 0 10 20 30 40 50 0 10 20 30 40 50 0 10 20 30 40 50 Time (s) Time (s) Time (s) 80 10 30 Desired Desired Adequate Adequate Adequate Adequate Desired Desired Desired Desired 0 0 ï 10 ï 5 ï 20 Pitch angle (deg) Lateral position (ft) Longitudinal rate (ft/s) ï 20 ï 10 ï 30 0 10 20 30 40 50 0 10 20 30 40 50 0 10 20 30 40 50 Time (s) Time (s) Time (s) 80 10 30 Desired Desired Adequate Adequate Adequate Adequate Desired Desired Desired Desired 0 0 ï 10 ï 5 ï 20 Pitch angle (deg) Lateral position (ft) Longitudinal rate (ft/s) ï 20 ï 10 ï 30 0 200 400 600 800 1000 1200 0 200 400 600 800 1000 1200 0 200 400 600 800 1000 1200 Longitudinal position (ft) Longitudinal position (ft) Longitudinal position (ft) Figure 21 . Time histories from VMS pilot ed simulations comparing Depart/Abort MTE flown with nacelle discrete - step and proportional rate inceptor Hover and low speed control without attitude change, as precision . This nacelle activity resulted in rate limiting of the with this TRC design, was found to be very effective at modeled actuators, and an ensuing PIO in the longitudinal removing objectionable pitch to heave coupling of large, control axis.
long tilt - rotor aircraft designs, and resulted in Level 1 The critical sub - phase of the Hover MTE maneuver was handling qualities.
found to be the deceleration segment. Deceler ation times Rotor tendency to flap back in response to nacelle rate were the only task performance parameters of the Hover induced a noticeable opposite pitch response, which some MTE left unrevised. Based on the observed sensitivity that pilots perceived to be counterintuitive. Aircraft hover aggressive decelerations could have o n the HQRs, i.e. if the handling qualities were improved by minimization of the pilots decelerated rapidly or they approached at a slightly longitudinal flapping in respon se to nacelle motion through quicker s peed before decelerating (Ref. 20 ), the respective the use of a crossfeed gain between nacelle rate and Cargo/Utility class desired and adequate task performance longitudinal cyclic . With this improvement, the TRC deceleration time requirements of 5 and 8 seconds may have architecture using automatic nacelle angle deflections with been too “tight”. Based on this, future investigations warrant rate limits of ±7.5 deg/s was shown to be a viable method of further consideration of this aspect to determine whether the p roviding precise longitudinal and lateral position control in current Cargo/Utility deceleration times of the Hover MTE hover and low speed whilst minimizing attitude changes. are appropriate to LCTR - sized classes of rotorcraft and their anticipated role.
Without this crossfeed , h over handling qualities ratings with the baseline control law were extremely susceptible to pilot Pilots were readily, and easily, able to adapt to a control control techniq ue, which is indicative of a handling qualities response type outside of the normal paradigm of rotorcraft cliff . Aggressive longitudinal position control with this flight control, which assumes attitude as the primary particular implementation of a closed - loop TRC control response to center stick input . Although pilot comments system demanded high rate nacelle rotations, with 7.5 – indicated that lack of attitude response felt “unnatural”, 12.5 deg/ s , or higher, typically required to achieve desired control performance in TRC was not compromised by use of the center stick . Furthermore, while the thumb stick Reference s controller proved to be adequate for the tasks at hand, [1] Blanken, C. L., Hart, D. C. and Hoh, R. H., position control through the center stick was generally “Helicopter Control Response Types for Hover and preferable.
Low - Speed Near - Earth Tasks in Degraded Visual Translational Rate Command was very effective at allowing Conditions,” presented at the American Helicopter lateral trans lations to be conducted with minimal off - axis Society 47th Annual Forum, Phoenix, AZ, May departures, effectively reducing the execution of the Lateral 1 991.
