fig1 is a side view of a conventional commercial aircraft illustrating the body , or fuselage 12 and the tail 14 . the problem exhibited by such aircraft to which the present invention is addressed is that the flexible structure is subject to accelerations resulting from external forces , such as gusts , exciting natural resonant modes of the body . of particular concern due to passenger discomfort are the lateral modes ( i . e ., into and out of the drawing ), which manifest as accelerations in the aft section , generally referred to in the figure with reference numeral 16 . the present invention is an active system for suppressing accelerations due to lateral mode excitation . first , an observability study was performed to determine a location on the aircraft that optimally senses accelerations resulting from excitation of the first lateral mode . in the example shown in fig1 which represents a study on a model 767 - 300 aircraft , the aft bulkhead was found to be an excellent location to observe the first mode . thus , an accelerometer 18 was positioned at this location . as discussed in this specification , the accelerometers employed are commercially available and produce an output digital signal representative of sensed acceleration . the output from the first modal accelerometer 18 is routed to the yaw damper 20 which is located in the electronics equipment bay 22 in the forward belly of the aircraft . also included in the electronics equipment bay 22 is the aircraft &# 39 ; s onboard inertial reference system ( irs ) 24 . the control signals produced at the output of the yaw damper 20 are routed to the tail 14 where they are applied to the actuation system for the aircraft &# 39 ; s rudder 26 . the yaw damper 20 , in the conventional manner , produces control signals to cause deployment of the rudder 26 to reduce undesired yaw of the aircraft . however , it has been found that the yaw damper does not suppress lateral accelerations on the aft section of the aircraft due to induced excitation of the natural modes . as described in detail with respect to the preferred embodiment illustrated in fig6 the first accelerometer 18 may be used to sense both first and second mode accelerations . this has the distinct advantage of utilizing less hardware , requiring less installation time and being more reliable , than the two - accelerometer embodiment illustrated in detail in fig5 . initially , a second order band pass filter was employed to filter signals from the digital accelerometer 18 to isolate accelerations due to excitation of the first body mode . this signal was then passed to the yaw damper where , acting in parallel to the yaw damper signal , it was applied to the rudder 26 to thereby suppress first mode induced accelerations . shown in solid line in fig2 is the power spectral density of the first mode accelerations resulting from the use of the second order filter in the yaw damper parallel path . it was found , however , that some information from both dutch roll and the second body mode excitations was passing through this path and exciting these two additional modes . hence , a fourth order band pass filter was used and , as shown in the dashed line of fig2 improved modal suppression was achieved . it was then found that the second body mode could be reduced by adding a second parallel path . the second path included a second digital sensor 28 which was positioned at the aft galley -- a position determined to optimally exhibit second mode accelerations . the output from the second accelerometer 28 was passed through a second order band pass filter to isolate the second mode signal . the second mode signal was then passed in parallel to the first mode signal through the yaw damper to the rudder . fig3 is a power spectral density graph illustrating the level of the lateral acceleration at the aft galley . here , the second order band pass filter is adequate to reduce unwanted frequency signals . the gains of the two parallel paths from the accelerometers 18 , 28 were adjusted to meet a stability criteria of a 6 db gain margin and a 45 ° phase margin . the dutch roll damping was found to be 0 . 96 ( a damping of 1 . 0 is considered highly damped ). in the presence of the two parallel paths , the yaw damper servo rate had to be increased from 35 radians per second to 70 radians per second to obtain satisfactory results . fig4 is a power spectral density plot illustrating lateral acceleration at the aft galley of the aircraft due to lateral gust inputs . here , the solid line represents the response of the aircraft as a result of the conventional ride quality yaw damper . the dotted line illustrates a prior art approach which was an active system that did not employ the positioned accelerometers . the dashed line illustrates the improvements achieved due to the present modal suppression system . fig5 is a detailed block diagram illustrating the preferred embodiment of the two - accelerometer modal suppression system according to the present invention . as mentioned with respect to fig1 the produced first and second digital accelerometer signals 18 , 28 , respectively , are filtered , in a manner to be described hereinbelow , then gain factored as a function of aircraft total airspeed ( also described hereinbelow ) and added in