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Cowl inlet sizing Formula?

patterson

Well Known Member
Patron
Ive searched the archives, but cant seem to locate a discussion of resizing/reshaping cowl inlets. I Wont be using round inlets, just standard. Im working on making a Rocket cowl fit a 4 cylinder motor. The inlet size was designed for 540 engine. Is there a way to determine how big the inlet(s) should be? Of course I want to get adequate cylinder head cooling but avoid unnecessary cooling drag. Does anyone have information that could help me calculate proper air intake? Im using a plenum.
Thanks
 
John Thorp was a recognized expert in engine installations - since his time serving as Lockheed's "engine liason" with all the WW2 engine manufacturers.

What few people know is that he advised Rajay Turbochargers in the 60s to 80s (approx). ( Aside - He related a story one time that with "boost off" one cylinder CHT would be high, but with boost on, a different cylinder would peak.)

In any case, he conservatively advised .35 sq in per HP, as any "oversize" of the inlet could be minimized, if not eliminated by careful design of the inlet, with particular attention to the inlet's radius. Most all cowl mfgs ignored the inlet radius. This position was later reinforced by a later NASA TR. (3405?)

FWIW
 
You could just leave the intake big, and reduce the exit area.

Dan Horton has written a lot about that.

Also a lot of info in the CAFE Foundation testing, might take a bit of effort to dig up however.
 
I'm working on making a Rocket cowl fit a 4 cylinder motor. The inlet size was designed for 540 engine.

It was designed to look good.

There is no equation which nets the perfect size, and inlet area isn't the whole picture anyway. Your first fundamental choice is between internal diffusion (high Vi/Vo, generally a small inlet area) or external diffusion (low Vi/Vo, generally a large inlet area). I believe low Vi/Vo is a far more practical choice for an RV's wide speed range, but high Vi/Vo can work.

Chris Z wrote a nice Sport Aviation article explaining some of the differences:

http://www.n91cz.net/Interesting_Technical_Reports/106-111_BuildingBasics.pdf

Round is merely convenient for sealing. As noted, internal and external radius is important. Entry radius on a Harmon cowl is probably inadequate.

Recommended reading, CR3405, and AIAA 80-1242.

https://www.danhorton.net/Articles/NASA_CR3405 Cooling.pdf

https://www.danhorton.net/Articles/AIAA_80-1242.pdf
 
hang in there

Vince will soon have a round inlet cowl that already has the round inlets - sized the same size and shape as DanH is using. Might be a bit large, but all you have to do is close off the outlet.

The fabricator is near here, so I go over and help him get things just so every now and then...the finished cowl will be very good indeed.
 
While we are talking about cowls

Hey Mark,
the IO-360 angle valve 200hp motor I'm using does not fit well in the current F1 cowling. I'm using a 2 and 1/4 inch prop extension. The front cylinders protrude by almost 2 inches. Better as it goes aft, but still requires RV4 like Cheeks. Please consider having your fabricator consider this as he builds the next one. Im guessing if the F4 is successful people will want the biggest 4 cylinder available.

Also, depending on how the CG works out, you might think about placing the engine / mount slightly more aft (to help with cowl fitment)

https://photos.app.goo.gl/LodQeEXjVYEjtTkS8
 
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Vince will soon have a round inlet cowl that already has the round inlets - sized the same size and shape as DanH is using. Might be a bit large, but all you have to do is close off the outlet.

Seriously, there are better and worse internal and external shapes, but no such thing as "too large". It would be closer to say "decreasing benefit". Each low Vi/Vo inlet makes its own phantom nose. Just imagine some softly rounded bazoombas out in front of those inlets, each made of increasing static pressure.

It was B-29, wasn't it? :D

Closer to Mooney Acclaim. But not to waste a good lesson...all the best round cowls are really just a big low Vi/Vo inlet.

Hey Mark, the IO-360 angle valve 200hp motor I'm using does not fit well in the current F1 cowling. I'm using a 2 and 1/4 inch prop extension. The front cylinders protrude by almost 2 inches.

No way I'd do that much glasswork and keep the shape, size, and location of the original Rocket inlet. If you really want a slot type for styling, fine, but move it outboard, and greatly increase the entry radius top and bottom. Still won't work as well in climb, but it will be better.
 
Well dang it!

Maybe coulda done this mod if we knew about it at the time. Aarrghhh. I know the std cowl will not fit around the angle valve engines - best thing I can think of is to make molds of a properly modified cowl, and have the parts available to builders. If you want to, you could be in the ‘modified cowl’ business!

Ya know, the inlets are moved up a bit and also outward - I would not be surprised if it would fit up to the angle valve engine as produced. You might need a different length prop extension?
 
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