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  #21  
Old 09-06-2016, 10:29 PM
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Blain Blain is offline
 
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Default Oh, sure

You start this thread AFTER I'm done with the baffles Thanks Dan.
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  #22  
Old 09-07-2016, 06:51 AM
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Toobuilder Toobuilder is offline
 
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Quote:
Originally Posted by Steve Melton View Post
silicone is my next upgrade. what is the difference between the blue and red silicone other than the color? thx
I've used both and I could not tell any difference. Just a note to be aware- the stuff I have here is the non reinforced "pure silicone" material. The other stuff has fabric imbedded. I've not tried the "tuck and stretch" method used here with the reinforced material, but I suspect it will be a bit less compliant.
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  #23  
Old 09-07-2016, 08:08 AM
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Keep 'em coming folks.

I'll bet what everyone really wants to see are...

(1) Used baffle seals. You know, the ones with some significant number of flight hours. Wear marks tell the tale about sealing success, or the lack.

(2) Details around the inlets, and in the area behind the spinner, the areas where most of the leakage is seen. It's critical to seal well in the inlet area, as that is where the highest pressures are found. Lose it there, and everything downstream suffers.

Example; this from NASA CR3405; pressure coefficients measured in four areas of an upper plenum volume:

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  #24  
Old 09-07-2016, 08:57 PM
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After posting pictures of my seals with a couple of "puckers" Dan admonished me and pointed out those are also know as air leaks. Back to the drawing board...now they fit nice. The reinforcement strips limits the use of silicone between the rivets.

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  #25  
Old 09-11-2016, 05:17 PM
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Bump. Added pics to post on page one
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  #26  
Old 09-12-2016, 02:47 PM
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Bill Pendergrass and I spent about an hour walking the lines at Triple Tree last Saturday, camera in hand, peering into the cowls of a great many RV's (and a few others).

The average baffle seals are abysmal.

Can't see very far back into the upper volume, so I didn't bother looking at seals above the cylinders and along the back wall. Here the focus was on sealing around the inlets.

This poor soul conveniently removed his cowl and went off somewhere. Sealing against the upper cowl appears to be non-existent at "A". The flap at "B" is riveted to the center baffle aluminum, so pressure blows the flap open, rather than against a hard surface for sealing. The outer end at "C" is just sort of stuffed into the area between the end of the upper fiberglass ramp and the inner cowl.



Predictably, given the state of sealing, the installation required louvers in the lower cowl:



Riveting the inboard seal to the aluminum wall (and hoping it will somehow seal against the backside of the fiberglass) must be on a published drawing somewhere, because it sure is common. If the wall and the fiberglass are offset from each other just so, and the aluminum extends up to or behind the fiberglass, it is possible to use the baffle rubber as a sort of gasket. It's not a good seal, just ok. Here you can just see wear marks at "A", indicating this seal runs in close contact with the glass. However, the upper seal, the one that runs around the upper edge of the wall, is a mess where it meets the ramp at "B":



The builder did not get the inboard flap just so on the other side, and the result is predictable. Pressure simply blows the flap in, away from the glass. It's a very common leak indeed.



Not all seals were terrible. Here's a nicely done outboard inlet. Note the curved piece at "A", which gives the seal on the lower cowl lip something to seal against. "B" is nice and tight against the ramp, and pressure blows it even tighter. The end tucks under the upper cowl lip at "C", and matches its curve. Although the tuck-under doesn't allow pressure sealing at this point, the fold doesn't allow it to blow open either, and the inlet flow can't blow it away from the ramp. Note the overlap at "D".

BTW, let's hope the builder injected some silicone sealant behind the curved aluminum piece at "A", or the air will just leak behind it.



This one is Bill's. It's an inboard doppelganger of the outboard seal shown above. Same nice tight fit, same lower seal overlap (gray), same tuck.



These are not the only bad and good ways to seal, but merely a few examples from this weekend's fly-in visit.
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Last edited by DanH : 09-13-2016 at 06:45 AM.
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  #27  
Old 09-12-2016, 03:48 PM
David Paule David Paule is offline
 
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Am I missing something? The reference that you quote in post #23, shows that the pressure is greater at the front center of the cowl than it is aft of the front baffle, and yet the photos showing the upper baffle strip such as the one in post #10, show that the top center strip is pressurized from the engine side of the baffle. I'm not quite seeing how there's greater pressure above the engine than there is from the nose area, but that's what the photo suggests.

