An Investigation of Large Tilt-Rotor Short-Term Attitude Response Handling Qualities Requirements in Hover

An Investigation of Large Tilt-Rotor Short-term Attitude Response

Handling Qualities Requirements in Hover

Carlos A. Malpica William A. Decker Colin Theodore NASA Ames Research Center Moffett Field, CA Chris Blanken Aeroflightdynamics Directorate (AMRDEC) U.S. Army RDECOM Moffett Field, CA Tom Berger UC Santa Cruz (UARC) Moffett Field, CA

Introduction

The development of both civilian and military rotorcraft typically involves meeting certain sets of specifications and guidelines that cover all phases of design and operation, including environmental, structural and performance requirements. Within these performance standards are flight control requirements, which include handling qualities requirements in addition to flight control system requirements. Design requirements and specifications for civilian rotorcraft may include the FAA Airworthiness Standards contained in Part 27 for Normal Category Rotorcraft and in Part 29 for Transport Category Rotorcraft. For military rotorcraft, the handling qualities and flight control system requirements may include criteria from MIL-H-8501 (Ref. 1) or more recently, from the U.S. Army’s Aeronautical Design Standard-33 (ADS-33) (Ref. 2) and MIL-DTL-9490E (Ref. 3). Although civilian rotorcraft are not certified to these military specifications, the standards embodied in ADS-33 represent “good” engineering practices and often translate into guidelines for use in flight control design for civilian applications.

The handling qualities requirements set forth in ADS-33 are well supported for VTOL currently flying aircraft ranging in size up to cargo class helicopters, but their direct applicability to significantly larger rotorcraft—such as Large Civil Tilt Rotor (LCTR2) (Ref. 4) and Joint Heavy Lift (JHL) configurations—has yet to be established. Of particular interest to this study are the short-term pitch, roll and yaw attitude response requirements of ADS-33 and how they relate to large tilt-rotors. This interest is driven by a previous experiment with a large tilt-rotor configuration (Ref. 5), which identified handling qualities issues resulting from large aircraft size; mainly from cockpit locations well ahead of the center of gravity (i.e., natural point of rotation) and from the impact of the reduced natural response frequencies of much greater moments of inertia. Here pilot commentary suggested that handling qualities—particularly in the yaw axis—degraded for high and low bandwidth cases, indicating that there is a limited range of acceptable bandwidths as a direct consequence of the higher inertias and pilot station offset from the center of gravity associated with this large tilt-rotor. A formal evaluation of the roll, pitch and yaw short-term response requirements—as they relate to handling qualities of LCTR2-sized tilt-rotors is found lacking.

A piloted simulation investigation was conducted using the NASA Ames Vertical Motion Simulator to study the impact of pitch, roll and yaw attitude bandwidth and phase delay on handling qualities of large tilt-rotor aircraft. Multiple bandwidth and phase delay pairs were investigated for each axis. The simulation also investigated the effect that the pilot offset from the center of gravity has on handling qualities. While pilot offset does not change the dynamics of the vehicle, it does affect the proprioceptive and visual cues and it can have an impact on handling qualities. The experiment concentrated on two primary evaluation tasks: a precision hover task and a simple hover pedal turn. Six pilots flew over 1400 data runs with evaluation comments and objective performance data recorded. The paper will describe the experiment design and methodology, discuss the results of the experiment and summarize the findings.

Sample Results

Data in the form of Cooper-Harper handling qualities ratings were collected for a wide pool of experimental pilots, painting a fairly consistent picture of the short-term pitch and roll attitude response characteristics for this class of aircraft. Level 1 handling qualities were not consistently attainable for a large aircraft of this class, i.e., with the pilot station located 40 ft ahead of the CG and an ACAH response type. Figure 1 shows optimal Handling Qualities Ratings tend to line up with the constant 1 rad/sec natural frequency line and phase delay lower than 250 milliseconds.

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^'•- Level 2 Level 3 (5) (3) 0.3 (4) (5) 5.

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It is suspected that increasing phase delay along this line will be conducive to increasingly worsening handling qualities. Increasing HQR gradients tend to point outward, from this 1 rad/sec constant line, in a direction orthogonal to the constant frequency lines, suggesting handling qualities are primarily driven by the natural frequency of the approximate second order response. OLOP boundaries (Ref. 6), which are primarily a function of natural frequency and hence are coincident with the constant frequency lines, fundamentally impose an upper limit on acceptable bandwidths. Above these boundaries handling qualities will only degrade as a consequence of actuator rate limiting.

However detrimental to the short-term attitude response handling qualities, rate limiting is not the only factor negatively impacting the performance of the aircraft. Not shown here, aircraft size, in terms of the location of the cockpit, is found to have a notorious effect on pitch and roll handling qualities. A general one-half to one HQR reduction throughout the test configurations evidences this for the 10 ft pilot offset case, when compared to the 40 ft pilot offset case. This resulted in a shifting of the suggested Level 2 boundary to lower frequencies, and in Level 1 ratings being obtained more consistently, particularly for natural frequencies of 1 rad/sec and above. Unfortunately only one pilot flew and rated the high frequency test points for both pilot offset distances. Both of these configurations were rated at 3 with the pilot station offset set at 10 ft, and 4.5 and 6 for the 40 ft offset, however.

Based on a yaw task developed during the exercise, preliminary results in Figure 2 indicate there is a very broad operational yaw axis bandwidth range where the aircraft will prove satisfactory.

Although pilots were able to push for much higher aggressiveness, this task would be considered representative of an actual mission an aircraft of this size would be required to perform. As expected from Ref. 5, pilot offset from the CG plays a significant role in the handling qualities, in particular at high bandwidths. At pilot offsets greater than 30 ft, the kinematical response of 0.4 9 1\ 0.3 (5) 4.

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o 0 0 0 0 0 0 ^^ , \ \ ^^^ '^ Level 3 (5)(4) (4) , (3) (3) (4) 0.1 5. 3.2 2.6 2.3 2.6  =0.1 ped 1 4. 4 Su gg sted Level 1 Acp d= O. 6 Level 2 boundaries Level 1 (40 ft offset) Bandwidth (rad/sec) Figure 2. Short-term yaw response handling qualities evaluations for a 40 ft pilot offset.

the aircraft is amplified into high accelerations, or side-forces, which severely interfere with the ability of the pilot to control the aircraft in order to capture a precise heading. Figure 3 shows an increase of two (2) handling qualities ratings between 20 and 30 ft for the high frequency test configuration shown in Figure 3.

Preliminary Conclusions

Based on a quick view of the objective task performance data and pilot evaluation comments several preliminary conclusions and observations may be made: 1.

Proprioceptive and visual cueing at cockpit locations much farther ahead of the center of gravity compared to currently flying cargo class rotorcraft (i.e., 30+ feet) have a significant negative impact on the short-term attitude response handling qualities of the aircraft in hover for both high and low bandwidths.

2.

Quickness of the attitude response (primarily in the yaw axis) associated with the high frequency configurations translates into objectionable load factors (a ride qualities issue) and unpredictable aircraft response (a handling qualities issue).

3. At low frequencies there is a general lack of control authority, with sluggish aircraft response lending itself to PIO, especially as pilots attempt the aggressive control techniques required to achieve desired position control. Pilot station offset obfuscates pilot perception of position, primarily due to the coupling of pitch and heave motions of the cockpit, and therefore it mainly serves the purpose of increasing task difficulty.

4. Control system response types such as translational rate control should be investigated for precision control in hover.

5. A broad range of acceptable yaw bandwidths was identified based on the proposed heading capture MTE. Reduction of the bandwidth requirement from 2.0 to 2.5 rad/sec, approximately, should alleviate structural and rotor design requirements.

6. The ADS-33 short-term attitude response handling qualities boundaries proved to be inadequate in predicting the handling qualities, independent of the position of the cockpit.

For small time delays, short-term response handling qualities appear to be driven fundamentally by the natural frequency of the commanded second-order system response.

Proposed boundaries therefore tend to follow the constant natural frequency lines. Effect of larger time delays should be investigated further.

7. The Open-Loop Onset Point (OLOP) criteria, currently based on fixed-wing data, proved to be a useful and accurate tool for predicting actuator rate limiting for rotorcraft.

Evidence suggests rate limiting fundamentally acts as an upper bound on all acceptable bandwidths (or natural frequencies). Additional flight data is needed to determine a more precise rotorcraft boundary.

8. Further maneuver and handling qualities task definition is needed for large hovering aircraft.

Additional results will be shown in the final version of the paper, in particular results for the pitch axis will complement those shown in the abstract. Further data analysis of the experiment results will also be performed in preparation of the paper.

References

[1] Anon., General Requirements for Helicopter Flying and Ground Handling Requirements, Military Specification MIL-H-8501A, September 7, 1961.

[2] Anon., "Handling Qualities Requirements for Military Rotorcraft", Aeronautical Design Standard-33 (ADS-33E-PRF), US Army Aviation and Missile Command, March 21, 2000.

[3] Anon., “Flight Control Systems - Design, Installation and Test of Piloted Aircraft, General Specification for,” MIL-DTL-9490E, U.S. Air Force, 22 April 2008.

[4] Acree, Jr., C. W., Yeo, H., and Sinsay, J., Performance Optimization of the NASA Large Civil Tiltrotor, NASA/TM-2008-215359, June 2008.

