Speed Stability — Page 131, Lesson 155

Let's pick up with speed stability. I want to walk you through what it means for an aircraft to be speed stable, and why it matters so much in real flying. First, the foundation. For an aircraft to be in steady flight, it must be in equilibrium — there can be no out-of-balance forces or moments. When an aircraft is trimmed to fly at a steady speed, thrust and drag are equal. So in steady flight, the term DRAG and the term 'THRUST REQUIRED' have the same meaning. That's a key substitution: instead of thinking of drag against indicated airspeed, as in Figure 6.16, we can think of 'THRUST REQUIRED' instead. Now, the scenario. For an aircraft in steady flight, if there is a variation in speed with no change in throttle setting — and that throttle setting is called 'THRUST AVAILABLE' — then depending on the trim speed, there will be either an excess or a deficiency of thrust available. Figure 6.20 illustrates this. Let's look at that figure. On the vertical axis we have DRAG or Thrust Required. On the horizontal axis, IAS. The curve dips down to a minimum at VMD — that's the speed for minimum drag. The figure is divided into three regions: a Stable IAS region, a Neutral IAS region, and an Unstable IAS region. There are two key points marked: point 'A' in the stable region, and point 'B' in the unstable region. And the figure shows Thrust Available as a horizontal line, with Thrust Excess and Thrust Deficiency labelled on either side of those points. Now let's work through point 'A'. If an aircraft is established in steady flight at point 'A', lift equals weight, and the thrust available is set to match the thrust required. If the aircraft is disturbed to some airspeed slightly greater than point 'A', a thrust deficiency will exist. If the aircraft is disturbed to some airspeed slightly lower than point 'A', a thrust excess will exist. This relationship provides a tendency for the aircraft to return to the equilibrium of point 'A' and resume the original trim speed. Steady flight at speeds greater than VMD is characterized by a relatively strong tendency of the aircraft to maintain the trim speed quite naturally — the aircraft is speed stable. So speed stability is an important consideration, particularly at speeds at and below VMD, most often encountered during the approach to landing phase of flight. Now point 'B'. If an aircraft is established in steady flight at point 'B', again lift equals weight and thrust available matches thrust required. But here's the difference. If the aircraft is disturbed and goes faster than the trim speed, there will be a decrease in drag, giving an excess of thrust which will cause the aircraft to accelerate. If a disturbance slows the aircraft below the trim speed, there will be an increase in drag, which gives a thrust deficiency causing the aircraft to slow further. This relationship is basically unstable, because the variation of excess thrust to either side of point 'B' tends to magnify any original disturbance. Steady flight at speeds less than VMD is characterized by a tendency for the aircraft to drift away from the trim speed — the aircraft is speed unstable. If a disturbance reduces speed, it will naturally continue to reduce. If a disturbance increases speed, it will naturally continue to increase. So the whole picture is this: above VMD, you're speed stable — disturbances get corrected naturally. Below VMD, you're speed unstable — disturbances get magnified. And that's why the approach to landing, where you're often operating at or below VMD, demands such careful attention to speed control.