Anderson, Seth B

English

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NATIONAL ADVISORY COMMITTEE 
FOR AERONAUTICS 
TECHNICAL NOTE
No. 1868
CORRELATION OF PILOT OPINION OF STALL WARNING
WITH FLIGHT MEASUREMENTS OF VARIOUS FACTORS
WHICH PRODUCE THE WARNING 
By Seth B. Anderson 
Ames Aeronautical Laboratory 
Moffett Field, Calif. 
Washington 
April 1949
https://ntrs.nasa.gov/search.jsp?R=19930082543 2020-06-17T22:00:57+00:00Z
I 
NATIONAL ADVISORY COH'ttTTEE FOR AERONATJTICS

TECICAL NOTE NO. 1868 
C0RELATION OF PflIOT OPINION OF STALL WARNING

WITH FLIGR'T MEASUREMENTS OF VARIOUS FACTORS
WHICH PRODUCE THE WARNING 
By Seth B. Anderson 
SUMMARY 
A study was made to correlate pilots' opinion of the stall warning 
properties of' 16 airplanes with a number of quantitative factors 
obtained from tiEie history records for speeds near the stall. The 
results indicate that, in general, the stall warning was considered 
satisfactory by the pilots when characterized by any of the following 
qualities: (a) airplane buffeting at speeds from approximately 3 to 
15 miles per hour above the stalling speed and of a magnitude to 
produce incremental indicated values of normal acceleration factor 
of 0.04 to 0.22, (b) preliminary controllable rolling motion of 0.04 
to 0.06 radian per second occurring anywhere within a range from 
approximately 2 to 12 miles per hour above the stalling speed, and 
(c) rearward travel of the control stick of at least 2.75 inches 
during the 15-mile—per—hour speed range immediately preceding the 
stall. The magnitude of the rolling velocity in the complete stall 
influenced the degree of buffet and stick movement required for 
satisfactory warning.
INTRODUCTION 
The need for the establishment of quantitative design criteria 
for describing the flying qualities of aircraft has been generally 
well recognized and met during the past several years. These criteria 
evolved from an analysis of a large volume of quantitative flight data 
correlated with pilot opinion and were reported originally in reference 1. 
Later these various criteria were adapted and revised to apply spec if i-
cally to military aircraft. 
At present it is generally possible to compare all the flying—
qualities characteristics with specific quantitative requirements 
except for the characteristics regarding stall warning and the behavior
2	 NACA TN No. 1868' 
of the airplane in the complete stall. Some work relating to quanti-
tative factors in the complete stall has been presented in reference 2. 
In order to provide a preliminary basis for a quantitative evalu-
ation of the stall—warning characteristics of an airplane, an effort 
is made herein to correlate a number of measurable factors with pilots' 
opinion of the adequacy of the stall warning. The data for this study 
were obtained from flight tests of various airplanes at the Ames 
Aeronautical Laboratory. 
INSTRUME1JTAT ION 
Standard NACA recording instruments syncbronized by an NACA timer 
were used to record normal acceleration, stick force, elevator angle, 
roll and pitch velocities, airspeed and altitude. The accelerometer 
was secured approximately at the test center of gravity for each air-
plane and the control—position recorders were located as close to the 
control surfacesas possible. In some instrument installations the 
pilot was able to identify the first indication of stall warning in 
the time history by means of a film marker. 
A free—swiveling airspeed head mounted approximately one chord 
length ahead of the wing leading edge was used in determining cali-
brated airspeed.
TESTS 
The data have been taken during stalls of 16 airplanes ranging 
from single—engine fighter to four—engine bomber types. The tests 
were made with the center of gravity at the normal position, within 
an altitude range of ,OOO to 12,000 feet, and in straight flight by 
gradually approaching the stall with the normal acceleration factor 
as close to unity as possible. For the various tests, pilots' notes 
were available in which opinions of the stall warning were expressed. 
The airplanes were flown by NACA test pilots having varied backgrounds 
of naval, military, research, and. commercial flying experience. Five 
pilots participated in the tests, although each airplane was not 
necessarily flown by all five pilots. 
