Nice Filtered Inlet

One of the best RV ideas I saw at OSH was this intake for an airbox with a large area K&N filter. The concept could probably be used with a modified Vans airbox for the vertical intake engines also.

The problem is how to put the lower cowl on the airplane while dealing with the connecting seals or ducts. Here the builder has fabricated an aluminum sleeve. It is inserted after the cowl is in place, slipping inside the orange duct leading to the airbox. It is then fixed into place in the cowl with two screws.

Am am deeply ashamed to say I can't remember whose airplane this was, despite talking with the builder for a few minutes. Claim it please, and kick me next time you see me

Great idea, and beautifully done.

I did something similar using a K&N conical filter. I cut off that protrusion on the left end:



I glassed up a ram airr intake diffuser with a 2-3/4" opening, expanding at a 7 degree angle on either side, ending up at 4" dia. I think it's about 11" long. My calculations told me these dimensions would slow the air down from my usual cruise speed of 200 mph to more like what the engine was sucking in. You can sort of see how it all fits together in this photo. The intake diffuser is connected to the airbox with a 4" piece of scat tube. The fiberglass airbox itself transitions from 4" round to a 4" wide x 3" tall rectangle, where the carb heat flapper resides. It's a balanced teeter-totter flapper: Ram air from the front and carb heat from the rear:



The K&N filter slides into the airbox itself. This RTV job is "temporary" (yeah, fooling myself again). That 2" scat duct was the carb heat, but I've changed that installation. The heated air still enters the airbox from the rear, though.

Well, not so similar...

The blue cowl is one piece. Slips on without airbox issues, after which the only remaining step is inserting the tube.

The 4" cone filter with limited pleat depth will be quite a lot more restrictive when compared to a large flat K&N with 7/8" pleats. Part of the appeal here is how well the builder enclosed the large airbox.

Looks like Kevin Taylors?s RV8 with DH inspired FAB.

Ron B

inlet diffuser?

Dan, How does the aluminum tube and orange rubber sleeve transition to the large face of the filter?

This is an area where the diffuser needs to be designed well. Short wide-angle diffusers can often perform better than a long shallow angle diffuser, but the curvature and expansion rate must be managed carefully.

Ron77 said:

Looks like Kevin Taylors?s RV8 with DH inspired FAB.

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That sounds familiar. Hope he checks in here and takes a bow.

scsmith said:

Dan, How does the aluminum tube and orange rubber sleeve transition to the large face of the filter?

This is an area where the diffuser needs to be designed well. Short wide-angle diffusers can often perform better than a long shallow angle diffuser, but the curvature and expansion rate must be managed carefully.

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The owner didn't remove the cowl, so I don't know the airbox shape. Nor did I note the tube diameter.

That said, the inlet velocity is probably low. For example, assume 3"D tube, 2400 RPM, 360 cu in displacement, and 0.95 VE. Velocity through the inlet would only be about 48 knots, thus diffusion at any practical cruise speed must be largely external, making the airbox entry shape not so critical. A 2"D would be about 68 knots.

Like the external shape at 3 and 9 o'clock? 6 o'clock is maybe a little sharp? Think there would there be a separation penalty there? The overall aesthetic is so attractive that I would be loathe to change it, regardless.

just make sure you have an alternate air setup....that is a great ice collector

Yes it?s mine!

Thanks for the nice comments Dan. Actually the silicone tube clamps to the aluminum tube and slides out a few inches to allow easy cowl removal. I made a DH inspired air box as well!

Kevin Taylor
N84KT

I used the same conical K&N filter (Bendix injection on Superior forward facing cold air sump, James long cowl). I cut down the rubber flange on the large end until it fit nicely in the fiberglass cone and RTV'd it in place. The fiberglass cone is then screwed into the face of the Bendix injection.



The front side of the cone attaches to the snout on the James long cowl with a neoprene ring and band clamp, easy to access and remove in about 30 seconds with the top cowl removed. This shot is kinda hard to see, looking down from the top but you can make it out.

KGT said:

Actually the silicone tube clamps to the aluminum tube and slides out a few inches to allow easy cowl removal.

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So the orange silicone tube slides into the airbox?

scsmith said:

Short wide-angle diffusers can often perform better than a long shallow angle diffuser...

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Doesn't that pretty much guarantee turbulent flow and the accompanying energy loss? My goal was to keep the flow laminar in the diffuser but you're saying that was wrong? Just asking because I don't know.

Yes on Silicone tube.

Silicone tube slides into the air box. I don't know about turbulent airflow but I cruise about 190 Knots TAS. This set up works well for me! As for ice build up...I don't fly in icing conditions! Get enough of that at work!

More pictures PLEASE

Kevin,

Please post pictures and share more details of your air intake, filter and airbox. Send me pictures if you don't want to bother with posting them, or send them to DanH and I bet he would post (and he's probably more reliable than I am ).

Regards,

snopercod said:

Doesn't that pretty much guarantee turbulent flow and the accompanying energy loss? My goal was to keep the flow laminar in the diffuser but you're saying that was wrong? Just asking because I don't know.

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It is common to confuse laminar vs turbulent flow with attached vs separated flow.

I absolutely guarantee that the boundary layer in an attached flow that is diffusing (slowing down) is a turbulent boundary layer. If the diffusion is done right, it is still an attached flow. The pressure recovery can still be very good.

DanH said:

That said, the inlet velocity is probably low. For example, assume 3"D tube, 2400 RPM, 360 cu in displacement, and 0.95 VE. Velocity through the inlet would only be about 48 knots, thus diffusion at any practical cruise speed must be largely external, making the airbox entry shape not so critical. A 2"D would be about 68 knots.

Like the external shape at 3 and 9 o'clock? 6 o'clock is maybe a little sharp? Think there would there be a separation penalty there? The overall aesthetic is so attractive that I would be loathe to change it, regardless.

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Dan makes a very good point - with a large enough inlet, most of the diffusion is external. Even a separated diffusion into the air box from the intake tube will have pretty good pressure recovery, because the bulk of the recovery has already occurred.

It does look like the inlet lip at 12 o clock and 6 o clock is rather fine, even though rounded. Likely a little separation bubble there.