An automatically variable control linkage and its effect on the lateral-control characteristics of a high-speed fighter ...

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Title:
An automatically variable control linkage and its effect on the lateral-control characteristics of a high-speed fighter airplane
Alternate Title:
NACA wartime reports
Physical Description:
12, 5 p. : ill. ; 28 cm.
Language:
English
Creator:
Murray, Harry E
Gillis, Clarence L
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:
Fighter planes   ( lcsh )
Ailerons   ( lcsh )
Aerodynamics   ( lcsh )
Genre:
federal government publication   ( marcgt )
technical report   ( marcgt )
non-fiction   ( marcgt )

Notes

Summary:
Summary: An analysis and a preliminary design were made for a control linkage that varies automatically with dynamic pressure. This device can provide greater lateral control than a fixed control linkage at all but one airspeed without additional aerodynamic balance. The mechanical construction should present no unusual problems and both the weight and volume of the device appear sufficiently small for use in single-seat fighter airplanes as well as in large machines.
Statement of Responsibility:
by Harry E. Murray and Clarence L. Gillis.
General Note:
"Report no. L-65."
General Note:
"Originally issued May 1944 as Restricted Bulletin L4E23."
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."

Record Information

Source Institution:
University of Florida
Rights Management:
All applicable rights reserved by the source institution and holding location.
Resource Identifier:
aleph - 003666890
oclc - 76036717
sobekcm - AA00006294_00001
System ID:
AA00006294:00001

Full Text

MFci+ L-WC


RB No. L4E23


NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS





WARlTIME REPORT
ORIGINALLY ISSUED
May 1944 as
Restricted Bulletin IAE23

AN AUTIAATICALLY VARIABLE CONTROL fKAGE AND ITS EFFECT
ON THE LATRAL-CONTROL CHARACTERISTICS
OF A HIGX-SPEED FIGHTER AIRPLANE
By Harry E. Murray and Clarence L. Gillis

Langley Memorial Aeronautical Laboratory
Langley Field, Va.







NACA


WASHINGTON
IJACA 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 65


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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/automaticallyyar001ang
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'ACA RB 'To. L4523


HIATIONAL ADVISORY COCi'ITT': FOR AERONAUTIS

F7E'STRCTOT''3 7IT. T ET i'

AN AUTOMATICALLY VARIABLE CO~iTFOL LINKAGE AiD ITS EFFECT

ON THE LATERAL-CONTROL CHARACTERISTICS

OF a HIGH-SPEED FIGHTER AIRPL.;E

By Harry E. :Murray and Clarence L. Gillis


SUMMARY


An analysis and a preliminary design were made for
a control lin-. -e that varies automatically with dyna:-ic
pressure. This device can provide greater lateral con-
trol than a fixed control linkage at all but one airspeed
without additional aerodynamic balance. The mechanical
construction should present no unusual problems and both
the weight and volume of the device appear sufficiently
small for use in single-seat fi-Q.ter airplanes as well
as in large machines.


INTRODUCTION:


The mechanical advant-i- of aileron control systems
in fighter airplanes has necessarily been a cc:7r:romise
between the control requirements at high and at low speeds;
the airplane has therefore had its optimum control charac-
teristics occurring in the middle sped range. In
contrast *;ith such a compr:o. .se, the critical control
conditions occur at minimum sped, when the airplane is
close to the ground and probably poorly trir.nmed, and at
high speed under fighting conditions. It is consequently
desirable that the control characteristics of fighter
airplanes be made more nearly optimum throughout the
speed range.

If the optimum control at any speed is to be realized,
it is necessary that the mechanical advantage of the con-
trol system be varied automatically with seed -.vithout
perceptible lag and independent of stick force, stick
position, -ir.6 normal acceleration. Th-e present paper
presents an estimate of the lateral-control characteristics










NACA RB No. L4E23


of an airplane equipped with a variable control l11- ae
cia ble of such a variation in mechanical advantage.
A preliminary design for the device is included.

Although the Tresent analysis deals only with
lateral control, sor.ewhat similar problems exist in
longitudinal and directional control; and the variable
control linkage can probably be expected to render a
similar improvement when applied to the elevator or
rudder.


