T= APPLICATION OF FLIGr =D SIMULATOR TETING TO

VTOL AIRCRAFT HANDLING QUAIZTES SPECIICATIONS

Paper to be presented at The

Society of Automotive Ensineersf

V/STOL Stability and Control Session,,

Manday April 8th, at the

Sheraton-Park Botel in Washington, D. C.

April 8-U1th

by

fre. J, Drinkwater, III and L. Stewart Rolls

(pGESI

(CODE)

NTIONAL TECHNICAL INOMTO SERVICE

-A FR ORT ORAD OEL {CATOcy,

https://ntrs.nasa.gov/search.jsp?R=19650076252 2020-07-31T00:06:07+00:00Z

INMRODTIION

~ V/STOL handling

qualities research J. ,., -­ , W-- as" OFu- various

simul tors and test aircrafti-T !,,, o"..... -o provide

guidance to the aircraft designer.

haI tk.. . I . Since several pre­

production prototype V/STOL aircraft have been completed and others are nearing

the flight test stages considerable effort is being expended to adapt the

research results to written specifibation for this class of aircraft. W

1)re are alb, a great variety of V/STOL types ol I

introduced by ach.lo a:..._ i-_ewpl .Ac Nevertheless, it is important to

translate as much of the research experience as possible into handling qualities

sPecifieations at-en early date. The Purpose of the specifications is to give

ihe operator some assurance that the mission capabilities of the aircraft, when

it is deliveredj, will not be unduly limited by its handling qualities and that

the aircraft cdU effeetively perf~rm the mission for which it vas designed.

The handling qualities requirements necessary to allow vertical take off

and landing for aircraft which differ rAdically from helicopters have been the

subject of numerous inveotigatiozis and wdtensiva bibliography would be required

to list all--of them, SMwver, even research data obtained while hovering under

visual Oonditione are difficult'to translate into meaningful specifications.

A geviev of the teats indicates apparent differences between simulator

results and even between flight test data as obtained by different researchers.

Very little is to be gaited by trying to resolve all of the differences in

results especially when flight and sLmulator data are compared. However, if we

consider that the simulators have assisted in directing us to the important

variables and then. look at the flight test results as being more definitive, we

see that vo have comd a long way towards defining the acceptable limits of

several Important control parameters for the visual hovering task. The develop­

mett of VTOL aircraft other than helicopters has made it necessary to consider

vfaluds of control power not always toand rotary daning which were gemrain

helicopter desigs and ydt are of primary importance to the VTOL aircraft. the

rotary ling provides, inherent pitch ahd roll damping -and relatively high control

powers are obtained without sacrificing performance. However most VTOL aircraft

will have very low rotary dampi about one or more axes without stability aug­

mentation and it hai been dffiult-to design VTOL aircraft to the values of contro:

power hich were in most cases readily obtained on helicopters.

SevetaL well defined handling qualities limitations were encountered during

the flight tests of the X-14A, research airplane and several other VTOL aircraft

while hovering in visual flight conditions. The purpose of th: paper is to

describe some of these limitatloua and to compare the results with the available

specifications for VTOL airplanes. ( )

Dlescription of aircraft:-. The basic X-14 airplane shown, in figure I was

modified by the NASA to p ovide it ith increased thrust, variable control power

and variable angular rate damping about all three axes. This VTOL airplane is

powered by two General Electric J*85-5 engines of 2200 lbs. thrust with cascade

.3.

divertear in the exhaust exits which allow the thrust to be vectored 900 to

the ' rotational axis for h r ~v'li~ht. Dual reaction control jets at

the wing tips end at the tail are suplied with 6gine -compressor bleed air to

Protide 0antrol. momots while hovering and in very low-speed. Onese

of nOzslee is lmiec1nically connected to tha control. stick and rudder pedals and

the second et is servo operated in response to signals from rate gyros and the

pilot's control position inputs through cockpit mounted potentiometers which

ailow peident variation of control power and demping about all three axes.

Additional circuitry is provided to cancel the engine Wyroscopic mmnePte.

Further details of this airplane eand its control system are described in refer­

ences 1 and 2,

Five pilots have flown this airplane through a wide range of control power

and damping characteristics and their opinions expressed in the Cooper rating

scale were fp6rted in TN D.1%8. The task involved was simply visual hovering

which incldes accelerating to about 20 knots in all directions in relatively

ligt winds' For this reason the control power-damping boundaries obtained are

very close to a minimum operational requirement. The boundaries for control

response about all three axes for the visual hovering task are shown in figure 2.

