REPORT No. 755

REQUIREMENTS FOR SATISFACTORY FLYING QUALITIES OF AIRPLANESBy R.R.Gxr,mxm

INTRODUCTION

The need for quantitative design criterions for describingthosequalities of an airplane that make Up satisfactory con-trollability, stability, and handling characteristics has beenrealized for severalyearn. Some time ago, preliminary studiesshowed that adequate data for the formulation of these cri-terions were not available and that a large amount of pre-liminary work would have to be done in order to obtain theinformation necessary. It was apparent that flight testsof the flying qualities of numerous airplanes -wererequiredin order to provide a fund of quantitative data for correlationwith pilots’ opinions.

Accordingly, a program was instituted which covered thevarious phases of work required. The first step involved thedevelopment of a test procedure and test equipment whichwould measure the characteristics on which flying qualitiesdepend. This phase of the work is reported in reference 1,although since that time the test procedure has been expandedand modiiied on the basis of additional experience and severalchanges have been made in the equipment used.

Another phase of the investigation has involved the meas-urement of the flying qualitiw of a number of airplanes. Theprocedure used has been in general in accord with thatdescribed in reference 1. At the present time (1941), completetests of this nature have been made of 16 airplanes of variedtypes. These airplanes were made available largely by theArmy and more recently by privah companies at the requestof the Civil Aeronautics Board. In addition, flyingqualitiesdata of more limited scope have been obtained from time htime on a number of other airplanes, the tests of which cov-ered only particular items of stability and control but which,nevertheless, augment the fund of data now available.

A third phase of the investigation, one which also has beenpursued throughout the duration of the project, has involvedthe analysis of available data to determine what measuredcharacteristics were significant in defining satisfactory ilyingqualities, what characteristics it was reasonable to require ofan airplane, and what infiuencs the various design featureshad on the observed flying qualities.

In order to cover this work adequately, ‘a number of pa-pers dealing separately with the various items of .%abili@and control are necessary. Several such papers have @enprepared or are in preparation at the present time. Detailedstudies of all items will require considerable time for com-pletion, but it is believed that the conclusions reached to dateare complete enough to warrant a revision of the tentativespecifications set forth in reference 1. As opportunity foradditional analysis occurs, it would be desirable to cover the

individual requirements at more length than is possible atthis time. & a result of further studies, it may also bedesirable to revise again the fly@qualities specificationsgiven here.

In addition to the actual specifications, the chief reasonsbehind the specifications are discumed. Wherever possible,interpretation of the specification is made in terms of the de-sign features of the airplane unless the subject is covered inreports of reference.

In formulating the specifications, every attempt has beenmade to define the required characteristics in easily memur-able, yet fundamental terms. It was necessary to considerall stability and control requirements in arriving at eachindividual item because of the varied functions of the indi-vidual controls and the conflicting nature of many of thesefunctions.

The specifications require characteristics that have beendemonstrated to be essential for reasonably safe and eilicientoperation of an airplane. They go as far toward requiringideal characteristics as present desia~ methods will permit.Compliance with the specification should tie satisfactoryflying qualities on the basis of present standards, althoughas additional lmowledge is obtained it may be possible todemand a closer appoach to ideal characteristics without inany way penalizing the essential items of performance.

FLYINGQUALITY REQUIREkD3NTS

It has been convenient to present the flyingquality re-quirements under the following individual headin~. Theyappear in the report in this order.

I. Requirements for Longitudinal Stability and Clmtrol

A. Characteristicsof uucontrolledlongitudinal motionB. Characteristicsof elevator control in steadyflightC. Characteristics of elevator control in accelerated

flight “D. Characteristicsof elevator control in landingE. (lharacteristim of elevator control in take-offF. Limits of trim change due to power and flapsG. Characteristics of longitudinal t “ “ g device

H. Requirements for Lateral Stability and Control

A. Characteristics of uncontrolled lateral and direc-tional motion

B. Aileron-control characteristicsC. Yaw due to aileronsD. Limits of rolling moment due to sideslipE. Rudder-control characteristics

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REPORT No. 755—NATIONAL ADvIsoFtYCowm’rrm FOR AERONAUTICS50

F.G.H.I.

Ya.v@g moment due to sideslipCrm-wind force characteristicl%t~m moment due to sidedipChamcteristim of rudder and aileron Irimming

devices.

III. Stalli& CJhnracteristics

These requirementspertnin to all flight conditions in whichthe airplane may be flown in normal or emergency operation,with the center of ~wavityat any point within the placardedEmits. Some of the specifications are based on the behaviorof the airplane at some specilkd airspeed. The airspeedin such casesshall be taken as the indicated airspeed. Whereminimum airspeed is referred to, unless otherwise stated, itshaIl be taken w+the minimum airspeed obtainable withfhps down, power off.

With the exception of part III of the requirements, whichdeals exclusively with characteristics at or close to the stall,the requirements pertain to behavior of the airplane in therange of normal flight speeds at a@w of attack below the~gle of a~~ck at ~h.ich the ~~ ~o~d occ~.

In the specifications which follow, the lower limits of thecontrol-force gradients are specified in terms of the abilityof the controls to return to trim positions upon release fromdeflected positions. This is a very desirable characteristicbecause it assures a control friction sdiciently low in com-parison with the aerodynamic forces to allow the pilot tofeel the aerodynamic forces on the controls. However, someadditional interpretation of the specifications is necessary,becauseno control systemcan be made entirely free of frictionand, therefore, there will always be some small deviationfrom return to absolute trim. At the present time, it is notpossible to & the allowable limits for these deviations. Itis known, however, that controls reasonably free from fric-tion, as measured on the ground, have satisfactory self-center~~ characteristicsin the air as long asthere is a definiteforce grndient. For elevato~ force gradients as low as O.O5pound per mile per hour have been satisfactory when the fric-tion was smalL For relatively smallairplanessuch asiighters,trainers, and light airplanes, it appears that about 2 poundsof friction in the elevator control system and 1 pound inthe aileron represent an upper limit. In several crises,-wherepush-pull rods with ball bearing- were used throughout thecontrol system+friction in both elevator and aileron systemshns been found to be under ~ pound.

