A comparison of the V/STOL handling qualities of the VAK-191B with the requirements of AGARD report 577 and MIL-F-83300

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NASA TP I 1494 , c.1

NASA Technical Paper 1494

Handling Qualities of the

VAK-191B With the Requirements

of AGARD Report 577 and MIL-F-83300

Seth B. Anderson

JULY 1979 , .

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TECH LIBRARY KAFB, NM OL34bb2-

NASA Technical Paper 1494

A Comparison of the V/STOL

Handling Qualities of the

VAK-lglB With the Requirements

of AGARD Report 577 and MIL-F-83300

Seth B. Anderson A m e s Research Ceirter Moffett Field, Califorilia National Aeronautics and Space Administration Scientific and Technical Information Branch A COMPARISON O F THE V/STOL HANDLING QUALITIES O F THE VAK-191B WITH THE REQUIREMENTS O F AGARD REPORT 577 AND MIL-F-83300 Seth B. Anderson Ames Research Center SUMMARY The handling qualities of the VAK-19 1B VTOL aircraft are compared with current V/STOL handling-qualities requirements. Generally, the aircraft’s handling qualities were superior t o other V/STOL fighter-type aircraft; however, several deficiencies would seriously affect shipboard V/STOL operation. These include poor hovering precision, inadequate mechanical control charac- teristics, nonlinear pitch and roll response, an uncommanded movement of the height (thrust) con- trol lever, low-pitch control sensitivity, excessive dihedral effect, and inadequate overall thrust response. The attitude-command control system resulted in reduced pilot workload during hover and low-speed flight.

The study disclosed gaps in the current handlingqualities requirements, particularly for operation aboard ships. AGARD Report 577 provides more comprehensive coverage than does MIL-F-83300 in the area dealing with STOL operation.

INTRODUCTION The possibility of using a V/STOL aircraft to meet the operational requirements of aviation and non-aviation ships and for other mission deployments has been studied by the U.S. Navy with increased interest in recent years. Because of the desirability to operate V/STOL aircraft from limited areas on small ships and under adverse environmental conditions, severe requirements are imposed on stability and control and handling qualities. Poor handling qualities increase pilot workload and can severely compromise mission effectiveness. Moreover, definitive handling-qualities requirements are needed to aid the design of future V/STOL aircraft.

Specifications for V/STOL handling qualities have been developed from a background of flight experience using a wide variety of V/STOL concepts. Two documents that are currently available t o aid in the design of V/STOL aircraft are AGARD Report 577 (ref. 1) specification and MIL-F-83300 (ref. 2). Neither of these documents has been operationally validated and many design requirements for shipboard operation are not covered.

In order to help define design requirements for future V/STOL fighter aircraft, the U.S. Navy, in a cooperative program with the Federal Republic of Germany (FRG), conducted a flight-test program on the VAK-19 1 B VTOL fighter aircraft. This aircraft, shown in figure 1 and described in reference 3, uses the lift plus lift-cruise VTOL concept and is equipped with a triply redundant, electrohydraulic, 1 00-percent authority, fly-by-wire (FBW) control system. As such, it represented Figure 1.- VAK-191B test aircraft.

an advanced state-of-the-art “operational-type” VTOL aircraft from which design requirements for future VTOL aircraft could be obtained. Of particular interest was an evaluation of the aircraft’s handling qualities t o determine how well it meets existing V/STOL handlingqualities requirements.

The purpose of this report is to: 1. Clarify V/STOL handlingqualities requirements with emphasis on shipboard operations for fighter aircraft.

2. Determine how well the handling qualities of the VAK-191B aircraft comply with existing handlingqualities specifications.

3. Identify omissions in the current handlingqualities specifications.

The report also compares the measured handling qualities of the VAK-19 1B aircraft with the requirements of AGARD Report 577 and specification MIL-F-83300; pilot comments of the capa- bilities and limitations of the VAK-191B are included. The flight-test program was limited by the experimental nature of some of the aircraft equipment; consequently, not all the handling qualities listed in the aforementioned references could be evaluated. From the information that was available, however, an attempt is made t o judge the results in terms of the requirements for a Navy VTOL fighter mission.

RESULTS AND DISCUSSION The various items in the V/STOL handlingqualities specifications are discussed below in the order outlined in AGARD Report 577 because the AGARD document was used by the US/FRG flight-test team as a guideline in conducting the flight-test program on the VAK-19 1B.

Tables 1 through 16 list handlingqualities specifications from references 1 and 2, correspond- ing values determined from tests of the VAK-19 lB, and pilot comments regarding handling charac- teristics of the VAK-19 1 B. Most pilot comments were taken from reference 4.

Characteristics of Control Systems Control breakout forces- The breakout forces, including friction and preload shown in table 1, indicate that the VAK-191B values fall within the range of values listed in the AGARD report (except for yaw), but are higher than those recommended for level 1 operation in MIL-F-83300. Pilots’ comments indicate a preference for lower forces for pitch and roll t o achieve the desired precision of control when using an attitude-command control system, particularly when hovering in ground effect (IGE). Relatively high yaw forces, when combined with the sluggish air- craft response in yaw, made sideslip control more difficult at low speeds.

Control force gradients (table 2)- Except for the yaw axis, the force gradients used on the VAK-191B are within the ranges specified. Even so, the control characteristics were not entirely satisfactory. For example, the pitch-force gradient was less than the maximum allowable. However, the pilot desired an even lower gradient because, with the attitude command type of control system used, it is necessary to continuously hold the stick forces to maintain pitch attitude t o achieve a translational velocity. This could result in less precise positioning, particularly for more demanding tasks such as shipboard operation. In addition, although the control-force gradient was linear, air- craft pitch response was nonlinear. This is due to the manner in which the pitch moments are pro- duced. (See figure 2 for schematic of flight-control system.) For the first 2 in. of stick travel, engine bleed air provides the pitch moments, after which lift-engine (LE) thrust modulation is added. As discussed more fully in the section on pitch control, nonlinear aircraft response increases pilot workload and results in less accurate positioning.

In roll, the breakout force and the force gradient relationship was considered unsatisfactory and did not meet AGARD-577 criteria. This is because the force required for 1-in. (2.54 cm) travel from trim is less than the breakout force. This results in poor control feel about trim. In STOL operation, the force gradient in roll was considered too large because the attitude-command control system required that the lateral forces be maintained while in turning flight. Reduced forces would improve precision of alignment in approach.

In yaw, the gradient was larger than allowed by both handlingqualities specifications. Because the breakout force is high, the gradient must also be high (greater than the breakout force) t o avoid control centering problems. The VAK-I 9 1 B could be improved in this regard by reducing the break- out force, which, in turn, would allow a reduced force gradient, thereby improving heading control accuracy and control of sideslip.

Control force harmony ratio (table 2)- Control force harmony (i.e., the ratio of maximum control force for one axis compared t o another axis) for the VAK-191B falls within the guidelines given in the AGARD report (no values are given in MIL-F-83300). Proper harmony is more impor- tant for attitude command systems because the forces are held for longer periods of time. A more accurate definition of control force harmony than that given in the AGARD report is needed, t o take into account the differences likely t o occur where more sophisticated (higher order) control systems are used and when side-stick controllers o r other unconventional control systems are used.

Height-control systems (table 3)- One of the more serious deficiencies of the VAK-191B was an uncommanded movement of the height-control lever (throttle) which could occur during certain hovering operations. This was due t o the pitch attitude stabilization system having the authority t o move the liftengine throttle (height control) against the pilot's hand t o reduce engine thrust t o stay within prescribed temperature limits. This uncommanded movement occurred on several occa- sions, while hovering in ground effect (IGE), when recirculation of the engine exhaust resulted in inlet temperature increases with resultant thrust changes. Another height-control system deficiency was the lack of an adjustable friction device. This is needed t o prevent unintentional height loss resulting from movement of the control when the pilot removes his hand to adjust other controls.

AGARD Report 577 requires that the height control remain fixed at all times unless moved by the pilot or some automatic system. A variable-friction adjustment is also required since less friction is desired for hover operations than for cruise. Specification MIL-F-83300 does not have specific requirements for height-control systems.

