During design, development, and performance evaluation of an aerospace vehicle, vehicle drag is estimated to :
- determine flight characteristics, such as range, endurance, maneuverability.
- Propulsion system sizing.
- Effect of payload/carriage of aircraft performance.
The design life-cycle majorly consists of three phases,(i) conceptual, (ii) preliminary, and (iii)critical design phases.
Drag estimate is refined at each of the design phases. Drag is estimated through theoretical/semiempirical methods and coarse CFD methods at the preliminary levels. At critical design, drag is estimated through wind-tunnel tests and actual flight tests.
The prediction, analysis, and simulations in axisymmetric vehicle configuration are performed in body axes, such as CA and CN.
The equation of motion for a point mass, in-plane, flat-earth trajectory are:
where CD and CL are defined in terms of CA and CN:
A numerical solution of these equations involves using CA and CN to obtain V, flight path angle, and height at each time step in the integration. The interpolation of the aerodynamic database obtains the aerodynamic coefficients. Prediction techniques must be capable of providing a large number of estimates quickly. Precision-guided weapons demand higher levels of accuracy and drag prediction.
Solving the Navier-Stokes equations, representing the actual flow is the best way to predict the drag accurately. The state-of-the-art numerical method, turbulent modeling, HPC are required for guaranteed and efficient solution of these flow equations, which has a long turnaround time.
However, prediction methods are used effectively with simplifying assumptions at the conceptual/preliminary levels to predict the drag values satisfactorily. Both viscous and inviscid categories are identified. The viscous category provides the boundary-layer solutions to estimate the skin-friction drag.
However, at transonic speeds, the wave drag cannot be separated from skin friction drag, and empirical results are often used. These simplified flow models are essential at the initial design phase for an aerodynamicist to intuitively ‘guesstimate’ the vehicle drag without resorting to costly and time-consuming wind-tunnel tests and high-fidelity CFD simulation.