Stalling — Page 153, Lesson 181

Right, let's get into the heart of the stalling chapter. We're going to focus on the reference stall speed, which is given the symbol VSR. This is a critical, precisely-defined speed that the aircraft manufacturer and the certification authorities use as the baseline for all stall-related performance. First, let's clarify the relationship between VSR and the 1g stall speed, VS1g. The 1g stall speed is the speed at which the wing stalls when the aircraft is pulling exactly 1g — that is, in straight and level flight. Now, here's the key point: on an aircraft that does not have a stick pusher, VSR is considered to be the same as VS1g. But on an aircraft fitted with a stick pusher, it's a different story. A stick pusher is a device that abruptly pushes the nose down at a selected angle of attack to prevent the aircraft from actually stalling. Because it's impossible to fly at speeds less than that at which the stick pusher activates, the reference stall speed, VSR, is defined as being 2 knots or 2%, whichever is the greater, above the speed at which the stick pusher activates. So the stick pusher prevents you from ever reaching the true stall, and VSR is set just above that activation point. Now, let's make this concrete with the "sample" aeroplane we looked at earlier in the book, on Page 76. For that aeroplane, the speed at CLMAX — that's the maximum coefficient of lift, the point of maximum lift before the stall — was 150 knots. This speed is designated VCLMAX. At 1g, VSR would therefore be 150 knots for that aeroplane. But VSR is not a fixed number. It's determined under a very specific set of conditions, and it will vary with each of them. Let me walk you through the conditions that define VCLMAX, and therefore VSR. First, VCLMAX is determined with zero thrust at the stall speed. That means the engine is not contributing any thrust at the moment of the stall — it's as if the propeller is just windmilling, not pulling the aircraft forward. Second, the propeller pitch controls, if applicable, are in the take-off position. This is about the blade angle of the propeller being set for maximum thrust, even though we just said thrust is zero. It's a specific configuration for the test. Third, the aeroplane is in other respects — such as flaps and landing gear — in the condition existing in the test or performance standard in which VSR is being used. So if you're testing for a take-off performance standard, the flaps and gear would be in their take-off configuration. Fourth, the weight used is the weight when VSR is being used as a factor to determine compliance with a required performance standard. So the weight is set to the value relevant to the performance requirement being checked. Fifth, the centre of gravity position is set to the position that results in the highest value of reference stall speed. This is the most conservative, worst-case CG position, because it gives the highest stall speed. Sixth, the aeroplane is trimmed for straight flight at a speed selected by the manufacturer, but that speed must be not less than 1.13VSR and not greater than 1.3VSR. So the manufacturer picks a trim speed within that band, and the aircraft is trimmed for straight and level flight at that speed. Then, starting from that stabilized trim condition, you apply the longitudinal control to decelerate the aeroplane so that the speed reduction does not exceed one knot per second. This is a very gentle, controlled deceleration — you're pulling back on the stick slowly to bleed off speed at a maximum rate of one knot per second. And finally, as I mentioned, in addition to all these requirements, when a stick pusher is installed, VSR may not be less than 2 knots or 2%, whichever is greater, above the speed at which the device operates. So VSR will vary with each of these conditions — weight, CG, configuration, trim speed, and so on. And there are additional factors that affect VSR, which we'll detail later: load factor, thrust in excess of zero, and wing contamination. Load factor is the g-force on the aircraft, thrust in excess of zero means the engine is actually pulling, and wing contamination means things like ice or frost on the wing surface. One final, very important point to remember: density altitude does not affect indicated stall speed. Density altitude is the altitude corrected for non-standard temperature, and it affects true airspeed, but the indicated stall speed — what you read on the airspeed indicator — remains the same regardless of density altitude. That's a key distinction for a pilot to understand. So, to summarise: VSR is the reference stall speed, defined under a precise set of conditions, and it's the speed you'll see referenced in performance charts and limitations. It's the baseline from which all stall-related speeds are derived.