November 23, 2018, ©. Leeham News: In the spring I ran a series of Corners which dealt with aircraft stability on a basic level (April 13 to June 8). It covered the aircraft’s basic stability modes in normal flight and described the basic helper systems one finds on aircraft, such as yaw dampers and autopilots. But we did not go deeper into aircraft stability problems and more advanced helper systems.
Given recent events, it can be interesting to dive a bit deeper into the pitch stability of an aircraft and common helper systems.
An aircraft’s airfoil is characterized by the air’s pressure distribution around the airfoil when the air flows past it.
Figure 2 shows two different types of airfoils, one conventional and one supercritical airfoil. The conventional is an often used historical airfoil and the latter the type of airfoils used in airliners flying today. The supercritical one has a different shape and therefore a different pressure distribution.
By curving less abruptly in the first part of the airfoil, the under pressure compared to ambient air (observe the Y-axis has the lower pressure at the top) is kept at a lower value than for the conventional airfoil and therefore it speeds up the air less passing over the airfoil. This gives it better high Mach characteristics.
We can also see the two airfoils have very different lift distributions over the chord of the airfoils (chord= the length axis of the airfoil). If we sum the pressures on the top and bottom sides (the top and bottom curves for both airfoils) we get different moments on the airfoils should we pivot them at half chord length. The conventional airfoil would like to pitch up and the supercritical pitch down.
Airfoils are characterized by their lift characteristics but also how their pitch moments vary when flown with a different angle of attack against the airstream. Figure 3 shows a typical lift force (here characterized by the lift coefficient Cl curve) versus angle of attack curve for an airfoil or aircraft wing.
At high angles of attack, caused by low speed or a high aircraft load factor, the wing flies closer to stall = maximum lift. Before reaching stall most wings start to shake from a partially separated flow. This is called buffeting.
When an airfoil or wing is characterized by CFD tools (Computer Fluid Dynamics programs) and later tested in wind tunnels, the lift curve but also the pitch moment curve is measured. Figure 4 shows a generalized pitch moment curve for two different airfoils or straight wings using such airfoils (the pitch moment is shown by the pitch moment coefficient Cm).
A flying wing will move in pitch around its Center of Gravity. The red wing is called pitch stable as any increase of Alpha from for example a gust would increase the lift (Figure 3) and at the same time increase the pitch down moment of the wing, Figure 4. This returns the wing to a lower Alpha. The wing is correcting the gust disturbance by itself.
The blue wing, on the contrary, will pitch up when hit by a gust which increases Alpha . This increases the Alpha further, which increases the pitch up moment, which….. This wing is unstable, it would ultimately flip over backwards.
We have now covered the basics around lift and pitch stability for a wing. In the next Corner, we start building an aircraft and look at its pitch stability.