Application of spring tabs to elevator controls

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Title:
Application of spring tabs to elevator controls
Alternate Title:
NACA wartime reports
Physical Description:
23, 7 p. : ill. ; 28 cm.
Language:
English
Creator:
Phillips, William H
Langley Aeronautical Laboratory
United States -- National Advisory Committee for Aeronautics
Publisher:
Langley Memorial Aeronautical Laboratory
Place of Publication:
Langley Field, VA
Publication Date:

Subjects

Subjects / Keywords:
Flutter (Aerodynamics)   ( lcsh )
Elevators (Airplanes)   ( lcsh )
Aerodynamics -- Research   ( lcsh )
Genre:
federal government publication   ( marcgt )
bibliography   ( marcgt )
technical report   ( marcgt )
non-fiction   ( marcgt )

Notes

Summary:
Summary: Equations are presented for calculating the stick-force characteristics obtained with a spring-tab type of elevator control. The main problems encountered in the design of a satisfactory elevator spring tab are to provide stick forces in the desired range, to maintain the force per g sufficiently constant throughout the speed range, to avoid undesirable "feel" of the control in ground handling, and to prevent flutter. Examples are presented to show the design features of spring tabs required to solve these problems for airplanes of various sizes. It appears possible to provide satisfactory elevator control-force characteristics over a large center-of-gravity range on airplanes weighing from about 16,000 to 300,000 pounds. On airplanes weighing less than 16,000 pounds, some difficulty may be encountered in obtaining sufficiently heavy stick forces for rapid movements of the control stick. Some special tab designs, including geared and preloaded spring tabs, are discussed. The geared spring tab is shown to offer a means of obtaining satisfactory ground control without introducing excessive variation of force per g with speed. By the use of spring tabs on elevators, the control forces may be made more closely predictable and the variation of stick-force characteristics among different airplanes of the same type may be greatly reduced. One of the principal objections to the use of spring tabs is the amount of weight required for mass balance to prevent flutter.
Bibliography:
Includes bibliographic references (p. 22).
Statement of Responsibility:
William H. Phillips.
General Note:
"Report no. L-122."
General Note:
"Originally issued October 1944 as Advance Restricted Report L4H28."
General Note:
"Report date October 1944."
General Note:
"NACA WARTIME REPORTS are reprints of papers originally issued to provide rapid distribution of advance research results to an authorized group requiring them for the war effort. They were previously held under a security status but are now unclassified. Some of these reports were not technically edited. All have been reproduced without change in order to expedite general distribution."

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University of Florida
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All applicable rights reserved by the source institution and holding location.
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aleph - 003613162
oclc - 71203365
sobekcm - AA00006299_00001
System ID:
AA00006299:00001

Full Text
iAc /1


NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS





WARTI IN ME RE PORT
ORIGINALLY ISSUED
October 1944 as
Advance Restricted Report I.H28

APPLICATION OF SPRING TABS TO ELEATOR COWEROIS
By Willia H. Phillips

Langley Memorial Aeronautical Laboratory
Laegley Field, Va.


WASHINGTON


NACA WARTIME REPORTS are reprints of papers originally issued to provide rapid distribution of
advance research results to an authorized group requiring them for the war effort. They were pre-
viously held under a security status but are now unclassified. Some of these reports were not tech-
nically edited. All have been reproduced without change in order to expedite general distribution.


L 122


DOCUMENTS DEPARTMENT


V


tii
.I;r
I i


ARR No. L4H28






































Digitized by the Internet Archive
in 2011 with funding from
University of Florida, George A. Smathers Libraries with support from LYRASIS and the Sloan Foundation


http://www.archive.org/details/applicationofspr001ang







MaCA AhR N-. TLHS 3":'

ITATIONAL ADVISORY COMMITTEE ?ORE rCE'ICHAUTICS


ADVANCE RESTRICTED REPORT

APPL1CATIT'?! ?'T? SPRIIN TABS TO ELEV.uTOR CONTROLS

By W-illiam H. Pniilips


S.W ARY


Equations are presented for calculating the stick-
force characteristics obtained with a soring-tab tyne of
elevator control. The main orobler.nm encountered in the
design of a satisfactory elevator scoring tab are to
provide stick forces in the desired range, to maintain
the force per g sufficiently c.rnstarnt throughout the
soeed range, to avoid undesirable "fecl" of the control
in ground handling, and tD prevent flutter. Zxmrr.ples
are presented t. show the design features of spring tabs
required to sol:.e thee n-oblems for airplanes of various
sizes. It -oo,:ars oocsi'lc to provide satisfactory
elevator contrjl-f.ni-'e characters sticks over a large
centtr-of-gravity range on airolanes weighing from about
16,000 to j00,OOC pounds. On airplanes cJeihing less
than 16,000 rio)nds, some difficulty may be encountered
in obtaining sufficiently heavy stick forcts for rapid
mrovem-ents of the control stic':.

Some special tab designs, including geared and
preloaded scoring tabs, are discussed. The geared spring
tab is shown to offer a means of obtaining satisfactory
ground control without introducing excessive variation
of force per g with seed.

By the use of spring tabs on elevators, the control
forces may be made more closely predictable and the
variation of 3tick-force characteristics among different
airplanes of the same type may 1-e greatly reduced. One
of the orincioal objections to the use of spring tabs is
the amount of weight required for mass balance to prevent
flatter.


