Effect of lateral shift of center of gravity on rudder deflection required for trim

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Material Information

Title:
Effect of lateral shift of center of gravity on rudder deflection required for trim
Alternate Title:
NACA wartime reports
Physical Description:
5, 4 p. : ill. ; 28 cm.
Language:
English
Creator:
Phillips, W. H
Crane, H. L
Hunter, P. A
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:
Bombers   ( lcsh )
Steering-gear   ( lcsh )
Aerodynamics -- Research   ( lcsh )
Genre:
technical report   ( marcgt )
federal government publication   ( marcgt )
non-fiction   ( marcgt )

Notes

Summary:
Summary: Tests of a single-engine scout-bomber airplane showed that the rudder deflection required for trim at low speed in the critical wave-off condition may be reduced approximately 10° by a lateral shift of the center of gravity equal to 1.8 percent of the wing span. The reduction in rudder deflection required for trim consists of the rudder deflection required to offset yawing moments from the ailerons and from the component of the weight in the direction of the longitudinal axis and the rudder deflection required to hold the sideslip angle necessary to maintain straight flight. The effect of the lateral loading must be taken into account in tests to determine the adequacy of the rudder for trim. The lateral center-of-gravity location is also important in the service operation of airplanes because, by suitable distribution of the useful load in the wings, the ability of the rudder to trim the airplane in critical power-on conditions may be markedly improved.
General Note:
"Report no. L-92."
General Note:
"Originally issued November 1944 as Restricted Bulletin L4I06."
General Note:
"Report date November 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."
Statement of Responsibility:
By W.H. Phillips, H.L. Crane and P.A. Hunter.

Record Information

Source Institution:
University of Florida
Rights Management:
All applicable rights reserved by the source institution and holding location.
Resource Identifier:
aleph - 003613453
oclc - 71209566
sobekcm - AA00006297_00001
System ID:
AA00006297:00001

Full Text
=F iA


RB No. L4I06


NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS




WARTIME E REPORT
ORIGINALLY ISSUED
November 1914 as
Restricted Bulletin L4I06

EFFECT OF LATERAL SHIT OF CENTER OF RAVITY
ON UD EFLEON RDN REQUIRED FOR TRIM
By W. H. Phillips, H. L. Crane, and P. A. Hunter

Langley Memorial Aeronautical Laboratory
Langley Field, Va.


NACA


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 92


DOCUMENTS DEPARTMENT


L- 92


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IACA RB No. L4I06 RESTRICTED

NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS


RESTRICTED BULLETIN


EFFECT OF LATERAL SHIFT OF CENTER OF GRAVITY

ON RUDDER DCFLE]TION REq.UIRED FORI TRIM

By W. H. Phillips, H. L. Crane, and P. A. Hunter


S3T.1I.TARY
SrT-,!-AiY


Tests of a single-en.ine s'cout-bomber airplane showed
that the rudder Jefl-ction required for trim at low speed
in the critical wave-off condition may be reduced anproxi-
matel 100 by a lateral shift of the center of gravity
equal to 1.3 percent of the win. span. The reduction in
rudder deflectin required for trim consists of the rudder
deflection required to offset vain; r ioments from the
ailerons and from the comoonent of the weight in the
direction of the longitudinal axis and the rudder deflec-
tion required to hold the sideslip angle necessary to
maintain straight flii.ht. The effect of the lateral
loading must be taken into account in tests to determine
the adequacy of the rudder for trim. The lateral center-
of-gravity locac on is also iminovttant in the service
operation of airplanes because,by suitable distribution
of the useful load in the wings, the abilit-,, of the rudder
to trim the airplane in critical power-on conditions may
be markedly improved.


I1TTRODUCTIOIT


Many modern single-engine airplanes have been found
to have rudder control that is inadequate for maintaining
straight flight at low speeds with nower en. Attemnts
have been made to increase the amount of rudder control
available by increasing tho rudder chord or by offsetting
the fin or the thrust line. Increasing the rudder chord
may result in excessive rudder ,"orces in maneuvers or in
rudder lock. The offset fin or thrust line produces an


RESTRICTED








NACA RB No. LrI06


asy1mnetricl configuration that, at high speeds, may
result in excessive rudder-force changes with speed due
to deformation of the surfaces under air loads.

Theoretical study indicated that a lateral shift of
the center of gravity should have an appreciable effect
on the rudder deflection required for trim at low speeds.
Flight tests were therefore made to determine the effec-
tiveness of this method of reducing the rudder deflection
required for trim.


1712 RESULTS AnD DISCUSSION


Two flights were made in a Brewster :KA-l airplane
equipped with modified tail surfaces (fig. 1). This
airplane was known to have marginal rudder control for
trim in some flight conditions and was therefore chosen
as representative of the type involved in the problem of
providing adequate rudder control in power-on flight at
low speeds. The center of g-av';ity was shifted 4.16 inches
to the right for the first flight and 4.16 inches to the
left for the second flight. This shift was accomplished
by aiymm.tric loading of fuel in the wing tanks.

The static directional-trim data presented in fig-
ure 2 were obtained from continuous records made while
the speed was gradually reduced from 100 miles per hour
to the stall in strlilht flight with the wins level.
This figure contains the results of three runs made with
each center-of-gravity location. The airplane was in
the wave-off condition with flaps deflected, landing gear
down, and maximum continuous power. Te', steady sideslip
characteristics in the wave-off condition at approxi-
mately 60 miles per hour are shown in figure 5. Measured
control-surface deflections. shown in figures 2 and 5
were not corrected for cable stretch, but errors from
this source are believed to be small.

