More than 100 research articles have referred to an experimental study by Feldkamp (1987) on the California sea lion, which mentions that its body resembles a symmetric airfoil, NACA 66018. We believe that perhaps an oversimplification has been made in this comparison. This conclusion may be arrived at by shifting the leading and trailing edges of the contour of the sea lion in order to move the chord line in the upward direction, such that the maximum thickness of the selected airfoil is close to that of a sea lion's body. It is pertinent to highlight that most of the research articles on different species of sea lions (Cheneval, 2005; Suzuki et al., 2014; Fish, 1998; Stelle et al., 2000) are related to its hydrodynamic drag and lift generated by its flippers. However, the effect of assuming resemblance to a symmetric airfoil reduces the additional hydrodynamic lift and the induced drag generated by the body.
If one closely observes the sketch used by Feldkamp (1987), it is obvious that a sea lion's upper and lower contours are different (Fig. 1A); this feature is unique to any cambered airfoil. In order to elaborate it further; this sketch was used as a baseline sketch and graphically digitized for unit length (L) by defining the maximum thickness (d) equal to 18% of unit length (Fig. 1B). The upper and lower contour lines were generated by using a 4th order polynomial fit and the camber line is also plotted to show the difference between a symmetric and a cambered airfoil. Feldkamp (1987) did not specifically mention the maximum diameter (d) of the body. The only known information is the lateral view of the body resembling a NACA 66-018 airfoil. Hence, the maximum thicknesses of 18% of the chord length and fineness ratio equal to 5.5, shown in Fig. 1A, are used in this work for defining a suitable cambered airfoil.
Sketches and image of the California sea lion body shape compared with airfoil cross-sections. (A) Sketch used by Feldkamp (1987) in which the dotted line shows the profile of the NACA 66018 airfoil. (B) Sketch drawn to show the camber in lateral view of the sea lion (black data points and black line). Red and blue lines indicate airfoil contour and mean camber line, respectively. (C) Geometric comparison of sea lion outline sketch (red) with NACA 66018 and cambered airfoils. (D) Pictorial view of California sea lion, taken in Zoo Negara, Kuala Lumpur.
Sketches and image of the California sea lion body shape compared with airfoil cross-sections. (A) Sketch used by Feldkamp (1987) in which the dotted line shows the profile of the NACA 66018 airfoil. (B) Sketch drawn to show the camber in lateral view of the sea lion (black data points and black line). Red and blue lines indicate airfoil contour and mean camber line, respectively. (C) Geometric comparison of sea lion outline sketch (red) with NACA 66018 and cambered airfoils. (D) Pictorial view of California sea lion, taken in Zoo Negara, Kuala Lumpur.
In order to select a cambered airfoil that best fits the shape of the sea lion at its nose, upper and lower contours and meet the requirements of maximum thickness and its location, RONCZ 1082 and FX S 03-182 were randomly chosen. These are two standard cambered airfoils which have maximum thickness of 18% at 40% chord and 18.2% at 40.2% chord, respectively (Airfoil Tools, 2014). Once the profile of these airfoils were plotted on the sketch used by Feldkamp (1987), it was observed that the lateral view of the California sea lion's body cannot fully resemble any engineering application airfoil (Fig. 1C). Although its body is streamlined, the lower contour of its body is somewhat bulkier than the upper.
Unfortunately, complete geometric details of the sea lion required to compare with different airfoils are still missing in the open literature. However, because of the differences between the upper and lower contours in lateral view, the body of the sea lion partially resembles a cambered or un-symmetric airfoil. This is also true for a sea lion lying on the ground, as observed at Zoo Negara, Kuala Lumpur, Malaysia (Fig. 1D). Also, it is well known that most of the terrestrial vertebrates have a vaulted dorsum which makes the body appear in the lateral view as a cambered airfoil. Based on the discussion above, it may be concluded that the sea lion's body resembles a cambered airfoil. Such a shape can be tailored for aeronautical applications such as the lifting fuselage of a hybrid buoyant aircraft.
Acknowledgements
The support of the Ministry of Science, Technology and Innovation (MOSTI), Malaysia, under the grant 06-01-08-SF0189 is gratefully acknowledged. Authors are grateful to the management of Zoo Negara, Kuala Lumpur for providing the opportunity to closely observe the shape of the California sea lion.
