An approximate empirical criterion, based on the projected side area and the mass distribution of the airplane, was formulated. The British results were analyzed and applied to American designs. A simpler design criterion, based solely on the type and the dimensions of the tail, was developed; it is useful in a rapid estimation of whether a new design is likely to comply with the minimum requirements for safety in spinning

Donlan, C. J.

Seidman, O.

English

NASA Technical Reports Server

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TECHNICAL NOTES 
I 
NATIONAL ADVISORY COMMITTEE FOR AERONAUTICE - -- -.. --. 
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AN APPROXIMATE SPIN DESIGN CRITERIOB FOR MOKITPWLlUlS 1 
By Oscar Seidman and Charles J..Dpnlan 
Langley Memorial Aeronautical Laboratory 
'Washington 
June 1939 
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NATIONAL ADViSORY COGITTEE FOR Albi%NAUTI& 
TECHNICAL NOTE NO. 711 
-m-M 
AN APPROXIMATE SPIN DESIGN CRITERION FOR MONOPLANES 
By Oscar Sefdman and Charles J. Donlan 
SUMMARY 
A quantitative criterion of merit has been needed to 
assist airplane designers to incorporate satisfactory 
spinning characteristics into new designs. An approximate 
empirical criterion, based on the projected side area and 
the mass distribution of the airplane, has been formulated 
in a recent British report. In the present paper, the 
British results have been analyzed and applied to America; 
designs. A simpler design criterion, based solely on the 
type and the dimensions of the tail, has been developed; 
it fsuseful in a rapid estimation of whether a new design 
is likely to comply with the minimum requfrements for safe- 
ty in spfnning. 
INTRODUOTION 
A considerable amount of information uoncerning the 
effects of dimensfonal and inertial design characterfstics 
exists in the literature on spinning. In general, however, 
the data are so presented that they are not dfrectly and 
quantitatively applicable to new designs. There is need 
for a satisfactory quantitative criterion to indicate 
whether a new design is lfkely to comply with the minimum 
requirements for safety in spinning. 
Such a criterion is developed in a recent British 
publication (reference 1). The present report is con- 
cerned with the application of the British criterion to 
American 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 than 
the original English criterion. 
.- 
. 
‘N.A.C.A. Technical'~Not'ei'No. 711 
BASIS OY.ENGLISB CRITERION 
i 
The complete .development of the British criterion is 
given in detail in reference 1. The basic considerations 
underlyitng the development are reviewed briefly in the 
following 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 measured 
by the difference IZ M Ix and expressed non- 
dimensionally'as Jz - Ix 
Ps(b/2>= ' 
where IZ and Ix 
are the moments of inertia about the Z and X 
body axes,. respectively; p is the density of 
the air; S; the wing area; and b/2-, the semi- 
span. The value of air density, p, used in 
this report is that corresponding to 15,000 feet 
standard altitude. h. 
(2) The resistance offered by the fuselage sfde area 
(exclusive of the rudder) while the aIrplane fe 
spinning, which Ls measured by CAx2, where A 
is an elementary area located at a distance x 
from the center of gravity of the airplane. Be- 
cause of its greater effectiveness, the area 
beneath the horizontal tail plane is multiplied 
by 2. (For conventional tail planes, this area 
is measured between the most forward and the 
most rearward portions of the tail plane.) The 
resistance of the fuselage to rotation is exi 
pressed in the form of a nondimensional "body 
damping ratio," defined as CAx2 
s(bfl' 
whero S 
and 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 of 
the unshielded rudder area to tho center of 
gravity of the airplane, 
r 
N.A.B.A. T'e&$i.~a~:;N.ot'e Na; ..Zl.l 3 
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In the computation.:of th!e*.%o&+ *damping ratio (BDR) 
and the unshislded. rud-der- folW8 coefficient ..(URVG), ft 
was assumed that the relative wfnd strikes the horizontal 
tail surfaces from below at an angle of 45' and that the 
air flow. diverges ?15O afterpa&ing t'he t&ii plane. (T-his 
assumption_ regardfng _th.e $ipergence of-4he-air flow above 
the tafl plane' fs ve'r%fie&by"the' flight- tests Gdescrfb8&' 
in referenc.8 2, ) Any areB -of the -ve'rt'ical 'tail within this 
divergent Wake is disregarded in the computat'%ons.., Figure 
1 illustrates the method used in evaluating 
VRVC. ' _ . : 
. ?DR .=a _ 
. 
A ' damping-power fadtor" 
p&duct ' 
(.D'PF) is defined as;the 
BDR x ,URVC., and this factor is plotted .agaQxs.t . . 
Iz - .1x 
I . 
