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A IAA-98-4265 PRELIMINARY STUDY OF RELATIONSHIPS BETWEEN STABILITY AND CONTROL CHARACTERISTICS AND AFFORDABILITY FOR HIGH-PERFORMANCEAIRCRAFT Marilyn E. Ogburn* NASA Langley Research Center Hampton, VA the aircraft. In the post-Cold War era, however, the Abstract emphasis has shifted dramatically to a more This paper describes a study that is being balanced design approach, for which cost is a primary influence on aircraft development efforts.
done as part of the Methods for Affordable Design (MAD) program within the National Aeronautics and NASA’s recognition of this shift in aircraft design Space Administration (NASA), for which the goal is approach has led to the establishment of an to develop design methods and information that enabling technology goal to cut the development contribute to reductions in the aircraft cycle time for aircraft in half. Within the NASA MAD development cycle time while increasing design program, selected design method technologies confidence throughout the design cycle. The are being developed to contribute to substantial product of the study will be a database of reductions in the development cycle time, and information that relates key stability and control therefore development cost, for high-performance aircraft.
parameters to affordability considerations such as air combat exchange ratio, safety of flight, and probability of loss of the aircraft or pilot. The overall The high-performance class of aircraft is unique in several aspects that significantly affect background and methodology are described, and preliminary results are shown for the first phase of the overall aircraft design process, including: (1 ) the use of unconventional configurations that the study to evaluate characteristics in the longitudinal axis. For these preliminary results a are greatly influenced by new technologies and simplified analytical model of the aircraft response demanding mission requirements, and (2) the requirement for air combat maneuvering, which to uncommanded nose-up pitching moments was can be dominated by flight in the nonlinear high developed and used to characterize the requirements for recoveries to controlled flight angle-of-attack regime and involve dynamic phenomena which can be difficult to predict, even conditions and to evaluate some parameters that affect the survivability of the aircraft and the pilot.
with the best computational and ground-based experimental tools. Both of these characteristics Introduction of this aircraft class can mean that there is a limited level of design knowledge in the early design High-performance aircraft traditionally have stages. Limited knowledge of the aerodynamics and flight dynamics of these unconventional high- been developed for maximum performance, with secondary consideration being given to the cost of performance aircraft concepts reduces the quality of configuration trade studies to define the *Senior Research Engineer, AlAA Associate optimum configuration, and increases the Fellow likelihood of unexpected technical issues late in Copyright 0 1998 by the American Institute of the design cycle, when the cost of configuration modifications is substantial.
Aeronautics and Astronautics, Inc. No copyright is asserted in the United States under Title 17, U.S.
The objective of the MAD Program is Code. The U.S. Government has a royalty-free license to exercise all rights under the copyright illustrated graphically in Figure 1. The conceptual, claimed herein for Governmental purposes. All preliminary, and detailed design phases are shown other rights are reserved by the copyright owner. on a time line, which indicates that increasing American Institute of Aeronautics and Astronautics most important characteristics for high- periods of time are devoted to each successive performance aircraft. However, the relative phase. As the design cycle progresses in time, the level of knowledge of a particular aircraft benefits (e.g., variations in air combat exchange ratio) derived from varying levels of aircraft agility configuration increases, but the degree of design freedom decreases due to rising (re-)design costs (provided by stable, commanded control moments) require better definition in order to and schedule requirements. A large payoff in determine the optimum level of agility as a function reduced design time can be obtained, however, if the same level of design knowledge can be of the cost (in terms of both money and the effect on other design characteristics) of providing the applied earlier in the design cycle, when design agility (see figure 2(a)). Preliminary assumptions freedom is greater and costs of changes are lower.
The strategy for achieving this payoff is to are being made that: (1) there is minimal air combat maneuvering capability provided by the controls if substantially increase the level of knowledge about a particular high-performance aircraft the available control moment is only enough to configuration early in the design cycle. This ensure flight safety (i.e., a small margin of objective will be accomplished by providing more controlled positive response to the pilot’s accurate, yet efficient, design tools for reliable use command) and (2) the air combat exchange ratio improves with increasing moment capability but earlier in the design process, especially in the levels off if the available moment becomes conceptual and preliminary design phases. An additional benefit of this strategy is decreased risk excessive. On the other hand, providing of an unsuccessful aircraft development program. degraded or unstable control characteristics may Therefore, the research efforts within MAD are save on some types of costs and/or enable the improvement of some other design characteristics, focused on methods to more accurately and rapidly predict the aerodynamics and flight but cause more aircraft and/or pilots to be lost (i.e., dynamics of high-performance aircraft.