Powered-control systems (table 3 ) - Another mechanical control system deficiency noted was the poor lateral damping of the control stick. The low value of viscous damping contributed t o a 17. D U P L E X S E R V O A C T U A T O R , E L E C T R I C A L L Y 1.

COCKPIT C O N T R O L GRIP S I G N A L L E D 2. POTENTIOMETER ( L O N G I T U D I N A L STICK) 18. A R T I F I C I A L - F E E L U N I T S A N D T R I M SERVOS 3. F O R W A R D PITCH-CONTROL R E A C T I O N N O Z Z L E 19. R U D D E R PEDALS 4. R E A R PITCH-REACTION N O Z Z L E 20. P O T E N T I O M E T E R ( R U D D E R PEDALS) 5. TAILPLANE-POWER A C T U A T O R S ( H Y D R A U L I C ) 21. F E E L U N I T 6. INTERCONNECT1 N G - C A M - L I NK A G E O U T P U T 22. C A B L E D R I V E T O R U D D E R Q U A D R A N T ( I N P U T ) 7. D U P L E X A C T U A T O R , H Y D R A U L I C A L L Y 23. D U P L E X SERVO, H Y D R A U L I C A L L Y OPERATED, OPERATED, E L E C T R I C A L L Y S I G N A L L E D E L E C T R I C A L L Y S I G N A L L E D (YAW A X I S ) (PITCH A X I S ) 24. R U D D E R - L I M I T E R C O N T R O L 8. T A I L P L A N E DAMPERS D U P L E X R U D D E R POWER A C T U A T O R 25.

9. R E A C T I O N - N O Z Z L E G E A R A N D Q U A D R A N T Y A W - C O N T R O L R E A C T I O N N O Z Z L E S 26.

10. F O R W A R D A N D R E A R N O Z Z L E 27. F L A P L E V E R I N T E R C O N N E C T I N G T O R Q U E S H A F T 28. T R A N S D U C E R 1 1 . C O N T R O L C O L U M N 29. D U P L E X S E R V O A C T U A T O R , H Y D R A U L I C 12. P O T E N T I O M E T E R ( L A T E R A L S T I C K ) 30. F L A P A C T U A T O R ( B O T H SIDES), H Y D R A U L I C 13. R O L L - C O N T R O L N O Z Z L E ( L E F T A N D R I G H T ) H Y D R A U L I C C O U P L I N G 31.

14. A I L E R O N - C O N T R O L D U P L E X A C T U A T O R , 32. SPRING L I N K H Y D R A U L I C A L L Y OPERATED, E L E C T R I C A L L Y 33. A I L E R O N - A N D FLAP-POSITION I N D I C A T O R S I G N A L L E D C A B L E D R I V E T O TENSIONER U N I T 15.

16. C O N T R O L INPUTS, M E C H A N I C A L I N T E G R A T I O N U N I T Figure 2.- Schematic of the flight-control system of the VAK-191B.

lateral pilot-induced oscillation (PIO). This was considered unsatisfactory; both handlingqualities references require that oscillations of powered-control systems be well damped.

Another control problem could occur following a powered-control system failure. The VAK-19 1 B mechanical backup system was judged t o be unsatisfactory for hover because of exces- sive backlash and large friction breakout and force gradients. Based on piloted simulator studies and tests on a pedestal test rig, failure of the power-control system would not allow a safe landing because of these poor mechanical characteristics. The need t o provide satisfactory control charac- teristics following a failure of the powered-control system is noted only in the AGARD report.

Trim systems- There is an obvious need to provide trim systems that operate fast enough t o keep control forces small during changes in aircraft configuration or speed and during any maneuver consistent with service use. The need t o maintain low control forces by adequate trim facilities would be more important for shipboard operation where greater precision of flight path and touch- down is required. As noted in table 4, both handlingqualities references have requirements in this regard. Pilots’ comments indicate that the pitch trim rate on the VAK-191B would be too slow for the forward and sideward positionings required in approaches t o a moving shipboard landing area.

As previously noted, the need t o trim out control forces is more important with attitude-command systems where forces must be maintained for longer periods of time t o hold a given aircraft attitude.

Additional concern was expressed over the inability t o prevent “runway” trim with the system used on the VAK-191 B aircraft. Also, in the event of a stability augmentation system (SAS) failure, it was not possible to provide trim capability.

Thrust-vector controls- Thrust-vector control characteristics should allow accurate control of flight path and airspeed as desired during transition as well as accurate ground positioning in hover.

A selected setting should be held indefinitely t o avoid unintentional drift and cross-trimming. The system used on the VAK-19 1 B provided rapid thrust-vector movement, but the tendency of the nozzles to drift slightly from their selected setting could affect hovering precision.

The requirement that the pilot’s hand be removed from the throttle when making thrust- vector nozzle adjustments was considered to be undesirable when operating close t o obstacles. As a result, the pilot preferred not to use nozzle adjustment for fore-and-aft positioning. This point is covered in greater detail in Sec. 1.8 of AGARD Report 577. N o requirement is listed for this item in M IL-F-83300.

Generally, the mechanical control characteristics of the VAK-19 1B were considered to be unsatisfactory by the pilots. This opinion was substantiated by the fact that the system did not meet many of the requirements stated in the two handlingqualities references. Although there were no major differences in the control-system specifications between the two handlingqualities refer- ences, AGARD Report 577 covered a broader scope of items peculiar t o V/STOL control systems.

A better design guide is needed on the mechanical control characteristics for advanced V/STOL control systems.

Longitudinal Stability and Control Pitch-control power- The total amount of pitch-control power required for V/STOL opera- tion depends essentially on three inputs: (1) how rapidly the aircraft must be maneuvered, (2) the magnitude of trim changes associated with power, flaps, or thrust-vector-angle changes, and (3) pitch-angle changes required t o correct for upsets due t o gusts, recirculation, or other disturbances.

As shown in table 5 , the total available power for pitch-angular acceleration (1 .O rad/sec2 ) on the VAK-19 1 B was larger than that required by the handling-qualities references and was considered adequate for hover out of ground effect. Pitch-attitude control of the VAK-19 1 B deteriorated when close to the ground, however, because recirculation of engine exhaust reduced the maximum differ- ential thrust available for pitch moments. Even relatively large amounts of control power may not be completely satisfactory for landing on a moving deck if pitch angular acceleration is the sole means of obtaining fore and aft positioning. Studies (ref. 5) of landing on a small platform under poor conditions (turbulence and heavy seas) indicate that a velocity command-control system was needed for a satisfactory pilot workload. A blending of pitch attitude for inner-loop control with velocity-command outer-loop control will undoubtedly be more desirable when operating with a pitching, moving deck.

Control-power requirements are specified differently in the two V/STOL handling-qualities (1) in terms of the indi- references. In AGARD Report 577, requirements are listed in two ways: vidual requirements for maneuvering, trim, and upset, and (2) as a range of values typical of those required for a wide variety of V/STOL aircraft, including trim, upset and an allowance for how rapidly the aircraft is intended t o be maneuvered. By considering each individual control-power need and the type of control system, the AGARD criteria are useful as a design guide and establish a more realistic total control power for a specific aircraft configuration. Specification MIL-F-83300 presents the requirement in terms of a pitch attitude change after 1 sec, as measured in flight tests, with the wind from the most critical direction. Neither of the handling-qualities guides specify translational control power values or means for relaxing angular acceleration capabilities in the event that velocity command-control systems are used. Clearly, there is a strong need t o improve pitch-control power requirements t o account for the use of more advanced control systems and t o reflect the more demanding needs for shipboard operation.

Pitch-control sensitivity- Control sensitivity has a major'influence on the pilot's impression of aircraft response and precision of control. With conventional control systems, pilots prefer t o use small, abrupt, high-frequency, control inputs t o adjust aircraft attitude t o obtain a desired transla- tion over the ground. If control sensitivity is too low, it creates the impression of sluggish response; consequently, the pilot compensates by making large, gross control motions that result in pilot fatigue and reduced accuracy of aircraft positioning. The VAK-I 9 1 B pitch-control sensitivity was 1.25" pitch-angle change in 1 sec for the first inch (2.54 cm) of stick movement. Both handling- qualities specifications require at least a 3" change in 1 sec per inch (2.54 cm) of stick deflection.

