An Examination of Handling Qualities Criteria for V/STOL Aircraft

NASA TN D-331

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TECHNICAL NOTE

D-331

AN EXAMINATION OF HANDLING QUALITIES CRITERIA .

FOR V/STOL AIRCRAFT By Seth B. Anderson Ames Research Center Moffett Field, Calif.

NATIONAL AERONAUTICS AND SPACE

ADMINISTRATION

I

WASHINGTON

July 1960 @ Ill % _tr IC NATIONAL AERONAUTICS AND SPACE ADMINISTRATION TECHNICAL NOTE D-331 AN EXAMINATION OF HANDLING QUALITIES CRITERIA FOR V/STOL AIRCRAFT By Seth B. Anderson SUMMARY A study has been undertaken to define handling qualities criteria for V/STOL aircraft. With the current military requirements for helicop- ters and airplanes as a framework, modifications and additions _ere made for conversion to a preliminary set of V/STOL requirements using a broad background of flight experience and pilots' comments from VTOL and STOL aircraft, BID (boundary-layer-control) equipped aircraft, variable sta- bility aircraft_ flight simulators and landing approach studies. The report contains a discussion of the reasoning behind and the sources of information leading to suggested requirements.

The results of the study indicate that the majority of V/STOL requirements can be defined by modifications to the helicopter and/or airplane requirements by appropriate definition of reference speeds.

Areas where a requirement is included but where the information is felt to be inadequate to establish a firm quantitative requirement include the following: Control power and damping relationships about all axes for various sizes and types of aircraft; control power_ sensitivity, damping and response for height control; dynamic longitudinal and dynamic lateral- directional stability in the transition region, including emergency opera- tion; hovering steadiness; acceleration and deceleration in transition; descent rates and flight-path angles in steep approaches, and thrust margin for approach.

INTRODUCTION For several years the NASA has been involved in the definition of handling qualities criteria for airplanes and helicopters. It was rec- ognized that handling qualities requirements are needed also for V/STOL aircraft to insure their safe and efficient operation. The purpose of this report is to suggest flying qualities requirements for V/STOL vehicles which could be used: (i) to guide prospective users in setting up speci- fications for any proposed operational V/STOL vehicle; (2) to judge the ability of various types of V/STOL vehicles to meet reasonable require- ments; and (3) to guide the flight test programs of various available V/STOL testbeds. Since the data which are available for the flight

conditions peculiar to V/STOLvehicles are incomplete, the requirements

presented herein are tentative, and it is anticipated that requirements

will be changed and added as more information becomesavailable.

To arrive at requirements for V/STOLvehicles, it w_s considered

expedient to use as a background the wealth of flying qualities informa-

tion contained in reference I for airplanes and reference 2 for helicop- ters. The information was examined in the light of possible V/STOL specifications to determine which areas were adequately covered and could be used directly and which areas needed furthez research. Modifications and additions to the airplane and helicopter requirements for conversion to V/STOL requirements were based on a broad background of flight results and pilots' comments (see pilot rating system, table I) from VTOLand STOL type aircraft, BLC (boundary-layer-control) equipped aircraft, variable stability aircraft, landing approach studies, and flight simulators. The VTOL aircraft consisted of the following: The Bell X-14 deflected turbojet (fig. i), the Bell XV-3 convertible helicopter (fig. 2), the Ryan VZ-3RY deflected slipstream (fig. 3), and the Vertol \Z-2 tiltwing (described in ref. 3). STOL experience was obtained from a rumber of aircraft (refs. 4 through 9) and included recent flight studies of the C-134A twin-engine cargo airplane equipped with a full-span BLC system (fig. 4).

In addition to the V/STOL specifications, the reasoning behind and the sources of information leading to the req_rements are discussed.

Those areas where the existing information is :'elt to be inadequate and where additional flight or simulator research :s required have been pointed out in order to formulate flying qualities req_irements with greater confidence.

In this study an effort has been made to consider three classes of aircraft; namely, light observation_ heavy surrei!lance or fighter, and tactical transport. The general form of reference i has been followed as closely as possible for organizational purpose_.

STOL operation as used in this report refers to flight at speeds below the power-off stall speed or below the mLnimum speed with all engines inoperative for aircraft not possessing an aer)dynamic stall (limited by control power, visibility, etc.) or below the 3peed at which it is possible to arrest sink rate to zero by aerodynamic mea_s alone (power off). In general, therefore, STOL operation is dependent on engine power to augment aerodynamic lift and change effective lift-dra_ ratio. VTOL operation implies the ability to hover out of ground effect over a given ground position in no wind.

DISCUSSION r The preliminary V/STOL requirements are crganized and presented in a form similar to that used in reference i. _'able II is a tabulation of the various handling qualities items along wi_h the appropriate airplane and helicopter requirements placed side by side for reference purposes.

These requirements have been paraphrased for brevity and can be reviewed in detail by referring to the appropriate numbered paragraphs in refer- ences i and 2. In the right-hand column are the V/STOL requirements.

Definitions of airplane classes and symbols can be found in the appendix.

In the following discussions the V/STOL requirements will be reviewed to point out the reasoning behind each and the areas requiring further research. In reviewing the V/STOL requirements, it should be kept in mind that they are not intended to be rigid military-type specifications, but rather those handling qualities which are felt desirable from what is known at the present state of the art.

Mechanical Characteristics of Control System Control friction and breakout force.- The relatively low values of friction presented in the table are based on the desirability of obtain- ing proper centering characteristics in a flight regime where the aero- dynamic restoring forces are absent. In addition_ it should be noted that during operation when the pilot can have only one hand on the control, the values for wheel control should be essentially the same as for a stick type of control. For power control systems in which there is both linkage friction and valve friction, an additional requirement is that the magni- tude of the linkage friction be at least twice the valve friction, the sum of the two not to exceed the values quoted for V/STOL aircraft. This relationship of linkage friction was chosen to avoid pilot-airplane instability as noted in reference i0.

The centering characteristics required are the same as those contained in the helicopter specification, chosen again on the basis of one-hand operation for either wheel or stick controls. For this type of system sufficient damping is needed to prevent undesirable cockpit control oscillations.

Cockpit control free play.- The amount of free play in the cockpit control has been specified in terms of percentage of full travel so as to include both stick and throttle type controls; ±i percent has been speci- fied for all types of control systems. Further work in this area will be required to define allo_alole values for specific types of control systems (i.e., acceleration or rate command) particularly in hovering flight where unpublished simulator results have shown this factor to be significant in the over-all suitability of the control system.

Artificial stability devices.- The general remarks for airplanes are qualitative and it is felt that a more quantitative approach is needed to define the allowable divergence rates for stability augmentation failure.

Accordingly, the values for helicopters (3.4.9a) are suggested as a start in this direction; however, it is felt that more research is needed in this area to define limits for V/STOL operation.