decrease survivability). Figure 2(b) illustrates a second database for which the assumption is being made that the probability of losing the aircraft The two types of products that are being developed within this program as design tools for as a consequence of having relaxed stability or predicting these aircraft characteristics and thus unstable characteristics increases with decreasing stability but is negligible if there is at least enough enabling reduced design costs are: control moment capability for flight safety. Figure (1) computational fluid dynamics methods that are 2(c) illustrates a third database for which the sufficiently fast, robust and resource-efficient to be assumption is made that the probability of losing used routinely in the early design phase and (2) flight dynamics design tools. Validated flight the pilot (and therefore the aircraft) increases with decreasing stability but is negligible if the unstable dynamics tools that can be confidently applied in the early design phase will allow key design issues control moment is not so great as to cause the pilot to be addressed with minimal cost and schedule ejection limits to be exceeded or otherwise prevent the pilot from ejecting. Another concern delay. The development of flight dynamics design tools focuses on phenomena that threaten flight for pilot survivability that can be related to the safety and survivability, especially during air stability of the aircraft is the acceleration at the combat maneuvering, and on defining the relative pilot’s station that can be generated by benefits of various levels of tactical agility. Existing uncommanded moments.
methods and data bases are the primary sources of information, augmented by computer simulation Information on the likelihood of and/or studies and additional experimentation, as design requirements that should help prevent required. these losses (as defined in terms of specific survivability parameters) as a function of the stability and control characteristics should aid in the This paper describes a database that is determination of the costs (e.g. acquisition and being developed as a design tool that defines the relationship between aerodynamic stability and other costs due to these losses) of providing other control characteristics and affordability gains that may be associated with less stable considerations. Some of the primary database designs. This database should also be useful for products and associated preliminary assumptions assessing the effects of the failure of propulsive or are illustrated in figure 2. For example, agility for other non-aerodynamic controls, which can cause effective air combat maneuvering is one of the unexpected decreases in stability and control American Institute of Aeronautics and Astronautics capability and therefore affect survivability. The 0 initial value objective of this research activity is therefore to define the relationships between key stability and P value at pilot’s station control characteristics and relevant affordability considerations such as those shown in figure 2 so r value for recovery that the design of these characteristics can be optimized with respect to the associated costs and Studv Approach and Methodoloav benefits. This objective will be accomplished by determining the stability and control characteristics The subject study is composed of two that affect the life-cycle cost for unstable aircraft major phases. An evaluation of the relationship between longitudinal stability and control and then developing a database to aid in making the associated design tradeoffs. parameters and affordability considerations is being conducted as the first phase of the study, and is illustrated in figure 3, which shows Nomenclature representative pitching moment curves for full- normal and axial accelerations, nose-down aerodynamic controls. Studies that all, ax g units were conducted jointly by NASA and the U.S. Navy to define required minimum values of C m static pitching moment coefficient nose-down aerodynamic pitching moment coefficient ( C , ) for relaxed static stability aircraft, for q pitch rate, deg/sec safe recoveries from high angles of attack and for tactical utility, have been (See curves labeled “1” in the figure.) A “safe” recovery
4 pitch acceleration, rad/sec2
was defined as one which, when commanded with full forward stick deflection at trimmed, stabilized, rates of change of pitch qddl, qdd, acceleration with time (defined as wings-level unaccelerated flight conditions, would negative), rad/sec3 (see fig. 7) occur with pitch acceleration values of less than or equal to zero in such a way that the pilot did not t time, sec doubt that the recovery would be completed.
X ratio of rates of change of However, the design guidelines derived pitch acceleration, qdd,/qddl from these studies have been considered to be too restrictive and costly for some future high- - X performance aircraft configurations. For these distance along longitudinal body axis from center of aircraft rotation, aircraft the design trend has been towards less ft aerodynamic pitch stability and more reliance on propulsive controls, in order to gain other design ratio of pitch acceleration to advantages such as fewer control surfaces and/or Y
maximum nose-up value, 4 / 4
low radar cross-sections. Therefore, to provide enough information for a more complete optimization of the minimum nose-down a angle of attack, deg aerodynamic control power, these earlier studies range of angle of attack in which A a have been expanded to include evaluations of the levels of pitch instability at which the survivability of uncommanded nose-up moments occur, deg the aircraft and pilot are adversely affected if a propulsive control failure or other situation results in an uncommanded nose-up pitching moment.
decrease in angle of attack (See curves labeled “2” and “3” in figure 3.) A required for recovery, deg similar study of the lateral-directional characteristics will follow.