[5] Blanken, C. L., Lusardi, J. A., Ivler, C. M., Tischler, M. B., Decker, W. A., Malpica, C.

A., Berger, T., Tucker, G. E., Höfinger, M. T., “An Investigation of Rotorcraft Stability – th Phase Margin Requirements in Hover,” American Helicopter Society 65 Annual Forum, Grapevine, TX, May 27-29, 2009.

[6] Duda, H., “Prediction of Pilot-in-the-Loop Oscillations due to Rate Saturation”, Journal of Guidance, Navigation, and Control, Vol. 20, No. 3, May-June 1997.

Cooper, G. E. and Harper, R. P., “The Use of Pilot Rating in the Evaluation of Aircraft [7] Handling Qualities,” NASA TN D-5153, April 1969.

[8] Lusardi, J. A., Blanken, C. L., Tischler, M. B., “Piloted Evaluation of a UH-60 Mixer th Equivalent Turbulence Simulation Model,” American Helicopter Society 59 Annual Forum, Phoenix, AZ, May 6-8, 2003.

[9] Blanken, C. L., Cicolani, L., Sullivan, C. C., and Arterburn, D. L., “Evaluation of ADS- 33 Using a UH-60A Black Hawk Helicopter,” presented at American Helicopter Society th 56 Annual Forum, Virginia Beach, Virginia, May 2-4, 2000.

[ 10] Fletcher, J. W., Lusardi, J., Mansur, M. H., Cherepinsky, I., Driscoll, J., Morse, C. S., Arterburn, D. R., and Kalinowski, K. F., “UH-60M Upgrade Fly-By-Wire Flight Control Risk Reduction using the RASCAL JUH-60A In Flight Simulator,” presented at the American Helicopter Society 64th Annual Forum, Montréal, Canada, April 29 – May 1, 2008.

[ 11 ] Aponso, B. L., Tran, D. T., & Schroeder, J. A. (2008). “Rotorcraft Research at the NASA Vertical Motion Simulator.” presented at the American Helicopter Society 64th Annual Forum, Montreal, Canada, April 29 - May 1, 2008.

[12] Blanken, C. L., Hoh, R. H., Mitchell, D. G., and Key, D. L, “Test Guide for ADS-33E- PRF,” U.S. Army RDECOM special report AMR-AF-08-07, July 2008.

[13] Lusardi, J. A., von Gruenhagen, W., Seher-Weiss, S., "Parametric Turbulence Modeling for Rotorcraft Applications, Approach, Flight Tests and Verification," presented at the Rotorcraft Handling Qualities Conference, University of Liverpool, UK, Nov 2008.

An Investigation of Large Tilt - Rotor Short - term Attitude Response Handling Qualities Requirements in Hover Carlos A. Malpica William A. Decker Colin R. Theodore NASA Ames Research Center Moffett Field, CA Chris L. Blanken Aeroflightdynamics Directorate (AMRDEC) U.S. Army RDECOM Moffett Field, CA Tom Berger UC Santa Cruz (UARC) Moffett Field, CA Abstract S hort - term pitch and roll attitude and heading handling quality requirements for large rot o rcraft in hover were investigated . The piloted simulation study, performed on the NASA - Ames Vertical Motion Simulator, focused on a large (heavy - lift) civil tilt - rotor aircraft. Five experimental test pilots representing the U.S. Army, Marine Corps, NASA, and rotorcraft industry evaluated the aircraft configurat ion for a range of bandwidth and phase delay values, and pilot offsets from the center of gravity, in moderate turbulence conditions, while performing modified versions of the ADS - 33 Hover and Hovering Turn MTEs. Pilot comments and aircraft - task perfo rman ce data were analyzed.

Level 2 and Level 3 handling qualities ratings were recorded using Attitude Command/Attitude Hold (ACAH) response type . Refinements to the Hovering T urn MTE were developed in order to make it consistent with the Limited Agility MTE category in the hover and low speed ADS - 33 requirements for large - amplitude attitude changes. E valuated against this task , y aw bandwidth shows that significant relaxation o f the Level 1 boundary from 2 to .25 rad/sec is possible to help account for large pilot offset from the center of gravity.

Notation Subindices Variables  cmd Command model, commanded a Acceleration vector lat Lateral cyclic K Command model sensitivity coefficients lon Longitudinal cyclic r Yaw rate (rad/sec or deg/sec) p Pilot s Laplace transform complex variable ped Pedal δ Pilot control input Abbreviations ζ Command model damping ratio ACAH Attitude Command/At titude Hold θ Pitch attitude (rad or deg)  AGL Above Ground Level ρ Pilot offset vector from the center of gravity (ft) BW Bandwidth τ Time constant or delay (sec) CG Center of gravity φ Roll attitude (rad or deg)  HQR Handling Qualities Rating ω Aircraft angular velocity vector LCTR Large Civil Tilt - Rotor ω Command model natural frequency (rad/sec) n MTE Mission Task Element ω Bandwidth frequency (rad/sec) BW OLOP Open - Loop Onset Point PID Proportional - Integral - Differential control RC Rate Command Presented at the American Helicopter Society 66th Annual Forum, TCL Thrust Control Lever Phoenix, AZ, May 11 - 13, 2010. This material is declared a work of UCE Usable Cue Environment the U.S. Government and is not subject to copyright protection .

VMS Vertical Motion Simulator Approach Introduction A piloted flight simulation investigation used the NASA - The development of both civilian and military rotorcraft Ames Vertical Motion Simulator (VMS) facility (Ref. 6 ) .

typically involves meeting certain sets of specifications and The cockpit was configured with stan dard inceptors and guidelines that cover all phases of design and operation, instrumen ts. A simple stability - derivative mathematical including environmental, structural and performance model p rovided direct control of experimental variables . A requirements. Within these performance standards are flight carefully tailored visual scene provided task cueing .

control requirements, which include handling qualities E xperimental test pilots provided evaluations in the form of requirements in addition to flight control system comments, and handling qualities ratings (HQRs) using the requirements. Design requi rements and specifications for Cooper - Harper rating scale ( Ref. 7 ). Evaluation tasks for the civilian rotorcraft include the FAA Airworthiness Standards piloted handling quality assessments used the ADS - 33 - contained in Part 27 for Normal Category Rotorcraft and in derived hover and low - speed flight demonstration Part 29 for Transport Category Rotorcraft. For military maneuvers .

rotorcraft, the handling qualities and flight con trol system requirements may include criteria from MIL - H - 8501 This simulation focus ed on a large civil tilt - rotor aircraft (Ref. 1) or more recently, from the U.S. Army’s similar to that described in Ref. 4. The aircraft model was a Aeronautical Design Standard - 33 (ADS - 33) (Ref. 2) and relatively simple stability - derivative type model with MIL - DTL - 9490E (Ref. 3). Although civilian rotorcraft are sufficient complexity to capture the key physics of a large not certified to these military sp ecifications, the standards roto rcraft in hover , including key nonlinearities such as embodied in ADS - 33 represent “good” engineering practices actuator pos ition and rate limiting. Model - following flight and often translate into guidelines for use in flight control control architecture was used to establish a family of design for civilian applications.

bandwidth and phase delay configurations . The following section describes the simulation model in greater detail.

The handling qualities requirements set forth in ADS - 33 are well supported for currently flying helicopters ranging in In addition to bandwidth and phase de lay variations in each size up to cargo class helicopters, but their direct control axis, the value of the p ilot f uselage s tation was applicability to significantly larger rotorcraft — such as Large varied to create multiple offsets from vehicle center of Civil Tilt – Rotor (LCTR2) (Ref. 4) and Joint Heavy Lift gravity to pilot . Changing the pilot fuselage station offset (JHL) configurations — has yet to be estab lished. Of did not change the vehicle response dynamics, but did particular interest to this study are the fundamental short - ch ange the visual s cenes and motion responses of the cab, term pitch, roll and yaw attitude response requirements of especially the linear accelerations resulting from yaw and ADS - 33 and how they relate to large tilt - rotors. This interest pitch control inputs .

is driven from a previous experiment with a large tilt - rotor configuration (Ref. 5), which identified handling qualities Simulation Model issues resulting from large aircraft size; mainly from cockpit locations well ahead of the center of gravity (i.e., natural An 11 - state, reduced - order, decoupled stability derivative point of rotation) and from the impact of the reduced natural bare - airframe model was employed. The model retained the response f requencies of much greater vehicle moments of key rotor - body coupling, but dropped the high frequency inertia. Here pilot commentary suggested that handling rotor modes and off - axis response, thereby allowing for qualities — particularly in the yaw axis — degraded f or high independent variation of th e feedback properties in a single and low bandwidth cases. This indicat es that there is a axis. A turbulence model in the form of the AFDD Control limited range of acceptable bandwidths a s a direct Equivalent Turbulence Input (CETI) model (Ref. 8) consequence of the higher inertias and pilot station offset provided realistic gust inputs . This aircraft model was from the center of gravity associated with this large tilt - updated from that used in a previous piloted flight rotor. A formal evaluation of the fundamental pitch , roll and simul ation experiment (Ref. 5) with increased inertial yaw short - term response requirements — as they relate to properties and corresponding control system gains aimed at handling qualities of LCTR2 - sized tilt - rotors is needed .

maintaining Level 1 stability margins and disturbance rejection characteristics. A detailed description and Objectives validation of the methods used to gene rate this model was published in Ref. 5 .

