10 Stability and Control

Power-off Stability

When the aerodynamic stability of a configuration is of interest, power effects are neglected and the stability is considered by a build-up of the contributing components.

Figure 10.24 illustrates a typical build-up of the components of a conventional aeroplane configuration. If the CG is arbitrarily set at 30 percent MAC, the contribution of the wing alone is destabilizing, as indicated by the positive slope of CM versus CL. The combination of the wing and fuselage increases the instability. The contribution of the tail alone is highly stabilizing from the large negative slope of the curve. The contribution of the tail must be sufficiently stabilizing so that the complete configuration will exhibit positive static stability at the anticipated CG locations.

		TYPICAL BUILD-UP OF COMPONENTS
10
Stability	CM	WING + FUSELAGE
Control and		WING ONLY
		CL
		AEROPLANE
		CG @ 30% MAC
		TAILPLANE
		ONLY
		Figure 10.24

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CM	50% MAC
	40% MAC (NEUTRAL POINT)
	30% MAC	CL
	20% MAC
	10% MAC

Figure 10.25

Effect of CG Position

A variation of CG position can cause large changes in the static longitudinal stability. In the conventional aeroplane configuration, the large changes in stability with CG variation are primarily due to the large changes in the wing contribution. If the incidence of all surfaces remains fixed, the effect of CG position on static longitudinal stability is typified by the chart in Figure 10.25. As the CG is gradually moved aft, the aeroplane static stability decreases, then becomes neutral then unstable. The CG position which produces zero slope and neutral static stability is referred to as the “neutral point”. The neutral point may be imagined as the effective aerodynamic centre of the entire aeroplane configuration, i.e. with the CG at the neutral point, all changes in net lift effectively occur at that point and no change in pitching moment results. The neutral point defines the most aft CG position without static instability.

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10 Stability and Control

Power Effects

Control and Stability 10

The effects of power may cause significant changes in trim lift coefficient and static longitudinal stability. Since the contribution to stability is evaluated by the change in moment coefficients, power effects will be most significant when the aeroplane operates at high power and low airspeeds such as during approach and while taking off.

DESTABILIZING

Figure 10.26

The effects of power are considered in two main categories. First, there are the direct effects resulting from the forces created by the propulsion unit. Next, there are the indirect effects of the slipstream and other associated flow which alter the forces and moments of the aerodynamic surfaces. The direct effects of power are illustrated in Figure 10.26. The vertical location of the thrust line defines one of the direct contributions to stability. If the thrust line is below the CG, as illustrated, a thrust increase will produce a positive or nose-up moment and the effect is destabilizing.

NORMAL FORCE DUE TO

MOMENT CHANGE

Figure 10.27

A propeller located ahead of the CG contributes a destabilizing effect. As shown in Figure 10.27, a rotating propeller inclined to the relative airflow causes a deflection of the airflow. The momentum change of the slipstream creates a normal force at the plane of the propeller. As this normal force will increase with an increase in aeroplane angle of attack, the effect will be destabilizing when the propeller is ahead of the CG. The magnitude of the unstable contribution depends on the distance from the CG to the propeller and is largest at high power and low dynamic pressure.

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W ING, NACELLE AND FUSELAGE

MOMENTS AFFECTED BY

SLIPSTREAM

DYNAMIC PRESSURE

AT TAIL AFFECTED

BY SLIPSTREAM

W ING LIFT AFFECTED

BY SLIPSTREAM

Figure 10.28

The indirect effects of power are of greatest concern in the propeller powered aeroplane rather than the jet powered aeroplane. As shown in Figure 10.28, the propeller powered aeroplane creates slipstream velocities on the various surfaces which are different from the flow field typical of power-off flight. Since the various wing, nacelle and fuselage surfaces are partly or wholly immersed in this slipstream, the contribution of these components to stability can be quite different from the power-off flight condition. Ordinarily, the change of fuselage and nacelle contribution with power is relatively small. The added lift on the portion of the wing immersed in the slipstream requires that the aeroplane operate at a lower angle of attack to produce the same effective lift coefficient. Generally, this reduction in angle of attack to effect the same CL reduces the tail contribution to stability. However, the increase in dynamic pressure at the tail tends to increase the effectiveness of the tail and may be a stabilizing effect. The magnitude of this contribution due to the slipstream velocity on the tail will depend on the CG position and trim lift coefficient.

DOWNWASH AT TAIL

AFFECTED BY

SLIPSTREAM DIRECTION

Figure 10.29

The deflection of the slipstream shown in Figure 10.29 by the normal force at the propeller tends to increase the downwash at the horizontal tail and reduce the contribution to stability.

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