Cowl exit size

[DanH]

Based on this reply of yours and one earlier in this thread I'm wondering if I'm hearing that you might be suggesting that the fabric that goes up and over the crank (just aft of the spinner) should be pointed forward, not aft. (???)

Certainly not. The seal sections that lie against the upper cowl behind the spinner should point inboard.

I think the way crabandy has it (pointed aft) is correct and would get pushed tighter against the glass when pressurized. Do you see it that way?
Thanks.

Exactly right.

Let's look at one of the photos Bob posted. It's a good example of normal, average baffle work....which, as quantified in NASA CR3405, doesn't actually seal very well.

The seal section outlined in green is the area discussed above. Upper plenum pressure will push it flat against the cowl fiberglass, except for the big pucker in the top center (red), which is a leak. Same for the little puckers on the back wall.

The vertical sections at each side of the inlet are a common approach, but fine examples of bad sealing. Look at the inboard section circled in red. When air pressure pushes against this section, it bends away from the fiberglass, opening a gap. What you want is seal rubber riveted to the back side of the glass, with the loose end extending rearward to overlap the aluminum and the upper seal. One piece is on the lower cowl, and one short section is on the upper cowl. Hopefully somebody will post a photo of this arrangement.

 

[BillL]

Eric,

Cheers,
Bob

Hi Bob. Does this twin 2" scat work well for the cooler? If it does, hurray!. If not, the entrance at the baffle is a sharp corner, meaning that the vena contracta will significantly reduce the effective diameter. It wouldn't take much and would greatly help flow to the cooler. Nice cooler valve too.

Just a thought.

 

[rvmills]

Let's look at one of the photos Bob posted. It's a good example of normal, average baffle work....which, as quantified in NASA CR3405, doesn't actually seal very well.

The seal section outlined in green is the area discussed above. Upper plenum pressure will push it flat against the cowl fiberglass, except for the big pucker in the top center (red), which is a leak. Same for the little puckers on the back wall.

The vertical sections at each side of the inlet are a common approach, but fine examples of bad sealing. Look at the inboard section circled in red. When air pressure pushes against this section, it bends away from the fiberglass, opening a gap. What you want is seal rubber riveted to the back side of the glass, with the loose end extending rearward to overlap the aluminum and the upper seal. One piece is on the lower cowl, and one short section is on the upper cowl. Hopefully somebody will post a photo of this arrangement.

SGTMAJ Horton , Good eyes as always! Those are the puckers I was talking about. The rubber was very supple there, and it almost feels as though they might be easily pressed into shape against the cowl. Wishful thinking perhaps, and I concur?very possible leak spots.

Those side inlet baffles are something I haven't seen him do before. I didn't look at the cowl, but his standard method is what you described in your last paragraph?I believe he did this for this build too, and those you circled are extras (though I'm not sure what for?I'll ask). I did it the way you describe too (on his mentoring), since I have standard rectangular inlets feeding my lid-covered plenum. I used screws instead of rivets, so the baffle material can be changed over time, and there is an AL brace behind the rubber. Here are a few older pics?post-induction inlet change, pre-exit tunnel change. Cowl is rough, but the inlet baffles are fairly easy to see. This is what you mean, correct?:

Here's a more recent one. Still not a great pic, as its the pic I posted to show the evidence I saw of a front seal going bad (since replaced). But it shows the upper cowl inlet baffles, to compliment the pics of the lower?and the inner cowl surface is a bit nicer at this point!

Another benefit of these seals is they bridge the small gap between the cowl and the inlet ramps?for those of us that don't have the cool round inlets and neoprene couplers (or the rubber impregnated glass ones! ?don't think I haven't thought hard about those! )

Hi Bob. Does this twin 2" scat work well for the cooler? If it does, hurray!. If not, the entrance at the baffle is a sharp corner, meaning that the vena contracta will significantly reduce the effective diameter. It wouldn't take much and would greatly help flow to the cooler. Nice cooler valve too.

Just a thought.

Hi Bill, good to hear from ya?been a while since RV-1 days eh! This is how my neighbor does most of his oil cooler set-ups for 8s. Performance is reported to be very good, though the one pictured is only 50 or so hours out of Phase I?so far so good though. They report good summer cooling with both scats feeding the cooler, and good winter OTs with one closed off, or partially closed off. I agree with you on curved surfaces being better, and will pass that along (thx!). Another place I think that could benefit from a more curved surface is the induction air inlet in the left inlet ramp (see the pic in Dan's quoted section). A nice radius there would help air flow into that snorkel with less losses. Credit to Steve Smith for teaching me that one!

Cheers,
Bob

 

[DanH]

Cowl is rough, but the inlet baffles are fairly easy to see. This is what you mean, correct?

There will be individual differences in execution, but yep, that's the concept.

…for those of us that don't have the cool round inlets and neoprene couplers (or the rubber impregnated glass ones)

Yes, some of us wimp out and avoid flap seal issues entirely...

 

[crabandy]

My inlet seals are setup like Bob's, I'll try to get some more pics when I pull the cowl again.
Erik,
A straight strip of baffling material around the lower inlet doesn't want to lay on the baffle ramp without puckering. I was able to put the lower cowling in place and slip a 5 or 6 in wide strip of baffle material in place on top of the engine baffle inlet ramp and around the lower inlet. I was able to move and position the baffle material so that it laid correctly, traced a line against the cowling on the baffling material, projected the line about an inch on each side for the overlap under the inlet and over the inlet ramp. It ended up being kind of a C or rainbow shape.
For the baffle material against the top of the cowl up front I laid my upper cowling upside down on a table and started fitting various shapes and pieces of baffle material to the ramps. Then camped it on the metal baffling and test fit the cowl, over and over and over....
The baffle material on the back half was a piece of cake though!

