This page shows examples of data from the NCAR C-130 for selected parts of a case study flight on May 18, 1998 near the SHEBA ice camp. They illustrate the the effects of riming on the performance of the anti-iced "small community" inlet that was used to bring ambient air to instruments inside the aircraft.

The "small community inlet" was used on the C-130 during the SHEBA project and on the NCAR Electra during the Lake-ICE project, Dec 1997-Jan 1998 (click on image to see bigger photo)

This inlet consists of a conical pipe facing the wind and a concentric shroud to align the air flow.

Both the cone and the shroud have embedded electrical heaters that operate continuously in flight. The inlet aperture at the small end of the entrance cone is 0.5 inch.  The shroud is 9" diameter and its axis is 8" from the aircraft skin.  At an air speed of 110 m s^-1, the probe samples isokinetically at ~850 liters per minute. The airflow distribution is as follows: air enters the aperture, expands from 0.5" to 1.0" outside the aircraft and expands again to 2.0" inside the cabin.  It then passes through a ball valve and a centrifugal blower and is exhausted out of the cabin. The valve is used to adjust the flow to approximate the isokinetic rate. The valve adjustment is done manually while checking INFLOWC on one of the data system displays.
 

During the period 22:00 to 23:40 on May 18, the aircraft made a series of level passes at altitudes from ~50m to 550m. The plot of INFLOWC in the bottom panel of this plot shows the effect of cloud water on the flow rate during eight cloud passes.  The aircraft was penetrating a shallow low level stratus cloud 200-450m AGL (HGM232) under a strong inversion.  In the cloud, supercooled liquid water (SLW) of ~0.1 to 0.2 g m^-3 was measured (PLWCC1) at -5 to -12°C (ATX).   While the aircraft was in cloud, rime ice built up on the sample inlet and choked off the air flow. Upon leaving cloud, the ice melted or broke off, and the sample flow was restored within ~30s. As long as the aircraft remained in the supercooled water cloud, the sample air flow decreased progressively.  Faster decreases were associated with greater LWC (click to see plot) .

A close-up view of the first cloud penetration (22:10) shows the 30s delay before INFLOWC was affected and the recovery period of ~30s.  Another close-up view of a longer penetration (22:57-23:04) shows similar responses.

"Interstitial" inlets that did not ingest cloud particles were also installed on the C-130.  They were basically 1/4" tubes flush-mounted to the skin of the airplane.  The anti-iced small community inlet was the preferable one for air sampling, but when problems with riming were recognized, the "interstitial" sample inlet was used. The interstitial inlet had other problems (not discussed here).

Although the PMS 2D-c probe showed only ~0.5 particles per liter, the CPI probe showed much higher values, ~50/L.  A large fraction of the CPI images were circular and up to 100µm in diameter, suggesting supercooled drizzle drops.  If so, the characteristics of icing may not be adequately represented as due to the LWC in 20µm cloud droplets.

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This material is based upon work supported by the National Science Foundation under Grant No. ATM97-14177 and by NASA grant NAG1-2063. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.
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