Under the Umbrella: Stability, Control, and Handling Qualities

Using the range and endurance testing described in the February “Test Pilot,” in March we explained level flight performance data reduction that produces useful tables and plots of speeds that result in an airplane’s maximum range, endurance, and specific range. We’ll take a break from performance this month and start discussing about flying qualities-those airplane characteristics that determine how easy or difficult it is to accomplish piloting tasks.

Performance sells. Period. Just look at the ads. Many of them ask, How fast? How slow? How far? How economical? The answers to these questions are important, but airplane performance is more important in some segments of aviation than others. Airlines operate on the bottom line, and fuel efficiency reigns. When jet fuel goes up a penny a gallon it adds megabucks to the bottom line.

In recreational aviation we fly for fun, not profit. We care about operating costs, cruise speed, and range, but a 200-knot cruise speed can quickly lose its luster if the airplane seems to be continually out of trim, constantly wiggles its way through the air, or takes unusual skill to fly. When this is the case, that 200-knot appeal can soon be reduced to gratitude only in that it means less time between takeoff and landing to be annoyed by unfriendly flying qualities.

Stability and Control  

Flying qualities are those characteristics that determine the ease or difficulty involved in accomplishing a task with the airplane. Flying-qualities testing involves the airplane’s static and dynamic stability and its control characteristics. No matter who’s at the controls, these things profoundly influence the accomplishment of every flying task.

Stability and control are two words frequently seen together, but they have different meanings. Stability is an airplane’s response to a disturbance. For example, if you slow the airplane with elevator alone to an airspeed different from the one it’s trimmed for, and you must maintain a pull force on the stick to hold this new, slower airspeed, the airplane is exhibiting positive non-maneuvering static stability. Generally, this is good.

If you didn’t need to hold any stick force to maintain the new, slower speed, the airplane would be neutrally stable. Imagine setting and keeping a cruise airspeed with a neutrally stable airplane. If you had to push the stick to keep the airplane from slowing even more, it would be demonstrating negative static stability. What would the landing flare be like in an airplane with negative static stability? Important stuff, and yes, there are airplanes that can exhibit each of these characteristics.

Control is what the pilot does to make the airplane change its condition. Push the stick to the left and the airplane rolls left. The farther you push the stick, the faster the plane rolls. Airplanes are designed for different applications, which means they have different control characteristics.

During a recent study exploring ground avoidance pull-up maneuvers in a Boeing 777, a yoke pull force of 135 pounds was required for a 1.7-G pull-up. That’s a lot of force, but that’s good because most passengers are probably happiest around 1 G. What if the Boeing’s stick force number existed in a fighter where 5, 6, or 7 Gs were routine?

To some extent stability and control work against each other. Too much stability with insufficient control can make an airplane feel stiff or sluggish. Too much control with insufficient stability can make a plane feel loose or twitchy. Designers seek the perfect balance of stability and control with the goal of creating a “good flying” airplane, and they follow standard stability and control numbers intended to achieve this result. Following these standards sometimes produces a good flying airplane, and sometimes it doesn’t. That’s why flying qualities testing is necessary, and it involves more than stability and control.

Handling Qualities

A new pilot’s opinion of how an airplane handles may differ greatly from an aerobatic champion’s, but it’s not because the plane’s stability and control characteristics are different. Handling qualities are based on the pilot’s opinion of how the plane flies when he or she attempts to do something. Setting a bank angle, maintaining airspeed during slow flight, and tracking the runway centerline during the landing roll are examples of where pilot opinion can vary widely. Like stability and control, handling qualities can be a subset of flying qualities. But unlike stability and control testing, which is typically documented with numbers, handling qualities are usually described with adjectives and adverbs.

Stability and control testing usually involves measuring the airplane’s response after making specific control inputs or measuring the control inputs needed for a specific airplane response or condition. How long it takes an airplane to roll 360 degrees using full lateral stick displacement is an example. Another is how much pedal force holds the airplane in a sideslip. Conversely, handling qualities always involves a pilot performing a task like capturing a bank angle, tracking the centerline, etc.

There are several big distinctions here. Evaluating handling qualities involves more than a single control input. When flying you make as many control inputs as necessary to accomplish the task. You make a control input, and the airplane responds. If the response is not what you wanted, you make another input based on the response to your previous input. This input-response-input loop continues until you achieve the desired result or abandon the task.

Another distinction between stability and control testing and evaluating handling qualities is that the latter involves the pilot’s level of effort to accomplish the task. Pilot effort comes in many forms: forces needed, control displacements, timing, and anything else that affects the pilot’s opinion of the ease of task performance.

Pilots expect to expend different effort levels for different tasks. You might portray a crosswind takeoff in a taildragger that requires 20 rudder inputs as easy, and an in-flight coordinated turn that needs even 10 rudder turns as difficult or, at the least, annoying.

