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Effect of Wing Contamination on Stall Speed

Refer to:

AIC 106/2004 “Frost Ice and Snow on Aircraft”, and

AIC 98/1999 “Turbo-Prop and other Propeller Driven Aeroplanes: Icing Induced Stalls”.

Any contamination on the wing, but particularly ice, frost or snow, will drastically alter the aerodynamic contour and affect the nature of the boundary layer.

ICE The formation of ice on the leading edge of the wing will produce:

a)Large changes in the local contour, leading to severe local adverse pressure gradients.

b)High surface friction and a considerable reduction of boundary layer kinetic energy.

These cause a large decrease in CLMAX and can increase stall speed by approximately 30% with no change in angle of attack.

The added weight of the ice will also increase the stall speed, but the major factor is the reduction in CLMAX.

FROST The effect of frost is more subtle. The accumulation of a hard coat of frost on the wing upper surface will produce a surface texture of considerable roughness.

Tests have shown that ice, snow or frost, with the thickness and surface roughness similar to medium or coarse sandpaper on the leading edge and upper surface of a wing can reduce lift by as much as 30% (10% to 15% increase in stall speed) and increases drag by 40%.

While the basic shape and aerodynamic contour is unchanged, the increase in surface roughness increases skin friction and reduces the kinetic energy of the boundary layer. Separation will occur at an angle of attack and lift coefficients lower than for the clean smooth wing.

SNOW The effect of snow can be similar to frost in that it will increase surface roughness. If there is a coating of snow on the aircraft it must be removed before flight. Not only will the snow itself increase skin friction drag, it may obscure airframe icing. Snow will NOT blow off during taxi or take-off.

The pilot in command is legally required to ensure the aeroplane is aerodynamically clean at the time of take-off. It is very important that the holdover time of any de-icing or anti-icing fluid applied to the airframe is known. If this time will be exceeded before take-off, the aircraft must be treated again.

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While the reduction in CLMAX due to frost formation is not usually as great as that due to ice formation, it is usually unexpected because it may be thought that large changes in the

aerodynamic shape (such as due to ice) are necessary to reduce CLMAX. However, kinetic energy of the boundary layer is an important factor influencing separation of the airflow and this energy is reduced by an increase in surface roughness. The general effects of ice and frost formation on CLMAX is typified by the illustrations in Figure 7.31.

Ice, frost and snow change the aerofoil section, decrease the stall angle and increase the stall speed

LEADING EDGE ICE FORMATION

UPPER SURFACE FROST

BASIC SMOOTH WING

C LMAX

CL

WING WITH FROST

WING WITH ICE

ANGLE OF ATTACK

Figure 7.31

The increase in stall speed due to ice formation is not easy to quantify, as the accumulation and shape of the ice formation is impossible to predict. Even a little ice is too much. Ice or frost must never be allowed to remain on any aerodynamic surfaces in flight, nor must ice, frost, snow or other contamination be allowed to remain on the aircraft immediately before flight.

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Stalling 7

Warning to the Pilot of Icing-induced Stalls

There have been recent cases involving loss of control in icing conditions due to undetected stalling at speeds significantly above the normal stalling speed, accompanied by violent roll oscillations.

Control of an aeroplane can be lost as a result of an icing-induced stall, the onset of which can be so insidious* as to be difficult to detect.

The following advice is offered on the recognition of, and the recovery from, insidious icinginduced wing-stalls:

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a)	Loss of performance in icing conditions may indicate a serious build-up of airframe icing	Stalling
	(even if this cannot be seen) which causes a gradual loss of lift and a significant increase
	in drag;
b)	this build-up of ice can cause the aeroplane to stall at approximately 30% above the
	normal stall speed;
c)	the longitudinal characteristics of an icing-induced wing-stall can be so gentle that the
	pilot may not be aware that it has occurred;
d)	the stall warning system installed on the aeroplane may not alert the pilot to the
	insidious icing-induced wing-stall (angle of attack will be below that required to trigger
	the switch), so should not be relied upon to give a warning of this condition. Airframe
	buffet, however, may assist in identifying the onset of wing-stall;
e)	the first clue may be a roll control problem. This can appear as a gradually increasing
	roll oscillation or a violent wing drop;
f)	a combination of rolling oscillation and onset of high drag can cause the aeroplane to
	enter a high rate of descent unless prompt recovery action is taken;
g)	if a roll control problem develops in icing conditions, the pilot should suspect that the
	aeroplane has entered an icing-induced wing-stall and should take immediate stall
	recovery action (decrease the angle of attack). The de-icing system should also be
	activated. If the aeroplane is fitted with an anti-icing system this should have been
	activated prior to entry into icing conditions in accordance with the Flight Manual/
	Operations Manual procedures and recommendations. If the anti-icing system has not
	been in use, then it should be immediately activated. Consideration should also be
	given to leaving icing conditions by adjusting track and/or altitude if possible.

*Insidious - advancing imperceptibly: without warning

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Stabilizer Stall Due to Ice

The tailplane is an aerofoil, and because it is thinner than the wing, it is likely to experience icing before the wing does. The effect will be the same as for the wing; the stall will occur at a lower angle of attack. The tailplane is normally operating at a negative angle of attack, producing a down load, so if the tailplane stalls and the down load is lost, the nose of the aircraft will drop and longitudinal control will be lost.

Stalling of an ice contaminated tailplane could be precipitated by extension of the wing flaps. Lowering the flaps increases the downwash, and this increases the negative angle of attack of the tailplane. If the tailplane has ice contamination, this could be sufficient to cause it to stall. Recovery procedure in this situation would be to retract the flaps again, thus reducing the downwash.

Effect of Heavy Rain on Stall Speed

Weight

Heavy rain will form a film of water on an aircraft and increase its weight slightly, maybe as much as 1 - 2% this in itself will increase stall speed.

Aerodynamic Effect

The film of water will distort the aerofoil, roughen the surface and alter the airflow pattern on the whole aircraft. CLMAX will decrease causing stall speed to increase.

Drag

The film of water will increase interference drag, profile drag and form drag. In light rain, drag may increase by 5%, in moderate by 20% and in heavy rain by up to 30%. This obviously increases thrust required.

Impact

An additional consideration, while not affecting stall speed, is the effect of the impact of heavy rain on the aircraft. Momentum will be lost and airspeed will decrease, requiring increased thrust. At the same time, heavy rain will also be driving the aircraft downwards. The volume of rain in any given situation will vary, but an aircraft on final approach which suddenly enters a torrential downpour of heavy rain will be subject to a loss of momentum and a decrease in altitude, similar to the effect of microburst windshear. (Chapter 15).

Stall and Recovery Characteristics of Canards

With the conventional rear tailplane configuration the wing stalls before the tailplane, and longitudinal control and stability are maintained at the stall. On a canard layout if the wing stalls first, stability is lost, but if the foreplane stalls first then control is lost and the maximum value of CL is reduced.

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