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Plenum help

motodave

Well Known Member
VAF,
I'm installing a Sam James plenum on my 7 with a I(O)360, James cowl and using the standard Vans baffles (I know that I need to modify these to adapt) to mount the plenum with screws/nut plates to secure it.

Question I'm not clear on is how high above the engine should it be, i.e. what is the clearance between engine and plenum? Should I just make this as high as possible? And, yes I will be mounting the oil cooler to the baffles at #4.

Appreciate any "experiences"!
 
You are right to put some thought into it.
You have to install the Vans baffling first, then fit the plenum.
Keep it as low as possible.....
The oil cooler will set one side.
The Cylinder the other...
But the big issue is the inlet ring alignment up front.
I have done 2 James plenums and had to hack up the James plenum to make it fit.
Also think about how the plenum will attach to the baffling. I installed a vertical flange the first time but it hard to seal. Second one, I installed a angle to allow the seal to be flat or horizontal, much preferred.
I looked at your pictures and you are doing great work.
 
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Thanks for the note Bob.

I wasn't sure if more volume above the engine is desirable or not and as you know the instructions are lacking... I'd like to avoid cooking my engine on the early flights. The intake holes look manageable (will need to cut a slit and bend upwards) but sealing the front of the plenum to the engine looks like a science project. I plan to get the standard baffles fitted and then see how the plenum fits that... then cut the hole for the oil cooler which per your post probably sets everything else.

How are your temps with the James Cowl/Plenum? Any pics of yours you could share?
 
plenum alignment

Hi Dave. I can tell you what worked for me. I let the alignment of the inlet rings dictate the height of the plenum. Using Van's baffle kit cut down after the height was established it all seems to work. It's on an RV-9 with IO-360 and after about 24hrs flying cht and oil temps have all been great....knock on wood.
(a lot)
danny
 
Clearance to cowl

I am far from an expert here but I do know you need some clearance between cowl and plenum to allow for the relative movement between the two ....how much ? I have read between 3/8" to 1/2" ...but I am interested also in some experience based data. Obviously more towards the outer extremities of the engine as farther from the mounts themselves
 
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Plenum

Even though neither is specific to the James plenum, posts on here by Dan Horton and articles in Kitplanes by Dave Anders have been helpful to me in creating my own plenum for non RV.
 
I've been doing research on this lately because I'm also getting ready to start the plenum install, and the Sam James instructions are not helpful at all. It seems that the recommended height is 3/8 to 1/2" above the fins. Most people seem to be aligning the front inlets and then raising the back half in order to not block off the oil cooler. This gives a smaller gap upfront, with a larger gap in the back. Suggested clearance from the top cowl is at least 1/2-3/4" as well to account for shaking on shutdown. If anyone has a direct link to pictures or articles on this, please post them up. Google only seems to find 1 or 2 write-ups about it, so if there's more info, I would love to read it.

Dave, please keep me informed if you start making progress. I'm working on induction this week, and hopefully will get the plenum installed next week or week after.
 
Will post pictures to my Flickr page as I make progress...

After I get the plenum done I will move to the induction, filter and alt air bypass. After all that I will prime everything, seal interior and protection with hear barrier.
 
Sure seems unlikely.

Yup, I likely misquoted those... let me go back and find the post I was reading.


The only "for sure" numbers published are in the sam's james instructions (for an IO-390) which say 1" clearance from the cylinders and 2.5" from the heads. But on a 360, the heads are taller than the cylinders, so its impossible to make those numbers work. They also state that you can't use the Van's supplied baffles and to cut out the templates, but then they don't provide the templates.

I think my plan is to just line up the inlets and make everything else fit with as few sharp corners as possible.
 
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Thanks for the note Bob.

I wasn't sure if more volume above the engine is desirable or not and as you know the instructions are lacking... I'd like to avoid cooking my engine on the early flights. The intake holes look manageable (will need to cut a slit and bend upwards) but sealing the front of the plenum to the engine looks like a science project. I plan to get the standard baffles fitted and then see how the plenum fits that... then cut the hole for the oil cooler which per your post probably sets everything else.

How are your temps with the James Cowl/Plenum? Any pics of yours you could share?

Pretty sure the Vans baffeling has the cut out established, with the reinforcement doubler ring.
The first one I did rubbed in the center....so the shaking of the engine was not an issue in the rubbing department.
The front needs several fiberglass cloth layups to make it fit the baffels and create places to screw it on.
Yes I can send pictures....PM me with your email.
 