For large airplanes not intended to maneuver where visualor instrument referent= are alwajs available, self-centeringchrmncteristksare not believed to be essential,altho~~h theyare very desimble. 1+ these airplanes control friction shouldbe kept as low as possible, although there is indication thatconsiderably more friction can be tolerated. A representa-tive amount of control friction for a transport or mediumbomber would be about 10 pounds in the elevator system nnd6 pounds in the ailerons.

Irreversible controls have somewhat Similarcharnctaristicsto controls with high friction; tliat is, they are not self-centering and therefore tend to destroy control f eel. They arenot considered desirable, although on very lnrge nirplanw

where the rates of deviation from steady flight nre slow tlmyha,vebeen used sumessfdly on ailerons.

I. Requirementsfor LongitudimdStabili@ and Control

Requirement (I-A). — Characteristics of uncontrolledlongitudinal motion.

When elevator control is deflected nnd released quickly,“the mibsequentvariation of normal acceleration and elevatorangle should have completely dimppemed after one cycle.

Reasonsfor requirement(I–A) .—The requirement specifiesthe degree of damping required of the short-period longitudi-nal oscillation with controls free. A I&h debweeof dampingis required because of the short period of the motion. Withairplanes having less damping than that specfied, the oscil-lation is excited by gusts, thereby accentuating their effectand producing unsatisfactory rough-nir characteristics. Theratio of control friction tb air forces is such thnt damping isgenerally reduced at high speeds. When the oscillationappears at high speeds ns in dives and dive puU-outs, it is,of course, very objectionable because of the nccslemtionsinvolved.

The short-period oscillations involve variations of thoangle of attack at essentially constant speed and should notbe confused with the well-lmown long-period (phugoid) oscil-lation, which involves vnriation of speed at m essentiallyconstantangle of attack. As shown by the testsof reference !2,the characteristics of the lntter mode of longitudinn~motionhad no correlation with the ability of pilots to fly nnairplane efficiently,the long period of the oscihtion makingthe degree of damping unimportant. Subsequent tests havenot altered this conclusion. The case of pure longitudinnl di-vergence of the airphme (static instability) will be coveredinter under requirements of the elevator control in stendyflight. No r~quirementfor damping of the long-period phu-goid motion appears justiiinble at the present time.

Design considerationa.-A theoretical nmdysis of this prob-lem (reference 3) has shown that the damping of thecontrol-free (short-period) oscillation is dependent chiefly onthe magnitude of the aerodynamic balance of the elevntorsand on the massbalance and moment of inertin of the controlsystem. The analysis shows thnt the damping is improved byreduc~~ the aerodynamic balance, increasing the mussbnlnnce, and reducing the moment of inertia. The introduc-tion of friction damping in the control system should, ofcours~ also be effective nlthough control friction is veryundesirable for other reasons.

Requirement(T–B) .—Characteristics of elevntor control insteady flight.

1. The variation of elevator nngle with speed should indi-cate positive static longitudinal stwbility for the followingconditions of flight:

a. With engtie or engines idling, flnps up or down, d nllspeedsabove the stall.

b. With engine or engines delivering power for levelt@ht with flaps down (ns used in landing nppronch),lnnd.iqggeay down, nt all speeds nbove the stall.

c. With engine or engines delivering full power with flnpsup at all speedsabove 120percent of the minimum speed.

REQ~M=NTS FOR SATI.SFACWORY

2. The variation of elevator control form with speed shouldbe such that pull forces me required at all speeds below thetrim speed rindpush forces are required at all speedsabove thetrim speed for the conditions requiring static stabili~ initem 1.

3. The magnitude of the elevator control force shouldeverywhere be sutlicient to return the control to its trimposition.

4. It should be possible to maintain steady flight at theminimum and maximum speeds required of the airplane.

Reasonsfor requirement(I–B) .—llmns 1 and 2 require posi-tive static stability for flight conditions in -whichthe airplaneis flown for protracted lengths of time, or where opportunityexists to establish a trim speed so that stable characterietiwcan be realized. Positive static stability is not consideredparticularly helpful to a pilot at very low speeds with fullpower on or with flaps extended with full power on, becauseof the large trim changes due to power usually experienced.The conditions me classed as emergency conditions becausein actual operation they are entered suddenly from approachconditions, where relatively little power is used. In thesecas.wthe elevatnr force and positiop change-s,due to appliedpower and change of flap setting, are usually far greaterthan any inherent stable or unstable force or position gradi-ents which exist due to the degree of static stability present.l?or these reasons, static stabili~ in these conditiom is notconsidered essential, at least not until trim changes due topower me reduced to much lower values than are experiencedat the present time. The magnitude of allowable trim changedue to power and flaps is covered later in requirement (I-F).

In other conditions of flight, however, static stabili~ isregarded as an essential flight chawcteristic. Item 1 per-tains to the elevator-fixed condition. This requirement in-sures that the airplane will remain at a given angle of attackor airspeed as long as the elevator is not moved, and providedthat disturbed motion of the airplsmeis not left uncontrolledfor long periods of time. Positive stability eliminates theneed for constant control manipulation in maintaining givenconditions and, furthermore, simplifies the control manipula-tion when rLspeed change is desired, because the directionof control movement required to start the rotation in pitchcorresponds to that required to trim at the new angle of at-tack A negative slope to the elevator-angle curve is a nece.s-snry requirement for elevator control feel, and the degree ofcontrol feel increases as the variation of elevator angle withangle of attack is increased (iwferenca 4). b general, itmay be said that the variation of elevator angle with angle ofuttack should be ne@ive and as large numerically as is con-sistentwith other requirements of elevator controL

Item 2 requires that the elevator-free static longitudinalstability shall always be positive. This specification ~ athat the airplane will not depart ffom a trim speed except asa result of definite action on the part of the pilot.