The 6.5 boundaries for each axis were determined in flight to be the minimum safe

values of control power and damping even under the nearly ideal test conditions.

Several accidents and near accidents which have occurred with. several test bed

aroraft whf-onpergtions were conducted beyond (with lower coutrolpoers)ti.

the limitiug yalues presented in this figure. It should be mentione however that

the yaw axis was the : from the safety point of view during these tests.

COWARISMaM MITSAX8501A

Iu 2nt oU-MZ~ tht esa'its pMsutta, iU reference 2 int* hading

~~ ~~'aS enbrat17 eecesOeM to e)press thme I a sinpler form 0e fOze OVA has beeA used is the heliopter iL t. 44eification "

• Zot v r rtion In control response with Ircraft

4 ?eduction In ratean O dtkifbd Am 14ftation ot the approowlte mment of Ijnert#i A vx G=nespecuLaton f r roll reaptse and the data Obtsatnearm the X1-14 fl-xt tests ih sh OA t figure 3, %a present roll reaxonse reviroent in referonce 3 is 96/(Ql300)lt/ dogres in 1/9 second; oweVar, a OqfrMent f~r lateral dispelaoent after ote second equal to or greater than 30/(v+10

degrees UasbqeeApropose and this response requyrement is shown in figure 3. since the. X,*I veiab 800 p8oUd the roll response reT;IqxO by thIs specificatlon

for this airplane -falls In a area vbi& was dete'moed from flight test to be aceputla for ouIjltedt.a Operatior for an aircraft of this weight,

The aiion, of the specification roll amcng re uirment of P5(lx) 0 7 .

ft lb/aeg/jec- still Vou2l4 provide Only limted operational apabillty waor~iatn to the flight test resuits.

')'rig th; ' lo0"eAt Of 'thi SIrr *fiti a found to be neesery to Increase the- lateval control powr fit 2slghtly less than I radiam/s e to almost 2 radlase . SoxLe ajust nto were made to the control s4sitivity vbhihinproved the Oontrlahii1* but satisfactory lateral control vas _not obtained mtll the

aircraft iled vwaa&*1th, the hisher ontrol power. It is-obvious tbat th4 helicopter ipedfications (ref, 3) hich mpasizss

the Weight of the aircraft in detetrin g Its control power do tot agree with the available WOL aircraft test fortation fo the roll axis.

A pterizo of the helicopter Specification for longitudinal response while

hoveri also Indicates an Inconsistency with the flight test results reported in

refere ce 2. This comarison is shown in figure 4. In this case) however$ the

military speeifications requires considerably more response for a 3800 ib. air­

*raft such as the X-14"-than the- test results indicate is necessary for normal

operation.

'me level 69 damping augmentation required is much higher than is required fopr

visual hovering as it also was for the roll axis. This level of damping might

very weil be required for certain missions but -the specification appears to require

this value for all. normal operations.

The results Or the X4141 stability and control frignt tests also =caoave

(figure 4) -thatp-given adequate control Power, visual hovering operation can be

conducted even with zero rate damping about any axis and with only the basic air-

Plane damping (to damping aumeatation) about all axes simultaneously. Flights

in a 12,000 3b. vectored thrust VTOL strike fighter also verified that the damping

level pmoviaed by the basic, airplane need not be augmeatei adin fact, experience

has shown that the control Poer and damping requirements for VMR hovering about

all axes were -the"same for this 19,000 lb. aircraft as they were for the X-1)#

w~hich vei~hea one-thir4 as much. To-direct qonclusigns can be reached from

these flight tests in the 4joo l b. to 12,000 lb. ran. Firet)- the basic air­

plane rotary damping is su'ficinet for vertical tshe off - landing and other VMR

hovering operations. Second, no reduction in the control power required for normal

operation was found due to the three-fold increase in weight although the sPedifi­

catign for rol. control Power (fig. 3) Indicates that a 30 percent reduction in the

6 .