VAF,
I'm installing a Sam James plenum on my 7 with a I(O)360, James cowl and using the standard Vans baffles (I know that I need to modify these to adapt) to mount the plenum with screws/nut plates to secure it.

Question I'm not clear on is how high above the engine should it be, i.e. what is the clearance between engine and plenum? Should I just make this as high as possible? And, yes I will be mounting the oil cooler to the baffles at #4.

Appreciate any "experiences"!

There are different generations of the James plenums. The original was based on the RV4 so it has a hump in the center, this is the limit how high it will fit. Also being one piece, the angle of how it fit across the front may define the rake to the rear. This may push your oil cooler quite low. If that happens you may be better off remounting the oil cooler off the aft baffles, like on the mount or firewall.

Some of these constraints can be reduced with modification of the plenum. Pictures as you begin fitting would help with more accurate assistance.
 
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Set the height by lining up the inlets with your cowl which will set the height above the two front cylinders. Then the rear is set by keeping the height about the same over the rear cylinders. The oil cooler will either need to be angled down when attached to the rear baffle or remote mounted. Plenty of pics online to get a visual idea.
 
Dave,
Bill built mine for me and I modified it to fit my engine and the James inlets. Here are a couple pics. PM me if you want more info. I am flying the next 2-3 days depending on weather so may not get back to you or I may have lots of time waiting weather to get across the Rocky’s!
 

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Take a good look at the above photos. There are significant technical differences between this one (from Bill Lane) and most of the James plenums I've seen.

A plenum lid is fundamentally a sealing device, but it also has to satisfy the requirement to convert available dynamic pressure to increased static pressure. Good installations have a measured coefficient of pressure above 0.8, and I'm pretty sure that is true for Rockwood's above. Believe me, not all installs meet that criteria. The optimum case slows the incoming air to near stagnation with little turbulence. High velocity flow past humps and bumps and sharp changes of cross section is unlikely to meet that criteria. Here note the steady expansion and clean transitions.
 
Plenum

Keep us posted Dave.
I have a RV Bits plenum to install soon.
Any tips would be appreciated.
 
I can say that all temps are very close to each other with this plenum and cooler now that the mags are correctly timed. All green. My plenum is very close to the cowl. It doesnt touch, but about as close as you can get.
 
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James is sending me a new plenum, seems I got the wrong one.

I’ve cut my baffles to Van’s spec, 3/8” - 1/2” to cowl, hopefully enough room to shake and be able to mount the plenum. Will update as I make progress.

Thanks for all the replies and pics! 😀
 
May I ask a question?

Hey. I like the idea of plenums. One thing I have been thinking about is the fact that the normal baffles are designed to be independent since they move. If you put a hard plenum on that is screwed onto all of the baffles doesn't that leave you with a problem since each cylinder has to have the ability to move a bit?
 
Doesn't seem to be a problem

I thought the same thing ....BUT three RV's on field with plenums report no issues with this
 
Hey. I like the idea of plenums. One thing I have been thinking about is the fact that the normal baffles are designed to be independent since they move. If you put a hard plenum on that is screwed onto all of the baffles doesn't that leave you with a problem since each cylinder has to have the ability to move a bit?

That's a valid point. The answer is found in choice of materials. Ordinary E-glass/epoxy is relatively low modulus. In lay terms, it is "stretchy". Carbon is high modulus, very stiff. Aluminums are somewhere in the middle. So, a fiberglass plenum lid offers some give, and wouldn't stress the aluminum baffle walls nearly as much as a carbon plenum, for example.

The vibratory movement of individual cylinders should also vary with cylinder pressure and size. I don't have any measured data for it, but I'm pretty sure my 8.9 390 cylinder heads move around more than the heads on a 150 horse 320.

Angle valve engines are very tight in an RV-8 cowl, with limited clearance at the front corners. The combined stress of cowl bumping, the plenum lid, and the rubber duct was cracking conventional (and very short) aluminum eyebrows over cyls 1 and 2, so eventually I just eliminated them and clamped the plenum directly to the heads. This scheme is dependent on the low modulus of glass/epoxy, and so far has worked out well.
.
 

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Frustrating

After waiting 16 months for the delivery of the plenum this is becoming a very frustrating experience.

