Report of the Second Piloted Aircraft Flight Control System - Acgsc.org

power operated syetem and that of the airplane made pilot operation
difficult and operation with the automatic pilot unsatiefactoxy.
An analyeis of the problem indicated that the difficulty could be
attributed primarily to the reaction epring rates of the sgsten.
The structure of the reaction vetem had been deaignqd for strength
in accordance wlth anticipated loads and not for rigidity and thus
accounted for a large portion of the elasticity. The compressibility
of the oil along with mansion of the power cylinder,
I~~Iraulic tubing ard other components also contributed to the
low overall spring rate of the qstem. Bending of a m and linkage8
in the feedback system ad entrained air in hydraulic oil
were recognised as other factors involved.
1t was also found that the original ralw, although providing
required gain at higher amplitudes, caused too high gain at low
amplitudes resulting in instability. Other probleuw included
backlaeh uxl play in the system which was attributed primarily to
bearings and the movenent of the power cylillder piston "0" ring
In its Nstardardm groove.
Various changes to the syetem were made in an effort to improve
performance. These included the design of a control valve whose
gain varied as the square of valve displacement, use of close
tolerance bearings ad bolte throughout, and use of more rigid
feedback ad control linkages. The reaction structure was also
etrengthened locally, but it was found that a large portion of
the fin contributed to the spring rate.
Althaugh some improvement was realited from this effort, it was
apparent tMt the performance was still less than desired. Figura
3 shows the frequency response curves of amplitude ratio ad
phaee of the sgstem with these inprovements.
The natural frequency of the airplane upon which this control
system was being used is shown by the dashed vertical line on
figure 3. .It can be eeen that the pawered surface control eyetam
phse lag wae 70° for 11/4 degree eurface amplitude. For
smaller surface amplitude8 the phase lag increased rapidly. In
this rmga of unplitndes the combined phaee lag of the control
syetsm and tht of the airplane approached a value so high that
mfflcient phaee lead could not be realised from m automatic
pilot to keep the closed loop sgstem stable.
To be on the safe
side the powered control system should ham less than kS degrees
phase lag, for q amplitude at which the rudder is effectiw,
at a frequency 5 times that of the airplane natural frequency.