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  #11  
Old 08-13-2018, 07:54 AM
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olyolson olyolson is offline
 
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Quote:
Originally Posted by dwranda View Post
Great pics Oly thanks! Looks like you did a very nice job with the baffles. Am I to assume that the round inlets of the cowl just feed into the squarish(is that even a word?) inlets with no real seal between the 2?
Correct- the round Aluminum inlets are mounted on the cowl and funnels the air into the front of the engine. The baffle seals are pretty tight and careful sealing of any other small areas with black gasket sealer completed the process. It’s amazing how useful popsicle sticks can be. They make a really nice edge to smooth out the seal in a uniform manner and lightly push the sealant into any little crevices. The old plenum was so small that I wasn’t getting enough air over the #2 cylinder to the oil cooler behind #4. Was constantly fighting oil temp issues. Deciding to ditch the plenum was kind of concerning but once making the plunge it turned out OK. During the process the oil cooler opening at the rear baffle was opened up a bit compared to the old plenum. The oil cooler was also mounted pretty low so it was raised up a couple inches and now gets nice fresh air and all the oil temp issues are solved.

Not 100% sure but evidently squarish is in fact a word, from the Webster dictionary:
Definition of squarish. : “somewhat square in form or appearance.”
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  #12  
Old 08-13-2018, 08:15 AM
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DanH DanH is offline
 
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A plenum lid is merely a sealing device. The primary reason to use a plenum lid is to make sealing near perfect, which guarantees all cooling air mass flow passes between hot parts. Any cooling mass bypassing the hot parts through seal leaks does nothing but provide drag.

The secondary reason is to relieve the cowl of the significant "balloon load" created by conversion of dynamic pressure to increased static pressure in the upper plenum space. Don't underestimate the load. I would not design for less than 230 KTAS/1000 feet/85%Q. That's about 150 lbs per square foot. If your lid is three square feet, that's 450 lbs.

I have seen plenum lids which don't seal very well at their perimeter, a mechanical design issue. Recall how the rear edge of a standard cowl will pooch outwards in flight? That's internal pressure. The edges of a plenum will do the same thing and leak if the attachment is poor.

I see a lot of plenum lids with sketchy sealing at the inlets, which negates the whole concept. May as well stick with flap seals.

I see quite a few with crappy duct connections between the inlet and the plenum space. Bad duct shape reduces dynamic pressure capture. As a general rule, if the cowl inlet area is small as compared to exit area (a high Vi/Vo inlet), the ducting shape becomes more critical. If it is large (a low Vi/Vo inlet), the duct shape is not critical, so much so that really low ratio Vi/Vo requires no duct at all, just a hole. See Mooney Acclaim, Cessna TTx, or a certain well known Rocket displayed at OSH this year, all of which have no plenum lid, BTW. Their flap seals, even if well done, will not match the leak rate of a really good plenum lid and inlet seal combination, but pressure recovery is very good. My next one will be a hybrid; I'd like to try molded seals with a reinforced upper cowl half.

For now, here's a low Vi/Vo inlet, urethane rubber and glass cloth flexible ducting, and a conventional fiberglass lid. Screws directly to the cylinder head ears on #1 and #2, with clamp plates. the rubber duct experiment is now at 800 hours. The system has near zero leakage. Easy to tell; look for black dirt tracks on the inside of the white cowl.





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Last edited by DanH : 08-13-2018 at 08:17 AM.
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  #13  
Old 08-13-2018, 10:10 AM
dwranda dwranda is online now
 
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That is just amazing craftsmanship Dan. I wish I had taken the time to come look at your plane in person at Oshkosh. Everything I do on this plane is new to me so I have to learn it. I am getting close to the fiberglass portion of the build with the canopy then cowl and plenum. I have never done any fiberglass work, but keep hearing it's not that hard. I would much rather buy a plenum from someone like Bill Lane but he may not be making them anymore. I also have no builder network near me so every task is on my own. I just get over the apprehension, look at a lot of builders sites and tackle each job as it comes along. I guess the plenum is no different. I figure I'll work on it this winter in my basement unless I can find a decent one to buy.
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  #14  
Old 08-13-2018, 12:37 PM
crabandy crabandy is offline
 
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My copy of DanH's plenum. I started with the standard baffle seal material and adapted the same previous metal baffling for the plenum. The plemum has a "Ultra Black RTV" gasket around the perimeter and underneath behind the spinner. Only found 2 minor leaks at the side metal baffling slip joint which was easily sealed with a little more RTV.





I currently would like to change the sealing of the boots to the plenum lid, right now I have to cut the RTV and re-seal when removing the plenum.





I really like the way my cowling slips into place with the inlet boots, no squeezing baffle material into place.





If I was doing it again I might try making inlet boots to fit the standard Van's cowling and use hinge along the backside of the plenum. The side is easy to get the screw gun on, but hinge would also work well. Next time I have the plenum off I plan on looking into a hinge type attachment for the front inlet boots to the plenum lid.
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  #15  
Old 08-14-2018, 09:40 AM
F1R F1R is offline
 
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Thanks to all for the cool(ing) photos, theory and examples. Keep the photos coming if you have any.

One of the reasons mine works is because of all the screws that do keep the top well sealed. I might try and find Torx drive or hex drive screws( to make it faster to service) and just keep on with what I have.
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  #16  
Old 08-14-2018, 09:52 AM
crabandy crabandy is offline
 
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Quote:
Originally Posted by F1R View Post
Thanks to all for the cool(ing) photos, theory and examples. Keep the photos coming if you have any.

One of the reasons mine works is because of all the screws that do keep the top well sealed. I might try and find Torx drive or hex drive screws( to make it faster to service) and just keep on with what I have.
I used the NAS 1801 Hex Head Screws for my Plenum.

http://www.aircraftspruce.com/catalo...es/nas1801.php
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