Reposition maneuver to a single axis task. This improvement [2] Einthoven, P. G., Miller, D., Irwin, J., McCurdy, provided the necessary reduction in workload warranting the B., Bender, J. , and Blanken, C. L., “Development Level 1 handli ng qualities r atings conferred.
of Control Laws for the Chinook Digital AFSC Attitude Command - Attitude Hold aircraft response type, in Program,” presented at the American Helicopter conjunction with two pilot nacelle position controllers, was Society 62nd Annual Forum, Phoenix, AZ, May evaluated for control of a large tilt - rotor aircraft in an 2006.
aborted departure . Difficulty in re - establish ing trimmed [3] Ir win, Josep h G., et. al., “ADS - 33E Predicted and hover within desired task parameters resulted in Level 2 Assigned Low - speed Handling Qualities of the CH - handling qualities ratings. Compared to a nacelle 47F with Digital AFCS”, American Helicopter proportional thumb controller alone, a discrete nacelle Society 63rd Annual Forum, Virginia Beach, VA, position controller was demonstrated to be a useful workload May 1 - 3, 2007.
reducer . This was achie ved by assisting the pilot in setting the nacelles back to the trim hover position . However , it was [ 4] Sahasrabudhe, V., Faynberg, A., Kubik, S., also found to be susceptible to occasional, inadvertent input Tonel lo, O., Xin, H., Engel, D., and Renfrow, J., by the pilot leading to conversion into a non - hover nacelle “CH - 53K Control Laws: An Overview and Some position. Analytical Results,” presented at the American th Helicopter Society 66 Annual Forum, Phoenix, AZ, May 11 - 13, 2010.
Conclusions [5] Brigadier, W. L., “Analysis of Control Actuator Based on a t horough review of the pilot evaluation A uthority Requirements for Attitude and comments and objective task performance data , and in light Translational Rate Command Augmentation of the discussion presented above, several conclusions are Systems for the XV - 15 Tilt Rotor Research established : Aircraft,” NASA TM 81243, December 1980.
• TRC using a form of longitudinal and lateral thrust [6] Blanken, C. L., Lusardi, J. A., Ivler, C. M., vectoring using nacelle tilt and p arallel lateral Tischler, M. B., Decker, W. A., Malpica, C. A., cyclic was shown to be a viable method of Berger, T., Tucker, G. E., Höfinger, M. T., “An providing precise position control in hover and low Investigation of Rotorcraft Stability – Phase Margin speed, as evaluated in revised Hover and Lateral Requirements in Hover,” American Helicopter Repos i tion MTEs.
th Society 65 Annual Forum, Grapevine, TX, May • The baseline TRC control law provided handling 27 - 29, 2009 .
qualities improvements compared t o ACAH, but encountered a tendency to PIO in the longitudinal [7] Malpica, C. A., Decker, W. A., Theodore, C. , axis in some circumstances .
Blanken, C. L., and Berger, T., “An Investigation of • An improved TRC with nacelle rate to longitudinal Large Tilt - Rotor Short - term Attitude Response cyclic crossfeed conferred Level 1 handling Handling Qualities Requirements in Hover.” th qualities by reduc ing the pitching response to presented at the American Helicopter Society 66 almost zero and improv ing the longitudinal rate Annual Forum, Phoenix, AZ, May 11 - 13, 2010.
system bandwidth.
[8] Acree , Jr., C. W., Yeo, H., and Sinsay, J., • Mechanisms for manual nacelle control were Performance Optimization of the NASA Large shown to be adequate, with Level 2 handling Civil Tiltrotor, NASA/TM - 2008 - 215359, June qualities attained in the Depart/Abort MTE.
2008.
Acknowledgements [9] Anon., "Handling Qualities Requirements for Military Rotorcraft", Aeronautical Design The authors would like to acknowledge Emily Lewis and the Standard - 33 (ADS - 33E - PRF), US Army Aviation rest of the NASA - Ames Vertical Motion Simulator staff for and Missile Command, March 21, 2000.
their dedicated and timely support . Participation of industry evaluation pilots was made possible thanks to the support [10] Lawrence , B., Malpica, C. A., and Theodore, C., provided by Dr. Micha e l J. Rutkowski, Director of the “The Development of a Large Civil Tilt - rotor National Rotorcraft Technology C enter. Simulation for Hover and Low - Speed H andling th Qualities Investigations,” presented at the 36 Methods and Flight Test Examples," AIAA, August European Rotorcraft Forum, Paris, Fr a nce, 2006.
September 7 - 9, 2010.
[11] Johnson, W., “CAMRAD II, Comprehensive Analytical Model of Rotorcraft Aerodynamics and Dynamics,” Johnson Aeronautics, Palo Alto, California, 1992 - 2009.
[12] Johnson, W., “Technology Drivers in the Developm ent of CAMRAD II,” American Helicopter Society, Aeromechanics Specialists meeting, San Francisco, January 1994 .
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