C.P. No. 334 (19.168)

A.R.C. Technical Report

C.P. No. 334 (19.168)

A.R.C. Technml Rermrr

MINISTRY OF SUPPLY

AERONAUTICAL RESEARCH COUNCIL

CURRENT PAPERS

Atmospheric Turbulence Encountered by Hermes Aircraft

BY

J.R.Heath-Smith, B.Sc.tEng.1

LONDON: HER MAJESTY’S STATIONERY OFFICE

1957

PRICE 3s 6d NET

C.P. No. 334

U.D.C. No. 551.551: 533.6.013.8(Hcrms)

Tcohnical Noto Structures 214

Jmuwy, 1957

ROYAL AlXtXiGT FSTABLIS~XXL' - -_-._. x-_.-, ..I_-- ---. "._.I

Atmospkric Turbulence cncountcrcd by Hcrxes aircraf't

J.R. Heath-Keith, B.Sc.(End

Accclcration rccolds wcrc obtained of the turbulcnco oncountcred by B-O.&C. Hermes aircrdt in 417,ooO miles of operational flyinS on routes ?romLondon to East and West Africa. It Is shown that turbulsnco doorunscs with increasing aitlttic. A result of the pilot's discretion in cholcc of altitude PJX? cowsc is that the gust frequency cncountercd during most of the cruise is about one third of tho averago atmospheric gust frcqucncy. BLtwccn 9,500 feet and 14,500 feet gusts wore oncountered four tkcs 35 fxqucntly over East and kcst African sectors as over European sectors.

LIST OF COIW!?JiTS

1 Introduction

2 Description of oquipmnt and flying

2.1 Instrument end installation 2.2 The flying covered bythe records

3 Variation in turbulcncc with sltitudc

4 Turbulcncc over different regions of the rout0

5 Conclusions

Aclmowlcdgmmts

References

LISTOFTABLES

SLmnnary of acceleration data

Distribution of rocording intervals according to 0.23g counts

Distribution of recording intervals aocording to aooeleration oolmts

Sunnnsry of gust speeds cncountored

Rclaiive time spent at nil altitdcs

Summary of accelerations from 9,500 ft to 14,500 ft

Comparison of 10 ft/scc gust frequency in diffeerent regions

LIST OF APPENDICES

Correotions to acceleration data and calculation of gust sped

LIST OF ILTJUSTR~IONS

Map of the routos flown

Distribution of recoded flying time in the year

Gust speed frequencies at different sltitldes

Variation of gust frequency with altitude

zt?E2

3

3

;

3

4

5

5

5

Table

I

II

III

Iv

V

VI

VII

I

Figure

I

-2-

1 1ntmducti0n

CountingAaco1erometoz-s were oarriodinHornx~ IV airor& of the British Overseas Airways Corporation and acceleration reoords wcm obtain& in the turbulcnoa oncountcimd in 417,000 miles of normal passenger serviw between London and Africa.

Tho records arc exomincd to dotordne the vsriation of turbulence with altitude and the diffemnce in turbulcnco between seators of the route in tha altittic band 9,500 ft to 14,500 ft.

2 Dosaription of aquipncnt end flving II

2.1 Instrument and Installation

The Counting Accelerometer' responds to the accelerations imposed on it in one dimation and rooords the- number of timrs oaoh of a series of acceleration levels ha5 been oxeeeded. Suoaossive counters ropresont levels at intervals of O.lg and resdings em givcz for a range of 1.2g to 2.9g for upward accelerations and from O&g to -0.9g for downwad accelerations. These values are nominal and have been corrected in this report except whom it is stated othorwiso. An altimctor, airsped indicator and cloak am grouped round the oounter disl and the whole assembly is photographed at regular intervals of 4ppmximatoly 10 minutes.

The CountingAoeelommz.ter wes rigidly attachedtotho airfreme close to the osntre of gravity in suoh an attittio that vertical accolcrations wore reoo~od during level flight.

2.2 The fltinp; covcrcd by the moor&

The roconis wmo mode betion April 1952 and Octobex 1953 on 487 flights covering 417,OKl milos of nod passenger service on routos bctwzcn London end East and Wost Africa. Dotnils of the main routes arc shown in Fig. 1. The distribution of rooording time between months of the yew is shown in Fig. 2. The instruments mro fitted at different times to vsrious airct-apt of the fleet.

