Contribution of the Component Surfaces

Thus, any pitching moment coefficient (CM) - regardless of source - has the common denominator of dynamic pressure (Q), wing area (S), and wing mean aerodynamic chord (MAC). This common denominator is applied to the pitching moments contributed by the:

•fuselage and nacelles,

•horizontal tail, and

•power effects as well as pitching moments contributed by the wing.

Wing

The contribution of the wing to stability depends primarily on the location of the aerodynamic centre (AC) with respect to the aeroplane centre of gravity. Generally, the aerodynamic centre is defined as the point on the wing Mean Aerodynamic Chord (MAC) where the wing pitching moment coefficient does not vary with lift coefficient. All changes in lift coefficient effectively take place at the wing aerodynamic centre. Thus, if the wing experiences some change in lift coefficient, the pitching moment created will be a direct function of the relative location of the AC and CG.

Note: The degree of positive camber of the wing has no effect on longitudinal stability. The pitching moment about the AC is always negative regardless of angle of attack.

Stability is given by the development of restoring moments. As the wing AC is forward of the CG, the wing contributes an unstable pitching moment to the aircraft, as shown in Figure 10.19.

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Figure 10.20

Fuselage and Nacelles

In most cases, the contribution of the fuselage and nacelles is destabilizing. A symmetrical body in an airflow develops an unstable pitching moment when given an angle of attack. In fact, an increase in angle of attack produces an increase in the unstable pitching moment without the development of lift. Figure 10.20 illustrates the pressure distribution which creates this unstable moment on the body. An increase in angle of attack causes an increase in the unstable pitching moment but a negligible increase in lift.

HorizontalTail

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

Figure 10.21

To appreciate the contribution of the horizontal tail to stability, inspect Figure 10.21. If the aeroplane is given an increase in angle of attack (by a gust OR control displacement), an increase in tail lift will occur at the aerodynamic centre of the tail. An increase in lift at the horizontal tail produces a negative (stabilizing) moment about the aircraft CG.

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For a given vertical gust velocity and aircraft TAS, the wing moment is essentially determined by the CG position. BUT, the tail moment is determined by the CG position AND the effectiveness of the tailplane. For a given moment arm (CG position), the effectiveness of the tailplane is dependent upon:

•Downwash from the wing.

•Dynamic pressure at the tailplane.

•Longitudinal dihedral.

Downwash from the wing and dynamic pressure at the tailplane will be discussed in due course, but the effect of longitudinal dihedral is shown below.

Longitudinal Dihedral

This is the difference between tailplane and wing incidence. For longitudinal static stability the tailplane incidence is smaller. As illustrated in Figure 10.22, this will generate a greater percentage increase in tailplane lift than wing lift for a given vertical gust.

This guarantees that the positive contribution of the tailplane to static longitudinal stability will be sufficient to overcome the sum of the destabilizing moments from the other components of the aeroplane.

AC

AC

4º INCREASE IN ANGLE OF ATTACK

DUE TO VERTICAL GUST

Figure 10.22

Figure 10.23

Downwash

It should be appreciated that the flow at the horizontal tail does not have the same flow direction or dynamic pressure as the free stream. Due to the wing wake, fuselage boundary layer and power effects, the dynamic pressure at the horizontal tail may be greatly different from the dynamic pressure of the free stream. In most instances, the dynamic pressure at the tail is usually less and this reduces the efficiency of the tail.

When the aeroplane is given a change in angle of attack, the horizontal tail does not experience the same change in angle of attack as the wing, Figure 10.23.

Because of the increase in downwash behind the wing, the horizontal tail will experience a smaller change in angle of attack, e.g. if a 10° change in wing angle of attack causes a 4° increase in downwash at the horizontal tail, the horizontal tail experiences only a 6° change in angle of attack. In this manner, the downwash at the horizontal tail reduces the contribution to stability.

Any factor which alters the rate of change of downwash at the horizontal tail (e.g. flaps or propeller slipstream) will directly affect the tail contribution and aeroplane stability. Downwash decreases static longitudinal stability.

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