Stability and Control — Page 273, Lesson 325

Let's pick this up right where the power effects leave off, because the jet engine case is the perfect bridge into the conclusions. We just saw how a propeller's slipstream can destabilize the tail. Now, essentially the same destabilizing effect is produced by the flow induced at the exhaust of turbo-jet and fan engines. Think of the jet exhaust as a high-velocity stream that drags air along with it, creating an induced flow field. Ordinarily, the induced flow at the horizontal tail of a jet aeroplane is slight, and it is destabilizing when the jet passes underneath the horizontal tail. So the geometry matters—if the engines are mounted low and the exhaust flows beneath the tailplane, that induced flow reduces the tail's effectiveness, which is destabilizing. Now, here's a key relationship to lock in: the magnitude of these indirect power effects on stability tends to be greatest at high CL, high power, and low flight speeds. High CL means high angle of attack, high power means a strong slipstream or jet flow, and low flight speed means low dynamic pressure. All three of those conditions amplify the disturbance at the tail. Let me now give you the conclusions to the effects of power, because this is a clean summary you'll want to remember. The combined direct and indirect power effects contribute to a general reduction of static stability at high power, high CL, and low dynamic pressure. It is generally true that any aeroplane will experience the lowest level of static longitudinal stability under these conditions. And because of the greater magnitude of both direct and indirect power effects, the propeller powered aeroplane usually experiences a greater effect than the jet powered aeroplane. So the prop aircraft is more susceptible to this stability loss than the jet. Now we move to a new influence on stability: high lift devices. An additional effect on stability can come from the extension of high lift devices—flaps and slats. High lift devices tend to increase downwash at the tail and reduce the dynamic pressure at the tail, both of which are destabilizing. Increased downwash means the tail sees a more downward-tilted airflow, reducing its effectiveness; reduced dynamic pressure means the tail gets less airflow energy to work with. However, high lift devices may prevent an unstable contribution of the wing at high CL. So there's a trade-off: they hurt the tail, but they can help the wing. While the effect of high lift devices depends on the aeroplane configuration, the usual effect is destabilizing. Here's the practical consequence: the aeroplane may experience the most critical forward neutral point during the power approach or overshoot, which is the missed approach. During this condition of flight, the static stability is usually the weakest, and particular attention must be given to precise control of the aeroplane. And here's the design implication: the power-on neutral point may set the most aft limit of CG position. So the aft CG limit is often dictated by the stability condition with power on, because that's when stability is weakest. Now let's shift to a new topic: control force stability. The static longitudinal stability of an aeroplane is defined by the tendency to return to equilibrium upon displacement. In other words, a stable aeroplane will resist displacement from trim or equilibrium. The control forces of the aeroplane should reflect the stability of the aeroplane and provide suitable reference to the pilot for precise control of the aeroplane. So the stick forces you feel are not arbitrary—they're a direct reflection of how stable the aircraft is. Let me walk you through the graph of Figure 10.31, which shows the effect of elevator deflection on pitching moments. The graph plots CM, the pitching moment coefficient, against CL, the lift coefficient. If the elevators of the aeroplane are held at zero deflection, the resulting line of CM versus CL for 0° depicts the static stability and the trim lift coefficient. So that zero-deflection line is your baseline stability curve. Now, if the elevators are held at a deflection of 10° up—which means the aircraft is trimmed at a lower speed—the aeroplane static stability is unchanged, but the trim lift coefficient is increased. So deflecting the elevator up shifts the trim point to a higher lift coefficient, meaning a lower speed, but the slope of the stability curve—the static stability itself—stays the same. The elevator deflection changes where you trim, not how stable you are. Let me make sure that's clear: the slope of the CM versus CL line represents static stability. Elevator deflection shifts that line up or down, changing the trim lift coefficient, but it doesn't change the slope. So a stable aeroplane stays stable regardless of elevator position; the elevator just sets the trim condition. That's the core of control force stability—the forces you feel reflect that stability, giving you the reference you need for precise control.