IO540 Air Intake Area

[Nev]

I’m using a Vans RV 6 air intake for my IO540. Obviously this intake is designed for the O360. My concern is about whether the intake will be large enough for the volume of air required.

The inlet on the Filtered Air Box that connects directly to the air intake IS designed for the IO540, and looks to be the same size. So if I enlarge the front air intake, I’m not sure that it’ll necessarily result in more air getting to the engine.

I estimate that I’ve got a frontal area of approximately 4000 sq/mm (6.2 sq inches) at the intake.

Interested in thoughts before I work myself into a frenzy with the Dremel

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[Mark Goldberg]

Nev, Bob Barrows could help you with an answer to this question. Mark

[Nev]

Thanks Mark, received a reply from Bob. I’d like to understand this better though (and not to take anything away from Bobs answer).

There’s a couple of things I don’t understand, and I know there’s plenty of you with knowledge far greater than mine.

My Vans filtered air box (where the filter sits in) has an opening of approximately 4.9 sq/inches.
My RV 6 air scoop has roughly the same size inlet, 4.9 sq/inches.

The throttle servo has an inlet diameter of 2.3 inches (radius 1.15 inches), giving an opening area of 4.15 sq/inches.

So the air scoop inlet area is already the same size as the air filter inlet, and larger than the throttle servo inlet.

By increasing the size of my air inlet on the front air scoop……do I gain anything? I’m not seeing it, but then I’ve got no experience in fluid dynamics. The issue I’ve got is that to increase my inlet area is a big job, so I’d rather only do it if I know for certain that I’ve got a problem that needs to be fixed to start with.

Any advice (as always) appreciated.

[Sir Newton]

I would look to the manufacturer for engine installation requirements for their engine. Aside from that on intake air I would spit ball it & stay 33% larger up stream while keeping the design as clean as possible.

[AZBearhawk272]

Nev

Looking at the photos, you are probably OK.
what you have there is a series of orifices that all add up like a series of electrical resistors.

Pressure drop at inlet, filter, throat, butterfly Venturi. All reduce the overall efficiency.
Keep all the air transitions smooth and sealed from leaks.
The added efficiency of efficient ram air at high power and cruise should offset the smaller inlet area.

If there is an air restriction you should be able to assess it via full power static run up and flight test down the road in the future.

How is the carb heat implemented?


Kevin D
# 272 KCHD AZ

[Nev]

Thanks Kev. Actually it’s fuel injected, so I’ve got an alternate air that bypasses the filter.

[svyolo]

Being that the Van's FAB and intake have already been used on numerous flying 540 Bearhawks successfully, I think I would say experience probably wins this one, and it works.

[Nev]

Thanks John. My concern was that it’s an intake designed for an O360, and mine is a 540. But I think you’re right.

[rv8bldr]

Nev, Michel Roy here in the Ottawa area has a Vans FAB on his O-540 powered 4pl. He has been flying for several years now and I have never heard him say he has any issues. That's why I am doing exactly what you and he are doing.

He has a YouTube channel with a bunch of short Bearhawk flying videos: (37) aerotangoRV7 - YouTube

Mark

[Aero_tango]

Nev, Michel Roy here in the Ottawa area has a Vans FAB on his O-540 powered 4pl. He has been flying for several years now and I have never heard him say he has any issues. That's why I am doing exactly what you and he are doing.

That’s right…my BH is an O-540 and I’m using the Vans 0-360/0-540 FAB (same box for both engines) and the RV6 air scoop…200 hrs+, no perceived problem with my set-up.
Mike

[Nev]

Ok, that’s the data point I needed to hear. Thanks!

[Battson]

I did a ram air calculation on Simon's post about the same, on the Bearhawk Facebook page. That calculation takes a little while to repeat, but long story short - it's not a simple yes / no answer.

It's a restriction which reduces or increases pressure in the manifold, and it depends on the airspeed entering the duct.

The engine will run if you block the intake with a foam plug, yes I have done that several times Point is, it will run FINE with a tiny intake, but it COSTS you power.

What you can calculate, is that at around 3.75" intake diameter you start to get ram air during normal cruising airspeeds.

[svyolo]

This thread sort of dovetails with the cooling thread. Intake area is one variable. But isn't distance from the spinner another variable? The farther the distance from the spinner, the more prop wash velocity/ram air effect?

For a Bearhawk, especially a 540 Bearhawk, is it even a factor? It will take off much shorter than it will land. They fly around at much reduced power settings compared to a 360 powered BH, so is efficiency even affected any, or at all.

I like the Vans FAB but my first attempt will be to use it as an airbox but supply the air from one of the cooling intakes. My throttle body is a couple of inches shorter than a carb. If that doesn't work I will order a Vans intake and modify it, or make my own. Quite a few builders have done the same. For me it is good enough, and I think proven enough.

[zkelley2]

The ram air effect of having a horizontal induction and a larger inlet hole starts to come into play mostly in cruise/cruise climb, not takeoff. You'll have a higher possible MAP than someone running a carb for the same altitude. So where I hit 21" with full throttle at say 8500' with my carb, Batson probably gets to 9 or 9.5k for 21". This would enable him to cruise at higher TAS than me, and burn less fuel doing it. He'd also climb faster since he's making more power all the way up.