X-14 test values should have been adequate, The lover -valuesvere tried onthe

heavier aircraft and found to be inqufficient. For these reasos it is believed

that the reductions it ootrol power with increased weight alloed by the present

specifiosti6n '600A "be misle&dheg to th6 dtsiiaer of veryfiavv- VTO acf. Several flight, tQt exjeriments are being developed which it is ,oped will provide,

us with more definitive 'resultsbut th~y are easplicated by the very large changes

in weight -wich dpa ent y, will berequtred in order' to. detect a change in the

control r~q~euirdmsa. It_ is of interest to note that ground simulato-s ae of

little valde in this area, At this, moit it is,tempting to .shift our rational­

izatiou to , size .dr later measurement lnstead of the weight effect but again

experimental results are not available for the hovering ease.

lhese apparently very high control "wer reuirements idave.the designer of

a future 100000 Jb. VT0 aircraft in. somewhat. of a q-.ndry, since a, =s gross weight and size the moments -of inertia will ,be so high that the thrust required for

control moments is,ften.an undesirable large pircentage of the-thrust reqired for

lift. 7urth~rmore, the cost of such.a venture isdo high that a puely*e~rimental

aircraft -is out of the question,- An approach which has been suggested is to build a large frame Vork ih tAO form of a coss having lift and cantrol engines mounted

at the eiio.f each arm. It is posabie, that significant information could be

phtaiued from li ited hovering tests to determine the influence of size ,and inertia

on the controI responsequrmnsb this method._ It Is easy -to dismisa this scheme -out of bsnj however, in the'light of the results obt&tmed to date an effort of this type may be required to provide guidande to the -designer of 'verv -lareVTOL.

aircraft

The to+-' -ntrOl -!over and rotary damping requirements have bean 0t4*sized

because of :their"Importance in meeting the response specified-In -rfevnces,2 and 3.

- 7­

Other control conditions of course are iorant when overall Uhdling qualities

are coadere4 and flijb~t test data are availeble Ioih sho4 the need, for

optimizing control ensitivrity' ~~ ~t~ e per inch of control dis.'

place~ent,. refe ee I, and the velocity stability parameter Mu .as reportd

inreference 5.

&6 flight tests of reference 5 shows that g velocity -stability is.

the most desired POtdition then in 90er hovering fight and that inereases in

velocity stabilitv not only increase the pilot"s control task but the moments

produoeaby velocity also reufe the control power available for msneuveringL

Sal 6is in. the velocity stab.ility of a VTOL aircraft, es±f easilyperedia subtract from the control Dower - euvering therefore the levels or

contr6l power foun' to be reqdired from the X-l14A flight tests are in excess of

the requiremento to irliti*wts pro4-doea by'-stability an& enginle power changes,

Reig.t #Q1ro-, In addition to the.need to control pitch, yaw, end. roll

of the VTOL afr-asft the pilot must: ala control height whtich becomes- independbut

of pitchattitude 'and dependent on thrutt response as hovering flight is attained.

studies of hei~ht control rsenurepeesncl as reprted in references 6 and 7 vere

conducted on sinulators 'a, provide an 'i.ication of; the influence n nildt oiiniol

of the Iportanut height control response paremeters. Increases in both available

thrust-eight ratio a( damping from near zer levels were found to be desired and

a collective typo height control sensitivi1ty of about .lg per -iq& spproacle t he

Optisa value. The .tnimMlevels of T/W sad height rate damping.ire difiiuli

to define yciau'e to bo so AuchtXhy"appeared a Anmetion of the bhvering task

requirements. Awing recent studies using, the Ames Height Control :SIMMU6r,

values of control power of less then I. "- and zero height rate dampilfg still,

alloved the hovering steadinis reqremepst of reference 3 to bOemet even thoup;

+he ability to dhan* height and sthiliza qUke.y had deteriorated. Me one

parameter vbich resulied in p (leoreas in hboveripg eveawiteso: and 1iekb erahte

a dangerous height control situation was a large inorease £ the first: rdor

response of tbrust to a stOeP height ,oont-,ol "InT

The siiiator which *as used -toiveatigate the effects of' the thnst

,response tiie donsatt is shown in -figurei -5. trivel -iIO zaThe ca~b nw'ib-ipL

velocity -h2 feit/aec-. y acceleratina 'f 4 96 € ci h ­be -atihined Te ca

4eight is, coijtrolled by x co:lletive layer &aad tr iad levs of' realionae

are -provided by, ad analogue oonp~ter -The rio- 6 the teats, of thie tbrust

tine edtaat *are-stiot in kigure v~c d±1I tild variatip. ratingm.ew in jalt

kOon r Sea ) tie ihe zeaeint . The&y theconstaifo aeaant. tsr"e!a5e"d

hovering tea less, require en o eference 5. That is t6 b-ld 4 !4i -t

i aInch or less o 'collctv.e Intion. :A total T/W of 1a rx wioviua-ana

the collective senitivity ight.z t1 inhl, The h eightat.aneziu V"ern te

"Mich represents a mpre severe onition than mst VTOL Adirhfat ,qouI bavd..