Longitudinal Stability and Control Stick fixed static stability.- Recent tests with variable-stability aircraft have indicated for some flight conditions that stick-fixed static stability is not required as long as stick force and dynamic requirements are met. For V/STOL airplanes_ however_ which are to operate extensively at low speeds, flight tests (see_ e.g., refs. ii and 12) have indicated the desirability of adequate stick-fixed staJility in the transition and landing regions. In addition, the pitch-up lefined in the helicopter specification (3.2._0) is considered undesirable if the instability occurs in the speed range below that for minimum drag. Here again_ flight experience (see ref. ii) in flying on the back side of the drag curve has indicated a particular need for stable stick-fixed and stick-free gradients in order to make satisfactory height adjustments along a desired flight path in landing approach. It is to be noted that smooth, steady flight is required throughout the speed range includirg maximum designated speed in rearward flight. Since rearward flight may prove difficult for some VTOL vehicles, further research is needed to estsblish limits compatible with various mission requirements.

In regard to BLC failure it is specified that failure of the BLC system shall not change the longitudinal stability characteristics suffi- ciently that a dangerous flight condition results. Although no quantita- tive values can be specified at this time_ flight experience with a number of BLC systems has indicated the desirabilily of minimizing stability changes due to BID_ particularly in landing approach where BLC effectiveness is derived from the main engine.

Elevator stick-force variation with sp(ed in unaccelerated flight.- Stick-free stability characteristics simila_ to those previously discussed for the stick fixed are desired. A stable _tick-force variation with speed is desirable over the complete speed _ange. The mild pitch-up pre- viously mentioned for the stick-fixed case _uld not be tolerated if it occurs on the back side of the drag curve. In addition_ the force rever- sal in airplane requirement 3.3.2.1 is considered too large. In order to aid in obtaining adequate precision control below the trim speed_ the requirement has been revised to state that _he reduction in force shall not decrease by an amount greater that the _riction force for the comparable airplane class.

Exception in transonic flight.- V/STOL aircraft which operate in or through the transonic speed range should me_% the characteristics specified for air!olanes (3-3.3).

Stability in accelerated flight.- For reasons similar to those stated in the discussion of stick-fixed static sta)ility_ a stable gradient of elevator position variation with normal acc_leration is specified for all fo_ard flight conditions. No requirement is felt necessary for rearward flight _here acceleration values would be snail.

Control effectiveness in unaccelerated flight.- The desirability of a margin in control effectiveness at each end of the speed range (noted in helicopter requirement 3.2.1) to cope with effects of longitudinal dis- turbances is well founded. The question of how much margin in needed for V/STOL aircraft throughout the speed range has yet to be determined _ith the desired accuracy. As a start_ ho_ever_ a helicopter requirement which states a margin of at least i0 percent of the maximum attainable pitching acceleration in hovering *z has been suggested for VTOL operation. For STOL operation it is felt that a quantitative requirement is necessary also to insure adequate control effectiveness throughout the speed range.

Further research is needed in this area_ however_ for a firm requirement to relate control effectiveness requirements to disturbing moments.

Control effectiveness in accelerated flight.- Because of the large A effects that engine power may have on the ability to develop maximum lift_ requirement 3.3.8 for airplanes has been increased in scope to include the effects of engine power.

Longitudinal response.- While no requirements have been specified for airplanes for the initial response of the longitudinal mode_ operation of V/STOL aircraft at i_¢ values of dynamic pressure will require a closer examination of the desirable values for the initial response characteris- tics. Accordingly_ the value from helicopter requirement 3.2.9 has been added as a first step in defining satisfactory response characteristics.

Further research is needed to authenticate this value for V/STOL operation.

Control forces in steady accelerated flight.- The stick-force gradients for V/STOL aircraft have been chosen to remain essentially the same as for airplanes (table in 3.3.9) except that the maximum force gradients for wheel controls should be low enough that during V/STOL operation one-hand operation is feasible. In general_ a major portion of V/STOL operation will be conducted at low values of acceleration and, therefore, the stick force gradients do not require as close scrutiny as for a high-speed fighter. It is felt, however_ to ease the task of pre- cision flying with V/STOL vehicles_ requirements dealing with control force magnitude_ linearity, and sense are highly desirable.

Control forces in sudden pull-ups.- Airplane requirement 3.3.10 _as originally intended to guard against overshooting a given acceleration in a sudden pull-up where relatively little control force is generated by control deflection. A requirement of this type is felt to be even more significant for V/STOL aircraft, particularly for control systems without power boost for which large inertia of the control system combined with small restoring forces at low dynamic pressure can result in poor precision in controlling the aircraft. Requirement 3.2.$ for helicopters_ which states that during and following a rapid displacement of the control_ the force acting to resist the displacement shall not fall to zero_ is felt to be unconservative. Therefore, in addition to airplane requirement iHereinafter an asterisk denotes an extension of reference 2 based on unpublished helicopter handling qualities studies and results of refer- ence 13.

3.3.10 the stipulation is included that the _tick force shall always lead

the acceleration by an adequate margin to provide satisfactory anticipation

of the resultant acceleration.

Control cross-coupling.- Control cross-coupling, peculiar to some helicopters without power boosted control sy_tems, destroys control har- mony. In an attempt to provide the pilot with the best possible control system_ the requirement is written to discourage any control force cross- coupling.

Longitudinal short-period oscillations.- For most airplanes_ the short period and the phugoid modes have widely different periods and are not coupled. At the low speeds of V/STOL operation, however_ the two modes may have similar periods; the combined effect of the short period and phugoid on the over-all aircraft behavicr must be such that the ensu- ing motion is satisfactory. Considerable flight and simulator experience has made it possible to establish more specific requirements for the short-period dynamic behavior of aircraft (see_ e.g., ref. 14). The results for airplanes as obtained from reference 14 and unpublished results from tests of a YF-86D variable-stability airplane are presented in figure 5 in terms of frequency and dampi_;g ratio. These data have been used to select a boundary for V/STOL aircraft in configuration P.

Data are not available to define a boundary for configuration PA. As indicated in figure 5, however; data obtain_d in landing approach for a number of fighter aircraft and helicopter r,_quirement 3.5.1.1 point out that lower frequencies and less damping may be acceptable for configura- tion PA. As a start, therefore; a helicop%_r requirement is suggested in _hich the damping ratio must be at least 0.)55 for periods less than 5 seconds.* Further research is necessary to define boundaries in config- uration PA for V/STOL aircraft. In an atte:mpt to define desirable maneuvering stability characteristic s_ helicopter requirements 3.2.11.1 and 3.2.11.2 are suggested.

Long-period (phugoid) oscillations.- T_e phugoid osciliation_ which is of relatively long period for airplanes in the cruise configuration_ has not had a specific damping requirement. At low speeds typical of STOL operation, however, the phugoid may become a problem as the period is reduced. The damping specifications foz satisfactory dynamic stability for helicopters require damping ratios ran_ing from 0.055 to -0.22 in the period range from 5 to 20 sec.* For the most part these data, which are based on a background of helicopter experience in the lateral-directional oscillatory mode and in the longitudinal mode, are qualitative in nature and it is felt that additional research is required in transition and landing approach to define with greater confidence satisfactory phugoid characteristics for V/STOL aircraft. Resets of simulated instrument flying with a variable-stability B-26 airplane (ref. 15) have indicated the desirability of the phugoid damping ratio being 0.15 or greater.

