Stability and Control — Page 301, Lesson 362

Let’s pick this up right where the material takes us: the power effect on static directional stability. I want you to hold onto the parallel from longitudinal stability, because the author builds this directly on it. The direct effect of power on static directional stability is confined to the normal force at the propeller plane. Let me unpack that. The propeller plane is the disc swept by the blades. The normal force is the force acting perpendicular to that disc. When the propeller is located ahead of the centre of gravity — which is the usual tractor layout — that normal force is destabilizing. Why? Because a force ahead of the CG, acting sideways, tends to push the nose further off the direction of flight rather than restore it. So the direct power effect is destabilizing. Now the indirect effect. The air in the slipstream behind a propeller spirals around the fuselage. That spiral flow is not straight back; it rotates. With a clockwise rotating propeller, that spiral slipstream produces a sidewash at the fin — and the sidewash comes from the left. Sidewash is the sideways component of airflow at the fin. So the fin sees air coming at it from an angle, not straight along the fuselage. That changes the fin’s effective angle of attack and therefore its sideforce. Here’s the key contrast: for a propeller-driven aeroplane, these indirect effects — the power-induced velocities and the flow direction changes at the fin, which together are called the spiral slipstream effect — are quite significant. They can produce large directional trim changes. But for the jet-powered aeroplane, the indirect effects are negligible, just as they were in the longitudinal case. So let’s summarise the contribution. The direct and indirect power effects together contribute most to static directional stability in the propeller-powered aeroplane, and usually only slightly in the jet-powered aeroplane. But in either case, the general effect of power is destabilizing. And the greatest contribution occurs at high power and low dynamic pressure. Dynamic pressure is the kinetic energy of the airflow — low dynamic pressure means slow speed or low air density. So the worst case is high power setting with low airspeed, which is exactly the climb or go-around regime. Now the critical conditions. The most critical conditions of static directional stability are usually the combination of several separate effects. And that combination which produces the most critical condition depends very much on the type of aeroplane. There’s no single universal worst case. Additionally, there is a coupling of lateral and directional effects — meaning the rolling and yawing motions interact — such that the required degree of static directional stability may actually be determined by some of these coupled conditions, not by the pure directional case alone. Finally, centre of gravity position. Here’s a nice relief: CG position has a relatively negligible effect on static directional stability. The usual range of CG position on any aeroplane is set by the limits of longitudinal stability and control — that’s the fore and aft limits you see in the weight and balance. Within that limiting range of CG position, no significant changes take place in the contribution of the vertical tail, the fuselage, the nacelles, and so on. So static directional stability is essentially unaffected by the variation of CG position within the longitudinal limits. So the takeaway: power is destabilizing, worst at high power and low dynamic pressure, and the propeller aeroplane feels it far more than the jet. And CG position, which dominates longitudinal stability, barely touches directional stability at all. That figure shows the sideslip angle at low and high angles of attack — it’s the geometry that sets up the fin’s restoring sideforce. Keep that in mind as we move on.