Although no airplane is completely stable, all airplanes must have desirable stability and handling characteristics. An inherently stable airplane is easy to fly, and it reduces pilot fatigue. This quality is essential throughout a wide range of flight conditions – during climbs, descents, turns, and at both high and low airspeeds. You will see that handling characteristics are directly related to stability. In fact, stability, manoeuvrability, and controllability are all interrelated design characteristics.
Stability is a characteristic of an airplane that causes it to return to a condition of equilibrium, or steady flight, after it is disturbed. If you are flying a stable airplane that is disrupted while in straight-and-level flight, it has a tendency to return to the same attitude. Manoeuvrability is the characteristic of an airplane that permits you to manoeuvre it easily and allows it to withstand the stress resulting from the manoeuvres. An Irplane’s size, weight, flight control system, structural strength, and thrust determine its manoeuvrability. Controllability is the capability of an airplane to respond to your control inputs especially with regard to attitude and flight path.
Large transport aircraft are designed to be very stable, since passenger comfort is a primary consideration. On the other hand, light training aircraft are designed to be more manoeuvrable, and somewhat less stable. The two major categories of stability are static and dynamic. Within each of these, there are subcategories called positive, neutral and negative stability. In addition since stability applies to all three axes of rotation, it may be classified as longitudinal, lateral or directional.
STATIC STABILITY is the initial tendency that an object displays after its equilibrium is disrupted. When you fly an airplane with positive static stability, it tends to return to its original attitude after displacement.
A tendency to move farther away from the original attitude following a disturbance is negative static stability. If an airplane tends to remain in its displaced attitude, it has neutral static stability.
DYNAMIC STABILITYdescribes the time required for an airplane to respond to its static stability following a displacement from a condition of equilibrium. It is determined by its tendency to oscillate and damp out successive oscillations after the initial displacement. Although an airplane may be designed with positive static stability, it could have positive, negative, or neutral dynamic stability.
Assume the airplane you’re flying is displaced from an established attitude. If its tendency is to return to the original attitude directly, or through a series of decreasing oscillations, it exhibits positive dynamic stability. If you find the oscillations increasing in magnitude as time progresses, negative dynamic stability is exhibited. Neutral dynamic stability is indicated if the airplane attempts to return to its original state of equilibrium, but the oscillations neither increase nor decrease in magnitude as time passes.
When you consider the overall stability of an airplane, remember that the airplane moves about three axes of rotation. Longitudinal stability involves the pitching motion or tendency of the airplane about its lateral axis. An airplane which is longitudinal stable will tend to return to its trimmed angle of attack after displacement. This is good, because an airplane with this characteristic tends to resist either excessively nose-high or nose-low pitch attitudes. If an airplane is longitudinally unstable, it has the tendency to climb or dive until a stall or steep dive develops. As a result, a longitudinally unstable airplane is dangerous to fly.
To achieve longitudinal stability, most airplanes are designed so they’re slightly nose heavy. This is accomplished during the engineering and development phase by placing the centre of gravity slightly forward of the centre of pressure.
The centre of pressure is a point along the wing chord line where lift is considered to be concentrated. For this reason, the centre of pressure is often referred to as the centre of lift. During flight, this point along the chord line changes position with different flight attitudes. It moves forward as angle of attack increases and aft as angle of attack decreases. As a result, pitching tendencies created by the position of the centre of lift in relation to the CG vary. For example, with a high angle of attack and the centre of lift in a forward position (closer to the CG) the nose down pitching tendency is decreased. The position of the centre of gravity in relation to the centre of lift is a critical factor in longitudinal stability. If the CG is too far forward, the airplane is. Dry nose heavy; if the CG is too far aft, the airplane may become tail heavy.
The location of the centre of gravity with respect to the centre of lift determines the longitudinal stability of an airplane.
When the airplane is properly loaded, the CG remains forward of the centre of lift and the airplane is slightly nose heavy. The nose-heavy tendency is offset by the position of the horizontal stabilizer, which is set at a negative angle of attack. This produces a downward force, or negative lift on the tail to counteract the nose heaviness. The downward force is called the tail-down force. It is the balancing force in most flight conditions.
The horizontal stabilizer is set at a negative angle of attack. This offsets the nose-down pitching tendency caused by the CG being located forward of the centre of lift.
With the exception of T-tail airplanes, additional downward forces are exerted on horizontal tail surfaces by downwash from the propeller and the wings. The strength of the downward force is primarily related to the speed of the airplane, but the position of the stabilizer in relation to the wings and the propeller also affects it.T-tail designs are not subject to the same downwash effect, simply because the horizontal tail-surface is above most, or all, of the downwash.
Power changes also affect longitudinal stability. If you reduce power during flight, a definite nose-down pitching tendency occurs due to the reduction of down wash from the wings and propeller. This decreases the associated downward force on the tail, and reduces elevator effectiveness. Although this is a destabilizing factor, it is a desirable characteristic, because it tends to result in nose-down attitude during power reductions. The nose-down attitude helps you maintain, or regain, airspeed.