;1TFPD 'DUCTION


Difficulties hdve been encountered in obtaining
desirable control-force characteristics on large or








NTACk ARR No. LLH28


high-speed airplanes, because the hinge moments on the
control surfaces must be very closely balanced and
because slight changes in the hinge-moment parameters
result in large changes in control forces. The
advantages of spring tabs in overcoming these difficulties
have been pointed out in reference 1 and other reports.
It has been recognized, however, that the use of a spring
tab on an elevator results in a decreasing value of the
stick force per g normal acceleration with increasing
speed that might be considered undesirable. An analysis
is presented herein of the effects of spring tabs on
elevator forces for airplanes of various sizes. The
results indicate that an elevator cluiooed with a
suitably designed spring tab may avoid any serious
disadvantage from this effect and may still obtain the
advantage of having the control forces predictable and
relatively insensitive to variations in the elevator
hinge-moment characteristics.


S'r3 30LS


W weight

b span

S wing area

c chord

f tail length

ST tail area

/d C
\ d/slope of lift curve of wing
da/w

E downwash .ngle

q dynarmic pressure

qT dynamic pressure at tail

T elevator effectiveness factor (6 C--
\6CLT/6aq/








i!CA A :;L, lio. L!1120


CL lift co -fficient

Vs stalling speed

I elevator moment of inertia

K ratio cf stick m,.' c. ..lt to elevator defl action,
Scab fixed; norl-ial.l; positive

K2 ratio of stic: :..ovrncnt to tab reflection,
Selc.vator fixcj.; normally negative

': ratio of stick force to tib angle at zero
'3 airspeed, elevator fi.;c:d; normally positive

H hinge moment
/ \
C, hinr -moc nt coc-ffic-J.znt q--2)
ch 1


68 cl.vateor a' dfl'ecction (positive -'. town)

6t tab ir- .Icet ion (pos.ti -, dco''ini)

x stick movcn n:t (casitive fo..'ard)

stick force ( ull .f'orct pe osi Aive)

a a.i.-l' of att~c!. of 'ii

a, anle of attack: of tail

p mass density of air
n normal acceleration in g units

g acccl.ration of Grvaviby (32.2 ft/sec2)
x distance between center of gravity and stick-
fi:xe- nuutral point in straight flight
(..ositiv when ce'ntcr of Lravity is ruarward)


C da- variation of elevator hin~E.-moment coefficient
( thef with anCle of attack of tail, measured with
tab free








NACA ARR No. LLH23


/dChe)
---d variation of elevator hinge-moment coefficient
\d6e/tf with elevator angle, measured with tab free

d distance between tab mass-balance weight and
tab hinge line

f distance between elevator hinge line and tab
hinge line




( 2
\da;/




ILT qT 1
ST?"
66e q
Subscripts

T tail

t tab

e elevator


EQUATIONSS ?F, ELEVATOR FORCES


The merthod of deriving the equations for the
elevator control force in maneuvers with a spring tab
will be briefly outlined. These equations are similar
to equations given in reference 2 but have been arranged
to give a clearer pn-'sical significance to the various
terms.

The change in elevator hbri -e momennt caused by any
change in angle of attack, elevator angle, or tab angle
is given by the following formula:

He 6 Che, Che c2 (1)6
He = ( T + Ae + A8t qT+eCe2 (1)
6se 6








iACA ARR No. L!'28 5


The c ;Ire sending change in tat hinge miorent is given by
the expression

t = + ^t )tt btt- (2)


The change in elevator angle end the corresoonding change
in angle of attack : at the tail both of v.hich enter into
the calculation of the change in elevator h.xnge ,.ment -
may be derived for any type of' maTeuver. The change in
tab angle reu'iured to insert in equation (i cdeonds on
the particular linl..ag. arrangement under c-n;"ideratiol.
The cre6ent dilcu",ion will c:nsiJer tne s-ri:ng-tsb
arrangesLent shown 'n figure 1. Fer th in arrangeii-nt, the
rriati-.n bet&,en .he si>?ck Pfrce, tre cleatDr h.inge
rrm.omnt, and L.e 1L hinle n.or,.cnt, wih'.n ti.. syctew1 is in
equ'ilibriun, is '.:cn o, the. formla

A^c
A' -

> (5)
v-t 4 L ?>- Z


in 'h.-ich the constants Jl anc K2 ar'; the gearing
ratios between the stick and elevator a d between the
stick and tab, respectively, defincd by the formula

Xs = Kl5e + :26t (4)

and the constant KY is the stiffness of the spring.
This sDring constant for an unpreloajded spring tab is
defined in t-rrms of tro stick force required at zer,
airspeed to deflect the tan with the elevator fixed; thus,

? = :5&t (5)

By simultaneous solutions of equations (1), (2), and (5),
the stick force required in any maneuver for an elevator
equipped with an unnreloaded spring tab !,&ay be derived.
The elevator force required to produce a given change in
acceleration in gradual pull-ups is used as a criterion
of the elevator control characteristics. In a pull-up,








NACA ARR No. L4H28


the change in angle of attack at the tail is given by
the formula



aT- dCr ) (6)



and the change in elevator angle required is given by
the formula


P,
Wx 1 (n 1) (7)