The objective of this effort was to investigate pitch, roll and yaw bandwidth and phase delay on piloted handling qualities The vehicle model was augmented to provide pitch and roll for LCTR2 - sized conf igurations. Another objective wa s to Attitude Command/Attitude Hold (ACAH) and yaw Rate investigate how the pilot offset from the center of gravity Command (RC) control response types . A simplified block affects handling qualities ratings. T he hover and low speed diagram view of the explicit model - following cont rol system Mission Task Elements (MTEs) from ADS - 33 need ed architecture used is shown in Figure 1 . The basic bare - review for large tilt - rotor aircraft and , if necessary, airframe vehicle model was augmented with turbulence and modifi cation to be more ap propriate for the maneuvering actuator dynamics models for added realism. The two main capabilities of these larger machines.

components of the control system are a feedforward path, comprised by the command and inverse plant models, and a feedback loop, consisting of a simple Proportional - Integral - first - order command model was used in the yaw axis to Differential ( PID ) controller. Provided with an estimate of achieve a Rate Command response type.

the control input, the objective of the feedback PID regulator − τ s cmd is to tr ack the commanded responses with minimal error.

r Ke cmd = (2) Some amount of time delay is introduced into the δ τ s + 1 ped ped commanded responses in order to avoid overdriving higher - order dynamics (rotor and actuator) that are not included in Pitch and roll bandwidth and phase delay values could the lower - order pseudo - inverse. As long as actuator therefore be set by varying the natural frequency, ω , and the saturation or rate limiting do es not compromise control n pilot input to response time delay, τ , in the command model authority, tracking, or model - following, performance is n independent of the command model transfer function, and transfer functions defined in Eq. (1). The yaw axis was depends only on the characteristics of the feedback loops.

treated analogously with the time constant, τ , and the ped This approach a llowed variations in the piloted bandwidth delay, τ , in Eq. (2) becoming the piloted response tuning cmd and phase delay of the aircraft to be examined , while parameters .

keeping t he inner - loop or feedback control law gains fixed at B ecause the handling qualities evaluation maneuvers are a baseline set of values such that the gain and phase margins, primarily visual tracking tasks , e xtra delay (not shown in the and the disturbance rejection characteristics of the aircraft bl oc k diagram) has been added to the closed - loop pilot input remained constant throughout the experiment. Quickness of to vehicle response to account for visual delay in the image response is determined by the natural frequency of the pitch processing hardware of the simulator . The visual delay ha s and roll command models or the time constant in the yaw been measured to be 47 milliseconds.

rate command model. Time delay parameters determine the moment of the onset of the response after the pilot The sensitivity gains define the steady - state ratio of the introduces a control input. Combined, these two parameters commanded response to pilot input. These were left define a unique combination of bandwidth and phase delay unchanged throughout the experiment, ensuring all control of the commanded response to pilot input (Ref. 9 ).

system configurations had identical low frequency gains, independent of the natural frequency or time constant. In the A n ideal second - order command model was used in the pitch frequency domain, varying these gains would have the effect and roll axes to achieve the A ttitude Command response of shifting, up or down, the magnitude curve of the closed - type.

loop attitude response to pilot input. While this has no impact on the bandwidth and phase delay values, it can play 2 − τ s n φ θ K ω ⋅ e cmd cmd n n a significant role on the handling qualities of the aircraft for (1) , = 2 2 δ δ s + 2 ζ ω s + ω pilot - in - the - loop maneuvering. Sensitivity gains were set at: lat lon n n n .2 rad/inch for pitch and roll, and .15 rad/sec/inch for yaw.

The pitch and roll command model dynamics were independently set, although care was taken to produce harmonious response characteristics. Similarly, a n ideal Turbulence Model Comman d Actuator Vehicle Inverse Vehicle Pilot " " Model Dynamics Model Plant response Input Equiv Feedback Delay !

" Figure 1 . Ove rview of the model - following control system architecture.

based primarily on fixed - wing data, proved in Ref. 5 to be a Model following performance useful tool for predicting actuator rate limiting for rotorcraft.

Model following performance is a measure of how well the actual vehicle response match es the commanded response generated by the command model. A detailed analysis of the ! = 0.42 sec, td = 0 sec ped model following performance as a function of uncertainty in ! = 0.22 sec, td = 0.1164 sec ped the inverse model was presented in Ref. 5. The optimized ! = 0.05 sec, td = 0.233 sec 400 ped regulator was show n to provide reasonable tracking performance over the frequency range of interest. While t he introduction of uncertainty to the inverse model allow ed the rotor - body coupling to somewhat degrade the model following performance , the effect on the closed - loo p bandwidth and phase delay was not significant. 200 Model Following Cost Previously, model following cost had been calculated by a mismatch cost function between the linearized closed - loop response and the linearized delayed command model response. Model following cost values less than 100 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 Sweep Amplitude (% of pedal input) indicate reasonable agreement, and values under 50 indicate nearly perfect agreement. For low bandwidth cases, where Figure 2 . Quantification of model following performance non - linearities such as actuator rate limiting are not an issue, for three commanded yaw rate response time constants.

this linear method provided an accurate value for the model following performance of the system. However, for the Pilot offset higher bandwidth cases, this tended to over - estimate the model following performance of the system. Therefore, the Given the kinematic response of the aircraft, the acceleration cost was calculated by sweeping the system, using several at the pilot station i n the simulator is computed by solving different frequency sweep signals of varying amplitudes, and the general motion equations with respect to the center of analyzing the commanded responses and actual responses.

gravity for any arbitrary pilot to center of gravity offset.

This generated a model following cost that accounts for the non - linearities in the system, as a function of sweep      ˙ a = a + ω × ρ + ω × ω × ρ (3)

( )

p CG amplitude.

Overlaid in Figure 2 is the model following cost for the yaw The VMS lab motion system then exe rcises the appropriate axis ( r / δ ) as a function of amplitude of the harmonic test ped combination of translation and rotation to generate the signal , in percent of pedal input, for three different levels of accelerations expected at the pilot station. The generated quickness of response ( τ = .42 sec, τ = .22 sec and ped ped visual scene is updated accordingly. The attitude dynamics τ = .05). Time delays of the commanded response for ped of the aircraft are independent of pilot offset, allowing for these three cases ensure bandwidth remained approximately independent evaluation of the effects of vestibular and visual constant within 2.5 – 2.7 rad/sec. Based on a linear system cueing variations on the handling qualities .

analysis, the mismatch cost between the closed - loop frequency response and the delayed command mo del Conduct of Test response remained constant at 96. The discrepancies with increasing amplitude of the input are directly associated with Facility actuator rate limiting, and are an indication that quick responding command model dynamics will degrade the The experiment was conducted in the NASA Ames Vertical model - following perform ance. The maximum throw of the Motion Simulator VMS , described in detail in Ref. 6 . The pedal inceptor is ± 2.69 in, indicating that it takes very little Transport Cab was used for its large horizontal field - of - control inputs (about a quarter of an inch) to excite these view, as seen in Figure 3 and Figure 4 . Traditional nonlinearities for the extreme command model configuration helicopter pilot - control inceptors, i.e., center stick and defined by τ = .05 sec . pedals , were provided for the right c ockpit seat . A tilt - rotor - ped specific vertical Thrust Control Lever (TCL) replaced the During optimi zation of the system feedback gains and collective control used in the previous experiment (Ref. 5) .

command model parameters, model following performance, P ilots could manually adjust the friction coefficient on the bandwidth and phase delay values were computed from a TCL to their preference . P rimary flight display and linear system analysis, and these would not capture the horizontal situation (hover) display, replicating the Army’s effects of non - linear phenomena such as rate limiti ng. The Common Avionics Architecture System (CAAS) displays, open - loop onset point (OLOP) criteria (Ref. 17) were were provided.

employed to account for the effect of actuator rate limiting on the predicted handling qualities. The OLOP criteria , standards were considered too “tight” and aggressive for this large of an aircraft. The increased altitude, and the associated degradation of the visual cues are contra st ed in Figure 4 for the utility class position in (a) and the large tilt - rotor one in (b) . The Hover MTE was used primarily for the evaluation of the pitc h and roll handling qualities. Definition of an appropriate task to evaluate yaw short - term attitude response handling qualities was necessary. The task utilized for evaluation was based on a simple 90 - degree turn about the aircraft center of gravity. The iterations towards more appropriate standards f or this large aircraft are discussed in the results section of this paper.

E valuation s w ere performed in a realistic turbulent environment designed to perturb the aircraft and force the pilots to increase their control activity to compensate for the Figure 3 . VMS two - seat transport cab overview.

ensuin g drift. The level of turbulence used in all evaluations corresponded to the “moderate” turbulence exercised in Ref. 5.

Matrix of Configurations A single combination of stability margins and disturbance rejection bandwidths , about 38 deg and 12 dB in all axes , was used for the entire experiment, which fixe d the control law gains of the attitude inner - loops. While these values do not me et the MIL - DTL - 9490E requirements , they were consid e red acceptable based on the trade - off analysis findings in Ref. 5 . The corresponding disturbance rejection bandwidths associated with these gains were approximately (a) 1.0 rad/sec in the lateral axis, .8 rad/sec in the longitudi nal, and .7 rad/sec in yaw.

