Inlet aircraft sampling

Figure 1. Representation of an aircraft aerosol sampling inlet showing flow streamlines for subisokinetic, superisokinetic, and isokinetic sampling. Under subisokinetic conditions large particles will be oversampled under superisokinetic conditions small particles will be oversampled and under isokinetic conditions, the particle size distribution will be preserved.

The ability of currently used aircraft probes to accurately sample aerosols has been questioned. Huebert et al. (8) conducted a comparative study of several different types of aerosol probes, all mounted on the same aircraft. The results suggested that substantial losses of particles occurred in all of the inlet systems. Because of the limited nature of the study, however, the causes of the aerosol losses could not be identified. The results of the Huebert study prompted a workshop to reexamine the entire issue of aerosol sampling from aircraft (9). An important conclusion of the workshop was that currently there is insufficient knowledge to adequately describe important characteristics of airflow and particle trajectories at flight speeds of aerosol sampling probes used on aircraft. 

An additional issue that seems obvious but that is frequently overlooked in installations is to ensure that the sampling line is not in a position to sample the aircraft engine exhaust or heater fumes and is not directly behind the propeller envelope. The sampling inlet should also be placed sufficiently far from the fuselage so that it is in the free airstream and not within the boundary layer of the aircraft. 

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