parallel to the ride quality yaw damper control law 20 produced signal . thus , the digital output from the first mode digital accelerometer 18 is passed both to the input of a low pass filter 52 and to the positive input of a summer 54 . the transfer characteristics for the various filter elemental blocks as described herein is illustrative of one preferred embodiment . it will be understood by those of ordinary skill in the art that the implementation of alternate filter embodiments will require a determination of the desired filter band pass characteristics and , therefore , an appropriate change to the elemental transfer functions . as used in these figures , the symbol &# 34 ; s &# 34 ; represents the laplace operator . the output from the low pass filter 52 is then multiplied by a constant in gain block 56 before being applied to the negative input of summer 54 . the output from summer 54 is , thus , the first mode digital accelerometer signal minus a defined low pass characteristic , i . e ., a high pass filtered accelerometer signal . the signal out of summer 54 is passed through a similar sequence of blocks 62 , 64 and 66 to blocks 52 , 54 and 56 , respectively . the output from summer 64 is , thus , the first mode accelerometer signal as passed through a second order high pass filter . the output signal from summer 64 is then passed through two successive low pass filters 70 , 72 to thereby realize the full fourth order band pass filtering of the first accelerometer signal . the fully fourth order band pass filtered first mode accelerometer signal is then passed through a gain schedule block 80 . since the ultimate control signal from the modal suppression system will be passed to the rudder of the aircraft , and inasmuch as the force produced by the rudder is a function of airspeed , the first mode accelerometer signal is gain amplified by a gain factor k new which reduces in value as aircraft total airspeed v tas increases . the resultant first mode signal is applied to the positive input of a summer 82 . the output signal from the second mode digital accelerometer 28 is passed both to the input of a low pass filter , comprised of blocks 90 , 92 , and to the positive input of a summer 94 . passed to the negative input of summer 94 is the output from the low pass filter 90 , 92 . thus , the output from summer 94 is the high pass filtered second mode accelerometer signal . the output from summer 94 is passed through a low pass filter 96 to thereby produce the first order band pass filtered second mode accelerometer signal . as with the first mode accelerometer signal , the band pass filtered second accelerometer signal is passed through a gain schedule block 100 . gain schedule block 100 multiplies the band pass filtered second accelerometer signal by a level which decreases with increasing aircraft total airspeed . the output from the gain schedule block 100 is passed to the minus input of the summer 82 . in reality , summer 82 sums the filtered first and second accelerometer signals . the sign difference in summer 82 simply reflects signal inversions that take place through the processing . thus , the output from summer 82 is a composite signal representing aircraft accelerations induced by excitation of the first and second natural modes . this composite signal is added to a yaw damper control 20 produced signal in a summer 110 . the yaw damper control law 20 receives four signals from the aircraft &# 39 ; s onboard irs system 24 . these signals are : a lateral accelerometer produced signal n y which is positive for lateral accelerations to the right of the aircraft , a yaw rate signal r which is positive for yaw rates forcing the nose to the right , a bank angle signal φ which is positive for the right wing deflecting down and a roll rate signal p which is positive for the right wing deflecting down . the yaw damper control law produces a signal β aft in accordance with the following equation ## equ1 ## where v tas = aircraft true airspeed , since the irs system is located in the electronics bay ( item 22 in fig1 ) the irs produced lateral accelerometer signal must be revised to reflect lateral accelerations at the aft bulkhead . where l x is the distance between the inertial reference system and the aft bulkhead and r is the yaw acceleration . thus , the lateral accelerometer signal n y is passed to the positive input of the summer 120 . the output from summer 120 is passed to an integrater block 122 , with the output from integrater block 122 being passed to the positive input of the summer 124 . applied to the negative input of summer 124 is the irs produced yaw rate signal r , as multiplied by a gain factor proportional to the distance l x . the output from summer 124 is passed through a unity gain buffer 126 , with the output from buffer 126 being applied to the negative input of summer 120 . thus , the output from buffer 126 is the desired n yaft , i . e ., aft bulkhead lateral acceleration signal . the signal n yaft is multiplied in gain block 128 by a level proportional to acceleration due to gravity divided by the aircraft &# 39 ; s total aircraft speed v tas , with the result being applied to a positive input of the summer 130 . the irs produced bank angle signal φ is first passed through a sine block 140 and then passed