How I built my airbox

[Battson]

Hi everyone,

I had a question about how I made my air intake and thought I would do a write-up.

I spent a long time looking at Bearhawks before we started on our build. I was torn between the masculine looks of machines with an airbox, and the clean lines of those without. We decided to do the clean lines thing, and hopefully gain some ram air in the process.

The design was inspired by James Aircraft, who offer a similar product for the Vans RV line. Sadly it wouldn't quite fit the Bearhawk by my estimate.

The first step was to rotate the throttle body by 95 degrees, using an elbow from Airflow Performance.



Then we fabricated an airbox from fibreglass to house a K and N conical filter. We used a simple paper plug to make this part. Note the conical part isn't centred on the throttle body, it sits higher up to clear the cowling.
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This part was then assembled and attached to the throttle body with screws and wide area washers.
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Next I adjusted the mouth to fit the desired inlet size - this could have been done earlier with a more complex plug. I also use a piece of pipe to determine where in the nose bowl to put the opening.
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I then fibre glassed the intake section of the nose bowl.
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I used a filler putty to form the curve on the outside. Of course you can use whatever shape or profile you like.

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I then used a small amount of filler to smooth out the inside to help the air flow smoothly, and installed nut plates to hold the two halves of the assembly together. The final addition was an alternative air supply door, to allow air to bypass the filter should the filter block with ice, dirt, straw / grass, heck - a birds nest - whatever. Of course this was a prototype, and there is room to improve if you were doing to repeat this approach, both cosmetically and functionally.

Note that large pieces behind the filter are a risk to the engine if they become loose and make their way into the intake manifold or cylinders. You need to ensure that everything is attached to something securely. This includes the filler putty.

Here's how the finished product looks when both sides join up.


Many of you will have spotted that our nose / cowl is longer than normal. It is only about 2", but it makes a lot of visual difference. This length has the added advantage of creating extra room in the cowling for this intake / airbox assembly.

[Chris In Milwaukee]

Great work, and very creative!

[mswain]

Thanks Battson, I find these type of detailed writeups extremely helpful to my own build process. Appreciate the efforts!

[Zac Weidner]

Did you consider installing a system similar to the RV-10, where the bottom portion of the two cooling intakes are used for the induction air?
How much modification would be necessary to install the entire cowling for the RV-10?


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[whee]

Nice work Battson. Always kinda wondered how you made the smooth lower cowl happen.

[Battson]

Did you consider installing a system similar to the RV-10, where the bottom portion of the two cooling intakes are used for the induction air?
How much modification would be necessary to install the entire cowling for the RV-10?


Sent from my iPhone using Tapatalk

Yes, we thought LONG and hard about where to put the intake.

Some people, like Robbie Staton, put their in the front baffle in the engine cooling air intake. You can just make out the filter in the right opening (viewer's right).


Others draw air from the back of the baffles through a tube, and face the throttle body backwards.

In both the above cases, I wasn't sure I would have room for a large enough air filter. I also didn't want to rob the engine of cooling air, as -540 Bearhawks seemed to be notoriously hard to keep cool.

I copied what I saw of Mike Araldi's Bearhawk (since sold to another person).

[Brad Ripp]

Battson, Is there another place to view the pictures of your air box design? They don't seem to exist on the forum any longer. If I can figure it out, I'd like to replicate.

[Battson]

Here's some more to keep things going for now, I will need to correct the original post.

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[whee]

Battson, How did you size you air filter? All the guidance I can find is for cars and applying the formula results in a pretty small filter. I’m just trying to be sure I don’t starve my engine for air.

[Battson]

Battson, How did you size you air filter? All the guidance I can find is for cars and applying the formula results in a pretty small filter. I’m just trying to be sure I don’t starve my engine for air.

I asked the same when I saw the filter on a Super Cub, they are twice the size of mine!
Jokes aside, we get pretty good MAP readings - ram air in fact - so I am happy with the way I calculated our one. It was not based on a proven method, just on first principles from what I learnt at university.

I started by calculating the volume of air the engine requires at full power. In short, displacement 8.8 L x 2750 RPM x 1/60 s/min x 1/4 inductions/rpm or something along those lines to get the air flow rate in litres per second.

Then I worked out the diameter of the air intake such that the prop wash should supply enough air. I assumed the speed of the prop wash was at standstill, then I added a fudge factor to make sure there was an abundance of air rammed into the intake. I think I might have found it was necessary to accept some loss from a standstill...