SYMBOLS


q dynamic pressure

A area of dynaric-pr.essure piston

F force on dynamic-pressure piston (qA)

X linear displaconent of dynamic-pressure piston

I number of effective coils of spring on dynamic-
pressure -niston

L length of stick from pivot to push-rod link

F, -relcad on spring of dynamic-pressure piston

L, length of stick below pivot when F = Fo
d diameter of wire of which spring is wound

D diameter of helical sprin-

J torsional modulus of elastici' of spring wire

6 aileron deflection, degrees

Os stick deflection, degrees

control-system mechanical -adt-antage
6

Is stick length

Is stick force









NACA RB No. L4E23


b

V
pb
2V

CL

C 6

C p
9


Ch

ChE

ba

Ca

P

ks

Ap


Cl,C2,C3

KO... K17


rolling angular velocity, radians per second
(plotted in deg-rees per second to conform with
usual practices)

span of airplane wing

forward velocity, feet per second

wing-tip helix angle of airplane in roll, radians


rolling-moment coefficient

rate of chaim.e of rolling-moment coefficient
with aileron deflection

rate of change of rolling-moment coefficient
with p
2V

bir.:ne-moment coefficient

rate of change of hinge-moment coefficient
with aileron deflection

aileron span

aileron root-mean-square chord

density of air, nound-seconds2 ner foot4

spring constant per coil of helical spring

pressure difference across dynamic-pressure
piston

constants used in determination of spring
characteristics

constants used in deterir-nstion of lateral-
control characteristics


BASIC ASS"flTTPTIONS iND CONDITIONS FOR -:- AJ.-.LYSIS


The analysis of the effect of the variable control
linkage is made on the basis of the following assumptions:

(1) The airplane has a rigid wing and control system










NACA RB No. L4E23


(2) The aileron hinge-morent characteristics are
linear with both deflection and angle of
attack

(3) The ailrron effectiveness is constant

The first assumption is strictly true only for low speeds,
and tho second and third hold for small aileron deflec-
tions; however, the accuracy is sufficient to rive a
preliminary prediction of the effect of the variable
linkage.

T'-. effect of the variable linkage on an airplane
having the geometric characteristics and performance of
the P-51, which was chosen as a representative fighter
airplane, was studied. The slope of the d.r'rnai-c hnr-'-
moment curve C and the ratio for the P-5 air-
5 CL
plane were estimated to be the following:

Ch = -0.00129


C76
= 0.0025 radian per degree
7p

All computations were made subject to the following
geometric characteristics of the P-51 airplane:

Wing
Aiaa, square feet ............................ 25.75
Span, feet .................................... 3 7.03
Root chord, inches .............. ........... .. 1 3. '.
Tin chord, inches .............................. 50.00
Taper ratio ................................... 0,45
Aspect ratio ................ ... .. ............. .815

Aileron:
Area, square feet .............. ........... ...... 6.70
Sp n, feat .............. ................ ....... 6.95
Chord, percent wing chord ..................... 13.7
Deflection, degrees ................... ....... 25
Distance from center line to outboard
and, semis. an ........................... .. 0.9.5
Distance trom center line to inboard
end, semisr .n ............................... 0.610
Root-rean-square cho,.:, square foot ........... 0.,.3










NACA RB No. L4F25


Stick:
Travel, inches at top ............................. 18
Length, inches ................................. 23
Total deflection, degrees ........................ 45


EFFECT OF VARIABLE LI::;1E ON; LATERAL-CO:I7OL

CHARACTERISTICS OF A HIGH-SPEZD FIGHTER AIRPLA.TE


In order that the control system of an airplane be
at an optimum mechanical advantage, the maximum allowable
stick force should always occur at the maximum stick
deflection. If such a condition is to exist throughout
a range of speed and atmospheric density, the mechanical
advantage of the control system must vary directly as
the square root of the dynamic pressure.

Equations for determination of lateral-control charac-
teristics of an airplane equi'-ed with a fixed control
linkage and the optimum variable control linkage have
been derived by the usual method and are presented in
appendix A for the conditions of no limitations and the
limitations of maximum stick deflection, stick force,
and aileron deflection. Formulas for the constants
involved in these equations are also presented. The
lateral-control characteristics shown in figures 1 and 2
were obtained from these equations.

The estimated lateral-control characteristics for
a 50-pound stick force at full stick deflection of a
high-speed fighter equipped with a variable linkage are
shown in figure 1. For comparison, these characteristics
of the same airplane equipped with a fixed control
linkage such that the maximum rolling angular velocity
occurs at about O.8Vmax are also given. Eeciase the
average maximum force exerted by pilots appears to be
approximately 50 pounds, the estimation was made on this
basis; however, a 50-pound stick force is not developed
on the part of the curves where aileron movement is
limited by maximum aileron deflection. From figure 1
it can be seen that the variable control linkage provides
more lateral control than the fixed control linkage at
all but one airspeed at any altitude.