This low pitch-control sensitivity of the VAK-19 1 B was judged inadequate because aircraft response was too sluggish for small control inputs. A further deterioration in precision of pitch-attitude con- trol due t o nonlinear response was apparent when larger control deflections were used. This aspect is discussed in the next section.

Linearity of aircraft response- Constant (linear) control effectiveness is desirable for two reasons: (1 ) the pilot may use control position to indicate margins available for trim and maneuver- ing, and (2) overcontrolling tendencies can occur when nonlinear characteristics are present because of the pilot’s inability t o predict the final aircraft response. AGARD Report 577 specifies that an aircraft’s response t o control input should be constant or, if not constant, should not abruptly increase or change sign. Specification MIL-F-83300 is more general, stating that no objectional nonliiiearities should exist.

The VAK-191 B response t o pitch-control inputs became nonlinear at approximately 5.08 cm (2 in.) of stick movement from neutral, due to the use of a combined bleed-air/thrust-modulation control concept. When small pitch-control inputs in the differential thrust control deadband were made, an oscillatory aircraft motion of about 0.5 Hz and 2” pitch amplitude resulted, which made precision hover more difficult.

Longitudinal-control characteristics in takeoff (table 6)- Rolling vertical takeoff (RVTO) and short takeoff (STO) are desirable operational modes, particularly for jet-lift aircraft, t o help reduce ground erosion, hot-gas ingestion, recirculation, and aerodynamic suckdown, as well as to improve payload capability by taking advantage of aerodynamic lift. Both handlingqualities documents specify that the effectiveness of the pitch control shall not restrict takeoff performance, and shall be sufficient t o prevent overrotation t o undesired attitudes.

For several reasons, the VAK-19 1B aircraft pitch attitude was difficult t o control accurately in takeoff runs. First, the large ground reaction moment associated with the bicycle landing gear geometry made it difficult to prevent pitch overshoots at liftoff in spite of the pilot’s anticipation of this change in pitch response. This pitch-up tendency was considered undesirable and limited the potential benefits of using aerodynamic lift for improved takeoff performance. A second factor that reduced pitch-attitude control effectiveness in takeoff was a mechanical feedback problem in the lift-engine throttle lever. Due t o the manner in which the high authority stability and augmentation system functioned, the throttle lever could push back the pilot’s hand if a reduction in thrust was required. This feedback occurred more frequently with smaller T/W values, with increased airspeed, and when the pilot introduced a commanded-pitch input at liftoff. This uncontrolled-for movement of the liftengine throttle lever was considered highly undesirable because it increased height-control coupling problems. There are no handlingqualities requirements in either reference that bear directly on this adverse-control-system feature.

The piloting takeoff procedure for RTO and STO should not be complicated t o the extent that height control (as affecting liftoff performance) is compromised. The complicated procedures that were required during takeoff for the VAK-19 1 B illustrate the problem. Because of the need t o start the lift engines during the takeoff roll (to avoid jet efflux erosion of the runway surface), many accurately timed pilot inputs were required (1 5 distinct steps) during the takeoff run. In addition, it was not possible to check for proper pitchcontrol functioning until after the aircraft had started its takeoff run. This increased pilot workload to the saturation point and compromised the accuracy of takeoff performance. This operational concern for downwash effects could be equally serious for shipboard operation. The need t o guard against complicated takeoff procedures and provisions for checking proper control functioning are covered adequately in the handlingqualities references.

Longitudinal-control characteristics in landing- The requirement for pitch-control effective- ness can become critical in short landings because of possible lift losses and adverse trim changes as the aircraft enters ground effect. As noted in table 7, both handlingqualities documents specify the need to provide adequate control power to adjust pitch attitude as necessary close t o the ground. In terms of total control power, the VAK-191B was satisfactory, since only a mild nose-up trim change occurred in entering ground effect. However, the lift-engine thrust modulation feedback previously discussed for takeoff could cause landing (touchdown) problems if very low touchdown speeds were used. For example, if a commanded attitude above approximately 5" nose-up was held or imposed at touchdown, the stabilization system could produce a throttle increase rapid enough t o snatch the throttle lever from the pilot's hand. Again this condition is not covered in either of the handlingqualities references.

Lateral-Directional Stability and Control Roll-control power- As discussed previously for pitch-control power, the total amount of roll- control power required depends on proper summation of the individual requirements for maneuver- ing, trim, and upset. As shown in table 8 , the roll-control power available on the VAK-191B for bank-angle control meets the criteria of both handlingqualities references and was judged to be adequate for hover in n o wind. In forward flight at low speeds, however,the bank-angle limit of k15" for full-stick throw (maximum provided with the attitude-command system) was judged t o be too small at speeds of about 60 knots, and would not allow a quick enough alignment from a nominal centerline offset.. Although AGARD Report 577 provides for different values of control power, depending on the type of control system employed, a maximum value of bank angle for STOL maneuvering should be included. In addition, as discussed later under "dihedral effect," the VAK-19 1 B did not have sufficient roll-control power for crosswind operation.

Satisfactory control of both lateral translation and bank angle would be particularly important for shipboard operation where alignment with a moving and canted deck can impose more severe roll-con trol requirements.

As discussed in the section on pitch-control power, there is evidence from piloted simulator studies (ref. 5) that direct translational control can greatly improve precision of control in approach and touchdown for shipboard operation onto a small platform in heavy seas. Translational control criteria for lateral positioning are included only in AGARD Report 577 wherein a lateral accelera- tion range of values between 0.08 t o 0.12 g in wings-level sideward flight is specified. Although the use of translational control greatly reduces the amount of angular acceleration control power required, some residual roll angular acceleration capability will be needed for wing alignment at touchdown. Further clarification is needed on the tradeoffs between translational and angular con- trol methods, including proper phasing out of translational control with increased forward speed.

Roll-control sensitivity- The optimum value of roll-control sensitivity depends, t o a large extent, on the vehicle dynamics. Generally, the pilot prefers quick response without overshoot or P I 0 tendencies. Specification MIL-F-83300 allows for a relatively large spread in sensitivities 0.03-0.14 rad/cm (4"-2O0/in.). AGARD Report 57.7 is more restrictive, specifying different ranges of values, depending on the type of control system used and the mode of operation.

Although the roll-control sensitivity of the VAK-191B falls within the bands presented by both handlingqualities specifications (see table 8), the system was not completely satisfactory.

First, when brisk maneuvering was attempted, a lateral P I 0 was encountered; and, second, the SAS introduced a nonlinear restoring moment in roll reversals. The reasons why the P I 0 tendencies were encountered with the attitude-command control system on the VAK-I 9 1B are not completely clear; however, as noted previously in the discussion on control systems, a contributing factor could be the poor mechanical damping characteristics of the control stick in lateral movements.

Linearity of aircraft response- As noted previously for pitch control, linearity of aircraft response t o control input is desired by the pilot. This is true t o a greater extent for the roll axis because of the need t o control bank angle more accurately, particularly for shipboard operation. In addition, with conventional control systems, the pilot tends t o use cockpit control position as an indication of control margins needed t o provide for trim changes and upsets or disturbances.

Although the VAK-I 9 1 B roll-coatrol system provided linear aircraft response with control deflection when the control was initially moved from the trim position, the response was nonlinear when the cockpit control was returned t o neutral position (see comments in table 8 and schematic of control system (fig. 2)). This was due to peculiarities of the SAS. When the cockpit control was returned t o center after an input of over 60-percent deflection, the roll servo reversed sign, intro- ducing a large roll overshoot that could not be precisely anticipated by the pilot. Neither of the handling-qualities specifications treat this type of nonlinear control system behavior adequately.

Cross-coupling (sideslip excursions)- Roll-control inputs for maneuvering at STOL operating speeds can cause large changes in heading (sideslip), pitch attitude, and vertical lift for most V/STOL configurations. Of these, the sideslip excursions have proved t o be the most important, and the need to limit sideslip excursions has been included in handlingqualities specifications from the start.

