Stability and Control — Page 265, Lesson 320

Let’s start with the downwash at the horizontal tail, because this is where the real subtlety of stability lives. I want you to picture the airflow at the tail. It is not the same as the free stream — the undisturbed air far ahead of the aeroplane. The flow at the horizontal tail has a different direction and a different dynamic pressure. Why? Because of the wing wake, the fuselage boundary layer, and power effects. All of those disturb the air before it reaches the tail. The dynamic pressure at the tail — that’s the pressure associated with the moving air, the force per unit area that the airflow exerts — is often greatly different from the free-stream dynamic pressure. In most instances, it is less. And that reduced dynamic pressure reduces the efficiency of the tail. So the tail simply doesn’t produce as much force as it would in clean, undisturbed air. Now here’s the key point about angle of attack. When the aeroplane is given a change in angle of attack, the horizontal tail does not experience the same change as the wing. Look at Figure 10.23. The reason is downwash. Behind the wing, the airflow is deflected downward — that’s downwash. When the wing’s angle of attack increases, the downwash behind the wing increases. So the tail, sitting in that downwash, sees a smaller change in angle of attack than the wing does. Let me give you the numbers from the text. Suppose the wing’s angle of attack changes by 10 degrees. That causes a 4-degree increase in downwash at the horizontal tail. The tail therefore experiences only a 6-degree change in angle of attack. Ten minus four. So the downwash at the horizontal tail reduces the tail’s contribution to stability. The tail is less effective as a stabiliser because it doesn’t feel the full angle-of-attack change. Now, anything that alters the rate of change of downwash at the tail will directly affect the tail contribution and therefore the aeroplane’s stability. The text gives two examples: flaps, and propeller slipstream. Both change how the downwash develops. And the bottom line is stated plainly: downwash decreases static longitudinal stability. Let me make sure that term is clear. Static longitudinal stability is the tendency of the aeroplane, when disturbed in pitch, to return to its original attitude on its own, without pilot input. Downwash works against that. It reduces the restoring tendency. Now let’s move to power-off stability. When we’re interested in the aerodynamic stability of a configuration, we neglect power effects. We consider the stability as a build-up of the contributing components — the wing, the fuselage, the tail, each contributing its own share. Figure 10.24 shows a typical build-up for a conventional aeroplane configuration. Let’s set the centre of gravity arbitrarily at 30 percent MAC. MAC is the mean aerodynamic chord — the average chord of the wing, the reference line along which we measure the CG position. So 30 percent MAC means the CG is located at 30 percent of that chord from the leading edge. Now look at the curves. The contribution of the wing alone is destabilizing. That shows up as a positive slope of the curve of CM versus CL. CM is the pitching moment coefficient — a dimensionless number representing the aeroplane’s tendency to pitch. CL is the lift coefficient. A positive slope of CM versus CL means that as lift increases, the pitching moment increases in a way that drives the nose further away from equilibrium — that’s destabilizing. When you combine the wing and the fuselage, the instability increases. The fuselage adds to the destabilizing effect. Now the tail alone — the tailplane only — is highly stabilizing. That shows as a large negative slope of the curve. A negative slope means the pitching moment opposes the disturbance — it restores. And here’s the crucial requirement. The contribution of the tail must be sufficiently stabilizing so that the complete configuration — the whole aeroplane — will exhibit positive static stability at the anticipated CG locations. In other words, the tail has to be strong enough to overcome the destabilizing wing and fuselage, and leave the aeroplane with a net restoring tendency. That’s the whole balancing act of longitudinal stability. The tail is the stabiliser that saves the configuration. So to tie it together: downwash at the tail reduces its effectiveness and decreases static longitudinal stability. And in the power-off build-up, the wing and fuselage are destabilizing, the tail is the strong stabilizer, and the tail must be powerful enough that the complete aeroplane is positively stable at the expected CG positions.