Item 3 requires thnt the elevator control be df-~nterhg, a

characteristic which is nece=ary for the attainment ofcontrol feel.

The reason for item 4 is obvious.Design considerations,-A detailed analysis of the static

longitudinal stability characteristics of various airplanes and

FLYING QUALITIES OF AIRPLANES 51

theinfluence of various design features on the observed char-acteristic is given in reference 5.

Requirement (I-C) .—Characteristics of the elevator con-trol in accelerated flight.

1. By use of the elevator control alone, it should be pos-sible to develop either the allowable load factor or the max-imum lift coefficientat every speed.

2. The variation of elevator angle with normal accelerationin steady turning flight at any given speed should be a smoothcurve which everywhere has a stable slope.

3. For airplanes intended to have high maneuverability,the slope of the elevator-angle curve should be such that notless than 4 inches of rearward stick movement is required tochange angle of attack from a CL of 0.2 to CL .O=k thema-neuvering condition of tligh~

4. As measured in steady turning flight, the change in nor-mal acceleration should be proportional to the elevator con-trol force applied.

5. The gradient of elevator control force in pounds perunit normal acceleration, asmeasuredin steady turning flight,should be within the following limits:

w For transports, ~heavy bombem, etc., the gradientahouldbe les thw 50pounds per g.

b. For lighter types; the gradient should be less than6 pounds per g.

c. For any airplane, it should require a steady pull forceof not lessthan 30pounds to obtain the allowable load f actor.Reasonsfor requirement(I-C) .—Item 1 of this specification

requires that sufficient elevator control should be availableto execute maneuvers of the minimum radius inherent in.the aerodynamic and structural desiowof the airplane. Sincethe curvature of the flight path is directly related to thenormal acceleration, it is obvious that the attainment ofeither the maximum lift codcient or the allowable loadfactor is the limt@g condition.

Item 2 is a requirement for stability in turning ilight. Air-planes that do not meet this requirement t~d to ‘dig in”and overshoot desired accelerations in maneuve~ eventhough every use is made of visual and instrument ref erences.

Item 3 specifies the amount of stability required of an ah-phme which must be maneuvered at or close to maximum liftwithout resort to visual or instrument references. It hasbeen demonstrated by tests of several fighter airplanes thatlongitudinal stability and control characteristics m specifiedare necessary for airplanes that require z l@h degree ofcontrol feel. The provision of such characteristics also re-duces the time required to change angle of attnck in enteringrapid turns or zooms due to the simplified control manipula-tion associated with a definitely stable airplane.

The linear stick-forca gradients specitied in item 4 me, ofcourse, very desirable as an aid to the pilot in obtaining theaccelerations desired. -

The numerical limits speciiied for the force gradients initem 5 are such that the minimum radius may be readily at-tained in any airplane. For pursuit types, gradients greaterthan 6 pounds per g were considered heavy by pilots. Forairplanes where the load factor is lower, such as bombers,transports, etc., which am not reqqired to maneuver con-tiIIUOUSIY,rt.ggdbnt of 60 po~ds per g ~ not =c~~e” TO

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52 REPORT NO. 755—NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS

insure against inadvertent overloading of the structure, the30-pound lower limit (item 5+) is nwessary. For pursuitairplanes with allowablp load factors of 9, this lower limitwould correspond to a gradient of about 4 pounds per g.For airplanes with lower load factors, such asbombers, &ns-ports, or light airplanes, the gradient in pounds per gwould be proportionately l@her.

Important design fachrs,-li turning fligh$ due to thecurvature of the fight path, a stabilizing effect is obtainedwhich increases the slope of the elevator-amgle curve overthat obtahied in straight fight. The stick forces requiredto maintain a given lift coefficient are considerably greaterthan those for straight flight, however, because the elevatoranglw are higher and because they are obtained at greater

# speeds. For this reason, it is necessary to specify the upperlimit of elevator-force gradients only for accelerated flight.

A Iinear rehtion between stick force and nonmd accelera-tion is always obtained provided the elevator-argle curve andhinge-moment coefficient curve have linear variations withangle of attack and deflection, respectively.

Requirement(I–D) .-Characteristics of the elevator con-trol in h.mding.

1. (Applicable to airplanes %ith conventional landinggear only. ) The elevator control should be sullicientlypowerful to hold the airplane off the ground until three-pointcontact is made.

2. (Applicable to airplanes with nose-wheel type landinggmr only. ) The elevator control should be sufficientlypowerful to hold the airplane from actwd contact with theground until the minimum speed required of the airpbme isattained.

3. It should be posible to execute the landing with anelevator control force which do= not exceed 50 pounds forwheel-type controls, or 35 pounds where a stick-type controlis used.

Reasonsfor requirement (I–D) .—For airplanes vvith con-ventional landing gear, the three-point attitude usually cor-responds closely to that for the development of minimumspeed for landing. In addition, an airplane alighting si-multaneously on main wheels and tail wheel is less likely tolewe the ground again as a result of possess&u verticalvelocity at the time of contact.

The renson for item 2 is obvious.The limits of allowable control force in landing were de-

termined from considerations of the pilot’s capabilities. Thelimit forces given are 80 percent of th~ which a pilot canapply with one hand to the Meraut control arrangementswith the control E? inches from the back of the sea~ (Seereferences4 and 6.)

Design factor%-The requirements of the elevator in pro-ducing three-point or minimum speed landings are by farthe most critical from a standpoint of control power. Flight-tes.tdata show that lo-iv-wingmonoplanes with flaps down re-quire about 10° more up elevator to land than to stall in com-parable conditions at altitude. Without flaps this incrementdue ti ground effect is not so great, and with high-wing mono-planes without flaps the larding frequently requires less ele-vator than the power-off stall at altitude.