The lateraldirectional cross-coupling characteristics of the VAK-19 1 B were evaluated in a speed range from 0 t o 120 knots, with primary emphasis at a 60-knot approach speed (table 9).

Although the adverse yaw was very small at 60 knots, coordinated turns were relatively difficult to execute because of the inability t o precisely control sideslip near zero by use of the rudder.

Aileron-only turns were preferred to minimize sideslip excursions. Because of the large dihedral effect (discussed next), sideslip excursions required relatively large lateral trim requirements that could saturate the lateral control system.

Both handlingqualities references state limits for turn coordination in terms of AP/A@, with a much larger allowable value stated in MIL-F-83300. The allowable values of sideslip excursion need improved definition, specifically for the instrument approach task and t o reflect how other related factors, such as heading lag in turn entries, roll damping, and dihedral effect influence pilot rating.

Dihedral effect- Although positive dihedral effect is desired by the pilot, the absolute amount tolerable depends on many factors, including the aircraft’s dynamics, turn coordination character- istics, roll-control power, and spiral stability. Because many VTOL aircraft inherently have too much positive dihedral effect at low speeds, it has been necessary t o restrict the amount allowed so that sufficient excess roll-control power is available for maneuvering and operation in turbulence and crosswinds. As shown in table 9, both handlingqualities references recognize the need t o limit positive dihedral effect. Specification MIL-F-83300 states that a margin of 50-percent roll-control power should be available t o the pilot.

The VAK-19 1B experienced a strong positive dihedral effect that limited crosswind operation and lateral sideslip maneuvers t o undesirably low values (approximately 10” sideslip at 50-knot forward speed). Because the roll-restoring moment is caused by a combination of factors - including liftengine momentum drag and aerodynamic and powerinduced flow characteristics over the wing - the amount of sideslip available t o the pilot before roll-control saturation occurs depends on engine power, angle of attack, and airspeed. The lack of a completely satisfactory system t o warn the pilot of control saturation was another concern. In a skidding turn, for example, the attitude command-control laws automatically provide roll control to compensate for asym- metric moments with the control stick centered. The pilot thus loses a natural warning of limited trim capability degrading pilot rating, even though the servo control position was presented on a headdown display. This nonlinear response (maneuvering deadband) can quickly lead t o a dangerous situation if roll upsets are encountered at low altitude.

A control margin t o guard against upset is difficult t o establish from the data base available.

The 50-percent margin noted in MIL-F-83300 is arbitrary and may unduly penalize some VTOL designs that are not required to be maneuvered extensively. Improved criteria are needed, taking into account multicontrol inputs and wind-direction considerations.

Yaw-control power and sensitivity- For most VTOL aircraft in hover, experience has indicated that relatively smaller amounts of control power are needed for the yaw axis than for the pitch and roll axes. Although the total value must include requirements for trim and upset,the major demand has been for maneuvering. The directional characteristics of the VAK-I 9 1 B at speeds below 20 knots were typical of other jet VTOL concepts in that n o directional stability was evident, and yaw response was sluggish (see table I O ) . In hover, the aircraft tended t o drift slowly out of 30 knots, positive directional stability was wind, indicating small directional instability. At about evident. The values shown in table 10, when compared t o the handlingqualities requirements, con- firm that both control power and sensitivity are too low at hover. As forward speed was increased, rudder effectiveness increased, and the aircraft became too sensitive for precise directional control.

This nonlinear response was due t o the fact that the gain for the yaw-rate command system remained constant regardless of forward speed. Because of this high yaw sensitivity even at low forward speeds (60 knots), the pilot preferred not to use rudder in turn entries. Large yaw/roll cross-coupling occurred during full-rudder-pedal-input turns because of the “aft only” location of the yaw reaction nozzles. On one occasion, the large rolling moment associated with lateral velocity during a large heading change maneuver saturated the lateral control system with no warning t o the pilot.

Hover and Vertical Flight-Path Characteristics Ground effect- During operations near the ground, most jet-type VTOL aircraft have experi- enced some form of unsteady dynamic behavior resulting from recirculation and impingement of engine exhaust gases on the undersurface of the aircraft. Depending in part on the type of control system used, precision of height control can be seriously degraded in landing and takeoff. The acceptable magnitude of disturbance will vary with both the mission and task.

The AGARD report criteria for ground-effect characteristics (noted in table 11) state the need t o provide satisfactory aircraft behavior in ground effect for any wind condition, including a safe landing capability in the event of a power-control system or SAS failure. No parallel requirement is contained in MIL-F-83300.

The VAK-19 1 B experienced random unsteadiness about all axes when hovering in n o wind at heights less than about 7 ft (2.12 m). Because of the attitude stabilization system used, pilot control inputs required to maintain position were minimum. Positive (favorable) ground effect was evident during level attitude, no-wind conditions, creating minor pilot complaint due t o nonlinear height- control response. Prolonged hover with a nose-up attitude, as might be required in a tail wind o r in crosswinds, had t o be avoided because of engine exhaust reingestion in the cruise engine inlet and rear lift-engine inlet, resulting in uncontrolled-for pitch attitude changes and loss of precision in height control. In addition, loss of the power-control system o r SAS, particularly in the roll axis, would make a safe landing questionable. The reason for this, based on piloted simulator studies and tests on a static test-stand pedestal, is that reversion t o the manual-control system increases the fric- tion (breakout) forces t o high values that would adversely affect precision of attitude control.

Ground-effect characteristics can be expected t o be of greater concern for platform operation from small ships when only part of the aircraft is over the landing platform. More operational expe- rience is needed t o define handlingqualities criteria for these conditions.

Hovering precision- Hovering precision is necessary t o ensure that a VTOL aircraft can operate in a confined space. The requirement for precise hover control will vary, depending on the type of aircraft, mission, and task.

AGARD Report 577 defines hovering precision in terms of the overall geometric dimension of the aircraft, allowing more leniency for operation out of ground effect (table 12). No requirement of this type is included in MIL-F-83300.

The VAK-19 1 B exhibited satisfactory hovering precision when hovering in n o wind in a level of the attitude-command control system. As previously noted, attitude by virtue of the benefits however, hovering with a nose-up attitude or in tail winds resulted in loss of pitch or height control, or both, due t o nonlinear thrust response associated with hot-gas ingestion. In addition, precision of height control varied with the margin in T/W available, due t o a nonlinear thrust requirement asso- ciated with an apparent positive ground effect, recirculation'. (ingestion) in the rear lift engine, and different thrust response-time constants between the front and rear lift engines. Pilot workload was relatively high because of the aforementioned items, and precision VTO characteristics would be considered inadequate for shipboard operations at low T/W values.

Vertical-thrust margins- Adequate vertical-thrust margins are needed for satisfactory height control t o establish safe climbout procedures and t o adjust sink rate as needed for touchdown.

As noted in table 13, both handlingqualities documents require a minimum T/W value of the order of 1.05 for takeoff. Vertical-thrust margin requirements in AGARD Report 577 criteria are more definitive, taking into account vertical-height damping. Vertical (height) damping levels affect the vertical-thrust margins needed, particularly if the heightdamping characteristics are nonlinear with respect to height from touchdown. For landing, larger values of T / W (up t o 1 . l ) are specified as vertical-height damping decreases t o lower values. For shipboard operations, it seems reasonable t o expect that generally higher T/W values (of the order of at least 1 . l ) may be needed t o cope with the added problem of a heaving ship deck. Additional information is needed t o more realistically specify vertical thrust margins, taking into account ship motion characteristics, simultaneous control usage, and environmental (wind shear) effects.

Height control for the VAK-191B was obtained by using the combined thrust variations of lift and the lift/cruise engine or by using a split-throttle arrangement to provide more gross thrust changes. Height control with matched throttles was considered satisfactory when a T/W of 1.1 was available and inadequate when T / W was at 1.05. With split throttles, height control was not satis- factory even with T/W on the order of 1.1, because of speed/attitude coupling problems associated with low throttle sensitivity, nonlinear height response due to recirculation, and ingestion of exhaust gases.

