Hopefully your answer to the above question is a resounding ‘No!’, especially if you happen to be in an aeroplane! Positive Stability is a desirable trait for just about every aircraft design, and is a certification requirement in many categories including LSAs. In smooth air it allows a plane to fly ‘hands free’ for a short period, useful if you need to refold a chart or rescue a dropped pencil, and in rough air it will help minimise the feeling that you are wrestling a wild animal. You can have too much of a good thing though and excessive stability can also make for an uncomfortable ride, as the aircraft will aggressively attempt to align its heading and angle of attack with every gust.
An aeroplane which lacks positive stability is not necessarily impossible to fly, but it will be hard work, requiring constant vigilance to prevent any small divergence from the desired flight path escalating into a complete loss of control. Reduced stability does however have its uses: Planes designed for aerobatics sometimes have marginal or even neutral stability and modern fighter aircraft designs achieve their extreme agility by deliberately incorporating negative aerodynamic stability which is tamed only by using fly-by-wire and computer controls to mask this lack of stability from the pilot.
What is Stability?
Cutting edge technology is all very nice but most of us are in the ‘fly-by-cable’ rather than fly-by-wire category, so let’s get back to basics; what is stability and how does it affect the way an aeroplane handles? In an aircraft context, stability is divided into two types, ‘static stability’ and ‘dynamic stability’. Static stability is a bit of a misnomer, it certainly doesn’t involve the aeroplane standing still and is actually the aircraft’s immediate response to a disturbance. If you pull back on the sick raising the nose slightly and then quickly release the control again, all by itself your plane will nose back down. This is positive static stability at work, returning the plane to its trimmed angle of attack.
Dynamic stability on the other hand is what happens next, i.e. the plane’s response over a longer time period. If the nose up attitude is held for a little longer allowing some airspeed to bleed off, releasing the stick will still result in the same initial nose down motion, but, providing the power and trim settings remain the same, the plane will accelerate and attempt to regain its original airspeed. In most aircraft this response will result in an overshoot of the trimmed airspeed, causing the plane to pitch back up again. If the aircraft has positive dynamic stability this cycle will then repeat with progressively smaller airspeed divergences until equilibrium is eventually reached back at the trimmed airspeed. Figure 1 illustrates this and the impact different types of pitch stability will have on an aircraft’s behaviour following the pull up manoeuvre just described.
Figure 1 – Different stability responses to an initial disturbance.
In addition to the two types of stability described above, aircraft stability is conventionally separated into the three planes of rotation, longitudinal (pitch), lateral (roll) and directional (yaw). This can prove confusing for the uninitiated – as rotation in the longitudinal plane occurs around a lateral axis and vice versa. On the bright side, at least directional stability is obvious!
Figure 2 – Stability is divided into three planes: Longitudinal, Lateral and Directional.
As pilots we tend to take longitudinal stability for granted, adjusting the trim setting tens or even hundreds of times a flight and then happily leaving the plane to obediently maintain our selected angle-of-attack without sparing it so much as a second thought. But consider for a moment just what a remarkable feature the ability to trim to a selected angle-of-attack really is. After all, how many other objects when released into the atmosphere at height not only fail to immediately tumble straight downwards, but instead shoot off sideways at over ten times their rate of vertical descent? By anyone’s standards that is quite unusual!
Compare the above to directional stability, which is a far more intuitive quality – basically amounting to ‘pointing in the direction you are moving’. You can put a tail on just about anything and it will be directionally stable, all that’s required is that the centre of area be separated from the centre of gravity and the weathervane effect will naturally align the object with the prevailing wind. Bombs and arrows are two obvious examples of directionally stable shapes. Despite their differences it could be argued that directional and longitudinally stability operate on the same principles – it’s just that directional stability is inherently trimmed to give a zero angle of attack.
Lateral stability is a very different beast to the other two. For high wing aircraft the pendulum effect from having the centre of gravity positioned below the wing naturally provides a degree of lateral stability, but for low-wing aircraft roll stability must be produced aerodynamically. Fortunately gravity also lends a hand by causing a plane which is banked to sideslip allowing wing dihedral to create a righting moment (more on this next time).
Striking the Right Balance
Most planes are highly stable in pitch and fairly stable in yaw, but lateral stability is often much less pronounced. This is primarily due to the interaction between roll and yaw – snappily titled ‘Lateral-Directional Cross-Axial Coupling’ – which forces a designer to make a stability compromise in order to get acceptable handling:
Aircraft with weak directional stability but good lateral stability inherently have good ‘spiral stability’ meaning they are resistant to entering a spiral dive due to pilot inattention, however this arrangement is prone to produce ‘Dutch Roll’ (a poorly damped snaking motion where the plane continually trades yaw and roll, causing the nose to follow a path which traces a sideways figure eight), excessive roll stability also results in a plane which feels cumbersome to manoeuvre, a trait not popular with pilots. On the other hand a plane with good directional stability but weak lateral stability will feel more agile and be resistant to ‘Dutch Roll’, but will possesses mild spiral instability causing the plane to gradually bank and fall into a spiral dive if left unattended. Striking a balance between the two arrangements described above is a challenge and designers usually aim towards the latter, minimising Dutch Roll and relying on the pilot to manage the slight spiral instability (which is usually so slow acting that it is dealt with subconsciously).
Having covered the basics of what stability is and its effects, next time I’m going to dig a little deeper and take a look at how a plane is actually designed to deliver the positive stability required, plus why weight and balance is so important.
Ultralight Design 