Loads Prediction at Early Stage of Water Landing of V-shaped Hull on Amphibious Aircraft
Creators: name:  Y. LU 
name:   T. XIAO 
name:   S. DNEG 
Publisher: name: Proceedings of  the 9th European Conference for Aerospace Sciences. Lille, France, 27 June - 1 July, 2022
Year: 2022
Subjects: FLIGHT PHYSICS
COMPUTATIONAL AERODYNAMICS
EXPERIMENTAL AERODYNAMICS

description: Amphibious aircraft is a special flight vehicle that can take off and land on both water and conventional runways on airports. Its functions can be essentially divided into two categories. The first one is basically for general aviation, i.e. firefighting, sea sightseeing tourism, etc. On the other hand, military missions, concerning marine patrol, anti-submarine, rescue and so on, can be considered as the second application. On the whole, due to its inherent operational situations, the most circumstances that the aircraft may face are open water field, leading to the water impacting load as the primary load conditions, which involves sorts of complicated physical and hydrodynamic phenomena. Moreover, the term of peak vertical acceleration is more significant, which not only causes mental problems for the crews but also it can lead to structural damages for the fuselage once it strikes the water, deserving more attentions amid the load characteristics about an amphibious aircraft landing on water. It is well known that less time to reach the peak value for larger initial vertical velocity, however, it will be beneficial and convenient for experimental test or simulation if there exists some theoretical or empirical formula to guide on the water entry phenomenon, especially for the complicated model like an amphibious aircraft or something else. What we want to do more in the present study is to build up kinds of relations which can be utilized to predict the load dynamics for the performance of an amphibious water landing. Numerical predictions, solving unsteady incompressible Reynolds-averaged Navier-Stokes equations enclosed by the SST k-? turbulence model, with the help of the momentum variation, first proposed by von Karman, and added mass theory are recommended as the better way to achieve the goal. Two distinct test cases of 2D symmetric wedge oblique entry and 3D cabin section vertical entry are presented first in order to validate the relationships between initial vertical velocity and vertical acceleration when acceleration itself reaches its highest point, as well as other corresponding parameters, viz., time, penetration depth, the ratio of velocity. The quantitative relations are investigated subsequently of hull landing on water for amphibious aircraft. Detailed results through the free surface and pressure distribution are provided to show the slamming effects, and their physical implications are discussed. The result shows that for the V-shaped sectional area of body impacting on water surface, the maximum vertical acceleration increases with the initial vertical velocity, and it is widely to acknowledge herein that the value of maximal acceleration is proportional to the square of the initial velocity, contributing to variation prediction of acceleration. By the way, for oblique entry, another situation that the effect of variant horizontal velocity on acceleration has also been investigated. It is meaningful to appreciate the maximum horizontal acceleration is a linear function of initial horizontal velocity itself, rather than the square value, which compromises with the theoretical regulation to some extent. The value 5/6 that corresponding velocity vs the initial velocity can be seen as a lower limit of the ratio, where data derived from all cases are in proximity to it when initial velocity rises. It indicates that a threshold value of initial vertical velocity need to be emphasized to make the theoretical result available, which can be seen as a modification to the theory.