43 Aircraft Stability & Control

Introduction

The subject of the stability and control of an aircraft or other flight vehicle is rather complex, partly because of the relatively difficult mathematics involved in explaining its nonlinear flight characteristics. Nevertheless, the basic principles of the subject can still be introduced to the student without straying too far into mathematics. While the professional practice of stability and control requires specialist knowledge, it is nevertheless essential for all aerospace engineers to develop a basic understanding of the fundamentals of this field and their importance to the aircraft design process.

The term “stability” when referring to a flight vehicle is the tendency for that vehicle to continue to fly in a prescribed flight condition, e.g., an aircraft is considered to be stable when it remains in the flight condition that the pilot intends the aircraft to be in. If the aircraft does something else, especially when the pilot removes their hands and/or feet from the flight controls, the aircraft would not be considered stable and may even be unstable in some cases. Most aircraft are inherently stable by design, but to a greater or lesser degree, so they can be safely flown by an average pilot without excessive workload. The term “control” means the ability to change the flight attitude or other conditions of the flight vehicle, i.e., to make the vehicle do what is commanded of it. Naturally, a flight vehicle’s inherent or natural “stability” and its “control” are inextricably linked.

Objectives of this Lesson

  • Appreciate the fundamentals of flight vehicle stability and control, and why stability is an essential requirement for flight.
  • Understand the differences between the static stability and dynamic stability of a flight vehicle.
  • Become familiar with flight dynamic terms like short-period and long-period responses, phugoid, Dutch roll, and spiral divergence.
  • Be aware of an airplane’s basic design features that contribute to its static and dynamic stability characteristics.

Trimmed Flight

As with all flight vehicles, the concern is with its stability and control about all three flight axes, as shown in the figure below, namely:

  1. Longitudinal stability and control is concerned with the response of the vehicle in the pitch axis.
  2. Lateral stability and control relates to the lateral axis or rolling motion.
  3. Directional stability and control relates to the yawing axis or directional (weathercock) motion.
Diagram of how the axis of an airplane can pitch, roll, and yaw.
An airplane can pitch, roll, and yaw. It is generally expected that the aircraft is, by design, statically and dynamically stable about all three flight axes.

While the responses and controls of any airplane tend to be coupled about the three axes to some degree, it is found that the pitch motion of an airplane is mostly decoupled from the roll and yaw responses. However, the lateral (roll) and directional (yaw) stability characteristics of an airplane tend to be significantly coupled with each other, and usually, one cannot be considered separately from the other for a stability and control analysis.

For an airplane to be in stable equilibrium at a particular flight condition, the net sum of all the forces and moments acting on the airplane must be zero, i.e., the position and attitude of the airplane will be in perfect balance about all three flight axes, namely pitch, roll, and yaw. For example, consider the equilibrium of an airplane flying straight and level at a constant airspeed and altitude, as shown in the figure below, a condition called steady trimmed flight or just trim. In steady trim, the lift on the airplane equals its weight, and for most purposes, the weight can be considered to act at a center of gravity location. The thrust (from the propulsion system) equals the aerodynamic drag at that weight, airspeed, and altitude. There can be no net forces and moments acting on the airplane about the center of gravity in the trim condition so that the airplane can be considered in a state of static equilibrium.

An airplane in static equilibrium or trimmed flight showing how the balance of forces and moments about the center of gravity must be zero.

The aerodynamic forces on the airplane can be considered to act at an effective location on each lifting component, i.e., the main wings and the horizontal tail. Of more significance is the lifting contributions, in aggregate, which can be assumed to act at a single point. The center of pressure is a convenient point because there is no net aerodynamic moment at this location, which in the field of aircraft stability and control is often referred to as the neutral point. The center of gravity is usually located in front of the neutral point (for stability), and the horizontal tail and flight controls are needed to create the necessary aerodynamic forces (and hence moments) to reach a balanced pitch or trimmed flight condition.

