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TECHNICAL NOTE
D-1233
EFFECTS OF COUPLING BETWEEN PITCH AND ROLL CONTROL INPUTS ON THE HANDLING QUALITIES OF VTOL AIRCRAFT By John F. Garren, Jr.
Langley Research Center Langley Air Force Base, Va.
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION
WASHINGTON March 1962
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION TECHNICAL NOTE D-1233 EFFECTS OF COUPLING BETWEEN PITCH AND ROLL CONTROL INPUTS ON THE HANDLING QUALITIES OF VTOL AIRC_ By John F. Garren, Jr.
SUMMARY Various VTOL aircraft configurations which have exhibited coupling between pitch and roll inputs, requiring a combination of longitudinal and lateral motions of the stick to obtain a pure pitch or a pure roll response, have often given rise to adverse pilot comment. In order to provide a basis for establishing handling qualities criteria with respect to cross controlling, both instrument and visual flight tests were con- ducted in which the pilot was required to cross control (use a combina- tion of longitudinal and lateral stick motion) to achieve a pure response.
The results of these tests indicate that for control phase angles (the angle through which the pilot must cross control to obtain a pure response) of up to 20 ° , the pilot is aware of coupling only when making large corrections or inputs. However, it was found that control phase angles greater than 35 ° resulted in unsatisfactory handling qualities for an aircraft with otherwise satisfactory to optimum characteristics.
INTRODUCTION Various VTOL aircraft configurations which have exhibited coupling between pitch and roll inputs, requiring a combination of longitudinal and lateral motions of the stick to obtain a pure pitch or a pure roll response, have often given rise to adverse pilot comment (ref. i).
There are a number of possible sources of cross coupling which give rise to the need for cross controlling. For example, two sources in helicopters are partial restraint of rotor flapping and variation of blade lag angle with power. The existence of such conditions results in an undesirable component of moment at right angles to the direction of the control input. Also, the overall effect of some types of mechan- ical augmentation equipment, being dependent on rotor speed, can produce coupling for certain flight conditions.
In theory 3 it is possible in somecases to removethe need for
cross controlling by simply skewing the pltch-roll control system. In
practice, however, such considerations as the change in lateral stick
trim position with forward speed limit the degree of built-in skewness
which can be tolerated. In fact 3 the shift in the lateral trim posi-
tion with speed can be so large (up to 45°) as to invite deliberate
skewing of the control in order to minimize the apparent change in trim
position. Therefore, the amount of cross coupling in the control system
normally represents the results of a design compromise.
L
The problem of gyroscopic cross coupling between pitch and roll has
been treated, in its own right, in reference 2. However, the response
produced by gyroscopic coupling (and also the technique of cross con-
trolling to compensate for gyroscopic coupling) is sufficiently unique
for the results of that investigation not to be generally applicable to
other types of coupling.
In order to provide a basis for establishing handling qualities
criteria with respect to cross controlling, flight tests were conducted
in which it was necessary for the pilot to cross control to achieve a
pure response. The investigation included both instrument and visual
flight conditions with the controls skewedby various amounts so that
cross-control angles ranging from 0° to 55° were required to obtain a
pure response.
TESTEQUIPMENT ANDPROCEDURES
Test Helicopter
The variable-stability helicopter shown in figure 1 was used in
the research flights of this investigation. Reference 3 describes the
variable control system of the test helicopter which makes it possible
to vary both the ratio of control momentto stick deflection (i.e.,
control power) and the apparent angular-veloclty damping about each of
the three principal inertial axes. The test vehicle is equipped to
record angular velocities about all three axes as well as to record all
control motions of the pilot. The general physical characteristics of
the helicopter are given in table I.
The in-flight simulation of cross coupling was achieved by skewing
the electronic plck-offs on the cyclic control stick (pitch and roll
control) with respect to the longitudinal axis. A sketch of the modi-
fication to the cyclic control stick used to simulate coupling between
control inputs is shown in figure 2.
Flight Conditions The effects of coupling between pitch and roll control inputs were evaluated during the following flight conditions: (i) Low-speed instrument-landing-system (ILS) approaches (2) Circling patterns which involved a series of maneuvers including a take-off, hovering in ground effect, transitions to and from forward flight, and a vertical landing (5) Square patterns (approximately 150 feet square) while main- taining constant heading into the wind (4) Roll reversals at 45 knots starting from an established bank angle of 30 ° and terminating in a bank angle of 50 ° in the opposite direction. Roll rates of up to 45 deg/sec were used.
These maneuvers were performed at cross-control angles of up to 55 ° except for the landing phases of the circling pattern which could not be performed safely with extreme coupling. Tests were made with the controls skewed both clockwise and counterclockwise.
