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Arnerlcan lnstltute of Aerona~lcs and Astrona~lcs . 20024

ULATION STUDY OF TWO TILT-WING CONTROL CONCEPTS

Lourdes G. Birckelbaw and Lloyd D. Corliss* NASA-Ames Research Center

Moffett Field, California

Abstract

A two phase piloted simulation study was conducted to investigate alternative wing and flap controls for tilt-wing aircraft. The initial phase of the study compared the flying qualities of both a conventional (programmed) flap and an innovative geared flap. The second phase of the study introduced an alternate method of pilot control for the geared flap and further studied the flying qualities of the programmed flap, and two geared flap configurations. In general, the pilot ratings showed little difference between the programmed flap and the geared flap control concepts, although differences between the two control concepts were noticed and are discussed in this paper. The addition of pitch attitude stabilization in the second phase of the study greatly enhanced the aircraft flying qualities. This paper describes the simulated tilt-wing aircraft and the flap control concepts, and presents the results of both the f is t and second phases of the piloted simulation study.

I. Introduction

Tilt-wings are a viable approach for Vertical and Short Takeoff and Landing (V/STOL) transports and other smaller V/STOL aircraft, because the tilt-wing concept lends itself well to reasonable efficiency in hover and to very good effi- ciency in cruise flight. A good technology base for tilt-wing aircraft exists. The Vertol VZ-2 was the fist tilt-wing aircraft to transition from hover to forward flight in 1958. Other tilt- wing aircraft included the Hiller X-18 (1958-1964), the Vought-Hiller-Ryan XC-142 (1964-1967), and the Canadair CL-84 (1965-1974). The XC-142 and the CL-84 flew mili- tary operational demonstrations.

Some significant issues associated with tilt-wing aircraft include wing buffet during decelerating or descending flight, a strong wing angle to speed dependence, wing generated pitch- ing moments, and the requirement for a tail rotor or tail thruster to provide pitch control at low speeds and hover.

A piloted simulation study was initiated at NASA Ames Research Center in response to renewed interest in tilt-wing aircraft for use in several applications including the U. S. Special Operations Command aircraft, the U. S. Air Force Advanced Theater Transport, NASA high speed rotorcraft stud- ies, and proposed designs for civil applications. A new look at tilt-wing aircraft was further motivated by advances in tech- nologies such as propulsion, materials, and flight control sys- tems which offer the potential to address shortfalls of previous tilt-wing aircraft.

* Aerospace Engineer, AIAA Member Copyright 0 1992 by the American Institute of Aeronautics and Astronautics, Inc. No copyright is asserted in the United States under Title 17, U.S. Code. The U.S. Government has a royalty-free license to exercise all rights under the copyright claimed herein for Governmental purposes. All other rights are reserved by the copyright owner.

Two piloted simulations of a transport size tilt-wing air- craft have been completed in the Ames Vertical Motion S i m ~ l a t o r ~ l ~ . ~ * ~ . This paper presents the results of both simulations.

The first simulation evaluated and compared the flying qualities of a conventional (programmed) flap and an innova- tive geared flap (the flap serves as an aerodynamic servo to position the wing incidence relative to the fuselage). The programmed flap was the control concept used by previous tilt-wing aircraft. The geared flap as first proposed by Churchill5 has the potential to eliminate the tail rotor or tail thruster required by previous tilt-wing aircraft in hover and low speeds for pitch control, and therefore, could result in a significant reduction in aircraft weight and complexity. Other control concepts such as monocyclic on the propeller have also been suggested as alternate low speed pitch control de- vices but they were not a consideration in this simulation study.

The second simulation redefined the pilot evaluation tasks and added control law refinements and then further evalu- ated the flying qualities of the programmed flap and the geared flap control concepts.

During the first simulation, the pilots controlled the geared flap with a rotary spring return beeper switch located on the throttle grip. This configuration was called "geared flap on the beep". During the second simulation, the pilots controlled the geared flap in two different ways. One configu- ration was the same as in the first simulation; the pilots controlled the geared flap with the beeper switch located on the throttle grip. In the other configuration the pilots controlled the geared flap partially through the beeper switch on the throttle and partially through the longitudinal stick. This configuration was called "geared flap on the stick'.

The combined objectives of both simulations were to:

1. Simulate a representative tilt-wing aircraft.

2. Develop control laws for the programmed flap, the geared flap on the beep, and the geared flap on the stick configu- rations.

3. Evaluate and compare the flying qualities of the three flap control configurations.

4. Determine the feasibility of eliminating the tail rotor or tail thruster using the geared flap concept.

This paper describes the simulated tilt-wing aircraft, the flap control concepts, and the experiment design including the simulation facility and the pilot evaluation tasks of both sim- ulations, The flying qualities results of both simulations are presented. Also included is a general discussion of control characteristics encountered with the geared flap configurations near hover and a discussion of the tail thruster control power usage by each configuration. The conclusions of the simula- tion study follow the results.

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11. Simulated Tilt-Wing Aircraft

The conceptual tilt-wing aircraft of this study was a mid- sized V/STOL (vertical and short takeoff and landing) trans- port aircraft, about two-thirds the weight of a C-130. A tail thruster was included to provide pitch control during hover and low speeds. A sketch of this conceptual aircraft is shown in Fig. 1. Table I lists many of the physical characteristics of the simulated aircraft.

Fig. 1 Simulated Tilt-Wing Aircraft

Table I. Physical Characteristics of Simulated Aircraft

General: Gross Weight _ _ _ _ __ _ _ _ _ _ _ _ _ _ _ 87,000 Ibs

10,000 lbs 1.15

40 psf

92 ft Overall Length Payload Thmsr/Weight Disk Loading Wing Loading

_ _ _ _ _ _ _ _ _ _ _ _ _ _ - _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ __ _ _ _ _ _ _ _ _ _ _ _ _ _ _

_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 66 psf _ _ _ _ _ _ _ _ _ _ _ _ __ _ _

Wing: 109 ft

1321 f?

2"- 105" 50-

41 %

0"- 60"

26 ft

Span _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ - k e a _ _ _ _ _ _ _ _ _ _ - _ _ _ _ _ _ _ _ _ _ - Mean Aerodynamic Chord -- -- -- -- 12 ft Tilt Range _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ Tilt Rates, Geared to Wing Incidence Pivot, Percent Chord -- -- -- -- -- --

Flap:

Propellers:

Horizontal Tail:

Range _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ Diameter _ _ _ _ __ _ _ _ _ _ _ _ _ _ _ _ _ _ _

46 ft 430 ft2

00- 280

0.6 rad/s2

Span _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ k e a _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ Mean Aerodynamic Chord -- -- -- -- 9 ft Tilt Range, Geared to Wing Incidence

Pitch Control Power -- -- -_ -- -- -- Tail Thruster

Aircraft Control Effectors

During hover and low speed flight, longitudinal control was provided by the tail thruster and wing incidence, and pitch control was provided by the tail thruster. Pilot preference and choice of longitudinal control technique near hover was

somewhat configuration dependent as discussed in the results of the second simulation. During conversion, the elevator, horizontal tail, and tail thruster provided pitch control. The throttle controlled altitude during hover and conversion. During airplane mode, all effectors worked conventionally.

