baffling question

I'm being baffleld by the baffles! Not really, but I have a question for those of you who have finished and have the prop and spinner mounted. According to the directions, the clearance between the forward air ramps and the cowling appear to be zero horizontally and 1/16 inch verticaly for the flexible cowl seal to come out over the inet ramps. It seems to me that it would be very dificult to mount the lower cowling with this arangement. I'm thinking of trimming the air ramps back to give more clearance for the flexible seal to ease lower cowling installation and removal. Mine is an a model and the cowling is already a bit tight to put on with the spinner backplate installed and no flexible cowl seal yet to deal with! Just wondering what those of you who are done with this step have experienced? ,

Yep you need to cut them back a bit. Think of raising the cowl into position and as you get up there close to the ramps you will need to reach in there and pull the seal material up on top of the ramps. I ended up with about 3/8" gap. I think this is an area where the install instructions just drop the ball and leave you hanging.

Randall in Sedona

The foward baffles all need to be cut back to clear the cowling fiberglass. I was asking the same questions when I got to this task.

Take a look at my builder's log.

Thanks, Thats what I thought. The way its drawn on the baffling plans would work fine if you remove the prop and spinner backplate every time the bottom cowl comes off, not really practical! Simply need room to manipulate the seal when installing the cowling. If there no other ideas then I guess I'll trim away and continue building!

Although you do need the fore and aft clearance between the glass and the aluminum baffle, it's not necessarily just the baffle that needs cutting.

On the outboard side you've installed a nice radius piece so the seal rubber will lay smoothly....excellent. So what were you planning at the inboard side? The rubber there also needs to be attached to the glass lip and lay against the tin. That's not possible with the glass at it's current length, given that the first bend in the center baffle tin angles it inboard, away from where the seal must lay.

Seems like many builders are hesitant to trim the fiberglass inlet flanges. In truth they don't need to extend any further aft than necessary to form a more or less horizontal flange tangent to the inlet radius. Left full length, It creates the above described sealing problem, and up in the top cowl, it moves the diffuser ramp rearward. It gets closer to the cylinder, an airflow pinch, right where an expansion is desired (left, below). Trimming the glass short moves everything forward (right).



Compare the short flange below with the OP photo. You can see how shortening the glass flange moves the inboard end of the seal (orange is rubber) forward so it can lay on the flat section at "A". Air pressure (red arrow) must push it against something if you expect it to seal.



Side note; another thread mentioned using a video camera to examine local flows within a cowl. I suspect the standard center baffle wall seen here is kinda dumb from an aero standpoint. The inlet flow has velocity, in particular if the cowl exit is large and the baffles have leaks. That sharp bend to go across perpendicular to the crankshaft probably makes the flow separate into turbulence at "B", which compromises pressure recovery...and we want all the pressure we can get. It would be an interesting spot to examine with tufts and a camera, if anyone is interested in that sort of thing.

There is no reason why the center baffle wall must have hard corners. It could instead describe a nice gentle parabolic curve (green lines).

Dan,

I'm curious, if one were to modify the center baffle wall to incorporate a parabolic curve as you suggest, how and where would it terminate? Would each modified vertical wall need to somehow mate with its counterpart from the other side?

J. Baker
RV8 Finishing

Thanks Dan for taking the time for analysis of the forward baffle area. Some well thought out comments and ideas for improvement! Ned

rockbottom said:

I'm curious, if one were to modify the center baffle wall to incorporate a parabolic curve as you suggest, how and where would it terminate? Would each modified vertical wall need to somehow mate with its counterpart from the other side?

Click to expand...

My own is one piece. Can't see a lot of it here, but he shape is obvious enough.

Very helpful drawings and explanation, Dan. I'm approaching this step with some trepidation, but less so with your info!