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How I built my airbox

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  • How I built my airbox

    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.
    20130707_145126.jpg


    20130708_090428.jpg

    20130708_094708.jpg

    This part was then assembled and attached to the throttle body with screws and wide area washers.
    20130727_130712.jpg


    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.
    ​​​​​
    I then fibre glassed the intake section of the nose bowl.
    20130713_103054.jpg

    I used a filler putty to form the curve on the outside. Of course you can use whatever shape or profile you like.

    20130713_150145.jpg


    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.
    Last edited by Battson; Yesterday, 03:10 AM.

  • #2
    Great work, and very creative!
    ​Christopher Owens, EAA #808438
    Project "Expedition"
    Bearhawk 4-Place Scratch Built, Plans #991
    Bearhawk Patrol Scratch Built, Plans #P313
    Germantown, Wisconsin, USA

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    • #3
      Thanks Battson, I find these type of detailed writeups extremely helpful to my own build process. Appreciate the efforts!

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      • #4
        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

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        • #5
          Nice work Battson. Always kinda wondered how you made the smooth lower cowl happen.
          I'm a Tapatalk user so I can't see your "comment"

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          • #6
            Originally posted by Zac Weidner View Post
            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).

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            • #7
              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.

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              • #8
                Here's some more to keep things going for now, I will need to correct the original post.

                IMG_20171210_162118.jpg

                20130831_214521.jpg

                20130622_191239.jpg

                20130622_191230.jpg

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                • #9
                  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'm a Tapatalk user so I can't see your "comment"

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                  • #10
                    Originally posted by whee View Post
                    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.

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                    • #11
                      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.
                      I'm a Tapatalk user so I can't see your "comment"

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                      • #12
                        Originally posted by whee View Post
                        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.....

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