Subscripts : max maximum value An experimental approach for the study has been defined and is shown in figure 4.
min minimum value Stability and control characteristics are being American Institute of Aeronautics and Astronautics evaluated, using analysis involving the aircraft completed no further increase in nose-down pitch rate is required and no positive pitch acceleration is equations of motion and computer simulation, as required. Parametric variations of these allowed.
characteristics and the resulting aircraft response to pitch commands are being modeled and As was mentioned previously, the computed. These computed motions are then recommended aerodynamic nose-down pitching moment requirement for safety of flight has been being evaluated with respect to life-cycle cost considerations such as flight safety, the survival of considered to be excessive for some applications the aircraft and the pilot, and tactical utility. The because of the high cost associated with it. A results will then be used to determine design more complete and therefore effective design tradeoffs between stability and control tradeoff can be made between this and competing requirements if the effect of variations in this characteristics and life-cycle cost considerations for high-performance aircraft.
requirement on the recovery characteristics can be made. One way to assess the effect of this Analvsis of Recoveries Evaluated in Earlier Studies variation is to compare the aircraft responses using relevant figures of merit. An example of this assessment is shown in figure 6, for which the time Previous studies have been used to develop guidelines for aircraft with nose-down to pitch the aircraft through a variety of angle of aerodynamic control power available throughout attack changes, Aar, was computed for a range of the angle-of-attack envelope. Guidelines were nose-down pitch acceleration values, according to the response model of figure 5. It is apparent that developed, based on the results of piloted simulation studies and flight tests, for recoveries for very small levels of nose-down pitch control the from high-angle-of-attack flight for two levels of time to pitch through even relatively modest response: (1) the minimum for safety, based changes in angle of attack can be greater than 10 primarily on whether or not the pilot had doubt that seconds, whereas for nose-down pitch the recovery would occur, and (2) response that accelerations of about .l rad/sec2or more, was considered to be useful for air combat recoveries of as much as 80 degrees of angle of maneuvering. (See pitching moment curves attack can be completed in less than 10 seconds.
labeled “1 ” in figure 3.) The response model that However, for more tactically useful levels of nose- down control power, and in particular for levels describes the motion during these recoveries is illustrated in figure 5. This model assumes the greater than about .2 rad/sec2,the recoveries, following: (1) the flight path (i.e. direction of the even those of up to 80 degrees of angle of attack, velocity vector) is constant so that the pitch rate is all take only about two to five seconds to accomplish. Therefore, within this upper range of the same as the time rate of change of angle of response there is not much additional time savings attack, (2) the airspeed and dynamic pressure are for making large angular changes in tactical constant, and (3) the command to recover the aircraft is initiated at stabilized, trimmed, situations as the control power is increased. The unaccelerated, wings-level flight conditions, such information in this figure is part of the information that there are no net forces or moments acting on that will be used for the development of the the aircraft.
proposed database product illustrated in figure The pilots who participated in the studies considered the short-term response following the Analvsis of Recoveries from Uncommanded initiation of the forward stick command to be the Nose-up Moments most important figure of merit in their assessment of the overall response. The guidelines for As aircraft static pitch stability decreases, minimum nose-down control moment versus angle there is more likely to be a range of angle of attack in which there is no available aerodynamic nose- of attack were therefore based on a response model with a constant pitch acceleration for most of down control moment, as shown in the curves labeled “2” and “3” in figure 3. If there is a stable the first two seconds. (About one-half second was break in the moment characteristics at the higher allowed for control surface actuator response time.) After two seconds and until the recovery is angles of attack in this range and the C, values cross from positive to negative, then there is a American Institute of Aeronautics and Astronautics deep stall trim angle of attack (not shown in the (1) If there is already sufficient nose-down pitch figure) at which nose-up but not nose-down rate present, there will be no pitch-up departure and the negative pitch rate will become less moments can be generated and above which nose-down moment can be generated. Pitch-up negative in the angle-of-attack range of the departures and hung stalls can therefore occur at uncommanded moment but not become positive these conditions, from which recoveries to low as the angle of attack decreases, so the aircraft will angles of attack may or may not be possible, recover.
(2) If sufficient nose-up pitch rate is present or can depending on a number of factors, including the flight motions present when this angle of attack be generated, the aircraft will pitch up to a high enough angle of attack so that sufficient negative range is entered or exceeded. A goal of the subject study is to develop aerodynamic pitch pitch acceleration can be generated for a recovery control guidelines for aircraft with these to occur.
(3) If there is insufficient positive or negative pitch characteristics which can be used to define the conditions under which an uncommanded nose- rate for a recovery to occur, the aircraft angle of up moment will result in the loss of the aircraft attack will converge to the deep stall trim point, because it will not recover to low angles of attack. from which a recovery may not be possible.