For extremely long periods, 50 seconds or longer, a damping ratio of -0.i0 was acceptable. For the period range in w_uich the phugoid is approxi- mately 15 seconds_ experience has shown t½_t a neutrally damped phugoid is acceptable only if the short period is satisfactorily damped also. In order to minimize the effects of longitudinal disturbances in V/STOL operation, the requirements specify a minimum damping ratio of -0.i0 for periods longer than i0 seconds.

Conventional longitudinal short and long period dynamics are confined to the vertical plane of motion. A longitudinal disturbance along the thrust axis has been encountered on one V/STOL aircraft. This longitudinal acceleration-deceleration characteristic which has a period of the order of i0 seconds is felt to be associated with the large diameter rotor system employed on the aircraft. Needless to say, this characteristic was considered unsatisfactory.

Longitudinal control effectiveness in hovering.- The ability to position VTOL aircraft accurately and rapidly over a given spot is a pri- mary consideration in defining control power and control sensitivity. 2 The effects of gust disturbances and aerodynamic and engine gyroscopic cross-coupling effects may further complicate the problem. In order to insure that adequate longitudinal control power is available for VTOL aircraft for maneuvering and gust disturbances during hovering, values for control power are suggested which were derived from the results in references 16 and 13 of tests of a variable-stability helicopter and include take-off, landing, hovering, quick stops, and fo_ard flight at various speeds. These results, which show the relationship of control power to aerodynamic damping, represent a significant improvement in analysis of hovering control for design purposes. Unpublished results obtained on a flight simulator with pitch freedom indicate that for the longitudinal case the minimum acceptable control power values were rela- tively insensitive to the amount of aerodynamic damping present. This was not true in the roll mode as will be discussed later. The control power specified for VTOL aircraft may not apply accurately to all sizes of VTOL aircraft since different sizes would be disturbed different amounts by gusts; however, until further research is conducted the values specified in the helicopter requirement which take aircraft weight into account are useful. No maximum limit on control power is felt necessary.

Longitudinal steadiness in hovering.- Helicopter requirement 3.2.2 was established in an attempt to set tolerable limits on the motion induced in the vehicle by downwash-ground interference effects. The motions, characterized by erratic darting and random unsteady behavior, are considered satisfactory in helicopter requirement 3.2.2 if only a small amount of control motion (±! inch) is required to hover over a given spot. Although this may give a rough measure of hovering steadi- ness, it is felt that control motion in itself is not representative of hovering steadiness since other factors_ such as control sensitivity and frequency of control motion, and amplitudes of excursions are also important in assessing hovering behavior. Further research is needed in this area to define acceptable hovering steadiness more quantitatively.

mControl sensitivity maybe defined as the slope of the control-power- deflection curve. For iine&r control characteristics the two terms may be used interchangeably.

One of the factors which has a direct effect on hovering behavior,

pal_ticularly in rough air, is the amount of aro_ular damping. In order

to insure satisfactory initial response characteristics following a longi- tudinal control input and to minimize the effects of external disturbances, a requirement for damping has been included. No maximum damping value is considered necessary. The damping values werc obtained from the results of a variable stability helicopter (refs. 16 _nd 13) and, as mentioned previously_ may require modification for larg_:r aircraft or for aircraft which would tend to be less disturbed by gust_;. Lower acceptable limits for damping in pitch have been demonstrated i_ recent unpublished simu- lator studies. F_rther research on gust dist_Lrbing effects is considered necessary, however, to determine a requiremen_ which more directly takes airplane size and type in consideration.

Height control in hovering.- The present helicopter requirement 3.2.3 for height control which specifies altitude control within +I foot with not more than ±i/2-inch movement of the collective control has been retained but is not considered completely definitive of height control for the same reasons as previously mentioned _or control in longitudinal steadiness. In addition, in order to develop satisfactory criteria for height control, research is needed to establi {h limits of control po_er_ sensitivity, and d_nping similar to those dev _loped for the aerodynamic controls. Other factors, such as ground suct :on effects, visibility_ thrust response (engine or stored rotor), and thrust margin should be considered in the over-all picture of factors influencing height control.

Additional research is required to provide s_fficient information relative to heigi% control for a more quantitative requirement.

Acceleration-deceleration cha_'acteristics.- The ability to accelerate and decelerate quickly in a safe and efficient manner at constant altitude or along a constant flight path angle is one If the important items affect- ing the utility of the VTOL vehicle. For a tactical transport capable of operating at high subsonic Mach numbers, the constant altitude requirement may be relaxed to fit the mission characteristics for this type vehicle.

From the flight tests conducted so far a hunker of points have been noted.

Although the vehicle must be able to accelerste rapidly, a limit on thrust rotation may be necessary to avoid _ing stal2 on some configurations. On the other hand., deceleration should not be !Jmited because of the necessity of maintaining high percent engine power few i to supply bleed air for reaction controls_ nor should deceleration bc limited by ability to main- taim trim with the longitudinal control. In addition, it should be possi- ble to decelerate rapidly without stalling o_ objectionable buffeting, and thrust response must be rapid enough to prewnt the aircraft from settling }_hen slowing down to hover. This was particl_arly true on one aircraft (ref. 17) which required a large, sudden inc:'ease in power for level flight. In this case the problem _as made m(,re difficult because avail- able power _as marginal. In addition to the aforementioned items_ some reasonable value of distance or time for dec,_leration is needed to define deceleration characteristics adequately. In the interim, until further research is completed, the requirement state_ that the deceleration should be compatiLle _{ith the mission requirement.

_C Conversion z and transition characteristics.- Transferring smoothly from thrust lift to aerodynamic lift is important to the success of the VTOL vehicle. Although only a limited amount of information is available from flight tests at this time, the following points have been noted.

Flexible operation depends on the ability to safely and readily stop con- version or transition in either direction. Both flight (ref. 3) and simulator results (ref. 18) have disclosed the desirability of minimizing pitch changes during conversion and transition. Large pitching moments may occur unless conversion controls are programed correctly with airspeed.

Another factor in transition is concerned with establishing an adequate speed margin between the speed at which the weight of the aircraft can be supported completely by the wing and the maximum forward speed obtainable with the thrust directed for hovering flight. This may be a problem for A some configurations for which acceleration is obtained by tilting the thrust vector forward. The large ram drag inherent in some types of propulsion systems may limit the maximum forward speed to undesirably low values. For safe operation it is highly desirable for wave-offs or land- ings to be possible with the critical engine inoperative at any time during transition. The aforementioned items have been placed in requirement form. Further research is required to arrive at more quantitative require- merits for conversion and transition.