The primary wing produces most of the lift on the airplane, but the tail may make some small increment. Hence, the neutral point is usually very close to the center of pressure of the main wing, which for the lift coefficients typical of flight, is near the 1/4-chord point. The horizontal tail acts like a smaller version of the main wing, but it can be used to give either positive or negative changes in the lift by means of the up/down elevator control. Because of the typically long moment arm from the horizontal tail to the center of gravity (but not always), only relatively small changes in the lift on the tail is required to produce significant longitudinal pitching moments.

Consider now a more forward position of the center of gravity. In this case, a bigger nose-down moment would be produced on the airplane, which would need to be compensated for by reducing the lift on the tail. Therefore, the elevator would need to be deflected up by the pilot by moving the control column aft and so reducing the aerodynamic upward force on the tail and so continuing to balance the net moments on the airplane. These small changes in aerodynamic forces and moments from the control surfaces are usually performed using trim tabs, as shown in the figure below, which can be actuated separately to trim the airplane and remove any residual forces from the pilot’s controls.

The use of the flight controls, elevator, ailerons and rudder, also provide the necessary forces and moments to create a trimmed flight condition. The flight control surfaces may also have smaller, auxiliary surfaces called “trim tabs” to allow the pilot to remove or “trim out” any residual forces on the controls.

The propulsion system can also affect the stability and control characteristics of the airplane. Propulsion will create a thrust vector, which may have a line of action offset from the center of gravity. It will also produce a pitching moment, i.e., a thrust/pitch coupling; this latter effect is illustrated in the figure below. Aircraft with underslung engines with thrust vectors well below the center of gravity are prone to this type of coupling, which can also be interpreted as an airspeed coupling. In this latter regard, changes in thrust setting will also cause changes in airspeed.

The effect of thrust/pitch coupling is particularly pronounced on airplanes with underslung engines below the wing, which will change with changes in engine thrust and so power setting.

Both the center of gravity and the airplane’s neutral point may (and generally will) change during flight. As fuel is burned off and the airplane’s weight changes, the center of gravity may move forward or aft, depending on the type of airplane and how it is loaded with the payload. Therefore, the airplane’s stability characteristics can (and often will) change as fuel is burned off, albeit slowly, and further trimming will be required. To reduce the trim drag on a large commercial airplane, fuel is pumped from one tank to another to manage the position of the center of gravity during flight rather than accepting the increased drag from the trim tabs. As shown in the photograph below, all-flying horizontal tails may also be used on airliners to trim out the pitching moments.

Photo of the root of the trimmable horizontal stabilizer on an Embraer ERJ-170. The markings UP and DOWN refer to the angles needed for “nose up” trim and “nose down” trim, respectively

The neutral point may also change with airspeed, especially in high-speed flight at higher Mach numbers. Approaching transonic and into supersonic flight, the center of pressure typically migrates aft on the wing from near the 1/4-chord to closer to the 1/2-chord. The resulting effect is a pronounced nose-down pitching moment. This effect has a particular name called Mach tuck, and it can be a stability and control issue for a supersonic airplane as they transition from subsonic to supersonic flight. Of course, these effects can often be trimmed out by using the elevator (or a trimmable tail surface). Still, there will be a limit to this control capability depending on the combination of the center of gravity and/or center of pressure movements during flight. On some larger aircraft, it is necessary to pump fuel longitudinally from one tank to another to keep the center of gravity between the required limits during supersonic flight.

Static Stability

Consider now the situation when the balance of forces and moment in the trim state is disturbed, such as by a gust caused by atmospheric turbulence. Such gusts can come from virtually any direction and can affect the airplane’s response about any axis, but vertical gusts typically have the largest effects on the aircraft’s responses. Therefore, it is initially convenient to describe the principles concerning the airplane’s longitudinal or pitching response because the responses in pitch are mostly clean and uncoupled from the responses in roll and yaw.

A vertically upward gust, the most common type of gust, will cause an increase in the angle of attack of the wing, increasing its lift, and the airplane’s natural reaction is that its nose will pitch up slightly. The consequence of this effect is that the airplane is no longer in stable equilibrium and will have deviated from its trimmed flight condition, as shown in the figure below. If the subsequent forces and moments generated on the airplane from the gust disturbance tend to return it to its trimmed condition, the airplane’s response would be referred to as being statically stable, as shown in scenario (b).