L e

In order to show the relation between the elevator
forces required with a scoring tab and the forces obtained
with a conventional elevator, the equations for the force
per g in a pull-up are derived first for an elevator
without a tab, then for an elevator with a servotab, and
finally for an elevator with a spring tab. In the case
of a conventional elevator, the change in elevator hinge
moment may be derived from equation (1). By use of the
values for auT and 66e obtained from equations (6)
and (7) and by setting 66t = 0, the force per g normal
acceleration is found to be


6 =_. Jr1jhe + h c be1 2 (8)
B- beCe (8)
6n K1 6OT 66e ]q

v'here


A, crP



(9)
B -= ": 1 P
BCLT q ''Zr
6e ST
86e 1







NACA ARR No. ~l{2S 7


The second case considered is that of a servctab,
v.hich is defined as the system shwn in figure 1 with
the spring oritted. In tills case, the stick force in a
pull-up may be otta'ned from equations (1), (2), and (5)
by setting the spring constant K equal to zero. The
relation obtaine. f'ir the force per ; is

.(f + _Ch cS,
8Oe. -- (I0 )
Pn } e


I it -
1 -- '-t
h bt

This equation d'ffsrs from that for the force without
a tab in tv.o ways. The first liffe_'er.ce is that the
terms ?Che6 T an' './,5 are rtola;tcd jo the
corrc-spo3nding values vhich wo'i.l be r, s3uled on the
elevator with the tab fr,-, i.'!l-: 1 o;- for the tab-
frec condition are Liven by the exo..c- si ns









\ 'Cht '-Che
dChe)t AChe f t
fde ,t 8Te ~ Cht


Tf the tab does not have any floating tendencies,
the values obtained with equations (11) are the same as
those obtained for the elevator with the tab fixed. The
second difference is that in the denominator a term Is
added which depends upon the ratio of the elevator
dimensions to the tab dimensions, the ratio of the
effectiveness of the tab to its aerodynamic hingt moment,
and the ratio between the tab and elevator gearing
constants. This added term, which in practical designs
may range in value from five to several hundred, effectively


i /








8 NACA ARR No. L)H23

divides the elevator stick force that would be obtained
without a tab by a large factor. The force per g for a
servotab, like that for the elevator without a tab, is
essentially independent of spe-d.

The force per g for an elevator equipped with an
unpreloaded spring tab is found to be

S, + Che e 1Che
+1 Ce\ T Che be qT 2
S+ + 8 + b




K2 ;70T bece K2T"3
1 + (12)
ACht Ch
K- 6btct2 Ft qTbtct


Three terms are added when the tab-soring constant is
taken into account. All three terms are seen to be of
the same form and contain the dynamic pressure qT in
the denominator. At very low speeds, therefore, these
three terms will be very large compared with any other
terms in equation (12) and, in this case, the equation
reduces to the form of equation (8), the force per g of
the elevator without a spring tab. At very high speeds,
the three adced terms in equation (12) approach zero and
the equation for force per g reduces to that derived for a
s.rvotab (equation (10)). The actual variation of force
per g with need for various values of the spring
constant K3 is shown for a typical spring-tab instal-
lation in figure 2.


DESIGN P)fL hVS


The main problems that arise in connection with the
design of a spring tab for an elevator are as follows:

(a) To provide stick forces in the desired range

(b) To iuaintain force per g sufficiently constant
through the speed range







NACA ARR No. LH28


(c) To avoid undesirable "feel" of control for
ground handling

(d) To prevent flutter

These four conditions will be shown to restrict the
design characteristics of a satisfactory elevator spring
tab to a rathi:r narr-w range for any particular type of
airplane.

Some additional discussion may be necessary to
clarify points (b) and (c). The force oer g obtained
with a servotab has been shown not to vary with speed.
A servotab has been found to be undesirable, however, because
the elevator does not fellow movements of the stick
smoothly when the airplane is on the ground or taxying
at law seed. The use of a spring tab provides a
mechanical connection between the stick and the elevator
and relieves this difficulty. One of the main problems
in connection with the design of a spring tab is to avoid
an undesirably large variation of force oer g with speed
in flight and still to provide a sufficiently rigid
connection between the stick and the elevator to give
control while the airplane is taxing. The variation of
force per g with speed in flight may be reduced to a
small value by using a spring sufficiently weak that, in
the normal-flight speed range, the control behaves
essentially as a servotab. It is necessary to decide
upon some criterion for the minimum. value of spring
stiffness required for control while the airplane is
taxying.

The resoonse of the elevator to a sudden stick
movement depends upon the elevator hinge moment that
results from a unit stick deflection. If the elevator
is held fixed, the variation of elevator hinge moment
with stick deflection for an elevator equipped with a
spring tab is given by the following equation:



"C he h 2
'He -KI K+ ?t qTb ce2 1 .t qTbtct
+ -- (15)
e.xs 2 .2 K2

At zero speed the elevator hinge moment comes entirely
from the spring but, as the speed increases, the







NACA ARR No. LL.H22


aerodynamic hinge moment due to tab deflection is added.
The initial ∠.lar acceleration of the elevator, which
occurs after a sudden stick movement, depends on the ratio
of elevator hinge moment to stick deflection divided by
the moment of inertia of the elevator about its hinge
line. In flight tests of a small fighter airplane, the
minimum value of spring stiffness required for satisfactory
feel of the controls on the ground corresponded to the
value (at zero airspeed)


1 =1 = = 200 foot--ounds per foot per slug-foot2
I ox3 K21

This value is, of course, many times smaller than the
degree of rigidity present in a conventional control
system but has nevertheless been shown to be satisfactory
for the case of the small fighter airplane. For a large
airplane, particularly one equi-oed with a tricycle
landing gear, elevator control should not be required
until speeds approaching the take-off speed are reached.
In such a case, then, a lower value of the ratio might

be acceptable at zero airspeed. The value of 1 6He
I Oxs
should, however, be reasonably large at speeds apnroaching
the take-off speed.