Height-control sensitivity and thrust response- Height (thrust) control sensitivity is a param- eter that should be optimized to provide precise vertical flight-path maneuvering. A number of studies have indicated that a height-control sensitivity of the order of 0.1 2 g/cm (0.3 glin.) of con- trol movement is desired. A range of values is given in AGARD Report 577 (0.04-0.16 g/cm (0.1-0.4 g/in.)) to take into account the influence of vertical-height damping. No requirement is listed in MIL-F-83300.

The VAK-19 1 B had inadequate thrust response with split-throttle (lift engines only) operation (0.016 g/cm (0.04 g/in.)). This low sensitivity resulted in excessive lags in height-control adjust- ments and was considered unsatisfactory. In addition, the front-lift engine had a faster thrust of nonlinear pitch- response than the rear engine; this caused problems in liftoff dynamics because attitude characteristics.

The thrust response of the lift engines was considered adequate; however, the lift/cruise engine response was inadequate, which led to overcontrolling tendencies. A particularly bad feature was the automatic limiting o f maximum power, which created a region o f throttle movement deadband.

Neither of the handling-qualities specifications covers these nonlinear response characteristics ade- quately. It would appear desirable to provide a warning of an approaching deadband condition. In addition, protection for nonlinear response characteristics in multiengine aircraft should be provided.

Transition Characteristics Transition handling characteristics can be more demanding for the lift plus lift/cruise concept because of the added pilot workload associated with lift-engine power management and the change in performance (power available for acceleration) due to the increase in lift-engine ram drag with increase in forward speed. The foregoing affect flight-path tracking precision, particularly for IFR operation.

Accelerationfdeceleration- The need t o have adequate control of longitudinal acceleration/ deceleration is important for a number of reasons, including time a t high specific fuel consumption, tactical considerations, and lift-engine life constraints. Each of the handlingqualities documents requires rapid and safe acceleration/deceleration capability for transition, without undue attention to loss of control o r departure from a prescribed flight path (table 14).

For the VAK-191B7 acceleration during transition from hover to conventional flight was exe- cuted by vectoring the lift/cruise engine nozzle a t a rate that depended o n the thrust margin avail- able. Because of the lift-engine ram drag, longitudinal acceleration changed markedly over the con- version speed range. At the initial part o f transition, the nozzles could be rotated more quickly, pro- ducing a longitudinal acceleration o f about 0.5 g, or a positive climb gradient, or both. At the higher end of the speed range, climb rate had to be reduced to zero to allow acceleration to 200 knots, a t which speed thrust was approximately equal to drag. Further acceleration could be achieved only after shutting down the lift engines (to reduce ram drag). This inability to accelerate to conven- tional flight in a climb profile would obviously be undesirable for a fighter mission. Conversely, however, decelerations from high speeds could be made quite rapidly. In the speed range u p to 120 knots, the attitude-command response characteristics were satisfactory and greatly reduced pilot workload.

Flexibility of operation- The need to provide the pilot with the option of aborting the transi- tion quickly is required in both handlingqualities documents (table 14). The VAK-191B met this requirement in principle, since the cruise-engine thrust vector could be adjusted as desired. But the available pitch-angle range of k 15" was considered inadequate due to lack of sufficient nose-up capability for steep climbs and quick stops. A value of 20" nose-up would seem t o be sufficient.

AGARD Report 577 has a more stringent additional requirement for allowing a safe landing o r wave-off in the event of a failure of a single engine with a multiengine aircraft or failure of a power- control system o r SAS. In this regard there would be difficulty with the AK-191B concept. With the engine arrangement used, failure of one lift engine, even at moderate approach speeds, would require the other lift engine t o be shut down for pitch balance. There would be insufficient thrust for a go-around with only the cruise engine. Because of the small wing (wing loading of 135 lb/ft2 (1 .O 1 hg/cm2 )), relatively high-approach speeds (200 knots o r greater) are needed for a conventional landing. As noted previously, failure of the SAS would not allow a vertical landing because of poor mechanical control systems characteristics.

Tolerance i n convcrsiorz (table 14)- The need to ensure against excessive pilot workload, high skill requirements, and excessive pilot attention in carrying out the transition is brought out in both handling-qualities documents. For the VAK-19 1 B, conversion requires relatively complex power management techniques, a large attitude (angle of attack) change as the lift engines are cycled in or out, and close monitoring of angle of attack (to minimize induced drag) because of the poor thrust margin available at the high-speed end of the transition corridor.

Control margins (table 15)- Sufficient control power must be available during transition to allow the pilot to maintain desired attitudes in turbulence and changes in flight-path angle as required for the mission. Specification MIL-F-83300 requires the control margin t o be such that 50-percent control margin must remain about any axis at any stage in transition. AGARD Report 577 criteria state that only a margin (beyond trim requirements) for maneuvering is required with the added proviso that simultaneous control input must be considered. Further work must be done to permit a realistic selection of control margins, taking into account the sensitivity t o gusts o f a given V/STOL concept and the amount of maneuvering required for the mission. I n addition, as noted in the AGARD report, a warning should be provided t o help guard. against reaching unsafe limits in angle of attack or sideslip during transition. I n this latter respect, the VAK-19 1 B system was unsatisfactory in that the approach to a loss-of-control situation was masked by the 100-percent authority SAS that tended t o create control (maneuvering) deadbands. Pilots did not consider the use of a CRT display in the cockpit, which indicated the amount of control power being used for SAS requirements, t o be satisfactory for operational use.

Trim changes (table 15)- The necessity that all trim changes required during transition be small in order t o reduce pilot workload and provide for precise flight-path control is noted in both handlingqualities documents. An additional point brought out in AGARD Report 577 which states that trim changes associated with switching from one control mode t o another should be satisfac- tory. In this regard, the VAK-191B was not completely satisfactory. In changing from the attitude mode t o the ratedamped mode, the pilot was required t o continuously apply nosedown pitch trim in accelerating transition and vice versa during deceleration. During control-mode transfer, a slight P I 0 tendency existed.

Miscellaneous Characteristics Control effectiveness during takeoff and landing rollout- The ability t o maintain a desired takeoff and landing rollout path under designated wind conditions is a requirement of both handlingqualities references (table 16). This is of particular importance t o shipboard operation where only limited space is available. The VAK-19 1B had generally good nose-wheel steering charac- teristics for takeoff; however, the “hot” (always engaged) nose-wheel steering was considered undesirable in crosswind landings because of inadvertent pilot inputs. In addition, the crosswind takeoff envelope was too low (10 knots). This was set by lateral-control-power limits required t o trim the large, positive dihedral effect and lift-engine gyroscopic moments. In addition, the bicycle gear arrangement made it more difficult to avoid overrotation and damage to the aft fuselage during takeoff. The requirement that a 360” turn be executed within a circle equal to the dimensions of the aircraft could not be met by the VAK-19 1 B.

Power checks before takeoff- The need t o check for proper control functioning before reach- ing takeoff power is self-evident, particularly with full-authority SAS control.. This was not possi- ble with the VAK-191 B under all wind conditions because of reingestion tendencies, as previously noted.

CONCLUDING REMARKS The handling qualities of the VAK-19 1B VTOL aircraft are compared with current V/STOL handling-qualities requirements. Generally, the aircraft’s handling qualities were superior t o other V/STOL fighter-type aircraft; however, several deficiencies would seriously affect shipboard V/STOL operation. These include poor hovering precision, inadequate mechanical control charac- teristics, nonlinear pitch and roll response, an uncommanded movement of the height (thrust) con- trol lever, low-pitch control sensitivity, excessive dihedral effect, and inadequate overall thrust response. The attitude-command control system resulted in reduced pilot workload during hover and low-speed flight.

The study disclosed gaps in the current handlingqualities requirements for operations aboard ships. AGARD Report 577 provides more comprehensive coverage than does MIL-F-83300, particularly in the area dealing with STOL operation.

