Control and Stability 10

Key Facts 1

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

Stability is the ________ of an aircraft to return to a _____ state of flight, after being disturbed by an external ______, without any help from the _____.

There are two broad categories of stability: ________ and ________ .

An aircraft is in a state of __________ (trim) when the sum of all forces is ____ and the sum of all ________ is zero.

The type of static stability an aircraft possesses is defined by its ______ tendency, following the removal of some disturbing force.

The three different types of static stability are:

a)_________ static stability exists if an aircraft is disturbed from equilibrium and has the tendency to return to equilibrium.

b)______ static stability exists if an aircraft is subject to a disturbance and has neither the tendency to return nor the tendency to continue in the displacement direction.

c)_________ static stability exists if an aircraft has a tendency to continue in the direction of disturbance.

The longitudinal axis passes through the ____ from _____ to _____.

The normal axis passes “vertically” through the ___ at __° to the ___________ axis.

The lateral axis is a line passing through the ___, parallel to a line passing through the ____ tips. The three reference axes all pass through the _______ ___ ________.

Lateral stability involves motion about the __________ axis (_______). Longitudinal stability involves motion about the ______ axis (_______). Directional stability involves motion about the _______ axis (_______).

We consider the changes in __________ of lift force due to changes in angle of ________, acting through a __________ point; the ___________ ______.

The aerodynamic centre (AC) is located at the ___% chord position.

The _________ pitching moment about the AC remains ________ at normal angles of attack. A wing on its own is statically ________ because the ___ is in front of the ___.

An upward vertical gust will momentarily ________ the angle of attack of the wing. The

________ lift force magnitude acting through the ___ will increase the ______ pitching moment about the ___. This is an ________ pitching moment.

The ________ is positioned to generate a _________ pitching moment about the aircraft ___.

Stability and Control

If the tail moment is greater than the wing moment, the sum of the moments will not be ____

and the resultant nose _____ moment will give an angular _________ about the ____.

The ______ the tail moment relative to the wing moment, the _______ the rate of return

_______ the original __________ position.

The tail moment is increased by moving the aircraft ___ forwards, which _________ the tail arm and decreases the _____ arm.

If the nose-down (_______) tail moment is greater than the nose-up (_______) wing moment, the aircraft will have _______ __________ stability.

The position of the CG when changes in the sum of the tail moment and wing moment due to a disturbance is zero is known as the ______ _____.

The further forward the ___, the ______ the nose-down angular __________ about the ___ - the ______ the degree of _____ __________ stability.

The _______ the ___ is forward of the ________ point will give a measure of the _____

longitudinal stability; this distance is called the static ______.

The greater the static margin, the ______ the _______ ___________ stability.

The ____ CG limit will be positioned some distance _______ of the _____ _____.

The distance between the ___ ___ limit and the neutral point gives the required _________

static stability ________.

An aircraft is said to be _______ if all ________ in pitch, roll, and yaw are equal to _____.

Trim ( __________ ) is the function of the _______ and may be accomplished by:

a)______ effort

b)trim _____,

c)moving _____ between the wing ______ and an aft located _____ tank, or

d)bias of a surface _______ ( ________ flying controls).

The term ____________ refers to the ability of the aircraft to respond to control surface displacement and achieve the desired ________ of flight.

A high degree of stability tends to reduce the ____________ of the aircraft.

The stable tendency of an aircraft resists displacement from ___ equally, whether by ____ effort on the controls ( ______ force) or _____.

If the CG moves forward, static longitudinal stability ________ and controllability _________

(stick force ________).

If the CG moves aft, static longitudinal stability __________ and controllability ________ (stick force ________ ).

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Stability and Control 10

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Control and Stability 10

Stability and Control

With the CG on the forward limit, static longitudinal stability is _______, controllability is ____

and stick force is _____.

With the CG on the aft limit, static longitudinal stability is _____, controllability is _______ and stick force is ____.

The aft CG limit is set to ensure a _________ degree of static longitudinal stability.

The fwd CG limit is set to ensure a _________ degree of controllability under the worst circumstance.

KEY FACTS 1 WITH THE MISSING WORDS INSERTED CAN BE FOUND AT THE END OF THIS CHAPTER.

Graphic Presentation of Static Longitudinal Stability

Static longitudinal stability depends upon the relationship of angle of attack and pitching moment. It is necessary to study the pitching moment contribution of each component of the aircraft. In a manner similar to all other aerodynamic forces, the pitching moment about the lateral axis is studied in the coefficient form.

M = CM Q S (MAC)

or

M

CM = Q S (MAC)

where:

M= pitching moment about the CG (positive if in a nose-up direction)

The pitching moment coefficients contributed by all the various components of the aircraft are summed up and plotted versus lift coefficient (angle of attack).

Study of the plots of CM versus CL is a convenient way to relate the static longitudinal stability of an aeroplane.

Figure 10.16 Graph A

Graph A illustrates the variation of pitching moment coefficient (CM) with lift coefficient (CL) for an aeroplane with positive static longitudinal stability. Evidence of static stability is shown by a tendency to return to equilibrium, or “trim”, upon displacement. The aeroplane described by graph A is in trim or equilibrium when CM = 0, and if the aeroplane is disturbed to some different CL, the pitching moment change tends to return the aircraft to the point of trim. If the aeroplane were disturbed to some higher CL (point y), a negative or nose-down pitching moment is developed which tends to decrease angle of attack back to the trim point. If the aeroplane were disturbed to some lower CL (point x), a positive or nose-up pitching moment is developed which tends to increase the angle of attack back to the trim point. Thus, positive static longitudinal stability is indicated by a negative slope of CM versus CL. The degree of static longitudinal stability is indicated by the slope of the curve (red line).

+ B STABLE

TRIM

NEUTRAL

CM

CL

UNSTABLE

Figure 10.17 Graph B

Graph B provides comparison of a stable and an unstable condition. Positive static stability is indicated by the red curve with negative slope. Neutral static stability would be the result if the curve had zero slope. If neutral stability existed, the aeroplane could be disturbed to some higher or lower lift coefficient without change in pitching moment coefficient.

Stability and Control 10

Control and Stability 10

Such a condition would indicate that the aeroplane would have no tendency to return to some original equilibrium and would not hold trim. An aeroplane which demonstrates a positive slope of the CM versus CL curve (blue line) would be unstable. If the unstable aeroplane were subject to any disturbance from equilibrium at the trim point, the changes in pitching moment would only magnify the disturbance. When the unstable aeroplane is disturbed to some higher CL a positive change in CM occurs which would illustrate a tendency for continued, greater displacement. When the unstable aeroplane is disturbed to some lower CL a negative change in CM takes place which tends to create continued displacement.

Figure 10.18 Graph C

Ordinarily, the static longitudinal stability of a conventional aeroplane configuration does not vary with lift coefficient. In other words, the slope of CM versus CL does not change with CL. However, if:

•the aeroplane has sweepback,

•there is a large contribution of “power effect” on stability, or

•there are significant changes in downwash at the horizontal tail,

noticeable changes in static stability can occur at high lift coefficients (low speed). This condition is illustrated by graph C. The curve of CM versus CL of this illustration shows a good stable slope at low values of CL (high speed). Increasing CL gives a slight decrease in the negative slope hence a decrease in stability occurs. With continued increase in CL, the slope becomes zero and neutral stability exists. Eventually, the slope becomes positive and the aeroplane becomes unstable or “pitch-up” results.

Remember, at any lift coefficient, the static stability of the aeroplane is depicted by the slope of the curve of CM versus CL.