Simulation Math Model

The longitudinal rigid airframe aerodynamic and dynamic characteristics were modeled completely. The aerodynamic model used a component buildup method to develop total forces and moments. Momentum theory was used to calculate propeller slipstream velocities which were then used with the "power-off" aerodynamics data to obtain "power-on" aerody- namic characteristics. Other elements in the math model in- cluded coupled-wing-body equations of motion, engine and propeller dynamics, programmed flap and geared flap controls, a generic second-order landing gear model, and a buffet bound- ary model. Pitch axis stabilization was augmented in rate only during the first simulation and rate plus attitude during the second simulation. The first simulation did not include a ground effects model, however the second simulation did include a developmental ground effects model. During the first simulation, the sirnulation model cycled real-time at a frame rate of 15 msec on a CDC 875. During the second simulation, the simulation model cycled real-time at a frame rate of 10 msec on a Vax 9OOO.

Wing buffet is a significant issue of all tilt-wing air- craft. The buffet onset was defined from wind tunnel data and was a function of the effective wing angle-of-attack and the flap setting. The progressive deterioration of the flying quali- ties as deeper buffet was encountered was not modeled. A typ- ical buffet boundary for the simulation is shown in Fig. 2 for a glideslope of -7.5". It should be noted that as tilt-wing air- craft transition from forward flight to hover, aerodynamic lift is replaced by powered lift and buffet onset becomes a ride quality issue. Recovery from buffet is immediate on applica- tion of power.

43 3 0 2 0 WING INCIDENCE, deg

0

Fig. 2 Simulation Buffet Boundary for -7.5' Glideslope

The lateral/dimtional dynamic characteristics were mod- eled using stability derivatives. The dominant features were high roll damping and the addition of turn coordination above 30 knots. This study concenuated on longitudinal flying qualities, hence, accurate modeling of the lateral-directional dynamics was considered less critical to the study.

2

A detailed description of the math model and the cou- pled-wing-body longitudinal equations of motion may be found in Ref. 4 and in a forthcoming NASA Technical Memorandum (Churchill, G.: Longitudinal Equations of Motion for Tilt-Wing/Rotor VSTOL Aircraft), respectively.

111. Flap Control Concepts

The programmed flap control concept uses a flap sched- ule that is basically a function of the wing incidence, although the pilot is provided an attenuation control. The pi- lot sets a desired wing incidence by using the beeper switch on the throttle which, in turn, sets the programmed flap de- flection through cam or electrical control. The wing is di- rectly driven by a hydraulic actuator, as shown in Fig. 3.

pilot input results in a flap setting which, in turn, ultimately controls the wing incidence relative to the fuselage. For ex- ample, an increase in flap deflection causes an unbalanced aerodynamic moment about the wing pivot which is balanced when the wing rotates down cancelling the moment through mechanical feedback to the flap through the windflap linkage.

Using the programmed flap concept, the aircraft needs a tail rotor or tail thruster to provide pitch control in hover and at low speeds until the elevator effectiveness is sufficient at higher velocities. The upsetting aircraft pitching moments are caused by the thrust offset from the fuselage center of gravity as the wing tilts, as shown in Fig. 5.

Win Tilt 6. Thrust /

\ Aircraft Pitchinel\

Fuselage Y

Fig. 3 Programmed Flap Control Concept

The geared flap control concept5 uses the flap as an aero- dynamic servo tab to control the wing incidence relative to the fuselage. A schematic of the geared flap control concept is shown in Fig. 4. The pilot sets a desired reference wing position (which through the control laws, results in a flap deflection which will then drive the actual wing incidence towards the desired position) through a beeper switch located on the throttle, through the longitudinal stick, or through a combination of the beeper switch and the longitudinal stick. Regardless of how the pilot controls the geared flap, the wing

Fig. 4 Geared Flap Control Concept

is essentially free pivoting (some damping is required) and is driven primarily by the forces generated by the flap deflections within the propeller slipstream. Friction and artificial damp- ing, as well as aerodynamic moments generated by aircraft motion, also affect the pivoted wing response. In short, the

Tail Thruster

Fig. 5. Tilt-Wing Pitching Moments Due to Wing Rotation

Using the geared flap concept, the potential exists to eliminate the tail rotor or tail thruster by using the essentially free-pivoting wing to provide both longitudinal and pitch con- trol. As already discussed, a flap deflection results in an un- balanced aerodynamic moment at the wing pivot which then results in a wing movement that cancels out the moment through mechanical feedback to the flap. Adding a biased moment at the wing pivot results in an additional flap deflec- tion. With proper control laws and windflap linkage, the moment created by the additional flap deflection has the poten- tial to balance out the aircraft pitching moment, thus elimi- nating the need for a tail thruster or rotor, or at least sig- nificantly reducing the pitch control power required from an auxiliary tail device.

IV. Simulation Study

Simulation Facilitv

Both simulations were conducted on the NASA Ames Vertical Motion Simulator (VMS). The VMS operational limits are f22 ft of vertical motion and, depending on cab ori- entation, f15 ft of longitudinal or lateral motion. Both simu- lations used the longitudinal orientation to focus on the longi- tudinal flying qualities of the aircraft. In the VMS the pilots can experience accelerations of up to f 2 2 ft/sec2 vertically, f13 ft/sec2 longitudinally, and f10 ft/sec2 laterally. A sketch of the VMS is shown in Fig. 6. More information on the VMS is available in a forthcoming NASA Technical Memorandum (Danek, G.: Vertical Motion Simulator

3

Fig. 6 Vertical Motion Simulator

Familiarization Guide Components and Systems, TM 103923).

CockDit Lavout

The same basic instruments were used for both simula- tions, although several instruments were arranged differently for the second simulation at the pilots' request. Glideslope and localizer information were added for the second simulation and were displayed around the attitude direction indicator (ADI). A new instrument was also added for the second simu- lation which combined both wing incidence and flap angle in- formation. In addition to the analog instruments, the first simulation displayed wing incidence digitally, and the second simulation displayed both wing incidence and speed digitally.

For both simulations the cockpit controls consisted of a center stick with a mm button, a left-hand throttle with a spring return rotary beep switch, rudder pedals, and a flap lever located to the left of the pilot and aft of the throttle. The flap lever was used only with the programmed flap configuration; the lever was graduated u, produce 0- 100% of the programmed flap schedule. During the first simulation, a stick shaker was installed to cue the pilot when buffet was encountered. During the second simulation, a seat shaker (no stick shaker was used) and an angle-of-attack warning light were installed to cue the pilot when buffet was encountered.