Izcreasing the time constant from zaro.to .3sedond causM&4,iiticble, dedie04M

in hovering steadinesse. A vau -or ,6 second caused Ove-oontaol4ug and a valuo

of 1;Z seconds required nearly: full'pl~lot attention. Neither cmdttioh met the

specification, for ~hVerifig zteadit as.- A-tite constait of 2*4Is aeaon rsutea

in a ~ oa large eaomuraiots. ohight oecuare an46,te hts

indiceated that 40 attempt at flight, should 4ba made vith this condit4ion

The time l'Astqves of hei&*d4.gttroI inpaIt .ad-eab -response goe~oeveraI test

conditions, are shov.u i. figures I to U.

9

The require nts for hoverini st adi~ess of reference 3 are Indicated on

the figures as dased lines.' When ential- zero time,constant isdpresezt all

the .u irAents are".xead1l met eIs shoA In figure 7. A tuhist -response time

constant ot 3 second results in a ,nticeable.increase in collective stick motion

but still le es thaa the h 1/2 inch-allmd. ai4ght rate exceedoed l/2 feet 'per

second by 4'sma. mount however qthe run Illustrated. in figure 8 height was

maintaine within a few inches. At- .6 segond all three steadines' rbquirements

were ecad.O iArshed -the-specificatilimits Furthir increasps in the

thrust response time constant resnlted in -ageneral decrease iin. overing controlL.

ability which is shown in figurei -9 1, . The- pilot .pinin variation with this

time const~t showh -in figure 6 indicates that maxmum-of.i..secomds is the lim!

for uornma. operaons u , secons is required tO meet, the elstiig specification!

Flight teot,'ata hja been presete& ch defines the VFR hoverin= control

pOV0e? requi 6 &W,orVTOL afrerdft- in.the, 4,000 ~.to a.066'$&. 'i§ei~ht clas

There ara largedifferences betweef -thls data .and the present-military specificatibi

for helicop.ters. . jigificant d e4nc e in thde control jower requirementi

were noted-. 'wth threed-fol" change in.alrcaft weight althou ie ;zstin ­

ftiation allws a 30 perteat reduotiOn it the, higher weight, Ze t~ts indicate

that further: Investigations ill he reqixira at much hiher "weighs-Bdwith large:

hovering tes.t -viclea in order tO -provide more realistic Ogidande tbr the 4aopign

of verSy large VTOI4 a~ircraft.

More than .10 iiOtU have flow 'the, X-14A jet Wft VTOL aircrsft, with zero rat

deoping about -each xies and with just-the basic airplae-d4=ping abQut all axes

siultanously !hir domeiints iudie th~at-'Siveh aev~ate OonftrQ1 powr

the U2U2te ra3e± ais 1fz'±u11 ye OPeratiofts- A

~i~e0 ti)j% of laoight contgol, response on a zov5ng base PI=U2~tor in~dicated'

tht .overing --t sdis' $. irkedly e~octed by the firs~t -ods tbruwt iesporme

tize tuiu , 'b Ipilots Xelt tbat--oIjJ.±e ojperatlon'sbs~ld bea ttemted

viha I ea)10t .s constant in e'X06s or a*Ac~d-that Opevatiou

a time. constant exceediing 1.2 secondsa vil be daneroas. -ZTes nJrntatianO

are iti greeri~nt Vith the have$.ing steadiuOsvsirmets:or th6.fn4t4 's peci­

f-icatioii fok l ecovters.

IRzNCS

L. O'Malley, Jazm~s A., Jr,, and Iandpbafr$ lee C.:--Me X-l1. VMLMxlftne. *AXDeigp Tool. WAPaper 93B, 1958,

2.- R11$,' L. Steiart, and Dintwater, Pred T., MI.fA'YUi~t Daterminatox O -the Atiiu~e Control 'Power, and , mPiut F60irennta for ' a a.Nvra TaaIk IA~ thq,-VAAbIh Stabilityan ono X-lAAA $toemab yeh4O24. - MSA-TN D45I8, 3_e .