The bandwidth and phase delay for the pitch, roll and yaw axes were varied through changes to the command model parameters whi le keeping the inner - loop feedback gains constant. All bandwidth and phase delay test point pairs are shown in Figures 5 – 7 , overlaid onto the hover and low speed ADS - 33 requirements for All O ther MTEs and Usable Cue Environment ( UCE ) greater than 1 . Boun daries separating the different handling qualities regions are shown. The bandwidth and phase delay include an additional 47 milliseconds to account for stick - to - visual delay. It should be noted that a small amount of extra delay is produced due to the m ismatch in the closed - loop and commanded responses introduced by model uncertainty .

(b) Therefore, even if zero delay is commanded, there will be a Figure 4 . Pilot’s view at precision hover station - keeping minimum value of phase delay present.

point. (a) Standard ADS - 33 test course (b) Modified test Stark differences in the on – and off – axis responses of an course at increased altitude aircraft in forward flight makes them more naturally distinguishable to a typical human pilot . T he same is not true Evaluation tasks in hover , where pilots desire from the aircraft similar response characteristics in both axes . Consequently, l arge The evaluation tasks included a modified precision Hover differences in the attit ude bandwidth or phase delay metrics MTE from ADS - 33 , and a hover pedal turn maneuver that between the roll and pitch axes result ed in objectionable was developed specifically for this simulation experiment .

disharmony of control. A direct implication of this is that R efinements to the Hover MTE performance standards for the roll and pitch attitude short - term response characteristics an LCTR - type aircraft documented in Ref. 5 were adopted.

could not be evaluated independently of each other, and The precision hover station - keeping box was ± 4 ft and therefore, every point in Figure 5 is paired to another in located at 55 ft AGL. These modifications were made Figure 6 (and vice versa) . While there are discrepancies necessary because cargo/utility maneuver performance between the pitch and the roll bandwidth and phase delay 0.4 !

values for ea ch pitch - roll pairing, each configuration was Level 3 !

tested for harmony of control before formal evaluations were Level 2 !

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noted that precise rotorcraft boundaries have not been 0.156 sec !

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determined due to lack of data. Although developed 0.2 !

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primarily based on fixed - wing data, these criteria proved to OLOP !

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to provide comparable results with existing utility class Bandwidth, ! (rad/sec) !

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Figure 5 . Roll bandwidth and phase delay test points Evaluation Procedure Five pilots provided evaluations during this experiment. A ll 0.4 !

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pilots were experienced experimental test pilots with Level 2 !

significant rotorcraft experience. Pilots were from the U.S.

Army, Marine Corps, NASA, and the rotorcraft industry.

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rotorcraft. All pilots were familiar with the Cooper - Harper Phase delay (sec) P !

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0 sec !

0.1 !

Handling Quality Rating (HQR) scale (Ref. 7) and with the 0.04 sec !

ADS - 33 evaluation tasks. All flew and evaluated a subset of 0.1 sec !

0.2 sec !

the bandwidth and phase delay combinations and pilot to 0 !

center of gravity offsets.

0 ! 1 ! 2 ! 3 ! 4 ! 5 !

Bandwidth, ! (rad/sec) !

Pilots completed at least two simulation sessions for training BW " in the overall experiment objectives, methodology, and Figure 6 . Pitch bandwidth and phase delay test points familiarization with the aircraft configurations prior to the start of formal evaluations. Task performance displays in 0.4 !

the VMS lab presented pilot - vehicle task performance in Command Level 3 !

terms of the desired and adequate standards for each MTE.

delay !

Level 2 !

This information was read back to the pilot after each 0 sec !

.1164 sec !

maneuver was completed, both during training and formal 0.3 !

! Level 1 !

.233 sec !

evaluation.

During formal evaluation sessions, pilots first flew the MTE - 0.2 !

bandwidth/phase delay configuration until cons istent performance was achieved and then at least three formal data Phase Delay (sec) runs were accomplished and recorded. If the pilot felt one of 0.1 !

# =0.1 !

these formal data runs was anomalous compared to the ped # =0.2 !

ped # =0.4 !

ped # =0.6 !

ped others, additional data runs were included to resolve the inconsistency. Only in rare cases, when it was evident the 0 !

aircraft configuration exhibited major control deficiencies, 0 ! 1 ! 2 ! 3 ! 4 ! 5 !

as would be the case with an HQR 9 or 10, or offered an Bandwidth, ! (rad/sec) !

BW " extremely uncomfortable ride quality were the formal evaluations suspended short of the three required runs. Data Figure 7 . Yaw bandwidth and phase delay test points collected and recorded include the aircraft control inputs and state data, task performance data, and pilot comments. A formal questionnaire was used to elicit pilot opinion about task aggressiveness/performance, aircraft characteris tics, and pilot workload. The pilots used the HQR scale to provide a qualitative evaluation of the configuration.

is required to better qualify boundaries for higher values of Results phase delay Results for pitch and roll short - term attitude response requirements will be presented first, followed by those for yaw. Data in the form of Cooper - Harper handling qualities 0.4 !

ratings will be presented for both cases. Pilot evaluation commentary will be included within the overall discussion Command delay !

Level 2 !

(sec) !

highlighting specific handling qualities or ride qualities Level 3 !

0 !

0.3 !

issues. A quantitative assessment of pilot co ntrol techniques !

0.0784 !

will complete the analysis. A brief discussion of the 5.7 4.3 5.4 0.1568 ! 6.1 evaluation task that was developed specifically for the yaw 0.2352 ! ! = 0.6 rad/sec !

n 0.2 !

6.8 4.7 case is included , also . For pitch and roll, a comparison 3.7 3.9 OLOP ! 4.1 Level 2 !

between the 40 ft and 10 ft offsets will be made. Discussion OLOP !

7.0 Phase delay (sec) 6.3 4.1 3.7 of yaw results for varying offset will focus largely on the Level 1 !

0.1 !

high bandwidth configurations.

7.8 ! = 1.0 rad/sec !

5.3 4.5 3.6 n Region of optimal Pitch and roll requirements handling qualities !

0 !

HQRs and Pilot evaluation for 40 foot offset . The Cooper - 0 ! 1 ! 2 ! 3 ! 4 ! 5 !

Harper handling qualities ratings paint ed a fairly consistent Bandwidth, ! (rad/sec) !

BW " pictu re of the short - term pitch and roll attitude response characteristics for this class of aircraft. Average ratings for a Figure 8 . Short - term roll response handling qualities subset of the matrix of configurations are shown in Figure 8 .

evaluations for a 40 ft pilot offset.

Shown in Figure 8 are the current ADS - 33 Level 1, Level 2 and Level 3 regions. Boundaries separating the three regions are included. Level 1 handling qualities were not consistently attainable for a large aircraft of this class, i.e., 0.4 !

with the pilot station located 40 ft ahea d of the center of ! = 0.5 rad/sec !

n Command delay !

Level 2 !

(sec) !

gravity and an ACAH response type , in moderate Level 3 !

0 !

turbulence . Figure 8 shows that better Handling Qualities 0.3 !

5.7 0.04 !

! 4.3 Ratings tend to line up with the constant 1 rad/sec natural 5.4 6.1 0.1 !

frequency contour for a given amount of phase delay . It is 0.2 !

! = 0.4 rad/sec !

n 6.8 suspected that increasing delay along this line will result in 4.7 3.7 3.9 0.2 !

4.1 OLOP !

increasingly worse handling qualities, in a manner that is Level 2 ! 7.0 6.3 4.1 3.7 qualitatively consistent with the current specifications.

7.8 Phase delay (sec) 5.3 4.5 3.6 Increasing HQR gradients tend to point outward fr om this 0.1 !

! = 1.0 rad/sec !

1 ra d/sec constant line, towards both higher and lower n frequencies, in directions roughly orthogonal to the constant Region of optimal handling qualities !

frequency lines. This result suggests there is a strong 0 !

correlation between the handling qualities and the natural 0 ! 1 ! 2 ! 3 ! 4 ! 5 !

frequency of the approximat e second order attitude response Bandwidth, ! (rad/sec) !

BW " of the aircraft to pilot input .

Figure 9 . Short - term pitch response handling qualities These same trends were observed in Figure 9 for the pitch evaluations for a 40 ft pilot offset.

bandwidth and phase delay values investigated; mainly the clustering of the better configurations around the 1 rad/sec Figure 10 shows the correlation between the natural line and below 250 milliseconds of phase delay into a frequency and the average Cooper - Harper handling qualities characteristic “thumbprint” pattern, plus the overall inability ratings for the large aircraft configuration (40 ft pilot offset).

to a chieve Level 1 handling qualities.