Principle of static stability, in this case with respect to pitch attitude: (a) Trimmed equilibrium flight, (b) Statically stable response, (c) Statically unstable response, (d) Statically neutral response.

However, if the forces and moments introduced by the disturbance tend to cause the nose to pitch up further, then the airplane would be considered statically unstable, which is the scenario (c). If the airplane is genuinely statically unstable, its subsequent motion may well cause a divergence of the flight path and, most likely, a departure from controlled flight. When the airplane remains indefinitely disturbed, as shown in scenario (d), then it is considered to have neutral static stability, but this is not a common characteristic of an airplane.

Dynamic Stability

If the airplane is statically stable, then the restoring forces and moments acting on it will cause the nose of the airplane to pitch down again after the initial disturbance. However, this desirable static response does not necessarily mean the airplane will immediately settle and reestablish its original trimmed state. So, the question becomes: What happens to the airplane response(s) in subsequent time, i.e., the dynamic response?

To this end, several possibilities could happen:

1. The airplane may continue to pitch nose-down and overshoot the initial trimmed state. Then the nose comes back up and returns toward trim but overshoots again. This process may continue in a series of nose-up and nose-down pitching motions. Suppose these oscillatory motions eventually damp out over time and cause the airplane to return to the initial trim. In that case, this decaying oscillatory motion means the airplane is dynamically stable.

2. The airplane does not overshoot trim and settles out quickly to reestablish its trimmed state, called subsidence. In this case, the airplane is dynamically stable, and the damping is said to be critically damped or to have a “deadbeat” response. Some airplanes exhibit this characteristic, but many do not because they would have to have large horizontal tail surfaces.

3. The airplane may continue with a continuous nose-up and down pitching motion, with the subsequent oscillations in pitch remaining at an almost constant amplitude. In this case, the airplane’s resulting “roller-coaster” dynamic response is said to exhibit neutral dynamic stability.

4. In a worst-case scenario, the airplane may respond with nose-up and nose-down pitching oscillations with increasing amplitude. This type of response would be referred to as being dynamically unstable. An unstable aircraft does not necessarily mean it is unsafe if the unstable tendency has a long period and can be controlled by the pilot. However, such an aircraft generally has inferior flying qualities and can impose a high workload on the pilot.

An airplane must be statically stable to be dynamically stable, i.e., a prerequisite for dynamic stability is static stability. Therefore, a statically unstable airplane will also be dynamically unstable. A statically and dynamically stable airplane is generally easier to fly. However, an airplane may be statically stable and dynamically unstable but still flyable, especially if the dynamic response is slow enough for the pilot to control employing appropriate flight control inputs. The dynamic response may also depend on the weight of the aircraft and the center of gravity location, as well as airspeed.

Longitudinal (Pitch) Stability

There are two forms of longitudinal dynamic and oscillatory responses that are found on airplanes: The long period dynamic response and the short-period dynamic response, as shown in the figure below for the pitch motion. On the one hand, the short-period response is typically highly damped and lasts less than a second. On the other hand, the phugoid mode of oscillation is a slow, long period, weakly damped pitch oscillation of the aircraft’s flight path over many seconds or even minutes.

Types of dynamic longitudinal responses: (a) Short-period response; (n) Phugoid (long period) response.

The term “phugoid” was initially coined by Frederick Lanchester; the word has a literal translation from the Greek meaning “fleeing,” so it is really a misnomer. The phugoid response is typically a weakly damped dynamic response, as shown more clearly in the figure below, but it is easily damped out by pilot control inputs and so is easily controlled even if the response is weakly divergent. The phugoid frequency depends on the airspeed of the airplane, and damping depends on the aircraft’s lift-to-drag ratio. The phugoid response tends to be very pronounced and weakly damped on airplanes with high lift-to-drag ratios, e.g., sailplanes, but is also easily controlled.

A representative dynamic response showing the short period (high frequency, heavily damped) and the long period (low frequency, lightly damped).