Steep descent characteristics.- The ability to make steep descents is important to the utility of the V/STOL vehicle. However_ flight tests have indicated that a number of fundamental problems must be solved if steep descents are to be feasible. These include aircraft disturbances due to wing stalling or rotor flow instability which occur in steep descents because of the high induced angles of attack. Another problem concerns the effects of the reduction in engine power required to obtain low effective L/D values for steep descents. This was disclosed by recent flight tests of an STOL aircraft which derives large lift gains from engine power. Unpublished results show that as engine po_er is reduced_ the minimum approach speed must be increased because the stall speed increases and the control power decreases (as a result of reduced slipstream velocity). In addition it should be possible to control atti- tude and rate of descent accurately for landing. In this regard sufficient visibility must be available to give the pilot the necessary cues for landing at a given spot. The requirement for angle of descent has been written in general terms since specific mission requirements will dictate approach angles and descent rates. More research and operational experi- ence is necessary to establish more firmly values for rate of descent com- patible with mission requirements. In this regard studies in reference 19 indicate that at least for helicopters it is not feasible to descent at rates greater than approximately i0 feet per second in steep approaches under instrument conditions.

Longitudinal trim changes.- The airplane requirement for trim change 3.3.19 has been followed in general but in addition wing sweep position and thrust direction are specified. Additional items may be required as SConversion refers to a configuration change such as wing and/or rotor tilting_ flap deflection_ thrust deflection_ wing translation_ etc.

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more experience is gained in this area. Maxinumallow_ble force changes

have been reduced to +i0 pounds for stick or _¢heelin an attempt to

minimize trim changes, thereby avoiding the necessity of operating trim

devices in addition to conversion devices. Although no direction of the

force changes has been specified, it may be desirable in certain cases

to specify a direction. For example, in studies of landing approach

techniques (refs. ii and 20) it _as found that flight path control was

improved if increases in engine power producei slight nose-up trim changes

and vice versa with negligible effect on airspeed. A desirable magnitude

of this trim change_as not determined, however, and information about a

preferable direction for the other items is rot available at this time.

_Longitudinal, lateral, and directional trim effectiveness.- The ability to trim the control forces to zero over the speed range including zero airspeed is important for V/STOL aircraft because of the extended periods of operation in the low-speed area.

Irreversibility of trim controls.- AirpTane requirement 3.5.5 is satisfactory in this regard.

Trim system failure.- Airplane requirement 3.5.6 is considered adequate for V/STOL aircraft.

Height control characteristics.- The us_ of collective pitch or throttle controls for height adjustment requires essentially the same mechanical characteristics as conventional s_ick controls since they are used in a similar manner for VTOL operation. The forces on the throttle type height control have, therefore_ been proportioned according to an average representative throttle length.

Longitudinal trim change due to sidesli0.- The maximum allowable longitudinal control forces for the various _irplane classes have been specified sufficiently low to be held with o le hand. It is felt that the longitu_inal trim change due to the side31ip for the conditions spec- ified in helicopter requirement 3.3-9 should not be so great that no margin in longitudinal control is available to cope with gust disturbances.

Accordingly, a margin equal to i0 percent of the maximum hovering angular acceleration is specified for VTOL operation. No requirement is specified for STOL operation; however_ a sufficient margin should exist for the same reasons. F_rther research is needed to define a margin for STOL operation and to determine the applicability of the l£-percent margin to all VTOL configurations.

Control effectiveness in take-off.- To insure that take-off performance is not _mduly compromised, airplane requirement 3.3.11 to adjust take-off attitude has been modified %o include all classes of STOL aircraft and to apply on sod and hard _urfaces. For VTOL operation the helicopter requirement 3.4.4.1 has been used except that the wind velocity has been deleted since this will vary with the mission require- ments of the vehicle. Experience in VTOL oi:eration has shown that it is

ii

desirable for the longitudinal control, which may depend on the main engine for power, to be powerful enough to adjust the attitude of the airplane so that the thrust vector is directed as necessary to prevent fore or aft translation during run-up to maximum power. In addition, in order to check for proper functioning (direction) of the controls it should be possible to observe control motion or the effect of control movement on the aircraft motion during run-up at reduced power. _ Longitudinal control force's in take-off.- The control force limits have been reduced in magnitude to permit one-hand operation during take- off and climb for either stick or wheel type control.

Control effectiveness in landing.- The longitudinal control shall be

A

powerful enough to land the airplane at designated wind conditions under a variety of approach conditions. For example, in steep descents when it may be necessary to reduce engine power significantly, the type of longi- tudinal control that derives it power, in part 5 from the main engine (such as reaction type using bleed air) must be able at reduced engine power to meet requirement 3.3.14 for airplanes. In addition, adequate control should be available to land the airplane safely at the minimum operating speed. The minimum operating speed for V/STOL aircraft is defined as the speed from which a safe landing can be made with the critical engine inoperative. The minimum operating speed is construed to apply to single- engine or multiengine vehicles. On multiengine VTOL aircraft, the minimum operating speed would be zero if it were possible to hover with the criti- cal engine inoperative. The term minimum operating speed as used through- out this report is felt to be a logical approach to safe operation of V/STOL vehicles. It is recognized that except in emergencies neither commercial helicopters nor military aircraft operate in such a manner that would prevent a safe landing if the critical engine failed.

Control forces in landing.- As mentioned previously, the maximum allowable longitudinal control forces have been kept low to permit one- hand operation for stick or wheel.

Control forces in dives.- In dive maneuvers where it is felt that V/STOL aircraft will not operate over prolonged periods, the force values for airplanes have been retained.

Auxiliary dive recovery devices.- No changes have been felt necessary from the airplane requirements for V/STOL aircraft.

Effects of drag devices.- Recent studies in landing approach (ref. 20) of a continuously adjustable thrust reverser on a single-engine jet fighter and unpublished data of thrust attenuators on a twin-jet trainer have sho_n the feasibility of this type of device for use as a flight path control during landing approach. When used as a flight path control it was found desirable that increases in reverser deflection (reducing for- ward thrust) should produce mild increases in nose-down trim with negligible change in airspeed.

Lateral-Directional Stability and Control Characteristics

Damping of the lateral-directional osci!_ations.- The airplane requirement 3.4.1 is based on research reported in reference 21. More recent work reported in reference 22 was primarily directed toward inves- tigating whether the requirement was too strip,gent for emergency operation.

These latter results are presented in figure 6 along with airplane require- ment 3.4.1. In the tests of reference 22, a variable-stability F-86E was used to make simulated landing approaches for various lateral-directional characteristics. Included in these studies was a rough air simulation obtained by sending random inputs to all controls. These tests disclosed that for the emergency condition (stabilizati(n devices inoperative), the A values in requirement 3.4.1 could be drastically reduced. In the landing approach configuration even slightly divergent oscillations were acceptable at the lower roll-to-yaw ratios. In addition_ there were indications that the parameter !/T1/2 would be more descriptive than i/Cl/2 to the pilot for rating damping. Other factors, such as adverse yaw, are know to influence the damping requirements. In view of the many variables which influence the lateral-directional damping and because these variables must be considered in operation of the V/STOL airc_'aft, further research is needed to extend airplane requirements to the low-speed region of the V/STOL vehicle. In the interim, the boundari_s noted on figure 6 are suggested. It can be noted that in line with the results of reference 22 for landing approaches the boundaries for V/S!_L aircraft have been shifted to reflect lower damping requirements_ Spiral stability.- From considerations s_ch as those discussed on spiral damping in reference 23 it is felt tha_ greater restrictions than those for airplanes may be placed on spiral dLvergence for STOL operation because heading changes associated with the s)iral mode will become more significant at lower speeds. Until further r_search is conducted to set limits for V/STOL operation, however, airplan_ requirement 3.4.2 is useful.