EXMt:PLES


Design consitderations.- In order to illustrate the
applic~tii n of sp1.ing tats to elevator controls of
airplanes of various sizes, the stick-force charac-
teristics in maneuvers have been calculated for four
airplanes ranging in size frmr a scout bomber to an
airplane weighing 500,000 pounds, which represents about
the largest type of airplane now bein; contemplated by
aircraft designers. In each case, a practical spring-
tab design has been srr'ved at that orjvides stick-force
characteristics which satisfy the requirements of
reference 3. These exar.nles show what design features
of a spring tab are required to obtain stick forces for
maneuvering within the range required for each class of
airplane and indicate also special problems that may
arise in the design of sprin- tabs for aircraft of







NACA ARR rlo. LH'28


particular sizes. The characteristics of the airplanes
chosen Vs examples are given in table I. Certain factors
that were considered in designing the spring tabs are as
follows-

(a) The spring stiffness has been selected on
the basis of providing satisfactory ground control
1 6He
by making the value of at zero airspeed
I Oz-;
: ;ual to or greater than 200 foot-pounds per foot
per slug-foot2 except where otherwise noted.

(b) A reasonable degree of aerodynamic balance
of the elevator, ccrresponding to a value of
be
= -0.002 or -0.005, has ben assumed so that

large elevator deflections .i:; be obtained without
having the tab size or deflection exceed practical
oChe
linlts. The value of = 0, which has been

used in all calculations, may be attained in
nractine by suitable choice nf tihe elevator contours.
he
Variation in the value of -- ill not, however,

alter the effects of the sorin'- 'ab but will simply
shift the stick-free neutral points in straight and
turning flight by the seie around for s spring tab
as for a conventional type of balance.

tc) The tab hinge-n.cment characteristics were
B -ht
assigned the representative values = -0.00G
GCht ?Cht cbt
or -0.005, = 0, and = 0. By suitable
S- e aT
modification of the tab design, considerable variation
in these values may be obtained. The effects of
such changes on tl-e stick forces may be determined
from formulas (11) and (12).
Scout bomber (weight, 16, 000 lb).- The variation of
forces per C with speed and with centur-of-gravity position
for a scout bomber weighing 16,000 pounds is shown in
figure 5. The desirable range of stick forces (shown by
cross hatching in figures) is indicated in accordance







NACA ARR No. L4H28


with the requirements of reference 3. A center-of-
gravity range of 10 percent of the mean aerodynamic
chord has been assum~ed.

The hypothetical curve of force per g at zero speed,
which also represents the force oer g throughout the
speed range when a spring tab is not used, shows that a
conventional elevator with the degree of balance used
would `lve heavy stick forces and an excessive variation
of force per g with center-of-gravity position. The
assumed scoring tab reduces the variation of force per g
with center-of-gravity position to an acceptable amount.
The variation of force ner g with speed, for the spring
stiffness chosen to give satisfactory ground control,
also appears to be desirably small. Somewhat larger
values of force oer g are obtained near the minimum
s,'oed, but this fact is thought to be unimportant because
the airplane stalls at low values of normal acceleration
in this speed range. The stick forces were generally
too low with a scoring tab alone but have been raised to
an acceptable value by the use of a small bobweight that
requires a pull force of about three pounds on the stick.

Although the combination of spring tab and bobweight
gives stick forces that satisfy the requirements, recent
flight tests have shown that such an arrangement might
be -considered unsatisfactory to the pilots because of
the lightness of the stick force required to make large
raid movements of the stick. This lightness, of course,
results fr.i the small effective value of 6Che/66e,
which is necessary in order to obtain a small variation
of force per g with center-of-gravity position. The
rei.uiJrement for llht stick forces over such a large
center-of-gravity range on an air-lane of this tnpe
seems, in fact, to be incor-o3tible with the pilot's desire
for forces large enough to prevent inadvertent movements
of the control stick.

The problem of nrroviding sufficient heaviness of
the control stick for quick movements (with the resultant
undesirable variation of force per g with center-of-
gravity position) when a scoring tab is used may present
some difficulties on an airplane as small as a scout
bomber. The following possibilities are available for
r.'ikng the forces heavier:







NACA ARR No. LL12S


(a) To decrease V2, the mechanical advantage
of the stick ovcr tne tao

(b) To increase the tab chord

(c) To increase GCht/Ait by use of strips on the
tab trailing edge

(d) To reduce the amount of aerodynamic balance on
the elevator

Of these possibilities, (a) and (b) may excessively
increase the amount of mass balance required to prevent
flutter, a subject that will be discussed in a later
section of Lhe paper. Only a limited advantage is
gained by method (c). Vietilod (d) will req-iire the use
of a larger tab to obtain large elevator deflections.
By a combination of these methods, however, it appears
practicable to obtain a sufAi'cieittly large centering
tendency of the stick on an airplane of the scout-bomber

class. For a given value uf i C-e at zero airspeed,
I OXs
changes (a), (b), and (c) give a favorable reduction in
the variation of force per g with speed.