Requirement(I–E) .-Charactaristics of elevator control intake-off.

During the take-off run, it should be possible to maintainthe attitude of the airplane by means of the elevators“atanyvalue between the level attitude and that corresponding tommiirmunlift after one-half take-off speed has been renched.

Reasons for requirement (I-E) .—The attitude of an r&plane for optimum take-off characteristics depends upon thecondition of the runway surfaca On smooth, hard surfaceswith low rolling friction the qhortesttake-off run is obtainedwith a tail-high attitude. Where rolling friction is high,however, it is advantageous to maintain m attitude whichgiVe9high lift,

Design considerations.-Adequata control of the attitudeangle during take-off depends more on the proper locotion ofthe landing gear with respect to the center of gravity thwnon the characteristics of the elevators themselves. This re-quirement certainly is not critical from a standpoint of ele-vator control. Au airplane that has sutlicienttail volume tobe stable and su5cient elevator control to perform three-point or minimum-speed landings should meet this require-ment easily, as long as the main landing-gem wheels areproperly located.

Requirenie.nt(I–F) .—Limits of trim change due to powerand flaps.

1. With the airplane trimmed for zero stick force at anygiven speed and using any combination of engine power andflap setting, it should be possible to maintain the given speedwithout exerting push or pull forces greater than those listedbelow -when the power and flap setting me varied in anymanner whatsoever.

a. Stick-type control-5 pounds push or pull.b. Wheel-@e controMO pounds push or pull.

2. If the airplane cannot be trimmed at low speeds withfull use of the trimming deviw, the conditions specifiedin item 1 should be met with the airplane trimmed fulltailheavy.

R&sons for requirement (I–F) .—It is desired that emer-gency manipulations of flaps or throttks do not requiresimultaneousadjustments of the trimming device. The forcelimits specified are approximately 80 percent of the maxi-mum that a pilot can apply with one hand. The one-hrmdlimit is necessary to allow the adjustment of bhrottles, flaps,or i&mming device while complete longitudinal control ismaintained. It i% of course, desirable that the trim changeabe less than the limiting values given, The ideal conditionwould be one where the stick forces required for trim werenot influenced by the position of the flaps or throttles.

It is also desirable that the control position required tomaintain a given speed or lift coefficient be independent ofthe power and flap position insofar as powible. It is not,however, believed reasonable or necessary to specify any def-inite limits at this time.

Designfactors.-Because of si&titaneous changes in down-wash, dynamic preswre at the tail, and pitching moment ofthe airplane less tail, the trim change produced by vmiotionsof power and flap setting are very dif6cult to predict. Sev-eral of the effects, however, have opposite signs, so tlmt with

REQ_MENTS FOR SATISFACTORY

aticient cure it should be possible to restrict the trim changesto Qreasonably low value. Wind-tunnel tests of a poweredmodel of the design under consideration would be n greathelp if not an absoluteessentialin this connection.

Requirement (I-G) .—Characteristics of the longitudinaltrimming device.

1. The trimming device should be capable of reducing theelevator control force to zero in steady flight in the followingconditions:

a. Cruising conditions-at any speed between high speedand 120percent of the minimum speed.

b. Landing condition-any speed betw~ 120 percentrmd140percent of the minimum speed.2, Unless changed manually, the trimming device should

retain rLgiven setting indefinitely.Reasonsfor requirement(I-G) .—It is, of course, desirable

to be able to reduce the elevator force to zero in conditionswhere the airplane must be flown for protracted lengths oftime. It is also desirable to be able to establish a trim con-dition within the allowable speed limits of the airplane sothrd release of the controls will not put the airplane in adangerous position.

The reasons for item 2 are obvious.H. Requirementsfor LateralStabilityand ControlRequirement (H-A) .—Characteristics of uncontrolled

lateral and directional motion.1. The control-free lateral oscillation should always damp

to one-half amplitude witJ& two cycles.2. When the ailerons are deflected and released quickly,

they should return to their trim position. hy oscillationsof the ailerons themselves shall have disappeared after onecycle.

3. When the rudder is deflected and released quickly, it“should return to its trim position. Anj oscillation of therudder itielf shall have disappeared after one cycle.

Reasonafor requirement(II-A) .—Because of its relativelyshort period, the lateral oscillation must be heavily damped.It is not logical to specify limits for the period of the oscilla-tion because the period ds dependent on factors covered byother specillcdions and also because the period is dependanton the size, speed, and weight of the airplane. The amountof damping spec%ed in item 1 has been obtained with allsatisfactory airplanw tested.

Items 2 and 3 of the requirement (H–A) are included toinsure ,stabili@ in the behavior of the lateral controlsthemselves.

Attention is called to the omission of a requirement forspiral stability. Tests have shown that the lack of spiralstability has not detracted from the pilot’s ability to fly anairplane e.fliciently. In fac~ it is very”difiicult to determinewhether an airplane is inherently spirally stable or not, be-cause divergence will occur with a spirally stable airplane ifperfect lateral and directional trim do not exist or if slightasymmetry in engine power occurs in a multienginedairplamFor these reasons Qlarge amount of inherent spiral stabilitywould be required to insure against lateral divergence underactual conditions.

Since it appears that the degree of spiral stability or in-stability is inconsequential or at least of doubtful importance

FLTING QUALITIES OF AIRPLANES 53

under actual conditions, it is desirable to avoid any such re-, quirement because the design conditions for spiral stabilityconflict with other factors known to be essentialin the attain--ment of satisfactory flying qualities.

