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It may be seen that, at the high inlet velocities necessary at high altitudes with radiators of reasonable size, the cooling drag power becomes very high.
Although it is, in general, desirable for inlet velocities to be low, a limit is set by the presence of this boundary layer, which will cause the flow to separate at the inlet If excessive deceleration is attempted (fig. 4).
The typical front underslung duct (fig. 3(a)) Is'also.more favorable than the rear underslung type because thefront underslung duct has only a relatively thin boundary layer at the Inlet; somewhat reduced inlet velocities
A further reason for limiting the inlet velocity is that, for very low inlet* velocity ratios, It beocmes Impossible to avoid high velocities over the lip, with the Resulting danger either of separaction Just hack of the minimum pressure region or of formation of a shook wave at high flight speeds.
Figure 6 (from reference 6)' shows, for example, acme pressure distributions over the lip of a oarbitrator Inlet with' different Inlet velocities.
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