The cowl is good but the plenum does not fit *at all*
- I had to extend it near cyl-2 for it to seal to the left fwd baffle.
- front does not fit. I am allready cutting away bits to re-glass later. I have seen the same thing on Mike Bullocks build log.

But the biggest problem is that the cowling sits very very low. In order to put the plenum inlet in line with the inlet rings, the plenum must be installed very low. The sparkplug connectors almost touches the plenum.

I understand that you need some volume above the cylinders to generate pressure for airflow. From the picture it looks like the Bill Lanes plenum has much more volume than the one I have from SJ.

@Motodave: Take a look at my picture... Isn't that the same one as you have? In that case I would have the wrong one too. (IO360-M1B)
Can you post a picture of the new one you got?

I remember one post where someone asks where to buy a plenum. DanH replied "Heck, build it yourself!".
The last few weeks, with this SJ plenum, this is exactly what I am thinking.
 

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That's a valid point. The answer is found in choice of materials. Ordinary E-glass/epoxy is relatively low modulus. In lay terms, it is "stretchy".... [snip] .....I just eliminated them and clamped the plenum directly to the heads. This scheme is dependent on the low modulus of glass/epoxy, and so far has worked out well.
There are two kinds of engine baffles / engine plenums, ones that have cracked and ones that WILL crack... :D

As you know large deflections during start and shutdown like a wet dog shaking water off, constant vibration in flight, jugs moving independently, bumping cowl, heat, pressure stressing the plenum and baffles, as well as dead weight like OIL COOLERS some keep hanging directly off the back of the baffle is a tough environment. Most baffles will at some point crack or fret or wear somewhere over time requiring repair.

You don't want leaky baffle/plenum but you want the fastener holes to be larger than the fastener to allow some movement, including torqued lightly. Nut plate locking feature and Loctite will keep them from backing out in theory (but should be checked frequently). A loose baffle fastener is not critical.

Sam James plenum is one piece and stiff. I went with Van's baffles, separate aluminum cover over "dog house" and individual plenum inlet fiberglass adapters mechanically fastened to plenum going to round, to connect flex duct (wet suit material) to cowl rings that are fixed. Everything can float or is not ridged. I copied this idea. Hearing of Sam James plenums always needed some "adjustment' made me go this way.

The Questair Venture https://en.wikipedia.org/wiki/Questair_Venture high performance plane first flew in 1987. I saw the first prototype at North West EAA Fly-in, Arlington WA state. The entire plenum or most of it was made of silicone material!!! I wonder how a "soft shell" semi ridged plenum could be made. It could be rubber or silicon with imbedded hard points bonded or vulcanized into the flexible plenum. I get what you are saying is the E-glass is flexible. A material even more flexible than E-glass but will not balloon would be possible? To make the E-glass work best (I am guessing) is have it thick where needed and thinner where flex is needed.

Per Van plans, traditional aluminum baffle around engine with soft seals up against the cowl might resistant to cracks if made properly, except if you hang a heavy oil cooler directly off the back of the baffle. However the standard Van baffle is not as efficient for reducing cooling drag as a plenum. What is your opinion or data on a Van's set up verses plenum sealed to round inlets ("An experimental investigation of the aerodynamics and cooling of a horizontally-opposed air-cooled aircraft engine installation" NASA, Texas A&M, MIss State U and later popularized by LoPresti in his speed products for certified plane). Has anyone one went from stock to modified and recorded good data? My question is it worth it?
 
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...the standard Van baffle is not as efficient for reducing cooling drag as a plenum.

True, if the standard baffle seals leak and the plenum does not. I've seen that comparison go either way.

What is your opinion or data on a Van's set up verses plenum sealed to round inlets

The subject is deeper than just "round inlets", CR3405 being a good illustration.

Has anyone one went from stock to modified and recorded good data? My question is it worth it?

Link to nice pictures of a stock James plenum and one of Bill's. Also a tutorial based on real data, further down the page.

https://vansairforce.net/community/showthread.php?t=148268&page=3
 
The subject is deeper than just "round inlets", CR3405 being a good illustration.
I am knowledgeable enough to know the magic is NOT just round inlets... it is the whole system from inlet to exit and countless details and thermodynamic, aerodynamic and fluid dynamic principles and tradeoffs. At some point there are diminishing returns, i.e., there is no free lunch. I have experimented enough with inlets, pressure plenums and exits to know it's not simplistic. Also flying many piston SE and twins and seeing the difference in cooling systems, oil cooler installations, cowl flaps, augmenter tubes has given me a respect for how complicated it is.

