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When my plane was inspected by FAA for
its Airworthiness, the inspector noted the minimal clearance between my spinner
and the cowling; it measured .010" to .015". He asked me if that would not allow
it to rub. I said that when I pull power in flight and the prop is pushed
back, it will rub, but not for very long. Think abradable seals on jet
engine fan and compressor blades. Now when I pull forward on the prop, it's
about .03" to .05". Why make it bigger? Engine cooling air from the high
pressure plenum tries to come forward around the crankshaft and exit radially
through the gap. This causes a flow that disrupts the smooth flow from the
spinner and causes it to turbulate back on to the cowling. It is actually
preferable to have the cowling 1/8" to 1/4" larger in diameter just behind the
spinner with that portion forming a "quarter-round". That forms a Coanda surface
which will turn the radial flow 90 degrees to the rear. It is the same principle
which is used to decrease drag on the horizontal surface by enlarging and
rounding the leading edge of the elevator (ailerons, too) to 10% greater
thickness than the stab immediately ahead of it, to cause the stab-elevator gap
flow to turn to the rear. See Hoerner's F.D. Drag, P. 5-13, Fig. 28 and
Stinton's "The Design of the Airplane", P. 444, Fig.
12.9d.
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