3. AncMu. i Z UO4pter Fli±ng an Oround Handling Qualitiis) General Reqre nt fez' Mi~itz'~at±6n B'p 7, 1961.pec LU.8$1A

4, $a*irq, zeymour, and TaPscott, Rdb .J.: Mse Effect Of Various Combinatto~A of Daxap4ne and Control, Poverzin icopter Handling ( aaltiea Ouring Zoth' lttrunient .ad Viaual 1rUght. NAN TS D58, 19$9.

5.. seckeli B.$ 'Xa r J. J.$ -u M1er ~0. B.: lbagitdipe and 5Qxlte ror Itevering.- Department of Aoniatioal Unversity)Aport No. 59I4, .%emnber 1961.

.xGeRGea onXd., and Waok-, 1Riabard P.: A Preliminary Piltad-%04uAtor endA FU~lbt Stucv. of &id~t Control Mouirents for VTOLAfrraft. NASA TN D.4.201.

1.Uz'~e, a . Jr.,. and AdnadAtwIan, ArtWer. I VTOL Hilaft *CQnbrol M'G~ra~ftinHverng a Determilled from Motion 'SImu3~tor Study. NAS TN D.14 St19 2.

40L 16 in

INPUl ,T~

roi

...... .i.E .A .St a m-.E . ME EA C - - B E T S C.A SS.A..L O E

OFFICIAL

NAIIONAL L-O 'i

AND SPACE AD ,lN iTAi ON

AMES RESEARCH CENTEA Moffett Field, California

z

6.40 9.50 13.40 19':- - i-"sec<.. ~-4 . . . . 0

< D *1

.ug-ft- 3---- / / 1170 (X-14)

0) o~ za0 IlS8

Ref. 2 ""­a ' /18 0C) , L f /__/ i j 25 .. {jf//jTL . 106

0

0 2 3 e

Lateral control power, radians/sec

of flight test results with MIL-H-8501A fo-the roll axis.-Figure 3.- Comparison

6.4-I sec 9.5' 13.40 19.00 TN D-1320 -4 / / N operationNormal

// Limited operation

C) 5/ c/oO © No operation

{ L / / // 0.7 Iy, slug-ftS.2D/=5

/(Iy 1 -­

/ / / X-14 (Spec.)L.E/

©®t / 1- --50

-- - - 106 2 35

Longitudinal control power, radions/sec

0 1

Figure 4.- Comparison of flight test results with MIL-H-8501A for the pitch axis.

i7,,

AND SAFETY DEVICE

0 ~iiiii

HEIGHT CONTROL TEST APPARATUS

to

9­

8 No operation

7 0

0o 6Limited operation

5

4

Normal operation No height rote damping

2 -1,15g maximum thrust/weight .lg per inch collective

0I 2 3

Lift - thrust time constant (first order).

Figure 6.- Pilot opinion bounaarIes Tor Tne imi migust first order

time constant with a motion simulator.

- -- - - - - - -

=' i 0

0

0

--j v V VV V V

010

0

2-­0- 2 4 6 8 10 12 14 .16 ,

Time, sec

Figure 7.- Hovering steadiness, results with variations in thrust response time constant.

N.tonl Aeronaut4 Space Admai.1straone

Ames R chCenter MAfeti F.ed, Calif.

0

' 0­

0 2 4 6 8 10 12 14 J6 I8

Time, sec

Figure 8.- Hovering steadiness results with voiations in thrust response

time constant.

National Aeron( and Space Administration Am search Cen er

Mr .*tt F eld.CahIF

-t.

"go 2 4 6 8 IO ,2 14 16 18

Time, sec

_Figure 9.-'Hovering stead[ness results with variations in thrust response time constant.

National Aeronautic Space Administration

Ames F :h Center

Mofe 1, C.liF.

.2

+ ­

-to

-ttoe co s an -I 14 6

2a o a e o au i s p c A m n s r t o

F..ure t,1 -,Hoverng stadne

Time sec s results with variations in trust response,

-timeetconstant.lif

39 0

0)0

in 10- -

Z 4)Z 0

4)

aT4,

60 4

Time, sec

Figure H.- Hovering steadiness results with variations in thrust response time constant.

National Aer cc and Space Admsinistration

Aries Research Center Maonett Field, Calit.

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