an approximate spin design criterion for monoplanes

8/8/2019 An Approximate Spin Design Criterion for Monoplanes

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-- .i TECHNICAL NOTESI NATIONAL ADVISORY COMMITTEE FOR AERONAUTICE- -- -.. --.-.

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- ---AN APPROXIMATE SPIN DESIGN CRITERIOB FOR MOKITPWLlUlS 1By Oscar Seidman and Charles J..DpnlanLangley Memorial Aeronautical Laboratory

'WashingtonJune 1939

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NATIONAL ADViSORY COGITTEE FOR Albi%NAUTI&TECHNICAL NOTE NO. 711

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AN APPROXIMATE SPIN DESIGN CRITERION FOR MONOPLANESBy Oscar Sefdman and Charles J. Donlan

SUMMARYA quantitative criterion of merit has been needed toassist airplane designers to incorporate satisfactoryspinning characteristics into new designs. An approximateempirical criterion, based on the projected side area andthe mass distribution of the airplane, has been formulatedin a recent British report. In the present paper, theBritish results have been analyzed and applied to America;designs. A simpler design criterion, based solely on thetype and the dimensions of the tail, has been developed;

it fsuseful in a rapid estimation of whether a new designis likely to comply with the minimum requfrements for safe-ty in spfnning.INTRODUOTION

A considerable amount of information uoncerning theeffects of dimensfonal and inertial design characterfsticsexists in the literature on spinning. In general, however,the data are so presented that they are not dfrectly andquantitatively applicable to new designs. There is needfor a satisfactory quantitative criterion to indicatewhether a new design is lfkely to comply with the minimumrequirements for safety in spinning.

Such a criterion is developed in a recent Britishpublication (reference 1). The present report is con-cerned with the application of the British criterion toAmerican airplanes.' An analysis of the results is pre-santed and a simplified criterion of spinning merit dev81-oped that, as far as American designs are concerned, con-forms better with full-scale and model spfnnfng data thanthe original English criterion.

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N.A.C.A. Technical'~Not'ei'No. 711

BASIS OY.ENGLISB CRITERIONi

The complete .development of the British criterion isgiven in detail in reference 1. The basic considerationsunderlyitng the development are reviewed briefly in thefollowing paragraphs.The spinning characteristics of an airplane are con-sidered to be affected by three major design factors.These factors are:

(1) The longitudinal distribution of mass as measuredby the difference IZ M Ix and expressed non-dimensionally'as Jz - IxPs(b/2>= ' where IZ and Ixare the moments of inertia about the Z and Xbody axes,. respectively; p is the density ofthe air; S; the wing area; and b/2-, the semi-span. The value of air density, p, used inthis report is that corresponding to 15,000 feetstandard altitude. h.

(2) The resistance offered by the fuselage sfde area(exclusive of the rudder) while the aIrplane fespinning, which Ls measured by CAx2, where Ais an elementary area located at a distance xfrom the center of gravity of the airplane. Be-cause of its greater effectiveness, the areabeneath the horizontal tail plane is multipliedby 2. (For conventional tail planes, this areais measured between the most forward and themost rearward portions of the tail plane.) Theresistance of the fuselage to rotation is exipressed in the form of a nondimensional "bodydamping ratio," defined as CAx2s(bfl' whero Sand b/2 denote the wing area and the semispan,respectively.

(3) The unshfolded rudder area, sxpressad nondiman-sionally as an "unshielded ruddor volume cooffi-*ient is equal to unshialded rudder area X 1-- ----xi72 > .where 1 is the distance from the controid ofthe unshielded rudder area to tho center ofgravity of the airplane,


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In the computation.:of th!e*.%o&+ *damping ratio (BDR)and the unshislded. rud-der- folW8 coefficient ..(URVG), ftwas assumed that the relative wfnd strikes the horizontaltail surfaces from below at an angle of 45' and that theair flow. diverges ?15O afterpa&ing t'he t&ii plane. (T-hisassumption_ regardfng _th.e $ipergence of-4he-air flow abovethe tafl plane' fs ve'r%fie&by"the' flight- tests Gdescrfb8&'in referenc.8 2, ) Any areB -of the -ve'rt'ical 'tail within thisdivergent Wake is disregarded in the computat'%ons.., Figure1 illustrates the method used in evaluatingVRVC. ' _ . :

. ?DR .=a _.A ' damping-power fadtor"p&duct ' (.D'PF) is defined as;theBDR x ,URVC., and this factor is plotted .agaQxs.t. Iz - .1x I .the p-itching parameter - 'Tho relative magnitudepS(b/2)'* 1: '

of the 'slope, I'npF) CpS(b/2)3l-: ;

----,Iz - Ix is,used by the Brftish asa figure of merit.

