Straight Ahead

Directional and lateral stability help make an airplane a good IFR platform. Heres why along with some tips on how to measure it.


by Greg Lewis

What are some of the flying characteristics that make an aircraft a good platform in instrument flight conditions? One aspect is static stability about the roll and yaw axes, typically referred to as lateral-directional static stability. As we define and explore this type of stability, well also describe ways for non-test pilots to assess their own aircrafts characteristics and thereby determine how well-suited it is for IFR.

The first article in this series on IFR platforms appeared in the August 2004 issue and detailed some of the contributors to, and benefits of, static stability with respect to pitch and airspeed. But there are many more aspects to an airplanes static stability than just its speed; many of them can make an airplane either a pleasure to fly or a real handful.

Static Stability
To re-establish some common ground from our first article, remember that the idea of static stability in any aspect of flying was explained by considering a ball in a cup, such as the one shown in Example 1. If the ball starts at the bottom of the cup and is then moved up one side and released, it will start moving downhill, back towards its starting point. Thats a definition of static stability: An object displaced from equilibrium tends to return towards equilibrium. The cup in Example 2, with its larger radius, is still stable, but the magnitude of its stability is less; the ball will start back toward its starting point less quickly. Example 3 is a ball on a flat table: if displaced, it wont start back at all but will rather stay in the displaced location and is thus neutrally stable. Finally, the definition of a statically unstable system is illustrated by the inverted cup in Example 4.

You could balance the ball on the top, but if it gets displaced and is released, the first motion would be away from its initial condition. To keep an unstable system steady in a real world environment requires constant input, and in the case of an aircraft that means constant pilot input (or computer input if you can afford a flight control system like that in the F-22 supersonic fighter!).

Lateral-Directional Coupling
Our first article focused solely on pitch changes and ignored any possibility of developing a roll or yaw when pitch changes were made. For the most part that isnt a bad assumption. Most aircraft are symmetric right and left, with some of Burt Rutans designs being notable exceptions. Because of that symmetry, pitch can be assessed in isolation. But not roll or yaw. A roll input in an aircraft typically causes yaw due to unequal drag on the two wings as a result of the changes in lift right and left. The yaw so produced is normally called adverse yaw because the yaw produces a roll in the opposite direction of the initial roll command. Rudder inputs made to yaw the aircraft also produce a roll in the same direction as the rudder input. This phenomenon-known as coupling-should be present in every general aviation aircraft because the FAA requires it to be that way in all Part 23 certified aircraft (those with 12,500 lbs or less maximum gross weight). Well discuss coupling in greater detail below when discussing how to assess your own aircraft, but for the moment lets analyze each of the two axes separately.

Directional Static Stability
Of the two, lateral and directional, directional static stability is far more important. Pilots can contend with an aircraft that has a small lateral instability, but a little directional instability could quickly lead to a complete loss of control if not corrected for immediately. Consider Figure 1. If the nose of the aircraft is yawed to the left, the aerodynamic forces due to the sideslip hopefully generate a yawing moment to the right. This is just like the ball in the cup in Example 1 on the previous page, a statically stable system. If the system ever became unstable, a slight yaw would produce forces that yaw the aircraft farther in the same direction, in turn producing greater forces that would cause a bigger yawing motion in the same direction which would … well, youve got the idea by now. The result of such an unstable system would be a fast, pure yaw divergence, causing the aircraft to quickly depart controlled flight. And this is exactly what happens when a tail-dragger ground loops-but more on that later.

So what contributes to directional stability? There are two big pieces. The bad actor is the fuselage. Looking at the fuselage in isolation, in a sideslip, is somewhat like looking at an airfoil at an angle of attack. A lift (sideforce in this situation) force develops and acts close to 25% of the way back from the leading edge, just like on most wings. If the mass distribution within the fuselage is close to uniform, then the center of gravity of the fuselage will be halfway back and the sideforce would push the nose further away from straight ahead. This is depicted by the left image in Figure 2, above, just like the ball on the top of an inverted cup. But the other big piece in this story is the vertical tail, and it provides a force that restores the nose into the wind, just like a weathervane or the feathers on an arrow as shown in the right image in Figure 2. So all the designer has to do is make sure that the restoring forces of the vertical tail generate moments that are bigger than those caused by forces on the fuselage and the aircraft will be directionally stable.

Put another way, to ensure maximum lateral stability, you can never have too much tail. The only downside to an excessively large vertical stabilizer would be extra skin friction drag in cruise. Most designs err on the side of large tails, to avoid directional stability issues.

Taildraggers suffer from ground loop problems during the early part of takeoffs and the latter part of landings because the aerodynamic restoring forces of the vertical tail are small at low speeds. On the ground, a taildraggers center of gravity is behind the pivot point of the main wheels. Thus, if you get a yaw disturbance at low speed, one of the laws associated with inertia-a body in motion tends to stay in motion-means that the CG wants to continue moving around the pivot point. Since the restoring sideslip forces are small at slow airspeeds, the result is an directionally unstable system. All taildragger pilots therefore need to be very active on the directional controls during low speed operations to prevent a groundloop or yaw departure.

Lateral Static Stability
Whereas the need for strong directional stability in an aircraft is clear under all conditions, the need for strong lateral stability is less obvious. One way to appreciate lateral stability is to understand that it causes a roll in the direction of rudder input. That is, if the left wing drops while your hands are full of approach plates and charts, you can raise the low wing by stepping on the opposite rudder. Its also what causes a rolling tendency if you dont center the ball during a power-on approach to a stall. But it may be less clear why that is called stable.

