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  #41  
Old 09-06-2021, 09:04 PM
agent4573 agent4573 is offline
 
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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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Last edited by agent4573 : 09-06-2021 at 09:07 PM.
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  #42  
Old 09-07-2021, 07:55 AM
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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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Last edited by DanH : 09-07-2021 at 07:58 AM.
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  #43  
Old 09-07-2021, 10:03 AM
agent4573 agent4573 is offline
 
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Quote:
Originally Posted by DanH View Post
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.
.
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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Last edited by agent4573 : 09-07-2021 at 10:09 AM.
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  #44  
Old 09-07-2021, 10:19 AM
Ralph Inkster Ralph Inkster is online now
 
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Quote:
Originally Posted by DanH View Post
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.
.
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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Last edited by Ralph Inkster : 09-07-2021 at 10:26 AM.
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  #45  
Old 09-07-2021, 10:49 AM
crabandy crabandy is offline
 
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Quote:
Originally Posted by Ralph Inkster View Post
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 arent and Im 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.
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  #46  
Old 09-07-2021, 12:34 PM
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DanH DanH is offline
 
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Quote:
Originally Posted by agent4573 View Post
Sam James doesn't require or suggest any fasteners across the front of the plenum ....
Yeah, I know. Doesn't trump physics.

Quote:
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...

Quote:
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.

Quote:
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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  #47  
Old 09-07-2021, 01:10 PM
rockwoodrv9 rockwoodrv9 is offline
 
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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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  #48  
Old 09-07-2021, 02:06 PM
BillL BillL is offline
 
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Quote:
Originally Posted by rockwoodrv9 View Post
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.
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  #49  
Old 09-07-2021, 03:28 PM
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DanH DanH is offline
 
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Quote:
Originally Posted by rockwoodrv9 View Post
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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  #50  
Old 09-07-2021, 06:32 PM
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Dan -

What material did you use to fabricate the ramp area?

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