COMPARISON OF BRITISH AND AMERICAN RDSULTSFigure 2, which is take-n from reference 1, is a plot

of Iz - IxDPF against -- for th8 22 British monbplanepS-(b/2)3 1 >designs submitted for iesting fn the British.free-spinning 9wind tunnel. The models are rated ai either "pa.Eised'I of V *"failed, 'I depending on th8'ir ability to:'meet. the require-ments of,a standard British mod81 recovqry test. In mostinstances where an'initial 'design i's represented as up-9atiafactor.y. a point will b8 found'representing the finalmodified version of that design.' It is obvious from thedispersion of poknts'that secondary factors not includedin the analysis influence the abilfty of a model to gas8the spin test. Nefertheless, a line has besn drawn suchthat no pass Doint lies below it (although fatlures maylie above it) and deffnes the minimum' requirement forsafety in spinning. It is implied that any design the acharacterZst&c p.oi.n$ .o$,,.mh+,+h .l:I.e,s, benoath- thfs line (i.e. ,:for which. th8 rat..Lo'- F- - '.(DP-$1 ]lCi?S(b/':)-~=!;;; .is io'ss than ; Ool > ---1 L.:f4$ 'f. .Xx * -, I .. : .s.,


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4 N.A.C.A. Technical Note No. 7'11

whereas, if its ch.aracteristfc point lies above this line,recoveries may be either satisfactory or unsatisfactory., Figure 3 is a plot of DPF against Iz - Ix--P.m/2? for 14American monoplanes tested in the N.A.C.A. free-spinningwind tunnel. The N.A.C.A. having no unique criterion ofspinning merit, the designs considered here have been da-noted as being IIgoodl' or Ifpoorfl spinners, partly on the

basis of- spin-tunnel results and partly on the basis ofpilots! reports. The fact that the English standard of re-covery was not utilized in the American classification mayaccount for the relatively greater percentage of Amerfcanairplanes appearing as satisfactory spinners. It will benoted that two good points fall below the British line forminimum safety in sptnning. If, however, a line (dottedline on fig. 3) is drawn through the point for atrplane 1,a separation of the Amerfcan airplanes fs effectod;'thisline has about one-half the slope of theBritish line.

ANALYSIS OF RIESULTSA detailed 'analysis of both the'BritLsh and the Amer-ican results was then made with the purpose of obtaininga simpler and more effective criterion.

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The individual factors that constitute the BritishDPF are plotted 'in figures 4 and 5. The olose groupingof pofnt.s in flqure 4 discourages the establishment of aspin criterfon on 'the basis of the BDR alone. Figure 6,on the other' hand, shows a greater dispersion of points andthe dotted horizontal line drawn through a value of URVCOf 0.013 effects a ,separation of passed and failed pointsthat is comparable with the separation previously noted onthe DPF chart (fig; 2). 'This result suggests that theURIC alone mfght prove as. satisfactory a criterion as themore complex DPF, ' which necessitates the consideration -

iof .BDR andQ-IX'pS(b~* --

It would appear that an alternative conclusion to the cBritish report might have stated that any model possess-ing a value for URIC. of less than 0.013 would be unlike- -ly to pass the.. model spinning r.equiremonts. This condi- .tion would have eliminated the necessity of consideringthe. body damping ratio and the inertia pitching parameter.


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N.A.C.A. Technical Note No. 711 6This discussion concludes the analysis of the Britishdata. The rest of the report fs concerned with the analy-.s.is of the results for the American monoplanes and theformulation of a criterion based on the unshielded rudderarea and the fuselage area directly below the horizontaltail surfaces.Figures 6 and 7 are plots of the factors constitutingthe DPF for the American monoplanes. Figure 6 could beused to segregate the good from the poor spinners but, as

will be shown later, the segregation is largely attribut-able to the area beneath the horfzontal tail surfaces andnot.to the BDR as a whole. In figure 7, the dotted hor-izontal line drawn through the value of URVC of 0.01indicates that satisfactory separation of good and poorspinners can be obtained for the American airplanes by Iconsidering the JRVC al-e, although the lfne of separa-tion is lower than that.&ed. for the British models infigure 5. Thus the value URVC = 0.01 might be used.toseparate new designs into two classifications; designshaving a value of URVC less than 0.01 may be consideredunlikely to pass the spinning requirements.In reference I., the importance of the fixed area be-low the horizontal tail surfaoes has been recognized by b. .

:Ei%=F doubling its contribution to the body damping dluifilrgts fnfluence is obscured, however, because itscontribution may be small even when doubled as comparedwith the body damping ratio. In order further to empha-sfze its importance, the .contribution to the body dampingratio of the fixed area below the horizontal tail surfaceshas been considered separately for the American airplanes:it is expressed as a-tail damping ratio, TDR = --FLBwhere sta/aaF is the total fixed area'below the horizontaltail and L is the distance from the centroid of this area,to the center of gravity of the airplane.

Values 'of thein figure .8. TDR for American monoplanes are shownIt will be noted that a separation. of thegoodfrom the poor spinners can be effected by using the'value TDR = 0.015. Ffgures 7 and 8 show .that the URVC1 and th,e TDR taJoen.separately effect similar separationso'f the American designs 5nto two groups. It is obviousJ that many possible combinations of these two factors 'oould.w - be used in devising an empfrica l criterion to segregate thepoor spinners. In order to emphasfge the importance of II


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6 N.A.C.A. Technical Note No.. 711having both unshielded rudder area and fixed area belowthe horizontal surfaces, it das decided to use the prod-uct URVC X TDR as a criterion of merit. It is appreci-ated that the resul% may not be va23d for unconventionaldesigns.