Steady sideslip conditions.- In order to adequately specify the conditions under which directional characteristics are to be checked, considerably more operational experience with various types of V/STOL vehicles must be acquired. For example, the z_ximum sideslip condition specified for helicopters is 45 ° , yet flight _t 90 ° sideslip is not uncommon. Until operational limits compatibl_ with mission requirements can be established more accurately, the combined conditions outlined in airplane requirement 3.4.3 and helicopter reqlirement 3-3-9 are suggested for V/STOL aircraft.

Static directional stability (rudder posLtion).- In general, it is desired that static directional stability be 3uch that increases in rudder deflection accompany increases in sideslip over the full sideslip range up to 90 ° . However, until further research is c_nducted to ascertain the feasibility of this criterion for VTOL operation, airplane requirement 3.4.4 (_r/_ > O) shall apply over the sideslip ranges specified.

Static directional stability (rudder force).- Characteristics similar to those discussed in the foregoing section on rudder position are desir- able for rudder force. As noted before, however_ until further experience has been obtained in this area_ a reduction is permitted in rudder force with increase in sideslip for sideslip angles greater than 15 ° from that for wings level. Because recent experience in STOL operation has indicated the desirability of keeping the reduction in rudder force to a minimum_ the airplane requirement which allowed the force to decrease but not to zero has been changed to allow reduction of rudder force to only one half the maximum value_ but not less than the friction value.

Dihedral effect (aileron force).- A similar reasoning to that used for rudder characteristics shotuld be applied to aileron (force and posi- tion) when operating V/STOL aircraft. In addition_ the aileron force should not exceed i0 pounds in keeping with one-hand operation. For transient type maneuvers_ such as wave off, negative dihedral effect (not to exceed i0 pounds) is permissible.

Dihedral effect (aileron position).- As previously discussed, linear position characteristics are desired over the sideslip angle range extend- ing to 90 ° sideslip. Further research is necessary for dihedral effect also to define requirements from a practical and operational standpoint.

In order to have available some margin of control for gust disturbances, it is recommended that positive dihedral effect never be so great that at maximum sideslip, less than i0 percent of maximum rolling acceleration is available for all classes of V/STOL aircraft at the minimum operating speed.

Side force in sideslips.- Airplane requirement 3.4.8 specifies that increases in bank angle accompany increases in sideslip. In addition to this it would be desirable to be able to define the minimum slope of bank angle versus sideslip which at a given airspeed would give the pilot an appreciation of the magnitude of sideslip angle. Sufficient information is not on hand, however_ to establish a revised requirement.

Adverse yaw.- The amount of adverse yaw tolerable for airplanes has been established at 15 ° as a representative value to restrict heading changes to a controllable value. Recent studies in landing approach (ref. 24) have shown, however_ that sideslip itself may not be indicative of a heading change in that appreciable values of sideslip can be obtained by merely rolling around a highly inclined longitudinal axis with little or no heading change. Since it may be necessary for STOL vehicles to use relatively large angles of attack to make steep approaches_ it is felt that a closer examination of allowable sideslip angles will be required to set limits for STOL operation.

Although in general_ favorable yaw has not been a major handling qualities problem of conventional airplanes, recent experience with a VTOL aircraft, in which favorable yaw due to lateral control deflection _¢as incorporated, has indicated the desirability of keeping this item to negligible values. There is not sufficient information at the present time to specify a maximum allowable value; however, the V/STOL require- ment has been _ritten to the effect that favorable yaw shall not be of sufficient magnitude to be objectionable.

Asyzmetric power (rudder free).- Airplane requirement 3.4.10 has been retained in essence except that the reference spced has been changed to include all speeds above that for minimum drag.

Directional control (symmetric power).- The requi_ememt for airplanes has been Modified to extend the speed range for _/STOL aircraft down to the minimum operating speed and to reduce the maximum rudder force to i00 pounds. This value is felt to be more compatible with precision of control and safety. For VTOL operation the initial trim condition is set at hover and no maximum force values are felt to be required. Additional research is needed to extend the i0 ° sideslip value given in airplane requirement 3.4.11.1 for landing to cover values more representative of V/STOL operation in cross winds.

Directional control (asymmetric power).- As before, the condition for minimum speed has been referenced to the minimum operating speed rather than a stalling speed. In addition, it is felt iLecessary to include the wave-off condition and a margin of rudder control to maneuver. The allow- able forces have been lo-_ered to a maximum value of i00 pounds for reasons previously discussed.

Directional control during take-off_ landin_ and taxi.- The directional control requirements for airplanes a:_d helicopters have been combined in an attempt to provide satisfactory d_rectional control for the maximum designated wind velocity in any direction for all classes of V/STOL aircraft. Additional testing undoubtedly will point out the relative merits of each V/STOL concept for operating under various wind conditions.

Directional control to counteract adverse y_w.- The airplane requirement has been changed to reference trim sLdes!ip angle and to lower the maximum allowable rudder force to i00 _ounds.

Directional control in dives.- Airplane reqlirement 3.4.15 has been changed slightly in regard to rudder force since it is felt that no distinction should be made for maximum allowable rudder force for various classes of V/STOL airplanes. A maximum value of i00 pounds has been selected for reasons previously discussed.

Directional steadiness in hovering.- As noted in the previous discussion on longitudinal steadiness in hovering, control motion in itself as used in helicopter requirement 3.3-3 is not felt to be adequate to define directional steadiness in hovering ovcr a given spot. Although this part of the requirement has been retained_ further research is needed in this area also for more suitable parameters ior measurement of directional steadiness.

It is recognized that directional damping will improve the hovering steadiness and, as discussed before_ the values derived from the helicopter tests of references 16 and 13 are used as a first choice. Additional research is needed to provide values representative of the requirements for various sizes and types of VTOL vehicles.

Directional control power in hovering.- Directional control power should from the flight safety standpoint be the least demanding compared to roll since directional rotation at touchdown is not as serious as side velocity. Yet in view of this, the amount of directional control power desired from tests of the variable stability helicopter (ref. 16) was large in comparison with that required for either pitch or roll. In this case the large amount of directional control power specified was felt to be due A in part to the high directional stability of the test vehicle and the par- ticular precision task used in the flight tests. As a result of additional studies, the values recommended in reference 16 have been reduced as noted f © in reference 13. Until additional research is comp!eted_ however, to establish the maximum amount of control power needed for other sizes and types of VTOL aircraft, the values noted in the helicopter requirement are suggested. An additional requirement is felt necessary to set a minimum directional control power value in hover since even for large aircraft a lower limit is needed for maneuvering. For this condition it is recom- mended that sufficient directional control power be available to establish a yaw displacement not less than 15 ° after one second for full control deflection.

