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NACA vents for engine air supply ?

Larry DeCamp

Well Known Member
I have an RV4 with SJ cowl and FI 0360. I have fabricated a custom fab and also fitted an AFP elbow with conical filter. Both are not great aesthetically to modify the cowl for clearance.
So, a rear facing servo fed by NACA scoops might be an option. They would supply two air sources with enough air for an 0360 and address the alternate air issue. What does history say about this air supply config ?
 
I believe the Sequoia Falco used rear induction on an O-320 / 360 through a large NACA scoop on the right side of the airplane. Seemed to work okay for their purposes.
 
Is just fitting stuff inside the cowl?

To what end Larry? I just measured the pressure in the filter inlet side of my SJ cowl and it was equal (within measuring accuracy) to dynamic pressure at about 160 kts.

Is it just to allow better fitting? There is a lot of room forward of the left inlet, you could route a tube down from there, if there is room of course.
 
There is a difference

The NACA scoop design makes no pressure; there would be no MP increase. A bluff body inlet would be a better plan, or a fully circular inlet - the P-38 hid circular inlets inside a 3:1 fairing...very interesting design. Seems a Mustang inlet (belly scoop) might make good pressure recovery - the RC guys in giant scale have that part available for about $40. Turning vanes might help too..

If I can recall the sizing for a round inlet for the 0-320 would be around 2"?
 
Plenum design criteria ?

Thanks for the info gents ! OK, so a scoop of some sort ( sides or bottom of cowl) appears functional. That leaves two unknowns for me:

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. So I will assume the equivalent cross section will work ? Also, it has been written that a modest MP boost like .5" only yields about 3 knots ( for gentlemen aviators not racers).

2-I have read that any air stream that becomes static ( as in a plenum ) becomes drag if not re-accelerated out of the cowl. So I assume any enclosure filled from a 3" equivalent air stream is as good as you can do ? I.E. if your engine RPM ( displacement ) is not using it, the balance is drag no matter if it is drawing from a box of air or a snout on the front of the cowl.

Comments to add to or correct my observations would be appreciated.
 
Look at other KNOWN inlets - the ones that work

DanH has the skinny on the round inlets that seem to allow laminar flow to the air that does not go into the inlet. I think his design follows what Boeing did for the B29 nacelle, and the ring mold he passed on was that particular airfoil. The papers on that testing and results are available.

No - a simple rounded lip does not 'do the trick'.

3" sounds closer to what a 540 might be able to use; the fellas with the good inlets and induction tell us that 1" of MP on the 540 is about 8hp, so I'd GUESS that 1" on your 360 would be about 5-6HP. So, the process is worth the time to do it.

I have the XL spreadsheet for an inlet calculator - drop me a note offline and I'll pass it on. It shows a 2.13"dia inlet on a 360 engine at 200MPH/2500RPM, or about 260.4 cu ft/hour/4.34 cu in/sec. Feel free to check the math - it does not come from MY thinking!

You might see if you can get pics of Dave Anders RV4?

As I prefer to put things: I have never seen and idea that is too good to copy!
 
IO360 parallel valve

RV7 Inlet (SJ) 2-7/8" ID Your air filter can lose quite a bit if too small.

Drag is not that the air slows down in transit, but the inefficiency of the drag internally. Then the low pressure at the exit vs inlet. Low pressure times area is drag. Pressure makes velocity, so either are good. (as SCSMITH is teaching me to calculate) The exit loss should be considered as a separate issue.

DanH has shown that a "large" (large relative to the exit) air inlet is not bad if the flow and exit conditions are tightly controlled (i.e. mass flow)
 
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.
 
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!
 
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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