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Cowl inlets

wirejock

Well Known Member
I'm working the cowl and thinking ahead to the plenum very soon.
It seems like there should be some sort of duct connecting the inlets to the plenum so maximum air flow goes into the plenum. However, how would the two cowls come together and seal to the duct? Maybe over thinking again.
I was thinking a horn shaped duct made of fiberglass with a seal of some sort. Since it's wider at the inlets, it wouldn't require overly complicated fastening to keep it from coming out. Easy to seal at the inlet. Assemble the cowls then insert the two ducts through the inlets. I just can't picture how to get a seal at the plenum. Maybe it just dumps with no seal? No idea.
I even considered wet suit material attached to both with a zipper seal between but that would be a real pain.
Any ideas would be appreciated.
 
Look st the JamesAircraft.com website to see how we do the plenum and connection to the inlets. Wetsuit material works good.
 
My plenum is DIY so a lot is involved in getting the seal right using the stock Van’s cowling. I settled on using two layers of BID impregnated with high temperature RTV silicone and cured in place. They will be two piece, top and bottom with an overlap. Attached to the cowling air inlets as in the plans. Just pull them through the gap when the cowling is in place and they will unfold and open up in place for a good seal. My bottom seal is done and I am working on the upper now. So far I am happy with the results. Gotta have a complete seal with no gaps, otherwise your defeating the main purpose of using a plenum.
 
Can you do a plenum with the snorkel air intake or do you have to have the lower cowling direct intake?
 
Cowl

Can you do a plenum with the snorkel air intake or do you have to have the lower cowling direct intake?

I have an M1B. No scoop. RV Bits plenum. I need a solid seal between the inlets and plenum to be fully effective. I may try impregnatting some fiberglass , as David mentions, with RTV to see what that looks like.
 
Larry,

Just use a small length of SCEET ducting between the back side of the inlets and the front side of the plenum/diffuser.

Make a bell shape for the plenum diffuser similar to what you did for the oil cooler.

Custom duct sizes and lengths available from Jonathan Kinney, Custom Duct -- two zero 7, six three 7, three zero zero zero. kinney AT fairpoint.net

He makes really nice ducts...
 
You have just begun to think, so not too much yet.

Since the interface is a racetrack shape, the flat sides would be an issue for wetsuit material. On my James cowl, the gap from the inlet-ring to the diffuser is only 1/8" or a 3/16" max. They don't touch from engine movement. If you try the wetsuit material make a buck the right shape and test it to see if it gets lifted by dynamic air pressures expected.

There is a nice one for Vans cowl on VAF. Edit: Good connection to the cowl, agree with DanH on the upper intersection trip point. It is a good idea to plot the areas for designing the expansion from inlet to head face. It pays off.
 
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You have just begun to think, so not too much yet.

Since the interface is a racetrack shape, the flat sides would be an issue for wetsuit material. On my James cowl, the gap from the inlet-ring to the diffuser is only 1/8" or a 3/16" max. They don't touch from engine movement. If you try the wetsuit material make a buck the right shape and test it to see if it gets lifted by dynamic air pressures expected.

There is a nice one for Vans cowl on VAF.

Wow! That is very nice.
 
Larry, Chis did some beautiful work. However, there is one issue. See the sharp, near 90 degree expansion? Flow separation is near guaranteed, which will reduce pressure recovery.

Compare with the plenum in post 31.
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Larry, Chis did some beautiful work. However, there is one issue. See the sharp, near 90 degree expansion? Flow separation is near guaranteed, which will reduce pressure recovery.

Compare with the plenum in post 31.
.

Got it. Looks like Rocky basically modified the plenum to incorporate the ducts.
Wonder what it looks like inside? So you're saying the air should flow straight into the plenum with no baffles up front?
 
My RV6 is similar to the Diamond set up with the inlets built into the lower cowl. Be warned, there is a lot of work and composite skills involved to achieve this. Cooling is pretty even across the cylinders and the urethane/fiberglass cloth ducts are holding up well so far (Only 30 or so flying hours tho' so early days).