Casting stability and control testing and evaluating handling qualities as independent endeavors is a bit misleading. Testing them separately is important, but their synergy is what affects pilots. You don’t care about stability derivatives while sideslipping over approach end trees, but you do care about getting that airplane straightened out, flared, and on the runway quickly following the sideslip. Those stability derivatives and control sensitivities represent many pieces in this steep approach puzzle, and it’s fun to take them apart to see how they fit together. It’s the complete flying qualities picture, however, that determines what you think of the airplane, and that includes how you and your airplane communicate.

Pilot-Airplane Interface

Pilots are connected to their airplanes through the flight controls. They transfer the force we apply to them to deflect the control surfaces that cause the airplane to pitch, roll, or yaw. From the cockpit, all we care about is the physical effort and mental attention needed to make the plane respond exactly the way we intend. Are the forces too high, the displacements too small, or the ergonomics awkward? Maybe it’s the sitting position, the shape of the stick grip, or even the instrument layout that hampers task accomplishment.

These are pilot-airplane interface issues, and they include all the cockpit environmental characteristics you must deal with when flying the plane. Because you must deal with pilot-airplane interface issues with every control input, they too can profoundly affect your opinion of the airplane’s handling qualities.

Pilot-airplane interface features are pilot-dependent. Stronger pilots may not object to high pedal forces. Larger pilots may feel cramped in a cockpit smaller pilots find comfortable. Arm and leg motions may feel awkward to long-limbed pilots but feel just fine to the smaller-statured. Your size and shape relative to your cockpit’s size and shape matter, and your strength matters, and your level of concentration matters.

Flying Qualities Umbrella

Stability and control and handling qualities are all under the flying qualities umbrella (see figure) and are a part of every maneuver you fly. Advertised performance may have been your airplane’s major selling point, but its flying qualities ultimately determine your flying enjoyment.

There aren’t many simple solutions to alter your airplane’s stability characteristics. You can modify the control system, but approach these explorations carefully. Incorporating a simple control system spring or servo tab will alter the control feel, but it can also adversely affect stability, stall protection, and even susceptibility to flutter.

If you haven’t bought your airplane yet, explore your candidate airplanes’ flying qualities during demo flights. Performing the same kinds of flying tasks you’ll do with your own plane is the best way to assess any plane’s handling.

Some tasks are common to all airplanes: taxi, takeoff, climb, cruise, descent, and landing. Some tasks are airplane-design specific. If you’re contemplating an aerobatic machine, evaluate its handling by flying aerobatics. Are the control forces and stick displacements manageable? Can you reach full stick displacement with the harness cinched tightly? Can you fly it smoothly, or does every control input feel jerky? Is it you or the plane? Are the plane’s responses to your control inputs predictable?

If you’re leaning toward a utility machine, you may want to sample its flying qualities at its maximum gross weight. If it has a large center of gravity (CG) envelope, its flying qualities will probably be significantly different with the CG at the forward and aft limits. How does it handle in crosswinds? Can you establish a steady sideslip, or must you continually work the controls to keep the pitch/roll/yaw oscillations bounded? How easy is it to transition from the slip to the flare? How about runway alignment? Which control are you working the hardest?

If you’re buying a fast cruiser, fly a portion of your demo flight high and fast. Is it easy to trim? Can you rest your hand on the stick without inadvertently causing a pitch or roll-even in light turbulence? Can you make a level turn without excessive attention and effort? Can you tell when you’re getting slow on final approach, or is the airspeed indicator your only clue? Simple questions, but it’s easy to not notice these characteristics during the thrill of your demo flight.

Exploring the flying qualities of your airplane candidates during demo flights can help narrow your choices. At the least, it should raise a few questions for you to pose to the manufacturer. And you might want to pose them to existing owners of your prospective airplane model. Do they have any flying qualities complaints?

If you’re already flying your airplane, you still might want to assess its flying qualities. Even if you don’t intend to modify your plane in an attempt to tailor its handling, there are advantages to a flying qualities exploration. Dissecting the control inputs and isolating the airplane response can help you pinpoint a problem.

For example, if the airplane overshoots or undershoots the bank angle you want to capture, try doing it with your feet off the rudder pedals. Is there adverse yaw? If so, maybe you’re not properly coordinating your roll. Experiment with different pedal displacements and timings to see if that helps. Perhaps your stick forces are uncomfortably high and you find it difficult to modulate a smooth stick release as you approach the target bank. Maybe the stick force is too light, again making small changes difficult to predict. Does the stick return to its wings-level position when you relax the force? If not, the ailerons may not return to their wings-level deflection either. In this case the airplane will continue to roll even after you stopped trying to make it roll.

By examining individual components of capturing a bank angle, you might be able to locate the source of difficulty. Once isolated, you may be able to come up with a solution that might be a physical adjustment to the airplane or a change to your piloting technique. Either way, you’ve solved the bigger problem of capturing a bank angle, and that should make your flying more enjoyable.

Next month we’ll continue with flying qualities by discussing flight control characteristics. Stick forces and displacements can profoundly affect what you think of your plane’s handling. Control feel also influences your opinions about the airplane’s stability, but not necessarily the airplane’s actual stability. We’ll explore the difference along with how your flight control system’s mechanical characteristics facilitate or complicate your flying.