Designconsiderations.-The theory of dynamic stability hasbeen rather extensively developed from amdhematical stand-point. The charts of reference 7 make the calculation of thedynamic characteristics a relatively simple matter, providedthe stability derivativ~ are lmown. In general, however, thestability derivatives are not known and cannot be estinmtedto a reasonable degree of accuracy, particularly with poweron.

On the basis of experience, however, it appears that thedamping requirement is not a critical design condition.There is every indication that whea other requirements ofiln area and dihedral are met, the uncontrolled lateral motionwill be satisfactory.

Items 2 and 3 of the requirement (H–A) me dependent,as was the elevator-free motion (requirement (I-A)), on thecontiol-Mnge moments, mass balance, and moment of inertiaof the control systems.

Requirement (II-B) .—&leron - control Characteristic(rudder locked).

1. At any given speed, the maximum rolling veloci~- ob-tained by abrupt use of ailerons should vmy smoothly withtie aileron deflection and should b~ approximately propor-tional to the aileron deflection.

2. The variation of rolling acceleration with time following an abrupt control deflection should always be in the cor-rect direction and should reach a maximum value not laterthan 0.2 second after the controls have reached their givendeflection.

3. The maximum rolling velocity obtained by use ofailerons alone should be such that the helix angle generatedby the W@ tip, pb/2V, is equal to or grem%rthan 0.07 where

p maximum rolling velocity, radians per secondb wing spanV true airspeed, feet per second

4. The variation of aileron control force with aileron de-fle~ion Shotid be a ~ooth curve. The force should every-where be great enough to return the control to trim position.

5. At every speed below 80 percent of maximum level-fightWSSCI,itshould be possible to obtain the specfied Vflueof pb/27 vvithout exceeding the following control-forcelimits :

w Wheel-type controls: &80 pounds applied at rim ofvvhwl.

b. Stick-type- controls: *30 pounds applied at grip ofstick.Reasonsfor requirement(H–B) .—Item 1 of this requirement

states an obviously desirable condition for any conixol; i. e.,that the response shall be proportional to ddiection.

Item 2 is desimmedto eliminata controls that are unsatis-factory from a standpoint of lag in the development of therolling momen~ or contiols in which the initial rolling ac-tion is in the vvrong direction.

Item 3 was obtained by corrdation of pfiots’ opinions andmeasured characteristics for some 20 different airplanes of

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54 REPORT NO. 755—NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS

various types and sizes (reference 8). It -was found thatpilots judged the adequacy of their lateral control on the.basis of the helix angle genernted by the wing tip of theairplane. Airplanes giving values of pb/2V less than 0.07were always conside~d unsatisbctory.

Item 4 is a requirement for self-centering characteristicof the laterid controL This is a necesary condition for dis-factory control feel.

The specification of item 5 wcs determined by the limita-tions of pilots in applying forms to the lateral controls.Lower forces ark .of course, desirable.

Designconsiderations.-Item 1 represents a normal charac-teristic of conventional flap-type &.lerons,provided they arenot deflected beyond the range -where their effectiveness islinear. Certain spoiler-type ailerons, however, have beanunsatisfactory becausa of their failure to meet this require-ment. In tiles w, the variation of effectiveness withdeflection was either markedly non.linenror such that appre-ciable movements of the control about the neutral point wererequired before the aileronsbecame e.t&tive.

Item 2 also is met by all conventional flap-type ailerons.Again, however, certain arrangements of lateral controlsthat depend on spoiIer action have proved unsatisfactory be-cause of lag or incorrect initial development of the rollingmoment. Detail information on various satisfactory andunsatisfactory spoiler types may be found in reference 9 andlater reports on the subjec~

The specification of the helix angle, pb/2~ ZO.07 of item 3,corresponds approximately to requiring a rolling momentcoefficient Cl of 0.035 or ~-ter. Actually since p6/2T isequal to the ratio of the rolling-moment coticient to thedamping-movement coeilicient-C,/C,P a criterion in terms ofCZ alone is not strictly applicable. The damping-momentcoefficient tends to decrease with increased taper of the wingrind to increase with increasit aspwt ratio. However, forthe aspect rntios and taper ratio likely to be used, thecriterion considered in terms of rolling-moment coefficientalonq OJzO.035, should be satisfactory. In several typestested, particularly the very large airplanes, contrcd-cablestretch resulted in a very serious 10S of aileron effectiveness.There is also indication that wing twist under the torsionalloads applied by ailerons should be considered in an inter-pretation of the rolling-moment coefficientrequired to obtainthe specilied value of pb/2T.

Item 4 sets the upper limit for aileron control frictionsince the ability of a control to center itself depends on therntio of the inherent force gradient to the frictional force.

The control-force limits of item 5 ar~ of course, criticalat the high speed specified. This requirement can be metby W@ existing design methods without servo control ormechanical booster systems excapt, perhaps, for the verylargest airplanes that appear at this time.

Requirement(II-O) .—Yaw due to ailerons.With the rudder locked at 110 percent of the minimum

speed, the sideslip developed as a result of full aileron de-flection should not exceed 200.*

●The measar@d sideelip angle on which this and subsequent speclfieationa areLmeedshould not he ccmfaeud with the an~le of bnmk of the airplarm The an@eof sideslip is simply that @wn by a vane free to pi~ot about a Tertical arie andnllne itMf with the reiatIve wind.

Reasons for requirement (II-C) .—Aileron yaw is respon-sible not onl~ for annoying heading changes m a result ofthe use of nileronsbut alscrfor a reduction of aileron effective-n~ unless the rudder is carefully manipulated to eliminatethe side.slip induced. This latter effect is also dependentoq the rolling moment due to sideslip (dihedral effect).