3 Variation in turbulonoe with sltittuao

The mcon3.s arc of avcrego duration 10.4 minutes and contain tho counts of aoccloration during the intcrvsl snd tho sped, altitude cmd weight of the sirct-sft at tha end of this intorval. Appendix I describes the correotions which sm made to thcsc msdinge and the methed283 of trsnslntingthe aocclerations into gust spools.

Table I is a summary of tho corolts of aoceleraticm grouped aueoding to speed, altitulo ard wuight. Tables II and III show the distribution of recoding intcrvds nocording to their content of aoeeleration increments. (This information is mquimd for subsequent work.)

On the basis of Tablo I an estimate is glvon in Table IV of the gust speeds in each altitude bard. Fig. 3 shows the gust fmquenoics end their variation with altitdas. Fig. 4 shows directly tho variation with eltitde of the frequency of gust spocds greater then 10, 15 snd 20 ft/soc E.&S.; the fmquencios of exoeding greater levels sm not inoltied as they were not moor&& in signifiosnt numbers over the whole altitude rage. Aa upwerd snd downward gusts u~ire found in about equal numbers Fig. 3 and 4 are based on tho sum of tho two, end thus give the frcqueney of a gust whcthcr it is up or down.

Table V shows the time spent at each altitula on the prindpal soutors of the routes snd it is socn that two bends were used for

-J-

cruising centred on about 12,000 ft and 18,ooO ft. Examination of the aonsecutivo flight recur& show-s that all flights oommsnced with a &orate climb at about 500 feet/minute to about 12,ooO feet whem the cruise amxmncd. In about two thirds of the flights the elroraft remained et 12,ooO feet for the whole oruise but in the remeirder of the flights, after 1 hour or ~)re at this altittie the aircraft olimbed to about 18,ooO feet to oompleto the oruiso. Flight at 18,ooO feet oawrred most frequently, but not exalusivoly, over high md.

The influence of the airaraft operating conditions on the shape of the amves of Fig. 4 is now discussed. As the curves are of &nil& shape attention will be oon

Y to the 10 ft/sec WC. It is assumed on the

basis of previous war that yosrly nverage atmospherio turbulenoe decreases exponentially with height snd therefore may be represented on the logarithmio sod.0 of FLig. 4 by a straight line. The position of this line is determined at low sltitties by the reoordea turbulence for the lowest altitude bat@ (sea level to 2,500 feet) beoause the aircraft flow at those altitudes in ell prroather conditians and, further, as the sirwaft 1~88 under ground control there wea little possibility of avoiding turbulence by deviating from course. Betvaen 2,500 and 7,500 feet the turbulence was reoorded entirely during climb snd descent, agsin in all weather ocmditions, but as svddence of turbulence by altoring course is considered more possible than at the lorwst altitties, the recorded turbulence is oxpeated to be less severe than the stnmspheria av%rsge. Thebends7,500- 12,500 feet and 12,500 - 17,500 feet oontain the aruising altitudes of the sircraft end the recuded turkulcnoe will be less severe than average to the extent of the pilot's ability to avoid turbulence by a chenge of course or altitude.

It wes stated previously that the airoraft cruised at 12,000 feet or 18,000 feet em3 it is possible that the pilot avoided the ororst turbulence at 12,ooO fact by olimbing to 18,COO feet. Thus the reoodd turbulence at about 12,OCO feet would be less scvcre than werage ad at 18,000 feet would be more sevcm than average.

These considerations explain why the observed values of Fig. 4 lie above the line suggwted for average turbulenca conditions. It is concluded that discretion on the part of the pilot is imports&in redwing the number of gust loads experienced by en zrb-araft. In the present instanoe the turbulence experienced by the aircraft is reduced generally by a factor of 3.

4 Turbulcnos over difforent reu,ions of the route

Table V shows the time spend at all eltitules during olimb, uruise sA de-t for three geographical regions. These regions sre defined in Fig. 1 and are related to the routes flown.

To compare the turbulcnoc over different geographical regions it is nooesssryto use samples whioh~ere obtained tier similar flying conditions in a restrioted eltitwle bend. Table V shows that the aruising moords in the bend 9,500 feat to 14,500 feet from fairly representative semples of turbulence in the different regions and a suznnary of the regional aoceler ations in this dtitde bend is given in Table VI. The numbera of gusts exceeding IO ft/sec end their relative frequencies in different geographiod regions are given in Table VII.