Ames Research Center National Aeronautics and Space Administration Moffett Field, Califomia 94035, March 9 , 1979 REFERENCES 1. V/STOL Handling - Qualities Criteria, Parts 1 and 2. AGARD Report R-577, 1970.

2. Military Specification: Flying Qualities of Piloted V/STOL Aircraft; MIL-F-83300. Dec. 31, 1970.

3. Traskos, R.; Schweinfurth, R.; and Anders, G.: USN/FMOD FRG VAK-191B Joint Flight Test Program. Final Report, Vol. 2. Aircraft Description and Flight Test. Aug. 1976.

4. Obermeir, L.; and Iles, J. E.: USN/FMOD FRG VAK-191B Joint Flight Test Program. Vol. 3 . Pilot Evaluations.

Aug. 1976.

5. Merrick, V. K.: Study of the Application of an lmplicit Model-Following Flight Controller to Lift-Fan VTOL Aircraft. NASA TP 1040, Nov. 1977.

I TABLE 1 .- CONTROL-SY STEM CHARACTERISTICS: CONTROL BREAKOUT FORCES I 1 I I ~ I

i AGARD report 1 MIL-F-83300 ~

VAK-19 1 B ' Pilot comments about VAK-19 1 B '

Item I I R-577 (ref. 1) (ref. 2) 1 Control breakout forces, lb 0.5- 3 0.5-1.5 2.2 too large for pre- Pitch breakout forces Pitch - pilot prefers to cision hovering operate out of breakout range 0.5-1.5 2.75 Roll force larger than desired, therefore ~ Roll 0.5- 3 difficult to determine trim 11.0 Yaw forces too large, difficult to con- Yaw 1-10 2-7 trol sideslip Value unknown; Lack of an adjustable friction device Height (throttle) 1-3 1-3 preset before flight; considered a serious deficiency not adjustable by pilot TABLE 2.- CONTROL-SYSTEM CHARACTERISTICS: CONTROL FORCE GRADIENT AND HARMONY RATIO ~~ AGARD report MI L-F-83 300 Pilot comments about VAK-19 1 B Item VAK-19 1 B R-577 (ref. 1) (ref. 2) Control force The force gradient shall not be less than the gradients, lb/in. breakout force 2.2 Prefer pitch-force gradient to be Pitch 1-3 0.5-3 reduced for attitude-command con- trol system because of need to hold control force t o maintain a given pitch attitude 0.5-2.5 2.1 Combined effects of high breakout Roll 0.5-1.75 forces and gradient result in poor feel and centering about trim - could con- tribute to lateral P I 0 tendencies noted in flight Yaw 2.5-10 5-10 13.5 High breakout and force gradient in yaw make accurate control of sideslip difficult when combined with sluggish yaw response in hover and high yaw sensitivity at 60 knots ~ Control force Pit ch/roll , No values stated Pitch/roll, 1 Good harmony important for attitude-

1 harmony ratio optimum 2 (response shall be

command control systems; pitch/roll harmonious) ratio should be 1 if forces are light, Yaw/roll, Yaw/roll, 6.4 2 if forces are heavier optimum 6 . . .

TABLE 3.- CONTROL-SYSTEM CHARACTERISTICS: HEIGHT CONTROL AND POWERED-CONTROL SYSTEMS MIL-F-83300 AGARD report VAK-19 1B Pilot comments about VAK-19 1B

1 Item R-577 (ref. 1 ) (ref. 2)

Height control (lift Uncommanded movement of height

I Height control Height control should

engine throttles) can controls caused by the pitch-control 1 systems remain fixed unless move against the system (thrust modulation feedback) moved by pilot or pilot’s hand t o pro- was considered unsatisfactory some automatic system vide attitude stabilization Lack of friction level control unsatis- No friction adjust- Adjustable friction device desirable ment provided factory because control can move inadvertently when pilot removes his hand to adjust nozzle vector angle Lateral P I 0 can occur if stick is Con t rol-sys tem lscillations of all Lateral-control sys- Powered-con trol released - lack of controlsystem oscillations should :ontrol systems shall tem lacks viscous systems damping is unsatisfactory not adversely affect )e well damped damping precision of control or cause pilot- induced oscillation Mechanical backup system not suit- Failure of powered- Failure of powered- able for hover due to large breakout- control system control system would result in rever- forces, friction, backlash, and force should not restrict operational maneu- sion to mechanical gradients vers required for linkage system mission N TABLE 4.- CONTROL-SY STEM CHARACTERISTICS: TRIM SYSTEM AND THRUST VECTOR CONTROLS ~~ AGARD report MIL-F-8 3 300 VAK-19 1 B Pilot comments about VAK-I9 1 B Item R-577 (ref. 1) (ref. 2) Trim devices shall Two trim rates avail- Trim rates not optimized Trim systems Trim operation able (values should be suffi- operate rapidly Trim rate too slow for hover operations enough to provide unknown) ciently rapid to pro- forces less than 1/3 vide low control the limit forces forces during con- figuration and speed changes Failure to level 2 and No provision t o pre- Inability t o prevent trim “runaway” Provision shall be made t o prevent 3 requirements vent trim “runaway” was considered unsatisfactory “runaway” trim include trim sticking and “runaway” Failure of SAS can No comments Failure of powered- prevent trimming control system or I SAS should not affect ability t o trim Variable rate (deg/ Variable-rate vector control was satis- Thrust vector Variable-thrust vec- controls toring rate is desir- sec) of thrust vector factory; nozzles tended t o drift from able; selected setting nozzles available by selected position - undesirable must be maintained proportional thrust- indefinitely; should vector lever be able to adjust movement thrust-vector control without compromisr ing ability to manage other controls TABLE 5.- LONGITUDINAL STABILITY AND CONTROL: PITCH-CONTROL POWER; PITCH-CONTROL SENSITIVITY; AND LINEARITY MIL-F-83 3 00 AGARD report VAK-191B Pilot comments about VAK-19 1 B Item (ref. 2) R-577 (ref. 1) 1 .O rad/sec2 ; 8 .O" Total control power is adequate but Pitch-con t rol 0.4- 0.8 rad/se c2 k3.0" attitude after 1 sec; k15" response was lower than desired in the power change after 1 sec (wind from most pitch attitude thrust-modulation deadband; need critical direction) available 20" nose-up and 15" nose-down attitude Pit ch-con t rol 3"-S0/in. 3"-20°/in. 3.S0/in. Pitch response is sluggish for small sensitivity inputs Linearity (a/c Constant; if not con- There shall be no Nonlinear in going Small pitch-control inputs within the response to stant, no abrupt objectionable from bleed air t o differential thrust deadband resulted control input increase or change nonlinearities thrust modulation ; in oscillatory aircraft motion in sign A/C oscillations of 0.5 Hz for small control inputs N TABLE 6.- LONGITUDINAL STABILITY AND CONTROL: LONGITUDINAL CONTROL N CHARACTERISTICS IN TAKEOFF AGARD report MIL-F-83 300 VAK-19 1 B Pilot comments about VAK-19 1B R-577 (ref. 1 ) (ref. 2) Longitudinal con- Control effectiveness Elevator control shall Pitch rotation Due to landing gear geometry, liftoff trol character- should not restrict not restrict takeoff obtained by lift- by rotation was accompanied by pitch istics in takeoff STOL operation; performance and engine thrust overshoot requiring corrective pilot shall be sufficient shall be sufficient to modulation action to initiate rotation prevent overro tation at the designated to undesirable I speed or at least attitudes.

2.0 sec before lift- off; adequate con- ture is highly undesirable because LE trol margin and throttle is unguarded pitch rate damping shall exist t o prevent rotation to undesir- able attitudes; con- trol shall be suffi- cient t o rotate in ground effect I Approximately 15 Due t o high workload associated with Satisfactory takeoffs distinct steps lift-engine start during takeoff roll, the shall not depend on required in STO pro- PR is 7; if wheel brakes are released complicated control ; con- manipulation trol travel during are started during 'I PR is 2 takeoff shall not , takeoff run t o mini- exceed 75 percent , mize ground erosion ,

of total travel i and hot-gas ingestion 1

TABLE 6.- CONCLUDED I I

1 AGARD report I MIL-F-83300

Item ' R-577 (ref. 1) (ref. 2) ! VAK-191B ~ ! Pilot comments about VAK-19 1B

!