If this is correct (i.e., it actually works that way) then I'd think that the spinner, cowl lips and flywheel must act as a sort of labyrinth seal all by themselves, especially compared to the inlet openings. The things that Crabandy is doing in his recent posts on his excellent build log (thanks, Andy!) will enhance that considerably.

Do we have any pressure data for the area behind the flywheel but in front of that front center baffle on RVs? Might be enlightening, especially if it differs significantly from CR-3405. At this moment I have a mental image of an RV engine installation with sensors all around it like a porcupine skin.

Thanks also for this great thread. This is a subject that I have a mere layman's knowledge of and it's pretty interesting.

Dave
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  #28  
Old 09-12-2016, 04:47 PM
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Quote:
Originally Posted by David Paule View Post
Am I missing something? The reference that you quote in post #23, shows that the pressure is greater at the front center of the cowl than it is aft of the front baffle....
I suspect you're misinterpreting Fig 45.

Text translation: In the climb condition (a) it shows a Cp of about 1.1 on the outboard side of the left inlet, dropping to about 0.4 on the inboard side of the left inlet. In the right inlet, the outboard side is at about 1.2 while the inboard side is at about 0.8.

In the cruise condition (b), Cp was fairly symmetrical for the two inlets, about 1.0 along the outboard walls, dropping to 0.9 along the inboard walls.

Neither illustration offers any Cp information for the area forward for the front baffle wall, around the case nose, flywheel, and propshaft.

Quote:
Do we have any pressure data for the area behind the flywheel but in front of that front center baffle on RVs?
With a propshaft seal in place, that area is simply part of the lower cowl volume, as is the accessory case area around the mags. Several of us have measured lower cowl pressure. Not surprisingly, it is less than upper cowl pressure, and greater than freestream static.

Andy's propshaft seal is not a new idea. Tom Martin flew a foam seal many hours on his Rocket, and I've got 500 or so hours on this rubber strip seal.



Lower cowl pressure pushes it against the ring gear carrier. You can see the burnished paint strip in this photo:

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Last edited by DanH : 09-12-2016 at 04:58 PM.
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  #29  
Old 09-12-2016, 08:01 PM
stancaruthers stancaruthers is offline
 
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Quote:
Originally Posted by Toobuilder View Post
I have not yet addressed the area around the ring carier, but as far as I'm concerned, this is how you do the perimeter:





One piece of rubber and even clamping at the fasteners assures minimal leakage. I have a few hundred hours on it now and the edges of the seal show perfect contact with the upper cowl.
I like this one
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  #30  
Old 09-12-2016, 09:14 PM
crabandy crabandy is offline
 
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Quote:
Originally Posted by David Paule View Post
Am I missing something? The reference that you quote in post #23, shows that the pressure is greater at the front center of the cowl than it is aft of the front baffle, and yet the photos showing the upper baffle strip such as the one in post #10, show that the top center strip is pressurized from the engine side of the baffle. I'm not quite seeing how there's greater pressure above the engine than there is from the nose area, but that's what the photo suggests.

If this is correct (i.e., it actually works that way) then I'd think that the spinner, cowl lips and flywheel must act as a sort of labyrinth seal all by themselves, especially compared to the inlet openings. The things that Crabandy is doing in his recent posts on his excellent build log (thanks, Andy!) will enhance that considerably.

Do we have any pressure data for the area behind the flywheel but in front of that front center baffle on RVs? Might be enlightening, especially if it differs significantly from CR-3405. At this moment I have a mental image of an RV engine installation with sensors all around it like a porcupine skin.

Thanks also for this great thread. This is a subject that I have a mere layman's knowledge of and it's pretty interesting.

Dave
I'm just a student of some of the idols on VAF, reading/learning/building/trying to copy what has been done before. Maybe someday I'll have an original idea, but it will always be a product of ideas/solutions that others have shared. I Thank them for sharing and challenging us all to do better.
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