The survivability of the pilot is in question if the pilot wants to but cannot eject, whether or not the One simple method of modeling the pitch- aircraft recovers. The guidelines that this part of up departure and recovery motions for the pitching the longitudinal study addresses are therefore moment characteristics just described is illustrated based more heavily on the flight dynamics that the in figure 7, and served as the basis for the aircraft and pilot experience rather than pilot preliminary analysis for this study. Desirable opinion of the response, which was the basis for characteristics for any such model are that it be the following: (1 ) generic, yet representative of the guidelines developed in the earlier studies.
unstable aircraft, (2) relatively straightforward for Preliminarv Model calculating or simulating the aircraft response, and (3) useful for developing design guidelines and In order to begin to develop nose-down tradeoffs. The most representative and useful pitch control design guidelines for aircraft that are model is one that models the static and dynamic pitching moment characteristics as functions of susceptible to pitch-up departures it is first necessary to understand the flight motions that angle of attack, for which a computer simulation occur during the departure and the requirements must be created in order to calculate the response for recovery. The general character of the motions time histories. A simplified model which uses information on the total pitch acceleration during a pitch-up departure and recovery is an uncommanded nose-up motion followed by characteristics versus time so that the response commanded nose-down motion. Pitch time histories can be calculated easily, using acceleration ( 4 ) can be related directly to the static simple equations, was developed and used as a aerodynamic moment ( C , , , ) , if the contribution of first step prior to the development of the pitch damping to 4 is not included. Consider the simulation, in order to perform a preliminary shape of the pitching moment curve versus angle evaluation of the aircraft responses and candidate of attack for full-nose-down controls, as shown in survivability parameters, including trends in the figure 3, the curve labeled “2”. Assuming that at results.
the higher angles of attack for the uncommanded nose-up moment there is a stable break in the As was the case for the earlier studies for curve such that there is a deep stall trim angle of recoveries without uncommanded nose-up attack, above which there is an increasing amount moments, this model assumes that the magnitude and direction of the velocity vector are constant of available nose-down control moment, a pitch-up and that there are no net forces acting on the departure and recovery can occur under certain conditions. Using aerodynamic pitching moment aircraft, so that pitch rate is the same as the rate of capability only, for aircraft with these nose-down change of angle of attack. Unlike the earlier model, control characteristics one of three outcomes will however, changes occur in pitch acceleration throughout each time segment of the motion occur as a consequence of encountering this which are linear with time, and the motion is uncommanded nose-up moment: initiated with an uncommanded nose-up pitch American Institute of Aeronautics and Astronautics change of angle of attack is maximized. The acceleration. It is also assumed that the response is completely symmetric with angle of attack such
middle dashed line indicates that 4 m,n, q = 0, and
that that there are no hysteresis or unsteady the maximum angle of attack occur simultaneously.
Using these simplifying equations, a preliminary aerodynamic effects as the aircraft pitches up and down through large changes in angle of attack. assessment was made of the pitch-up response The complete time history as shown in figure 7 and recovery requirements and some associated parameters that affect the survivability of the aircraft represents the motions generated during the and pilot. The preliminary analysis that has been second of the three outcomes of an done is described in the following sections.
uncommanded nose-up moment, as described earlier in this section, for the case in which there is just enough positive pitch rate during the initial Recoverv Requirements and Characteristics pitch-up departure for a recovery to occur. The static pitching moment characteristics that could Pitch rate. - Examination of the pitch generate such a time history are therefore similar to recovery model just described reveals that the those shown in curve “2” of figure 3. The negative aircraft will not recover from an uncommanded acceleration that occurs on the stable part of the pitch-up moment to a recovery angle of attack of a, curve at the higher angles of attack continues until or less unless the magnitude of the pitch rate during the pitch-up moment is greater than a value the nose-up pitch rate is completely arrested and the maximum angle of attack is reached. As the that is determined by the values of 4, 4 m a x v qddl, angle of attack then decreases, the character of and qdd,. (Recall that earlier studies defined nose- down guidelines for an initial pitch rate of zero.)
the pitch acceleration reverses so that it becomes less negative and then positive as the angle of When the uncommanded moment is encountered attack returns to the region of the uncommanded at angles of attack above that for 4 , , , , this value of nose-up moment. The aircraft will recover to an pitch rate required for recovery is computed as angle of attack below this region if the nose-down follows: moment generated at the maximum angle of attack is high enough that a negative pitch rate is maintained as the angle of attack decreases.
