Crim, Almer D

Reeder, John P

Whitten, James B

English

NASA Technical Reports Server

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~ NATIONAL ADVISORY COMMITTEE 
~ FOR AERONAUTICS 
TECHNICAL NOTE 2761 
~"rr2S'52 
BUfllNESS AND 
TECHNICAL BRANCH 
INSTRUMENT - FLIGHT RESULTS OBTAINED WITH A 
COMBINED -SIGNAL FLIGHT INDICATOR 
MODIFIED FOR HELICOPTER USE 
By Almer D. Crim, John P . Reeder, and 
James B. Whitten 
Langley Aeronautical Laboratory 
Langley Field, Va. 
Washington 
August 1952 
GOVT. Du~. 
https://ntrs.nasa.gov/search.jsp?R=19930083536 2020-06-17T20:18:46+00:00Z
1M NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS 
TECHNICAL NOTE 2761 
INSTRUMENT -FLIGHT RESULTS OBTAINED WITH A 
COMBINED- SIGNAL FLIGHT INDICATOR 
MODIFIED FOR HELICOPTER USE 
By Almer D. Crim, John P. Reeder, and 
James B. Whitten 
SUMMARY 
A commercially available flight indicator which combines heading, 
bank- angle, fuselage-pitch- attitude, and, if desired, altitude and radio-
aid information was flight - tested in order to determine its suitability 
for helicopter instrument flying . The indicator was installed in 
the instrument panel of a single - rotor heli copter specially equipped 
for blind flying and was evaluated by performing prescribed maneuvers 
with conventional instruments and with the combined-signal flight 
indicator. 
The instrument was then modified by adding a fuselage -rate - of-pitch 
signal and by decreasing the bank signal required for canceling a heading-
deviation signal. 
The use of the unmodified indicator for helicopter blind flying 
waS found to result in less fatigue to the pilot and to require less 
concentration than the use of conventional instrumentsj also, the 
ability to maintain a given heading was greatly improved . The addition 
of the rate - of- pitch signal and increased bank sensitivity resulted in 
more pr ecise control of heading , altitude, and airspeed, as ,yell as a 
reduction in amplitude and frequency of control motion . 
INTRODUCTION 
The National Advisory Committee for Aeronautics is engaged in a study 
of problems associated with helicopter instrument flying in order to deter-
mine the flying qualit ies and flight aids that are necessary for all -weather 
operation. Preliminary results of this program were reported in refer-
ence 1, wherei n the conclusi on was reached that both the flying qualitie s 
of the helicopter and pilot's instruments would require improvement before 
satisfactory helicopter instrument fli ght would be possible throughout the 
2 NACA TN 2761 
speed range. In view of these results, an investigation of the instru- ~ 
ment problems appeared desirable before the determination of satisfactory 
stability and control characteristic s for instrument flight is attempted. 
One approach to the problem of providing more suitable instruments 
is t o combine on a single indicator information that is usually obtained 
from several different instruments and to present this information in a 
manner easily interpreted by the pilot. A commercially available 
instrument emp loying such a presentation was obtained and modified 
along line s suggested by the problems reported in reference 1. Some 
results of flight tests obtained with this instrument, which for con-
venience is referred to a s a combined-signal flight indicator, are 
given in the pre sent paper . 
DESCRIPTI ON OF FLIGHT INDICATOR 
Basic Instrument 
The comb i ned- signal f l ight indicator is a gyroscopic flight and 
navigation instrument which combines heading, bank-angle, fuselage-
pitch- attitude , and, if desired, a l titude and radio- a id information 
and presents it to the p ilot on a two - element indicator. As may be 
seen in figure 1, the face of the indicator consists of a horizontal 
bar and a vertical bar which move at right angles t o each other and 
a fixed background reference marked wi th suitable scales . The 
vert i cal bar is actuated by heading signals from a gyroscopic compass , 
bank s i gna l s from a vertical gyro, and, at the discretion of the pilot, 
radio track s i gnals . The horizontal bar is actuated by fuselage -pitch-
att i tude signal s and, if des ired , by a constant- altitude or radio 
glide - path signal . Figure 2 illustrates the sensing of the indicator 
bars for various conditions . 
Flight by means of the indicat or is accomplished, once the desired 
fl i ght path has been se l ected, by keeping the two crossbars centered, 
or zeroed, over the reference marker of the instrument. Displacement 
of the vertical bar due to bank angle, heading, or radio - track devia -
tions may be cance l ed by banking the aircraft in the proper direction . 
