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Static longitudinal stability - Steady as She Goes?

By Ed Kolano

In September we concluded our discussion of airspeed calibration by presenting a few readers' ideas and techniques for using GPS in this process that appear to be technically sound.

GPS is easier to say than non-maneuvering longitudinal static stability, but this mouthful just ain't that complicated. Still, "static long-stab" is an important stability characteristic, and it could be a bit insidious in how it affects your flying.

When talking about airplanes there are several different types of stability. You're probably familiar with the two basic labels-static and dynamic.

Static stability addresses whether your airplane initially tends to return toward its pre-disturbed condition. For example, when you move the pendulum of a grandfather clock away from its hanging position and release it, its first move is back toward its hanging position. This is positive static stability.

Dynamic stability addresses whether your plane actually does return to its pre-displaced condition and how it does or does not get there. Release that clock pendulum, and it will swing back and forth several times, eventually coming to rest at its original position-positive dynamic stability.

While static and dynamic stability are the two basic kinds of stability, there are additional classifications for the longitudinal (pitch) axis and the lateral-directional (roll-yaw) axes.

Longitudinal static stability involves maneuvering (traditionally called "maneuvering stability" or "man-stab") and non-maneuvering (traditionally called "long-stab" or "long-stat") flight. Both arise from your airplane's pitch response to an angle of attack change, but they are assessed differently. Man-stab is your plane's response during accelerated flight (when you're pulling Gs). Long-stab describes your airplane's initial tendency following a deviation from its trimmed airspeed.

Say you've trimmed the plane for 100 knots in straight and level flight. You'd expect to hold a little back-stick to fly 90 knots and a little forward-stick to fly 110 knots, if your airplane exhibits positive longitudinal static stability. Holding back-stick implies that if you let go the plane would initially tend to accelerate back to its initial trimmed airspeed and vice versa for the forward-stick, faster airspeed case.

If you have to push the stick to maintain 90 knots, your airplane would be statically unstable at that flight condition. The implication here is that if you release your push, the plane's initial tendency would be to decelerate more, moving further away from its trimmed airspeed. Clearly an undesirable, nonintuitive situation.

If you released the stick and the plane remained at 90 knots or 110 knots when it was initially trimmed for 100 knots, it would appear to exhibit neutral static stability. I say "appear" because other factors-like control system friction-might be overpowering your plane's static stability.

Figure 1 shows a static stability plot for airplanes with positive, neutral, and negative static stability.


Why Not Re-trim?

Why would you fly off-trim? Well, you always fly off-trim, until you re-trim. Whenever you change your flight condition, the airplane's trim requirement probably changes, and you re-trim after establishing the new condition.

For example, you approach the downwind leg of a landing pattern at 100 knots and slow to 80 knots on downwind. Power and airspeed changes often cause pitching moments, which you counter by trimming. When those moments are balanced, you don't have to hold forward or aft stick to remain at that airspeed. After you slow to 80 knots you know you have to re-trim because you're holding back-stick. This is probably the most important aspect of long-stab-the stick force cue that tells you the airplane is not at its trimmed airspeed.

Pilots receive information through every sensory channel. We process and react to these separate puzzle pieces without even thinking about them. Some cues to an inadvertent airspeed change are wind noise, engine noise, vibration changes, coordinating rudder requirements, and stick force.

During cruise flight this stick force cue has limited utility because you're not typically holding any stick force here. Once established at your cruise altitude and airspeed you trim out the stick force. If your airspeed deviates, you wouldn't know it through stick-force feedback because you're flying "hands-off." Under these conditions the airspeed indicator and altimeter will most likely be your first clues to an inadvertent airspeed deviation.

There are other phases of flight where stick-force feedback can be your primary cue to an airspeed deviation. Let's say you're on a long final approach to land at Oshkosh during EAA AirVenture. Three airplanes are ahead of you, and who knows how many are behind you.

The radio is jammed with the controller telling the plane on the runway to take it to the end, the plane about to touch down that it needs to remain aloft until the orange cones, the guy behind him to land on the runway's left side, and the guy between that guy and you to slow down and take the right side. He's also barking out similar orders to a few planes behind you.

Naturally, most of your visual attention is directed outside the cockpit. You're sacrificing your normal instrument scan to enhanced seeing, avoiding, and following the controller's directions. If you find yourself holding back-stick during all this head-on-a-swivel activity, it probably means you've slowed down. Your airplane's positive static stability tells you that through stick-force feedback.

Another example is approaching minimums during an actual instrument approach. While in the clouds you established a steady airspeed/power/glideslope relationship, and all of your visual attention is directed toward your flight instruments. As you approach your go-around decision, you start to look outside for the runway.

If you're barely emerging from the cloud base, catching an occasional glimpse of ground beneath you but still can't see the runway ahead, you might devote more attention looking for the runway and less attention to your instruments. Noticing you're pushing on the stick tells you you're flying faster than your established approach speed and you've probably pushed your plane below the glideslope or minimum descent altitude.

Piloting chores during these critical phase-of-flight examples are more difficult in an airplane without positive static stability.

An airplane with neutral static stability provides no force cue at all to a changed airspeed. Such an airplane maintains whatever speed you like hands-off. In this case, you have to rely on alternate cues like noise and vibration changes and keep a diligent eye on the airspeed indicator.