The requirement for aileron yaw expressed in this mannerclearly separates satisfactory characteristics from those con-sidered unsatisfactory by pilots and, moreover, hns the meritof relating the factors responsible for aileron yaw in ILfundam-ental manner. The limiting condition of 20° sideslipseems surprisingly higl+ but the number of Satisfnctoly aiwplanes that develop sideslip angles substantially this greatcannot be ignored. Th~ requirement, however, is written tocover the critical low-speed conditions. At cruising speeds,compmable tests would give sideslip angles of the orderof 5°.

Design considerations.-The sideslip due to ailerons ischiefly dependent on the aileron yawing moment, the yawingmoment due to Tolling, the dihedral effeot, and tho direc-tional .stabili@ of the airplane. Complknce with the re-quirement depends mairdy on the provision of srdlicientdirectional stability, since the nileron ymving moment andthe yawing moment due to rolling are determined by theaileron power. Of course, the designer has some controlover the adverse aileron yawing moment through the useof dMerential in the control system and by incensing theprofile dmqgof the up aileron. These effech, however, nregenerally small in comparison with inherent yawing mo-ments due to ailerons and roiling veloci~, which are ahmysadverse in sign.

The required amount of directional stability is simply thntwhich will give an equilibrium of the yawing moments ator below the angle of sideslip specified. The adverse aileronymving moments can, of course, be determined in the windtunneL The yawing moment due to rolling for wings ofvarious plan forms is given in the charts of referenm 10.

Requirement (H–D) .—Limits of” rolling moment due tosidesdip (dihedral effect).

1. The Tolling moment due to sideslip as mensured by thevariation of aileron deflection with angle of side-dip shouldvary smoothly and progressively with nngle of sidedip nndshould everywhere be of a sign such that the aileron is nl--ways required to depress the leading wing as the sideslipis increased.

2. The variation of afleron stick force with angle of side-slip should everywhere tand to return the aileron controlto its neutral or trim position when relensed.

3. The rolling mornant due to sidesdipshould never be sogrent that ‘a revenxd of rolling veloci~ occurs as a resultof yaw due to ailerons (rudder locked).

Reasonsfor requirement(H-D) .—Item 1 insures that theroll due to Tudder will always be h the correct directionand that any lateral divergemmwill not be of a rapid type.It is also a necessary but not a SufEcientcondition for theability to raise a vving by menns of the rudder.

Item 2 is required to insure that the rolling moment dueto sideslip vi-illbe of the correct si.w with controls free. The

REQuHuHMENTS FOR SATISFACTORY

ability of the conbrol to self-center here again is a require-ment for control feel.

The reason for item 3 is obvious.Design considerations.—Wind-tunnel data showing the ef-

fects of fhps, wing plan form, and fuselage-wing arrange-ment on the rolling moment due to sideslip are given in refer-ences 11 and n?. These results are generally substantiatedbyflight test. With single-engine low-wing airplanes, however,the dihedral effect in sideslips made to the left sometimesbecame negative at low speeds with power on, even thoughit was satisfactory with power off or with power on at higherspeeds. Low-wing monoplanes generally required from 4°to 8° more geometric dihedral angle than high-wing mono-planes to obtain the same effective dihedral effect. On air-pkmes with the trailing edges of the wing swept forward,flaps reduced the effective dihedral and, where the trailingedge of the wing was a continuous straight line, flaps hadlittle or no effect on the dihedral effect..

In order to meet item 2, the friction in the aileron controlsystem must be low and the aileron required to overcomethe rolling tendencies in the sideslip (dihedral effect) mustexceed that at which the ailerons would tend .to float due tothe spanwise angle-of-attack variation.

The upper limit of the rolling moment due to sidedip (itemII-II-3) is dependent on the yaw due to ailerons (itemII-C-1) and the power of the aileron control -(item II-B-3).

Requirement(JI-E) .—Rudder-control characteristics.1. The rudder control should everywhere be sntliciently

powerfd to overcome the adverw aileron yawing moment.2. The rudder control should be suiliciently powerful to

maintain directional control during take-off and landing.3. On airplanes with two or more enginw, the rudder con-

trol should be sntliciently powerful to provide equilibriumof yawing moments at zero sideslip at all speeds above 110percent of the minimum take-off speed with any one engineinoperative (propeller in low pitch) and the other engineor engines developing full rated power.

4. The rudder control in conjunction with the other con-trols of the airplane should provide the required spin-recovery characteristics.

6, Right rudder force should always be required to holdright rudder deflections, and left rudder force should alwaysbe required to hold left rudder deflections.

(3.The redder forces required to meet the above rudder-control requirementsshould not exceed 180 pounds (trim tabsneutral).

Reasonsfor requirement(H-E) .—The reasons for these va-rious items me obvious. Item 1 must, of COWS%be metif satisfactory turns are to be made at low speeds unless, ofcourse, the directional stabili~ is very great. Item 2 repre-sentsone of the most important functions for rudder control,although if a tricycle kmding gear is used it becomes muchless important.

.,

Items 3 and 6 should insure adequate control over asym-metric thrust following engine failure subsequent to take-ON It does not seem necessary to retain directional controlbelow the speed specilled because of the probability that

FLYING QUiLITIES OF AIRPLANES 55

Maral instability due to stalling would set in flrk The180-po&d force limit specitied is about 90 percent of themaximum that an average pilot can apply.

Design considerations,-The rudder power needed to meetitem 1 of the above requirement can be determined in thesame manner that the directional stability required byaileron yaw was found (requirement (II-C) ).

In at least one instance, item 2 of the above requirementswas met without any rudder control. This was accom-plished by using a tricycle landing gear and by eliminatingthe rudder-position variation with speed and power. How-ever, due to the inherent instability of conventional land@gear, a certain amount of rudder control dur&~ take-off andlanding will always be required when this amzmgement isused, even though the rudder-trim change due to power orspeed were eliminated. Just how much rudder is neededhere is not known. The elliciency of the brakes, type of tailwheel (lockable or free-swiveling), and the magnitude ofthe inherent ground-looping tendency undoubtedly enterinto the problem. Also, in landing, the stalling character-istic of the airplane may have an important bearing. Onthe basis of data on hand, however, it appears that a ruddercontrol that is sufficiently powerful to meet the other re-quirements outlined should generally be satisfacto~ from astandpoint of ground handling.