Satisfactory control feel night possibly be provided,
even on an airplane that has no variation of force per g
with center-of-gravity position, by suitable inertia
weights or damoing devices in the control system. Several
systems for accomplishing this result have been proposed,
but none has yet been tested in flight.

Medium bomber (weight, 50,000 lb).- The stick-
force characteristics of a medium bomber weighing
50,000 pounds with the assumed spring-tab design are
shown in figure L. The spring stiffness, chosen on the
basis of ground control, provides a sufficiently small
variation of force per g with speed. The stick forces
lie within the desired limits. It is believed that the
centering tendency of the control stick associated vith
these forces would be considered sufficiently large,
although no tests have been made of an airplane of this
size to verify this belief.

Heavy bomber (weight, 125,000 lb).- The calculated
stick-force characteristics of a heavy bomber (weight,
125,000 lb) are shown in figure 5. In order to obtain







N1CA ARR No. 14H28


stick forces within the desired range, a tab of rather
narrow chord and an increased value of K2 (the
mechanical advantage of the stick over the tab) have to
be used. When these measures are adopted, it is no
longer possible to meet the criterion for ground control

S e at zero seed = 200 foot-pounds per foot per
s 2xs
slug-foot2) and still maintain a sufficiently small
variation of force per g with speed. Although the spring
stiffness required to obtain the characteristics shown
in figure 5 is greater than the stiffness used on the

smaller airplanes, the value of 1 iHe at zero sped is
I 6xs
considerably reduced but reaches a value of 200 at a speed
of 80 miles per hour. This condition would probably be
acceptable, however, on a large airplane with a tricycle
landing gear.

Airplane of 300,000 pounds weight.- The calculated
stick-force ciharactcristics of an Airplane weighing
approximately 300,000 pounds are shown in figure 6. On
an airplane of this size, considerable care must be
taken to balance aerojcinanically both the elevator and
the tab in order to obtain sufficiently light stick

forces. A very s:all value of 1 -He at zero soped
I 6xs
must also be accepted in order to avoid excessive
variation of force oer g with speed. The value of
1 6le for this tab arrangement exceeds 200 at soeeds
I 6xs
above 102 miles per hour.

The stick forces on an airplane of this size depend
rather critically on the elevator and tab hinge-moment
characteristics. In view of the rather limited data
available at present on the hinge-moment characteristics
of tabs, special tests would undoubtedly be required to
develop a design that provides the desired hinge-moment
parameters. The degree of balance rtuired is not so
high that small variations in contours among different
airplanes would cause excessive variations in the stick
forces. It therefore apoears that a spring tab :nay be used
to provide satisfactory elevator control on an airplane of
at least 500,000 pounds gross weight. The limiting size








NACA ARR IT LL4H2_


of airplane that could be adequately controlled by this
means is difficult to estimate, inasmuch a3 factors
such as the response of the elevator Lo stick movements,
rather than the magnitude of the stick forces, would
probably set the ucper limit on the size 3i airolane that
could be cDrAtrolled. The increasing importance of the
elevator inertia on large airplanes is caused oy the
fact that the moment of inertia of the elevator tends to
increase as aoproxirrately the fourth power of the linear
dim.,nsion, whereas the aerodynamic hinge moments due to
the tab vary as the cube of the linear dimension.


DTSCTSSION OF EXAMPLES


The ability of the scoring tab to provide desirable
stick-force characteristics over a large center-of-
gravity range on airplanes weighing between about
16,000 and 300,000 pounds has been shown by the preceding
xamiioli-s. The lo;::-r l.iriit on the sizt :.f airplane that
can be controlled is determined by the requirement for a
definite centering tendency of the control stick. The
upoer limit is not clearly defined but probably is set
by the ability of the elevator to follow rapid stick
rmovemetnts.

One advantage of the spring-tab control is that any
variation in the stick-force characteristics between
airplanes of the same type, caused by slight differences
in the contours of the elevators, wuuld be much less Ior
a spring-tab elevator than for an elevator equipped with
a conventional uype of balance such as a balancing tab
or an inset hinge. This difference may be explained as
follows: In order to obtain desirable stick forces with
a conventional tyoe of balance, the elevator hinge-
moment Darameters eChe/MSe and Ch!e/,IaT must be
reduced to very small values. Variations of these
parameters caused by slight differences in the elevator
contours are likely to be of the sanc order of magnitude
as the desired values. Such variations would cause changes
in the stick-force characteristics of 100 percent or
rore. In the case of the spring tab, howFvcr, a high
degree of balance of the elevator is not required. The
stick forces are reduced to desirable values by the
action of the cab. A pronorly designed spring tab has
been shown to act essentially as a servotab at normal
flight specds. The formula for the force per g with a








NACA ARR No. L1H28


servotab (equation (10)) shows that the force per g is
reduced by a large factor in the denominator that depends
on the tab and linkage characteristics. The effects of
any variations in the values of 6Che/iaT and BChe/66e
will be reduced by the same ratio. Tnasrnuch as this
ratio varies from about 1:10 in the case of the scout
bomber to 1:100 in the case of the 500,000-nound airplane,
the scoring tab should effectively eliminate any difficulties
caused by variations in elevator hinge-moment parameters.

