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Aerodynamic Balancing

The floating characteristics of a control surface depend onthe hinge-moment characteristics and

As we will see later, the stick force also depends on thehinge- moment characteristics.

Too low values of the hinge-moment would make thecontrol highly sensitive to small disturbances

whereas too high values would make the controls sluggishto operate.

Therefore, a careful design of the hinge-moment

parameters C h and Che is necessary to achieve a properbalance between these two conflicting requirements.

The control of these hinge-moment parameters is calledaerodynamic balancing.

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Pressure Profile with Elevator withdeflection

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Set-Back Hingleline

When the hingeline is at the controlsurface leading edge,both C h andCheare negative.

If the hingeline is moved further aft,both Ch and Che become morepositivebecause the control surfaceforward of the hingeline produces anopposing moment to that produced bythe surface aft of the hingeline. Thenet hinge moment which is thealgebraic sum of these two moments,

is greatly reduced. This type of aerodynamic balance is

called set-back hinge balance

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Horn balance

Horn balance (Fig. 3.45b) issimilar to the set-back hinge,except that all the area ahead ofthe hingeline is concentrated on

one part of the surface.

The horn balance makes Chand Che less negative thoughthe effect on Cheis morepronounced than in the case ofset-back hinge method.

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Internal Balance

The internal balance (Fig. 3.45c) is a modification of the set-back hinge method. The inside of the airfoil has to be vented tothe external pressures so that the pressures acting on thebalancing area provide the necessary balancing effect.

The effectiveness of this balance can be increased by sealing

the gap between the leading edge of the control surface andthe structure of the airfoil.

The amount of balance can be adjusted by properly venting theseal.

This type of balance has a greater effect on Chethan Ch

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Beveled trailing edge

type of balance has a greater effect on C hl . ethan Ch The shapes of the leading and trailingedges of the control surface also have an effect

on the hinge moment parameters Ch

and Che

For example, beveling the trailing edge alters

the parameter Che because the pressuredistribution over the control surface is altered.

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Tab balancing the control surface is by deploying an additional control

surface called tab.

The tab is much smaller in size compared to the elevator and isusually deflected in the opposite direction as shown in Fig. 3.45e.

Even though the tab is small in size, the pressure changes caused byits deflection produce appreciable moments about the elevator

hingeline. Because the tab moves only when the elevator moves, it has little or

no effect on Ch

The tab has the added advantage that it can always provide just theright amount of balancing as needed. Hence, the possibility ofcontrol surface overbalance is almost ruled out in this case.

However, it has a minor disadvantage that an opposite deflection ofthe tab results in a small lift loss, hence a reduction in overallcontrol effectiveness.

This type of tab is also known as a trim tabbecause it is used to trimthe stick forces as we will study later.

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Numerical

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