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

explicit set of specifications-or a more quantitative description of
the behavior we desire from our system. Some of you ~lvyr say that quantitative
specificstiona can not be written--instead we nust be satisfied with euch
specifications as the statement-The airplane mst fJy satisfactorily.
This field of engineering is not alone in this difficulty of writing a
cpantitative set of specifications. The seruomechaniem field in general
is plagued by the same difficulties. I have heard maqy tin108 the et,atemsnt--
We want a real fast servo that has practically no aror--or somuthing
equally vague. If there is to be logic to the eyetom design, the first
step involves making this specification quantitative. Indeed, when this
is done, the problem is half solved.
In particular, what specifications are appropriate to the aircrai"t
control problem? We consider still, for the sake of explicitness, only
the longitudinal control system. 1i-1 addition, we consider on.
the
performance specifications, taking the word performance in its conmon,
narrow meaning, and excluding such factors as the maxhm~ allowable
acceleration, etc. We consider only specifications governing the etabillty
and dynamic performance in general of the aircraft undsr small daflections
of the control wface.
The specifications, it seems to me, should include at least the following
quantities: the relative damping of the overall aircraft system, the damped
(or undam~al) natural frequencies af oscillation, the low-frequency gaia of
the system, and the thm lags in the various parts of the system. The firat
two quantities, relative damping and oscillation frequency, dot elmine the
relative stability . The third quantity, the low-f requency gain, datemines,
essentially, the effectiveness of the feedback, the abillty of the systan to
continuously calibrate itself, reduce the effect of nonlinearities, aDd control
the effect of corrupting signals entering the system (for example, wind gusts).
Thg last quantity, the time lag In the various system components, measures
the speed of response of our alrcraft.
Other quantities will of course have to be specified qventuaUy,in the
complete problem. For example, we are interested not only In the low-frequency
responee of the system to wind gusts, but also the overshoot and sattlhg timb
associated with the response to a gust of wind. However, the above four
parameters will give us a basis for the remainier of this paper.
Ql the bade of specifications of this sort, at least the prelhhaxy
design of the syetm can be aarried out on a logical basis. The dismssion
here is samewhat difficult, because the two aepecte of control and stability
can not be divorced, but I shall try to indicate the general tra of a
method which we think is reasonable for design. Mrst, let us consider the
stability problem. does the system basically tend toward instability?
And does the stability problem become more difficult as the speed of the
aircraft increases. In the very simple system we are conaideriag here (shown
' in Fig. 1) significant phase lags In the system are introduced from two