The effect of a variation of the stick force exerted
by the pilot rather than of altitine is shown in fig-
ure 2, which presents lateral-control characteristics










1ACA RB No. L4E23


similar to those of figure 1. :.er: this stick force is
greater than 50 pounds, figure 2 shows both the rolling
velocity and the helix angle to be independent of stick
force on the variable control linkage. Similarly, the
loads on the wing and ailerons resulting from aileron
deflection are independent of stick forces greater than
50 pounds at any speed.

With a fixed control linkage the wing and ailerons
are desi--red to sustain some maximum load, which corre-
sponds to a certain constant stick force and, at high
seed, to a stick deflection less than maximum. If
the ailerons tend to overbalance at high speeds or if
the strength of the pilot is excessive, deflections and
loads be--ynd design values will occur with this system.

hern the automatically variable linkage is used,
the maximum possible aileron deflection decreases with
speed and is determined not by a stop at the aileron
but by maximum stick deflection. This maximum stick
deflection then corresponds, for shy speed, to a definite
load which cannot be exceeded regardless of the strength
of the pilot or any tendency of the ailerons to over-
balance.


PR2Li;TIiTRY DESIGN OF a DEVICE TO aiCCOMPLISH

THE- REQUIRED LT.TE:G3 VARIATION


A mechanism for electrically varying the stick
mechanical advantage and a control unit for relating
the mechanical advanta-c to the dynamic pressure according
to the characteristics of the spring located in back of
the dynamic-pressure piston are shown schematically in
figure 3. This control unit supplies power to a
reversible direct-current motor through the breaker
points which are moved, by means of a flexible cable,
a distance proportional to the linear displacement of
the variable link. Such an electrical system (fig. 3)
for varying the control l:n;.:' e has no extremely
delicate cr complicated :'rts and should operate quite
reliably without perceptible lag, as does a similar but
more complicated mechanism for varying the pitch of
constant-sre-d propellers.

The pressure cell shown in fig-i. 3 was mounted with
its axis parallel to the lateral a:.i of the airplane in










NACA RB No. L4E23


order that the effect of inertia forces resulting from
normal and longitudinal accelerations might be eliminated.
It is conceivable that such a device might hunt, in which
case a brake could easily be installed as indicated in
figure 3.

If this mechanism is to produce a variation in
mechanical advantage directly proportional to the square
root of the dynamic pressure, the length of the lever
arm L (fig. 4) of the aileron push-rod link must vary
inversely as the square root of the dynamic pressure.
The d=n3..ic-pressure piston must thus move according to
the following relation, which is developed in appendix B:


S= (1)
(e X)2

In order that such a force-deflection relation shall
exist with the constant-diameter-coil spring shown in
figure 3, the number of coils in the spring must vary
with deflection as follows:

I ksX (C2 X)2
I (2 (2)
C3 Fo(C2 X)2

If the relations shown in figure 4 between the
constants and variables involved in the link system are
used, the constants of equations (1) and (2) can be
evaluated, as explained in appendix B. Physically,
such a variation in the number of effective coils of
a spring can be achieved with a constant-diameter
helical spring by variation of the helix angle along
the length in order that the coils of the spring will
gradually fall against each other as the spring is
compressed; the effective number of coils are thereby
decreased according to the relation given. Typical
curves of the force and the shortening characteristics
of the spring fulfilling these relations are shown in
figure 5. Although only the constant-diameter helical
spring was considered in this analysis, the same
characteristics can be obtained from any one of several
other springs.

No special power supply is required by either the
motor or the control unit; hence, it has been estimated










NACA RB No. L4E23


that the entire device can be installed in a high-speed
fighter for an additional weight of about 10 pounds.
If, however, the power supply failed or the mechanism
became damaged by gunfire, the pilot might conceivably
be left in the high mechanical-advantage ra ne without
sufficient aileron deflection to make a safe landing.
A manual method of operation may therefore be required
in case of an emergency.

The additional lateral control made available by
the variable linkage will result in an additional
wing torsional load which increases with speed. If,
at speeds near terminal velocity, the additional
loads become nndesirably large they can be reduced
by reducing the stiffness factors of the dynamic-
pressure spring and, consequently, the available
aileron deflections corresponding to these speeds.

Because of the imorovemesnt in lateral control
indicated by the analysis and of the simplicity of the
required mechanism, it is si'i.- ested that the auto-
matically variable control linkage be tested in flight.


CONCLUSICUS


From an analytical investigation of the effects
of a variable control linkage on the lateral-control
characteristics of a fighter airplane and from a
preliminary design of this device, the following
conclusions are indicated:

1. The automatically variable control linkage
can provide more lateral control than a fixed linkage
at all but one airspeed without additional aerodynamic
balance.