Thanks for link with info...

My thought is on "Sam James" system (for lack of better term) Vs. Van's Baffle System, is the Sam James system is less cooling drag making for higher top and cruise speed for almost equivalent cooling at higher cost and effort to build it. I know you have written about the EXIT being very important. I agree. My comment is based on stock cowl exit air.
 
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The cowl is good (agree) but the plenum does not fit *at all* (agree)
- I had to extend it near cyl-2 for it to seal to the left fwd baffle.
- front does not fit. I am already cutting away bits to re-glass later. I have seen the same thing on Mike Bullocks build log.

But the biggest problem is that the cowling sits very very low. In order to put the plenum inlet in line with the inlet rings, the plenum must be installed very low. The sparkplug connectors almost touches the plenum.

I understand that you need some volume above the cylinders to generate pressure for airflow. From the picture it looks like the Bill Lanes plenum has much more volume than the one I have from SJ.

@Motodave: Take a look at my picture... Isn't that the same one as you have? In that case I would have the wrong one too. (IO360-M1B)
Can you post a picture of the new one you got?

I remember one post where someone asks where to buy a plenum. DanH replied "Heck, build it yourself!".
The last few weeks, with this SJ plenum, this is exactly what I am thinking.

Yours looks same as mine, way to low in back if the inlets are correct... conversely if you raise the back up the center hits the cowl and inlets do not line up.... I posted pics earlier but ours appear same. I have a I(O)360-M1B Thunderbolt with "new" stock Vans baffles, very standard set up.
 
If it does not fit, make it fit. We are building something. And not necessarily tied to one solution provided by a “manufacturer”. If someone does not like the product purchased for whatever reason, return it. Then build it from scratch. We should not look at everything as plug and play. Think about it more as a starting point or something to deviate from.

EXPERIMENTAL=Recreation and education. Certified=Plug and play (and pay)

click on the cowl section on the signature below for gouge. You can do it. I knew zero when i started.
 
A couple pics showing how I made the inlets match my engine. Hint: the foam does not melt as easy as I thought it would. The brush works getting the remains if the foam!
 

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Thanks for the pics! So you sanded to shape and then epoxy/fiberglassed over the top then hollowed out the insides I assume and glassed the inside to smooth it out? How many layers of glass were needed? What did you do for the body of the plenum? Any particular foam you would recommend? Last question did mold the intakes separate to body? I like the West 105 system epoxy.
 
Thanks for the pics! So you sanded to shape and then epoxy/fiberglassed over the top then hollowed out the insides I assume and glassed the inside to smooth it out? How many layers of glass were needed? What did you do for the body of the plenum? Any particular foam you would recommend? Last question did mold the intakes separate to body? I like the West 105 system epoxy.

I sanded it to shape and used some bondo to make it smooth and covered with wax. I wish I had covered it with tape of plastic wrap. I did make them separate and then attached them to the plenum. I used west system epoxy to smooth the inside and lots of work to get the foam out. Gas or acetone helped but I think the bondo didn’t help.

It may have been easier to make a mold and use great stuff foam and cut closer to shape. I can’t remember how many layers i did. It was several. I enjoyed the project but it was a pain getting rid of the foam.
 
For the inlets shown above, I have switched to flat wrap card stock as an in place mold. The geometry for a "circle to square adapter" is quite simple (many aircraft fuselages are based on the concept - do a google search). Scissors, masking tape and some card stock gets you formed up in a few minutes. Once you like it, glass over the top. One more layer on the inside and you are done.
 
Thanks for all the great replies. Given what I have and what I want (good cooling / low drag) and with inspiration from Dan and Rockwood... I think I will build my own plenum to match my engine/baffles set up to the cowl. If I can do it I will likely end up with a better solution. Key take away is lots of volume and smooth transitions.

Standby as I get started down this patch.
 
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If you are starting over, let me offer the above as a "simple" alternative.

Plenum top is .016 aluminum sheet secured with piano hinge all around. Inlets are circle to square adapters formed in place with card stock and masking tape, glassed over.
 