through a gain block 142 having a gain factor proportional to g / v tas . the output from gain block 142 is applied to a positive input of a summer 144 . applied to the other positive input of summer 144 is the irs produced roll rate signal p as gain factored in gain block 146 by a factor proportional to the aircraft &# 39 ; s angle of attack divided by g . the output from summer 144 is applied to the positive input of a summer 150 . applied to the minus input of summer 150 is the irs yaw rate produced signal r . the output from summer 150 is applied to a positive input of summer 130 . the output from summer 130 is the β signal , in accordance with the equation given above . the β signal is then passed through a gain schedule block 160 . gain schedule block 160 has a gain factor k . sub . β which is a function of dynamic pressure q c . since the force produced by deployment of the rudder is a function of altitude , the gain set by gain schedule block 160 assures proper rudder deployment for a given aircraft altitude . the summer 110 sums the yaw damper control law signal with the band pass filtered first and second mode accelerometer signals . thus , the output from summer 110 is a composite signal representing modal acceleration due to flexible body oscillation ( from the accelerometers ) and rigid body oscillation ( as processed by the yaw damper control law ). this digital composite signal is then further filtered through a notch filter 200 which has a transfer characteristic designed to &# 34 ; notch out &# 34 ; higher mode flutter producing signals . the signal passed through notch filter 200 is then digital - to - analog converted in a d / a converter 202 . this signal is then fixed gain amplified in a gain amplifier 204 and low pass filtered , to further filter off flutter inducing signals by analog filter 206 . the control signal from analog filter 206 is then passed to the yaw damper servo 208 which , through appropriate linkage 210 , is coupled through a linkage summer 212 to the rudder power control unit 214 . in the normal manner , redundant systems are provided on the aircraft such that an output from a second , totally redundant yaw damper 220 also passes through appropriate linkage 222 through the linkage summer 212 to the rudder power control unit 214 . fig6 is a detailed block diagram illustrating an alternate preferred embodiment of the invention wherein a single accelerometer is used to sense both the first and second natural modes of the aircraft . as discussed hereinabove , this embodiment has the advantage of utilizing less hardware and thereby being less costly and as such has distinct commercial advantages . the system as shown is fig6 is identical to the fig5 embodiment with respect to the signal produced out of the yaw damper 20 and the resultant processing through to the rudder power control unit 214 . the only difference is in the processing of the signal from the single accelerometer and , as such , only this difference will be discussed hereinbelow . it was found that the signal produced by the first accelerometer ( accelerometer 18 of fig1 located at the aft bulkhead ) could be used to sense both first and second mode accelerations . thus , in the fig6 embodiment the output from the first digital accelerometer 18 is passed both through a low pass filter , comprised of blocks 300 , 302 and to the positive input of a summer 304 . in this way , the output of summer 304 is the first order high passed accelerometer signal . the signal out of summer 304 is then passed through a low pass filter comprised of blocks 306 , 308 which , along with the signal out of summer 304 passes through a summer 310 . as such , the output from summer 310 is the accelerometer signal as passed through a second order high pass filter . the output from summer 310 is then passed through a notch filter 312 . fig7 is a chart depicting both the gain and phase characteristics of the overall filtering of the accelerometer 18 produced signal through the notch filter 312 . as can be seen , this composite filtering provides band pass filtering for the first mode and high pass filtering for the second mode and the right phase for each mode . as in the embodiment of fig5 an analog filter 206 used downstream thereby completes the band pass filtering for the second mode . also , as described above in the fig5 embodiment , the resultant out of the notch filter 312 is passed through gain schedule block 320 to vary the produced control signal as a function of aircraft true airspeed . this signal is then summed with the yaw damper control signal 20 as described in the fig5 embodiment and then processed as a rudder control signal , also described with respect to fig5 . in summary , an improved aircraft lateral modal suppression system has been described . this system uses one or more fixed accelerometers to directly sense accelerations due to natural mode induced oscillations and is further responsive to aircraft true airspeed to produce a control signal to deploy an aircraft control surface to suppress modal accelerations . whereas preferred embodiments of the invention have been described in detail , it should be apparent that many modifications and variations thereto are possible , all of which fall within the true spirit and scope of the invention .