During the first simulation, a four-window computer- generated imaging (CGI) system provided the external view of the airport environment. The four windows were arranged left, center, right, and lower right. During the second simula- tion, a three-window CGI system provided the external view of the airport environment. The three windows were arranged center, right, and lower right. The CGI system used in the second simulation (CTSA) was a generation newer than the CGI system used in the f is t simulation (DIG 1) and had many

features that enhanced the overall image quality.

Study Confimrations

During the first simulation, two flap control configura- tions were evaluated by the pilots; these were programmed flap (PF), and geared flap on the beep (GFB). During the sec- ond simulation, a third flap configuration was added, geared flap on the stick (GFS).

The PF, GFB and GFS configurations all used the spring return rotary beep switch embedded on the throttle grip to control the wing tilting mechanism. Release of the beep switch resulted in a constant value of the last resulting wing incidence. In the PF configuration the pilot beep switch in- put generated a wing rate command. In the GFB configura- tion the pilot beep switch input generated a reference (desired) wing incidence which through the control laws then resulted in a flap setting that drove the wing incidence towards the ref- erence wing incidence. In the GFS configuration the pilot beep switch input and the longitudinal stick input were com- bined to generate a reference wing incidence which then re- sulted in a flap setting that drove the wing incidence towards the desired wing incidence. For the latter configuration the pi- lot had full authority of wing tilt on the beep switch and a limited authority on the longitudinal stick. The stick author- ity translated to about 2" of wing per inch of longitudinal stick for wing incidences of 25"-105" and was scheduled from 2"-0" for wing incidences less than 25". It should be noted that with no longitudinal stick activity, the GFB and the GFS configurations yield the same aircraft characteristics.

Evaluation Tasks

This section describes the evaluation tasks of the first and second simulations, respectively.

The evaluation tasks during the first simulation were outbound transition, descending decelerating inbound transi- tion to hover, hover station keeping, and STOL landings.

Qutbound Transition - The aircraft was positioned over a predetermined location on the runway at 50 ft altitude in hover. The pilots smoothly increased power and ascended to 100 f t altitude, then incrementally lowered the wing while trying to maintain altitude. After a wing incidence of 40"-45" and a velocity around 80 knots, the rest of the wing could be lowered more aggressively to gain speed and altitude. The task ended at 180-200 knots and 500 ft altitude.

Descending Decelerating Inbound Transition to Hover - The aircraft was positioned initially downwind of the runway at 500 ft altitude and 12,000 ft to the left of the runway with 200 knots velocity. The pilots slowed the aircraft velocity to about 180 knots and lowered the landing gear on the down- wind leg. On the base leg, the pilots descended to 300 ft alti- tude, slowed the velocity to about 100 knots and raised the wing incidence to 10". On the turn to final, the pilots ex- tended full flaps (programmed flap configuration only), slowed the aircraft velocity to about 70 knots, and raised the wing to 20". On the final approach, the pilots incrementally raised the wing, adjusting power accordingly, and slowed the velocity to about 35 knots. A desired glideslope was not specified, the pilots were allowed to use whatever glideslope they preferred. As the pilots approached the hover position above the touch- down point, they descended to 50 ft altitude and continued to raise the wing as appropriate. The task ended when the pilots brought the aircraft to a hover and landed.

4

Hover Station Keeping - The aircraft was positioned over a checkerboard pattern to the right of the runway at 50 ft altitude in hover. The pilots attempted to maintain position and altitude for 3 minutes.

- The aircraft was positioned initially at 500 f t altitude and 5,000 ft to the left of the runway with the landing gear down. This task was evaluated using four differ- ent initial velocities (60, 50,40, and 35 knots). Each initial velocity corresponded to a specific wing incidence (depending on the flap configuration) which remained constant through- out the approach. The task ended when the aircraft landed at the target position.

The evaluation tasks were redefined for the second simu- lation to emphasize the flying qualities of tilt-wing aircraft during conversion and hover within the boundaries permitted by the math model (Le., primarily longitudinal flying quali- ties). The baseline altitude was chosen at 70 ft to avoid con- figuration-specific ground effects and because 70 ft was con- sidered a reasonable reference altitude for the large simulated aircraft. The tasks were bounded by specific performance stan- dards, thereby permitting a better application of the Cooper- Harper pilot rating method6. The four tasks and their perfor- mance standards are described below.

Level Inbound Transition to Hover - The aircraft was po- sitioned initially short of the runway threshold at 70 ft alti- tude with 93 knots velocity. This initial velocity corresponded to 9" of wing angle in the programmed flap configuration and to 16" of wing angle in the geared flap configmtions (for the same velocity, the wing angles are different because of differ- ent flap settings). The pilots decelerated the aircraft to arrive at a hover over the designated end position while trying to maintain 70 ft altitude, level pitch attitude, and avoiding buf- fet. The pilots were allowed to use whatever wing tilt rate they preferred. The performance standards are listed in Table 11.

Table II. Performance Standards for Level Inbound Transition to Hover Task

~

Pitch Attitude

The buffet time shown in Table II (and Tables I11 and V) represents the total accumulated buffet time. Only the bound- ary between desired and adequate buffet time was specified.

Descending Decelerating Inbound Transition to Hover - The aircraft was positioned initially 6OOO ft short of the run- way at 800 f t altitude. The initial wing incidence (46" for programmed flap configuration and 52" for the geared flap con- figurations) was selected to yield a speed of 40 knots, hence investigating only the final stages of deceleration where buffet considerations were minimized (see Fig. 2) and where differ- ences among the control configurations were maximized. The pilots captured the -7.5 deg glideslope using both electronic guidance (glideslope and localizer guidance on the ADI) and the visual approach slope indicator (VASI) lights on the run- way and established a nominal sink rate of 550 ft/min. At 400 ft altitude, the wing incidence angle was increased to de-

celerate, and power was added as necessary to remain on the flightpath. The pilots decelerated the aircraft to a hover at 70 ft altitude over the designated end position while maintaining level pitch attitude and avoiding an overshoot of the final end position. The pilots were to avoid buffet as much as possible by using low deceleration rates and by avoiding low power settings. The performance standards are listed in Table III.

Table III. Performance Standards for Descending Decelerating Inbound Transition to Hover Task

Pitch Attitude

The word "dot" used above refers to the glideslope guid- ance markers on the ADI.

Hover Station KeeDine with Turbulence - The aircraft was positioned over a predetermined location on the runway at 70 ft altitude in hover. The turbulence level was severe at 8 ft/sec rms in all three axes. The pilot attempted to maintain position for 70 sec, using whatever technique he preferred (wing incidence, pitch attitude adjustment, or a combination of the two). The performance standards are listed in Table IV.

Table IV. Performance Standards for Hover Station Keeping with Turbulence Task

Longitudinal Reuosition - The aircraft was positioned initially short of the runway threshold at 70 ft altitude in hover. The pilots began a forward translation, achieving a wing angle that was 40 deg less than the initial wing angle at hover, then started decelerating back to a hover, and ended the task in hover at 70 ft altitude over the designated end position. The pilots were to maintain 70 ft altitude and level attitude, avoid buffet, and arrive at the end position without overshoot. The performance standards are listed in Table V.