The data of figure 2(a), obtained with the center
of gravity 4.16-inches to the left of the thrust line,
show that a rudder deflection of approximately 200 was
required to trim the airplane at 50 miles per hour
(5 mph above the stalling speed). Corresponding data
of figure 2(b), obtained with the center of gravity
4.16 inches to the right of the thrust line, show th.t
a rudder deflection of approx Lutely 100 was required








NACA RB No. L4T06


for trim at this speed. For the total shift of 3.32 inches
(1.3 percent of the wing span) in tne center of ravilty,
the reduction in the rudder deflecticn required for trim
at 50 miles per; hour v.as therefore 1i0. These values were
obtained from the flight rurs that showed closest agree-
ment in angle of banrk. Usinr the average data for all
three runs in each flight v.ould give somewhat greater
values for the reduction in rudder deflection. This
increase in the values for the reduction in rudder deflec-
tion is thought to be ceased by the small difference in
the angle of bank between the two sets of runs.

The rudder forces for trim at the stall were decreased
from 65 pounds to 53 pounds by shifting the center of
gravity to the right. It should be noted that the direc-
tion of propeller rotation was normal, that is, clockwise
when viewed from the rear.

At 50 miles per hour, the totsl aileron angle required
for trim w-as 150 to the right with the left center-of-
gravity location and 15 to the left with the right
center-of-grevity location. The aileron angle was always
well within the available range of tL'O. Since the
dihedral effect was neutral as is sho:.'n in figure 5 by
the fact that the eileron angle does rnot very appre-
ciably with sideslip angle, none of the change in total
aileron angle required for trim was caused by the differ-
ence in sideslip angle. The difference in sideslip angle
for the two center-of-gravity locations affected the
rudder deflection required for trim, however, because
of the inherent directional stability of t*,e airplane.

The effect of lateral loading on the rudder deflec-
tion required for trim is believed to be caused by yawing
moments that result from the aileron deflection required
for trim and from the thrust required to overcome the
component of weight in the direction of the longitudinal
axis. When the rudder is deflected to offset these
yawing moments, the side force developed on the vertical
tail causes a change in sideslip angle that requires a
further change in the rudder deflection for trim. Calcu-
lations based on theory indicate that these three sources
of yawing moment are of about equal importance and that
the estimated values are of the right order of magnitude
to explain the observed effects.

The changes in rudder and aileron deflection caused
by the change in lateral loading may be seen to decrease








ITACA RB Ho. L4I06


rapidly with speed an indication that asy~netrical con-
trol deflections become very small in flight at high
speed. The corresporj.ig control forces would therefore
be expected to be small at high speeds. In this respect,
lateral shifting of the center of gravity is thought to
be preferable to offsetting the fin or the thrust line.

The use of a lateral shift of the center of gravity
to reduce the rudder deflection required for trim with
power on requires rudder deflection in the opposite
direction to trim the airplane at low speeds -',ith ;ower
off. Calculations show that this rudder deflection is
relatively small because the maximum lift coefficient
with power off is generally much smaller than with power
on. :.hn the center of gravity is shifted laterally, a
cbl-nie in aileron :rnjle is required to maintain the
wi:n? level while the normal acceleration is increased
in turns or pull-ups. A total Pileron 1,-rle of about
5.80 would be required in pull-ups of the XSBA-1 air-
plane to the stell at maximum level-flight speed with
the center-of-gravity locations used in the tests. A
more asymmetrical center-of-gravity location then was
used in the present tests of the XSBA-1 airplane appears
to have been tolerated on several airplanes in service,
which have a lateral center-of-gravity shift with normal
change in loading more than twice as great as that tested
on the XSBA-1 airplane. Because r;.c.y airplanes have
provision for carrying part of their useful load in the
wings, the ability of the rudder to trim these airplanes
in critical power-on conditions may be improved by suitable
distribution of this load. It should be noted that an
airplane having counterrotating propellers, which nor-
mally needs no rudder deflection for trim, m:- require
considerable rudder deflection for trim at low speed if
the center of gravity is shifted laterally.


CONCLUDING REMARKS


T'he data indicated that a lateral shift of the
center of gravity is an effective method of overcoming
inadqua.to rudder control in power-on flight at low








N.CA RB !To. iL.106 5


speeds. Tr te. ts to determine the adequecy -2 th. e rL.dder,
tn? lateral location cf th:e center 0' of g'vty .ust be
taken in.o account.


Langley '.enorial Ae-rc nutica LEboratory
Nerional Auvisory Cor.n.mittete frL Aec, nr-ut!ls
Langley Field, Vc.













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(a) Center of gravity 4.16 Inches to left of thrust line.
Figure 2. Static ditectional-trim and Icngitudtill s tability character-
wstics in wave -off condition (flaps deflected; landing gear down;
maximum continuous power). Rudder trimming tab neutral;
Brew ster XSBA -7 airplane.


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(b) Center of gracvty 4.16 inches to right of thrurt line.
Figure 2. Concluoed.


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Sidesip angle,deg Right


figure J. Steady Sideslip chardcteristics in uVve- off condition
at approximately 60 miles per hour (flaps deflected; landing
gear down; maxirnum continuous power Center of graviLy
4.16 inches to right of thrust kne; Brewster XSBA-1 airplane.


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