The relative fmquenoies idioate that recoded turbulence is of the ssnm intensity in East and West Africa ti is about four times greater than over Europe. This result is not necessarily true of average atmospheric turbulence sa the extent to which+& pilot avoids twbulence depends on faators in which the geographical regions msy differ, suoh as frequency of the turbulenoe and essooiated cloud conditions, congestion of dr routes, etc.

-4-

5 Conclusions

Examination of gust accalcration records fromHcrmes aircraft flying between London end East and West Africa led to the following conclusions.

(i) Thwe is a continuous deorcsse in the number of gusts with increasing altitude from sea love1 to 20,000 feet.

(ii) The pilot's discretion in choice of altitude and course with rGgnrd to weather conditions results in the turbulence expsriencod by the aircraft during cruising flight being about one third as frequont 89 avcrege atmospheric turbulence.

(iii) At modcratc cruising altitudes the Hermes nircraf't encountered gusts about four times as frequently over East Africa and West Africa as over Europo.

AcknowlodRemonts

Thsnks are due to tho British Overseez Airweys Corporation for their co-operation in lnstdlling and servicing the instruments.

PO. -

1

2

3

4

REE'ERENCES

Author Title etc.

J. Taylor Accelorometw for detorrmning aircraft flight lads. Engincoring. 11th and 18th April, 1952

Air Publication 970, Chapter 203

J.X. Zbroeck Gust Alleviation Factors R & M No. 2812 June, 1950

J.R. Heath- Turbulcncc encounter& by Comet I aircraft Smith Current Paper No. 248, 4956

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APPENDIX I

Comcctions to acceleration data and calculation of gust speed

The aata arc oonsccutivc redings of airspeed, altitude and weight at an avcragc intcrvol of IO.4 minutes end the counts of aocelaration during each interval. The airspcod is real to the nearest 10 knots, I.&S., the ciltitudc is rcsii to the nearest 1,000 feat, I.C.A.V., above sea level and tho weight ie estimated to the noarcst 1,000 lb.

Corrections

Those I‘CCO~~S &i& WY contain the effects of ground 10ads DIP discadcd with the result that on an avcragc the first and la& 5.2 minutes of each flight are cxcludcd from the analysis.

The airspocd associated with the acceleration counts is the average of the initial and final airapoed of the interval, rounded doull to 10 knots. The altitude assoclatod with tho counts is also tho average of the initial ana final readings, rounded down, except where the change in altitua0 was less than 2 nominal units of 1,000 feet during the interval, in which oaae the final altitude of tha interval is used.

The acceleration thresholds are corrected for the instrument error which is given in the footnote to Table I.

It is assumed that thr, normal acceleration at the instrument position is that at the centre of gravity of the aimraft. The rccordcd aocclorw tions arc trnnslatod. into vertical gust speeds by the following method.

Aircraft weight is divided into.lO,OOO lb loads centred on 64,500 lb, 74,500 lb and 04,500 lb.

Airspeed bandwidth is 10 knots.

The altitude range is divided into a sea level band up to 2,500 feet and bands of 5,000 feet width centred on 5, IO, 15 and 20,000 feet.

Tho counts of acceleration are grouped according to the altitude, weight ma spcea of the aircraft ma the gust speed OOrrcspOnaing with oath group is found from the f0rmula:

U= Anw . F & p. aoV

U cqu+vdcnt vc&icR3. gust spocd

Ang nozmcJ accelorntion inorcment

W wing loading

-6- .

F gust alleviation factor*

PO air density at sea level (I.C.A.O.)

a0 slope of the lift curve at low spaed

V indicated airspeed

By graphical interpolation tha counts are raferred to gust speeds of &, IO, 15, 20 . . . . ft/sec and a gust distribution is obtained for each altitude band. The mlesgc flown in each bend is cokulated and gust spcdra are obtained in tams of the avcrago distance between gusts excscding different magnituaos.

* The gust is aasumcd to increase linearly toits maximumvalue in a

horizontal distsncc of 100 fcot. The slloviating factor is caloulatcd as a

function of the mess paramtar pg = '", where p is air density and E is gPaao

tho mean wing chord. ~owance is mad0 for the effect of wing aspect ratio on the rate of growth of lift. No allowance has been made for the effect of compressibility on the unsto&y lift function.

