Prediction of the Effect of Engine Geometry on Aircraft Smoke Visibility

The visibility of aircraft exhaust smoke derives principally from light absorption and, hence, varies with smoke concentration and exhaust diameter. Because of complex exit aerodynamics and jet entrainment, however, effective exhaust diameters differ from actual. Entrainment rapidly transforms the exit profiles of smoke concentration to self-preserving Gaussian ones and causes slow decay of visibility. The initial visibility of the self-preserving jet is shown to depend on the mass emission rate of smoke times the square of ambient gas density and gross jet thrust. This expression also describes, within about 10 percent, the actual exit visibility for current engine types. Quantitative criteria are also derived for the engine spacing, at which coalescence of parallel jets influences visibility, and for the magnitude of effect.