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Cowl Inlet Dimensions

DanH

Legacy Member
Mentor
A favor please...need the inlet dimensions for a standard RV7 cowl. Got one handy?

Please measure from flat portion to flat portion, inside the inlet, not out at the leading edge lip. See illustration below. Looks like four measurements to establish area.
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I bought an "RV-7" cowl from a -7 builder some years ago and it was in fact a RV-6 part number delivered with his -7 kit. I'll dig it out of the loft if you dont get a quicker response. But unless there was a change later on, it looks like the -6 and -7 are the same. Might open the aperature of your "ask" to include the -6 peeps.
 
I went for a round flange shape behind the spinner first, and than relatively equal inlet shape second, when fitting the front cowls on my planes. The openings didn't always come out exactly the same shape & dimensions caused by any minor mismatch at the firewall/cowl joint.
I dealt with any obvious inlet shape issues later.

Edit- Dan, re-reading your OP, I suspect you are probably doing another cooling airflow study?
 
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Dan,
Pic and area of my vertical induction cowl. I found my inlets were noticeably smaller than my buddy’s horizontal induction “smooth” cowl. Maybe the horizontal cowl inlets were sized bigger for possible 200 hp?
 

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Dan, re-reading your OP, I suspect you are probably doing another cooling airflow study?

Well, ballpark stuff. Here it looks like 19.6 sq in per inlet, about 70% of the low velocity inlets I use.

Dan, Pic and area of my vertical induction cowl. I found my inlets were noticeably smaller than my buddy’s horizontal induction “smooth” cowl. Maybe the horizontal cowl inlets were sized bigger for possible 200 hp?

Measured at the leading edge, or way back inside the inlet?

I don't know how the good folks at Vans settled on inlet sizes. I suspect it was iterative, with a heavy dose of tradition.
 
Dan,
If memory serves me correctly it was at the aft edge of the fiberglass inlet at 3/16-1/4” gap at the metal baffling. Visually the front of my inlets were slightly larger and tapered backwards in a bit constricting area a couple inches back. The front is way harder to measure accurately, the aft edge I measured has to be pretty close.
This pic should show the side as well of front view of my measurements, smallest at the aft edge of the fiberglass cowling, aft at the cylinder face and middle between the two.
 

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Well, ballpark stuff. Here it looks like 19.6 sq in per inlet, about 70% of the low velocity inlets I use.

Following your lead, I made 6" ID fiberglass rings for my inlets. I don't have my scratch paper numbers from then but I remember calculating that I had increased inlet area by about 25% from the RV-6 green cowl. I then proceeded to decrease exit area by about 30%. I added cowl flaps but don't use them often - mainly just for taxi in when I'm doing a fuel stop to try and make the hot start go better.

Ed Holyoke
RV-6
 
Well, ballpark stuff. Here it looks like 19.6 sq in per inlet, about 70% of the low velocity inlets I use.

Measured at the leading edge, or way back inside the inlet?
Dan why are you measuring Van's cowl inlet at the back or exit. The total area is measured at the tangent of the radius. Yes? It depends on where the air is stagnating. With the prop beating the air to death the dynamics are not exactly axiomatic. The effect of the diffuser and plenum are also a factor (along with exit air). It's a complex system really.

I don't know how the good folks at Vans settled on inlet sizes. I suspect it was iterative, with a heavy dose of tradition.
Dan, Mr. Richard VanGrunsven is a degreed engineer. If you mean by tradition, he had access to all the engine cooling data developed by NACA, as well as what others did, I am sure. Rules of thumb for inlet to exit ratios have been known for a long time. There are always compromises and design decisions. I think Richard is pretty good at sizing things, but also easy to build, inexpensive, light, simple and works well overall. As pointed out his inlet sizes may have changed over the years with different cowl plugs. The early planes were all made around 150-160 HP O-320's and wood props. As the newer models came in designed for higher HP and metal CS props I am sure Van's increased the inlet and exit size. It would be cool to survey all the different models. Plan on Sun N Fun 2022, may be I'll bring a measuring wand and note pad. Ha ha.

1977 Mississippi State Paper by Miley, S. J. funded by NASA showed (as you are aware) is interesting and made many great observations. It popularized the round inlets which are not magic just have some advantages. Van's RV-3 came out 1971 and RV-4 1979 and RV-6 around 1985. On my RV-4 I made aerodynamic plugs I taped into the cowl inlet near the prop hub, reducing inlet area while meaning change in temperatures. Almost impossible to measure improved airspeed, but I am 100% sure it was good for "micro knots" ha ha. I came in first on the spruce copper state dash in the 160HP category (with 150 HP). I also put on a tiny tail wheel (roller blade) and took off the VOR antenna.

My RV7 O360 180HP has two 4.6875" inlet ring with 4.0625" outlet.... about as small as you want to go for 160-180HP.
 
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Dan why are you measuring Van's cowl inlet at the back or exit. The total area is measured at the tangent of the radius. Yes?

Sure. Problem is, how do you get all survey respondents to use the same measurement points, given tangents are not well defined? So, asking them to use the flat at the back of the inlet offers consistency. I can add back the additional area on my own, if necessary. Rarely is, because comparisons require only apples to apples.

Dan, Mr. Richard VanGrunsven is a degreed engineer.

Indeed he is. Are you trying to say there is only one conclusion, understood only by the degreed?

Rules of thumb for inlet to exit ratios have been known for a long time.

Believed for a long time, recently discounted. I've attached a scan below, from Dan Raymer's 2012 work, available here:

https://www.amazon.com/Simplified-A...45711451&sprefix=daniel+raymer,aps,155&sr=8-5

Read the attached a few times. There is a lot to consider in those three little paragraphs.

BTW, Raymer's "Conceptual Design" is a reference standard well worth having:

https://www.amazon.com/Aircraft-Des...45711655&sprefix=daniel+raymer,aps,155&sr=8-3

As I've previously written here, inlet-to-exit ratios are all but useless in reality. Given good detail design, both low and high Vi/Vo inlets can be made to work well, meaning very different exit/inlet ratios.

On my RV-4 I made aerodynamic plugs I taped into the cowl inlet near the prop hub, reducing inlet area while meaning change in temperatures. Almost impossible to measure improved airspeed, but I am 100% sure it was good for "micro knots" ha ha.

Nothing in CR3405 suggests inlet plugs would be of value. The above result was entirely predictable. The primary goal for an inlet is maximum conversion of available dynamic pressure to increased upper plenum static. Exit size governs mass flow and velocity.

Here's an email written to a friend circa 2008, maybe 2009. I didn't arrive the current cowl configuration via the million monkeys approach.

https://www.danhorton.net/VAF/Cowl Inlet Dimensions/Cooling notes3.txt

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Dave and I talk, but going well back, credit for initially pointing me in the right direction goes to Steve Crow. Dr. Crow is a very smart fellow.
 
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