The pitch motions during each segment of
where x = qdd,/qddl and y = 41 4 , , , . When the
continuously changing pitch acceleration during uncommanded moment occurs at an angle of the pitch-up and recovery, using this model, are attack at or below that for 4 , , , , this value is described by simple one-degree-of-freedom computed as: equations, assuming that there is a constant rate of change of pitch acceleration with time (qdd = constant) and that the velocity vector does not change direction such that the pitch rate equals The absolute value of the pitch rate required for recovery at any point in time is equivalent to the rate of change of angle of attack:
area under the 4 versus time curve between t = 0
(where 4 = q = 0) and that time, as depicted in
4 = (qdd) t + 4, figure 7. Therefore, for a given set of values for
q =[add) t2+ 4, t + q , q,,,, qddl, and qdd,, the highest absolute value
2 of pitch rate is required for recovery if the
a = [add) t3 + 4, f + q , t + uncommanded moment occurs at an angle of
6 2 attack near the deep stall trim point (a > a for 4 , , , ,
4 = y = 0), and is indicated in the time history of
The character and time histories of the motions are figure 7 by q , , , and - q , , , . In contrast, nearly zero pitch rate is required for recovery if the then completely determined by specifying the values of a,, q , , 4,, and qdd, which is determined uncommanded pitch-up moment occurs near the by the values of 4 , , , , qddl, and qdd,, as shown in lowest angle of attack of the pitch-up region ( a < a figure 7. Thefirst and third dashed lines on the
for 4 , , , 4 = 0, which is also the recovery angle of
figure indicate that when 4 = 0, the magnitude of
attack, a,). Therefore, I q , , , I for recovery is
the pitch rate is at a maximum and the rate of computed as: American Institute of Aeronautics and Astronautics must then take place before the recovery is completed, and the time to recover (i.e., the time that it takes for the angle of attack to return to a,) is One way to help ensure a recovery, then, computed as follows: according to this response model, would be for the
t,= 4 , , , - 2 ( ~ + 1 +(1 + x ) ~ )
aircraft to have or be able to generate a pitch rate
such that I q I > I q , , , I during the time that the
qdd, angle of attack is within the range of the Figure 10 shows the results for the time to recover uncommanded nose-up moment. Recoveries from deep stall trim conditions may also be versus qddl =qdd,(i.e., x = l ) , for a range of possible if there is enough nose-up control power values of 4 , , , of .05 to .3 rad/sec2. The results available to generate the pitch rate required for the show that the time to recover increases an le of attack to reach that for 4 m,n. Values for significantly as the slopes qddl and qdd, become I q ! = I q , , , I are plotted in figure 8 versus rate of more shallow (less negative) and as the maximum change of pitch acceleration, for the case that uncommanded pitch-up moment increases. The qdd,= qddl, for several values of 4 , , , . As the time to recover to a = a, approaches or exceeds maximum pitch-up moment 4 increases and/or one minute in some cases, so for aircraft designs the rate of change of pitch acceleration decreases, with these characteristics there could be concerns about the potential loss of the aircraft and/or the the absolute value of the pitch rate required for a recovery to occur increases. This result suggests need for the pilot to eject.
that a design tradeoff could be made between the aerodynamic pitching moment characteristics and The effect of differences between values the pitch rate required for a successful recovery. of qddl and qdd, on the time to recover is
illustrated in figure 11 for two values of 4
and The effect of a difference between qddl three values of x. The results show that as 4 increases, significant differences can occur in the and qdd,on the pitch rate required for recovery is illustrated in figure 9, for two values of 4 and time to recover for a given value of qdd, as the ratio three values of the ratio (x) of qdd, to qddl. The of the slopes is varied between .5 and 2. The difference in t, due to variations in this ratio is also
figure shows that as 4 , , , increases, significant
differences can occur in I q , , , I as the value of x is higher for the more shallow values of qdd,.
varied between .5 and 2 for a given value of qdd,.