Items 3, 4, and 5 do not appear to require additionaldiscussion.

Requirement (II-l?) .—Yawing moment due to sideslip(directional stability).

1. The yawing momants due to sidedip (rudder fhed)should be snflicient to rwtrict the yaw due to ailerons to thelimits spectied in requirement (H-G1).

2. The yawing moment due to sideslip should be such thatthe rudder always moves in the correct direction; i. e., rightrudder should be required for left sidwlip and left ruddershould be required for right sidedip. For angles of sideslipbetween * 15°, the angle of sideslip should be substantiallyproportional to the rudder deflection.

3. The ymvingmoment due to sideslip (rudder free) shouldbe such that the airplane will always tend to return to zerosidesdip regardless of the angle of sideslip to which it hasbeen forced.

4. The yawing moment due to sideslip (rudder free withairplane trimmed for straight flight on symmetric power)should be such that straight flight can be maintained bysidesl.ipping at every speed above 140 percent of the mini-mum speed with rudder free with extreme asymmetry ofpower possible by the loss of one engine.

Reasonsfor requirement(II-F) .—The reasons for item 1 wecovered in discussion under requirement (IC-C).

Item 2 of this requirement states a desirable character-istic for any control; i. e., the responseshould be proportionalto the deflection.

Item 8 is designed to insure satisfactory directional sta-bility, particularly at large angles of sideslip where verticaltail stalling has frequently led to trouble. This requirementfollows directly from the results of reference 13.

,- .“. ‘L,., ,.+ ..-:+:---- -..-–.~L. _ ,. .:.-. . . . . . . . . . .— _. . .

66 REPORTl?O. 755—NA’lhONALADVISORYCOIK~ FORAERONAUTICS

Item 4 is included to prevent the directionnl divergencefollowing an engine failure from being excessively rapid.Although the ability to fly with rudder frea on asymmetricpower is probably not in itself important, it is undoubtedlystrongly related to the rate of divergence and therefore therequired quickness of action on the part of the pilots whenthis emergency occ~

Design considerations.-The directional stability requiredto fuliill item 1 has been discussedunder requirement (II-O).

General discussion of the factors that determine the finarea required to meat items 2 and 3 of this requirement isgiven in reference 13. Howev=, the interference eilects ofwing-fusehi=w position, vertical tail arrangement, etc., areso ~-at that wind-tund tests wouId appear a necessaryaid to design for these requirements. Since the directionalstabili@ at large a@es of sideslip, however, is related tothe manner in which the flow break down on the verticalsurfaces, and on its effect on the floating characteristics ofthe rudder, the scale of the test should be kept as ~g-eataspossible.

Requirement(H–G) .—Cross-wind force chmacteristics.The variation of cross-wind force with sideslip angle, as

measured in steady sideslips, should everywhere be such thatright bank accompanies right side.dip.and left bank accom-panies left sideslip.

Reasonsfor requirement(II-G) .—tJnder normal conditionsin a sideslip or skid, a force is produced which acts towardthe backward-lying wing tip. Since the actual angle ofsideslip cannot be observed by the pilot, the cross-wind forcedeveloped alIovw appreciation of the fact that side-slipexistsbecause of the lateral acceleration which occurs. Iq steadysideslips the cross-wind force is balanced by a componentof the weight of the airplane, so that an angle of bank re-sults. The greater the cross-wind force the greater is theangle of bank. An approximate relation between angle ofbank @ and the cross-wind furm may be written as follows:

~.fi-I Cross-wind forceWeight of airplane

In addition to providing the pilot with “feel” of the side-slip or skid, the lahral attitude from vvhich it is possible torecover with the rudder alone (without permitting a head-ing changg) is directly related to the magnitude of the cross-&d f orw. Obviously, a positive dihedml effect is alsonecesary f or the perf ormance of this maneuver, but the f actremains that turning toward the low wing will always occurif the lateral attitude from which recov~ is attempted ex-ceeds that which can be-held in steady sideslip with fullrudder.

For th~ and other reasons, large values of cross-windforce are desirable and more riggd specification than thatgiven would lead to better flying qualities. On. the otherhand, it is not known whether this could be done withoutincreasing the drag of the airphme.

None of the airplanes tested to date has failed to meet therequirement as written. It is included, however, becausethere is indication on the basis of wind-tunnel tests fiat some

future designs may actually develop cross-wind force of op-posite sign to that normally experienced. Obviously, thiscondition could not be tolerated.

Requirement(II-H) .—Pitching moment due to sidedip.As measuredin staady sideslip, the pitching moment due to

sideslip should be such that not more than 10 elevator move-ment is required to maintain longitudinal trim at 110 percentof the minimum speed when the rudder is moved 5° right orleft from it+position for straight flight.

Rtwons for requirement (H-H) .—A pitching-momentchange due to sideslip is undesirable because it requires tlmtthe elevator as well as the rudder must be coordinated withthe ailerons. Also, since sideslip of considerable amountsmay be carried inadvertently, a marked variation of pitchingmoment with sideslip will tend to produca inadvertmt m@e-of-attack changes. The condition is critical at high lift co-efficients,so compliance with the specifications given shouldautomatically insure satisfactory characteristics at higher

$peeds”Detign considerations.-It is believed that the change in

pitching moment with sideslip occurs as a result of the down-wash chane~ expwienced by the horizontal tail as it movesfrom behind the wing center. In most cases, the momentproduced is a diving moment because of the relatively highconcentration of do-wmmwhat the wing center due to the pro-peller or partial-span flaps. It has also been noted that themagnitude of the pitching moment due to sideslip progres-sively decreased as the angle of attack was reduced, presum-ably becauw of the corresponding reduction of downwshangles.

