Lateral Dynamic Effects

Spiral Divergence

Spiral divergence will exist when static directional stability is very large when compared to the “dihedral effect”.

The character of spiral divergence is not violent. The aeroplane, when disturbed from the equilibrium of level flight, begins a slow spiral which gradually increases to a spiral dive. When a small sideslip is introduced, the strong directional stability tends to restore the nose into the wind while the relatively weak “dihedral effect” lags in restoring the aeroplane laterally. The rate of divergence in the spiral motion is usually so gradual that the pilot can control the tendency without difficulty.

Dutch Roll

Dutch roll will occur when the “dihedral effect” is large when compared to static directional stability.

Dutch roll is a coupled lateral and directional oscillation which is objectionable because of the oscillatory nature.

When a yaw is introduced, the strong “dihedral effect” will roll the aircraft due to the lift increase on the wing into wind. The increased induced drag on the rising wing will yaw the aircraft in the opposite direction, reversing the coupled oscillations.

Aircraft with a tendency to Dutch roll are fitted with a Yaw Damper. This automatically displaces the rudder proportional to the rate of yaw to damp-out the oscillations.

If the Yaw Damper fails in flight, it is recommended that the ailerons be used by the pilot to damp-out Dutch roll. Because of the response lag, if the pilot uses the rudder, pilot induced oscillation (PIO) will result and the Dutch roll may very quickly become divergent, leading to loss of control.

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Dutch roll is objectionable, and spiral divergence is tolerable if the rate of divergence is low. For this reason, the “dihedral effect” should be no more than that required for satisfactory lateral stability.

If the static directional stability is made adequate to prevent objectionable Dutch roll, this will automatically be sufficient to prevent directional divergence. Since the more important handling qualities are a result of high static directional stability and minimum necessary “dihedral effect”, most aeroplanes demonstrate a mild spiral tendency. As previously mentioned, a weak spiral tendency is of little concern to the pilot and certainly preferable to Dutch roll.

The contribution of sweepback to the lateral dynamics of an aeroplane is significant. Since the “dihedral effect” from sweepback is a function of lift coefficient, the dynamic characteristics may vary throughout the flight speed range.

When the swept wing aeroplane is at low CL the “dihedral effect” is small and the spiral tendency may be apparent. When the swept wing aeroplane is at high CL the “dihedral effect” is increased and the Dutch roll oscillatory tendency is increased.

Pilot Induced Oscillation (PIO)

Certain undesirable motions may occur due to inadvertent action on the controls. These can occur about any of the axes, but the most important condition exists with the short period longitudinal motion of the aeroplane where pilot control system response lag can produce an unstable oscillation. The coupling possible in the pilot/control system/aeroplane combination is capable of producing damaging flight loads and loss of control of the aeroplane.

When the normal human response lag and control system lag are coupled with the aeroplane motion, inadvertent control reactions by the pilot may furnish negative damping to the oscillatory motion, and dynamic instability will exist.

Since short period motion is of relatively high frequency, the amplitude of the pitching oscillation can reach dangerous proportions in an unbelievably short time.

When pilot induced oscillation is encountered, the most effective solution is an immediate release of the controls. Any attempt to forcibly damp the oscillation simply continues the excitation and amplifies the oscillation. Freeing the controls removes the unstable (but inadvertent) excitation and allows the aeroplane to recover by virtue of its inherent dynamic stability.

Mach Trim

As speed increases beyond the Critical Mach number (MCRIT), shock wave formation at the root of a swept-back wing will:

•reduce lift forward of the CG, and

•reduce downwash at the tailplane.

Together, these factors will generate a nose-down pitching moment. At high Mach numbers, an aircraft will become unstable with respect to speed; instead of an increasing push force being required as speed increases, a pull force becomes necessary to prevent the aircraft accelerating further. This is potentially dangerous. A small increase in Mach number will give a nose-down pitch which will further increase the Mach number. This in turn leads to a further increase in the nose-down pitching moment. This unfavourable high speed characteristic, known as “Mach Tuck”, “High Speed Tuck” or “Tuck Under” would restrict the maximum operating speed of a modern high speed jet transport aircraft.

To maintain the required stick force gradient at high Mach numbers, a Mach trim system must be fitted. This device, sensitive to Mach number, may:

•deflect the elevator up,

•decrease the incidence of the variable incidence trimming tailplane, or

•move the CG rearwards by transferring fuel from the wings to a rear trim tank.

by an amount greater than that required merely to compensate for the trim change. This ensures the required stick force gradient is maintained in the cruise at high Mach numbers.

Whichever method of trim is used by a particular manufacturer, a Mach trim system will adjust longitudinal trim and operates only at high Mach numbers.

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Key Facts 2

Self Study (Insert the missing words, with reference to the preceding paragraphs).

Positive static longitudinal stability is indicated by a _______ slope of CM versus CL. The degree of _______ longitudinal stability is indicated by the ______ of the curve.

The net pitching moment about the ________ axis is due to the contribution of each of the component _________ acting in their appropriate _____ fields.

In most cases, the contribution of the fuselage and nacelles is ____________.

(Page 259) Noticeable changes in static stability can occur at high CL (low speed) if:

a)the aeroplane has __________.

b)there is a large contribution of ‘______ effect’.

c)there are significant changes in __________ at the horizontal tail.

The horizontal tail usually provides the ________ stabilizing influence of all the components of the aeroplane.

__________ decreases static longitudinal stability.

If the thrust line is below the CG, a thrust increase will produce a _______ or nose ___ moment and the effect is ___________.

