Addition of Nacelle Specific form factor to drag estimation

Desired capability or behavior.

Add a new form factor relationship specifically for Nacelles to generate more accurate drag buildup.

FLOPS (and Aviary) uses the same form factor relationship for both Fuselages and Nacelles in the EDET (Empirical Drag Estimation Technique) drag buildup. Most modern nacelles have a very low fineness ratio (much lower than fuselages), with the result that the EDET form factor relationship results in very high form factor estimations for Nacelles, and therefore too much drag is apportioned to the Nacelles.
Raymer offers an alternative form factor relationship for Nacelles which could be coded in to provide better nacelle form factors:

Nacelle Form Factor = 1 + (0.35 / fine)
[where fine = fineness ratio = max length / max diameter for the body]
(See graph below)

Aviary form factor relationship is in subsystems>aerodynamics>flops_based>skin_friction_drag.py

form_factor[idx_body] = F[0] + fine *
(F[1] + fine * (F[2] + fine * (F[3] + fine * (F[4] + fine * (F[5] * fine + F[6])))))

Form factor fit coefficients:

self.F = np.array([ 4.34255, -1.14281, .171203, -.0138334, .621712e-3, .137442e-6, -.145532e-4, 2.94206, 7.16974, 48.8876, -1403.02, 8598.76, -15834.3, 4.275])

If this was simply added with no other corrections it would just result in lower overall drag for every aircraft. This may not be desirable since the EDET method was calibrated to real aircraft with a calibration factor. Further analysis is required to assess if this calibration factor should be updated to preserve the top level drag coefficient, with modified breakdown underneath, or if a simple correction to the nacelle drag is sufficient.

In either case results will diverge from FLOPS with identical inputs.

Is your feature request related to a problem? Please describe.

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Associated Bug Report

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