NACA vents for engine air supply ?

[Larry DeCamp]

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 ?

[RhinoDrvr]

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.

[Bob Martin]

Pretty sure the Grumman Tiger has rear induction.
Lopresti has some pics online too.
http://www.loprestiaviation.com/tiger-cowl.html

Thinking this is a single set up not a dual but food for thought.

[BillL]

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.

[F1Boss]

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"?

[cajunwings]

Originally Posted by Bob Martin

Pretty sure the Grumman Tiger has rear induction.
Lopresti has some pics online too.
http://www.loprestiaviation.com/tiger-cowl.html

Thinking this is a single set up not a dual but food for thought.

The Tigers also had a side draft carb making them a natural for a side scoop.


Don Broussard

[Larry DeCamp]

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.

[Toobuilder]

Larry, this might give you some ideas:

https://www.homebuiltairplanes.com/f...engines.31231/

[F1Boss]

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!

[BillL]

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)