Similarly, the indicat ions of deviation in pitch attitude, altitude, 
or radio glide path, as shown on the horizontal bar, may be canceled 
by nosing the aircraft up or down . In any case , as long as the 
indicator remains zeroed the aircraft is either on or is approaching 
the desired f light path . 
As an example of the principle of the combined- signal flight indi -
cator, a deviation from an established heading will cause a displacement 
------~~~ - -_ . . . -
, 
NACA TN 2761 3 
of the vertical pointer) but by banking the aircraft in the proper direc-
tion) the pilot can return the bar to center immediately. Because the 
bank angle required to cancel the heading signal is proportional to the 
heading deviation, the pilot must gradually reduce the amount of bank, 
as he approaches the desired course, in order to keep the indicator bar 
zeroed. The heading correction is thus made smoothly and without over-
shoot. In order to maintain altitude or follow a radio glide path a 
similar technique is employed) that of nosing the aircraft up or down 
in response to indications of the horizontal pOinter. 
In order to avoid the possibility of overcontrolling, the constant-
altitude and glide-path signals to the indicator are limited so that in 
no case will more than 60 of pitch-attitude change be required to cancel 
the deviation signal) whereas the sum of the heading and radio track 
signals are limited to a value that will never require more than 200 of 
bank to cancel the deviation signal. 
Modif ica t ions 
Inasmuch as the flight indicator just described was designed for 
use in airplanes) certain modifications to the instrument were made in 
order to evaluate its usefulness for helicopters. These modifications 
consisted of adding a fuselage - rate-of-pitch signal to the horizontal 
bar and reducing the bank angle required to cancel a given heading-
deviation signal. 
As a means for enabling the pilot to anticipate changes in attitude 
of the helicopter) indication of rate of change of attitude about all 
three axes of the machine would seem desirable. However, the difficulty 
of maintaining low airspeeds and the large and frequent longitudinal 
control motions sometimes necessary suggested the possibility of signifi-
cant improvement by simply adding a fuselage - rate - of-pitch signal to the 
combined-signal flight indicator. Accordingly, an electrical signal 
proportional to the rate of fuselage pitch was added to the horizontal 
bar of the indicator, the instrument thus providing the pilot with 
information about both attitude and rate of change of attitude. Means 
were provided whereby the pilot could vary this fuselage-rate-of -pitch 
signal from zero to its maximum value, the maximum value giving full-
scale deflection of the bar for a rate of pitch of 17 degrees per second. 
As noted in reference 1) the helicopter) because of i ts relatively 
low speed) experiences very high rates of turn at bank angles considered 
normal for airplanes. The flight indicator was therefore further modified 
by increasing the sensitivity of the instrument in roll and thus reducing 
the bank angle required to cancel a given heading- deviation signal. 
Since the bank angle necessary for a given rate of turn is a function 
------ - -
4 NACA TN 2761 
of forward speed, means were provided whereby the pilot could select 
different values of instrument sensitivity . These values ranged from 
the normal setting f or an airplane, which required a bank angle of 200 
to cancel a maximwn heading- deviation signal, to one which required 
only 50 of bank to accomplish the same purpose. 
TEST MEI'HODS 
The combined-signal flight indicator was installed in the rear 
instrument panel of a single-rotor helicopter (fig. 3) equipped with 
dual controls and a hood, or curtain, which permitted the rear pilot 
to fly without outside visual reference. The instrwnent was evalu-
ated by comparing results of a series of prescribed maneuvers flown 
with conventional instrwnents, with the unmodified flight indicator, 
and with the indicator after modif ication. The conventional instru-
ments used for this evaluation consisted of an artificial horizon, 
directional gyro, altimeter, and airspeed, turn-and-bank, and rate-
of - climb indicators (fig. 4) . 
Since the combined- signal flight indicator shows only deviation 
from a desired flight path or changes from trim attitude, other instru-
ments are still necessary for cross - checking and for establishing 
desired flight condition s . However, when flight records were desired 
by using only conventional instruments, the combined-signal flight 
indicator was covered . In most of the cases, records of a given 