The short-period oscillatory response mode, as also shown in this figure, is a much higher frequency and highly damped oscillatory response, often appearing in the airplane’s dynamic response after encountering gusty air or from the application of quick elevator inputs, such as during landing. But the short-period response is usually unnoticed by the pilot and does not have to be controlled. All three flight axis will typically show a short period dynamic response, which in all cases is quickly damped out.

However, in some circumstances, the pilot may inadvertently excite the short-period responses, such as during landing or in severe turbulence when quick, deliberate movements of the control stick are being made to adjust the aircraft’s flight attitude. In particular, a landing requires significantly increased attention on the controls from the pilot. In some cases, the pilot’s control inputs may become entirely out of phase with the aircraft’s short-period response, resulting in a pilot-induced oscillation (PIO). A PIO is just one type of Aircraft-Pilot Coupling (APC) effect. A PIO is a hazardous flight condition because the pilot’s inputs may cause the short-period response to become quickly divergent, resulting in a crash.

Sources of Longitudinal Stability

The typical steady (static) pitching moment contributions about the center of gravity for a conventional airplane as a function of the angle of attack are shown in the figure below. In this regard, a conventional airplane is one with a single wing and tail combination. There is a net-zero pitching moment at the trim angle of attack, \alpha_{\rm trim}. The sign convention is that positive moments are nose-up moments, i.e., d M_{cg} /d\alpha, is positive nose-up, tending to increase the wing’s angle of attack and so having a destabilizing effect on the airplane. Notice that different pitching moment contributions (both in magnitude and in sign) are caused by the various components of the airplane, e.g., the wing, the fuselage, and the tail all produce different aerodynamic effects. Therefore, other moments are produced by these components about the center of gravity.

The primary components of an airplane that will affect the longitudinal stability.

The wing lift component by itself is destabilizing in that it produces a nose-up moment about the center of gravity, i.e., the slope of the moment curve, d M_{cg} /d\alpha, is positive for the wing by itself. Likewise, the fuselage has a destabilizing effect. However, it can be seen that the horizontal tail produces a powerful nose-down moment about the center of gravity with a negative slope of the moment curve, providing a significant stabilizing effect, hence the name “horizontal stabilizer.”

The combined effect of all the components on the entire airplane is a negative d M_{cg}/d\alpha slope, making the airplane statically stable. Generally, a larger horizontal tail will produce a more statically stable airplane, but the physical position of the tail on the fuselage relative to the center of gravity (and other things) plays an important role too. In practice, the area of the tail surfaces must be enough to give a sufficient pitch and directional stability to the airplane. Still, too much stability will also make the airplane less maneuverable as well as tail-heavy.

Center of Gravity Effects & Limits

The center of gravity location is critical on an airplane because it has a powerful effect on its stability and control characteristics, e.g., if the center of gravity moves with respect to the neutral point or if the neutral point moves (because of compressibility effects) with respect to the center of gravity. If the center of gravity moves progressively aft (toward the tail), such as when fuel is burned off, then the moment curve slope becomes less negative and will eventually become zero at the neutral point; in this case, the airplane will have neutral static stability.

If the center of gravity is moved even further back, then the airplane will become unstable. On some airplanes, this behavior can become a severe problem if the center of gravity moves too far rearward such as when a load is discharged in flight, e.g., weapons, cargo, parachutists, etc. Likewise, if the center of gravity moves toward the nose, the moments will need to be trimmed out by using up elevator control inputs or a horizontal tail with trim capability. Eventually, suppose the center of gravity moves too far forward. so that the upward elevator displacements on the tail surfaces will not be enough to compensate. In this case, the airplane cannot be trimmed and will become unflyable, nosing down toward the ground and building up airspeed, often with a catastrophic outcome.

The movement of the center of gravity of the airplane will also affect its static stability.

It is clear, therefore, that engineers must carefully establish the center of gravity limits (fore and aft) on an airplane to ensure that it falls within an acceptable range for safe flight and that any center of gravity movements during the flight will not compromise its stability and control characteristics. An example of a center of gravity chart for an airplane is shown in the chart below. Based on the estimated takeoff weight and the calculated center of gravity location, the values must lie within the aircraft’s defined and certified envelope to be safe to fly.

