Modeling of unsteady flow about a fire fighting aircraft dropping the water bomb

Abstract

Unsteady incompressible potential flow over the whole aircraft dropping the water bomb is sequentially solved using low order panel methods. Constant strength source and doublet panels distributed over aircraft surface, constant strength doublet panels over flat wake and constant strength source together with linear strength doublet panels distributed over the vertical water column, being able to properly imitate the downward water outflow of growing velocity, are used to solve the Dirichlet's boundary condition. To be able to evaluate the influence of history of motion on pressure distribution, this unsteady approach was preceded by classical, steady flow analysis, both for aircraft without water column and aircraft with water column of assumed, finite length. The main focus was to analyze the effect of water release onto the values of lift, pitching moment and downwash. Comparison of the steady flow parameters with the selected parameters, taken from experimental investigations, validated the analysis. Assuming that aircraft flight path is horizontal and is not influenced by the change of aircraft mass, it is found that unsteady flow, induced by water outflow, has negligible influence on aircraft aerodynamic characteristics. Numerical solution of the dynamic equations of aircraft motion under the assumption that aerodynamics is so-called quasi-steady, shows that aircraft response on water bomb dropping can be very abrupt, depending on design parameters and/or elevator and throttle control.