Table V. Performance Standards for Longitudinal Reposition Task

Pitch Attitude

5

Task Environment and Visual Cues

First Simulation - All the tasks were performed visually without the aid of a flight director and under daytime calm conditions. No visual enhancements were added to the com- puter generated database.

Second Simulation - The tasks were evaluated in day- time calm conditions with the exception of the hover station- keeping task which included turbulence. The tasks were per- formed visually, except for the descending decelerating transi- tion to hover, which could be performed both visually and with the aid of the glideslope and localizer information dis- played on the ADI.

During the second simulation, in addition to an im- proved visual system, several visual cues were added to aid the pilots. VAS1 lights were added to help the pilots maintain the -7.5" glideslope during approach. Runway cracks and tire marks were added to aid in depth perception and to add realism. Several vertical pylons consisting of stacked colorcoded 10 ft cubes were added along the edge of the runway to provide height information. STOL runway markings were superim- posed over the main runway. The STOL runway markings were used to define task end positions. Task end positions were also marked by a truck or an arresting gear on the right side of the runway where they were easily seen from the lower right cockpit window (chin window).

Evaluation b e d ure

First Simulation - All evaluation pilots attended a brief- ing where they were instructed on the study objectives and the evaluation tasks. The pilots were allowed to practice the eval- uation tasks until they felt they were ready for evaluations. During evaluations they were asked to use a Cooper-Harper Rating scale card6. The pilots were allowed to give half and quarter ratings between 1-10,

Second Simulation - All evaluation pilots attended a briefing before flying the simulator where they were intro- duced to general tilt-wing aircraft characteristics. At the brief- ing they also received a handout which included aircraft and simulator familiarization tasks, evaluation task definitions, performance standards, a Cooper-Harper rating scale card and a check list of topics to comment on before rating the configu- rations.

The pilots were allowed as much time as they needed to familiarize themselves with the aircraft and the simulator be- fore evaluating the tasks. During the evaluation runs the pi- lots were encouraged to give comments as they performed the task. Before enunciating their decisions through the rating scale card the pilots were required to comment on specific air- craft characteristics, perceived task performance, and pilot workload.

Pilots were allowed to give half ratings between 1-3,4- 6, and 7-9. Use of 3 5 , 6 -, and 9 5 was not allowed because

they represent important boundary conditions.

1 1 1 2

Evaluation Pilo&

First Simulation - Nine evaluation pilots participated in the study. Six pilots had experience with fixed wing aircraft, and three had experience with helicopters. Three pilots also had experience with powered-lift aircraft; one of these pilots also had experience flying the XC-142 tilt-wing.

Second Simulation - Six evaluation pilots participated in the study. They all had extensive experience with fixed wing aircraft and helicopters; five also had powered-lift aircraft experience. Four pilots had experience flying the XV-15 til- trotor; one of these pilots also had experience flying the V-22 tilwotor. One pilot also had experience flying the CL-84 tilt- wing.

V. Results

The results of the first simulation are presented first, and are followed by the results of the second simulation. Following the results, a general discussion of control charac- teristics encountered with the geared flap configurations near hover and a discussion of tail thruster pitch control power usage by each configuration are included.

Pitch axis stabilization was augmented in rate only during the first simulation and rate plus attitude during the second simulation. Attitude augmentation was an improve- ment which greatly alleviated the pilot pitch axis control workload. This effect can be seen in the pitch activity in Fig. 7. With the addition of pitch attitude stabilization in the second simulation, the pilots were allowed to direct their full attention to longitudinal maneuvers through wing control, and hence, rarely commented on pitch axis control problems.

First Simulation 100

3 80

f ;; 4

$ 0 x -4 -8 E

0 5 10 15 20 TIME, sec

Second Simulation

3 a

E 100

80

60

40 0 5 10 15 20

4 3 s i o

E u -4

-8 0 5 10 15 20

TIME, sec

Fig. 7 Time Histories Before and After Pitch Attitude Stabilization

During the first simulation, the pilot ratings exhibited large variation. This was probably due to loose constraints on task performance definitions and to different levels of pilot training. The task definitions also left many decisions up to the pilots. For example, a desired glideslope for the descend-

6

ing decelerating transition to hover task and for the STOL landings task was not specified. Also, task and simulator fa- miliarization run times varied from pilot to pilot, and the task evaluation procedure was not very structured,

During the second simulation, the evaluation tasks were defined more completely and desired performance standards were identified for each evaluation task. Aircraft and simula- tor familiarization tasks were defined and practice runs were monitored to assure that each pilot attained a similar training level. This, coupled with better instructions on general tilt- wing characteristics, led to better trained pilots; consequently, the pilot ratings exhibited less variation. Also, the pilots were required to comment on specific aircraft characteristics, perceived task performance, and workload, so that better air- craft flying qualities information was obtained.

The terms "pilot compensation" and "workload" as re- ferred to in the results are defined by ref. 6. Pilot compensa- tion is "the measure of additional pilot effort and attention re- quired to maintain a given level of performance in the face of deficient vehicle characteristics". The workload is "the inte- grated physical and mental effort required to perform a speci- fied piloting task". Pilot compensation and workload com- ments in this paper are based solely on the pilot subjective comments.

The pilot performance (desired or adequate, as defined in Tables 11-V) during the second simulation tasks was measured during evaluation runs. Comments on the task performance are based on actual data and not on pilot comments.

First Simulation Results

Outbound Transition

During the first simulation, the pilots were allowed to use whatever wing tilting technique they preferred (incremental or aggressive wing movements) to transition from hover to airplane mode and back. The maximum wing rate was lO"/sec. The flying qualities pilot evaluations for this task are summarized in Fig. 8 for each flap configuration.

0 0

COOPER-HARPER PILOTRATING io

Inadequate, 9

00 O Satisfactory

1

Programmed GeareedHap f lap on the Beep

Fig. 8 Pilot Evaluations of Outbound Transition

Programmed Flag - During mid-conversion, the aircraft experienced large pitch down moments. At the lower wing incidences the aircraft response to a wing movement was a heave response. One pilot, who was a former XC-142 project pilot, noted that both the nose down moment and the heave response were similar to the XC-142 aircraft behavior.

The pilot workload was associated with altitude control

and trying to minimize the large nose down attitudes encountered during mid-conversion. Unfortunately, some pilots experienced significant nose down attitudes (as much as -20") during climb. Pitch oscillations were sometimes encountered while trying to correct this problem. Throttle sensitivity and heave damping were low, and sometimes caused overcontrol while monitoring altitude.

Geared Fla! on the Beeg - The first major difference from the programmed flap configuration noted by the pilots was that the initial aircraft response to a forward wing com- mand was a longitudinal acceleration transient in the rearward direction. This aircraft response is discussed in the nonmini- mum phase response subsection of the results.