Plotting results in this fashion obfuscates the effect of delay The preponderant influence of the natural frequency on the in the response. Diff erences in the average handling ratings is further evidenced by the approximate loci of the qualities rating for configurations with similar natural Level 2 – Level 3 boundary lines suggested by the data.

frequencies are caused by the different values of delay in the Boundary lines separating the Level 2 and Level 3 ratings in response. For example, configuration X (Figures 5 and 6) the low frequency reg ion for the particular test correspond sits around 200 milliseconds of phase delay, w hile cases Y roughly to natural frequencies of about .6 rad/sec for roll, and Z possess about 262 – 284 milliseconds. C onfigurations and in between .4 – .5 rad/sec for pitch. At lower frequencies, with roll and pitch attitude natural frequencies below configurations were deemed too sluggish in response to pilot .6 rad/sec and .4 rad/sec, respectively, were all found to input, consistently resulting in intolerable workloads and possess Level 3 qualities. Better handling qualities ratings adequate performance not being achieved. Additional data were foun d to be between .8 and 1.2 rad/sec, but Level 1 handling qualities were not attainable and the average rating main differences were in degre e of severity, with task scores tended to rise with increasing natural frequency. performance not compromised as strongly in the lower phase delay case.

It should be pointed out that these maneuvers were carried out in moderate turbulence con ditions. These results are not Deficiencies in the handling qualities were noted primarily surprising, necessarily, considering the pilot is required to in the longitudinal axis, with the ability to compensat e for decelerate what is essentially a very large aircraft into a longitudinal drift through control of pitch attitude relatively tight hover box ( ± 4 ft) with ACAH response type deteriorating due to an objectionable heave coupling . This and no Position Hold. pitch - heave coupling has been shown to be an issue closely associated with the location of the cockpit relative to the The severe unexpected degradation of ride and handling center of gravity of the aircraft and it is hence a clear qualities for high frequency is contrary to the experience indicator of the potential impact of aircraft size on the with current utility type rotorcraft. Pilots clearly handling qualiti es.

encountered, in cases X, Y and Z, an aircraft that was highly The second issue impacting the ability of the pilots to intolerant of pilot control aggressiveness , particularly in the longitudinal axis. This was due to both an objectionable maintain longitudinal position was rate limiting of the actuators. Case Z is a Level 2 OLOP specification in pitch, sensitivity to control (characterized by a disproportionate amplitude and quickness of the response to pilot input), and which explains the actuator rate limiting frequently experienced by the pilots who flew it . Rate limiting was a high propensity to rate limiting of the actuators.

Com bined, these characteristics yielded an unpredictable experienced in all three high frequency configurations, however. The usefulness of OLOP specifications to predict aircraft response, both in its initial and its mid - term response. I n practice these qualities translated into increased actuator rate limiting for utility size aircraft was observed in Ref. 5. The preponderance of rate limiting for cases X and difficulty to cancel out aircraft position drift and a heightened visual cue patt ern scan frequency necessary t o Y may be a further indicator of increased pilot activity resulting from variations in control technique required for ascertain the hover position. As such, both of the “measurable” handling qualities elements , performance and the 40 ft offset .

workload were directly impacted . Only by substantially Pilot cutoff frequencies. Pilot cutoff frequency, determined decreasing their control gains were some of the pilots abl e to from the spectral analysis of the inceptor p osition time effect the precise maneuvering necessary to achieve desired histories — during the 30 second precision hover hold task performance. This situation would not be representative subtask — is a measure of pilot operating frequency, and of an average pilot using normal control technique.

considered a good estimate of pilot crossover frequency for Compounding the handling qualities issues discussed above, pilot - in - the - loop tasks ( Ref s . 1 1 – 1 3 ) . The concentration of pilots complained in general about an uncomfortable good handling q ualities ratings around 2.1 rad/sec in Figure roughness of ride and this oftentimes factored decisively in 11 (a) indicate pilots consistently preferred the vehicle roll the rating process.

dynamics that allowed them to operate at this frequency.

Pilots appeared to be more tentative in pitch, with the mean long itudinal control cutoff frequency for these optimal control system configurations dropping to about 1.6 rad/sec ( Figure 11 (b)). In general, o perating at higher frequencies 10 !

excited objectionable deficiencies in the aircraft respons e Roll !

!

9 !

Pitch !

qualities.

Case Z !

8 !

One particular case stands out in Figure 11 (b) as an 7 !

Case Y !

exception where pilots , on average, used a higher crossover 6 !

frequency of about 2.85 rad/sec , ye t rated this configuration 5 !

Case X !

to have borderline Level 1 – Level 2 handling qualities .

4 !

However, l arge standard deviation of .89 rad/sec in the 3 !

longitudinal control cutoff frequency, along with a 2 !

Average Cooper-Harper Rating 1.72 rad/sec minimum and 4.27 rad/sec maximum, indicate 1 !

significant variability in the control techniques. Reviewing 0 !

0.0 ! 0.5 ! 1.0 ! 1.5 ! 2.0 !

pilot comments , it is evident that ride quality, though Natural Frequency (rad/sec) !

described as “very rough”, was not weighed into the HQR score for this particular configuration . Pilots indicated they could operate the aircraft with continuous, but small Figure 10 . Correlation of natural frequency of the amplitude, control inputs. This is consistent with the commanded second order response with average Cooper - quantitati ve c utoff frequency measurements. W hile pilots Harper handling qualities ratings for 40 ft pilot offset disliked the ride qualities of the aircraft, they liked the fact that the control system allowed them, under the appropriate The effect of delay on these configurations can be implied control techniq ue, to achieve reasonable accuracy in task from the subtleties in pilot evaluation. Case X elicited performance .

similar general comments from pilots as Y and Z did. The combined with higher frequencies is the likely contributing factor to the objectionable response characteristics reported Command delay !

10 !

by pilots for the 40 ft cockpit position offset . It was found 0 sec !

!

9 !

0.0784 sec !

during the experiment that pilots adjusted to these 8 !

0.1568 sec !

configurations by reducing to a minimum t heir control 7 ! 0.2352 sec !

inputs. Similarly, at lower frequencies the magnitude of 6 !

response is too small, requiring the pilots to adjust control 5 !

technique by increasing the amplitude of their control inputs.

4 !

This characteristic, combined with the overall sluggishness 3 !

of response is the principal cause for the handling qualities 2 !

deficiencies experienced at low frequency.

Average Cooper-Harper Rating 1 !

It should be noted that stick sensitivity, inasmuch as it 0 !

0 ! 1 ! 2 ! 3 ! 4 !

causes the magnitude curve to shift up or down, could be Pilot cutoff frequency (rad/sec) !

varied in an attempt to correct fo r these deficiencies. Care should still be taken not to increase the low frequency gain (a) excessively, since doing so tends to increase the pilot stick forces beyond acceptable levels.

Command delay !

10 !

0 sec !

!

9 !

0.04 sec !

8 ! 0.1 sec !

" 0.2 sec !

7 !

6 !

! %" 5 !

4 !

! $" 3 !

2 !

&'()*+,-./0-12 Average Cooper-Harper Rating Frequencies for ! #" 1 !

better HQRs 0 !

0 ! 1 ! 2 ! 3 ! 4 !

! !"

" Pilot cutoff frequency (rad/sec) !

! $5 (b) ! 7" Figure 11 . Correlation of Handling Qualities Rating with ! 365 pilot cutoff frequency, (a) lateral axis, and (b) ! 3!"

?@'>./0-.(2 longitudinal axis ! %%5 Pilot operating points defined by the a verage input cu toff ! %4" ! 3 " 3 3" 3" 3" frequencies are overlaid on to the attitude fr equency response 89.:,.);</09'-=>.;2 curves shown in Figure 12 and Figure 13 . Results indicate pilots tended, on average, to operate at a frequency near the Figure 12 . Average roll attitude frequency responses for roll bandwidth, so that the mean roll response phase is 40 foot offset approximately – 131 degrees, and thus effectively creating 49 degrees of phase margin for the pilot - in - the - loop closure .

In pitch control, pilots appeared to decrease their input frequency and back away from the bandwidth, operating at a phase margin of about 61.4 degrees. However, results indicate that pilot operating frequency for the ACAH response type was, in general, primarily driven by the bandwidth.

The average magnitude of the roll and pitch frequency response for the set of preferred configurations is .054 ( - 25.4 dB) and .037 ( - 28.6 dB). More importantly , the magnitude of the frequency responses for the different pilot op erating points shows a monotonically increasi ng trend , such that forcing the aircraft response at higher frequencies resulted in higher amplitude attitude oscillation per unit of stick displacement. This larger attitude response magnitude, response, resulting in major control deficiencies. In the extreme case, pitch and roll natural frequencies of .22 rad/sec and .31 rad/sec, respectively, produced a " marginally uncontrollable aircraft. Pilots required maximum control authority, defined by inceptor displacement, just to ! %" maintain control of the aircraft much less to adequately perform the task. Data are insufficient t o formulate ! $" complete Level 2 – 3 boundaries dividing these two regions with any certainty. A high phase delay configuration with &'()*+,-./0-12 Frequencies for ! #" an HQR 7 and a low phase delay one with an HQR 8 were better HQRs evaluated by only one pilot, such that Level 2 – 3 boundaries ! !"

established wo uld not be representative of a wide population " of pilots.

! $5 ! 7" 0.4 !

Suggested Level 2-3 Command delay !

boundary (40 ft offset) !

! 365 (sec) !

! = 0.55 rad/sec !

n 0 ! ! = 0.45 rad/sec !

! 3!" n ?@'>./0-.(2 0.0784 !

0.3 !

!

! %%5 0.1568 !