ANALYSIS OF DATA 
The current flying—qualities specifications state that the 
approach to the complete stall shall be accompanied by a definite
NACA TN No. 1868	 3 
stall warning consisting of one or more of the following, preferably 
the first: (1) buffeting and. shaking of the airplane and controls, 
(2) marked increase in rearward travel of the control column or marked 
Increase of control force for further speed reduction, (3) prelimInary 
development of the stall through small amplitude pitching and. rolling 
motion. The requirements also state that the stall warning shall occur 
at a speed not less than 1.05 or more than 1.15 times the stalling 
speed for each of the various flight configurations. 
quantItative values of the following factors were considered in 
an attempt to provide correlation with pilot opinion of the adequacy 
of the stall warning: (1) the amount of airplane buffet indicated by 
the accelerometer, (2) the amount of shaking of the elevator shown by 
the control position recorder, (3) the amount of elevator buffet shown 
by the stick force recorder, (14.) the maximum amplitudes of roll and 
pitch velocities, and. (5) the amount of change in elevator stick posi.-. 
tion and control force prior to the stall. 
During the analysis it was found that a number of the afore-
mentioned items did. not correlate with the pilot's impression of the 
adequacy of the stall warning. Control shaking shown by the elevator 
control—position recorder did not give a good indication of the result-
ant feel of the control stick experienced by the pilot probably because 
of play and/or stretch in the control system. Another item which was 
eliminated was the amount of buffet shown by the force variation at 
the control—stick grip. This showed no consistent correlation with 
buff et or shaking of the controls possibly because of friction in the 
control system and the fact that the force recorded depended upon the 
amount of restraint supplied by the pilot. The measurement of small 
amplitude pitching motions was not used since in no case was pitching 
motion encountered without the presence of either rolling motion or 
airplane buffet. For this reason it was felt that measuremant of 
factors tending to produce pitching motion were covered satisfactorily 
by the measurements of either preliminary rolling motion or airplane 
buffeting. Sufficient data were not available to establish a relation-
ship between pilot opinion of stall warning end the amount of increase 
in control force for further speed reduction. The only items found 
which produced consistent quantitative measurements and could be 
correlated with pilot observations of the stall warning were normal 
acceleration, rolling velocity, and elevator—control position. 
It should be noted that the absolute magnitude of the values 
given by the accelerometer are in error due to Inability of the 
accelerometer to measure true changes In acceleration except for 
very small amplitudes or frequencies far lower than its natural 
period. For the range of buffeting frequencies encountered
14.	 NACA TN No. 1868 
(approximately 6 to 114. cycles per second), the accelerometer readings 
were approximately correct for incremental values of 0.05 from a mean 
value of about 1, but increased in error with increase in incremental 
acceleration so that at an incremental acceleration factor of 0.25 the 
low—frequency incremental acceleration values were 7 percent low and 
the high—frequency incremental values were 25 percent low. 
RESDLTS MID DISCUSSION 
Pilot opinion of the stall warning has been correlated with the 
results of the measurements obtained from the accelerometer, roll-
turnmeter, and elevator—control position recorder, and are presented 
herein. In the discussions it should be noted that the acceptance of 
each stall warning is based. on pilot opinion of that particular type 
of warning. 
The results of the measurements obtained by the accelerometer 
are shown in figure 1, in which the indicated values of the incremental 
normal acceleration factor Az (measured from a mean value), due to 
buffeting at various speeds above the stall, are shown with pilot 
opinion of the stall warning. In this figure, the points of initial 
buffet and. buffet at stall have, in most cases, been connected by a 
straight line, since it was found that the variation of incremental 
indicated acceleration factor with airspeed was approximately linear. 
Boundaries have been drawn showing the satisfactory and unsatisfactory 
regions as determined from the comments of several pilots. 
The incremental value of Az measured at the first point of 
airplane buffet was approximately 0.0 11. which, judging from the indica-
tion given by the pilot's film marker, was the smallest change in 
acceleration that the pilot was able to detect. The ability of the 
pilot to detect the lower limit of buffeting is influenced by a number 
of factors such as its frequency, the presence of other transverse 
vibrations, and by the intensity of noise (reference 3). In. this 
regard jet—propelled aircraft would possibly offer more ideal condi-
tions for detecting stall warning produced by buffeting. 