The pilot workload was again associated with altitude and attitude control. At lower wing incidences the aircraft re- sponse to a wing movement was a heave response. Pitch down moments were again encountered; however, the majority of the pilots felt that the nose down attitudes during mid-con- version were not as severe as experienced with the pro- grammed flap configuration.

Descending Decelemting Inbound Transition to Hover

The flying qualities pilot evaluations for this task are summarized in Fig. 9 for each flap configuration.

Programmed Fla - The pilot workload was associated with controlling pitch attitude, altitude and glideslope. Buffet was often encountered at around 40"-50" of wing incidence. High pilot compensation with power was required to offset the heave response to wing movements. Wing incidence movements closer to the hover incidence produced less heave and more drag. A pilot noted that it was unlike flying the Harrier or the tiltrotor because of the "barn door" (increased drag) effect caused by the large wing deflections.

COOPER-HARPER PILOTRATING 10

improvement required

7

Programmed Geared R a p flap on the Beep

Fig. 9 Pilot Evaluations of Descending Decelerating Inbound Transition to Hover

The heave response to wing changes noted by the evalu- ation pilots is a typical tilt-wing aircraft response during con- version from cruise to hover. The following is an excerpt7 about the CL-84. "In order to decelerate, the wing tilt angle must be increased, and the thrust reduced to prevent balloon- ing (heave). As the speed reduces, the thrust-power required increases. Thus, the pilot must find the matching rates of wing tilt angle and power increases to perform a smooth, level deceleration."

Geared FlaD on the Beep - The pilot workload was again

7

associated with controlling pitch attitude, altitude and glides- lope. The approach could not be rushed, or the glideslope was difficult to control. Some pilots felt that the altitude changes due to the heave response resulting from wing movements were more exaggerated in this configuration than for the pro- grammed flap configuration. When reducing power to main- tain altitude, buffet was often encountered. Some pilots got into lag situations and overcontrolled power.

Inadequate, 9 improvement required

Adequate, improvement s warranted

Hover Station KeeDing

- -

1 -

6 - 0 - 0 r 0

~

0 Satisfactory .

Fig. 10 Pilot Evaluations of Hover Station Keeping

Promammed Flag - Height control was precise. The pi- lot workload was low. Pitch, roll, and yaw controls were pre- dictable. The pilots had difficulty visually holding position over the checkerboard pad and encountered tracking divergence that ranged from 50 ft to over 100 Et.

Geared Flap on the Beep - Two pilots could not detect a difference between this configuration and the programmed flap configuration. However, two other pilots felt that height con- trol was not as precise in this configuration as it was with the programmed flap configuration. The pilots again had diffi- culty visually holding position over the checkerboard pad. The tracking divergence ranged from 50 ft to over 100 ft.

STOL Landings

The flying qualities pilot evaluations for this task are summarized in Figs. 11 and 12 for each flap configuration.

The speed and wing incidence angle combinations shown for this task were based on trimmed flight values achieved with pitch attitudes in the range of k 6".

R O W a m m e d F l ~ 60 knots, 20" wing incidence - The pilot workload was

low. There was some initial maneuvering in altitude and ve- locity, but the overall approach was smooth. Pilots could control glideslope and velocity well by a combination of mi- nor throttle adjustments and pitch attitude.

50 knots, 30" wing incidence - The pilot workload in- creased. The aircraft was less responsive because of lower dy- namic pressure. The pilots were distracted by the closeness to the buffet boundary. The flight control technique was the same as above: pitch attitude was used for velocity control, and power was used for glideslope control. Some pilots were

COOPER-HARPER PILOT RATING

Inadeuuate, 9

0 Programmed Flap

60 knots 50 knots

Fig. 11 Pilot Evaluations of STOL Landings

COOPER-HARPER PILOT RATING

Inadequate, ': improvement required

7

0 Programmed Flap Geared Flap

40 knots 35 knots

Fig. 12 Pilot Evaluations of STOL Landings

tending to overcontrol power. * 40 knots, 35" wing incidence - More degradation was

noticed between this case and the last. The pilot workload in- creased and was associated with monitoring angle of attack and sideslip angle (aircraft was departing directionally during turns) and with trying to avoid buffet which was encountered on the base leg and on the approach.

0 35 knots, 40" wing incidence - Large nose down atti- tudes were required to avoid buffet on the base leg and on the turn to final. The pilot workload was associated with trying to avoid buffet and monitoring angle of attack and sideslip. The flight control technique was the same as the previous cases. The sink rate was low and the final approach was easy.

60 knots, 44" wing incidence - The pilot workload was associated with trying to avoid buffet which was encountered on the base leg and on the final approach. The flight control technique was the same as the programmed flap configuration: power was used for glideslope control, and pitch attitude was used for velocity control.

0 50 knots, 47" wing incidence - The pilot workload was similar to the previous case. The flight control technique was the same.

0 40 knots, 52" wing incidence - The pilot workload in- creased from the previous case and was associated with main- taining zero angle of attack and sideslip and trying to avoid buffet. To avoid buffet, the pilots were flying faster than they wanted. The control technique was a little different: the pi- lots led with pitch attitude and followed with power.

Geared FlaD on the BeeB

8

0 35 knots, 54" wing incidence - The pilot workload was about the same as the urevious case. The Dilots again flew

speed predictability near hover degraded compared to the PF Configuration (The reason for this is discussed in the hover re-

faster than desired to avoid buffet. -

Second Simulation Results

Level Inbound Transition To Hover

The flying qualities pilot evaluations for this task summarized in Fig. 13 for each flap control configuration.

COOPER-HARPER PILOT RATING

Inadequate, 1 improvement required

7

- v

sults for this configuration and in the nonminimum phase re- sponse subsection at the end of the results). To avoid the de- graded speed predictability, some pilots accomplished the final hover acquisition by establishing the hover wing incidence early and then using attitude for final position capture. In general, the pilots achieved desired performance for altitude, pitch attitude and heading, and adequate performance for lateral position and buffet.

Geared Flap On The Stick (GFS) - The aircraft character- istics were similar to the PF and GFB configurations, except that some pilots noticed a small degree of coupling between wing movement and pitch response. The pilot compensation and workload were similar to the GFB configuration.

The time spent in buffet was about the same as the GFB configuration (an average total buffet time of 8.4 sec). The

are

Adequate, +- 00 oooo ....

0. 0 5L & improvement

warranted 00 00

Satisfactory

1 L

Programmed GearedFlap GearedFlap flap on the Beep on the Stick

Fig. 13 Pilot Evaluations of Level Inbound Transition to Hover

Proprammed F l a (PF) - At low wing incidence, the short term response to a wing incidence change was a heave response. Hence, coupling was noticed in wing tilting to ver- tical response. A wing incidence increase from wing posi- tions closer to hover incidence produced less heave and more drag. Altitude and minor pitch oscillations were experienced.