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If it does not fit, make it fit. We are building something. And not necessarily tied to one solution provided by a “manufacturer”. If someone does not like the product purchased for whatever reason, return it. Then build it from scratch. We should not look at everything as plug and play. Think about it more as a starting point or something to deviate from.

EXPERIMENTAL=Recreation and education. Certified=Plug and play (and pay)

click on the cowl section on the signature below for gouge. You can do it. I knew zero when i started.

When you are at the finishing kit stage of your build, it is very obvious that building an RV-7 is not "Plug and play". ;)
However, I did not purchase a "plenum starter kit". I purchased a plenum for an IO360-M1B with superior cold sump, horizontal induction for a Sam James long cowl.

Returning merchandise from Europe to the USA is an administrative nightmare besides high freight charges. I usually accept the damage and carry on.

I have decided to go-ahead with the plenum I have. After some work it is sitting at the correct height now. My earlier criticism for the plenum to be sitting very low is not justly. It has to sit this way to provide clearance to the upper cowling.

The RH plenum inlet does not line-up with the R-cowl inlet ring. It has to move 1/2 inch outboard. I think the flexible transition can correct that, I have a 2.5 inch prop extension so there is enough room, but I think I should have them lined-up to prevent cooling drag.

Does anyone has a tip which technique to use to adjust the plenum inlet?
 

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There is a post somewhere on VAF with a guy that cut 7/8th of the way around the offset inlet. He then heated up the last inch or so with a heat gun and drove a wedge into the slit to offset the front and align it. With the wedge still in place you add a few layers around the slit, then remove the wedge and fill in that gap.

I spent about 10 hours today fitting the baffles and getting the plenum into its initial position. I ended up with 1/2" clearance over the top in the center, and a ton of room over the cylinders. The plenum was at least 1/2" low in the oil cooler area and the rear flange didn't line up at all, so I just cut the entire rear 1" off the plenum. I also had to make 4 holes that will eventually be reglassed with "bubbles" in order to clear my fuel injection lines (SDS fuel injection). The front inlets are vertically inline, but my right one is 1/4" or so inset just like yours. There's also a dent in the side of it. I don't think I'm going to worry about the offset, but I'm planning on trimming back 3/8-1/2 inch to remove the dent. I trimmed most of the front flange away as well and will re-lay glass in order to seal to the motor case and the remaining portions of the inlet ramps. There's huge gaps where the plenum transitions to the ramps/cylinders on the front. I kept about an inch of the ramps outside of the inlets so I can put a fiberglass flange over them and then screw everything down with a few plate nuts in the ramps. Unfortunately, in order to get the center clearance correct, there's no way I have room for the oil cooler. I'm running a showplanes dynafocal mounted 13 row oil cooler, and in order to fit the opening I'm going to have to cut most of the plenum away from the top of cylinder 3 and reglass it about an inch higher. If you look close at one of the pictures, you can see the blue sharpie on the baffle behind cylinder 3. That's where I have to raise the plenum up to in order to fit the oil cooler opening. I was able to measure for the first few cleco's in the fiberglass flange on the sides though, so the installation position is at least repeatable from this point forward.

I think getting the oil cooler hole cut and positioned is the last thing I have to do before rivetting the baffles together and starting the foam shaping for hte new glass. Hopefully I'll have all the new glass in place next weekend and can start final trimming and sealing. The metal baffles still need to be final trimmed as well, but that's waiting until the oil cooler is in place, because I really don't want to take too much off there and have to reorder the rear baffle.
 

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Here's the last shot of the plenum before I start the rebuild process. Aluminum as all been final trimmed and next step is to get the rear flange made up. It has to be laid up upside down, so everything is coming off for that. Once it's on and trimmed I'll make some foam bumps and glass those in along with the leading edge layup.
 

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Shot of mostly finished plenum. I didn't spend a ton of time doing the finishing work since almost no one will ever see it, but it was good enough for a coat of primer. I'm going to do a more complete post in my build thread in case anyone in the future is looking for extra info on this subject as well. It'll end up on the blog in my signature sooner or later (likely later) when I get a chance to get that caught up to reality as well.
 

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If the plenum and intake system are efficient, i.e. a high percentage of available dynamic pressure is converted to increased static pressure above the engine, force pushing upward on the lid is quite high.