Dan, definitely something to strive for. Thank you for all of your helpful post! Now that I have an actual flying airplane I won't mind doing more tinkering on my next project.

 

[crabandy]

Finally pulled the cowling again and took some pics, my lower inlet baffling is per plans and looks like Bob's. The lower inlet baffling covers the bottom, extending up the sides with an ear that lays against the upper inlet ramp. Incoming air "should" push this material down onto the lower metal inlet ramp, against the metal side of the baffling and against the top of the fiberglass inlet ramps. Wear marks show everything seems to be sealing as well as the baffle material can at the inlets.

I did have 2 puckers in the baffling I know are leaking, 1 above the oil cooler and 1 across the front center of the baffling. Like Bob said in the previous post the material is supple and should seal but it doesn't want to conform to the cowl properly. Before I replace the baffling pieces I thought I would try some RTV to help hold the proper shape. I put a piece of tape on the backside of the baffling to hold the proper shape, I then ran several "ribs" of RTV horizontally to hold the shape. Hopefully the Wx is decent tomorrow and I'll see if it worked.

 

[erikpmort]

Wow lots of posts since I've been back ! Glad to see people contributing and learning.

I have a lot of work to do. It's amazing how much there is to do and learn even though I didn't build it.

I asked the guy I bought this plane from about the high Chts, he said to me- it's a high performance plane, you can't expect to run it wide open very long without getting hot. Glad he wasn't the builder ! What a lame attitude. After everything I've learned from this site, I'm definitely going to make it happen !what I mean by that is run wide open and stay within spec.

 

[crabandy]

Good luck on getting your CHT's down, spend more time thinking and fitting than actually attaching the material and you'll be fine. Don't be afraid to cut out/modify/re-glass your existing upper intake ramps so you can get a decent seal. I would plan on 2 or 3 work sessions of 2-3 hours each on planning and fitting material once the upper intake ramps are fixed. The ultimate solution is shown in DanH's gorgeous pic, but we all have to start somewhere!

Just as a reference, my newly rebuilt engine has 5 hours on it and in a climb if I keep the IAS above 130 knts I never see 400 degrees CHT, 100-125 IAS will net 2 cylinders in the 405-413 range. Full throttle in cruise yields 340-380 degree CHTS, I'm really looking forward to the engine breaking in and temps dropping. I've been lucky breaking it in with cooler temps, but I was really surprised that I get the same CHT's at 18 or 60 degrees OAT.
Good luck!

 

[rvmills]

With well sealing baffles, you should be able to run in all but the more extreme conditions and stay within limits. If it works in formation on 100 plus days or for six laps at Reno or 30-45 minutes in a SARL race, it can work in most normal ops for sure! To add to Andy's pics, here are a couple more of my inlet ramp baffles...I replaced one during my condition inspection, just completed:


The baffle material has a natural curve to it from the roll. I was taught to cut the material so the curve will help it lay down against the inlet ramp:

Here's the removed seal to show the size:

Attached to the cowl:

Backside of the seal and brace:

Lower cowl in place:

I do similar on the upper cowl, to seal all around the ramp and plenum top. Just one technique, hope its helpful to others!

Cheers,
Bob

 

[DanH]

Well done Andy. Those look pretty good.

Bill Woods did really nice baffle seals on the last RV-8 he built (blue nose P-51 paint scheme). Sold now, but is the new owner here?

 

[David-aviator]

No one is having CHT problems in the Midwest right now, it is COLD.

If nothing else works, move.

Don't want to move, get good baffle seal like Dan says and increase exit area. It's a matter of physics and will work.

 

[nucleus]

Plenum Plenum Plenum

I think that a plenum is easier, cools better and is low maintenance.

 

[rzbill]

I don't need no stinking plenum.

As has been said here already, if you do the baffles right, they will work very well.

 

[MauiLvrs]

It's a matter of physics and will work.

And math ...

 

[erikpmort]

thought i would repost some updates-

been working on the baffles a bit- getting more help this week but still need improvements.

installed a louver on each side of the lower cowl and saw a good improvement but not enough for these temps we are having now.

I have read and read and read and i am starting to believe that I need to confirm if i have the lean carb issue- i am going to run this test i pulled from an old post-

Post by GMC jet pilot
EGT is relevant to each other and frankly Lycoming does not give a MAX EGT.

If able try to switch probes, but before you do that (since things look OK to me), you need to check for being to LEAN which you already sound like you suspect.

The acid test:
climb to 8,000-8,500 feet density altitude (approx)
WOT - side open throttle
mixture full rich
note all EGT's and start to lean very slowly and pause
note EGT rise
note the cylinder "first to peak" and note that peak EGT temp

(The test is done, but you can keep leaning and see the 2nd, 3rd and last to peak or lean till rough.)


Take the FTP "first to peak" and calculate the difference or delta between FULL RICH and PEAK EGT. About 150F min difference at 75% power, rich EGT to peak EGT. You might get away with a little less than 150F, but 80-100F is too little. The solution that works time and time again is to remove the main jet and drill it out one or two or three drill #'s larger. Usually worse case is three # sizes larger but you may only need one or two # sizes larger. Some guys can't lean at all with out getting immediate EGT drop.

End quote-

So if this confirms i am in need of rejetting- who would you recommend i have do it? any other details i should consider?

i still have not cut the cowl- well if you don't count the louvers

 

[CharlieMike]

Just getting ready for my condition inspection and was think of opening my cowling exit up while the cowling is removed. Will use this thread while inspecting the firewall forward. I've noticed in many of the replies they are from tailwheel aircraft where the cowling exit is much cleaner vs my -6A with all of the NLG and support brackets. (side note - Not even remotely thinking of change the location of the third wheel.)

I have already inspected baffle seals and corrected leaks, but always room for possible improvements.