maneuver made with the various instrument combinations were obtained 
during the srune flight . 
Two pilots, both experienced in visual and instrument helicopter 
flying , performed the flight tests . The evaluation wao based on the 
opinions of the pilots and r ecords, obtained with standard NACA recording 
instruments , of heading, airspeed, altitude, control position, pitching 
velocity , and rolling velocity . 
The radio -aid channels of the flight indicator were not used in 
the present investigation, since the necessary receivers were not 
available in the test helicopter. 
RESULTS AND DISCUSSION 
Unmodified Flight Indicator 
A comparison of flight records of maneuvers performed with conven-
tional instruments and with the unmodified flight indicator reveals that 
,. 
NACA TN 2761 
the most outstandi ng difference is in heading deviation. As noted in 
reference 1, precise control of helicopter heading is difficult under 
instrument conditions, particularly at low speeds. Figures 5(a) and 
5(b) show records of heading, altitude, and airspeed made during a 
maneuver involving straight and l eve l flight while changing airspeed 
from 65 to 25 knots. The record made with conventional instruments 
5 
shows large, inadvertent heading variations, at one point exceeding 200 • 
In comparison, headings are held within very close limits when the same 
maneuver is flown with the combined- Signal flight indicator. Examina-
tion of the records showed no significant change in frequency or amplitude 
of control motion between the two maneuvers and little difference in 
altitude and airspeed. The opinion of the pilots was, however, that the 
flight indicator made instrument flying less fatiguing and required less 
concentration than with conventional i~struments alone. 
As previously mentioned, a constant-altitude signal is available, 
at the discretion of the pilot, in the flight indicator. Any deviation 
from an established altitude causes a displacement of the horizontal 
bar, which may be returned to zero by a pitch signal produced by nosing 
the aircraft up or down. If changes in airspeed are not objectionable, 
this procedure normally provides an easy method for correcting small 
altitude deviations. However, at speeds below that for minimum power 
(about 45 knots for the test helicopter), a decrease in airspeed 
requires more power to maintain level flight, and any attempt to 
correct an altitude loss simply by raising the nose of the helicopter 
results in decreased speed and further loss of altitude . The pilots 
concluded that the constant-altitude signal , in its present f onn, was 
useful only above 45 knots and could lead to control difficulties at 
lower speeds . For this reason, the constant -altitude signal was not 
used when comparing the combined- signal flight indicator with standard 
instruments ; however, the horizontal bar of the indicator still provided 
fuselage -pitch information . 
Modified Flight Indicator 
-
The addition of a fuselage - rate -of -pitch signal to the combined-
signal flight indicator enab l ed the pilot t o maintain more precise 
control of the helicopter under instrument conditions. Figures 5(b) 
and 5(c) show the flight records of heading, altitude, and airspeed 
made with the unmodified flight indicator and with the same instrument 
after a rate-of-pitch signal was added . Although the previous comparison 
of conventional instruments and the unmodified flight indicator 
(figs. 5(a) ~nd 5(b)) showed little difference in airspeed and altitude 
variations, such variations are noticeably reduced after addition of the 
rate-of-pitch signal. Also of interest is the improvement in heading 
control when the rate-of -pitch signal is used. This improvement 
app~rently results from the fact that, when less difficulty is experienced 
6 NACA TN 2761 
with longitudi na l control , more time is available for the pilot to devote 
to di rectional contr ol. 
The rate -of -pitch s i gnal could be varied in flight from the maximum 
available, which pr oduced full - scale deflection of the horizontal bar for 
a rate of pitch of 17 degrees per second, to zero. The magnitude of the 
rate - of-pitch signal did not appear to be critical and a value which pro-
duced half-scale defl ection for a pitching rate of 17 degrees per second 
was found t o be satisfactory (this was the value used in obtaining the 
records shown in fig . 5(c)). Full - scale deflection of the horizontal 
bar of the indicator may a l so be obtained by a 150 pitch-attitude change. 
The second modification t o the flight indicator, that of increasing 
the bank sensitivity and thus reducing the bank angle required to cancel 
a given heading deviation, resulted in improvement in the ease and 
