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FLOW INDUCED BY

JETFAN EXHAUST

Figure 10.30

Essentially the same destabilizing effect is produced by the flow induced at the exhaust of turbo-jet/fan engines, Figure 10.30. Ordinarily, the induced flow at the horizontal tail of a jet aeroplane is slight and is destabilizing when the jet passes underneath the horizontal tail. The magnitude of the indirect power effects on stability tends to be greatest at high CL, high power and low flight speeds.

Conclusions to the Effects of Power

The combined direct and indirect power effects contribute to a general reduction of static stability at high power, high CL and low dynamic pressure. It is generally true that any aeroplane will experience the lowest level of static longitudinal stability under these conditions. Because of the greater magnitude of both direct and indirect power effects, the propeller powered aeroplane usually experiences a greater effect than the jet powered aeroplane.

High Lift Devices

An additional effect on stability can be from the extension of high lift devices. High lift devices tend to increase downwash at the tail and reduce the dynamic pressure at the tail, both of which are destabilizing. However, high lift devices may prevent an unstable contribution of the wing at high CL. While the effect of high lift devices depends on the aeroplane configuration, the usual effect is destabilizing. Hence, the aeroplane may experience the most critical forward neutral point during the power approach or overshoot/missed approach. During this condition of flight, the static stability is usually the weakest and particular attention must be given to precise control of the aeroplane.

The power-on neutral point may set the most aft limit of CG position.

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Control Force Stability

The static longitudinal stability of an aeroplane is defined by the tendency to return to equilibrium upon displacement. In other words, a stable aeroplane will resist displacement from trim or equilibrium. The control forces of the aeroplane should reflect the stability of the aeroplane and provide suitable reference to the pilot for precise control of the aeroplane.

	EFFECT OF ELEVATOR DEFLECTION
	CM	ELEVATOR	10
		DEFLECTION
	+	TRIM FOR	Control
			and
		10º UP
	TRIM FOR 0º		Stability
	-		CL
		CG @ 20% MAC

Figure 10.31

The effect of elevator deflection on pitching moments is illustrated by the graph of Figure 10.31. If the elevators of the aeroplane are held at zero deflection, the resulting line of CM versus CL for 0° depicts the static stability and trim lift coefficient. If the elevators are held at a deflection of 10° up (aircraft trimmed at a lower speed), the aeroplane static stability is unchanged but the trim lift coefficient is increased.

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As the elevator is held in various positions, equilibrium (trim) will occur at various lift coefficients, and the trim CL can be correlated with elevator deflection as shown in Figure 10.32.

		TRIM CL VERSUS ELEVATOR DEFLECTION
		CG LOCATION
		10% MAC
		20% MAC
		UP
	10	30% MAC
		CL
	Stability
		40% MAC
	Control and	(NEUTRAL POINT)
		DOWN

Figure 10.32

When the CG position of the aeroplane is fixed, each elevator position corresponds to a particular trim lift coefficient. As the CG is moved aft, the slope of this line decreases, and the decrease in stability is evident by a given control displacement causing a greater change in trim lift coefficient. This is evidence that decreasing stability causes increased controllability and, of course, increasing stability decreases controllability.

If the CG is moved aft until the line of trim CL versus elevator deflection has zero slope, neutral static stability is obtained.

A change in elevator position does not alter the tail contribution to stability

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TRIM AIRSPEED VERSUS ELEVATOR DEFLECTION

	STABLE
UP	UNSTABLE
	EQUIVALENT
	AIRSPEED
DOWN

Figure 10.33

Since each value of lift coefficient corresponds to a particular value of dynamic pressure required to support an aeroplane in level flight, trim airspeed can be correlated with elevator deflection as in the graph of Figure 10.33.

If the CG location is ahead of the neutral point and control position is directly related to surface deflection, the aeroplane will give evidence of stick position stability. In other words, the aeroplane will require the stick to be moved aft to increase the angle of attack and trim at a lower airspeed and to be moved forward to decrease the angle of attack and trim at a higher airspeed.

It is highly desirable to have an aeroplane demonstrate this feature. If the aeroplane were to have stick position instability, the aeroplane would require the stick to be moved aft to trim at a higher airspeed or to be moved forward to trim at a lower airspeed.

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There is an increment of force dependent on the trim tab setting which varies with the dynamic pressure or the square of equivalent airspeed. Figure 10.34 indicates the variation of stick force with airspeed and illustrates the effect of tab setting on stick force.

		EFFECT OF TRIM TAB SETTING
	PULL
	0			EAS
		1	2	3
	CG @ 20% MAC
	PUSH

Figure 10.34

In order to trim the aeroplane at point (1) a certain amount of up elevator is required and zero stick force is obtained with the use of the trim tab. To trim the aeroplane for higher speeds corresponding to points (2) and (3), less and less aircraft nose-up tab is required.

Note that when the aeroplane is properly trimmed, a push force is required to increase airspeed and a pull force is required to decrease airspeed. In this manner, the aeroplane would have positive stick force stability with a stable “feel” for airspeed.

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EFFECT OF CG POSITION

		CG POSITION
	10% MAC		TRIM
PULL
	20% MAC		SPEED
	30%	MAC
0	40%	MAC	EAS
	50%	MAC
PUSH

Figure 10.35

If the CG of the aeroplane were varied while maintaining trim at a constant airspeed, the effect of CG position on stick force stability could be appreciated. As illustrated in Figure 10.35, moving the CG aft decreases the slope of the line of stick force through the trim speed. Thus, on decreasing stick-force stability it is evident that smaller stick forces are necessary to displace the aeroplane from the trim speed. When the stick force gradient (or slope) becomes zero, the CG is at the neutral point and neutral stability exists. If the CG is aft of the neutral point, stick force instability will exist, e.g. the aeroplane will require a push force at a lower speed or a pull force at a higher speed. It should be noted that the stick force gradient is low at low airspeeds, and when the aeroplane is at low speeds and high power and has a CG position near the aft limit, the “feel” for airspeed will be weak.

	EFFECT OF CONTROL SYSTEM FRICTION
PULL	TRIM SPEED
	BAND
0	EAS
PUSH	FRICTION FORCE
	BAND

Figure 10.36

Control system friction can create very undesirable effects on control forces. Figure 10.36 illustrates that the control force versus airspeed relationship is a band rather than a line. A wide friction force band can completely mask the stick force stability when the stick force stability is low. Modern flight control systems require precise maintenance to minimize the friction force band and preserve proper feel to the aeroplane.

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