7.0 4.0 0.2352 ! 3.0 3.0 ! %4" Level 3 ! Level 2 !

! 3 " 3 3" 3" 3" 0.2 ! 6.0 4.0 2.5 89.:,.);</09'-=>.;2 3.0 OLOP !

Level 2 !

9.0 OLOP ! 6.0 6.5 4.3 4.0 3.2 4.0 Phase delay (sec) Level 1 !

Figure 13 . Average pitch attitude frequency responses 0.1 !

for 40 foot offset ! = 1.0 rad/sec ! 8.0 6.0 5.0 4.2 3.3 n Suggested Level 1-2 Suggested Level 2-3 boundary (10 ft offset) !

HQRs and pilot evaluation for 10 foot pilot offset . The boundary (10 ft offset) !

0 !

10 ft cockpit offset corresponds to existing utility class 0 ! 1 ! 2 ! 3 ! 4 ! 5 !

helicopters upon which much of the current testing for ADS - Bandwidth, ! (rad/sec) !

BW " 33 requirements is based. HQR results for 10 ft cockpit offsets are shown in Figure 14 (roll axis) and Figure 15 Figure 14 . Short - term roll response handling qualities (pitch axis). Also shown are the boundaries suggested for evaluations for a 10 ft pilot offset.

the 40 ft offset. Aircraft size, in terms of the location of the cockpit, was found to have a noticeable effect on pitch and 0.4 !

roll handling qualities. A general one - half to one HQR ! = 0.35 rad/sec !

n ! = 0.3 rad/sec !

n improvement throughout the test configurations for the 10 ft pilot offset case, when compared to the 40 ft pilot offset case 0.3 !

7.0 Suggested Level 1-2 is evidence of this. This is shown as a shifting of Level 3 ! 4.0 3.0 boundary (10 ft offset) !

Level 2 !

3.0 configurations to lower frequencies, and in Level 1 rat ings Level 3 !

being reported more routinely, in particular for natural 6.0 Command delay !

4.0 0.2 !

2.5 (sec) !

OLOP ! 3.0 frequencies of .8 rad/sec and above. The main implication 9.0 Level 2 ! 6.0 6.5 4.3 0 !

4.0 3.2 stemming from this shift in the Level 2 – 3 boundary is that 4.0 8.0 0.04 ! 6.0 5.0 Phase delay (sec) 4.2 3.3 the 10 ft offset aircraft configuration can possess more 0.1 !

0.1 !

0.2 !

sluggish attitude r esponse characteristics before performance ! = 1.0 rad/sec !

n Suggested Level 2-3 is compromised or pilot workload becomes intolerable. boundary (10 ft offset) !

Suggested Level 2-3 boundary (40 ft offset) !

More importantly, it points to a fundamental difference in 0 !

the nature of the control technique required from the pilots.

0 ! 1 ! 2 ! 3 ! 4 ! 5 !

Bandwidth, ! (rad/sec) !

Control configurations with roll natu ral frequency between BW " .45 – .55 rad/sec and pitch natural frequency between .3 – Figure 15 . Short - term pitch response handling qualities .35 rad/sec, for phase delay below 175 – 200 milliseconds, evaluations for a 10 ft pilot offset.

were nominally rated with an HQR 6 or 6.5 . Pilots consistently indicated the aircraft possessed such A pproximate Level 1 – 2 boundaries suggested are roughly objectionable handli ng characteristics that extensive based on the average Cooper - Harper handling qualities compensation was required just to achieve adequate ratings gradient, and in this sense are traced to represent performance. Issues were due mainly to sluggishness in the constant 3.5 ratings. Unfortunately, data collected around response and overall lack of controllability. Corresponding the fringes of the test configuration matrix were too sparse to bandwidths were within 1.23 – 1.7 rad/sec for both p itch and establish any sweeping conclusions. These configurations roll. Reducing natural frequency further (and consequently comprise all high bandwidth and phase delay regions.

the bandwidth too) caused significant degradation of the Certainly, available data was deemed insufficient to according to ADS - 33, yet pilot opinion for these estab lish accurate and representative Level 1 boundaries for configurations was divided. Similarl y, point P should be phase delay above 250 milliseconds. The available data do solidly Level 1, but a significant sub - set of the experimental appear to suggest a curving of the boundary line in a manner pilots who tested this configuration found it to be consistent with the current ADS - 33 specifications. unsatisfactory . While possessing higher bandwidth and lower phase delay than R and S, respectively, points O and P Configurations indicated by p oints O, P, R and S in appe ared to the pilots to be in general more sluggish and Figures 5 and 6 appear to be Level 2 based purely on the unpredictable than the former two configurations.

average Cooper - Harper HQRs, whereas points Q and T are more suggestive of Level 1 aircraft. Table 1 summarizes the These findings reinforce the observation that a stronger number of pilots who rated these co nfigurations, as well as dependency on natural frequency of the response , rather that maximum and minimum ratings obtained, for the specific bandwidth , tended to drive the han dling qualities of the subset of bandwidth and phase delay points identified by aircraft, and indeed pilot ratings for the 10 ft offset show letters O through T. Point O is the only configuration that very good correlation in Figure 16 with results from earl ier can be confidently categorized as Level 2 . Bandwidth and handling qualities investigations (Ref. 1 4 ) that evaluated the phase delay values for this configuration would place it right effect of ideal second order system responses.

on the ADS - 33 Level 1 – 2 roll boundary, but squarely in the Configurations with average pilot ratings between 3.5 and Level 2 region for pitch, and hence, consistent with the 4.5 straddled the Level 1 boundary, which is not an assigned rating. Statistical variation for points P through T unreasonable expectation considering these dividing lines suggests that these command model configurations could be separate the ratings into Level 2 configurations that ensure rated Level 1 or Level 2 with a similar probability. Points R de sired task performance was not compromised.

and S should, theoretically, be solidly Level 2 configurations Table 1 . Cooper - Harper ratings for specific high bandwidth and low phase delay cases Bandwidth Natural frequency Cooper - Harper Rating Test No. of (rad/sec) (rad/sec) case ratings Pitch Roll Pitch Roll Avg Min Max O 0.45 0.55 1.94 2.00 4 5.00 4.0 7.0 P 0.6 0.64 2.17 2.22 3 4.17 3.0 5.0 Q 0.8 0.8 2.4 2 2.49 4 3.25 3.0 4.0 R 0.45 0.62 1.83 1.76 4 4.25 3.0 5.0 S 0.6 0.72 2.0 6 2.00 3 4.00 3.0 5.0 T 0.8 0.9 2.2 9 2.28 5 3. 20 2.0 4 .0 3 ! 10 !

Level 1 ! 10 ft offset !

Attitude !

9 !

Large phase delay !

Systems !

40 ft offset !

Only 1 pilot rated !

8 !

= 1.0 # n Rate ! 7 ! !

2 !

" Systems !

) = 1 6 !

sec 1/T / 5 !

rad ( n 4 !

HQR>4.5 - Roll !

!

Average HQR 1 !

HQR>4.5 - Pitch !

3 !

3.5<HQR<4.5 - Roll !

2 !

3.5<HQR<4.5 - Pitch !

HQR<3.5 - Roll !

1 !

HQR<3.5 - Pitch !

0 ! 0 !

0 ! 1 ! 2 ! 3 ! 4 ! 5 ! 6 ! 7 ! 0.0 ! 0.5 ! 1.0 ! 1.5 ! 2.0 !

-1 2 # ! (sec ) ! Natural Frequency (rad/sec) !

n n Figure 16 . Correlation of average HQRs with second Figure 17 . Correlation of natural frequency of the order system response characteristics commanded second order response with average Cooper - Harper handling qualities ratings (pitch axis) 10 ! " 10 ft offset !

9 !

Large phase delay !

40 ft offset !

Only 1 pilot rated !

! %" 8 !

7 !

!

6 ! ! $" 5 !

&'()*+,-./0-12 4 ! ! #" Average HQR 3 !

2 !

! !"

" 1 !

! $5 0 !

0.0 ! 0.5 ! 1.0 ! 1.5 ! 2.0 !

! 7" Natural Frequency (rad/sec) !

! 365 ! 3!"

?@'>./0-.(2 Figure 18 . Correlation of natural frequency of the ! %%5 commanded second order response with average Cooper - ! %4" Ha rper handling qualities ratings (roll axis) ! 3 " 3 3" 3" 3" 89.:,.);</09'-=>.;2 Correlations, shown in Figure 17 and Figure 18 , of the average Cooper - Harper ratings with the natural frequency of Figure 19 . Roll attitude frequency responses for 10 foot the commanded second order response for pilot offsets of 10 offset and 40 ft reveal a sharp divergence of the handling qualities ratings for the 40 ft cockpit location for natural frequencies over .7 rad/sec, compared to the 10 ft configuration, which achieves Level 1 in a manner consistent with existing utility class helicopters.

" Pilot cutoff f requencies. An increase, in both axes, of the pilot cutoff frequencies for this shorter fuselage ! %" configuration, shown in Figure 19 and Figure 20 , indicate that pilots felt comfortable driving the 10 ft offset ! $" configuration at higher frequencies even though both &'()*+,-./0-12 configurations had identical attitude frequency responses.

! #" Maximum average cutoff frequencies incr eased from 2.43 rad/sec to 2.96 rad/sec, in the lateral axis, and from ! !"