Errors in the predicted stick-force characteristics
for a proposed spring-tab design, caused by failure to
obtain the desired elevator hinge-moment characteristics,
are likewise reduced by this ratio. As a result, the
control characteristics of a soring-tab elevator should
be more closely predictable than those of a conventional
elevator, especially on a large airplane. This advantage
is somewhat offset by the fact that the stick forces
obtained with a spring tab depend on the hingei-rcomctnt
parameters of the tab, as well as of the elevator. At
present, information on the hinge-moment characteristics
of tabs is not very complete.

The spring tab should provide an effective means of
control in high-speed flight, especially as regards
recovery from high Mach number dives, where the control
forces on a conventional elevator may become excessive.
It is known that trim tabs may be used to recover from
dives, at least at the Mach numbers reached by present-
day airplanes, but this procedure is known to be extremely
dangerous because, when the airplane reaches lower
altitudes and 'ach numbers, excessive accelerations may
be experienced before the trim tabs can be returned to
neutral. The spring tab directly controlled by the
stick should eliminate this difficulty. Furthermore, the
stick forces with a spring tab would not be 11'uly to
become excessive in the oull-out. The effects of
comoressibility may in many cases be considered as a
larz. rearward shift of the neutral point (of the order
of 21 to 50 percent of the mean aerodynamic chord).
Figures 3 to 6 show that such a shift would lead to
excessive stick forces for recovery with a conventional
elevator but to reasonable forces for a spring-tab
control. In order to effect recovery, the elevator and
tail would have to be built sufficiently strong to
withstand the large loads imposed.








NACA ARR No. L4H28


PREVENrION OF FLUTTER


h theoretical investigation of the flutter of
spring tabs is presented in reference 4 and the practical
results are given in reference 5. These reports show
that both the elevator and tab must oe mass-balanced
about their hinge lines and that the tab mass-balance
weight must be placed closer to the tab hinge line than
a certain distance defined by the relation


d (14)
: 1
1 --


In order to be most effective, the tab mass-balance
weight should be placed about half this distance ahead
of the tab hinge line. Equation (14) shows that, if the
mechanical advantage of the stick over the tab K2 is
reduced to a small value, the tab mass-balance weight
must be olaced so close to the tab hinge line that a
prohibitively large weight may be required. Equation (4)
indicates that K1 and K2 cannot be reduced simul-
taneously without unduly decreasing the stick travel.

A small value of the mechanical advantage of the
stick over the tab has been shown to be advantageous on
small airplanes in order to orovide sufficiently large
stick-force gradients and small variation of force
per g with seed. An exoerimental investigation to
detern.ine the validity of equation (14) is, therefore,
urgently required. Because of effects of flexibility in
the control linkages, the aDplicability of equation (14)
is open to some question in cases in which :2 is small.
In some instances spring tabs without mass balance have
been used without the occurrence of flutter. Special
devices with a smaller penalty due to weight have also
been proposed to prevent flutter.


STICK-.FORCE CHARACTERISTICS IN STRAIGHT FLIGHT


In figures 3 to 6, the rear limit of the assumed
center-of-gravity range was taken as the stick-fixed
(actually, elevator- and tab-fixed) neutral point in








NACA ARR No. LhH28


6Ch
straight flight. Because was taken equal to zero,
6aT
this point also represents the stick-free neutral point.
For all center-of-gravity positions ahead of this point,
the stick-force variation with sneed will be stable and
the gradient will be reduced by the spring tab in the
same proportion as the maneuvering forces. The effects
ACh 6Cho
of changes in the hinge-moment parameters --h and
06e 6aT
and the effects of altitude on the neutral point and
maneuver point may be shown to follow the same rules with
a spring tab as with a conventional elevator.


SPECIAL SPRING-TAB AF.RA.iGEENITS


The formulas set up for the stick forces obtained
in maneuvers with a spring tab may be used to determine
the characteristics of several special arrangements.

Tab controlled independently of elevator.- The
mechanism for a tab controlled independently of elevator
is shown diagramatically in figure 7(a). Tniis arrangement
is a special case of the previously used system in which
the elevator gearing constant K1 equals zero. The
stick-force characteristics may be found from equations (12)
and (15) by setting K1 equal to zero.

If K1 equals zero, the value of K2 must be large
enough to require full stick travel for full.tab
deflection. For airplanes weighing about 53,000 pounds
or less, a small value of K2 was required to provide
sufficiently heavy stick forces. The tab controlled
independently of elevator would therefore be considered
satisfactory only on large airplanes. Formula (13)
furthermore indicates that, when K1 = 0, the elevator
w.ll not be constrained to follow stick movements at
zero airspeed no matter how stiff a spring is used. The
system of figure 7(a) will thus have no advantages over
a servotab from the standpoint of ground control. The
spring should therefore be Dmitted in order to avoid a
force per g that varies with speed. This system is more
likely than an ordinary spring tab to result in instability
of the short-period oscillation of the airplane with
stick fixed, tccause the stability of the elevator itself









NACA ARR No. LLH2RS


with stick fixed is essentially the same as with stick
free. As a result, the dynamic stability of the airplane
with stick fixed is no greater than with stick free.
With a conventional scoring tab such as that shown in
figure 1, on the other hand, the effective restoring
moment on the elevator with stick fixed is greatly
increased by the leading action of the tab, so that the
stick-fixed dynamic stability of the airplane is close
to the elevator-fixed value. The only benefit that
appears to result from the use of the system of figure 7(a)
is a nossibile reduction of stick forces on a very large
airplane because of the increased allowable mechanical
advantage of the stick over the tab. Use of tnis
alternative does not appear to be necessary, however, for
the largest airplane considered i00,0UC'O pounds weight).