Increasing power has the opposite effect. It causes increased downwash on the horizontal tail surfaces and tends to force the nose of the airplane to rise. The influence of power on longitudinal stability also depends on the overall design of the airplane. Since power provides thrust, the alignment of thrust in relation to the longitudinal axis, the CG, the wings and stabilizer are all factors. The thrustline is determined by where the propeller is mounted and by the general direction in which the thrust acts.
In most light general aviation airplanes, the thrustline is parallel to the longitudinal axis and above the CG. This creates a slight pitching moment around the CG. If thrust is decreased, the pitching moment is reduced and the nose heaviness tends to decrease. An increase in thrust increases the pitching moment and increases nose heaviness. Notice that these pitching tendencies are exactly the reverse of the pitching tendencies resulting from an increase or decrease in downwash. This thrustline design arrangement minimizes the destabilization effects of power changes and improves longitudinal stability.
Although the tail-down is excellent for longitudinal stability and balance, it is aerodynamically inefficient. The wings must support the negative lift created by the tail, and the negative angle of attack on the stabilizer increases drag. If an airplane design permitted two lifting surfaces, oath the wings and the horizontal stabilizer, aerodynamic efficiency would be much greater.
The canard design utilizes the concept of two lifting surfaces. A canard is a stabilizer that is located in front of the main wings. Canards are somewhat like miniature forward wings. They were used in the pioneering days of aviation, most notably on the Wright Flyer, and are now reappearing on several original designs.
Since both the main wings and the canard produce positive lift, the design is aerodynamically efficient. A properly designed canard is stall/spin resistant. The canard stalls at a lower angle of attack than the main wings. In doing so, the canard’s angle of attack immediately decreases after it stalls. This breaks the stall and effectively returns the canard to a normal lift-producing angle of attack before the main wings have a chance to stall. Ailerons remain effective throughout the stall because they are attached to the main wings. In spite of its advantages, the canard design has limitations in total lift capability. Critical design conditions also must be met to maintain adequate longitudinal stability throughout the flight envelope.
CENTRE OF GRAVITY POSITION
The position of the centre of gravity (CG) is another key factor in longitudinal stability. Distribution of weight in an airplane determines the position of the CG. The weight includes the basic weight of the airplane itself, as well as the weight of fuel, passengers, and baggage. Since the weight of the airplane and the distribution of that weight normally are fixed, the CG location is largely determined by what you put into the airplane, and where you put it. For example, if you load heavy baggage into an aft baggage compartment, it might cause the CG to shift to an unfavourable position. This could be critical, so let’s look briefly at what happens when the position of the CG shifts beyond acceptable limits.
As you might expect, for an airplane to be controllable during flight, the CG must be located within a reasonable distance forward or aft of an optimum position. All airplanes have forward and aft limits for the position of the CG. The distance between these limits is the CG range (Figure 1-50).
When the CG is within the approved range, the airplane is not only controllable, but its longitudinal stability also is satisfactory. If the CG is located near the forward or aft limit of the approved CG range, a slight loss of longitudinal stability may be noticeable. However, stabilator (elevator) effectiveness is still adequate to control the airplane during all approved manoeuvres.
CG TOO FAR FORWARD
If you load your airplane so the CG is forward of the forward CG limit, it will be too nose heavy. The condition gets progressively worse as the CG moves to an extreme forward position. This actually makes the airplane too stable. Eventually, stabilizer (elevator) effectiveness will be insufficient to lift the nose.
An airplane loaded to its aft CG limit will be less stable at all speeds.
An airplane becomes progressively more difficult to control as the CG moves aft. If the CG is beyond the aft limit, you will be unable to lower the nose to recover from a stall or spin (Figure 1.51).
CG TOO FAR AFT
A CG located aft of the approved CG range is even more dangerous than a CG that is too far forward. With an aft CG, the airplane becomes tail heavy and very unstable in pitch, regardless of the speed (Figure 1.52).
CG limits are established during initial testing and airworthiness certification. One of the criteria for determining the CG range in light airplanes is a spin recovery capability. If the CG is within limits, a normal category airplane must demonstrate that it can be recovered from a one-turn spin; and a utility category airplane that is approved for spins must be recoverable from a fully developed spin. The aft CG limit is the most critical factor. As the CG moves aft, stabilator effectiveness decreases. When the CG is at the aft limit, stabilator effectiveness is adequate; but when the CG is beyond the aft limit, the stabilator mY be ineffective for stall or spin recovery.