Moderate to considerable pilot compensation was re- quired in power management to offset the heave response to a wing change. Moderate pilot compensation was also required to predict speed towards the hover end position.

Speed predictability was better in this configuration than with the geared flap configurations. Some buffet was encoun- tered. The final hover acquisition was accomplished by some pilots by overcontrolling wing position to achieve zero speed and then resetting the wing incidence required for hover. In general, the pilots achieved desired performance for altitude, pitch attitude and buffet, and desired to adequate performance for heading and lateral position.

Geared FlaD On The Beep (GFB) - The aircraft character- istics were similar to those described above for the PF config- uration. Considerable pilot compensation was required in power management to offset the heave response to wing changes and to avoid buffet.

Some pilots felt the heave response to initial wing change was reduced compared with the PF configuration; one pilot noted that the "heave response to initial beep (wing tilt) was much better than (the) programmed flap, coupling (was) not as bad". Another pilot felt the throttle usage to control the heave response was lower and thus an "improvement over the programmed flap, especially during (the) initial part of (the) task, (and the ride) seemed a little smoother".

All pilots agreed that time spent in buffet increased in this configuration compared to the PF configuration (an aver- age total buffet time of 8.0 sec vs. 2.1 sec, respectively). The

speedpredictability close to hover and the final hover acquisi- tion technique were similar to the GFB configuration. In gen- eral, the pilots achieved desired performance for altitude, pitch attitude and heading, desired to adequate performance for lateral position, and adequate performance for buffet.

Buffet Comments - The increased time spent in buffet with the geared flap configurations is most likely due to lower flap settings than the programmed flap configuration for simi- lar wing angles. Examination of time histories showed that buffet was encountered during the mid-wing-incidence range of 35"- 60" for both the PF and the GFB configurations. During this mid-wing-incidence range, the flap range was 20"- 40" for the PF and 5"- 20" for the GFB.

Increase in leading and trailing edge flap deflections on the CL-84-1 improved the buffet boundary of the aircraft8. Also, one of the methods proposed by results of flight inves- tigations of the VZ-2 to alleviate buffet was larger flap deflec- tionsg.

Descending Decelerating Inbound Transition To Hover

The flying qualities pilot evaluations for this task are summarized in Fig. 14 for each flap control configuration.

Proerammed Flap - Control power, sensitivity and oscil- lations were not issues. Most pilots felt the workload was low because the task was slow and glideslope control only re- quired power regulations. However, two pilots noticed a cou- pling between wing movement and vertical response and felt that the workload was high due to poor heave predictability. Examination of the smp charts showed that the reported heave control difficulties were associated with large abrupt wing movements.

The nominal task definition was such that beginning at 400 ft, deceleration could be accomplished slowly and smoothly with slow monotonic wing and power increases to maintain glideslope. In these circumstances, the differences among the three flap configurations were minimal.

Largely because of the task structuring, no buffet was encountered. The final hover acquisition technique used by some pilots was the Same as the PF configuration in the pre- vious task. In general, the pilots achieved the desired perfor- mance standards.

Geared FlaD On The Beep - The aircraft characteristics and pilot workload were similar to those described above for the PF configuration.

One pilot noted that he "felt the glideslope tracking was the tightest so far" compared to the other two flap configura-

9

COOPER-HARPER PILOT RATING

Programmed GearedFIap GearedFIap R a p on the Beep on the Stick

Fig. 14 Pilot Evaluations of Descending Decelerating Inbound Transition To Hover

tions. Another pilot said that "height control was easier than with the PF configuration". Again, because of task structur- ing no buffet was encountered. The final hover acquisition technique used by some pilots was the same as for the GFB configuration in the previous task. In general, the pilots achieved the desired performance standards.

Geared Flag On The Stick - The aircraft characteristics and pilot workload were similar to those described for the PF and the GFB configurations.

Since the task definition required that the pitch attitude be maintained level and hence longitudinal stick activity was minimal, the GFS configuration showed only subtle differ- ences from the GFB configuration. In general, the pilots achieved the desired performance standards.

Hover Station Keeping; With Turbulence

The flying qualities pilot evaluations for this task are summarized in Fig. 15 for each flap control configuration.

Programmed F l a ~ - Control power, sensitivity, and cou- pling were not issues. Moderate to considerable pilot com- pensation was required for height and position control. One pilot felt that controlling altitude while trying to maintain position was a highly iterative process, "...constantly beeping (moving) the wing for longitudinal control" while at the same time, "using multiple throttle inputs to control altitude".

In general, the pilots achieved the desired performance standards except for the longitudinal position limits which were exceeded by all the pilots. In many cases the pilots were unable to perceive the longitudinal drift because of the limited visual cues.

As mentioned earlier the pilots were allowed to use whatever technique they preferred (wing incidence, pitch atti- tude, or a combination of the two) to regulate longitudinal position. In this configuration the majority of the pilots pre- ferred controlling their longitudinal position with wing inci- dence. This technique preference has been noted before by CL-84 pilots, "For forward and aft translation the pilots preferred to use wing tilt while holding the fuselage level. This was smoother, easier and more natural than tilting the whole aircraft."1°

Geared Flap On The Bew - The aircraft characteristics were similar to those described above for the PF configura- tion, except that the initial longitudinal response to a wing movement was sluggish.

COOPER-HARPER PILOT RATING

0

Programmed Geared Flap Geared Flap Flap on (he Beep on the Stick

Satisfactory

Fig. 15 Pilot Evaluations of Hover With Turbulence

The workload and pilot compensation associated with height and position control were also similar to the PF con- figuration, except that the lag between wing movement and perceptible longitudinal aircraft response required moderate to considerable lead compensation.

Longitudinal control predictability was lower than with the PF configuration and was a result of the degraded speed predictability characteristic of both geared flap configurations near hover. As with the PF configuration, in general, the pi- lots achieved the desired performance standards except for the longitudinal position limits which were exceeded in all but two of the evaluated runs. Again, many pilots were unable to perceive the longitudinal drift because of the limited visual cues.

One pilot evaluated this task on three separate runs: one with turbulence in all three axes, one with no lateral turbu- lence, and one with no turbulence. The pilot flying qualities

ratings were 3,2 - and 1 -, respectively. Unlike the PF configuration, most pilots preferred using

pitch attitude over wing incidence to control longitudinal po- sitioning. One pilot evaluated this task using both techniques and rated the pitch attitude technique a 5 and the wing incidence technique a 7 where the degradation was primarily attributed to a delay in longitudinal response leading to oscil- latory longitudinal characteristics. This delay stems from a characteristic of the geared flap configurations near hover where a forward wing tilt results in an initial rearward acceler- ation, giving a response which is nonminimum-phase-like in nature. This response characteristic led to the pilots' choice of controlling position through attitude, but as one pilot noted the pitch attitude technique would not be acceptable for such a large aircraft, "...the pitch activity would certainly be disconcerting to passengers". This response characteristic will be discussed further at the end of this section.

