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FAB seal to cowl inlet.

Zero4Zulu

Well Known Member
I see people attach the baffle material on the Filter air box. Is there any reason not to attach the baffle material on the cowl inlet instead? Seams it would be easier to install the lower cowl when the seal would simply come up to the FAB when in position. I have plenty of length to my cowl inlet to attach the baffle material. Is there a sealing issue with this approach?
 
Generally you want the seal facing agsinst the flow. Attached to the cowl, the baffling would not seal as effectively since only the flex of the material would oppose an air leak.
 
Is there a sealing issue with this approach?

There is a sealing issue with either approach, and imperfect sealing decreases deltaP available for cooling. Given the airbox moves laterally at start and shutdown, there is also an issue with the flap seal being too stiff, so airboxes crack, or get loose on the throttle body.

The standard approach has flown a lot of hours, but we can do better.
 
Ditto Dan's comment

As Dan stated,
Be careful not to make the whole affair too tight. I made mine (on a -4) too tight, and the torque movement of the engine eventually caused the carb to loosen, resulting in backfiring, erroneous power and almost an engine out situation. I reworked everything to get the seal/joint a good balance between adequate seal, and allow movement. I also mention to others to periodically try and move the FAB with your fingers through the inlet which confirms the carb security.
 
I have done a couple of installations exactly as the OP described, attaching a U shaped seal on the cowl inlet, with a hose clamp, and a flat piece on the top of the FAB inlet.
Unless you need to have that air tight perfect fit and that extra miniscule amount of pressure in the inlet, I have found it to work just fine.
Actually, if you think about it, it allows for the movement you get from starting and shutting down without causing stress on the FAB, since the seal mount is split between the cowl and the FAB.
I flew my RV6 for 900 hours, with this type of installation, and never had any issues related to that.
YMMV.
 
Split

What Dan said. However, I believe that the convenience of splitting the baffle material outweighs the loss of pressure. Here is mine:
lvQSTfTlKN_QcTFxN9q3aIgP87pdF7ajuG83eaLpmBWNLDD7gojq2Zj7KU0gCugbRB1S-Cjno6SdaFQq5TVsL0Okq-h-AV_Mi-RHTWlA9nFiI0unb3YjXttb9Y8T8KYwdxoiByIfCdYdTUGDaAN_JZhYuCXRAtIVi8hciSWMDI9YLb6raXMTPOa7vB7XzJ7bjOvImYr8pQzbue_erBdQeWD0j-1i01dR1FuRscbnqg7iWt--v-BLLQZhJId7gn4JXldFMluG-IoEd2HWg_WtudqK_L9z4-mH3o-_bZvSWlHzdE9yOzymTNBd7BwDjpFi-cfkWho4SvtNl78iUqSTx-Zm-Z0CguGURB_1BF9Q6Vvf2MC7mdxnthFgKNTvzzmS8rPkLa8NlIkYN3IKMF7TxZqQA7X4tWI3XYTd7SlM-kFolsOZa0_-hJRC9tdLiMCnoP6_M-0DbtPQcfq_cVUc-lmMymUN02h0qORpEQ8rbEqwgthFCkVVmPn5D3MyrSYtDWRA4i3c3cI6pYtWYuuAs7AVFRFP1XmwfDexR1ZcIvq9C4y-A5jfUSFzuAjqu4k70qE-_QknaEQeuxiSNuG-LhLIR18wL5qxTETSuO5O9L6L7qE1lM3ktUgFUZ6A_GLMN5pkG0MulwfUf3AknilfEjKE_SOxF6uP_zyE6WzkUCILY94D13PnBos=w800
 
Thanks for the replies. I have about a 3/8" gap between my FAB and the cowl inlet. My engine is a conical mount so I believe it will have less movement at start up and shut down than a dynafocal type. My baffle material is the light weight stuff so it should flex easily also.

I like Johns idea in the photo with the sides fastened to the air box and the bottom baffle material attached to the cowl inlet. Thanks for the photo John! Unless there was significant amount of hot air being drawn in to the carburetor in flight I doubt any decline in performance would be noticed.

My main concern is just making the bottom cowl easier to remove. I don't want to hesitate removing the cowl at times just because it's hassle.
 
Unless there was significant amount of hot air being drawn in to the carburetor in flight I doubt any decline in performance would be noticed.

Issue isn't drawing hot air into the carb. It is pressurizing the lower cowl volume.
 
I did the split.

My lower seal ( red silicone) wrapped around sides and bottom one peice of the cowl inlet plenum . So as you slid lower cowl up it cradled the FAB.

The top piece of seal (red silicon) was attached to FAB aluminium. A second aluminium strip on top of seal with nut plates sandwitched seal down. The seal was cantilevered out and fairly stiff. As cowl was in position the two seals closed up. The gap between the cowl inlet near plenum (fiberglass) and FAB was only an inch or so.

Cowl installation was a breeze, and dont think I had much loss. Going from alt air to ram-air in cruise at 8500 feet I got a full 1" MP rise.

Dan is right any air leak in lower cowl reduces cooling... but if leak is small and engine cools you are good. Cooling drag increase too small to measure is my guess.
 
Attaching seal to nozzle

If you don?t attach the lower portion of the seal directly to the FAB, how are folks attaching it to the cowl nozzle?

I did the split.

My lower seal ( red silicone) wrapped around sides and bottom one peice of the cowl inlet plenum . So as you slid lower cowl up it cradled the FAB.

The top piece of seal (red silicon) was attached to FAB aluminium. A second aluminium strip on top of seal with nut plates sandwitched seal down. The seal was cantilevered out and fairly stiff. As cowl was in position the two seals closed up. The gap between the cowl inlet near plenum (fiberglass) and FAB was only an inch or so.

Cowl installation was a breeze, and dont think I had much loss. Going from alt air to ram-air in cruise at 8500 feet I got a full 1" MP rise.

Dan is right any air leak in lower cowl reduces cooling... but if leak is small and engine cools you are good. Cooling drag increase too small to measure is my guess.
 
Scat tube

I have seen another builder connect the bottom cowl nozzle (air intake to the fab/carb) using a length of scat tube. He had modified the lower cowl to make the nozzle removable; it was secured to the rest of the cowl with 3 or 4 countersunk screws. When removed, the nozzle pulls out of the cowl with a length of scat tube attached. I assume the scat tube must slide over some sort of spigot on the fab box; I didn't see that part.
This seemed like a good solution because it isolates the cowl from the engine kick when it starts, via the flexible scat tube, whilst also making for a good path for fresh air to enter the fab box. But there's more fibre glass work and time, though, that's the down side.
 
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