The difference in the pitch rate required due to Anale-of-attack ranae of uncommanded variations in this ratio is also higher for the lower nose-up moment. - Another characteristic of the absolute values of qdd,. pitch response that is of interest and can be related directly to any design requirements for Time to recover. - Given that the pitch rate aerodynamic pitching moment versus angle of attack is the range of angle of attack within which requirements are satisfied so that a recovery occurs, another potential survivability parameter there is no aerodynamic nose-down moment would be the time that it takes for the aircraft to available. This range is shown as Aa in the pitch recover from the initial angle of attack that the response model of figure 7 and can be calculated uncommanded pitch-up moment occurs to the from specified values of 4 , , , , qddl, and qdd, as angle of attack below the region of this follows: uncommanded moment. One concern would be that if the recovery takes too long to occur, the aircraft may lose so much altitude that it crashes or the pilot must eject before the recovery is
Figure 12 shows how Aa varies with 4 , , , and
completed. Using the recovery model described previously, the time to recover the aircraft is qdd, = qddl (i.e., x = 1). As would be expected, as maximized for the case in which the the rate of change of pitch acceleration becomes less negative and/or the maximum pitch-up uncommanded pitch-up moment takes place in the presence of a small positive pitch rate at a = a,, moment 4 increases, the value of Aa increases
when 4 = 0 (i.e., at the beginning of the time
and could become quite large for some unstable aircraft configurations. This characteristic is related history of figure 7), and no additional nose-up motion is commanded. The entire time history to the uncommanded pitch-up and recovery American Institute of Aeronautics and Astronautics characteristics and could be included as part of the Figures 14 through 16 show computed analysis for design tradeoffs if, for example, it is so values of these incremental accelerations at the large that the aircraft is susceptible to pilot’s station versus pitch acceleration. A value of 20 feet is used for the distance of the pilot from the uncommanded nose-up moments in an unacceptably significant portion of its flight center of rotation to illustrate the phenomenon envelope. because this value is representative of current fighter aircraft.
The effect of differences between values Axial Acceleration of qddl and qdd, on Aa is illustrated in figure 13 for
and three values of x. As was two values of 4
seen for the pitch rate required for recovery (figure Values of Aax,pversus I 4 1 , time, and 9) and the time to recover (figure 1 l), as 4 , , ,
I q I are shown in figure 14, assuming that pitch
increases, significant differences can occur in Aa
rate is generated by a constant value of 4 acting
for a given value of qdd,, as the ratio of the slopes over time. Negative values of ax,pare commonly is varied between .5 and 2. The difference in Aa referred to as “eyeballs-out g”, and several g units due to variations in this ratio is also higher for the can be very uncomfortable for the pilot. The
results show that for nominal values of I 4 I that
lower absolute values of qdd,.
act over a few seconds, the pitch rates generated Analvsis of Incremental Accelerations are not so large as to create more than small values at the Pilot’s Station of Aax,p. However, there would be concern for the pilot’s welfare if, for example, a pitch-up departure Comwtation and recovery occurred during which excessive time was spent recovering the aircraft while it One potential concern for the design of experiences large pitch rates. Unstable pitching unstable aircraft that is related to the probability of moment characteristics that include large values of loss of the aircraft because the pilot must eject ern,, and/or small rates of change of pitch and/or loss of the pilot because of inability to eject acceleration with time ( I qddl I and I qdd, I ) not only result in high pitch rates that are is the axial, normal, and lateral accelerations (g’s) that the pilot can experience during rapid aircraft requiredlgenerated for recovery from pitch-up motions. If these accelerations and/or the rates of departures (see figures 8 and 9) but also result in g onset are excessive, then the pilot may feel lengthy times to recover (see figures 10 and 11).
uncomfortable, want to eject, and/or be unable to eject. The survivability of the pilot from the The average onset rate of the incremental axial acceleration can be determined by dividing standpoint of g tolerance could be an issue whether or not the aircraft experiences a departure the value of Aax,pby the time to generate it. For from controlled flight and whether or not it recovers example, the average incremental onset rate for from the departure. The (incremental) acceleration
e= .25 rad/sec2sustained for 15 seconds is about
that the pilot feels relative to the center of aircraft -.6 g/sec (see figure 14). The onset rate will be rotation is proportional to the distance between highest at the end of the motion, so for this the center of rotation and the pilot’s location. In example, at t = 14 sec (after the pitch rate has particular, for pitch motions only, if it is assumed reached 200 deglsec), Aax,p = -7.6 g units, but that the pilot location is forward from the center of during the next second more than 1.1 additional rotation and the vertical and lateral offsets are negative g units are generated, or almost twice the small, then incremental axial and normal average onset rate.
accelerations (due only to the rotation) will be experienced by the pilot according to the following Normal Acceleration equations: Figures 15 and 16 show the incremental - normal acceleration at the pilot’s station versus
Aax,p = - (4’ xp) / g , where q = 4 t, for constant 4
nose-up (figure 15) and nose-down (figure 16)
pitch acceleration. For the range of values of 4
previously evaluated (0 to .3 rad/sec2),figure 15 shows that less than .2 g of incremental normal acceleration is produced. If, however, starting with American Institute of Aeronautics and Astronautics that levels which are suitable for the definition of
4 = 0, a constant rate of change of pitch
the minimum for safe, controlled recoveries and
acceleration (i.e., I qddl I or I qdd, I ) of as much as
tactical utility could be defined. A simple analytical .3 rad/sec3is sustained for as long as
30 seconds, then 4 values of as much as model was developed and used to represent the
general character of the pitch motions during 9 rad/sec2(Le., Aan,p = 5.6 g units, with a constant g onset rate of slightly less than .2 g/sec) will be uncommanded nose-up moments and recoveries generated just from the pitching motion. This that can occur for aircraft that are longitudinally motion would be of concern with respect to the unstable. This one-degree-of-freedom model pilot’s positive normal g tolerance, therefore, assumes that these motions consist only of during those portions of pitch-up departures and periods of nose-up and nose-down pitch acceleration with constant rates of change of pitch recoveries with sustained positive rates of change of pitch acceleration that generate high values of acceleration with time, pitch rate equal to the rate of change of angle of attack, and no hysteresis or 4.