Hovering turns in winds.- The requirement for helicopters 3.3.6 which specifies 360 ° turns over a given spot in a 30 knot wind has been relaxed for VTOL aircraft to match the mission requirements for a given vehicle, since it is felt that rearward and sidewise flight at 30 knots may not be required for some VTOL concepts. To assure an adequate margin of control umder these wind conditions the margin in yaw displacement in one second specified for helicopters is used. These values were derived from the results of references 16 and 13 and included an attempt to take into account the weight of the aircraft. There are indications, however_ from tests of different sized helicopters that equal margins of control may be required regardless of the weight of the aircraft. This philosophy, pointed out in reference 27, suggests that, in general, all VTOL vehicles regardless of size must maneuver into similar areas with equal ability and, therefore_ control power and control margins must be suitable for this kind of VTOL operation. Additional testing is felt required to check more fully the effect of aircraft size or weight. In the interim, the requirement has been modified to set as a minimum a yaw displacement value of _o after one second, regardless of the aircraft size.

Directional control sensitivity.- As noted in previous discussions, it is felt that the directional control characteristics including sensi- tivity require further study to define requirements for aircraft of various sizes and weights. In the interim the sensitivity value of helicopter requirement 3.3.7 has been recommended.

Directional control in power-off flight (sutorotation).- This requirement has been revised to include all tyl es o£ aircraft by refer- e_cing to the min_num speed as defined in the stall section. In addition, it _as felt necessary to specify a minimum accc:ptable value for rate of turn.

Lateral steadiness in hovering.- For reasons discussed previously_ further studies are felt needed to define requ:-rements in addition to that specified for the amount of lateral control mo_,ion required to hover over a given spot on the ground.

Lateral control power in hovering and in ::orw_rd flight.- It is recognized that both control power and damping are important for satis- factory lateral control characteristics. The !_ignificance of the rela- tionship of lateral control power to damping _as sho_n initially for fighter aircraft in the results of reference 26. These results_ from both flight and simulator tests_ showed that pilot opinion deteriorated at lov_ values of roll control po_<er and at low values of damping. At high values of roll pov_er there was a loss of precision of control due to sensitivity.

At low d'_nping the control behaved as an accel,_ration command control with <ulsatisfactory characteristics. A summary of _he results of reference 26_ _{hich represent both flight and simulator test3_ is plotted in figure 7 in terms of LSa$ama x and T. Included in figure 7 are data points from a Rumber of V/STOL aircraft. In addition, the l%teral control criteria of reference ip are presented (assuming 5 inches of stick travel) and also unpublished results of moving base simulator tssts. The latter sets of data represent both hovering and low-speed forward flight. It can be noted that the lines of constant pilot opinion (see table II for number defini- tions) forming the boundaries are approximated by lines of constant bank angle in one second. It can be shorn that the parameter pb/2V is not suitable for design purposes since it does no± take into account roll dsamping and indicates that increased roll rate s are required as speed is increased. These considerations are not borne out by the pilot opinion data in figure 7 obtained from reference 26.

The data in figure 7 show as would be exyected that greater control power _as demanded at io_¢ values of T for airplane flight where evasive t}qpe maneuvers are made compared to that requJ red for hovering or transi- tion flight (typified by the larger T value_). In addition_ the results indicate that greater control power is requir(d as damping is increased in order to avoid the feeling of a stiff or slug_;ish aircraft. With regard to d_mpi.ug, the simulator results indicate that T values of the order of 4 seconds };ere considered satisfactory for ho_ er. These simulator results _ez'e obtained _ith no disturbing effects_ ho_ver_ and, in addition, the pilot had to cope _£ith only one degree of fre_:dom. Although a number of V/STOL aircraft are being florin with essentia;ly zero damping_ these fligb_ts are conducted under still-air conditi.)ns and it is felt that for practical VTOL operation damping is necessary.

C On the basis of the foregoing_ the requirements for lateral control for configuration P have been rewritten to delete the parameter pb/2V used in airplane requirement 3.4.16 and include the roll time constant T and the use of a given bank angle obtained in one second chosen according to the lines of constant pilot opinion. A helicopter requirement which takes airplane weight into account is used for lateral control for hover and transition (ref. 13). As discussed previously for directional control, a lower limit is felt necessary to prevent undesirably low roll perform- ance for heavy aircraft. The roll damping specified for low speeds is that from the helicopter requirement since this is the best information available.

The foregoing applies to rolling perfo_nance for full lateral control.

A Recent flight experience with the XV-3 has sho_n that particularly in hovering where roll damping is generally small, the variation of rolling acceleration with lateral control displacement should be essentially linear 6 over the control deflection range. In additionj a sensitivity requirement which is essentially that specified for helicopters (3.3.14) is used to avoid overcontrolling tendencies in hover and low-speed flight.

It is recognized that additional research is needed to more clearly define lateral control requirements for all V/STOL concepts and sizes.

For example_ lateral velocity can be obtained either by tilting the thrust vector_ by banking the aircraft, or by remaining vel_ical and supplying a side thrust. For aircraft with large inertia about the roll axis the lat- ter method may be more practical when possible performance losses are considered. It is felt_ however_ that roll displacement may provide the pilot with an important cue in a quickening sense and may, therefore_ be desirable for satisfactory lateral positioning. To clarify the necessity for physical roll displacement in hovering_ further research is required.

Roll response.- The requirement for time delay in obtaining roll response is necessary to cover aerodynamic lags inherent in some spoiler systems. It is felt_ however_ that for the classes of V/STOL aircraft considered herein_ the requirement for time delay in attaining the maxi- mum roll acceleration should be independent of the class of aircraft and should preclude the possibility of incorrect initial rolling direction.

Peak lateral control forces for rolling performance.- The peak lateral control forces for rolling performance have been _ritten to conform with one-hand operation in approach and landing where frequent use of the control is required.

Lateral wheel throw limits.- The use of ±90 ° for wheel throw with one hand operation may prove undesirable; however_ until sufficient information is obtained to justify a change to a smaller value_ the aircraft require- ment has been used _ith the added stipulation that full throw shall be readily obtainable with one hand.

Peak lateral forces for various maneuvers.- The requirements for helicopters and airplanes have been combined to express a maximum lateral force not to exceed 20 poumds for stick or wheel for V/STOL operation.

Lateral trim changes and effectiveness.- The use of a fixed value of lateral stick movement to define a trim change is not considered adequate for V/STOL aircraft since lateral force or margin available is not taken into account. It is felt preferable_ therefore_ to specify the ability to balance the airplane laterally for the various, conditions with an allowable maximum force change and to include a margin of control of i0 percent of the maximum attainable value to cope with disturbances.

Lateral control effectiveness in dives.- The airplane requirement has been used umaltered.

Control cross-coupling and transient effects.- The airplane requirement 3.5.7 is intended to provide protection from excessive loads at high speeds generated by inertia cross-coupling effects. The maneuver for airplanes specifies rolls through 360 ° which is considered too large to be applicable to all V/STOL aircraft. Accerdingly, the roll displacement has been stated to conform with the mission designation for each aircraft.

In addition_ as discussed previously for longitudinal control_ lateral control forces acting to resist displacement _hall not decrease appreciably with control displacement.

