Stalling — Page 179, Lesson 212

Let's pick up with the undercarriage and how it changes the stall. I want you to picture the aircraft in a stall, hanging on the wing's lift. Now, when we lower the landing gear, we're not just changing the shape of the aircraft; we're changing the balance of forces. Look at Figure 7.25. With the undercarriage down, the profile drag from the gear increases. And here's the key: that drag acts below the centre of gravity. Because it's pulling back and down below the CG, it creates a nose-down pitching moment. The aircraft wants to pitch its nose down. To hold the nose up and maintain the attitude, the tail must work harder. We have to increase the tail down load. Now, if the tail is pushing down more, the wing has to produce even more lift to balance that extra downward force. So the total lift required goes up. Now, you might think the CG moves when the gear extends, and it does, but the book tells us that CG movement due to the direction the undercarriage extends has an insignificant influence on stall speed. The dominant effect is that increased profile drag. The bottom line, and this is the rule to remember: extending the undercarriage increases stall speed. Now let's move to engine power, which is a bigger and more subtle story. The certification rule CS-25.103(b) tells us that VCLMAX is determined with zero thrust at the stall speed. VCLMAX is the maximum lift coefficient speed, the stall speed in the landing configuration. So the baseline stall speed is measured with the engines at zero thrust, and it's assumed the weight is entirely supported by lift. But in reality, if you apply thrust close to the stall, the aircraft is in a nose-high attitude. That means the thrust vector is tilted upward. So part of the thrust is now pointing up, and that vertical component of thrust helps support the weight. If thrust is helping hold the plane up, less aerodynamic lift is required. And less lift required means you can fly slower before the wing stalls. The book lists the most important factors in this relationship: engine type, whether propeller or jet; the thrust-to-weight ratio; and the inclination of the thrust vector at CLMAX. Let's take propellers first, because they have an extra effect. Look at Figure 7.26. The propeller slipstream is moving faster than the free stream flow, depending on the thrust developed. So at low airspeeds and high power, the dynamic pressure inside that slipstream is much greater than outside. Higher dynamic pressure means much more lift. So at a given angle of attack and airspeed, the propeller's lift is greatly affected. This has a real operational consequence. If you're in the landing flare and you suddenly reduce power, you lose that slipstream lift. The book warns that a significant reduction in lift could result in a heavy landing. Conversely, a judicious blast from the engines can avoid that potentially heavy landing. Now the jet. A typical jet doesn't have that induced flow from a propeller slipstream. So the only significant factor is the vertical component of thrust, shown in Figure 7.27. Since that vertical component helps support the weight, less aerodynamic lift is needed. If the thrust is large and given a large inclination at maximum lift angle, the effect on stall speed can be very large. And because there's very little induced flow from a jet, the angle of attack at stall is essentially the same power-on as power-off. So the summary rule: power-on stall speed is less than power-off. And the book flags that this will be significant when we study windshear in Chapter 15. Keep that in mind — it's a connection we'll come back to.