From there, I worked on a constant cross-section calculation until the air reached the filter. Once the air was upon the filter - this is a conical filter with a dome at the top to smooth the intake airflow - I calculated the shape of the airbox such that the air would slow down over the length of the filter.

That dictated an airbox size which worked for my installation, so green light at that stage. Then I looked at the pressure drop across the filter size closest to what I calculated. That turned out OK, so I went ahead on that basis.

It was quite involved. Given I have always had an interest in fluid dynamics, I didn't mind. Your mileage may vary.

[whee]

Thanks Battson! My method for calculating the volume of air the engine needs was the same and that was the part I wondered about. The K&N filter that meets the flow requirement is half the size of my factory filter.

[Battson]

Thanks Battson! My method for calculating the volume of air the engine needs was the same and that was the part I wondered about. The K&N filter that meets the flow requirement is half the size of my factory filter.

That's where I applied my "fudge factor" to ensure I didn't lose any ram air effect. Double the size
I have seen some small filters feeding the -540, the Staton machine pictured above is one such install.
When you look at the flow rate, that engine drinks in a huge volume of air.....

[Russellmn]

Just so ya know, that filter is bigger than what many of us use for 1000hp+ turbo cars. You're definitely not losing power through the filter.

[robcaldwell]

I've reached the stage of my build where I will be fabricating my airbox around the conical K&N E-0995 air filter. In my case, I did purchase the airbox kit from James Aircraft (https://www.jamesaircraft.com/products/air-filter-kits/). I will, however, be modifying the base plate to square off the bottom to clear the cowling.

Battson I could not tell from your pictures, did you allow for an Alt Air opening? I think I will include a hinged door.


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[Battson]

Yes I put a hinged door on the base plate, above the metal flange of the fuel servo.
Remember that the cowling rises up when you're flying. You need at least 1/4" clearance between the bottom cowl and the airbox.

[robcaldwell]

Ok great! Looks like I will have at least 5/8" clearance between the airbox and the bottom cowl.

I am casting a fiberglass base plate that will account for the opening offset and will place my Alt Air door on the port side instead of on top. Pics to follow...

Thanks so much!

[Bcone1381]

I hope I have room to do this with my Patrol.

[svyolo]

I am a month or two from starting on an engine cowl, but am trying to figure out what to do for an air intake. I prefer either the forward facing cone filter or the Van's FAB. The only problem with both is the engine is moving one way, and the cowling is doing something completely different. The cone filter (Battson style or James Aircraft) is much closer to the prop shaft, so the linear displacement is much less than the FAB hanging off the bottom of the Carb/TB when the engine moves. But it still moves.

I was thinking about using one of those systems, except instead of connecting to the cowl with a flexible connection, put a 2.5 or 3 inch air intake in both air intake ramps. Those would connect to the air box with glass/CF tubes. Everything is bolted to the engine, which moves as one big piece. My EFI TBI has a throat of about 2.5 inches. Hopefully doubling that at the intake would eliminate any intake losses, or maybe give a small boost if I am lucky.

I think the forward facing cone box makes this whole thing very small and compact. Has this been tried? I know some aircraft have the air intake in one of the cowl inlets.

[Bcone1381]

The cone filter (Battson style or James Aircraft) is much closer to the prop shaft, so the linear displacement is much less than the FAB hanging off the bottom of the Carb/TB when the engine moves.

I am curious.....Do you have a concern with the intake being close to the prop shaft? What do you think is ideal? Do you want the intake further aft (like a vertical intake with the Vans FAB provides) or do you want the intake just further away from the center of the prop rotation?

This photo shows an intake that might be a touch lower (further from the center of prop rotation). The lip of the intake is quite close to the propeller as blade rotates past it and (if I recall correctly...this is book knowledge, no actual testing that I've done) can give a boost in manifold pressure to the air intake.

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[svyolo]

Most of my concern is structural and leaking ram air into the bottom of the cowl. Having the cowl inlet closer to the shaft will hopefully have less movement than farther away like the Vans FAB. Easier to seal, and less likely to leak air.

Not connecting the air box to the cowl at all eliminates all of that. I am pretty sure that is what I am going to try first. I think I will just try the James Aircraft Mk3 air box and modify it. I would make my own but the James has an alternate air source already fabricated. I will graft on a fiberglass/CF inlet on each side, and close off the end.