Lateral and directional control force cross-coupling effects_ which are peculiar to some helicopters, are considered undesirable as noted in a previous discussion of longitudinal control. The helicopter requirement has been reworded to eliminate any control force cross-coupling characteristics.

Control for spin recovery.- The requirement for airplanes has been made more general to include all aircraft capsble of being spun and to cover possible effects of engine power on control power. The relatively high control forces allowed for recovery are considered satisfactory in view of the emergency nature of the maneuver.

Stalling Characteristics Required flight conditions.- Because the stalling characteristics are of particular interest in the transition _egion_ it is felt necessary to include_ in addition to the standard airpl_ne configurations_ a check of the stall behavior in wave-off. In addition_ the large effects which engine power and BLC may possibly have on the stalling behavior require flight tests with engine power for shallow an_ steep descent approaches and BLC on and off.

Definitions of stalling speed.- The stalling speed for conventional airplanes is defined in reference i as the minimum speed attainable in flight_ and is normally associated with breakdown of air flow over the wing immediately after the maximum over-all trim lift coefficient is attained. The complete stall is characterized by large magnitude pitch- ing or rolling or by a decrease in normal acceleration in turning flight.

Stalling speed for STOL airplanes which fall into the conventional stall category will be strongly dependent on engine power_ thrust angle_ or slipstream magnitude and; therefore_ stalling speed in configuration PA will vary appreciably depending on whether a shallow or steep descent is being made.

For V/STOL aircraft which do not possess a conventional stall_ the A stalling speed may be defined as in airplane requirement 3.6.2.1 or 3.6.2.2 with an addition for V/STOL operation. Accordingly; the minimum operating speed has been added which was previously defined.

Stall warning requirements.- Although the stall w_rning characteristics defined in airplane requirement 3.613 shall be generally applicable for V/STOL aircraft with conventional stalling behavior_ it is felt that the expression of airspeed at which the warning is felt as a percentage of stall speed is inadequate at low airspeeds. Flight experience under STOL conditions has pointed out that for low stall speeds the pilots desired a minimum fixed margin in speed above the stall to have sufficient margin for safety from stalling due to finite gust dis- turbances. For this ptu_oose a 5-knot minimum value for sts ll warning margin is specified. A similar relationship applies to the minimum landing approach speed; however_ in this case a lO-knot minimum speed margin from the stall is desired.

For aircraft which are limited in longitudinal control (defined in airplane requirement 3.6.2.1) and others where a conventional stall can- not be obtained_ no stall warning has been specified provided no dangerous flight behavior occurs.

Requirements for acceptable stalling characteristics.- The stalling characteristics in airplane requirement 3.6.4 have been revised to be more stringent in the landing approach and landing configurations. In this area it is felt necessary to limit the maximum allowable initial roll- off at the stall to the roll angle at which a wing tip or pod may strike the ground when the aircraft is resting on the landing gear. This philos- ophy_ which extends from a variety of flight experience in landing approach_ is intended to place a more practical limit on the allowable roll-off at the stall.

For the case of failure of the BLC system_ the allowable magnitude of angular displacement has been relaxed to permit excursions to 30 ° pitchdown; roll_ or yaw_ provided_ however_ no dangerous flight characteristics arise.

2O

Prevention of the complete stall and definition of recovery

characteristics are fe_t to be covered adeq_tely by airplane require-

merit 3.6.4.1 with the addition of the effect_ of engine power on control

effectiveness.

Performance (engine) considerations.- Because of the closer tie-in of engine operation to flight characteristics for V/STOL aircraft_ it is considered desirable to include the effect of engine operation in certain areas of flying qualities requirements. Some of the items to be considered include the following: Engine power changes over the range used operation- ally should not appreciably affect control power of reaction controls or other controls (including BLC) which derive _ heir effectiveness in part from the main engine. Engine thrust respons_ shall not compromise the A ability to hold altitude in hover or in goin{i from transition to hover.

Power controls shall not require complicated procedures for power changes.

Thrust control shall be fine enough to permit: control of flight path by the use of engine controls.

Although the effect of thrust to weight ratio is normally considered a performance characteristicj the effect on _ he over-all flying qualities should not be overlooked. In particular_ th_ results of flight tests of a number of jet aircraft in landing approach (ref. ii) have indicated the necessity that the thrust veight margin _T/_ be at least equal to or greater than 0.12 at the minimum approach sp_ed. Further tests are needed to redefine this item for V/STOL operation.

Gyroscopic effects.- Because of the greater ratio of engine gyroscopic inertial moments to airplane inertial moment_ characteristic of VTOL air- craft and because of the low aerodynamic dam_ing available_ engine gyro- scopic coupling effects can have an appreci_ le effect on airplane dynamic motions. From the flight experience gained on V/STOL aircraft thus far (see, e.g., ref. 27) it would appear that gy_'oscopic coupling effects can- not be tolerated to any appreciable degree. Accordingly_ a requirement to minimize the effects of gyroscopic couplilg has been included for V/STOL aircraft.

CONCLUDING REMARKS The results of a study of handling qual:ties of V/STOL aircraft have indicated that the majority of V/STOL requir_:ments can be defined by modi- fications to the current military helicopter and airplane requirements in part by appropriate use of reference speeds. Since the available data for the flight conditions peculiar to V/STOL veh: cles are incomplete_ a number of the requirements can only be presented in qualitative form. Areas where a more firm quantitative requirement is felt necessary include control power and damping relationships about all ax_s for various sizes and types of aircraft_ control power_ sensitivity_ dam_,ing_ and response for height control_ dynamic longitudinal and dynamic la_:eral-directional stability in transitiom imcluding emergencyoperation; hoverimg steadiness;

acceleration and deceleration in transition; characteristics im steep

approaches; amdthrust margin in approach.

AmesResearch Center

National Aeronautics amd SpaceAdministratiom

Moffett Field_ Calif., May 23_ 1960

APPENDIX

NOTATION

For purposes of this report_ V/STOLairplanes are divided into the

following classes:

Class I - Light observation

Class II - Heavy surveillance ard fighter

Class III - Tactical transport

Configurations used for V/STOLairplanes are similar to those for

airplanes (ref. i).

Symbolsused in this report are defined as follows:

b

wing span_ ft

rolling moment

CZ

rolling moment coefficient_ qSb

CZp

_(pb/2V)' per radian

CI/2 number of cycles to damp to half amplitude

Ca

cycles required to double _aplitude

F

cockpit control force, ib inertia, slug-ft 2 qSb 2 2VIx C_p_ per sec initial rolling acceleratio_ for full lateral control

l_a_amax

input_ radians/sec 2

n_Aa normal load factor, in g umits

limit load factor

nL

rolling velocity, radians/sec

P

helix angle, radians

2V

dynamic pressure, ib/ft 2 q S wing area, sq ft time to damp to half amplitude_ sec T1/2 true airspeed, ft/sec V indicated airspeed Vi stalling speed VS

v _ sin

W airplane gross weight, ib sideslip angle_ deg AT thrust margin W elevator angle_ deg Se _Sr slope of rudder deflection -sideslip curve S@ damping ratio (fraction of critical) pitching velocity_ radians/sec pitching acceleration_ radians/sec 2 cr density ratio i roll time constant, - _--_ sec © bank angle_ deg rolling acceleration_ radians/sec 2 rolling parameter, deg/ft/sec

I vel

angle of yaw_ deg Sub script s aileron a elevator @ airplane configurations L_ TO, _70_ etc.

r_dder r roll_ ya_ and pitch axes_ r_spectively x,y_ z C REFERENCES i. Anon. : Military Specification - Flying Qualities of Piloted Air- planes. MIL-F-8785 (ASG), Nov. 12, 1957.

