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  #11  
Old 05-06-2019, 06:07 PM
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DanH DanH is offline
 
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Quote:
Originally Posted by Larry DeCamp View Post
1-I have read calculations that a 3" dia. intake is all the air a 180HP 0360 can use and the SJ cowl has a 3" hole.
Assume a very high Vi/Vo inlet...small diameter, with a good internal diffuser if you expect any manifold pressure increase due to captured dynamic pressure. Air requirement for your 360 is:

[(360/2)x2700]/60 = 8100 cubic inches per second, assuming 100% volumetric efficiency.

The RV-4's VNE is (IIRC) 200 MPH, which would be 3520 inches per second.

8100/3520 = 2.3 sq inches required, or a diameter of roughly 1.75".

Note in this calc the air is passing through the inlet at freestream velocity, thus any conversion of dynamic pressure to increased static pressure must happen internally...in a really well shaped diffuser airbox. Prettiest one I've seen is on Paulo Iscold's recent speed record airplane.

This tiny pitot inlet offers the best chance of maximized external streamlining. However, at any speed less than VNE, the 1.75"D inlet size would be a choke, one reason we usually only see 'em on bespoke racers.

The 3" inlet works out to match about 65 MPH, which means it would not be a choke in slow speed full power climb, and it would pick up some external diffusion at higher speeds. There is no one perfect answer. Personally I'd go with 2.5"D for my RV-4 if committed to internal diffusion and low external drag.

A low VI/Vo inlet works better across the speed range, and doesn't require nearly so much internal airbox space. More later.
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  #12  
Old 05-06-2019, 07:09 PM
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Larry DeCamp Larry DeCamp is offline
 
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Default Wow, thanks Dan

As usual !
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RV-3B flying w/ carb / Pmags / Catto 2b / Steam
RV-4 fastback w/ Superior XP360/AFP/G3X/CPI/Catto3b
Clinton, IN
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  #13  
Old 05-07-2019, 07:52 AM
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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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  #14  
Old 05-07-2019, 10:15 AM
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Larry DeCamp Larry DeCamp is offline
 
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Default Air box size and cowl clearance

Thanks again Dan for the excellent guidance. I already built the airbox with airway dimensions equal to a 3" dia cowl opening. I should mention here for others i addressed the aluminum plate cracking and glass bottom chafing by using through bolts to the servo with tubes on the bolts inside the filter to control compression. Of course bolt heads are saftied.
I thought the new glass to rehab the cowl cutout was not handsome. The irony is an elbow with the servo horizontal requires the same width cowling bubble to clear fuel fittings and linkage.

So, the box is ample size for a 2 1/2" inlet you suggest. My only unknown at this point is how much to clearance the cowl "rehab" for shake clearance ? I have roughed in 1" pink foam but some guys have suggested 1/2" to 3/4" is OK. The only really cool looking option would be a rear facing horizontal servo on an elbow with a belly scoop, but the air box/ filter would be demanding
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Clinton, IN
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