Judgment of whether or not the stall warning is satisfactory at 
the first indication of airplane buffet is dependent not only on the 
initial amplitude but also on the speed above stall where buffet first 
begins and on how rapidly the buffet increases in amplitude with 
decreasing airspeed. A region on the right of figure 1 is shown where 
buffeting did not serve satisfactorily as a stall warning for a number 
of reasons. At speeds in excess of approximately 15 miles per hour 
above the stall the buffeting was too far removed from the complete
NACA TN No. i868	 5 
stall to serve as a satisfactory warning of the approaching stall. 
While it was not possible t ,o accurately establish this speed due to 
lack of data the point selected corresponds approximately to that used 
in the current flying—qualities specifications. Another unsatisfactory 
aspect of the early buffet is that the buffet might build up to such 
great magnitude as the stall is approached that the pilot fears struc-
tural damage to the aircraft. This region.where structural damage to 
the airplane was expected is shown in the upper part of figure 1. 
In addition, large magnitude buffeting occurring over a long period 
before the stall would be particularly objectionable to a pilot in 
landing. 
The boundary for the lowest speed above the stall where the pilot 
would accept buffeting as a satisfactory stall warning is somewhat 
obscure due to the interaction of several factors. However, the general 
trend of data seems to indicate that buffet should occur approximately 
3 miles per hour above the speed of stall for a satisfactory warning of 
the approaching stall. In some cases it was found that the pilot would 
consider the warning satisfactory with little or no airspeed margin 
above the stall as shown on the extreme left—hand part of figure 1. 
This condition was probably due to the fact that sometimes a finite 
time delay existed between the attainment of a minimi. airspeed value 
and the complete flow breakdown producing the stall. This time delay 
amounted to as much as 3 or 11. seconds for some airplanes. During this 
period if the pilot experienced buffeting and the roll—off In the complete 
stall was not too severe, the warning may have been judged satisfactory. 
The tendency for the behavior of the airplane in the complete 
stall to influence pilot opinion of the stall warning is shown in 
figure 2 where the amount of buffet and the roll—off in the complete 
stall are given with pilot opinion of the stall warning. These data 
(fig. 2), judging only from the magnitude of buffet at the complete 
stall appear to indicate that correspondingly larger values of buffet 
are required for satisfactory warning as the magnitude of roll—off in 
the complete stall increases. Another point of interest shown in this 
figure, which ordinarily may not be realized, is that relatively large 
amplitudes of buffet can occur with large values of roll—off at the 
stall.
The values of preliminary rolling motion preceding the stall, 
presented with pilot opinion of the stall warning, are shown in 
figure 3. These data indicate that the rolling velocity must be at 
least 0.02 radian per second (1.15 deg/sec) to be perceptible to the 
pilot as a stall warning. It is also shown that the magnitude of the 
rolling motion must be at least 0.011. radian per second to be considered
6	 NACA TN No. 1868 
satisfactory as a stall warning. Because ample data were not avail-
able, no attempt was made to establish an upper limit for the value 
of rolling velocity required for a satisfactory stall warning. In one 
case at a rolling velocity of 0.22 radian per second the pilot con-
sidered. the roll-off to be more characteristic of the complete stall 
rather than the stall warning as he stated, "roll too large to be 
considered as a warning." From this it appears that the upper limit 
of the value of rolling velocity which could be used as an acceptable 
stall warning would not exceed 0.22 radian per second and would probably 
be somewhat lower. Indications are that the pilots generally did not 
consider rolling-velocity values in excess of approximately 0.06 radian 
per second as a satisfactory stall warning since information was non- - 
existent above this value. The results shown in figure 3 appear to 
indicate little or no influence of change in airspeed on the magnitude 
of the value of rolling velocity required for a satisfactory warning. 
From the data available, the speed range over which stall warning was 
satisfactory has been established as 2 to 12 miles per hour above the 
stalling speed. In most cases the rolling motion was of an oscillatory 
nature with a frequency of the order of 0.3 cycle per second. In a 
few cases the acceptance of the rolling motion as a warning was influenced 
by the presence of another type of stall warning; that is the pilot 
stated, "stall warning occurred in the form of rolling and buffeting 
of the airplane.?? 