Geared FlaD On The Stick - The aircraft characteristics and the pilot workload were similar to the GFB configuration. The stick to pitch sensitivity seemed low to some pilots.

As with the the GFB configuration, most pilots preferred using pitch attitude to control longitudinal position. In general, the pilots achieved the desired performance standards except for the longitudinal position limits which were exceeded in all but one of the evaluated runs.

One hypothesis concerning the GFS configuration was that it would reduce pitch control requirements and hence, pitch activity might be lower than with the GFB configura-

1 1 2 2

10

tion. However, examination of data did not show reduced pitch activity compared to the GFB configuration. This is probably due to the current level of control law development and to not having enough wing authority on the longitudinal stick (about +lo% only).

Longitudinal Reposition

The flying qualities pilot evaluations for this task are summarized in Fig. 16 for each flap control configuration.

COOPER-HARPER PILOT RATING

Programmed Geared Flap Geared Flap Flap on the Beep on the Stick

Fig. 16 Pilot Evaluations of Longitudinal Reposition

Programmed Flap - The short term response to a wing incidence change at the lower wing angles was a heave re- sponse. Control power and sensitivity were not issues. Altitude control was a little oscillatory. Coupling was no- ticed between wing movement and vertical response and to a small degree between wing movement and pitch response.

Low to moderate pilot compensation was required to an- ticipate the heave response to a wing change and to predict speed towards the end of the task. The workload was primar- ily in the vertical axis trying to maintain altitude. One pilot noted that, "...conditions were ideal and that any complica- tions due to wind, turbulence or visibility would significantly add to the workload."

Just as for the level inbound task, the final hover acqui- sition was sometimes accomplished by overcontrolling wing position to achieve zero speed and then resetting the required wing incidence for hover. In general, the pilots achieved the desired performance standards.

Geared Flap On The Beep - The aircraft characteristics were similar to those discussed for the PF configuration, ex- cept that the initial longitudinal response to a forward wing movement from the hover position was sluggish.

Moderate to high pilot compensation was required to lead the heave response with throttle and to arrive at the end position with the right speed.

Height response was not very predictable. Speed pre- dictability degraded compared to the PF configuration. Using the wing incidence technique for the final hover acquisition, one pilot got into a divergent position P I0 (pilot induced oscillation) "that could not be suppressed with any amount of compensation" (the rating was a 7). Time histories showed that the flap was at the lower limit during most of his hover acquisition which caused a distorted wing flap response. In general, the pilots achieved desired performance standards for altitude, heading and buffet, and desired to adequate perfor-

mance for pitch attitude. Geared Flap On The Stick - The aircraft characteristics

were the same as discussed above for the GFB configuration, except that the available tail thruster pitch control power was not enough if the wing tilting technique was very aggressive. In one case, the aggressive wing tilting technique resulted in loss of control (this is discussed below).

Pilot compensation was similar to the GFB configura- tion, but the workload was higher than the GFB configura- tion. One pilot explained, "(The) workload was a bit higher as a result of (increased) vertical response to wing change, (I) had to predict (Le., anticipate response) more strongly". Another pilot also perceived a "slight increase in vertical re- sponse to wing change", and said it made "the vertical ride a little bumpier".

In general, the pilots achieved desired performance stan- dards for altitude, heading, and buffet, and desired to adequate performance for pitch attitude.

Initially, the tail thruster pitch control power for this configuration was k 0.3 rad/sec2 which was half the pitch control power of the other two configurations. Several pilots evaluated this configuration without encountering any tail thruster pitch control power limits. However, one pilot did encounter loss of aircraft control because of the reduced tail thruster control power, "...an overshoot was developing which required continuous wing beep (wing movement). As power was increased to account for the loss of wing lift, the power- pitch coupling response became apparent and objectionable. It was countered with stick input but when the flaps reached the deflection limit a divergent pitch PI0 rapidly developed that resulted in loss of control after 2 oscillations." This resulted in the flying qualities rating of 10. The tail thruster pitch control power of the GFS configuration was increased to k 0.6 rad/sec2 (the same as the other two configurations), and the problem did not occur again. The same pilot using the same aggressive wing tilting technique evaluated the task again and the rating was a 5.

Nonminimum Phase Response

As mentioned earlier, when the geared flap configuration is at high wing angles, the initial response to a forward wing tilt command is a longitudinal aircraft acceleration transient in the rearward direction (hence, a nonminimum phase response). The rearward acceleration transient is the result of a transient increase in force (lift) on the wing caused by the initial flap deflection in the propeller slipstream.

Fig. 17 shows the time histories of a conversion from hover for three geared flap configurations and a programmed flap configuration. Fig. 17a is a time history from the first simulation and Figs. 17b, c, and d are time histories from the second simulation. The flap activity and rearward pilot longitudinal acceleration (AXP) for the geared flap may be seen in Fig. 17a, b, c, and particularly in Fig. 17a where there is no damping about the wing pivot. By comparison, Fig. 17d for the programmed flap does not show any rearward pilot acceleration. Fig. 17b and c show the similar aircraft charac- teristics of the geared flap configurations when no longitudi- nal stick is used with the GFS configuration.

The nonminimum phase response was reduced by the addition of damping about the wing pivot in the second simulation as comparisons of Figs. 17a and b show. With the addition of damping, the longitudinal response was felt as more of a hesitation than a reversal. An associated delay in

11

% d

d >

8 E- ti d >

(a) Geared Flap on the Beep

(Without Damping) 90 80 70 60 50

60 40

20

0

4

2

0

-2

20 1 5 10 5 0

0 5 10 15 TIME.

(c) Geared Hap on the Stick

(Damping Added) 90 80 70 60 50

0 5 10 15 60 40

20 0 0 5 10 15

4

2

0

-2

20 15 10 5 0

0 5 10 15

0 5 10 15 TIME. sec

3 U

3 -u

N s

(b) Geared Flap on the Beep

(Damping Added) 90 80 70 60 50

60

40

20 0

4

2

0

-2

20 15 10 5 0 0 5 10 15

TIME, sec

(dl Programmed Flap

M 90 8 80

70 60 ’ 50

15 5 10

3

d

-u

% d i 10 1 5

4 N

s 2

$ 0

-2

il 4 20

E- 10 t i 5

15

0

TIME, sec

Fig. 17 Time Histories of Flap Control Configurations

12

the velocity response is also evident on Fig. 17 for all of the geared flap time history cases. As mentioned in the results, this longitudinal response to a wing tilt command was objectionable to the pilots in hover because it resulted in degraded velocity and position predictability.

Tail Thruster Pitch Control Po wer Us=

In the simulation study the maximum pitch control power of the tail thruster was 0.6 rad/s2 for both the pro- grammed flap and the geared flap on the beep configurations. As already discussed, the maximum pitch control power of the geared flap on the stick configuration was initially 0.3 rad/s2, and was later increased to 0.6 rad/s2. Table VI compares these values to the V/STOL Handling Qualities Criteria1 and to previous tilt-wing aircraft7.12.

