Clagett, Harry P

Drake, Hubert M

English

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C o' PX RESEARCH MEMORANDUM 
EFFECTS ON THE SNAKING OSCILLATION OF THE

BELL X-1 AIRPLANE OF A TRAILING-EDGE

BULB ON THE RUDDER
By Hubert M. Drake and Harry P. Clagett 
S DOCUMENT ON LOAN FROM THE nL Tgley Aeronautical Laboratory 
Langley Field, Va. 
NATIONAL ADVISORY COMMITTEE FOR AENUTrCS 
	
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v	 k4cupgpt	 ç4 -	 Information affecting the National DeO s'e of the United .States within the NATIONALAL ADVISORY COM
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NATIONAL ADVISORY COMMITTEE 
FOR AERONAUTICS 
WASHINGTON 
January 16, 1951 
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NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS 
RESEARCH MEMORANDUM
EFFECTS ON THE SNAKING OSCILLATION OF THE 
BELL X-1 AIRPLANE OF A TRAILING-EDGE 
BULB ON THE RUDDER 
By Hubert N. Drake and Harry P. Clagett 
SUMMARY 
A rudder bulb was installed on the trailing edge of the rudder of 
the Bell X-1 airplane having the 8-percent-thick wing and 6-percent-
thick tail. Several flights were made to investigate the effects of 
the bulb on the snaking oscillation at Mach numbers between 0.75 and 
1.0.
It was found that the rudder bulb had no noticeable effect on the 
snaking oscillation over the Mach number range tested.. 
INTRODUCTION 
The NACA is cooperating with .the U. S. Air Force in a program of 
flight research in the transonic speed range utilizing the Bell X-1 air-
plane having an 8-percent-thick wing and a 6-percent-thick tail. The 
airplane is maintained by the Air Force and flown by an Air Force pilot. 
NACA recording instruments are installed and the data reduction and 
analysis is performed by NACA personnel.
	
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In the course of this program, a bulb was placed on the rudder 
trailing edge to increase the tendency of the rudder to. float with the 
relative wind and, thus, increase the damping of the snaking oscillation 
which is experienced with the X-l. Such a bulb was installed for several 
flights and the results of these tests are presented in this paper.
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CN
	
