An optimization procedure for helicopter rotor aerodynamic performance is presented. This optimization procedure is centered on the numerical optimizer CONMIN, a gradient-based method that minimizes a functional under constraints. The optimizer has been coupled to a 3D Navier–Stokes CFD solver elsA, and applied to helicopter rotor optimization in hover.
The optimization chain and its components are first described. Several validations and applications are then presented starting from the 7A and 7AD rotor geometries with optimization of the different blade shape parameters (twist, chord, sweep and anhedral distribution). The efficiency and the robustness of the method are then tested for some more complex applications starting from the ERATO rotor. Finally a synthesis is made showing that the optimization chain is an helpful tool for the design of new helicopter rotors.
The design of helicopter rotor blades is a very complex
task involving many domains and disciplines such as aerody-
namics, acoustics, dynamics, which is not presently achieved
in a single shot. Some attempts have been done in the recent
past years to build optimization tools by coupling an opti-
mizer algorithm with a performance analysis code for theifferent disciplines involved in helicopter design. A com-
plete review of design optimization applied to helicopter is
presented by Celi in [6]. A large part of the previous studies
was devoted to aeroelastic and dynamic optimization such
as [8,21] in which the purpose is to reduced vibratory loads
and dynamic stresses. The aerodynamic models involved in
this optimization are simple models based on the lifting-line
theory and eventually free-wake model.
Concerning aerodynamic optimization, up to now, the
aerodynamic performance are also taken into account through
models based on 1D momentum theory or lifting-linemethods [10,13,19,22]. In parallel, the CFD methods have
reached sufficient maturity to compute very accurately heli-
copter rotor aerodynamic performance in hover. These CFD
methods are now currently applied during the design cycles
of advanced geometry blades. The current CFD codes effi-
ciency (CPU consumption, robustness,
...
) enables to use
them in automatic optimization chains. Such optimization
strategies involving Navier–Stokes solvers have been ap-
plied in aeronautics on fixed wing configurations [11,17],
and via adjoint formulation on aircraft configurations [9,15],
and have demonstrated their efficiency to be successfully in-
tegrated in design cycles. Only few authors apply numerical
optimization coupled with CFD codes on rotary wing, these
automatic design tools being only applied for turbomachin-
ery problems in quasi 3D and 3D [4,7].
This paper describes an aerodynamic optimization strat-
egy for helicopter rotor blades shape, based on the coupling
of an optimizer (gradient method) with a 3D Navier–Stokes
solver. For that purpose the CONMIN optimizer [18] has
been coupled to the CFD object-oriented
elsA
software [5],
developed by ONERA. The optimization procedure is fo-
cused on aerodynamic performance in hover flight condition,
since it is one of the critical point for the power and thrust
required. In addition, for hover flight condition, only steady
computations are required; at the present time, unsteady
computations would not enable such a coupling because of
a very high CPU time consuming. For comparison purpose
and in order to assess the necessity to use CFD methods in
such an optimization tool, applications are also made with
CONMIN coupled with a lifting line code, HOST [2].
The description of the different functionalities of the op-
timization procedure is presented. First the general strategy
for conducting the optimization, and the choice made are
also explained. The validation of the coupling is performed
on the optimization of linear aerodynamic twist of the 7A ro-
tor, since the solution of this simple problem is well-known.
Further applications on the optimization of the chord, sweep
and anhedral distributions are then presented, and an analy-
sis of the new geometries given by the optimizer is proposed.
Finally the optimization chain is initialized by a more modern rotor shape: ERATO, for which the optimization of the
blade tip is shown. A more complex optimization of the ER-
ATO rotor involving several parameters and design variables
concludes the study.
