IO540 Air Intake Area

[Bcone1381]

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.

The advantage of leaving the servo vertical seems to be a more standardized installation of the engine control cables.

Have you researched or considered a Cobra Head Intake Elbow? https://www.intakehoses.com/cobra-head-elbows.html

I've got the Vans FAB on the shelf. When I get to the piont of building the cowl, I thought I'd check my clearance below the Bendix FI servo and see how things might look. I also might try my hand using an English wheel to put a slight compound curve in the lower cowl to give more space there. I dream and am curious of others thoughts.

[dramsey]

Here is a reference I used in checking how good RV’s intake design was when I was helping a neighbor with his RV10 intake. The author is an aeronautical engineer for a navy contractor and widely respected in the Lancair community. Using his designs he modified the Lancair 360 cowling to gain 5 knots and reduce CHt’s 40 degrees. He basically takes the intake volumetric flow and divides it by the low end of the aircraft velocity to get the area of an intake for which the velocity ratio is 1 (ratio of intake velocity to aircraft velocity). Then he goes to the next larger size intake. The use of a ratio of 1 requires a great defuser downstream of the intake. I think van’s qualifies. Because people often dont have the physical space to have great diffusers, I see elsewhere that designers use 0.3 to 0.7 for the velocity ratio and target in the middle of that range. If you dont gently diffuse you can lose a lot of energy due to eddy turbulence from a rapid expansion. I did expand the van’s intake slightly to get as much RAM as possible. The objective of a good design is to come into the intake with maximum kinetic energy and slow that flow as much as possible inside the intake to convert it to potential energy or RAM pressure. At the RV speeds thats about 1 inHG available Ram. At wide open cruise in the bearhawk its about 0.6-0.7 in Hg available Ram.

https://www.n91cz.com/InductionSizing/Induction_Inlet_Sizing_rev0.pdf

I should add a suggestion to make Chris’s calcs more accurate. An engine has an approximate 85% volumetric efficiency. So every 2 rotations it actually only pulls in 360x0.85 cubic inches of air. But his rounding up fixes that oversight.

Anyway, since this topic will come up in the future, decided to tack it on to Nev’s thread.

[dramsey]

I did apply this to my Bearhawk 5 build with a 540. Calcs showed 3.2 inch diameter inlet, which I will upsize to 4 inches pending understanding of how much space I have to design a great defuser. That gives me a velocity in range of other literature but still requires a mediocre diffuser to not waste the available ram.

Dennis R

[Battson]

That is about what I ended up with, using quite a different (less sophisticated) approach. Around 3.75" seemed to be the sweet spot, which is the circular duct size I used for my intake.

I am finding I get somewhere between 0.5 to 1 inch of ram air, given or take for measurement error, at wide open throttle.