normal-force coefficient 
M	 Mach number 
13	 sideslip angle 
p	 rolling velocity, radians per second 
p	 rolling acceleration, radians per second squared 
W	 airplane weigit,' pounds 
S	 wing area, square feet 
q	 dynamic pressure, pounds per square foot 
AIRPLANE AND INSTRUMENTATION 
The Bell X-1 airplane is a single-place rocket-propelled monoplane 
designed for flight research in the transonic speed' range. A detailed 
description of the airplane has been presented in reference 1 and a 
three-view sketch is shown as figure 1. 
Synchronized NACA instruments were used to record normal, transverse, 
and longitudinal acceleration, rolling velocity and acceleration, side-
slip angle, altitude, airspeed, and rudder, aileron, elevator, and sta-
bilizer positions. The rudder-position transmitter was sufficiently 
sensitive to indicate rudder movements as small as 	 and was fastened 
directly to the rudder. Because of lack of space, no pedal-force recorder 
could be installed. 
A rudder bulb consisting of half-round strips, 1/2 inch in diameter, 
was fastened alo'ngthe entire trailing edge of'the rudder except. in the 
region of the trim tab. A photograph of the installation is shown in 
figure 2.
TESTS, RESULTS, AND DISCUSSION 
In flight tests of the X-1 airplanes a small-amplitude, lightly 
damped lateral oscillation has been encountered over very nearly the
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entire range of Mach numbers at which the airplane has been flown. 
Examples of the oscillation and calculation of the lateral stability of 
the X-1 are given in reference 2. A number of possible causes were 
suspected and investigated. It was thought that fuel sloshing might be 
a contributing cause until flights with propellant tanks empty showed 
that the snaking also occurred in this condition. It was suspected that 
small motions of the rudder might cause the snaking at subsonic Mach 
numbers so flights, were made with the rudder fixed. The results of 
these flights are given in reference 3 and one of the figures from this 
reference presenting a rudder-fixed lateral oscillation of the X-1 air-
plane having the 10-percent-thick wing and 8-percent-thick tail is shown 
as figure 3. It was concluded that small rudder motions and fuel sloshing 
were not the primary causes of the oscillation although they might be 
contributing causes. In order to further investigate the effects of the 
rudder .on the snaking, the U. S. Air Force requested that tests be made 
on a rudder trailing-edge bulb. The purpose of the bulb was to modify 
the rudder hinge moments to increase the tendency of the rudder to float 
with the relative wind. Several flights were made with the bulb installed 
on the X-1 having an 8-percent-thick wing and a 6-percent-thick tail. 
Records were obtained of lateral oscillations occurring in steady flight 
and as a result of rudder disturbances. 
Time histories of lateral oscillations occurring in climbs are pre-
sentéd in figure 4 at Mach numbers of about 0.8 and 0.9 and in level 
flight in figure 5 at a Mach number of 0.99. These records were obtained 
with rockets operating and with varying amounts of fuel aboard as detailed 
in the figure legends. Time histories of oscillations resulting from 
rudder disturbances are presented in figure 6 for power-on flight at a 
Mach number of about 0.98 and in figure 7 for.power-off flight at a Mach 
number of about 0.79.. Although a roll turn-meter record was obtained for 
all of these time histories, a defective timer prevented synchronization 
of all but the time history shown in figure 7. 
The rudder traces of figures a) and li. (b) and the first part of the 
trace of figure 7 indicate that the rudder is moving as the airplane 
oscillates. No pedal forces were obtained so it is not possible to say 
whether the pilot was moving the rudder as a result of the oscillation. 
The rudder traces in figures 5, 6, and the last part of the , trace of 
figure 7 do not show any rudder movements that would be expected to 
materially affect the oscillation. In particular, the time history.' of 
figure-7 is of interest. The rudder initiates the oscillation and. con-
tinues to move as the oscillation damps to time 14 seconds when the rudder 
motion stops. The lateral oscillat1oi of the airplane, however, continues 
with nearly constant amplitude. This agrees with the data of reference 3 
in which it was shown that fixing the rudder hadlittle effect.
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CONCLUSIONS 
As a result of several flights of the Bell X-1 airplane with a rudder 
bulb installed, it was found that the rudder bulb had little effect on the 
snaking oscillation of the airplane at Mach numbers between 0.75 and 1.0. 
Langley Aeronautical Laboratory 
National Advisory Committee for Aeronautics 
Langley Field., Va..
REFERENCES 
1. Williams, Walter C., Forsyth, Charles M., and Brown, Beverly P.: 
General Handling-Qualities Results Obtained during Acceptance 
Flight Tests of the Bell XS-1 Airplane. NACA RM L8A09, 1948. 
2. Drake, Hubert M., and Wall, Helen L.: Preliminary Theoretical and 
Flight Investigation of the Lateral Oscillation of the X-1-Airplane. 
NACA HM L9F07, 1949. 
3. Drake, Hubert M.: Effects on the Lateral Oscillation of Fixing the 
Rudder and Reflexing the Flaps on the Bell X-1 Airplane. NACA 
RM L50IO5, 1950.
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Figure 2. -
 Photograph of the installation of a rudder bulb fastened along 
the trailing edge of the rudder.
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Figure 3.- Snaking oscillation of X-1 airplane with rudder fixed; 10-percent-
thick wing and 8-percent-thick tail; M = 0.84; CN = 0.19; altitude, 
28,000 feet to 5,000 feet. (From reference	 A





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Effects on the Snaking Oscillation of the Bell X-1 Airplane of a Trailing-Edge Bulb on the Rudder

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