" 2.85 rad/sec to 3.37 rad/sec in the longitudinal. T hese increments imply that pilots generally opted to operate at ! $5 smaller stability margins compared to those with the 40 ft ! 7" offset : 45 degrees in roll and 55 degrees in pitch on average.

! 365 Predicted vertical (heave) acceleration response magnitudes ! 3!"

?@'>./0-.(2 of the cockpit at the pilot cutoff frequencies for both pilot ! %%5 station offsets are shown in Figure 21 . Pilots operated at ! %4" these high frequencies for the 10 ft offset , essentially ! 3 " 3 3" 3" 3" doubling the attitude frequency response magnitude, with 89.:,.);</09'-=>.;2 respect to the better cases for the 40 ft configuration . This was achieved without inducing the objectionable ride and Figure 20 . Pitch attitude frequency responses for 10 foot control characteri stics associated with the 40 ft cockpit offset location as evidenced by the kinematic response of the cockpit in Figure 21 .

indicate there is a very broad operational yaw axis bandwidth range where the aircraft will prove satisfactory.

)/in) 10 feet Although pilots were able to push for much higher 2 40 feet aggressiveness, this task would be considered representative of an actual mission an aircraf t of this size would be required to perform.

0.4 !

# =3.6 !

ped 0.3 !

!

4.8 2.7 3.2 3.8 Suggested Level 2-3 5.1 boundary (40 ft offset) !

7.2 0.2 !

0 Heave acceleration at pilot station ((ft/sec 0.5 1 1.5 2 2.5 3 3.5 3.5 2.4 2.6 2.6 4.5 Average pilot cutoff frequency (rad/sec) Level 3 !

Phase Delay (sec) 0.1 !

# =0.1 ! 3.3 Figure 21 . Predicted heave acceleration response 2.3 2.7 ped # =0.2 !

ped # =0.4 !

ped 5.7 3.3 2.6 # =0.6 !

magnitude for pilot cutoff frequencies Level 2 !

Suggested Level 1-2 ped boundaries !

Level 1 !

(40 ft offset) !

0 !

Yaw requirements 0 ! 1 ! 2 ! 3 ! 4 ! 5 !

Bandwidth, ! (rad/sec) !

BW " Task development . The previous experiment (Ref. 5) pointed to the need for a formal evaluation of yaw response Figure 22 . Short - term yaw response handling qualities characteristics for large hovering aircraft. A project pilot evalu ations for a 40 ft pilot offset using a series of 180 - degree pedal turns evaluated the yaw axis response for that experiment . For the formal A minimum bandwidth of .25 rad/sec was required to eva luations of the current experiment, an early effort was the generate satisfactory yaw control for p recise heading capture development of a formal yaw evaluation task (MTE maneuvers. Anything below this frequency produced definition). The initial yaw evaluation task proposed enough sluggishness in the yaw response that control required a complete, 360 - degree, turn about the pilot station.

characteristics became unsatisfactory for precision capture This task was dismissed quick ly due to lack of appropriate of the desired heading.

visual cues. Pilots found the workload in the lateral and longitudinal channels to be extreme, masking the yaw For the minimum delay case this .25 rad/sec b andwidth is characteristics.

associated with a time constant ( τ ) of 3.6 seconds. At ped these time scales, handling qualities are largely insensitive to A simple pedal hover turn was developed to independently phase delay. A dditional 116 – 233 millisecond s of delay in evaluate the yaw axis response dynamics. The Hovering the response do not result in substantial degradation of the Turn MTE in ADS - 33 for Cargo/Utility class was modified h andling qualities. It is expected that y aw control will to better suit the very large aircraft being evaluated. A 90 - eventually be lost in the limit when bandwidth approaches degree turn was performed with pedal input alone, in zero, as the time to build up desired rates would become turbulence but without steady wind. The lack of wind excessive and meeting the performance metrics impossible.

compensated for t he lack of a hover position hold system It is noted that while th e current ADS - 33 short - term yaw that likely would be provided for the large aircraft size. The response boundaries are not supported for the yaw task 90 degree heading change was sufficient for the evaluation defined for this experiment, for low bandwidths the same and well supported by available visual cues. Task standards trends are observed, mainly that increasing yaw bandwidth retained the time standards of t he Cargo/Utility MTE: 15 sec result in improving handling qualities.

for desired, and 20 sec for adequate performance. This effectively halved the desired maximum yaw rate to the The size of th is class of vehicle presents unexpected order of 9.5 deg/sec. This rate was consistent with the kinematic issues for high yaw bandwidth configurations, Limited Agility MTE category in the hover and low speed which have not been anticipated in current specifications.

AD S 33 requirements for large - amplitude attitude changes.

Degradation, into Level 2, of the short - term response Although pilots were able to fly the aircraft more handling qualities was observed for bandwidth and phase aggressively than this rate, this maneuver was considered to delay pairs beyond a line defined by rate response time be more appropriate to an aircraft of this size, in terms of the constants in the order of .45 – .5 seconds. Increasing the agility required. F inal heading capture tolerances were set quickness of response even further, and hence, indirectly, the at ±3 deg, which forced enough aggressiveness and precision bandwidth , down to response time constants of .1 sec , or from the pilots to evaluate the different issues at hand.

less , resulted in major control deficiencies that put the aircraft, with the 40 ft pilot offset, squarely in Level 3 Pilot evaluations. Based on the yaw task developed for this handling qualities. It should be noted that the quickness investigation, average HQR scores sho wn in Figure 22 commanded at these levels far exceeded the ability of the "!

control system to track aggre ssive pilot commanded @373-0$ * responses due to rate limiting of the actuators. In this sense, actuator rate limiting played a significant role in the ) degradation of the handling qualities. Figure 23 shows the ( direct impact the ideal, or commanded, second order system =98<380>9/,?: ' @373-0# response parameters play on the handling qualities.

!

& % Command delay !

10 !

$ @373-0" 0 sec !

!

9 !

67389:30;..<38 0.1164 sec !

# 8 !

.233 sec !

" 7 !

! "! #! $! %! &!

6 !

+,-./0.1123/04133/5 5 !

4 !

Figure 24 . Effect of cockpit position on HQR scores for 3 !

high b andwidth response 2 !

Average Cooper-Harper Rating 1 !

0 !

0.0 ! 2.0 ! 4.0 ! 6.0 ! 8.0 ! 10.0 !

Time histories. Figure 25 illustrates different control Time constant (sec) !

strategies employed as pilots adapt ed to different bandwidths for the 40 ft pilot offset. Pure time delay for all cases was Figure 23 . Correlation of average Cooper - Harper .233 second. The prolonged rate buil dup typical of low handling qualities ratings with natural time constants of bandwidth can be observed for the 0.24 rad/sec case the commanded first order rate response for 40 foot ( τ = 3.6 sec), as the pilot commands a full pedal step input ped offset and the commanded rate takes 5 – 6 seconds to develop. This lack of response forces him to do full pedal reversal to try to Delay in the response had a more significant impact on the get the airc raft to stop. This type of control shaping was handling qualities at bandwidths over 1.5 rad/sec. All three considered undesirable for the rate command system configurations, e.g., with bandwidth between 2.5 and modeled. These characteristics drove this control 2.7 rad/sec, jumped one full handling qualities Level for configuration into Level 2, considering the pilot was still every 116 milliseconds of time delay added to the able to achieve the desired performance. With 1.37 rad/sec commanded response dynamics. Pilots consistently of bandwidth ( τ = 0.64 sec) pilots can now generate the ped described the extreme (i.e., Level 3) case as having an desired rates in a more responsive manner. The improved unpredictable initial response to input. This is not an controllability brings the aircraft into Level 1. While pilots unexpected result considering the strong susceptibility to could generate rates quickly in the 2.12 rad/sec actuator - rate limiting for the high quickness required of the ( τ = 0.21 sec) case, lateral kinematic issues start showing ped commanded response ( τ = .05 sec). This situation was ped up and pilots tended to slow down the maneuver by reducing unavoidable with the current actuator design (rate limits), the maximum rates generated and consequently the overall considering the magnitude of the time scales associated with task performance is compromised. These characteristics the commanded response dyn amics. In order to achieve a become even more exaggerat ed, for the given delay, as desired commanded response bandwidth it is necessary to bandwidth increases to 2.68 rad/sec ( τ = 0.05 sec) and ped progressively reduce the time constant of the response in higher.

order to compensate for increasing response delay in the loop. In the extreme case this creates a distorted response Inasmuch as the offset distance amplifies the lateral pilot where the delay in the response, to a ste p input, e.g., can be station accelerations, or more precisely their time rate of several orders of magnitude greater than the rise time, i.e., change, Figure 26 shows that pilots undergo similar the time to reach 63.2% of the steady state rate .

adjustment s of their control technique for increasing pilot offsets. The jerk felt in response to a pedal input relates The yaw kinematic response of the aircraft for pilot offsets more to the rate of change of the acceleration, rather than the of 30 ft and greater is amplified into large lateral magnitude of the acceleration itself. It is noted that all four accelera tions, or side - forces, at the pilot station, which configurations, having the same bandwidth, exhibited severely interfere d with the ability of the pilot to capture a similar high frequency jerks in the response when pilots precise heading. Figure 24 shows, e.g., an increase of two attempted the heading capture maneuver. The reduction in (2) full handling qualities ratings between 20 and 30 ft for amplitude makes the maneuver more tolerable t o the pilots.

the high frequency test configuration defined by 2.68 rad/sec bandwidth and .193 sec of phase delay (time constant 50 milliseconds and 233 milliseconds of delay).

