Last month I began on the subject of Weight and Balance, starting with some fairly obvious points about weight.
This month and for a while longer, the more complex second half: balance.
Balance affects three important areas of aircraft performance: stability, control authority and stall/spin recovery.
STABILITY--STATIC
There are two kinds of stability: static and dynamic. Both require definition, so it’s time for a short physics lesson.
Static stability refers to the immediate tendency of a system after it is first disturbed. If the tendency is to resist and attempt to reverse the disturbance, the system is said to be statically stable. A common example used to illustrate this concept is that of a marble lying at the bottom of an open bowl. When the marble is pushed up the side, it resists the input and “wants” to return toward the bottom, where it started.
If the bowl is turned over, with the marble balanced on the very top, any disturbance will result in a demonstration of static instability: the marble will “want” to roll down the side and continue on.
By extension, when you raise the nose of an airplane from trimmed level flight, the nose will “want” to drop back down if the plane is statically stable in pitch. Similarly, the plane will resist any nose down input by pushing back up.
Static stability is a critically important design consideration for airplanes: without it, the plane becomes potentially very dangerous to operate. Once the design stage is complete, the degree of static stability is determined by CG location.
Important note: this critical concept is usually confused with dynamic stability and then incorrectly presented as a consequence of “tail down force.”
STABILITY—DYNAMIC
Static stability is the starting place, but by itself does not guarantee that a system will actually revert to its original state after a disturbance. That is, the nose may want to drop after being raised, but there is no guarantee the airplane will return to level flight. When we examine what actually happens over time, we are looking at issues of dynamic stability.
The nose may simply continue to oscillate, first down, then up, then down, and so on. If the oscillations remain constant, the plane is demonstrating neutral dynamic stability. If the oscillations increase in magnitude, the plane is dynamically unstable.
Finally, if the oscillations diminish until level flight is restored, the plane is dynamically stable. This is the ideal.
STATIC CONSEQUENCES
Put in the simplest terms, static stability determines what a system wants to do; dynamic stability determines what it does.
To be certified, airplanes must be statically stable. As stated, the determining factor is CG location: as that point moves backwards toward the tail, the airplane becomes less and less stable. At some point, well aft of the legal limit, the airplane becomes statically neutral and potentially dangerous to fly: when you raise or lower the nose, it tends to stay where you put it, leading rapidly to problems of airspeed and angle of attack.
A statically unstable airplane is even worse: in this extreme, when you push or pull on the yoke, the system tends to reinforce the input, “snatching” the control and making an immediate counter-effort necessary to prevent an extreme attitude from developing. Management would require constant vigilance in VMC (ask any helicopter pilot about hovering), and would probably be impossible in IMC.
DYNAMIC CONSEQUENCES
The consequences of dynamic neutrality or instability are much less severe: irritation rather than nightmare. Dynamically neutral aircraft want to return toward level pitch, but aren’t very good at it. That is, they continue to oscillate until settled down by deliberate control inputs. In a dynamically unstable aircraft the oscillations actually increase in size unless countered.
In each case, the consequence is relatively mild, making further examination of dynamic issues unnecessary at this point. This is ironic, as nearly all teaching materials, including those produced by the FAA, discuss CG location in terms of dynamic stability—and then explain it incorrectly.
More—much more—to come.
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