I Need to functionally check liftengine

trol character- 1 off operation, desir-

performance for proper pitch-control istics in takeoff 1 able to check for functioning during engine run-up (continued) proper control func- urgently exists tioning during run-up at less than takeoff thrust h, w TABLE 7.- LONGITUDINAL STABILITY AND CONTROL: LONGITUDINAL CONTROL CHARACTERISTICS IN LANDING ~~ ~ ~~ ~ AGARD report MIL-F-83300 VAK-I 9 1 B Pilot comments about VAK-19 1B Item 2) I R-577 (ref. 1) (ref.

Elevator cockpit con- Attitude-control Pitch control satisfactory for short Longitudinal Pitch control in con- system and ME noz- landings; ground-cushion effect notice- junction with other Lrol shall be suffi- control zle vectoring able, particularly on shallow characteristics in controls, should be ziently effective that allowed setting of approaches; cushioning effect landing capable of flaring the geometry-limited pitch attitude and enhances flight safety on steep aircraft and achiev- touchdown attitude approach speed; a approaches due to low landing gear ing desired landing can be obtained in limit of 4.8" y strength, but adversely affects pre- attitude from both proximity to ground existed at 60 knots cision of touchdown.

steep and shallow Va due t o landing approach angles If a 5"-6" attitude is commanded at gear strength; a touchdown, SAS control laws are such reduction in sink rate that LE throttle lever can be snatched occurred when enter from pilot's hand ing ground effect, requiring a further power reduction t o achieve touchdown at the desired spot i- TABLE 8.- LATERAL-DIRECTIONAL STABILITY AND CONTROL: ROLLCONTROL POWER, SENSITIVITY, AND LINEARITY MIL-F-83300 AGARD report VAK-19 1 B Item Pilot comments about VAK-19 1 B R-577 (ref. 1 ) (ref. 2) ' Roll-control 0.4- 1.5 rad/sec* k4.0" attitude 1.4 rad/sec2 ; 13.4" Adequate for hover with no wind; power change after 1 sec after 1 sec; for full I inadequate for full flight envelope 6, k 15" roll attitude ? 15" maneuvering; need more than available SAS on roll angle for STOL maneuvering Roll-cont rol 3- 5" /in. 4-2Oo/in. 5 A" /in, Lateral response too sensitive in sensi ti vity hover tending toward P I 0 Linearity Constant; if not There shall be no Abrupt change in Roll overshoot on recovery from side- :onstant no abrupt objectionable non- roll response when step maneuvers was objectionable and increase or change , linearities in air- lateral control was could lead t o PI0

in sign I craft response returned to neutral

in sidestep maneuvers SAS can exert 100- Loss of lateral control masked from percent control pilot by capability of 100-percent authority authority SAS t o create control deadbands near fullstick deflection TABLE 9.- LATERAL-DIRECTIONAL STABILITY AND CONTROL: CROSS-COUPLING AND DIHEDRAL EFFECT AGARD report MIL-F-83 3 00 1 B Pilot comments about VAK-191B Item VAK-19 R-577 (ref. 1 ) (ref. 2) Aj3/@, maximum APIA$ is essentially Satisfactory, little sideslip induced in Cross-coup1 ing At reference speed zero in transition aileron-only turns in speed range u p to for STOL opera- change in sideslip tion, APIA$ not angle to initial peak speed range 120 knots to exceed 0.3 to 0.5 bank angle not to or 20" sideslip exceed I .65 angle Dihedral effect Positive dihedral Positive dihedral At 100 knots and Operational crosswind envelope of limited so that suffi- effect should never a = 5", / 3 limited t o 10 knots unsatisfactory; no warning of cient roll control be so great that more 5 " , at 50 knots, sideslip limits, unacceptable for to correct than 50-percent roll- a = lo", j 3 limited operational use remains control power needed to I O " for gusts, upsets, and maneuvering for sideslip excursions TABLE 10.- LATERAL-DIRECTIONAL STABILITY AND CONTROL: YAWCONTROL POWER, SENSITIVITY, AND CROSS COUPLING MIL-F-83300 AGARD report VAK-19 1 B Pilot comments about VAK-19 1B Item R-577 (ref. 1 ) (ref. 2) I 0.4 rad/sec2 (30'1 Yaw considered too sluggish in hover

! Yaw-control power 0.35-0.8 rad/secZ k6.0" attitude

sec yaw rate) 12" change after 1 sec after 1 sec full rudder input 0.085 rad/sec2/in. Yaw-control sensitivity was low in Yaw-con t rol 0.08-0.2 rad/sec2 /in. 6.0-23.0' head- 4" in 1 sec/in. 6, hover but heading-control accuracy sensitivity ing change after was adequate 1 seclin. 6, Yaw-rate command Constant yaw rate per inch of rudder pedal deflection regardless of forward system gain constant speed made turns difficult and result- over transition speed ing sideslip excursion reduced lateral- range to 120 knots control availability for maneuvering Large lateral velocity Preferred not to use rudder for turn Cross coupling Rolling moment entries because of large rolling variation with yaw developed during full moment due to sideslip rate shall be posi- rudder pedal input tive but not require turns more than 50- percent roll control to trim for 6rmaX

TABLE 1 1 .- VERTICAL FLIGHT CHARACTERISTICS: GROUND EFFECT

AGARD report MIL-F-83300 VAK-19 1 B Pilot comments about VAK-I 9 1 B Item R-577 (ref. 1 ) (ref. 2) Ground effect Downwash/ground Positive ground effect Height control difficult due to non- interference should evident in no wind, linear response associated with posi- not result in unsatis- stationary hover; tive ground effect, recirculation factory character- maneuvering or tail- (exhaust ingestion) in rear lift engine istics during hover wind conditions resulting in pitch attitude changes, and could result in consequent reduction in height control and STOL recirculation in rear power operations Following failure of a power control system or SAS, safe landing should be possible - .

I TABLE 1 2.- VERTICAL FLIGHT CHARACTERISTICS: HOVERING PRECISION AGARD report MI L-F-833 00 Item VAK-19 1 B Pilot comments about VAK-19 1B R-577 (ref. 1) (ref. 2) ~~ Hovering precision It should be possible No requirement Precision VTO (no Precision of height control varied with to takeoff, hover wind) requires a T/W more than expected; at low T/W continuously ICE, slight nose-up atti- (1.05), vertical response is nonlinear and land, all within tude adjustment after and controllability is questionable if an area 1.1 X span liftoff t o stop for- combined with rapid pitching maneu- and length of ward drift; due to vers due to LE throttling; precision aircraft engine placement VTO characteristics considered inade- used on this VTOL quate for shipboard operations at

concept, recircda- T/W < 1.1 ; not possible to meet spot-

tion can occur with ting accuracy factor of 1.1 possible engine surge and reduction in T/W It should be possi- Spotting accuracy was improved OGE; ble t o hover contin- attitude stabilization system charac- uously OGE within teristics were satisfactory an area 1.2 X span and length of aircraft w TABLE 13.- VERTICAL FLIGHT CHARACTERISTICS: VERTICAL THRUST MARGIN AND THRUST RESPONSE MIL-F-83 300 AGARD report VAK-19 1 B Pilot comments about VAK-I 9 I B Item (ref. 2) R-577 (ref. 1 ) Steady-state T/W Height control Height control with matched throttle Maximum T/W avail- Vertical thrust available, shall not Jbtained by com- (lift/main engines) was satisfactory able at the most margins be less than 1.05 bined thrust of lift when T/W = 1.1 ; but inadequate when critical value of md cruise engines; reduced to 1.05; height control with

r/W available can go split throttle resulted in speed/altitude

from 1.1 down t o coupling due t o reduced throttle sensi- 1 .O in ground effect tivity, exhaust geometry, and asym- metric liftengine response resulting in unacceptable height control Split-throttle opera- Height con .:ol Normal acceleration, height-control lags; desire approxi- tion resulted in a sen- sensitivity and (glin.) should be in mately 0.075 g/in. per throttle sitivity of 0.04 g/in.