In an effort to determine the effect of the uncoupled aircraft control-response characteristics on the rate at which the controlla- bility deteriorates as the coupling is increased, the tests were per- formed both with the basic aircraft characteristics and with three times the control power and damping of the basic aircraft. The original values of control power and damping generally represent handling quali- ties which are marginally satisfactory to unsatisfactory for most maneu- vers. On the other hand, the increased control power and damping result in relatively good handling qualities.
The pilots were instructed to rate the overall controllability of the aircraft for each test condition. The Cooper pilot-opinion rating system, described in reference 4 and used in the evaluation, is pre- sented in table II. Using this rating system, the pilot assigned to each test condition a numerical rating of 1 to l0 with the smallest num- ber signifying optimum conditions and the largest number signifying cat- astrophic conditions.
RESULTS AND DISCUSSION The test results are presented in figure 3 as a plot of cross-control angle against pilot rating of overall controllability. The solid-line
curve represents results obtained with the control power and damping
increased about all axes) the dashed-line curve represents results
obtained wlth the basic control power and damping of the aircraft. No
difference in pilot opinion was noted for coupling in one direction as
opposed to the other.
For a given amount of coupling, the poorest rating of overall con-
trollability was consistently obtained during the landing maneuver. In
fact, thls was the only maneuverwhich could not be executed at 5_ ° cou-
pllng. At this extreme value of coupling it was not possible to steady L the aircraft for a sufficient period of time to permit touchdown. One other maneuver in which coupling appeared particularly objectionable was the rapid turn reversal. In this case, the degree of objection was attributed to the fact that the large control inputs caused the pilot to be more aware of the coupling. For the remainder of the tests (ILS approaches, square patterns, and the circular patterns excluding the landing phases), the pilot rating appeared to be reasonably independent of the maneuver.
Figure 3 indicates that the overall controllability was considered unsatisfactory at a cross-control angle of approximately 3_ ° for the air- craft configuration with increased control power and damping; whereas a control phase angle of 2_ ° was considered unsatisfactory wlth the basic control characteristics. It should be noted, however, that even wlth zero coupling, the basic aircraft is considered onlymarginally satisfactory.
Pilot commentary indicated that a control phase angle as large as 20 ° was only slightly noticeable except for maneuvers in which large corrections or inputs are required - for example, the rapid roll rever- sals. This conclusion may also be inferred from figure 3 which indicates an insignificant change in pilot rating as the control phase angle is increased from 0 ° to 20 °.
CONCLUSIONS Based on a flight investigation in which coupling between pitch and roll control inputs was simulated and in which the pilot was required to use a combination of longitudinal and lateral stick motion to achieve a pure pitch or a pure roll response, the following conclusions are drawn: i. For control phase angles up to 20 °, the coupling problem is gen- erally insignificant and the pilot is aware of an undesirable control response only when making large corrections.
2. Control phase angles greater than 35 ° result in unsatisfactory handling qualities for an aircraft with otherwise satisfactory to optl- mum characteristics.
3. Landing is the most critical maneuver from the standpoint of tolerating coupling.
Langley Research Center, National Aeronautics and Space Administration_ Langley Air Force Base, Va., January 23, 1962.
L i REFERENCES I. Connor, Andrew B., and Tapscott_ Robert J.: A Flying-Qualities Study of a Small Ram-Jet Helicopter. NASA TN D-186, 1960.
2. Garren, John F., Jr.: Effects of Gyroscopic Cross Coupling Between Pitch and Roll on the Handling Qualities of VTOL Aircraft. NASA TN D-812, 1961.
3. Salmirs, Seymour, and Tapscott, Robert J.: Instrument Flight Trials With a Helicopter Stabilized in Attitude About Each Axis Individually.
NACA TN 3947, 1957.
4. Cooper, George E.: Understanding and Interpreting Pilot Opinion.
Aero. Eng. Rev., vol. 16, no. 3, Mar. 1957, PP. 47-51, 56.
TABLE I.- PHYSICAL CHARACTERISTICS OF THE TEST HELICOPTER Gross weight, ib .............. . ......... 5,500 Moments of inertia: L Pitch, Iy, slug-ft 2 ..................... 7,000 Roll, IX, slug-ft 2 ...................... 2,000 Yaw, IZ, slug-ft 2 ...................... 5,000 Rotor rotational speed, radians/sec .............. 19.4 6.5 Height of rotor hub with respect to center of gravity, ft .
Control travel: Longitudinal cyclic, in ................. 13.6 Lateral cyclic, in ...................... 13.6 Pedal, in .......................... 4.75 Basic control power: Yaw, ft-lb/in, of control travel ............... 4,140 Basic damping: Pitch, ft-lb/radians/sec .............. 2,495 Roll, ft-lb/radlans/sec " " i ] . . 2,495 Yaw, ft-lb/radians/sec . . . . . . . . . . 10,600 O o o 0 0 o o L C o 9 0 ,4 +._ .,-i e_ ,.-4 0 I1) I1) o o +_ ,-I o 4.)
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