$ $ % % , , " " " " 6 7" 76 %" 6 7" 76 %" ! E4F#,2)3 7",0))( C)= 9,))&(%-")&*2(,+ ! % B)='+,9>CD(,/9>5 ! % B)='+,9>CD(,/9>5 %",0))( F#$ 6)78&#"+% 4",0))( F%7 7##/-55"6-% , , ! $ ! $ $",0))( $%2%#"+%- F"6 %" %" "1(+* 76 76 7" 7" 6 6 " " " " 6 7" 76 %" 6 7" 76 %" @'A,*'(),/=)?12)35 @'A,*'(),/=)?12)35 4-,+2-, ! 6 ! 6 5%-,3%-&(*)7+ 746 746 &##/-55"6-1-55 "$$#%::*;%2%:: ;" ;" $6 $6 <)'=9>?,/=)?5 <)'=9>?,/=)?5 " " 6 7" 76 %" 6 7" 76 %" !" !"

5 5 4 5%-,3%-&"'()*+,-% 4 #" #" 4-,+2-,%&'()"*+,- "2-&0#%1,%234 $" $" !"#$%./-0+-123 %" %" " " " " 6 7" 76 %" 6 7" 76 %" ! %" ! %" 89:),/2)35 89:),/2)35 ! $" ! $" !"#$%&"'()*+,-%. !"#$%&'()"*+,- ! #" ! #" &'()*'+,-)*.,/0(12)3 &'()*'+,-)*.,/0(12)3 /*$/&0#%1,%234 !"#$%./-0+-123 ! !" ! !"

Figure 25 . Effects of bandwidth on hover turn Figure 26 . Effect of pilot offset for high bandwidth maneuvers ( 40 foot offset ). response on hover turn maneuvers.

high er frequ ency cases ( over 1.2 rad/sec pitch and Conclusions roll commanded response natural frequenc ies and under .5 sec yaw rate commanded response time A piloted flight simulation was performed on the NASA - constants ) translate d into objectionable impulsive Ames Vertical Motion Simulator (VMS) to investigate the load factor rate s (a ride qualities issue) and applicability and potential refinements of the current ADS - unpredictable aircraft response (a handling qualities 33 short - term attitude and heading handling quality iss ue).

requirements to large (heavy - lift) tilt - rotor aircraft. Five experimental test pilots representing the U.S. Army, Marine 3. Actuator rate limiting acted as a fundamental upper Corps, NASA, and rotorcraft industry evaluated the Large bound on acceptable bandwi dths in all axes.

Civil Tilt - Rotor (LCTR2) configuration for a range of 4. For low response frequenc ies there was a general bandwidth and phase delay values, and pilot offsets from the lack of control authority in all axes , with sluggish center of gravity, in moderate turbulence conditions, while aircraft response lending itself to excessive primarily performing revised versions of the ADS - 3 3 Hover w orkload , especially as pilots attempt the high and Hovering Turn MTEs. Analysis of objective aircraft - amplitude aggressive control techniques required to task performance data and pilot evaluation comments ach ieve desired position control. Pilot station offset suggests the following conclusions and observations: obfuscates pilot perception of position in the hover 1. Attitude Command/Attitude Hold response type task , primarily due to the coupling of pitch and was investigated for hover control of an aircraft heave motions of the cockpit, and therefore it with a large (i.e., 40 ft) pilot offset from the center mainly serves the purpose of increasing task of gravity in mo derate turbulence environmental difficulty.

conditions. Level 1 handling qualities, given these 5. Pilot cutoff frequency for ACAH response type was experimental constraints was not achievable.

naturally constrained to the phase bandwidth, plus 2. Quickness of the attitude response ( in all axes, but or minus a margin of error. Higher bandwidths led primarily in the yaw axis) associated with the pilots to operate at control frequencies that excited Qualities Conference, University of Liverpool, UK, high a mplitude and frequency oscillation. Nov 2008.

6. The ADS - 33 short - term pitch and roll attitude [ 9 ] Blanken , C . L. , and Pausder, H. - J., “ Investigation response handling qualities boundaries did not of the Effects of Bandwidth and Time Delay on support the assigned handling qualities for this Helicopter Roll - Axis Handling Qualities ”, Journal vehicle , i ndependent of the position of the cockpit. of the American Helicopter Society , Vol. 39 , No. 3, For small time delays, short - term response handling July 199 4 .

qualities appear to be driven fundamentally by the [ 10 ] Duda, H., “Prediction of Pilot - in - the - Loop Oscil - natural frequency of the commanded second - order lations due to Rate Saturation”, Journal of system response. Proposed boundaries therefore Guidance, Navigation, and Control , Vol. 20, No. 3, tend to follo w the constant natural frequency lines.

May - June 1997.

7. A broad range of acceptable yaw bandwidths was [ 11 ] Tischler, M. B., Remple, R. K., "Aircraft and identified based on the proposed heading capture Rotorcraft System Identification: Engineering evaluation maneuver. Relaxation of the Level 1 Methods and Flight Test Examples," AIAA, August yaw bandwidth requirement from 2.0 rad/sec to 2006.

. 25 rad/sec was possible to help account for the large pilot offset from the center of gravity.

[ 12 ] Atencio, A., "Fidelity Assessment of a UH - 60A Simulation on the NASA A mes Vertical Motion Simulator" , NASA - TM - 104016, September 1993.

References [ 1 3 ] Lusardi, J. A., Blanken, C. L., and Braddom, S. R., [1] Anon., General Requirements for Helicopter Flying “UH - 60 External Load Handling Qualities and Ground Handling Requirements, Military Evaluation,” Presented at the 35th European Specification MIL - H - 8501A, September 7, 1961.

Rotorcraft Forum, Hamburg , Germany, Sep 22 - 25, [2] Anon., "Handling Qualities Requirements for 20 09.

Military Rotorcraft", Aeronautical Design [ 1 4 ] Hoh, R. H., and Ashkenas, I. L., “Development of Standard - 33 (ADS - 33E - PRF), US Army Aviation VTOL Flying Qualities Criteria for Low Speed and and Missil e Command, March 21, 2000.

Hover,” NADC - 77052 - 30, Dec 1979.

[3] Anon., “Flight Control Systems - Design, [ 1 5 ] Blanken, C. L., Cicolani, L., Sullivan, C. C., and Installation and Test of Piloted Aircraft, General Arterburn, D. L., “Evaluation of ADS - 33 Using a Specification for,” MIL - DTL - 9490E, U.S. Air UH - 60A Black Hawk Helicopter,” presented at Force, 22 April 2008.

th American Helicopter Society 56 Annual Forum, [4] Acree, Jr., C. W., Yeo, H., and Sinsay, J., Virginia Beach, Virginia, May 2 - 4, 2000.

Performance Optimization of the NASA Large [1 6 ] Blanken, C. L., Hoh, R. H., Mitchell, D. G., and Civil Tiltrotor, NASA/TM - 2008 - 215359, June Key, D. L, “Test Guide for ADS - 33E - PRF,” U.S.

2008.

Army RDECOM special report AMR - AF - 08 - 07, [5] Blanken, C. L., Lusardi, J. A., Ivler, C. M., July 2008.

Tischler, M. B., Decker, W. A., Malpica, C. A., [17] Fletcher, J. W., Lusardi, J., Mansur, M. H., Berger, T., Tucker, G. E ., Höfinger, M. T., “ An Cherepinsky, I., Driscoll, J., Morse, C. S., Investigation of Rotorcraft St ability – Phase Margin Arterburn, D. R., and Kalinowski, K. F., “UH - 60M Requirements in Hover,” American Helicopter th Upgrade Fly - By - Wire Flight Control Risk Society 65 Annual Forum, Grapevine, TX, May Reduction using the RASCAL JUH - 60A In Flight 27 - 29, 2009.

Simulator,” presented at the American Helicopter [6] Aponso, B. L., Tran, D. T., & Schroeder, J. A.

Society 64th Annual Forum, Montréal, Canada, (2008). “Rotorcraft Research at the NASA Vertical April 29 – May 1, 2008.

Motion Simulator.” presented at the American Helicopter Society 64th Annual Forum, Montreal, [18] Lusardi, J. A., Blanken, C. L., and Tischler, M. B., Canada, April 29 - May 1, 2008.

“Piloted Evaluation of a UH - 60 Mixer Equivalent Turbulence Simulation Model,” Am erican [ 7 ] Cooper, G. E. and Harper, R. P., “The Use of Pilot th Helicopter Society 59 Annual Forum, Phoenix, Rating in the Evaluation of Aircraft Handling AZ, May 6 - 8, 2003 .

Qualities,” NASA TN D - 5153, April 1969.

[ 8 ] Lusardi, J. A., von Gruenhagen, W., and Seher - Weiss, S., "Parametric Turbulence Modeling for Rotorcraft Applications, Approach, Flight Tests and Verification," presented at the Rotorcraft Handling