During the conceptual design phase, the aircraft stability and control derivatives (aerodynamic coefficients) can be estimated by using fast computational means. Aerodynamic potential codes like the Vortex Lattice Method (VLM) or the Doublet Lattice Method (DLM) are very easy to use and are capable of estimating these coefficients accurately as well as providing remarkable insight into wing aerodynamics and components interaction. Compared to the VLM, the DLM (originally used for aeroelastic computations) allows prediction of the steady as well as unsteady stability and control derivatives.
The relationships involving these coefficients and the airplane's dynamic behaviour are well known, like for example the one relating the pitch damping derivative and the damping ratio of the Short Period mode. Previous research work (see references 1 and 3 for further details) based on panel methods, has modelled analytically the rigid aircraft aerodynamic coefficients, leading to an interesting result:
“A singular combination of longitudinal stability derivatives, independent of the centre of gravity location, could describe, to a great extent, the longitudinal behaviour of a rigid aircraft in flight.”
It turns out that this “aerodynamic constant” is virtually spread over each longitudinal stability and control relationship, like for example the transfer functions characterizing the aircraft response to control surfaces deflections and gusts inputs. In the linear region of the flight envelope, this Aerodynamic Invariant seems to be important for the airplane longitudinal stability and control problem.
By using a standard DLM code, the objectives of the current research work are:
To assess the importance of this “aerodynamic constant” as well as its predictive nature on longitudinal dynamic parameters, like the frequency and damping ratio of the Short Period mode or the manoeuvre point location.
To investigate, among a wide variety of conventional and unconventional configurations, the value of this invariant in order to establish a relationship between the aircraft geometry parameters and its longitudinal Handling Qualities.
To conclude, in conventional aircraft conceptual design phase, design for Performance is done before design for Handling Qualities.
Taking this into account, a last question can be formulated:
Could this “aerodynamic constant”, and hence the Handling Qualities analysis, be used (in parallel to Performance analysis) in the conceptual design phase for suggesting new aircraft configurations capable of achieving further Performance benefits?