Example of an allowable center of gravity location for an airplane, which will be verified by flight test before certification.

The pilot must be sure that the airplane is loaded correctly before the flight with all passengers and cargo, etc., takes off and that the center of gravity location is within limits. Indeed, many airplanes have crashed because they were not loaded correctly, and the airplane either became unstable during flight or the control forces became too high.

Lateral (Roll) Stability

Lateral stability and control refer to displacements about the longitudinal or roll axes. For example, an airplane has lateral static stability if, after a disturbance is applied, the aircraft rolls and acquires a bank angle but simultaneously generates new aerodynamic forces and moments that tend to reduce the bank angle and bring the airplane back to the initially trimmed flight condition. All airplanes have at least some inherent lateral stability, although because roll and yaw responses are coupled, the resulting dynamic response characteristics tend to be more complicated.

It is well known that using dihedral on the wings is a powerful means of providing an airplane with increased static lateral stability, as shown in the figure below. Just a few degrees of dihedral can make marked improvements to lateral stability. The horizontal tail may also have some dihedral and may contribute to the lateral stability and limit any coupling to the yaw response.

However, using dihedral tends to enhance the coupling between yaw and roll control inputs (i.e., rudder and aileron, respectively), so when the airplane yaws in one direction and develops a sideslip angle, it also tends to roll in that same direction. The application of ailerons to produce a roll response and initiate the turn is also accompanied by different changes in lift and drag on each wing (port and starboard). The consequence is to cause a yaw response in the opposite direction to the turn. The resulting behavior is called adverse yaw and is particularly pronounced on airplanes with long wingspans. The piloting solution is to first lead the turn using the rudder to compensate for the adverse yaw response when the ailerons are applied. A sailplane, with its long wings, is particularly susceptible to adverse yaw effects and must be flown with constant use of rudder inputs to lead the roll (aileron) inputs so as to prevent adverse yaw.

The position of the wing relative to the center of gravity also affects lateral stability, i.e., airplanes with a high-mounted wing versus a low wing. A high-wing airplane design naturally tends to have better lateral stability than a low-mounted wing because the center of gravity is below the wing’s center of pressure, thereby giving a form of pendulum stability. This behavior is why dihedral is not a common design feature on high-wing airplanes. Some airplanes with high-mounted wings may use wings with only partial dihedral, such as on the outer wing panels, but this feature is rare in airplane designs.

Wing sweep is used on high-speed airplanes to reduce compressibility drag, but wing sweep gives significant increases in lateral stability. In fact, a combination of wing dihedral and wing sweep often gives an airplane too much lateral stability. Therefore, airplanes with highly swept wings may use anhedral to negate the inherent lateral stability caused by wing sweep. Furthermore, on a large/heavy airplane with a high-mounted wing configuration (e.g., the C-5 Galaxy), there is usually excess pendular roll stability, so the use of anhedral wings is common. The use of anhedral on a fighter aircraft is needed to maintain its agility and maneuverability, as shown in the photo below. Fuselage and vertical tail effects may contribute to or detract from the airplane’s lateral stability, but this depends on the shape of the fuselage and the side of the vertical tail.

The use of anhedral wings is often needed on higher-speed aircraft to limit the amount of lateral stability caused by sweeping back the wing and so allowing the aircraft to have better agility.

Directional Stability

Dynamic responses are also associated with an airplane’s directional (yaw) or weathercock motion. However, as already mentioned, the static lateral (roll) and directional (yaw) stability characteristics of an airplane are coupled, i.e., a roll response causes a yaw response and vice-versa. This coupling also affects the dynamic response, i.e., what happens to the airplane at longer times after a disturbance in roll or yaw. This so-called cross-coupling between the static directional and lateral static stability can give rise to three important dynamic responses: a directional divergence mode, spiral divergence mode, and the “Dutch Roll” mode.