High lift devices tend to _________ downwash at the tail and _____ the dynamic pressure at the tail, both of which are ___________.

An increase in TAS, for a given pitching velocity, _________ aerodynamic damping.

The aeroplane with positive manoeuvring stability should demonstrate a steady _______ in stick force with ________ in load factor or “__”.

The stick force gradient must not be excessively ____ or the aeroplane will be difficult and tiring to manoeuvre. Also, the stick force gradient must not be too _____ or the aeroplane may be overstressed inadvertently when light control forces exist.

When the aeroplane has high static stability, the manoeuvring stability will be _____ and a ____

stick force gradient will result. The ________ CG limit could be set to prevent an excessively high manoeuvring stick force gradient. As the CG moves aft, the stick force gradient _________

with ____________ manoeuvring stability and the _______ limit of stick force gradient may be reached.

At high altitudes, the high TAS ________ the change in tail angle of attack for a given pitching velocity and _________ the pitch damping. Thus, a decrease in manoeuvring stick force stability can be expected with _________ altitude.

A flying control system may employ _______ springs, _____ springs or ____ weights to provide satisfactory control forces throughout the speed, CG and altitude range of an aircraft.

While static stability is concerned with the initial tendency of an aircraft to return to equilibrium, dynamic stability is defined by the resulting _______ with _____.

Stability and Control

An aircraft will demonstrate positive dynamic stability if the _________ of motion ________

with time.

When natural aerodynamic damping cannot be obtained, _________ damping must be provided to give the necessary positive dynamic stability.

The longitudinal dynamic stability of an aeroplane generally consists of two basic modes of oscillation:

a)_____ period (phugoid)

b)______ period

The phugoid oscillation occurs with nearly constant ______ of _______.

The period of oscillation is so great, the pilot is easily able to counteract ____ _____ oscillation.

Short period oscillation involves significant changes in ______ of _______.

Short period oscillation is ____ _______ controlled by the pilot.

The problems of dynamic stability can become acute at _____ altitude because of _________

aerodynamic ________.

To overcome the directional instability in the fuselage it is possible to incorporate into the overall design _______ or ________ fins.

The _____ is the major source of directional stability for the aeroplane.

A ___ - tail makes the fin more effective by acting as an “____ plate”.

Because the _______ fin stalls at a very much higher angle of attack, it takes over the stabilizing role of the fin at large angles of sideslip.

___________ produces a directional stabilizing effect, which increases with increase in CL.

_________ fins increase directional stability at _____ angles of attack. Landing clearance requirements may limit their size, require them to be retractable, or require two smaller ventral fins to be fitted instead of one large one.

Generally, good handling qualities are obtained with a relatively _____, or ____ positive, lateral stability.

The principal surface contributing to the lateral stability of an aeroplane is the _____. The effect of geometric _________ is a powerful contribution to lateral stability.

A low wing position gives an ________ contribution to static lateral stability.

A _____ wing location gives a stable contribution to static lateral stability.

The magnitude of “dihedral effect” contributed by the vertical position of the wing is _____

and may require a noticeable dihedral angle for the _____ wing configuration. A high wing position, on the other hand, usually requires ___ geometric ________ at all.

The ______ back wing contributes a positive “dihedral effect”.

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An aircraft with a swept-back wing requires _____ geometric dihedral than a straight wing.

The fin contribution to purely lateral static stability, is usually very ______.

Excessive “dihedral effect” can lead to “______ roll,” difficult rudder coordination in ________

manoeuvres, or place extreme demands for _______ control power during crosswind take-off and landing.

Deploying partial span flaps gives a _________ dihedral effect.

A swept-back wing requires much less geometric dihedral than a straight wing. If a requirement also exists for the wing to be mounted on top of the fuselage, an additional “dihedral effect” is present. A high mounted and swept-back wing would give excessive “dihedral effect”, so

_________ is used to reduce “dihedral effect” to the required level.

When an aeroplane is placed in a sideslip, the lateral and directional response will be ________, i.e. sideslip will simultaneously produce a _______ and a ______ moment.

Spiral divergence will exist when static directional stability is very _____ when compared to the “dihedral effect”.

The rate of divergence in the spiral motion is usually so ________ that the pilot can control the tendency without _________.

Dutch roll will occur when the “dihedral effect” is ______ when compared to static directional stability.

Aircraft which Dutch roll are fitted with a _____ Damper. This automatically displaces the rudder proportional to the _____ of yaw to damp-out the oscillations.

If the Yaw Damper fails in flight, it is recommended that the ________ be used by the pilot to damp-out Dutch roll.

If the pilot uses the ________, pilot induced oscillation (PIO) will result and the Dutch roll may very quickly become _________, leading to loss of _______.

When the swept wing aeroplane is at low CL the “dihedral effect” is small and the ______

tendency may be apparent. When the swept wing aeroplane is at high CL the “dihedral effect” is increased and the ______ _____ oscillatory tendency is increased.

When pilot induced oscillation is encountered, the most effective solution is an immediate

_______ of the controls. Any attempt to forcibly damp the oscillation simply _________ the excitation and _________ the oscillation.

Higher TAS common to high altitude flight _______ the _____ of ______ changes and reduces aerodynamic ________.

Mach Tuck is caused by ___ of lift in front of the ___ and _______ downwash at the tail due to the formation of a __________ on a swept-back wing at _____ Mach numbers.

The Mach trim system will adjust ___________ _____ to maintain the required _____ _____

gradient and operates only at _____ Mach numbers.

KEY FACTS 2 WITH THE MISSING WORDS INSERTED CAN BE FOUND ON page 326.