Chapter 11 - Ground Effect

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WILJAM FLIGHT TRAININGChapter 11.

Ground Effect

Introduction

When an aircraft flies close to a surface, e.g. ground or water, the lift, drag and stabilitycharacteristics are significantly affected. The changes that occur are collectively known asground effect, and are noticeable whenever an aircraft is one wingspan or less above thesurface. The closer to the surface the more pronounced the effect.

The Characteristics of Ground Effect

Ground effect takes place because the surface interferes with, and alters the airflow pattern

around the wings. Primarily the surface restricts the formation of the wingtip vortices(Fig. 11.1).

FIG. 11.1

This results in a reduction in the amount of induced downwash behind the wing, andincreases the wings effective angle of attack (Fig. 11.2).

INCREASED LIFT

INCREASED EFFECTIVEANGLE OF ATTACK

FIG. 11.2

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WILJAM FLIGHT TRAININGIt also alters the pressure distribution around the wing, and the amount of lift developed. Thisoccurs because the change in effective angle of attack increases the wings coefficient of lift(CL), and thus its lifting capability at any given angle of attack. The aircraft will also stall at a

lower angle of attackwhen flying in ground effect. (Fig.11.3).

CLMAX

CL

COEFFICIENTOF LIFT

ANGLE OF ATTACK

AIRCRAFT OUT OFGROUND EFFECT

AIRCRAFT INGROUND EFFECT

FIG. 11.3

The magnitude of the wing tip vortices, and thus downwash, also determine the amount ofinduced drag produced by the wing. Thus the closerthe aircraft is to the surface, the greaterthe reduction in induced drag, at any given angle of attack (Fig. 11.4).

60

50

40

30

20

10

00 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1

RATIO OF WING HEIGHT TO SPAN, hb

PERCENTAGEREDUCTION

ININDUCED DRAG

FIG. 11.4

The reduction in induced drag is approximately 1% at a height of one wingspanabove asurface, and approximately 50% at a height of one tenth of the wingspan . This effect is

therefore significant during the take-off and landing phases of flight when induced drag may

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WILJAM FLIGHT TRAININGaccount for over 80% of the total drag. The overall reduction in drag also reduces the amountof thrust required to maintain a given airspeed (Fig. 11.5).

FIG. 11.5

THRUSTREQUIRED

(N)

AIRCRAFT OUT OFGROUND EFFECT

AIRCRAFT INGROUND EFFECT

VELOCITY, KNOTS

The second characteristic associated with ground effect is the change in aircraft trim andstability. This occurs because the downwash acting behind the wing is deflected by thesurface, thus altering the angleat which the airflow meets the tailplane(Fig. 11.6).

DOWNWASH

DOWNWASH PATH OUTOF GROUND EFFECT DOWNWASH PATH INGROUND EFFECT

DOWNWASH

REDUCEDDOWN

DOWN

LOADLOAD

FIG. 11.6

This places the tailplane at a less negative or increased angle of attack, and reduces thedownload, acting on the tailplane. In this condition the aircraft will experience a nose-downpitching moment, although high T tailed aircraft do not respond in this manner, since thetailplane is outside the influence of the downwash.

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WILJAM FLIGHT TRAININGThe Influence of Ground Effect on Landing

When an aircraft enters ground effect during the landing phase of flight the sudden increase inliftand reduction in dragwill cause it to experience a floating tendency. The aircraft will alsotend to pitch nose-down due to the reduction in download on the tailplane.

The Influence of Ground Effect on Take-Off

During the take-off phase of flight as the aircraft leaves ground effect the wing tip vorticeswill rapidly grow in magnitude, producing an increase in downwash behind the wing. Thiswill cause the lift produced at a given angle of attack to suddenly reduce, and the associatedrapid increase in induced drag may prevent a successful take-off. The increased downwashacting on the tailplane will increase the download, and a nose-up pitching moment will result. Itis therefore important that the take-off is not attempted at speeds less than the designated take-off speed, because even though the aircraft may be able to fly in ground effect, it will be unableto climb out of it. The aircraft may even sink as it flies out of ground effect, and will resettle

on the ground.

The Influence of Ground Effect on Trailing Edge Flaps

When flying in ground effect with the trailing edge flaps deflectedthe surface will increase theadverse pressure gradient over the flap, and will also induce earlier separation of theairflow (Fig. 11.7).

FLOW SEPARATION AT FLAPDUE TO HIGH PRESSURE ON GROUND

FIG. 11.7

This will significantly reduce the maximum lift capability of the wing, compared to that obtainedoutside ground effect. This is one of the main reasons why STOL aircraft are manufacturedwith a high wing configuration.

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Chapter 11 - Ground Effect