unsteady aerodynamic effects. The parameters The pitch-up departure and recovery that are used to define the numerical response model of figure 7 shows that the minimum (most characteristics of the model are the maximum negative) value of 4, e,,,, is encountered at the positive uncommanded pitch acceleration and the maximum angle of attack achieved during the time rates of change of the pitch acceleration.
recovery, and has a greater absolute magnitude Using this preliminary model, the pitching
than 4 , , , . The value of 4 , , , is related to that of
moment characteristics were varied and several 4 max as follows: parameters were examined which are related to life-cycle cost considerations, such as the survivability of the aircraft and/or the pilot. As Negative incremental normal acceleration values at would be expected, as the maximum value and the pilot’s station are generated by negative values time of the uncommanded nose-up acceleration increased, the values of the survivability
of 4, so at the time that emin occurs the negative
incremental acceleration is maximized. Figure 16 parameters worsened. A successful recovery from shows the relationship between Aan,p(the most an uncommanded nose-up pitching moment (and negative value encountered, for which 4 = 4 , , , ) therefore the survival of the aircraft) occurs only when there is a minimum pitch rate, the absolute and the specified pitch acceleration characteristics value of which is determined by the specified pitch
of emax (based on its relationship to e , , , ) and
acceleration characteristics and the current pitch x (= qdd,/qddl). Normal accelerations of zero and negative g can be uncomfortable to the pilot, so acceleration. One design tradeoff which could be
excessively negative values of 4 mln (because of defined would be between the longitudinal
stability characteristics of the aircraft and the pitch high values of gmaXand/or x), especially if they are rate generation capability. The time to recover the sustained, would cause concern and therefore aircraft to angles of attack below the region for could be a consideration for design tradeoffs.
uncommanded nose-up pitching moments was Concludina Remarks also evaluated because excessive recovery times can mean that the aircraft may lose too much The overall methodology and altitude before it recovers, so that it crashes or the pilot must eject. Another characteristic of interest experimental approach have been defined and a is the range of angle of attack within which no preliminary analysis has been performed to nose-down aerodynamic pitching moment is examine the effect of unstable static aerodynamic pitching moment characteristics on selected life- available. If this range is excessively large or cycle cost considerations such as the survivability includes angles of attack at which a pitch-up of the aircraft and/or pilot, for high-performance departure would be especially troublesome then it aircraft. This work is being done as part of NASA’s may be included with other design tradeoffs. The MAD program. This study extends the results from survivability of the pilot was also addressed by earlier studies of more stable aircraft for which at examining the incremental axial and normal least some nose-down aerodynamic control power accelerations at the pilot’s station generated by pitching motions. If the aircraft pitch characteristics was always available at high angles of attack such American Institute of Aeronautics and Astronautics are sufficiently unstable such that the pitch rates 3. Ogburn, Marilyn E.; et. al.: Status of the Development of High-Angle-of-Attack Control and/or accelerations generated during a pitch Margin Requirements. NASA High-Angle-of-Attack departure and/or recovery are high and/or sustained, then the pilot’s g tolerance could form Projects and Technology Conference, NASA Dryden Flight Research Facility, Edwards, CA, the basis for another necessary design tradeoff.
April 21-23, 1992. NASA CP-3137, Volume 2, This study will continue in order to define pp. 99-147.
databases of design information, including tradeoffs, for safety/survivabiIity and tactical utility 4. McNamara, W. G.; et. al.: Navy High Angle of of high-performance aircraft. After phase one of Attack Pitch Control Margin Requirements for Class IV Aircraft. NAVAIRWARCENACDIV the work is completed for the pitch axis, databases will be developed for the lateral-directional axes. Technical Memorandum 91 -167 SA, To complete the longitudinal study, a more realistic June 25, 1992.
and useful model of pitching moment 5. Lackey, J.; et.al.: Flight Test Validation of Navy characteristics versus angle of attack will be High-Angle-of-Attack Pitch Control Margin developed and used for computer simulations of Requirements for Class IV Aircraft.
pitch response to uncommanded nose-up moments. The definition of the key survivability NAVAIRWARCENACDIVTechnical parameters and the associated motion Memorandum 92-2 SA, July 27, 1992.
requirements in pitch will be completed and guidelines for the specific design of the pitching 6. Lackey, James B.; and Hadfield, Christopher A.