As a pilot there are certain actions you can take to prevent an aft CG position. You can make sure the heaviest passengers and baggage, or cargo, are loaded as far forward as practical. Lighter passengers and baggage normally should be loaded in aft seats or compartments. The main thing you must do is to follow the airplane manufacturer’s loading recommendations in the POH. If you do this, your airplane will be loaded so the CG is within approved range where longitudinal stability is adequate and at the same time, where you can control the airplane during all approved manoeuvres. Two important points to remember are that a CG beyond acceptable limits adversely affects longitudinal stability, and the most hazardous condition is an extreme aft CG
Stability about an airplane’s longitudinal axis which extends nose to tail, is called lateral stability. If one wing is lower than the opposite wing, lateral stability helps to return the wings to a wings-level attitude. This tendency to resist lateral, or roll movement is aided by specific design characteristics of an airplane. Four of the most common design features that influence lateral stability are dihedral, keel effect, sweepback, and weight distribution. Two of these, keel effect and sweepback also help provide directional stability about the vertical axis.
The most common design for lateral stability is known as wing dihedral. Dihedral is the upward angle of the airplane’s wing with respect to the horizontal. When you look at an airplane, dihedral makes the wings appear to form a spread- out. Dihedral is usually a few degrees.
If an airplane with dihedral enters an uncoordinated roll during gusty wind conditions, one wing will be elevated and the opposite wing will drop. This causes an immediate side slip downward towards the low wing. The side slip makes the low wing approach the air at a higher angle of attack than the high wing. The increased angle of attack on the low wing produces more lift for that wing and tends to lift it back to a level flight attitude.
Lateral stability is also provided by the keel effect. During flight the vertical fin and side area of the fuselage react to the airflow very much like the keel of a ship. The keel effect is the steadying influence exerted by the side area of the fuselage and the vertical stabilizer. When the airplane rolls to one side, a combination of the pressure of the airflow against the keel surface, along with the airplane’s weight, tends to roll the airplane back to a wings-level attitude.
In many airplanes, the leading edges of the wings do not form right angles with the longitudinal axis. Instead the wings taper backwards from the root of the wing to the wingtips. This is called sweepback. In high performance airplanes with pronounced sweepback, the design helps maintain the centre of lift aft of the CG. In light training airplanes sweepback helps improve lateral stability.
Sweepback may also aid in directional stability. If an airplane rotates about its vertical axis or yaws to the left, the right wing has less sweep and a slight increase in drag. The left wing has more sweep and less drag. This tends to force the airplane back into alignment with the relative wind.
You have no control of the design features that help maintain lateral stability, but you can control the distribution of weight and improve lateral stability. For example most training airplanes have two fuel tanks, one inside each wing. Before you takeoff on a long flight, you normally fill both tanks. If you use fuel from only one tank, you will soon notice that the airplane wants to roll towards the wing with the full tank. The distribution of weight is uneven and lateral stability is affected. In this case you can easily correct the imbalance in weight by using an equal amount of fuel from the full tank.
Stability about the vertical axis is called directional stability. Essentially, an airplane in flight is much like a weather vane. You can compare the pivot point on the weather vane to the centre of gravity pivot point of the airplane. The nose of the airplane corresponds to the to the weather vane’s arrowhead, and the vertical fin on the airplane acts like the tail of the weather vane (Figure 1.56).
EFFECTS OF LATERAL AND DIRECTIONAL STABILITY
Although airplanes are designed with stabilizing characteristics which lighten your workload while you are flying, there are normally some undesirable side effects. Two of the most important ones are Dutch roll and spiral instability.
Dutch roll is a combination of rolling/yawing oscillations caused by either your control input or by wind gusts. In a typical case, when equilibrium is disturbed, the rolling reaction precedes the yW, and the roll motion is more noticeable than the yaw motion. When the airplane rolls back towards level flight in response to the dihedral effect, it continues to roll too far and side slips the other way. Each oscillation overshoots the wings-level position because of the strong dihedral effect. Dutch roll actually is the back-and-forth, rolling/yawing motion. If the Dutch roll is not effectively dampened, it is considered objectionable.
The alternative to an airplane that exhibits Dutch roll tendencies is a design that has better directional stability than lateral stability. If directional stability is increased and lateral stability is decreased, the Dutch roll motion is adequately suppressed. However, this design arrangement tends to cause spiral instability.
Spiral Instability is associated with airplanes that have strong directional stability in comparison with lateral stability. When an airplane with spiral instability is disturbed from a condition of equilibrium, a side slip is introduced. In this case, the strong directional stability tends to yaw the airplane back into alignment with the relative wind. At the same time the comparatively weak dihedral effect lags in restoring lateral stability. Due to the yaw back into the relative wind the outside wing travels faster than the inside wing and, as a result, more lift is generated by the outside wing. The yaw forces the nose of the airplane down as it swings into alignment with the relative wind. The net result is an over-banking and nose down tendency which generally is considered less objectionable than Dutch roll.
As you can see even a well designed airplane may have some undesirable characteristics. Generally designers attempt to minimize the Dutch roll tendency, since it is less tolerable than spiral instability. Because of this, many airplanes have some degree of spiral instability which h generally is considered acceptable.
Courtesy of Jeppesen Sanderson CO, USA, 1991 (PRIVATE PILOT MANUAL)