Geared spring tab.- The mechanism for a geared spring
tab is shown diagramatically in figure 7(o). This device
differs from an ordinary scoring tab in that, when the
elevator is moved (at zero airspeed) with the stick free,
the tab deflects with respect to the elevator in the same
manner as a conventional geared tab or balancing tab.
The stick-force characteristics for a geared spring tab
rnay oe calculated by means of the same equations as those
derived for an ordinary spring tab, if certain substitutions
are made for the hinge-moment parameters )f the elevator.
These substituted values may be interpreted physically
as the hinge-moment parameters of an elevator equipped
with an equivalent balancing tab, which is defined as a
balancing tab that has the same gearing ratio as would
be obtained on the geared spring tab with stick free at
zero airspeed.

By means of a geared scoring tab, the force per g at
low airsoced may be reduced without decreasing the force
per g at high speed and without reducing the response of
the elevator to raid stick movements on the ground.
This device, in fact, presents the theoretical oosslbility
of obtaining a force per g that does not vary with speed
regardless of the spring stiffness used. This result may
be attained by making 6Che/6aT and PCht/6aT equal to
zero and by using a tab-gearing ratio such that the force
per g at low speed is reduced to the value which would
be obtained at very high speed, where the characteristics
of a servotab are approached. In practice, it is unlikely
that the exact values of hinge-moment characteristics
required could be obtained. Some variation of force
per g with speed would result if these characteristics









NACA ARR No. LH28


differed slightly fr-m the desired ones. The variation
of force per g with snced would be smaller, however, than
that obtained with an ungeared spring tab with the same
spring stiffness. It therefore appears that the stick-
force characteristics shown in figures 3 to 6 could oe
rimproved by the use of geared spring tabs. Stiffer
--prings, providing improved ground control,could be used
alternatively for the same variation of force per g with
speed. Errors in obtaining the desired value of 6Ch.,-/'6e
for the geared spring tab may be compensated by adjustment
of the tab linkage by trial on the actual airplane.

Preloaded spring tsb.- If the tab spring is preloaded-
to prevent deflectri of the tab until the stick force
exceeds a certain amount, the stick force per g will equal
that of the elevator without a spring tab up to the mnint
where the stick force reaches the preload. Beyond this
point, the force per g will equal the force calculated
for an unoreloaded s.rling tab. ?Th force variation with
acceleration will therefore be nonlinear, a characteristic
usually considered'to be undesirable.

If friction is present in the tab system, an
unpreloaded spring tab may not return to a definite
equilibrium position and, as a result, the pilot may
experience difficulty in maintaining a specified trim
speed. A small amount of preload nmay be used to center
definitely the tab in trimmed flight and thereby to
overcome this difficulty. In view of the mecha.-ical
complications involved in the use of a preloaded spring,
as well as the nonlinear force characteristics mentioned
previously, it appears desirable to avoid the necessity
for preload by reducing friction in the tab system to a
minimniun.


CO II LSI'3I ON


..n analysis of the effects of scoring tabs on elevator
forces if 'irolanes of various sizes has indicated the
follo.. '.. cl asions:

L. t- tha use of spring tabs, satisfactory elevator
controi- 'jo, ca&a.acteristics may be obtained over a
large cencr-- ..-ravit rance on airplanes varying in
weight f:- i about 16,U00 to at least. 5CO,000 pounds.









IACA ARR No. L.'H23


2. The scoring tab offers the possibility of greatly
reducing the changes in stick forces that result from
small variations in contours of the elevators on different
airplanes of the same type.

5. The elevator control-force characteristics
resulting from the use of a scoring tab should be more
closely predictable than those with other types of
aerodynamic balance such as a balancing tab or inset-
hinge balance: in order to take advantage of this effect,
however, more conolete information on the hinge-moment
characteristics of tabs is required.

4. One of the chief objections to the use of spring
tabs is the amount of weight required for mass balance to
prevent flutter. Exoerimental work is recommended in
order to find means of reducing the amount of balance
weight required.


Langley Memorial Aeronautical Laboratory
National Advisory Committee for Aeronautics
Langley Field, Va.








TACA ARR No. L1H28


REFERENCES

1. Gates, S. B.! Notes on the Spring Tab.
Rep. Lo. B.A. 1665, British R.A.E., April 1941.

2. Greenberg, Harry: Calculation of Stick Forces for
an Elevator with a Spring Tab. NACA RB No. L4p'07,
1944.

5. Anon.: Stability and Control Requirements for
U. S. Army Airplanes. AAF Specification, June 10,
1943.
4. Frazer, R. A., and Jones, W. P.: Wing-Aileron-Tab
Flutter (Parts I and II). 5668, 0.251,
British N.P.L., March 17, 1942.