Table VI. Pitch Control Power

I 0.45

CL-84-1 1.2 XC-142 NIA

Since pitch control power usage information was more readily available for the second simulation, the discussion on pitch control power usage in this section refers to results ob- tained during the second simulation only.

During the second simulation, two inputs determined the pitch control power used, the pilot's longitudinal stick input and the SAS (stability augmentation system) input. The lon- gitudinal stick input to tail thruster command logic was the same for each of the three configurations. The SAS input was added to the longitudinal stick input, and the combined pitch control power was limited to 0.6 rad/s2. The tail thruster was not phased out at the higher velocities.

The maximum pitch control power usage during all the evaluated runs of each task of the second simulation is sum- marized in Table VII. The values in the table are meant to provide an indication of the pitch control power used and to allow a discussion of the subject. For the hover case, the maximum pitch control power used with the PF and the GFB configurations is broken down according to pilot longitudinal positioning technique (Le.. wing or stick).

It is important to note that in most cases the maximum pitch control power encountered was an isolated "spike" in the data and might not necessarily be the best way to compare control power usage among the configurations, especially since the control laws are still developmental and not finalized. This was particularly me in the case of the geared flap configurations where aggressive wing tilting caused the flap into a position limit (0' if tilting up or 60' if tilting down) which would then result in a large spike increase in pitch control power. Aggressive wing tilting also increased the pitch control power usage of the programmed flap, but be- cause the programmed flap was scheduled, flap position limits were never encountered and the increases in pitch control power were not as large as with the geared flap. In the case of the geared flap configurations, it may be that the change in momentum caused when the wing rate was suddenly arrested

Table VII. Maximum Pitch Control Power Encountered During Evaluation Tasks (rad/s2)

GFS (windstick) 0.45 Turbulence GFl3 (stick)* I N

* Preferred technique for configuration 1 One evaluated run only

by a flap stop was not handled properly in the math model. Comparison of the values shown in Table VI1 do not

show a reduction in pitch control power used by the geared flap configurations cornpared to the programmed flap configu- ration. However, the values in Table VI1 are pitch control power results of the flap concepts at the current stage in de- velopment and should not be taken as conclusive.

VI. Summary of Results

1. The pitch attitude stability augmentation system (SAS) added to the flap configurations during the second sim- ulation was a significant improvement over the pitch rate SAS of the f is t simulation, and greatly alleviated the pilot workload associated with pitch axis control.

2. In general, during the first simulation, the pro- grammed flap configuration showed level 1-2 flying qualities during all the tasks except hover station keeping and the 60 knots STOL landing case, which showed level 1 flying quali- ties. The geared flap configuration showed level 1-2 flying qualities during all the tasks, except the 60 and 40 knots STOL landing cases which showed level 2 flying qualities.

3. In general, during the second simulation, the pro- grammed flap configuration showed levels 1-2 flying qualities during all the tasks except during the hover station keeping with turbulence task which showed level 2 flying qualities. The geared flap configurations generally showed levels 1-2 flying qualities during the descending and decelerating transi- tion to hover and the hover station keeping with turbulence tasks, and level 2 flying qualities during the level inbound transition to hover and the longitudinal reposition tasks.

4. The results did not show a reduction in tail thruster pitch control power usage for the geared flap configurations compared to the programmed flap configuration.

13

VII. Concluding Remarks

The pilot ratings showed that in general, the pro- grammed flap and the two geared flap configurations had simi- lar flying qualities.

The results showed the advantages of pitch attitude aug- mentation for tilt wing aircraft.

Although many of the aircraft characteristics were simi- lar among the three flap configurations, the geared flap con- figurations had degraded velocity predictability near hover due to an adverse acceleration transient response to a wing com- mand, as discussed in the results. This transient response was reduced in the second simulation by the addition of damping about the wing pivot. Further design improvements associ- ated with wing damping as well as the location of the wing pivot relative to the wing aerodynamic center of pressure are warranted for the geared flap configurations to further reduce the adverse acceleration transient response to a wing com- mand.

The results did not show reduced tail thruster pitch con- trol power usage for the geared flap configurations compared to the programmed flap configuration. A more controlled study of pitch control power requirements is warranted where characteristics such as wing pivot location, pivot damping, flap gearing, and flap limits are varied for maneuvering flight. Also, it may be that the geared flap potential for eliminating or reducing the tail thruster pitch control power requirements can only be achieved through a higher bandwidth response, such as with a smaller, two propeller configuration.

References

1. Guerrero, L. M., Corliss, L. D.: "Handling Qualities Results of an Initial Geared Flap Tilt Wing Piloted Simulation", S A E Paper 911201, April 1991.

2. Guerrero, L.M., Corliss, L.D.: "Initial Piloted Simulation Study of Geared Flap Control for Tilt-Wing V/STOL Aircraft", NASA TM 103872, October 1991.

3. Birckelbaw, L. G., Corliss, L. D.: "Phase I1 Simulation Evaluation of the Flying Qualities of Two Tilt-wing Flap Control Concepts", S A E Paper 920988, April 1992.

4. Totah, J.: "Description of a Tilt Wing Mathematical Model for Piloted Simulation", Presented at the 47th Annual Forum of the American Helicopter Society, Phoenix, Arizona, May 1991.

5. Churchill, G.: "Evaluation of Geared Flap Control System for Tiltwing V/STOL Aircraft", Boeing Report No. D8-2076, AD 712 645, January 1969.

6. Cooper, G. E., Harper, R. P. Jr.: "The Use of Pilot Rating in the Evaluation of Aircraft Handling Qualities", NASA TN D-5153, April 1969.

7. Michaelsen, 0. E.: "Application of V/STOL Handling Qualities Criteria to the CL-84 Aircraft", AGARD Conference Proceeding No. 106, June 1972.

9. Pegg, R.J., Kelley, H.L., Reeder, J.P.: "Flight Investigations Of the VZ-2 Tilt-Wing Aircraft With Full-Span Flap", NASA TN D-2680, March 1965.

10. Bernstein, S.: "CL-84 Tilt Wing Applications", The Sixth Congress of the International Council of the Aeronautical Sciences Paper No. 68-45, September 1968.

11. Anon: "V/STOL Handling Qualities Criteria I - Criteria and Discussion", North Atlantic Treaty Organization, Advisory Group for Aeronautical Research Development Report No. 577 Part I, December 1970.

12. Anon: "V/STOL Handling Qualities Criteria I1 - Documentation", North Atlantic Treaty Organization, Advisory Group for Aeronautical Research Development Report No. 577 Part 11, December 1970.

8. Phillips, F. C.: "The Canadair CL-84 Experimental Aircraft Lessons Learned", AIAA-90-3205, September 1990.

14