For example, dynamic pressure at 175 KTAS and 8000 ft is 81.6 lbs per sq ft. A good system should convert more than 0.8 to static pressure in the plenum, so 81.6 x 0.8 = 65.28 lbs/sq ft. Check your own dimensions, but the lid here is probably something like 18 x 30, or about 3.75 sq ft. 65.28 x 3.75 is 245 lbs lifting the lid...and that's just cruise speed at altitude. 200 knot VNE at 1000 ft is 131.68 dynamic, thus 395 lbs.

So, two points. First, the lid should be considered a membrane structure, with particular attention paid to stiffness as well as strength. It will try to blow up like a balloon. Second, fasteners should be adequate for the load, as well as spaced so that flexibility of the components does not allow leakage.

Here we can't see the fastener arrangement down the sides and across the back, but we can see the front. There is a significant span across the engine case with no support, and the ends of that span are tied to baffle tin with what looks like two #8 screws in tabs at an angle to the load.

Imagine this lid hung upside down from its perimeter fasteners, in an open frame, so that 395 lbs of water or sand could be heaped into it. How much would it deflect? Do you think that unsupported span would gap open? Even a small gap means a large leak if it extends for some length, and there's not much point in a plenum lid which leaks even just a little bit.
.
 

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If the plenum and intake system are efficient, i.e. a high percentage of available dynamic pressure is converted to increased static pressure above the engine, force pushing upward on the lid is quite high.

For example, dynamic pressure at 175 KTAS and 8000 ft is 81.6 lbs per sq ft. A good system should convert more than 0.8 to static pressure in the plenum, so 81.6 x 0.8 = 65.28 lbs/sq ft. Check your own dimensions, but the lid here is probably something like 18 x 30, or about 3.75 sq ft. 65.28 x 3.75 is 245 lbs lifting the lid...and that's just cruise speed at altitude. 200 knot VNE at 1000 ft is 131.68 dynamic, thus 395 lbs.

So, two points. First, the lid should be considered a membrane structure, with particular attention paid to stiffness as well as strength. It will try to blow up like a balloon. Second, fasteners should be adequate for the load, as well as spaced so that flexibility of the components does not allow leakage.

Here we can't see the fastener arrangement down the sides and across the back, but we can see the front. There is a significant span across the engine case with no support, and the ends of that span are tied to baffle tin with what looks like two #8 screws in tabs at an angle to the load.

Imagine this lid hung upside down from its perimeter fasteners, in an open frame, so that 395 lbs of water or sand could be heaped into it. How much would it deflect? Do you think that unsupported span would gap open? Even a small gap means a large leak if it extends for some length, and there's not much point in a plenum lid which leaks even just a little bit.
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A few counter points. Sam James doesn't require or suggest any fasteners across the front of the plenum and most of the pictures online of SJ plenum installs haven't modified the nose at all for extra retention or to stem the massive amount of airflow that can escape there. While I agree that there could be more support, there's plenty of people run SJ plenums without hinge line fasteners 360 degrees around the plenum. I put a total of 12 screws around the perimeter of the plenum and there are some areas where I'm a little concerned about flex being an issue, so that may increase to 14 or 16 in the future.

I would question the difference between the pressures you're saying are required for convective cooling vs what Lycoming states is required. Lycoming quotes 5.5" H2O pressure difference between upper and lower halves to cool 150 hp and 6" for 180 hp. Let's just assume it's 7" H2O for 200 hp because they didn't post that number. That equates to 28.6 psf, 31.2 psf, and 36.4 psf. You're saying any type of small leak will be an issue at twice the required pressure differential than Lycoming says is required. You could conceivably have a leak that opened at speed, lowered your plenum pressure by almost half and still have enough airflow for cooling per the manufacturer spec.

I've attached the picture from the kit planes article showing their SJ plenum install, also no hold-downs across the front of the engine. But like all things experimental, if this plenum doesn't work in flight, I'll fix it and try again. I think it'll be fine though.
 

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If the plenum and intake system are efficient, i.e. a high percentage of available dynamic pressure is converted to increased static pressure above the engine, force pushing upward on the lid is quite high.

For example, dynamic pressure at 175 KTAS and 8000 ft is 81.6 lbs per sq ft. A good system should convert more than 0.8 to static pressure in the plenum, so 81.6 x 0.8 = 65.28 lbs/sq ft. Check your own dimensions, but the lid here is probably something like 18 x 30, or about 3.75 sq ft. 65.28 x 3.75 is 245 lbs lifting the lid...and that's just cruise speed at altitude. 200 knot VNE at 1000 ft is 131.68 dynamic, thus 395 lbs.
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Very interesting prospective I've not thought of before. This dynamic pressure load would also apply to stock baffling systems resulting with lifting the top cowling in flight visible on some planes along top rear edge. I always thought it was pressure build up behind the engine caused by turbulent air flowing towards the bottom of the firewall.

