Stability and Control — Page 273, Lesson 324

I want to walk you through the indirect effects of power on stability and control. This is a really important distinction, because when we talk about power effects, we split them into two main categories: direct and indirect. The direct effects are things like the thrust line itself and the forces it generates. But the indirect effects — the ones we're looking at now — are the changes power makes to the airflow over the rest of the aeroplane. And here's the key point I want you to hold onto: these indirect effects are of greatest concern in the propeller powered aeroplane, rather than the jet powered aeroplane. Why is that? Because a propeller physically accelerates a large mass of air backwards, and that creates what we call the slipstream. The slipstream is that accelerated column of air flowing over the aeroplane. In a propeller aeroplane, this slipstream creates velocities on the various surfaces which are different from the flow field typical of power-off flight. In other words, the airflow over the wing, the nacelle, and the fuselage is no longer the same as it would be if the engine were producing no thrust. Since the various wing, nacelle and fuselage surfaces are partly or wholly immersed in this slipstream, the contribution of these components to stability can be quite different from the power-off flight condition. Let me break that down. The nacelle is the streamlined housing that encloses the engine. So we have the wing, the nacelle, and the fuselage — all of these surfaces are sitting in that accelerated slipstream. Now, ordinarily, the change of fuselage and nacelle contribution with power is relatively small. So we don't worry too much about those two. The big changes come from the wing and the tail. Here's the first major effect. The added lift on the portion of the wing immersed in the slipstream requires that the aeroplane operate at a lower angle of attack to produce the same effective lift coefficient. Let me unpack that. The slipstream increases the dynamic pressure — the kinetic energy of the airflow — over that part of the wing. More dynamic pressure means more lift for the same angle of attack. So to achieve the same lift coefficient, CL, the aeroplane can fly at a smaller angle of attack. Generally, this reduction in angle of attack to effect the same CL reduces the tail contribution to stability. Why? Because the tail's contribution to stability depends on the angle of attack it sees. Reduce that angle, and the tail's stabilizing effect diminishes. But here's the counteracting effect. The increase in dynamic pressure at the tail tends to increase the effectiveness of the tail and may be a stabilizing effect. So we have two opposing forces at the tail. On one hand, the lower angle of attack reduces the tail's contribution. On the other hand, the slipstream increases the dynamic pressure at the tail, which makes the tail more effective. And the magnitude of this contribution due to the slipstream velocity on the tail will depend on the CG position and trim lift coefficient. So the centre of gravity position and the trim lift coefficient — the lift coefficient the aeroplane is trimmed to fly at — both determine how strong this tail effect actually is. Now there's a second indirect effect, and it comes from the direction of the slipstream. The deflection of the slipstream shown in Figure 10.29 by the normal force at the propeller tends to increase the downwash at the horizontal tail and reduce the contribution to stability. Let me explain that. The propeller doesn't just push air straight back. It also exerts a normal force — a force perpendicular to the thrust line — on the air, which deflects the slipstream. This deflection increases the downwash at the horizontal tail. Downwash is the downward component of airflow behind the wing. More downwash at the tail means the tail sees a more downward-directed relative airflow, which reduces its contribution to stability. So this is a destabilizing effect. So let me summarise the whole picture for you. In a propeller aeroplane, the indirect effects of power are significant because the slipstream changes the flow field over the wing, nacelle, and fuselage. The wing immersed in the slipstream produces more lift, so the aeroplane flies at a lower angle of attack, which reduces the tail's stabilizing contribution. But the increased dynamic pressure at the tail can increase tail effectiveness, depending on CG position and trim lift coefficient. And finally, the deflection of the slipstream by the propeller's normal force increases downwash at the tail, which reduces the tail's contribution to stability. These are the indirect effects of power, and they're why a propeller aeroplane's stability characteristics change so noticeably with power setting, whereas a jet aeroplane, with no such slipstream, is far less affected.