-.7-

TAB23 II

Distribution of moordinn intorvels according to 0.23~ Counts

y- Number of Acceleration . Inoremsnts Exaeeding 0.23g

Nun&x of intervels (IO.4 min) containing a given nhnbcp of aaocloration incrdnknts

t Altitude bend (feet)

i

T 5313 106

5: 29 II 7

II 11 5

5"

i

:

;

f

'; 1 3 I

-25m= 7500

y: 16 10 6

l

5"

:

2 1 0 2 2 2 2 0 0 0 0 0 I 1 0 0 0 1 0 I

o- 2500

21

: 5

: 1

:

: 2 0 1 0 2 2 1 0 0 2 I 1 1 0 0 0 0 0 I Cl

:

I

-

0 1

:

3887 70 33 16 14 7 5

2 : ; 2 I 0 I 2

:"

: 0 2 0 0 0 0 0 1 0 0 0 0 0

ii 1

Y

‘; 6

I

’ i

0 2 1 0 1 0 1 I 1 1 1

L

; 66

t 5664 1 4066 ~. 59

- 11 -

TABLF: III

Distribution of recording intcmals according to acccleratlon oounts

Nmbcr I of !

Nmbcr of intcrvels (IO.& min)! centm.niq a given number of accderaticn~incrcmnt~

slsts / greater than I 1 0.238 0.3363

0 1

:

i.

7 0

2 II 12

:; 15 16 17

:; 20 21 22

z 25 26 27 20 29

:i” 32 33

;z 39

s 42

I fQ-24 232 i 119 I

69

;: ‘8 I 21 24 i2

0.43B i 0.5253

11020 I 80 I 18 8 / 7 ;

?jO87

:I: 5 1

- 12 -

TABLEIV

su5narv of pust spcds encountered

Estimated number of times a given gust sped vms cxuxdcd

Vertical gust swcd ft/scc.E.kS. (+Up. -Down) : -7.5 ;! 7.5 ' IO i 15 I 20 ' 25 / 30 ' 35

Relative the sacnt at dl altitudes in different ronions

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54

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74

91

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----L- 53 i7ct ’ 159 4437 23 1 632 346 8 4161 I

I

Time In 10.4 mlrute unl?.s

- 14-

TABIZVII

comparison of 10 fwsec J?Jlst frcouQncv in different ncoaraphicsl retions

Flight sector Goographiod~ mood&

Flights region

I London-Rome Rom-Cxiro LordonJTripoli Romdripoli

1, Europa , 128,200 285 450

4 Cairo-Khsrtoum Khartoum-Entebbe I I :;

2.E t gz rica j 441,200 1 44.66 / 96 1

Tripoli-Ksno Ksno-Lagos Lagos-Acra

I 45 13 J .west 121 6

j 21,100 Afrioa

174 /

Entebbc-Nairobi ';

\ Dar-es-SslaamEntobbc &South small mileage reconkd I Nsirobi-Lwdcn 18 ' Africa in this region

t I

- 16 -

Y.~.W78.CP.33U.K3 - Printed ,n areat erita.m

I

R

.------- DIVISION INTO REGIONS

I

I

----‘-

FIG. I. MAP OF THE ROUTES FLOWN.

lid 1952

1953

TOTAL FLY\NG HOURS

2 001

FIG. 2. DISTRIBUTION OF RECORDING TIME IN THE YEAR.

l0.000~ I I I 1 I

I I I I I I

I I I I

IO

I

0.1 0 IO 20 30 40

VERTICAL GUST SPEED FT./SEC. E.A.S.

FIG.3. GUST SPEED FREQUENCIES AT DIFFERENT ALTITUDES.

TO I REDUCTION IN TURBUL FROM SIDEWAYS

0 5 IO I5 110 ALTITUDE IN 1000’S OF FEET ABOVE SEA LEVEL.

FIG. 4. VARIATION OF TURBULENCE WITH ALTITUDE IN TERMS OF

GUSTS EXCEEDING IO, IS AND 20 FT/SEC. E.A.S.

C.P.$lc$ 334

A.R.C. Technical Report

SO. Code No. 23-9010-34

C.P. No. 334