One way to understand this concept is to consider an aircraft in hands-off, trimmed flight. If a gust upsets the aircraft in roll, the loss of lift would allow the aircraft to fall, resulting in sideslip from the same side as the roll upset. (If thats hard to understand at first, imagine the extreme-an upset to 90 degrees of right bank, with the aircraft falling, which produces some wind in your right ear, or sideslip from the right). So an upset in roll to the right, hands off, produces sideslip from the right which is the same as what you get if you had stepped on the left rudder, which hopefully produces a left roll, righting the aircraft back towards wings level-a statically stable system!

And what contributes to lateral static stability? There are many factors, but the biggest two in general aviation aircraft are dihedral and the wings position relative to the fuselage (high- or low-mounted wings).

Dihedral can be defined as when wingtips are higher than the wing root relative to the horizontal, as shown below in Figure 1. If the wingtips were lower than the root it would be called anhedral. Dihedral is so important to lateral static stability that anything that causes a stable roll due to sideslip is called dihedral effect.

To illustrate this concept, imagine a component of the relative wind coming from the left. That part of the wind would strike the bottom of the left wing and the top of the right wing. The resulting sideslip would cause a left, or stable, roll back to level (remember, wind in the left ear causing a left roll is stable).

The other big factor is the wing position. Figure 2 on the opposite page depicts a rear view of an airplane with a high wing upset in roll to the left. The roll has produced a component of wind from the left. This wind from the left strikes the fuselage and goes around equally towards the top and bottom, except in the vicinity of the wing root where the flow is blocked. This blockage causes the pressure under the left wing to increase which in turn raises the low wing.

Just the opposite happens in an aircraft with a low-mounted wing, as shown in Figure 3. The increased pressure forces the low wing down. Therefore, high-wing aircraft will tend to have good lateral static stability while low wings contribute to poor lateral static stability.

These two major factors-dihedral angle and wing position-along with several other smaller factors, are combined into a total that will give a modest amount of lateral stability. Aircraft with low wings tend to have dihedral and aircraft with high wings generally have zero dihedral, or even a small amount of anhedral.

But sometimes the pieces dont add up exactly as envisioned by the designer and the aircraft may not have sufficient lateral stability to meet the certification rules. One way to solve the problem after hardware has been built is to add an aileron/rudder interconnect (ARI), which may be sufficient to meet regulatory requirements. Many general aviation aircraft have ARIs.

Critical Conditions
In our article on speed stability, the CG location was shown to be the most important variable affecting stability that the operator could control. For lateral and directional stability, CG wasnt even mentioned above. And thats because it is relatively unimportant. An aft CG is critical for directional stability because it results in a smaller distance arm for the vertical tail forces to work on (Moment = Force x Distance). But the range of longitudinal CG travel is almost insignificant compared to the total distance back to the tail; inches compared to 15 or 20 feet, typically. As a result, CG travels are very small and dont have a significant effect on directional stability. A major factor though, can be a lateral imbalance, especially on lateral static stability. For this reason, certification tests have to be done with the maximum lateral imbalance allowed by the aircraft flight manual.

Critical flight conditions are usually slow speed and, especially for lateral stability, flaps down. Flaps are normally on the inboard section of the wing. Therefore, with the flaps down the majority of the lift is closer to the fuselage, which means that anything that changes the lift between the wings to create a stable roll will be acting through a smaller arm and thus be less effective.

Real-World Tests
Three easy tests to see if your aircraft is laterally and directionally statically stable are summarized on Page 21. Both cruise and approach configurations should be tested; normally, low-speed cases are the more critical for stability in general aviation aircraft.

The first test would be to conduct a steady-heading sideslip. Use right rudder to yaw the aircraft; youll end up banking left to keep a steady heading. If you have both right pedal force and deflection, plus left aileron force and deflection, thats a stable result. Increases in pedal deflection should result in increases in all other parameters. But use caution: Prolonged sideslips could result in fuel starvation. Worse, if airspeed decays toward a stall, an inadvertent spin could result. But modest, steady-heading sideslips right and left should demonstrate whether or not your aircraft is laterally and directionally stable.

An even easier test for directional stability is to perform a modest flat turn-put in some rudder and keep the wings level with aileron. Releasing the rudder from a flat turn should cause the nose to promptly return to straight into the wind, just like a weathervane.

The easy test for lateral stability is to see if the rudder can be used to pick up a wing. The FAA does it by starting from a steady heading sideslip and releasing the lateral control, but it is just as reasonable to start in a shallow turn and see if you can use rudder alone to raise the low wing.

How Much is Enough?
You would like as much directional stability as you can get; its hard to have too much. As long as you have enough rudder control power to align the nose with the runway during crosswind landings then the directional stability is not too high.

Lateral stability is another issue. Too much lateral stability would make crosswind landings difficult in that you could run out of opposite aileron while trying to maintain a steady heading sideslip to landing. In fact, an argument could easily be made that neutral lateral stability would be the best for crosswind landings because you wouldnt have to hold down the wing on approach.

Even though the FAA requires lateral stability, military specifications allow some instability. As long as no more that 50% of the lateral control in the unstable direction is used during landings, the military is satisfied. But a good reason for the FAAs requiring a laterally stable aircraft may be the need to provide an alternate way of controlling roll in the event of aileron failure. As long as you can keep the wings level using rudder alone, your lateral static stability is probably sufficient.

Other Considerations
A significant factor not covered in this article is the effect of power on stability. Power effects on both lateral-directional stability and on speed stability covered previously can be significant. These effects will be covered in a future article. Well also cover dynamic stability, as that can have a very dramatic impact on how the pilot perceives an aircrafts flying qualities.

Also With This Article
“Visualizing Stability”
“Directional Stability”
“Lateral Stability And Airplane Design”
“Lateral/Directional Stability Test”

-Greg Lewis is deputy director and an instructor pilot at the National Test Pilot School, Mojave, Calif.,


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