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Literature Review of Planing Craft Design - The Ground Effect

Preliminary fluid flow experiments for the ground effect were performed on a scale model of a tunnel racing boat by Reif and Geunther [1978]. These experiments were classified as aerodynamic and hydrodynamic, using both a wind tunnel and then a towing tank. This study demonstrated that the aerodynamic drag forces of modern tunnel boat hulls had become a significant percentage of the total drag forces. With this in mind it became apparent that the major improvements in performance would come from any aerodynamic changes, as opposed to alterations to the hydrodynamic form.

Modern tunnel hull racing boats literally fly over the water, with only the trailing edges of the sponsons and the lower unit of the engine in the water. For this reason it can be seen why the residual drag is small relative to the frictional hydrodynamic drag, and that a thorough knowledge of the aerodynamic forces acting on the hull may be of value for the optimisation of performance.

Reif showed that the aerodynamic drag coefficient increased as angle of attack was increased for low Reynolds numbers, but was mainly independent of angle of attack at high Reynolds numbers. The residual drag coefficient was seen to decrease as the Froude number was increased. The pressures on the upper and lower surfaces of the deck were measured and it was found that a net positive lift force was present due to lower pressure on the upper deck. The centre of pressure of this lift force was estimated to be at about 58 per cent of the chord from the leading edge.

Since the estimation of how much lift, or angle of attack, to be built into the hull has a profound effect on the performance of the hull, the primary objectives of a second study [Reif 1985] were to obtain accurate measurements of the aerodynamic lift, drag, and pitching moment on the scale model tunnel boat hull. From these measurements a set of nondimensional design curves were developed.

Chronologically, detailed investigations of the ground effect began around 1960. For example Bagley [1961] investigated the pressure distribution on two dimensional wings near the ground, using both model tests and a mathematical model. Other tests were conducted on aerofoils, with end plates, in the ground effect [Carter 1961] and showed the following:

1.   As the ground was approached, the aerofoils experienced an increase in lift-curve slope and a reduction in induced drag; thus an increase in lift-drag ratio resulted.

2.   Near the ground, the addition of end plates to the aerofoil resulted in a further increase in lift-curve slope and reduction in induced drag which resulted in a large increase in lift-drag ratio.

3.   As the ground was approached, the profile (frictional) drag remained essentially constant for each aerofoil.

4.   At positive angles of attack, the static longitudinal stability was increased as the height above the ground was reduced.

Around this time, channel flow ground effect vehicles (including SES), as shown in Figure 2.06, were being studied. Performance calculations for such vehicles were analysed using aerodynamic theories such as momentum theory, exponential theory, vortex theory and conformal mapping theory [Strand, Royce and Fujita 1962]. 

Figure 2.06: Basis of a channel flow ground effect vehicle.

(After [Strand, Royce and Fujita 1962])

By 1970 the term 'ram wing vehicle' was being used to describe these ground effect vehicles and the theory had been developed to incorporate a flat plate aerofoil translating over a ground plane with sinusoidal bumps. The problem of a three dimensional wing travelling over a flat ground plane was also being investigated [Barrows and Widnall 1970].

Tuck [1979] has been at the forefront of research into ground effect theory and considered the nonlinear unsteady one dimensional theory for wings in extreme ground effect. Extending his previous work, Tuck [1981] continued to investigate the steady flow around a thin aerofoil-like body in close proximity to a plane ground surface. This was based on a one dimensional, but nonlinear gap region flow, matched to the trailing edge, which could have significant flap-like appendages, as shown in Figure 2.07. It is noted that in the presence of the ground effect there is a coupling between pitch and heave, and hence an unstable situation occurs.

Figure 2.07: Aerofoil in ground effect, with trim tab. (After [Tuck 1981])

In further work [Tuck 1984], ground effect theory was applied to a simple case of an air supported vehicle over water. This treatment uses one dimensional aerodynamic theory and also replaces the ground plane by a free surface which is depressed in the gap region, as depicted in Figure 2.08.

Figure 2.08: Aerofoil operating in ground effect over a free surface.

(After [Tuck 1984])

Next Chapter; The Theoretical Model...

There is plenty more of this review, to be published at a later date. Contact JB if you have any particular questions.

REFERENCES

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