On the plus side, you never have to trim a neutrally stable airplane. You'll have to move the stick forward or aft to cause an airspeed increase or decrease, but once at the desired airspeed, you can simply release the stick. Some military fighters are designed with this feature. Not having to re-trim as the pilot accelerates 300 knots to engage a ground or air-to-air target reduces the pilot's workload. Not really the same motivation in the homebuilt world.

Let's say you've trimmed for straight and level flight at 100 knots. Then you slow to and maintain 80 knots without re-trimming. Your airplane has positive static stability, so you must hold, say, 5 pounds of back-stick force. If your airplane has 6 pounds of friction in its longitudinal (pitch) control system, you might be able to release the stick at 80 knots and the airplane will stay at 80 knots. In this case the friction masks the airplane's positive stability and makes it appear to be neutrally stable.

If you slow to 70 knots, where it takes 8 pounds of back-stick, you'd have to keep some pull force on the stick because the required stick force is greater than the friction that is helping hold the stick back. This airplane appears to be neutrally stable within a band of airspeeds around the trimmed airspeed and positively stable outside that trim-speed band.

Flying an exact speed within that trim-speed band can be difficult and frustrating. Let's say the trim-speed band is 15 knots, and you want to cruise at 100 knots. You set the power to the cruise setting you know will provide 100 knots, and the airplane settles on 105 knots. So you apply a little back-stick, and the plane slows to 100 knots.

If you release the stick here, the plane will likely continue to decelerate because the friction will hold the elevator where it was for the deceleration, even after you release your pull on the stick. So you push a little to accelerate, and the same thing happens in reverse. It turns into a game of trial and error, nudging the stick, waiting to see what airspeed eventually settles out, and then nudging again. Frustrating.

Military fighters don't have this problem because their flight control computers maintain hands-off 1-G, zero-pitch-rate flight, and military pilots establish the desired airspeed with throttle. If you try this in the high-friction, computerless airplane, every throttle change will probably pitch the airplane nose-up or nose-down, and your altitude will change. An autopilot with an altitude-hold feature should allow this throttle/speed trick to work.

You can fly a statically unstable airplane just as you can balance a broomstick vertically on your up-turned palm. How long you can do it is another story. Balancing the broomstick takes total visual concentration and constant hand-eye coordination. Fortunately, statically unstable airplanes are usually only mildly unstable and usually only in a particular configuration. Still, the pilot must dedicate more time to airspeed control in these airplanes, and that means less time is available for other necessary piloting tasks. And yes, a few popular kit planes have mildly unstable characteristics during certain flight phases.

The farther aft an airplane's center of gravity (CG), the less stable it is. This is why there are limits to the aft end of the allowable CG range. Figure 2 shows the effect of CG on a static stability plot. The forward CG limit is usually determined by how much nose-up authority the elevator has. The worst case for the forward CG limit is typically the landing flare in ground effect with the gear down, and flaps down in ground effect, all of which create nose-down pitching moments the elevator must overcome during the flare.


What's Enough?

How much stability is good is a good question. The airplane should be stable enough to provide reasonable stick force cues to an off-trim condition. It should be stable enough to remain at its trimmed airspeed rather than wander off. But it shouldn't be so stable that it takes two hands on the stick to fly a temporary off-speed condition.

Federal Aviation Regulation 23.171 requires general aviation airplanes to have positive static stability under most flight conditions: "The airplane must be longitudinally, directionally, and laterally stable…. In addition, the airplane must show suitable stability and control 'feel' (static stability) in any condition normally encountered in service, if flight tests show it is necessary for safe operation."

How much "control feel" is suitable? The FARs don't say for small airplanes, but for transport airplanes it sets a minimum 1 pound of stick force for every 6 knots off the trimmed airspeed.

The stick force versus airspeed gradient is not a true measure of an airplane's static stability, but it is an operational cue pilots use. You can increase the stick force by adding springs to your longitudinal control system; this doesn't make the airplane more stable, but it can provide a more obvious force cue to the pilot.

Adding springs is not a simple solution because they will affect every control input you make, and the results may be good for static stability feel and bad for dynamic stability, flutter, and other airplane behaviors.

The FARs also address the trim-speed band. General aviation airplanes must have a trim-speed band no larger than 10 percent of the trimmed airspeed. In other words, when the airplane is trimmed for its 70-knot final approach speed, it can have a 7-knot trim-speed band, and when it's trimmed for its 180-knot cruise speed, it can have an 18-knot trim-speed band. Neutral and negative static stability are not allowed in certificated general aviation airplanes.

What's right for your airplane depends on how you fly your airplane. Neutral static stability might be good for an aerobatic airplane that undergoes large airspeed changes quickly when maneuvering.

IFR pilots could benefit from an airplane with off-airspeed stick force cues should they inadvertently neglect the airspeed indicator in favor of the ILS needles during an instrument approach.

Day VFR fliers can get by with a mildly unstable airplane, but they should be aware of that instability and maintain a rigorous airspeed indicator scan, particularly near the ground or other airplanes.

This month we explained what non-maneuvering longitudinal static stability is and gave a few examples of how it can affect you when flying. Next month "Test Pilot" will explain how to test your airplane to determine its static longitudinal characteristics.

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