the p-itching parameter - 'Tho relative magnitude 
pS(b/2)'* 1: ' 
of the 'slope, 
I'npF) CpS(b/2)3l 
-: ; 
----, Iz - Ix is,used by the Brftish as 
a figure of merit. 
COMPARISON OF BRITISH AND AMERICAN RDSULTS 
Figure 2, which is take-n from reference 1, is a plot 
of Iz - Ix DPF against -- for th8 22 British monbplane 
pS-(b/2)3 1 > 
designs submitted for iesting fn the British.free-spinning 9 
wind tunnel. The models are rated ai either "pa.Eised'I of V * 
"failed, 'I depending on th8'ir ability to:'meet. the require- 
ments of,a standard British mod81 recovqry test. In most 
instances where an'initial 'design i's represented as up- 
9atiafactor.y. a point will b8 found'representing the final 
modified version of that design.' It is obvious from the 
dispersion of poknts'that secondary factors not included 
in the analysis influence the abilfty of a model to gas8 
the spin test. Nefertheless, a line has besn drawn such 
that no pass Doint lies below it (although fatlures may 
lie above it) and deffnes the minimum' requirement for 
safety in spinning. It is implied that any design the a 
characterZst&c p.oi.n$ .o$,,.mh+,+h .l:I.e,s, benoath- thfs line (i.e. ,: 
for which. th8 rat..Lo'- F ,- - '.(DP-$1 ]lCi?S(b/':)-~=!;;; .is io'ss than ; Ool > --- 
1 L.:f 4$ 'f. .Xx * -, I .. ': 
. s., 
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 14 
American monoplanes tested in the N.A.C.A. free-spinning 
wind tunnel. The N.A.C.A. having no unique criterion of 
spinning 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 of 
pilots! reports. The fact that the English standard of re- 
covery was not utilized in the American classification may 
account for the relatively greater percentage of Amerfcan 
airplanes appearing as satisfactory spinners. It will be 
noted that two good points fall below the British line for 
minimum safety in sptnning. If, however, a line (dotted 
line on fig. 3) is drawn through the point for atrplane 1, 
a separation of the Amerfcan airplanes fs effectod;'this 
line has about one-half the slope of theBritish line. 
ANALYSIS OF RIESULTS 
A detailed 'analysis of both the'BritLsh and the Amer- 
ican results was then made with the purpose of obtaining 
a simpler and more effective criterion. 
* 
-L 
. 
The individual factors that constitute the British 
DPF are plotted 'in figures 4 and 5. The olose grouping 
of pofnt.s in flqure 4 discourages the establishment of a 
spin criterfon on 'the basis of the BDR alone. Figure 6, 
on the other' hand, shows a greater dispersion of points and 
the dotted horizontal line drawn through a value of URVC 
Of 0.013 effects a ,separation of passed and failed points 
that is comparable with the separation previously noted on 
the DPF chart (fig; 2). 'This result suggests that the 
URIC alone mfght prove as. satisfactory a criterion as the 
more complex DPF, ' which necessitates the consideration - 
iof . 
BDR and Q-IX' 
pS(b~* -- 
It would appear that an alternative conclusion to the c 
British 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 considering 
the. body damping ratio and the inertia pitching parameter. 
N.A.C.A. Technical Note No. 711 6 
This discussion concludes the analysis of the British 
data. The rest of the report fs concerned with the analy- 
.s.is of the results for the American monoplanes and the 
formulation of a criterion based on the unshielded rudder 
area and the fuselage area directly below the horizontal 
tail surfaces. 
Figures 6 and 7 are plots of the factors constituting 
the DPF for the American monoplanes. Figure 6 could be 
used 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 and 
not.to the BDR as a whole. In figure 7, the dotted hor- 
izontal line drawn through the value of URVC of 0.01 
indicates that satisfactory separation of good and poor 
spinners can be obtained for the American airplanes by I 
considering the JRVC al-e, although the lfne of separa- 
tion is lower than that.&ed. for the British models in 
figure 5. Thus the value URVC = 0.01 might be used.to 
separate new designs into two classifications; designs 
having a value of URVC less than 0.01 may be considered 
unlikely 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 dluifilrg 
ts fnfluence is obscured, however, because its 
contribution may be small even when doubled as compared 
with the body damping ratio. In order further to empha- 
sfze its importance, the .contribution to the body damping 
ratio of the fixed area below the horizontal tail surfaces 
has been considered separately for the American airplanes: 
it is expressed as a-tail damping ratio, TDR = --FLB 
where 
sta/aa 
F is the total fixed area'below the horizontal 
tail and L is the distance from the centroid of this area 