2. T.'Tn the automatically variable control linka,.ge
is used, the design loads can be made to occur at maximum
stick deflection, which corresponds to a constant stick
force within the limit of the pilot's stror:nth. Further
aileron deflection with a corresponding overloading of
the structure cannot therefore result from excessive
nilot strenct- or a ten .-ncy of the ailerons to over-
balance at high seeds.









NACA RB To. L4E23


3. A manual method of operation may be required in
the link-variation mechanism in case of a power failure
or mechanical difficulties.

4. TC'e device has no extremely delicate or com-
plicated parts, should operate without perceptible lag,
and probably can be installed in a high-speed fighter
airplane for an additional weight of about 10 pounds.


SUGGESTION FOR FUTURE T T :.-:CH


Inasmuch as the conclusions indicate that definite
improvements in lateral control may be expected crom the
use of a variable control linkage, it is recommended
that such a unit be constructed, installed in an air-
alane, and tested in flight.


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









10 YACA RB "~-. L4E23


APPENDIX A

LATERAL-C OTROL CA-.RACTERISTICS


The basic formulas for computing the lateral-control
characteristics may be conveniently tabulated as follows:


The "-: ,st:,,ts K:O .
the following relations:


. *.17 ay t evaluated from


constant depending upon aileron-stick linlkzce


KI =-

2
}K2 = 1I


K8 = K11:09max


K9 = K2K8Osmax


1 2
`5 7- s 'h6 a KO O"0


K7 = K5-O smax


- 101
12 = I


K10K2
13 K
K5

K15 = K56max

K16 = K16rax

K17 = K26max


smax









NACA RB No. L4E23 11


APPENDIX B

DE'?r; ,II'.TION OF CONSTANTS FOR SPRING EQUATIONS


From fiFure 4 it can be seen that

L = Lo C1X

If the mechanical advantage is to vary directly as the
square root of the dynamic pressure, however, the
following;: condition must be true:
KO
L=-
Vq
= Lo ClX

and, therefore,

F =qA
C3

(C2 X)2

where
Lo
C2 CI


C3 =C A

For a helical spring

F= (i X + Fo

where
d4J
k =
s 8D3

From these conditions
I kX
F Fo
or
k X(C X)2

C3 Fo(C2 X)2









TACA R No?. L4E'.-


Figure 5 shows a typical curve of I plotted
against X and a correspo'":d:i: curve of F plotted
against X for the following numerical values of the
constants:

C2 = 4

k = 4
k =4

Figure 5 represents no particular setup, however, and
is given only as an exanpls, because C2, 03, and ks
will depend upon the dimensions of the device designed
and tho properties of the spring.







NACA RB No. L4E23


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a


C


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I


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60--- --0


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NATIONAL ADVISORY
.1 COMMITTEE FOR AERONAUTICS


.I

0 ___ _0








.04- Variable linKag.qe
= Fixed linkaqe. -

'O 90--- -400--0


True airspeed, mph


Figure 1.-Lateral-control characteristics of a high- speed
fighter airplane having fixed and variable linkages at
three altitudes with a maximum 5tick force of 50pounds.


Fig. 1


zoo







NACA RB No. L4E23


S4 14C
o0
- IZc

t IOC
0

S80c
C
S60
o0
40


True airspeed, mph
NATIONAL ADVISORY
COMMITTEE FOR AERONAUTICS

Figure 2.-Lateral-control characteristics of a high-speed
fighter airplane having fixed and variable linkages at sea,
level with three stick forces. Maximum 5ticK deflection
occurs with the variable linKage when Fs-50 pounds.


F I
__ _(Ib50 )
60
50


S40 >



L40
/ ^ '^r-^-, ^--


Fig. 2









NACA RB No. L4E23 Fig. 3









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a )C 0 S < 0









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to \\\




el <















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Z'
iB -y u








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V2






NACA RB No. L4E23


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cyl//icer


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NATIONAL ADVISORY
COMMITTEE FOR AERONAUTICS.


7'yure 4.- 7-he re/a/o/bn hetwen Mhe
d/so/acemrnL of the dyncam/' --pressucre
o z'on and 7he /oca'Ion of 'he varabL/e
//n^.,


Fig. 4






NACA RB No. L4E23


D/splacemen 7 of dcyr7fmi -presure pr o~, X
Figure -.-7Typ/i4o force on'd shor fr~9ny
char acter /Al of dyn/amc -prssure spor/ny


Fig. 5










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