The data of figure 14. show the rearward movement of the control 
stick (at the grip) for the 15-ni1e-per hour range immediately above 
the stall correlated with pilot opinion of the stall warning. For 
ease in evaluating this type of stall warning, the stick movement-
airspeed variation has been presented as a straight line. In figure 1j. 
three regions are shown defining the acceptability of stick movement 
as a stall warning, since it was found that the pilots' opinion of this 
stall warning was influenced by the amount of roll-off experienced in 
the complete stall. The regions shown in figure 11. have been established 
with aid of data from figure 5 which show the relationship between the 
stick travel and the roll-off at the stall for the last 15 miles per 
hour above the stall correlated with pilot opinion of the stall warning. 
In region (1) of figure 14., a stick movement greater than 2_3/14. inches 
was considered satisfactory for values of rolling velocity up to 0.3 
radian per second. (For the tests covered. herein, data were nonexistent 
for large values of stick travel with roll-off velocities beyond 0.3 
radian per second. See upper portion of fig. 5). In region (2) the 
stall warning was found to be marginal, depending on the magnitude of 
the roll-off at the stall; that is, the warning was satisfactory for 
small values of rolling velocity and unsatisfactory for large values 
(data taken from the middle region in fig. 5). One and three-quarters
NACA TN No. 1868
	
7 
inches or less of stick travel was unsatisfactory as a warning 
regardless of the degree of roll—off at the stall (region 3). 
CONCLUSIONS 
A correlation of pilot opinion of the stall—warning character-
istics of' 16 airplanes with quantitative factors taken from time 
history records at speeds near the stall indicated the following: 
1. In general, stall warning was considered satisfactory by 
the pilots when any of' the following conditions was present: 
(a) airplane buffeting occurring at speeds from approximately 3 to 
15 miles per hour above the stalling speed and resulting in an 
incremental indicated acceleration factor of 0.014 to 0.22; (b) pre-
liminary controllable rolling motion from 0.04 to 0.06 radian per 
second occurring anywhere within a range from approximately 2 to 12 
miles per hour above the stalling speed; and (c) rearward movement of' 
the control stick at the grip o± at least 2.75 inches during the 15-
mile—per—hour speed range immediately preceding the stall. 
2. The degree of' buffeting and rearward movement of the control 
stick considered satisfactory as a stall warning was influenced by 
the maitude of the rolling velocity in the complete stall. 
Ames Aeronautical Laboratory, 
National Advisory Committee for Aeronautics, 
Moffett Field, Calif. 
REFERENCES 
1. Gilruth, R.R.: Requirements for Satisfactory Flying Qualities 
of Airplanes. NACA Rep. 755, 1914.3. 
2. Abzu.g, Malcolm J.: Flying Qualities Requirements for Military 
Aircraft. Aeronautical Engineering Review, vol. 5 no. 10, 
Oct. 1946, pp. 16-22. 
3. Best, Stanley G.: Propeller Balancing Problems. SAE Journal, 
vol. 53, no. 11, Nov. 19145, pp . 648-659.
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NACA TN No. 1868 
Pilot op/n/on of stall, warning 
+ Satisfactory 
- Unsatisfactory 
Region 
® Satisfactory for all values of 
roll-off at the stall (up to 
03 rad/sec). 
® Marginal depending on roll-
off at stall. 
® Unsatisfactory regardless of 
roll-off at stall 
.	 6 
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0	 2	 4	 6	 8	 /0.	 /2	 /4/6
Speed above stall, mph 
Figure 4.- Correlation of pilot opinion of stall warning with rearward 
movement of control stick in theY speed range immediately above 
the stall.
NACA TN No. 1868	 13 
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foiling velocity at stall, rod/sec 
FIgure 5.- Correlation of pilot opinion of s/all warning with 
rearward movement of control stick for the last /5 
miles per hour above the s/all and roil off at the stall. 
—4-
____
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stall warning 
+	 Satis factory 
-	 Unsatisfactory 
_____ _____ _____ 
—+------+----
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Correlation of pilot opinion of stall warning with flight measurements of various factors which produce the warning

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