Directional Divergence

Directional divergence results from a directionally unstable airplane, as shown in the figure below. When the airplane yaws or rolls into a sideslip, side forces on the airplane are generated, and the yawing moments that arise can continue to increase the sideslip and result in significant yaw angles. Recovery is accomplished by the normal use of flight controls. However, the concern here also is that the vertical tail can stall for steep angles, which reduces its aerodynamic effectiveness to rudder inputs, thereby making recovery difficult. For this reason, not only is the sizing of the vertical tail important to give sufficient directional stability, i.e., in providing adequate surface area, as well as controlling its high yaw angle of attack behavior.

The initiation directional or spiral is from a perturbation in side force, such as from a gust for rudder input.

Spiral Divergence

The spiral divergence mode tends to be pronounced on an airplane that is very stable directionally (about the axis yaw) but is not as stable laterally. This behavior relates to the size of the vertical fin versus the amount of dihedral. The tendency to exhibit spiral divergence is reduced by increasing the dihedral on the wing. When the airplane is in a bank, the aerodynamic forces tend to turn the plane more deeply into the bank, and the nose drops, resulting in an ever-tightening downward spiral with increasing airspeed. However, the use of wing dihedral tends to slow the development of a spiral instability mode. Most airplanes exhibit a spiral instability mode, but it is usually slow to develop, and the airplane is easily recovered to a level flight attitude by the normal use of appropriate flight control inputs.

Directional or “spiral” divergence is an unusual behavior but it can result from an undersized vertical fin, i.e., the vertical fin has insufficient area to give adequate “damped: directional stability.

Dutch Roll

The so-called “Dutch Roll” is a weaving dynamic response mode coupled with both the directional and spiral divergence modes. For most airplanes, the lateral stability is always fairly good, whereas the directional stability can be much weaker. This behavior is especially so if the tail is even slightly undersized during design, which tends to be a rather common approach to save airframe weight; airplane designs generally always become tail-heavy.

If a sideslip disturbance occurs, then the airplane yaws in one direction and the airplane rolls the other away in the form of a weaving motion, as illustrated in the figure below. While this motion is fairly low-frequency and usually well-damped, it is incredibly annoying, if not uncomfortable, for the crew and passengers. In addition, on some airplanes, the Dutch roll mode can become weakly divergent. In this case, a yaw damper, which is part of the flight control system (autopilot), is used to automatically damp out the Dutch roll mode and improve the flying qualities; the damper is switched on soon after takeoff and switched off again just before landing.

The Dutch-roll dynamic mode is a coupled roll-yaw behavior but is usually well damped on most airplanes but can also be damped out almost completely by the action of flight control system called a yaw damper.

The vertical tail provides most of the static directional stability of an airplane. An airplane is said to possess lateral directional or yaw stability if, after a disturbance is applied, the aircraft yaws but simultaneously generates new aerodynamic forces and moments that tend to damp out the yaw displacement. However, the combined size and depth of the fuselage and the vertical tail’s height, area, and shape ultimately affect an airplane’s directional stability characteristics. For example, suppose an airplane experiences a sideslip angle. In that case, the fuselage produces a side force that tends to increase that angle, similar to what the fuselage does for the pitch response.

In this regard, deep fuselages with boxy or elliptical cross-sections tend to be much worse in terms of directional stability than fuselages with circular cross-sections. During the design process, such issues are often explored by wind tunnel testing. A design goal is to shape the fuselage to minimize the static instability and so minimize the size of the tail, hence saving on airframe weight. Fuselages with more area forward of the airplane’s center of gravity tend to have less directional stability. A good example is the Boeing 747 or the C-5 Galaxy, which have large huge vertical tails to compensate for the large “hump” on the forward fuselage. The Airbus A380 had similar design issues. The somewhat “disproportionately” large vertical fin and horizontal tail sizes of the A380 or the Boeing 747SP are also distinctive, as shown in the photograph below.

The disproportionally large tail on the shortened fuselage of the Boeing 747SP is needed to give the aircraft sufficient static and dynamic stability.

