• International Journal of Technology (IJTech)
  • Vol 13, No 1 (2022)

Numerical Analysis of Floatplane Porpoising Instability in Calm Water During Takeoff

Numerical Analysis of Floatplane Porpoising Instability in Calm Water During Takeoff

Title: Numerical Analysis of Floatplane Porpoising Instability in Calm Water During Takeoff
Muhammad Hafiz Nurwahyu Aliffrananda, Aries Sulisetyono, Yuda Apri Hermawan, Achmad Zubaydi

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Cite this article as:
Aliffrananda, M.H.N., Sulisetyono, A.Hermawan, Y.A., Zubaydi, A., 2022. Numerical Analysis of Floatplane Porpoising Instability in Calm Water During Takeoff. International Journal of Technology. Volume 13(1), pp. 190-201

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Muhammad Hafiz Nurwahyu Aliffrananda Department of Naval Architecture, Institut Teknologi Sepuluh Nopember, Surabaya 60111, Indonesia
Aries Sulisetyono Department of Naval Architecture, Institut Teknologi Sepuluh Nopember, Surabaya 60111, Indonesia
Yuda Apri Hermawan Department of Naval Architecture, Institut Teknologi Sepuluh Nopember, Surabaya 60111, Indonesia
Achmad Zubaydi Department of Naval Architecture, Institut Teknologi Sepuluh Nopember, Surabaya 60111, Indonesia
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Abstract
Numerical Analysis of Floatplane Porpoising Instability in Calm Water During Takeoff

In the operation of floatplanes, porpoising must be avoided, but it is a common occurrence during takeoff, as it induces longitudinal instability and compromises the safety and comfort of passengers. The mechanism of porpoising and the factors or variables that influence the occurrence of this phenomenon are explored in this study. Based on a review of the literature, the position of the longitudinal center of gravity (LCG) and the deadrise angle were found to be the two most significant variables affecting the porpoising phenomenon. The mechanism of porpoising was simulated using a numerical model based on computational fluid dynamics (CFD). The simulation findings were then compared to the results of a related model’s towing tank experiment. With five velocity differences, a validated computational model was used to analyze the impact of LCG ordinates and deadrise angles on the frequency of porpoising. Compared to the LCG 50% length overall (LOA) configuration, the floater with an LCG 53% LOA configuration caused a higher heave porpoising amplitude by 4% for the floater with a 10° deadrise angle and 1% for the floater with a 20° deadrise angle at all speed variations. However, the pitch porpoising amplitude produced by the floater with an LCG 53% LOA configuration was found to be 4% higher than the LCG 50% LOA configuration for the floater with a 10° deadrise angle and -1% higher than the LCG 50% LOA configuration for the floater with a 20° deadrise angle. The results showed that the higher heave and pitch porpoising amplitude was generated by a low deadrise angle and a shift in the floater’s center of gravity toward the bow.

CFD; Deadrise; Floatplane; Porpoising; Take-off

Introduction

    Seaplanes can operate on both land and water, making them suitable for short-distance travel (Xiao et al., 2020). and operations near coastal lines to reduce traffic at municipal airports while improving connectivity between secluded islands (Ito et al., 2016). Except for specific operations and restricted areas, seaplanes have been widely overlooked in modern aviation. Nonetheless, this aircraft model has begun to regain its appeal due to the robust advancements in computational abilities (Ito et al., 2016). Floatplanes, which are an amphibious type of seaplane, are equipped with submerged floats or pontoons underneath their fuselage. A numerical study in 2020 analyzed the optimal separation ratio (S/L) configuration of a catamaran hull design most suitable for the N219 floatplane by implementing a sailfish’s hydrodynamic qualities to minimize the total hull resistance caused by hull interference (Yanuar et al., 2020). This interference is also caused by changes in pressure and flow velocity around the catamaran hulls, which are affected by the form factor (Utama et al., 2021). This has been proven in a numerical study that aimed to clarify the reduced total resistance in a traditional catamaran by modifying the hull form in line with the Lackenby method (Iqbal and Samuel, 2017). The floats or pontoon’s purpose is to provide lift force; any inconsistencies in the lift force led to dynamic instabilities, such as porpoising. The porpoising phenomenon, a rhythmical pitching motion on amphibious vessels equivalent to a swinging sensation (Gudmundsson, 2013), is commonly experienced by floatplanes during takeoff (Aliffrananda and Sulisetyono, 2021). Operability problems, such as increased takeoff time and track length (Gudmundsson, 2013), incidents during takeoff (Federal Aviation Administration [FAA], 2004), inconvenience to pilots and passengers (Faltinsen, 2005), and damage to the floater’s construction (Abrate, 2011), are outcomes of porpoising. A deeper understanding of the complexity of these problems is required to determine methods that can minimize their occurrence.

