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Old 05-07-2019, 06:52 AM
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DanH DanH is offline
 
Join Date: Oct 2005
Location: 08A
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As with cooling, a low Vi/Vo inlet won't require as much real estate inside the cowl. The precise shape of the airbox isn't critical because the flow is slowed externally, out in front of the inlet ring. There is no aircraft velocity at which the inlet diameter is choked; velocity through the inlet is always slower than freestream.

Much of the freestream approaching the inlet will ultimately flow out and around the inlet housing, rather than through the hole. So, the downside is generally a much large bulge on the cowl to accommodate both the large inlet diameter and the required external shape to avoid separation. (I should note that my own airplane is likely not correct in this regard, so don't copy that particular shape. It's probably too sharp-edged. We learn as we go.)

Re diameter of a low velocity ratio inlet, I made some airbox pressure measurements last fall which seemed to confirm the following approach.

As before, determine volume demand from displacement and RPM. Pick a diameter to examine, calculate its area, then divide volume by area to find velocity due to air demand.

Now pick an airspeed and altitude of interest. Subtract intake demand velocity (above) from freestream velocity. Determine dynamic pressure for the difference, at the altitude and temperature of interest. Add it to local static pressure and the result is airbox pressure.

Again, there is no one perfect answer. Larger diameters will increase airbox pressure, but also require a larger housing, thus probably add external drag. In the end some of decision depends on physical integration with the cowl, airbox, and engine. In my own case, 4" matched the short airbox and filter size with which I was working. I rank practical very highly; whatever you do, make it easy to R&R the cowl!
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