stability lecture - lateral

8/13/2019 Stability Lecture - Lateral

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AIRCRAFT STABILITY AND CONTROL

STATIC LATERAL STABILITY AND CONTROL

BY

Y.K.SINHA


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SIDE SLIP AND YAW ANGLE


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LATERAL STABILITY

Stability about the aircrafts longitudinal axis,

which extends from the nose of the aircraft to its

tail, is called lateral stability. This helps to stabilizethe lateral or rolling effect when one wing gets

lower than the wing on the opposite side of the

aircraft.

It is the ability of the aircraft to recover from a rollwithout pilots intervention.


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For a aircraft to be laterally stable

Clshould be positive


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There are four main design factors that

make an aircraft laterally stable:

Dihedral

Wing placement on lateral stability

Vertical tail placement on lateral stability

Keel effect and weight distribution Sweepback


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ESTIMATION OF DIHEDRAL EFFECT

If a disturbance causes one wing

to drop relative to the other, thelift vector rotates and there is a

component of the weight acting

inward which causes the

airplane to move sideways in this

direction. When wings havedihedral, the wing toward the

free-stream velocity, hence the

lower wing, will experience a

greater angle of attack than the

raised wing and hence greaterlift. There results a net force and

moment tending to reduce the

bank angle.


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Positive sideslip (nose left) creates an upward

velocity on right wing and downward velocityon left wing

Equivalent of downwash

Increases angle of attack over right wing,decreases angle of attack over left wing

Results in a rolling moment to the left

The stabilizing effect of this configuration is

known as the dihedral effect.


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EFFECT OF WING PLACEMENT ON LATERAL STABILITY


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EFFECT OF VERTICAL TAIL PLACEMENT ON LATERAL STABILITY

INTERFERENCE OF WING ON VERTICAL PALNE


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During flight, the side area of the

airplane's fuselage and vertical finreact to the airflow in much the

same manner as the keel of a ship.

That is, it exerts a steadying

influence on the airplane laterally

about the longitudinal axis. Such

laterally stable airplanes areconstructed so that the greater

portion of the keel area is above

and behind the center of gravity.

Thus, when the airplane slips to

one side, the combination of the

airplane's weight and the pressureof the airflow against the upper

portion of the keel area (both acting

about the CG) tends to roll the

airplane back to wings-level flight.

EFFECT OF VERTICAL TAIL PLACEMENT ON LATERAL STABILITY OR

KEEL EFFECT


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Sweepback

The secondary effect of sweepback is to improve

lateral stability. When a side-slip

occurs, the lower wing presents a larger span as

seen from the direction of the approaching air,

and as with dihedral, the effect is to roll the

aircraft back towards the horizontal.

In general, as the sweepback angle is increased

the dihedral angle will be reduced.


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Airbus380

HARRIER


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EFFECT OF FLAP DEFLECTION ON LATERAL STABILITY


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EFFECT OF POWER ON LATERAL STABILITY


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LATERAL CONTROL


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SPANWISE LIFT DISTRIBUTION

DUE TO AILERON DEFLECTIONS

LOAD DISTRIBUTION DUE

TO ROLLING VELOCITY


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At a given helix angle, the distance

to complete a roll remains constant

regardless of forward speed.

The helix angle is the ratio of tip

velocity in roll, pb/2, to the aircrafts

forward velocity, V.

E ti ti f L t l C t l P


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Estimation of Lateral Control Power

Strip IntegrationAssumptionThe airplane to be a single degree of

freedom in roll about X AxisThe wing is considered to be a rigid structure with therolling moment arising due to aileron deflection (a)anddamping moment due to angular velocity (p)

The two parameters rolling moment due to aand damping

moment due to p can be evaluated by strip integration

method


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Note: For a given aileron deflection pb/2V is constant

For a tapered wing


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Aileron Reversal


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End

stability lecture - lateral

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