(Capt., CAF): Pitch Control Margin at High Angle of moment curve, based on these requirements, will Attack - Quantitative Requirements (Flight Test be developed. Consideration will be given to the applicability of forces acting on the airplane, Correlation with Simulation Predictions). AlAA various types of coupling, and kinematic effects to Flight Test Conference, Hilton Head, SC, August 24-26, 1992. AlAA Paper No. 92-4107.
the study. The parameters that affect ejection envelope limitations will be examined. The study 7. Lackey, J.; and Prater, D. (Lt., USN): Limited will also be extended to include extremely Navy Flying Qualities and Performance Evaluation unstable configurations that are susceptible to autorotation in pitch (i.e., tumbling). As the of the NASA F/A-18 HARV. NAWC-AD Technical research progresses, consideration will be given to Memorandum 93-11 SA, June 21, 1993.
the inclusion of information in the database 8. Ogburn, Marilyn E.; et. al.: Status of the products that is more closely related to Validation of High-Angle-of-Attack Nose-Down manufacturing costs.
Pitch Control Margin Design Guidelines. AlAA Atmospheric Flight Mechanics Conference, References Monterey, CA, August 9-11, 1993. AlAA 1. Nguyen, Luat T.; and Foster, John V.: Paper No. 93-3623.
Development of a Preliminary High-Angle-of-Attack Nose-Down Pitch Control Requirement for 9. Ogburn, Marilyn E.; et. al.: Flight Validation of High-Performance Aircraft. NASA TM 101684, Ground-Based Assessment for Control Power Requirements at High Angles of Attack. Fourth February 1990.
High Alpha Conference, NASA Dryden Flight 2. Ogburn, Marilyn E.; et. al.: High-Angle-of-Attack Research Center, Edwards, CA, July 12-14, 1994.
Nose-Down Pitch Control Requirements for NASA CP-10143, July 1994, Volume 2.
Relaxed Static Stability Combat Aircraft. NASA High-Angle-of-Attack Technology Conference, NASA Langley Research Center, Hampton, VA, October 30-November 1, 1990. NASA CP-3149, 1992, Volume I, Part 2, Paper No. 24.
1 0 American Institute of Aeronautics and Astronautics . b a p l r a l r d d configuration Outcome
. *+bl . ' d Simulation
'. Rrgn
L F d m
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1 -
bc W p h N -
Tactical, Safety, Survivability Figure 1. - Objective of MAD program Criteria combat probability Figure 4. - Experimental approach exchange of losing ratio stable aircraft stable poor control 0
u control
0 safety opti- moment o safety moment min mum
goodL min
(b) probability of losing aircraft (a) combat exchange ratio versus control versus control moment moment + .
probability of losing pilot stable control moment min
q - 0 1
(c) probability of losing pilot versus control moment + , Figure 2. - Examples of database products
-
+ I r c G I t 1 I I 1 , 1 UIIVJ~ fa u C ~ I c 0 1 2 3 4 Time lrom stick input, sec anlo i t mui Figure 5. - Recovery model used for guidelines - developed in earlier studies Figure 3. - Effect of full-nose-down aerodynamic pitching moment characteristics on survivability American Institute of Aeronautics and Astronautics Figure 6. - Time to recover using recovery model developed in earlier studies Figure 8. - Pitch rate required to assure recovery; qddl = qdd2 Figure 9. - Pitch rate required to assure recovery; Figure 7. - Preliminary pitch-up departure and qdd1 qdd2 recovery model American Institute of Aeronautics and Astronautics
r
Figure 10. - Time to recover from pitch-up departure; qdd, = qdd, Figure 12. - Range of angle of attack for uncommanded nose-up moment; qdd, = qdd,
m
Figure 11. - Time to recover from pitch-up departure; qdd, n qdd, Figure 13. - Range of angle of attack for uncommanded nose-up moment; qdd, n qdd, 1 3 American Institute of Aeronautics and Astronautics Id, tadwr?
Figure 14. - Incremental axial acceleration at the pilot's station due to pitch motion; - %=20fl Figure 16. - Incremental normal acceleration at the pilot's station due to pitch motion; - xp = 20 ft 0 0.05 0 . 1 0 . 1 5 0.2 025 0.3 Figure 15. - Incremental normal acceleration at the pilot's station due to pitch motion; - xp = 20 ft 1 4 American Institute of Aeronautics and Astronautics