I might revisit the plenum concept, not so much for cooling, but for saving those ugly cowling bulges some planes wear.
 
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Very interesting prospective I've not thought of before. This dynamic pressure load would also apply to stock baffling systems resulting with lifting the top cowling in flight visible on some planes along top rear edge. I always thought it was pressure build up behind the engine caused by turbulent air flowing towards the bottom of the firewall.

I might revisit the plenum concept, not so much for cooling, but for saving those ugly cowling bulges some planes wear.

After the high pressure air flows through the engine cooling fins/oil cooler it pressurizes the lower cowling that causes the cowling to bulge on the top rear edge. Leaks in baffling/plenum lead to more pressure in the lower cowling and less differential between the upper and lower. My cowling has a slight bulge where the hinges aren’t and I’m running a plenum.
At 140 KIAS My lower cowling pressure is about 2” H20, upper plenum is 14” H2O. If 2” bulges the small spaces between cowling fasteners imagine what 14” would do across the front of the engine.
 
Sam James doesn't require or suggest any fasteners across the front of the plenum ....

Yeah, I know. Doesn't trump physics.

I would question the difference between the pressures you're saying are required for convective cooling vs what Lycoming states is required.

Nothing I wrote above addresses the mass flow required for cooling. I've only addressed the pressure which will be present if the system exhibits a good pressure coefficient, Cp as defined in CR3405.

In fairness, I did assume a Cp of 0.8, but based on a pressure dataset from another James user, you may not need to worry about much more than 0.6.

The second assumption is a large cowl exit, i.e. lower cowl pressure near freestream. If the exit is restricted so pressure is above freestream, the load on the plenum lid would be reduced. To use Andy's example, 2" H2O lower cowl pressure would reduce plenum lid loading by 10.4 lbs/sq ft. With a plenum "lower cowl" pressure is also that above the plenum lid.

Now, as for cooling...

Lycoming quotes 5.5" H2O pressure difference between upper and lower halves to cool 150 hp and 6" for 180 hp.

Show me the quote. In the meantime, I've attached an Lycoming cooling chart. There is no one deltaP value for a given HP, because (1) density varies with altitude, (2) heat transfer varies with air temperature, and (3) there is no reference to a resulting CHT.

But like all things experimental, if this plenum doesn't work in flight, I'll fix it and try again. I think it'll be fine though.

Itwill cool, but so will flap seals. The only reason to install a plenum lid to attain zero leakage, without which it is a PITA for no return. I'm merely suggesting the application of fundamental principles.
 

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I attached a pic of the front of mine and one on a Glastar. I have plenty of fastners and the Galstar has none. I can only vouch for mine and that all my temps are great and within a few degrees of each other. My exit may be bigger than some since I have a cut for my nose wheel leg. I may put a removable cover over the cut. Probably pick up another 10 knots or so.
 

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I attached a pic of the front of mine and one on a Glastar. I have plenty of fastners and the Galstar has none. I can only vouch for mine and that all my temps are great and within a few degrees of each other. My exit may be bigger than some since I have a cut for my nose wheel leg. I may put a removable cover over the cut. Probably pick up another 10 knots or so.

Looking good Rocky, and your oil cooler is higher to get full coverage of virgin air for cooling! Dans references are covered with your center forward attachments.
 
I attached a pic of the front of mine and one on a Glastar. I have plenty of fasteners and the Glastar has none.

It obviously leaks (see below). But it is very decorative ;)

First version of mine, circa 2009, attached the span above the forward case to a formed flange using vertical screws into nutplates. Couple hundred hours and the loads fatigued the formed flange enough to start breaking it from the ends where load was concentrated.

So I revised the lid by adding a 90 degree flange with four horizontal screws in shear. The flange stiffened the edge, and the assembly is really stiff when screwed to the front bulkhead, which is bracketed to the case spine.

This iteration has seen 230 knots (yes, I was careless), meaning 150 lbs/sq ft with a Cp over 0.85, or nearly 500 lbs without failure.
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