2. Anon.: Military Specification - Helicopter Flying Qualities_ Require- ments for Military Specification. MIL-H-8501, Nov. 5, 1952.

3. Thomas_ Lovic P._ ZII: A Flight Study of the Conversion Maneuver of a Tilt-Wing VTOL Aircraft. NASA TN D-I_3 _ 1959.

_B Cooper_ George E._ and Innis_ Robert C.: Effect of Area-Suction-Type Boundary-Layer Control on the Landing-Approach Characteristics of a 35 ° S_ept-Wing Fighter. NACA RM A55KI4_ 1956.

.

Anderson_ Seth B._ Faye_ Alan E._ Jr._ and Innis_ Robert C.: Flight Investigation of the Low-Speed Characteristics of a 35 ° Swept-Wing Airplane Equipped With an Area-Suction Ejector Flap and Various Wing Leading-Edge Devices. NACA RMA57GIO_ 1957.

t Bray_ Richard S._ and Innis_ Robert C.: Flight Tests of Leading- Edge Area Suction on a Fighter-Type Airplane With a 35 ° S_eptback Wing. NACA RMA55C07, 1955.

To Anderson_ Seth B._ Quigley_ Her_ey C._ and Innis_ Robert C.: Flight Measurements of the Low-Speed Characteristics of a 35 ° Swept-Wing Airplane With Blowing-Type Boundary-Layer Control on the Trailing- Edge Flaps. NACA RMA56G30, 1956.

.

Quigley_ Hervey C._ Anderson_ Seth B._ and Innis_ Robert C.: Flight Investigation of the Low-Speed Characteristics of a 45 ° Swept-Wing Fighter-Type Airplane With Blowing Boundary-Layer Control Applied to the Trailing-edge Flaps. NACA RMA58E05, 1955.

o Qtuigley_ Hervey C._ Anderson_ Seth B._ and Innis_ Robert C.: Flight Investigation of the Low-Speed Characteristics of a 45 ° Swept-_ing Fighter-Type Airplane With Blowing Boundary-Layer Control Applied to the Leading- and Trailing-Edge Flaps. NASA TN D-321_ 1960.

i0.

Brown, B. P.: Ground Simulator Studies of the Effects of Valve Fric- tion_ Stick Friction; Flexibility and Backlash on Power Control System Quality. NACA Rep. 1348_ 1958.

ii.

Drinkwaterj Fred J._ III_ and Cooper_ George E.: A Flight Evaluation of the Factors Which Influence the Selection of Landing Approach Speeds. NASA MEMO I0-6-58A, 1958.

12.

Rhoads_ Donald W.: A Qualitative Flight Test Investigation of Some Handling Qualities of Liaison Type Airplanes. Cornell Aeronautical Lab., Inc._ Rep. TC-1214-F-I, Sept. 1959.

2O

13. Tapscott, Robert J. : Criteria for Con±rol and Response Characteris- tics of Helicopters and VTOL Aircraf± in Hovering and Lo_ Speed Flight. IAS Paper No. 60-51, Jan. 1960.

14.

Chalk, Charles R.: Additional Flight Ivaluations of Various Longitudinal Handling Qualities in a Variable-Stability Jet Fighter.

WADC TR 57-719, Jan. 1958.

15. Newell_ Fred, and Rhoads, Donald W.: 1_light Evaluations of the Effect of Variable Phugoid Damping in a JTB-26B Airplane. WADS TR 56-223, Dec. 1956.

16.

Salmirs, Seymour, and Tapscott, Robert J.: The Effects of Various Combinations of Damping and Control ]tower on Helicopter Handling Qualities During Both Instrument and Visual Flight. NASA TN D-55_ 1959.

17. Deckert, _4. H., and Ferry, R. G.: AFF!I_ Flight Evaluation of the XV-3. Preliminary Report. Air Forc_ Flight Test Center.

July 22, 1959.

18.

James, Harry A., Wingrove, Rodney C., i_izhauser, Curt A., and Drirak_¢ater, Fred J., III: Wind Tunnel and Piloted Flight Simula- tor Investigation of a Deflected-SlipstreamVTOL Airplane, the Ryan V_-3RY. NASA TND-89, 1959.

19. Reeder, John P., and Whitten, James B.: Notes on Steep Instrument Approaches in a Helicopter. (NACA paper presented at the 12th Annual Helicopter Society Forum, May 1956).

20.

Anderson, Seth B., Cooper George E., ald Faye, Alan E., Jr.: Flight Measurements of the Effect of a Cont_ollable Thrust Reverser on the Flight Characteristics of a Single-E_gine Jet Airplane. NASA MEMO 4-26-59 A, 1959.

21.

Liddell, Charles J., Jr., Creer, Brent Y., and Van Dyke, Rudolph D., Jr.: A Flight Study of Requirements for Satisfactory Lateral Oscillatory Characteristics of Fighter Aircraft. NACA RMASIEI6 , 1951.

22.

McNeill, Walter E., and Vomaske, Richard F.: A Flight Investigation to Determine the Lateral Oscillatory Damping Acceptable for an Air- plane in the Landing Approach. NASA MEMO 12-I0-58A, 1959.

23. Rhoads, D. W.: Flight Evaluations of the Effect of Variable Spiral Damping in a JTB-26B Airplane. Correll Aeronautical Laboratory Rep. TB-IO94-F-I, Oct. 19, 1957.

24.

White, Mauriee D., and Innis, Robert C.: A Flight Investigation of the Low-Speed Handling Qualities of a Tailless Delta-Wing Fighter Airplane. NASA MEMO 4-15-59A, 1959.

25. Putnam, K. V.: Some Human Engineering Aspects of Several Unconventional Aircraft. Paper presented to the joint meeting of the AGARD Aeromedical and Flight Test Panels. Athens, Greece, _y 11-15, 1959.

26.

Creer, Brent Y., Stewart, John D._ Merrick, Robert B., and Drink_ater, Fred J., llI: A Pilot Opinion Study of Lateral Control Requirements for Fighter-Type Aircraft. NASA MEMO 1-29-59A, 1959.

27. Davis, W. B.: General Flying Qualities in the Transition Flight Regime Based on Flight Test Results, USAF Model X-13 Airplane.

Ryan Aeronautics Rep. 6954-10, April i, 1958 • ,--4 O +_ 0 0 Q© © O O O O O ,-t CI O r-I ,-4

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A-25685 Figure 2.- The XV- 3 convertiplane.

A-26052 Figure 3.- The VZ-3RY deflected sl_pstream airplane.

A-26297 Figure 4.- The C-134A STOL airplane.

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