5. Collar, A. R.: The Prevention of Flutter of Spring
Tabs. Ren. 0. S.M.E. 3249, British R.A.E.,
May 1945.








NACA ARR No. L4H28


TABLE I.- CHARACE.RISTICa OF VARIOUS AIRPULANEi


Scout
bomber


Medium
bomber


Heavy
bomber


hAONAL ADWSOR
COMMIWfTi uOR AERmOmIC
300,000-pound
aLrplane


Scale, ft 1 0 o 0 190 9 1 200

u, lb 16,0uO 50,000 125,000 300ci, )
b, ft 49 89.3 14.3 22. 5
S, eq ft 400 1000 2275 5003
c, ft 8.16 L.1 8 15.90 22.35
1, ft 20 35 50 75
ST, sq ft 100 200 455 1000
( L,) per radLan 4.2 4.5 4.6 4.7

i 0.5 0.55 0.57 0.60
da
qT/q 1.0 1.0 1.0 1,o
be. ft 20 34 50 75
ae, ft 1.8 2.2 3.2 4.8
b., ft 5.0 7.55 15.0 26.2
oa, ft 0.50 0.80 0.60 0.666
r 0.5 0.5 0.5 0.5
S. per radian 1.7 1.7 1.7 1.7

I, slug-f 2 0.5 1.5 7.0 35
Ki, ft per radian 1.80 1.80 1.80 1.80
12, ft per radian -0.60 -0.45 -1.20 -1.20
E3, Ib per radian 33.3 100 124 200

-h, per dg 0 0 0 0
doQT

6 per des -0.003 -0.003 -0.003 -0.002

S par deg -0.003 -0.003 -0,003 -0.003

'ht, per deg 0 0 0 0

per deg 0 0 0 0

c6 per deg -0.005 -0.005 -0.005 -0.003
6 6 t,________ I______ I______ I_______ I__________


-s-,


4--i







NACA ARR No. L4H28 Fig. 1












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'













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NACA ARR No. L4H28


5 =o




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I 0a
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em

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Fig. 2






NACA ARR No. L4H28


I I I I I
of- elrc7i/y ,Vosi/~of,


I _


-4rs--


.C


I rffI--~bSL^|, / Lr\,i4s4
0 /00 200 300 400
/ndicted o/irspeed, M/1ph


NATIONAL ADVISOR(
COMMITTEE FOR AERONAUTICS





Speed, -P7
/_ Sf/ck-f/fred neu
___ / in stra/ht f
I Ira! f


I 30b ,


1 I .1
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*-- 4 -~ IY V


~Psill


/d limnfs


0







40



0


Desired
//md/ts


Back


Center-o/J-fv/fy position, percent ACA.C


Figure 3.- Variation of force per g with speed and with center-of-gravity
position for scout bomber (weight, 16,000 lb).


I I
Cenler-


tao/ polnt
// ht


4e 0,-.O3
^e


IO
/00O


-8
Forward


T 1


--


Fig. 3


,errc~.~~t MIA~C.


-r~C i






NACA ARR No. L4H28


I I I I
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I I
posihtn -peyrct-af~AAC


-/ ___


f_ ~ ,! I / / I I/y / .= '
y' / I / / ,, / / "


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I NATIONAL ADVISOF Y
COMM nEE FR AERON UTICS.





/0 \ /Yo Srr

300 400
._ |X


J-


-6 -4 0 4 &
Forward Back
Center- of- rav/fy pos/f/on, percent V/.A C.


Figure 4.- Variation of force per g with speed and with center-of-gravity
position for medium bomber (weight, 50,000 lb).


\
\


/ited


80



40



0


Desred
hmt


/00



80



40



40



20


i \ IL
'\ 0 --


SticA- fixed f eutra/
pOint if straight
f// / I


7/


fabjc, -.03
SCe---


V esired
':j /,, ifs


Fig. 4


-7u/


..J/L.J






NACA ARR No. L4H28


I C
\ Ceter-


I I I I
# grOYvi/ oosl//onr


1/



\8
' I I0 I


I I I kC [ ;
0 /00 Zo0 300
indicatedd a/rspeed, mnph


400


NATIONAL ADVISORY
COMMITTEE FOR AERONAUTICS


\ pee

0





4-00



300



200



/00


\


- -- __- 1\ ii


t -I I


SAjringq tab,
e =-. 003
a &


L eszrecd
9A- /., .;-


-8 -4 0 4- 8
Forward Bac/r
Center- of-groarty position, percent /A.C.
Figure 5.- Variation of force per g with speed and with center-of-gravity
position for heavy bomber (weight, 125,000 Ib).


\


I
ercenft IAC.







Desired limits


400



200


/0 1
03 0p


sf/cAi-fixed
/0i str~a, hf


n'e41fva/po/n7'


--I I \ I


r/,,,,,, _, 1~~1 1 /I


Fig. 5


p


/1,.


IF/'/it /






NACA ARR No. L4H28


Cent-of-yera- i
1/ I 1a i i


-0



0\


position, percent MA.C.
I I I


- J I our. I I I


/00


200


300


Li*Ses/red //1rn'ts
400


d/cafdted o/rspeed, mph
NATIONAL ADVISORY


Forward Back
Center- a-granty pos/ ton, petent MA.C.


'.Figure 6.- Variation of force per g with speed and with center-of-gravity
position for 300,000-pound airplane.


400
GOO


200



0


"4
t'






k
15
9
2:
LP


1 I 1 I 1 I I


Fig. 6








NACA ARR No. L4H28 Fig. 7


















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Ut DIVERSITY OF FLORIDA
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