Figure 9 illustrates the method used in evaluating atail damping-power factor (TDPF) defined 'as the productof TDR and URVC. This TDPF is plotted in figure 10for 14 American monoplanes and effects a satisfactory sep-aration of good and poor spinners.

It may be concluded on the basis of figure 10 thatmonoplanes poss8ssing a TDPF of less than, say, 0.00015are not likely to exhfbit satfsfactory recovery charac-teristics. On the other hand, it is felt that a TDPP inexcess of 0.00015 is, tn itself, insufficient to insuresatfsfactory spin characteristics,Similar-re-su lts were obtained for American biplanes,but the critical value of the TDPF appeared to be some;what lower and.1888 distinct than for the monoplanes.Lack of sufficient data prevented the calculation ofthe :TPDF, for the British designe.

DISCUSSION AND CONCLUSIONSIn the present. state of knowledge:-no criterion is:available that will infallIbly predictthe recovery char-acteri,stics of a new airplane design. As shown in thetext, -hopPever;,it. is possible t-0 formulate empirica l cri-terions that are helpful in establishing the minimum d8-sign requirements for safety in spinning. It is believedthat the tafl damping-power factor (TDPF) developed inthe text is a sfmple practical method for rapidly estimat-

ing whether a new design is likely, to comply with the min-imum requirements for safety 'in spinning and it fs recom-mended that no new monoplane design be constructed whichpossesses a TDPF of less than 0.00015. It should not beassumed, however, that a design which has a satisfactoryTDPF will necessarily exhibit good recovery charactorfs-tics, as other factors not her8fn.consider8d may influencethe results. . . i.Langley Memorial A8ronautica.l Laboratory; 'National Advisory Committee for Aeronautics,Langley Field, Va., May 1, 1939.


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I.A.O.A. Teomoel liote Ho. ?ll Figr. 1,Q

A&se of motionoentrold of8ection to 0.g.Of airplane Body Aroaundsr tUlplan0 Tin%dy dapl&dng rati0 = Alxl~+A~x~a+---Ala~a~ + 2cAlm3~+---Alex18a) + A17q7a+---Asma

.8, wing area 8(b/a) a

Unehlelded rudder +olums ooeffloient * %aded LrmDOroes ring area

To 0.g. of Urplane, zFigare l.- Yetbod of ooqmtetion of damping oosffiotents.

Tall dasplng- I Tall den@=power faotor Unnblelded rudderret10 x rolums ooeffialentTDPT = FL28(b/@ = Wl + RataB(b/a)

Figure 9.- Yetbod Of aoqnat1llg Pail darping-pomr flAotor.


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P.A.O.A. Teotmioal Iote IO. 711 Fig. a,&4

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OrigInal deniv Pssred temt

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Plgom a.- variation of dainplY -pomsr faotor dth inertia pitohing psrametar for 22Brltlmh momplaner bon refermoe 1).

a.513 - IxP Ebb)3PIgum 3.- Varlatlon of damping-powor factor rith Inertia pitching pazrcaeter for 14American monoplaner.

Ia.61% IxFigure C.- Variation of body damping ratio with lnmtis pltohlng parameter for alBrltlmh monoplanes (From reference 1).


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t. Taohnlcal rote IO. 711 rim.05 I15 I

I Origins1 deelpa 1o 8 Y% V Pamed test c,o -04 - A ?Uled teat 8T3 I

5 1.a I@9 I I I I I I0 t.5 1.0 1.6 a.0 a.5 3.012 - IXps(blaI5Figure 5.- Varistlon of unehislded rudder volum coefficient with inertia pitohlngparameter for aa Brltleh mcnoplanse (Worn reference 1).47-l-r l- 1 $4I I 1 I

I0I0 Good apinnerA Poor spinner

0 .5 1.0 1.5 2.0 a.5 3.012 - IxH Tlgure 8 p 8(b/i3)32 .- Yarlatlon of body dmplng ratio tith inertia pftobing parameter for 14Ammsriaah mnoalPndn.---,

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Pigus 7.- Varlatlon Of unehlelded rudder Wlnm cosfflalent with inertia pltohiagparameter for 14 herioan monoplahsr.


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H.A.O.A. TeoMoal Note HO. 7ll rig.. 8,lO

zs -1x .-p Sib/8)3Pigme 8.- Verlatloa of tail damplng ratio rith ine rtia pltohing prameter for14 Amerlosn monoplanm.

____,__. -_ -- __._d ---- A----.------- -.8.oool 8 10*A

I 2 I s1 $2 ,0 '5 1.0 _1.5- a.0 8.5 S.0I -Ip S(b/a) 3

Plgure lO.- V.srlatloa of tall ""p lng-power fhotor with laertla pitohiing prruetErfor 14 Amerloan mopop sums-

an approximate spin design criterion for monoplanes

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