Requirement (II-I) .—Power of rudder and aileron trim-ming devices.

1. Aileron and rudder trimming devices should be pro-vided if the rudder or aileron forces required for str~ightfight at any speed between MO percent of the minimumspeed and the maximum apeed exceed 10 percent of themaximum values specitled in requirements (II-B-5) find(IGE-0), rwpectively, and unless these forces at cruisingspeed are substantially zero.

2. Multiengine airplanes should possessrudder and derontrimming devices sutliciently powerful, in addition, to trimfor straight flight at speeds in excess of 140 percent of theminimum speed with maximum asymmetry of engine power.

3. Unless changed manually, the trimming device shouldretain a given setting indefinitely.

Reasonsfor requirement(IX-I) .—The reasons for the itemslisted above are obvious.

m. stfdling Characteristics1. The approach of the complete std should make itself

unmistakably evident through any or all of the followingconditions:

a. The instability” due to stalling should develop in ngradual but unmistakable manner. ,

b. The elevator pull form and rearward travel of thecontrol column should markedly increase. -

c. BrMeting and shaking of the airplane and controls

REQUIREMENTS FOR SATISFACTORY FLYING QUALITJXS OF AIRPLANES 57

produced either by a gradual breakdown of flow or throughthe action of some mechanical warning dwica shouldprovide unmistakablevrarningbefore instability develops.2. After the complete stall has developed, it should be

possible to recover promptly by normal use of controls.3. The three-point landing attitude of the airplane should

be such that rolling or yawing moments due to stalling, noteasily checked by controls, should not occur in landing, eitherthree-point or with tril-iirst attitude 2° greater than that forthree-point contact.

Reasonsfor requirement(lIf) .—The items of this require-ment are in keeping with all others given; i. e., it demandsall that can be obtained with existing knowledge and yet iseufliciently rigid so that any airplane that complies withthe specification will be reasonably safe in terms of ourpresent standards. Since there is never occasion in the nor-mal operation of an airplane for a pilot to stall intentionally,such characteristics that provide warning of the stall, aregiven first importance. If the warning is unmistakable, therelative violence of the actual stall 10SWmuch of its sig-nificance because it would then occur only as an inten&onalact on the pati’ of the pilot and at a safe altitude. Item 2 is‘bcluded to tie that recovery from an intentional stall canbe promptly made.

Item 3 is an outgrowth of some experiauce in studyingground-handling problems. In most cases, poor stallingcharacteristics are troublesome in landing. because of wingdropping either during the actual landing flare or after theairplane has alighted during the landing run. In other casesthe wing stall has influenced the flow at the vertical &l insuch a manner that powerful yawing moments have devel-oped. Unless the stall itself can be made to develop in agentle manner, the cure for these characteristics can be ef-fected by preventing the occurrence of the ‘stall altogetherin the landing maneuver.

LANQIZJY MEMORIALAERONAUTICAL LABORATORY,

NATIONAL ADWSORY COMMITTEE FOR &RONAUTIOS,LANGLEY l?mm, VA., March .%j,194.1.

REFERENCES

1. f30ul~ E A.: preliminary Inmst&ationofof Airplanes. h7ACARep.No.700,1$!40.

2. Soul+ Hartley&: FlightMasnrementsoftndinal Stability of Several Airplanes and

the FlyingQualities

the Dynamic lkmgl-a Correlation of the

MeasurementswithPiloW Observations of Handling Oharac-teristice. NACA Rep. No. 678,1936.

3. (Meenberg, Harry, and Skrntleld, Leonard: A Theoretical Investi-gation of Longitudinal Stability of Airplanes with Free tinholaIncluding l!lffect of Friction in Control System. NAOA Rep.No: 79% 1944

4. Gough, M. N., and Beard, “A. P.: Limitations of the Pilot in Apply-ing ForcEKJto Airplane Controls. NACA TN No. 560, 1936.

5. Wlruth, R. R, and Whit% M. D.: +mlys”is and Prediction of lkngi-tndinal Stability of Ai.rPlaneJL NACA Rep, No. 71.1,1941.

6. ~CATOY, TVillhunE : ~nm ~OKWS Applied by Plloia to Wheel-Type Controls. NAOA TN No. 623,1937.

7. Zimmerman, Charlea H.: An Analysis of Lateral Stability inPower-Off Flight with Charts for Use in Design. NACA Rep.No. 669,1937.

S. @lruth, R. R. and Turner, W’. N.: Lateral Control Required forSrdlafactory Flying Qualities Based on Flight Tests of NumerousAh@anes. NACA ReP. No. 716, 194L

9. Weick, Fred E., and Jones, Robert T.: R&mm6 and Analysis ofN. A. 0.& Lateral Control Remarch. NAOA Rep. No. 605, 1937.

10. Peareon, Henry A+ and Joneaj Robert T.: Theoretical StabRi&and Control characteristics of Winge with Various Amountsof Taper and ‘3Mst. NACA Rep. A’o. 636, 193?3.

11. Bnmber, M. J., and House, R. O.: Wind-Tunnel Investigation ofEffect of Yaw on I.ateral-StabiUty characteristics. I—3i’ourN. A, C. A. 23012 Wings of Various Plan Porms with andwithout DihedraL NAOA TN No. 703, 1939.

M?.Bamber, ht. J9 and House, R. O.: Wind-Tunnel Investigation ofEffect of Yaw on Lateral-Stability Characterlsffcs. 11-R&tangnlar N. A. C. A, 23012 Wing with a circular Fuselage and aFin. NACA TN No. 730,1.939.

13.Thompson, l?. IA, and G1lrnth, R. R: Notes on the Stalling ofYertkal Tail Surfnce8 and on Fin Design. NACA TN No. 778,1940.