Ventral and/or dorsal fins are used on some airplanes to augment the directional stability and/or reduce the tendency to develop a Dutch roll, especially at high airspeeds. Adding more vertical tail area using a dorsal fin extension or ventral tail area, as shown in the figure below, provides increased directional stability but at the price of some minor structural weight.

Diagram of how the axis of an airplane can pitch, roll, and yaw.
Ventral and/or dorsal fins are used to augment the directional stability and/or reduce the tendency to develop a Dutch roll, especially at high airspeeds.

Nevertheless, dorsal or ventral fins are simple, lightweight structures and can be resized quickly and inexpensively during flight testing. As a result, they are often helpful in achieving the needed levels of directional stability for the airplane without having to embark on a complete redesign of the vertical tail to resolve deficiencies with the flight dynamics. The photograph below, for example, suggests that additions were made to the aircraft after the first flights to improve its flight stability characteristics.

The addition of ventral and dorsal fins can help to achieve the precise levels of directional stability without increasing the size (area) and weight of the primary stability surfaces. In this case, the addition of so many tail surfaces seems almost an afterthought.

Handling Qualities Assessments

Handling or flying qualities is used in the study and evaluation of an aircraft’s stability and control characteristics. Assessments of handling qualities are critical to the flight of the aircraft and are related to the ease of controlling an airplane in steady flight and various types of maneuvers. The “ease” of controlling the aircraft will include the forces needed to be applied to the various controls that the pilot must move or otherwise actuate during flight.

Test pilots and flight test engineers use the Cooper-Harper handling qualities rating scale to assess aircraft handling and flying qualities in setting. The numerical scale ranges from 1 to 10, with a value of 1 indicating the best handling characteristics and a value of 10 being the worst, i.e., unflyable.

The Cooper-Harper aircraft handling qualities rating scale.

The scale is subjective, so several test pilots and engineers are usually used in the evaluation of aircraft handling qualities. Specific mission task elements (MTEs) are usually defined for the aircraft in question (based on its intended purpose), which are a set of “role-relatable” or representative tasks. The test pilots and flight test engineers then evaluate the aircraft’s handling qualities for these MTEs, amongst other criteria.

The handling qualities are judged not just by reference to the aircraft’s role but by reference to the MTE and the skill level expected of an average pilot. Usually, handling qualities assessments rated less than Level 3 are considered unacceptable for a modern aircraft, and changes to the aircraft and/or the flight control system will likely be required. Particular attention is usually focused on exploring deficient handling qualities in the form of pilot-induced oscillations (PIO) or, in general, the possibility of any type of Aircraft-Pilot Coupling (APC) effects. In addition, there are derivative MTEs used for specialist military flight operations such as air-to-air refueling, operations from ships, etc.

Summary & Closure

Stability and control are crucial factors in the design of any flight vehicle. Aerospace engineers need to understand the basics of stability and control to ensure safe and efficient flight. For an airplane, static stability about all axes is necessary for safe flight and good handling, which is usually achieved during the design stage. Dynamic stability is explored and documented during flight testing, and changes to the design may be made if needed to improve stability. Adding aerodynamic surfaces like ventral or dorsal fins may also be necessary to improve stability characteristics.

5-Question Self-Assessment Quickquiz

 

For Further Thought or Discussion

  • Review and comment on the tail design for the Beech 1900 turboprop airplane.
  • Can you fly a statically unstable aircraft? Explain carefully.
  • During flight testing, it was found that a new military fighter airplane has a very pronounced spiral instability mode. Consider the options as to whether this is a problem and whether it should be mitigated.
  • It is found during flight testing of an airplane that, under some conditions, the phugoid mode is mildly divergent. Discuss whether this is a problem or not.
  • Besides PIO, do some research to find out more about Aircraft-Pilot Coupling (APC) events.
  • If the yaw damper on a jet aircraft fails during flight, are there likely to be any issues of concern?
  • Consider some types of handling qualities assessments that might be needed for an uncrewed aerial vehicle.

Other Useful Online Resources

To dive further into the stability and control of aircraft, then visit the following web sites: