Stability and Control — Page 286, Lesson 341

Let's talk about the landing control requirement. This is one of the three principal conditions that demand adequate longitudinal control, and it's the one that applies as you're bringing the aeroplane down onto the runway. The core idea is this: at landing, the aeroplane must have sufficient control power to ensure adequate control at specified landing speeds. So it's not just about having enough control authority in general — it's about having it specifically at the speeds you'll actually use when landing. Now, here's the critical part. The most demanding situation exists when three things are true at once: the centre of gravity, the CG, is in its most forward position, the flaps are fully extended, and the power is set at idle. Why does that combination create the toughest case? Because that configuration provides the most stable condition. And a highly stable aeroplane is the most demanding of controllability — the more stable it is, the harder it is to force it to change attitude, so you need more control authority to overcome that stability. But the landing control requirement has one particular difference from the manoeuvring control requirement of free flight. As the aeroplane approaches the surface, there's a change in the three-dimensional flow over the aeroplane due to ground effect. Let me explain what ground effect does. A wing in proximity to the ground plane will experience a decrease in tip vortices and a decrease in downwash at a given lift coefficient. So the air flowing off the wing is less deflected downward than it would be in free flight. Now, why does that matter for the tail? Because the tail sits in that downwash. When downwash decreases, the tail sees a different flow, and that tends to increase the static stability. And here's the key consequence: the reduction in downwash at the tail produces a nose-down moment from the reduction in down load on the tail. So the tail, which normally carries a downward load to balance the aeroplane, has less of that down load in ground effect, and that creates a nose-down pitching tendency. So what does the pilot have to do? The aeroplane just off the runway surface will require additional control deflection to trim at a given lift coefficient. In other words, you need more elevator deflection to hold the same lift condition near the ground than you would in free flight. And that's why the landing control requirement may be critical in the design of longitudinal control power — you have to size the elevator to handle this ground-effect condition. Let me give you a concrete example of how significant this is. A typical propeller powered aeroplane may require as much as 15° more up elevator to trim at CLMAX in ground effect than in free flight. CLMAX is the maximum lift coefficient — the highest lift condition, which is exactly what you're dealing with in the flare just before touchdown. So 15 degrees of extra up elevator is a substantial demand on the control system. There's also a caution here about trim tabs. In some cases the effectiveness of the elevator is adversely affected by the use of trim tabs. If trim is used to excess in trimming stick forces, the effectiveness of the elevator may be reduced, which would hinder landing or take-off control. So over-trimming to relieve stick force can actually rob you of elevator authority when you need it most. Now let me tie this back to the bigger picture. Each of the three principal conditions requiring adequate longitudinal control — and the landing case is one of them — are critical for high static stability. If the forward CG limit is exceeded, the aeroplane may encounter a deficiency of controllability in any of these conditions. That's the danger: push the CG too far forward and you may simply not have enough control authority to handle the aeroplane. And that leads to the two fundamental limits that define the CG envelope. The forward CG limit is set by the minimum permissible controllability. The aft CG limit is set by the minimum permissible stability. So the forward limit protects you from losing control authority, and the aft limit protects you from losing stability. The landing control requirement is one of the conditions that drives that forward limit — because ground effect demands extra elevator deflection, it's often the landing case that sets how far forward the CG is allowed to go.