The risk of porpoising was evaluated in a towing tank using high-speed craft models and two variations of deadrise angles (Day and Haag, 1952). A higher deadrise angle led to an increase in the critical trim, while a higher speed led to a decrease. The results were plotted to represent the longitudinal stability limits induced by different deadrise angles (Savitsky, 1964). In 1999, a program was developed to predict the porpoising instability induced by various deadrise angles, and the longitudinal center of gravity (LCG) (Sulisetyono, 1999) was validated against previous experiments (Day and Haag, 1952). A similar experiment conducted on a speed range of 3–15 m/s found that porpoising began to occur at 5 m/s, with the period decreasing over the increase in speed. The highest porpoising period occurred at 5 m/s while the lowest began at 15 m/s (Katayama, 2004). Another experiment on stepped and regular hulls (Sajedi and Ghadimi, 2020) found that the stepped hull decreased the average of the ship’s heave and pitch motion and the porpoising amplitude. At high speeds, the start of porpoising was delayed on the ship with a stepped hull while at the same Froude number, the ship with a regular hull had begun porpoising (Sajedi and Ghadimi, 2020).

    A numerical simulation of porpoising in a large seaplane model (Duan et al., 2019) concluded that the porpoising amplitude started to increase over time. Another similar numerical study was done on a modified N219 aircraft turned into a floatplane with a catamaran hull configuration, and the variations in LCG and deadrise angle were investigated to analyze the effects of LCG and deadrise angle on porpoising (Aliffrananda and Sulisetyono, 2021). The results were compared to a high-speed craft motion experiment (Sajedi and Ghadimi, 2020). A low deadrise angle generated a higher lift force, causing the floater to experience heave faster than other floater models with higher deadrise values. However, the LCG increased the heave and pitch period during porpoising if the LCG was near the stern of the floater (Aliffrananda and Sulisetyono, 2021). From these studies (Duan et al., 2019; Aliffrananda and Sulisetyono, 2021), the conclusion that amplification will continue to happen due to porpoising oscillation if the plane’s position remains uncorrected can be drawn. Despite previous studies on porpoising, studies of its occurrence in floatplanes are still scarce. This paper addresses this research gap by engaging in further analysis of the porpoising phenomenon in floatplanes. The purpose of the analysis is to determine the optimal configuration of both LCG coordinates and the deadrise angle of floaters to mitigate the porpoising phenomenon. This phenomenon was simulated by utilizing computational fluid dynamics (CFD), variations of five traveling speeds, two LCG coordinates, and two deadrise angle variations. 

Conclusion

    CFD has proven its ability to accurately simulate the porpoising phenomenon on the floaters of a floatplane during takeoff operations. High-speed fluid flows, as the focus of the simulations, were modeled using the unsteady flow RANS equation and the  turbulence model. Structured hexahedral mesh and deformation mesh methods were utilized to accommodate the floater’s motion. The numerical computational setup was validated against the ITTC experiment using the DTMB 5415 models. Based on the dynamic features imposed by the porpoising motion on the floater seen at various speeds, a few conclusions were obtained. First, at both deadrise angle variations, LCG near the floater's bow generated higher heave and pitch porpoising amplitudes. Second, the low deadrise angle gave rise to a higher heave and pitch porpoising amplitude. Finally, the porpoising periods for both motions followed the same pattern, with the values decreasing after reaching its peak. However, higher deadrise angles resulted in overall lower porpoising periods.

Acknowledgement

    The authors would like to thank the National Research and Innovation Agency (BRIN) of Indonesia and the Institut Teknologi Sepuluh Nopember (ITS) for funding this study under the scheme of “Basic Research Awards” with contract number 3/AMD/E1/KP.PTNBH/2020.

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