[Pbruce]

Svyolo, I’m envisioning a scat tube feeding the Vans air box from a hole in the somewhat horizontal ramp at the bottom of the cooling inlet. Is that what you are planning? 3” diameter scat? I’m trying to visualize the options. I like that you don’t have to put another hole in the cowl. I have seen installations like this from a distance and I like the look. Does this prevent ram air benefit (if indeed you are tapping off of then horizontal dam at the base of the cowling inlet)? Also, does it effectively rob any cooling air from the cylinders and oil cooler? Any benefits, besides less holes to cut and cosmetic preference?

[svyolo]

I think I have seen Mooneys with the outboard part of the cooling intake looks like it feeds the airbag. I was going to try something similar, Or the bottom ramp.

From reading I get the idea that the cooling inlets are larger than required and lots of air spills out close to the spinner. There might be a little ram effect, but that isn't really my goal. I think I can keep almost the entire FAB within the cowl. At most I might need a little bump.

I might start out with Skeet tubing as it supposedly flows better. If it works I will make a composite tube.

[Pbruce]

Good idea tapping off the somewhat under-utilized cooling inlets. Makes a lot of sense to me. I see lots of guys locating oil coolers there too.

[svyolo]

We'll see how it goes. My backup is using the FAB as intended.

[Nev]

I like the Vans FAB but my first attempt will be to use it as an airbox but supply the air from one of the cooling intakes. My throttle body is a couple of inches shorter than a carb. If that doesn't work I will order a Vans intake and modify it, or make my own. Quite a few builders have done the same. For me it is good enough, and I think proven enough.

I think this will work very well, although having just finished mine, it would be very difficult to contain the FAB inside the cowl without it protruding. It was how I wanted to do it, but being a first timer I didn’t want to do it more than once. You know what I mean. In the end I used the Vans method. It’s at least as much work, maybe a lot more.

If I was going to run the intake off the baffle (great idea), a consideration would be to mount the fuel servo horizontally. Then use a conical filter. I’d also put a spring loaded (automatic) alternate air door between the filter and the servo. That way, the alternate air bypasses the filter, and doesn’t occupy dash space for something that is seldom (if ever ) used.

One consideration would be to face the fuel servo aft. In that case, the standard Vans cables would probably fit. If facing forward, they’ll need to be custom.

[svyolo]

My throttle body is at least 2" shorter than a carb. Not sure about a fuel servo for MFI. I believe a fuel servo and carb are similar by design to be close to interchangeable. I think between moving the FAB aft, and at an angle, it will fit, or almost with just a bump. Going down and aft gives more room than up and forward.

All that being said, there is wisdom in doing what has already been done many times before. Like you said it takes less time. I am at three years and you are at one so you are wiser than me. I am close to building the cowling, and I will spend exactly 1 day seeing if my idea will work. I have wasted too much time already.

[Pbruce]

Hmm. I think I’ve seen an RV8 with the setup you describe. In my case, I’m going to use a standard updraft MS carburetor on my Barrows Lyc O-360. I know our engine packages will differ, but do you think this arrangement might fit, Nev? Any potential traps that you can see with the benefit of your experience?

By the way, thanks for the vids. Excellent! I really enjoy your construction and your videography, and most especially your commentary! Please keep it coming as you fly your airplane around your amazingly beautiful home islands.

[Nev]

I know our engine packages will differ, but do you think this arrangement might fit, Nev?

Pat I’m not really familiar with how the carb sits inside the cowl and whether it can be mounted horizontally (if I’m understanding correctly) so maybe others will chime in too. However next time I post a video update I’ll show the area around my fuel servo and FAB so you can get a good idea of how it all fits for my installation.

I’ve kept my fuel servo vertical, and used the Vans FAB. That’s a standard “Vans” installation, and the air intake accommodates where the FAB penetrates below the cowl.

I think it could possibly also be done by facing the fuel servo rearwards, but I’ve just had a closer look at it and it may interfere with the cooling airflow outlet and also clearing the exhausts.

Both Battson and Rob Caldwell (and probably others) got very innovative and used a 95° elbow to rotate the fuel servo forward into a horizontal position. This also pushes the servo forward and space becomes very tight. The Vans FAB is only designed for the vertical installation, so I think the guys have used a conical filter in front of the fuel servo. Battson has moved his cowl 2” forward, which looks pretty good and this makes room for the filter. Rob has made a small blister under his cowl to accommodate his filter. This is how I originally planned to do mine. The issue for me is that I can see how tight it’s going to be up front and I wasn’t sure if I could pull it off first time. It’s something I might revisit later.

Perhaps one way might be to do a similar installation to Battson and Rob, but when making the air filter housing, put a curve in it and run scat tube up to the inlet ramp. With luck they might chime in with their thoughts- whether there’s enough room to do this.

[Pbruce]

Ok thanks. It sounds like there isn’t much room. I’ll probably mimic what you did, but I’m going to leave the carb exactly as it normally is: vertical. I don’t think you can do anything else with it, but if so, I’m still not doing it. Your install still looks neat and aesthetically appealing to my eye. Looking forward to the next vid-thanks again.