accuracy of turning t o and estab l ishing a desired heading. Figure 6 
shows typical time histories at 25 knots of turns to selected headings 
using the combined- signal flight indicator. Figure 6(a) illustrates the 
high rate of turn (about 14 degrees per second) involved and the conse-
quent overshooting of the desired heading for an instrument sensit ivity 
which required 200 of bank to cancel a maxDnum heading-deviation signal. 
In contrast, figure 6(b) shows the better control resultin§ from a lower 
rate of turn when the required bank angle was reduced to 6. Although 
the instrument was noticeably sensitive to small disturbances at the 
latter setting, flight records indicate that the frequency and magnitude 
of control motion was reduced, particularly at low speeds, by using the 
smaller bank angles. 
Inasmuch as the most satisfactory value of instrument sensitivity 
in bank i s a function of forward speed, some compromise is necessary if 
a fixed value is used. In the present tests an instrument setting of 
100 bank to cancel a maximum heading-deviation signal was found to be 
satisfactory over the speed range from 25 to 75 knot s. 
CONCLUSIONS 
A flight indicator which combines heading, altitude, bank-angle, 
and pitch information was evaluated by means of instrument flight in a 
single - rotor helicopter, and the following conclusions were reached: 
1. The use of the unmodified combined-signal flight indicator for 
he l icopter blind f l ying resulted in less fatigue to the pilot and 
required le ss concentration than with the use of conventional instru-
ments alone. In particul ar, the ability to establish and maintain a 
given heading was greatly improved. 
, 
NACA TN 2761 7 
2. The addition of a fuselage-rate-of-pitch signal to the original 
instrument resulted in more precise cont rol of airspeed, altitude, and 
heading . 
3. The bank angle required to cancel a given heading-deviation 
signal of the instrument should be smaller for the helicopter than the 
airplane. Bank angles considered satisfactory for airplanes resulted 
in excessive rates of turn for the test helicopter. 
4. A constant-altitude signal, as used in the test instrument, is 
undesirable for the helicopter at low speeds, although helpful at high 
speeds. At speeds below that for minimum power, maintenance of constant 
altitude by means of simple attitude changes is not feasible. 
Langley Aeronautical Laboratory, 
National Advisory Committee f or Aeronautics, 
Langley Field, Va. , May 23, 1952. 
REFERENCE 
1. Crim, Almer D., Reeder, John P., and Whit ten, James B.: Initial 
Results of Instrument-Flying Tria l s Conducted in a Single -Rotor 
Helicopter. NACA TN 2721, 1952. 
8 NACA TN 2761 
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Figure 1 .- Comb i ned- signal f l ight i ndicator . 
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Figur e 2. - Sensing of flight i ndicator for various conditions . 
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Figure 4.- Instrument panel used in evaluating combined-signal flight 
indicator. 
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Head ing dev iat ion, 
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NACA TN 2761 
Alt i tude, ft 2000t~==~==~====~====~==~ 18  
(a) Standard instruments. 
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Heading deviation, 0 
deg - 5 
70~ _ _ ___ 
Ai rspeed, knots 5 0 
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A l t i tude, ft 
2000~ ________________ ~ __________ --
1800 
~ ______ L-______ L-____ ~L-____ ~ ______ ~ 
(b) Unmod if ied fli ght ind i cato r. 
Head ing dev ia tion, 6 
deg 
Airspee d , kn ots 
A l t i t ude , f t 
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7 0t----..--~ 
5 0 
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2000~ ________________________ ___ 
1800 
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Ti me, sec 
(c) Fli gh t in dicator with pitch r ate. 
100 
Figure 5 ·- Comparison of maneuver (straight and level flight with change 
i n air spe ed from 65 to 25 knots) flown with standar d instruments and 
comb i ned- signa l f light i ndicator . 
3M NACA TN 2761 13 
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(a) 20° bank cancels maximum heading- deviation s ignal. 
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Time, sec 
(b) 6° bank cancels maximum heading- deviation signal . 
Figure 6. - Heading records obtained during 90° turns . Airspeed approximately 
25 knots . 
NACA-Langley - 8-13-a2 - 1000 


Instrument-flight Results Obtained with a Combined-signal Flight Indicator Modified for Helicopter Use

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