,to the center of gravity of the airplane. 
Values 'of the 
in figure .8. 
TDR for American monoplanes are shown 
It will be noted that a separation. of the 
goodfrom the poor spinners can be effected by using the' 
value TDR = 0.015. Ffgures 7 and 8 show .that the URVC 
1 
and th,e TDR taJoen.separately effect similar separations 
o'f the American designs 5nto two groups. It is obvious 
J that many possible combinations of these two factors 'oould 
.w -- be used in devising an empfrical criterion to segregate the 
poor spinners. In order to emphasfge the importance of II 
6 N.A.C.A. Technical Note No.. 711 
having both unshielded rudder area and fixed area below 
the 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 unconventional 
designs. 
Figure 9 illustrates the method used in evaluating a 
tail damping-power factor (TDPF) defined 'as the product 
of TDR and URVC. This TDPF is plotted in figure 10 
for 14 American monoplanes and effects a satisfactory sep- 
aration of good and poor spinners. 
It may be concluded on the basis of figure 10 that 
monoplanes poss8ssing a TDPF of less than, say, 0.00015 
are not likely to exhfbit satfsfactory recovery charac- 
teristics. On the other hand, it is felt that a TDPP in 
excess of 0.00015 is, tn itself, insufficient to insure 
satfsfactory spin characteristics, 
Similar-re-sults 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 of 
the :TPDF, for the British designe. 
DISCUSSION AND CONCLUSIONS 
In 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 the 
text, -hopPever;,it. is possible t-0 formulate empirical cri- 
terions that are helpful in establishing the minimum d8- 
sign requirements for safety in spinning. It is believed 
that the tafl damping-power factor (TDPF) developed in 
the 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 which 
possesses a TDPF of less than 0.00015. It should not be 
assumed, however, that a design which has a satisfactory 
TDPF will necessarily exhibit good recovery charactorfs- 
tics, as other factors not her8fn.consider8d may influence 
the results. . . i 
. 
Langley Memorial A8ronautica.l Laboratory; ' 
National Advisory Committee for Aeronautics, 
Langley Field, Va., May 1, 1939. 
N.A.C.A. Technical Note No. 711 7 
REFERENCES 
1. Finn, E.: Analysis of Routine Tests of Monoplanes 
in the Royal Aircraft Establishment Free Spinning 
Tunnel. R. & M. No. 1810, British A.R.C., 1937. 
2. Scudder, N. F., and Miller, ti. Pa: The Nature of Air 
Flow about the Tail of an Airplane in a Spin. 
T.N. No. 421, N.A.C.A., 1932. 
. 
. . 
I.A.O.A. Teomoel liote Ho. ?ll Figr. 1,Q 
A&se of motion 
oentrold of 
8ection 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 LrmD Oroes ring area 
To 0.g. of Urplane, z 
Figare l.- Yetbod of ooqmtetion of damping oosffiotents. 
Tall dasplng- I Tall den@= 
power faotor 
Unnblelded rudder 
ret10 x rolums ooeffialent 
TDPT = 
FL2 
8(b/@ = 
Wl + Rata 
B(b/a) 
Figure 9.- Yetbod Of aoqnat1llg Pail darping-pomr flAotor. 
P.A.O.A. Teotmioal Iote IO. 711 Fig. a,&4 
*.. 
* 
OrigInal deni 
v Pssred temt 
- - 
i.0 1.5 a.6 a.5 3.0 
IL-IX 
P &da) 3 
Plgom a.- variation of dainpl 
Y 
-pomsr faotor dth inertia pitohing psrametar for 22 
Brltlmh momplaner bon refermoe 1). 
a.5 
13 - Ix 
P Ebb)3 
PIgum 3.- Varlatlon of damping-powor factor rith Inertia pitching pazrcaeter for 14 
American monoplaner. 
I 
a.6 
1% - Ix 
Figure C.- Variation of body damping ratio with lnmtis pltohlng parameter for al 
Brltlmh monoplanes (From reference 1). 
11.A.O.1 
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t. Taohnlcal rote IO. 711 rim 
.05 I 
15 I 
I 
Origins1 deelpa 1o 8 Y 
% V Pamed test c 
,o -04 - A ?Uled teat 8 T3 
I 
5 1.a I@9 I I I I I I 
0 t .5 1.0 1.6 a.0 a.5 3.0 
12 - IX 
ps(blaI5 
Figure 5.- Varistlon of unehislded rudder volum coefficient with inertia pitohlng 
parameter for aa Brltleh mcnoplanse (Worn reference 1). 
47-l-r l- 1 $4 I I 1 I 
I 
0 I 0 Good apinner 
A Poor spinner 
0 .5 1.0 1.5 2.0 a.5 3.0 
12 - Ix 
H Tlgure 8 
p 8(b/i3)3 
2 
.- Yarlatlon of body dmplng ratio tith inertia pftobing parameter for 14 
Ammsriaah ImnoalPndn. ----, 
,1.5 
%-Ix 
p8(b/213 
. 5,6,7 
. 
Pigus 7.- Varlatlon Of unehlelded rudder Wlnm cosfflalent with inertia pltohiag 
parameter for 14 herioan monoplahsr. 
H.A.O.A. TeoMoal Note HO. 7ll rig.. 8,lO 
zs -1x .- 
p Sib/8)3 
Pigme 8.- Verlatloa of tail damplng ratio rith inertia pltohing prameter for 
14 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.0 
I -I 
p S(b/a) 3 
Plgure lO.- V.srlatloa of tall 
""p 
lng-power fhotor with laertla pitohiing prruetEr 
for 14 Amerloan mopop sums- 


An approximate spin design criterion for monoplanes, 1 May 1939

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