thrust response the range 0.1 to 0.4 Front lift engine has faster thrust response Time constant of combined throttle response varied be- more than 0.3 sec tween 0.4 and 0.7 sec , TABLE 13.- CONCLUDED AGARD report MIL-F-83 300 VAK-19 1 B Pilot comments about VAK-19 1 B Item R-577 (ref. 1) (ref. 2) No requirement Height control No requirement Automatic limiting Pilot should be warned of approaching sensitivity and of maximum power deadband operation and a thrust is incorporated thrust response margin of about 10 percent should be (continued) resulting in a region provided on throttle deadband Deadband resulted in excessive degradation of height control due to nonlinearity and reduced sensitivity w TABLE 1 4.- TRANSITION CHARACTERISTICS: ACCELERATION/DECELERATION N FLEXIBILITY O F OPERATION AND TOLERANCE IN CONVERSION AGARD report MI L-F-83300 VAK-19 1 B Pilot comments about VAK-19 1 B Item R-577 (ref. 1 ) (ref. 2)

I

Maximum decelera- Need no more than 0.15 t o 0.18 g Acceleration/ Acceleration and It should be possible deceleration deceleration values to accelerate rapidly tion limited to capability deceleration for night or up to 0.5 g in level and safely to Vcon 0.25 g IFR operation flight are desired and decelerate Acceleration capabil- Acceleration inadequate in upper tran- rapidly and safely; It should be possi- ity reduced from a sition speed range; transition mode time taken for these ble to accelerate con- maximum of 0.5 g at required constant pitch trim to pre- maneuvers t o be des- tinuously from a takeoff t o 0 g at vent pitch-up during acceleration ignated by mission rolling takeoff to 200 knots requirements Vcon and vice Trim pitch force Trim requirements should be mini- versa change unknown mized in transition due t o high pilot workload Flexibility of Direction of transi- It should be possible Transition performed Transition could be stopped or I operation tion should be t o stop and reverse by positioning cruise reversed at any time easily reversible the transition engine nozzle quickly and safely I Failure of a single- A liftengine failure Conventional landing would be ex- engine, SAS, or would require a con- tremely dangerous because of poor power-control system ventional landing flight-path control and field length shall still allow a safe requirements landing on wave-off , Tolerance in ' Changing from hover It should be possible Conversion requires Conversion sequence too complex and conversion to conventional t o change from hover complex liftengine pilot workload too high for opera- operation, large atti- , flight and vice versa to Vcon and vice tional use

1 with low pilot versa without exces- , tude change, and

workload sive pilot skill and close airspeed control attention .._ < I

TABLE 15 .- TRANSITION CHARACTERISTICS: CONTROL MARGINS, TRIM CHANGE

MIL-F-83300 AGARD report VAK-19 1 B Pilot comments about VAK-19 1 B Item R-577 (ref. 1) (ref. 2) Margins in control

j Control margins

remaining displayed on CRT True value of control Control positions do Approach to loss of control was margin available not indicate margins masked from pilot by capability of should be apparent of control power 100-percent-authority SAS t o create to pilot available deadbands near full-control stick throw Trim changes on Trim change should Trim change Trim changes Transition mode required constant pitch trimming t o prevent pitch-up control-mode switch- be small and gradual unknown during acceleration and vice versa; over shall be small, and longitudinal con- trol force not exceed trim requirements during transition gradual, and com- 15 lb pull and 7 lb should be minimized to reduce pilot patible with trim push workload.

rate available During control mode transfer a slight P I 0 tendency existed W W TABLE 16.- MISCELLANEOUS CHARACTERISTICS AGARD report MIL-F-83300 VAK-19 1 B Pilot comments about VAK-19 1 B Item (ref. 2) R-577 (ref. 1) ~ ~~ Excellent nose-wheel steering on take- Maintain desired Aileron, elevator, and Bicycle gear with 3on tro 1 rudder cockpit con- outriggers off, but "hot" (always engaged) nose- effectiveness path and attitude by normal use of cock- trol and other normal wheel steering limited crosswind during pit controls and means of control op,eration on landing (see text) takeoff, landing steering controls in shall be adequate t o rollout Bicycle landing gear arrangement can designated cross- maintain a straight result in overrotation tendency and wi lids path on the ground damage to aft fuselage after takeoff or other landing sur- face in crosswinds up Nose wheel steering Directional control during takeoff was t o 35 knots by rudder pedals wit1 good two ranges of sensi- tivity - low range foi takeoff and high range k40" for taxi Aft position of main gear results in increased nose-wheel lift/holdoff speeds for STOL operations which reduces wing lift effectiveness Outrigger landing gear design enhances 1 It should be possi- It should be possible Limited nose wheel ble to make a 360" to make 360" taxiing steering range deck motion and gust capability; how- ~ turn within a circle turns within a circle restricted t o +40" ever, combined gear geometry and non- equal to major whose radius equals wheel steering characteristics would I dimension of air- major dimension of result in excessive turning radius for i n designated craft aircraft in winds up LPH operation wind conditions t o 35 knots , I c TABLE 16.- CONCLUDED AGARD report MIL-F-83300

I Item VAK-19 1 B Pilot comments about VAK-19 1B

R-577 (ref. 1) (ref. 2) I Power checks For VTO, STO, and Lift engine run-up It is necessary t o check out function- prior t o takeoff RTO, it should be to functionally ing of fuel control, flight controls.

possible to check for check out engines and engine power before takeoff proper control func- was not possible

tioning before ' under static

reaching takeoff conditions power

I

w cn .... . . ..

~. - ~ . ...

2. Government Accession No. 1. Report No. 3. Recipient's Catalog No.

NASA TP-1494 I __ .. . - . _ .

4. Title and Subtitle 5. Report Date A COMPARISON O F THE V/STOL HANDLING QUALI- J u l y 1 9 7 9 6. Performing Organization Code TIES O F THE VAK-191B WITH THE REQUIREMENTS O F AGARD REPORT- 577 AND MIL-F-83300 7. Authorb) 8. Performing Organization Report No.

A-71 17 Seth B. Anderson 10. Work Unit No.

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9. Performing Organization Name and Address 505-1 0-32 Ames Research Center, NASA 1 1 . Contract or Grant No.

Moffett Field, Calif. 94035 13. Type of Report and Period Covered .

12 Sponsoring Agency Name and Address Technical Paper National Aeronautics and Space Administration 14. Sponsoring Agency Code Washington, D.C. 20546 _. _ _ -_ I 5 Supplementary Notes 16 Abstract The handling qualities of the VAK-I91B VTOL aircraft are compared with current V/STOL handling-qualities requirements. Generally, the aircraft's handling qualities were superior t o other V/STOL fighter-type aircraft; however, several deficiencies would seriously affect shipboard V/STOL operation. These include poor hovering precision, inadequate mechanical control characteristics, nonlinear pitch and roll response, an uncominanded move- ment of the height (thrust) control lever, low-pitch control sensitivity, excessive dihedral effect, and inadequate overall thrust response. The attitude-command control system resulted in reduced pilot workload during hover and low-speed flight.

The study disclosed gaps in the current handling-qualities requirements, particularly for operation aboard ships. AGARD Report 5 77 provides more comprehensive coverage than does MIL-F-83300 in the area dealing with STOL operation.

. - - 7. Key Words (Suggested by Author(s1) 18. Distribution Statement Aerodynamics/Performance Unlimited Stability and Control Handling Qualities STAR Category 08 ... = ~ - . - ~ 22. Price' 9. Security Classif. (of this report) 20. Security Classif. (of this page) 1 21. ~0~;- Pages Unclassified

1 $4.00 Unclassified

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'For sale bv the National Techrwsl Information Service. Sprinqfield, Virginia 22161 NASA-Lanql ey , 1979 THIRD-CLASS BULK RATE Postage and Fees Paid National Aeronautics and National Aeronautics and Space Administration ~ Space Administration NASA451 Washington, D.C.

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