The top is relatively easy to remove for spark plug access but the inlet ducts need to be removed for the lower cowl to come off. Not difficult but does add about ten minutes to the job.

https://photos.google.com/album/AF1.../AF1QipOiUzVxUjSB4RXriEucVV917waDPbzXpzwkhYHC

https://photos.google.com/album/AF1.../AF1QipPH7qBAUiKCqi0w2qZZ3EkMl-m0MUY55N3zqy-C

Clive Whittfield
Auckland
New Zealand
 
Larry, if you are absolutely sure you want to do a plenum, ignore the following.

I was drooling over these plenums on VAF and from time to time saw one "live" at KOSH or SNF. They look fantastic!

I was pretty sure I would build a plenum, but when the time came, I was in very strong "get 'er done" mode - needed to get that aircraft in the air. I'm really happy I did not build a plenum, for the following reasons:

  • Would have added a lot of build time - plus tinkering
  • Makes it harder to inspect the top of the engine
  • My cooling is fantastic with standard baffles - some say too good
  • My speeds are good - but who can have too much speed?

I still think plenums are sexy, and many are impressive works of art, and I'm delighted that people are experimenting with them. Just wanted to share my experience, in case you are also considering taking the more well-worn path of baffles.
 
Mickey has a good point. A plenum is just a sealing device. Leakage means less deltaP and more cooling drag, so sealing is good.

However, more than few plenums are wasted effort because their connection to the inlets violates various fundamentals, the result being poor conversion of dynamic to static. Better sealing, less pressure recovery = no net gain.

Again, a lot depends on the inlet velocity. If slow, the shape of the internal components connecting the inlet to the plenum is not critical. Very low Vi/Vo inlets generate excellent upper cowl pressure with no connecting ducts at all, i.e. flow just dumped into a sealed space. If inlet flow is a high percentage of freestream velocity, everything aft of the inlet plane becomes critical. Any humps, bumps, edges, corrugations, and sudden section changes deteriorate pressure recovery.

The -7's stock inlet is neither low Vi/Vo or high Vi/Vo, and it has a pressure gradient across its face. Mostly it's shaped to be conventional. There's nothing crazy wrong with it, as it works, but it's not an optimum approach.

So, go big or go conventional. A truly optimized inlet-plenum-baffle-exit system wouldn't start with the standard inlet. So maybe we should ask "Can I craft really good flap seals?". If yes, there is no great advantage to a plenum with a stock cowl. Serious advances require adjusting the system.

Got it. Looks like Rocky basically modified the plenum to incorporate the ducts.

Bill Lane designed those ducts in accordance with good principles. The plenum is just a lid. The ducts are everything if velocity is significant.

Wonder what it looks like inside? So you're saying the air should flow straight into the plenum with no baffles up front?

I'm saying there should never be a sharp section change unless flow is slow. Slow usually requires external diffusion and a throttled exit.
 
However, more than few plenums are wasted effort because their connection to the inlets violates various fundamentals, the result being poor conversion of dynamic to static.

I hope that people will take Dan's' comment to heart.

I personally know of a few instances where people spent a huge amount of additional time designing and building there own plenum system only to discover they had poor cooling performance when the stared flying.

Both ended up switching over to standard baffling and their cooling problems were solved.

One of the common errors with DIY plenum design is having too tight of a gap between the plenum top and the #1 cyl.
Since #1 is pretty close to the air inlet on the cowl, it is the most difficult transition. Any amount of area lost between the cowl and the plenum top is cross sectional area that is lost for airflow. This loss doesn't occur with the standard baffling (as long as the inlet ramp has been located properly) because the top of the plenum is the inside surface of the cowl. I have looked into a lot of inlets on airplanes with plenums and have seen that this is an obvious pinch point when it should instead be shaped so that it is expanding in cross section.
 
Education

I really enjoy these projects. Yes, the stock configuration is fine and I get builders trying to disuade me from a plenum. I still enjoy figuring something out and maybe even doing a fair job at it. Come take a look at my airplane and canopy sometime. I've learned a ton from folks like Dan, Dave, Scott. So let's keep ideas coming and encourage creativity.
 
So let's keep ideas coming and encourage creativity.

Assuming you wish to stick with the conventional slot inlet, and given your interest in glasswork, I'd suggest a hard look at Ron Moring's RV8, or Darwin Barrie's Grand Champion RV-7.

Ron's inlet and plenum below. I'm sure he would never post these pictures himself, given he is such a bashful fellow....
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Plenum

Assuming you wish to stick with the conventional slot inlet, and given your interest in glasswork, I'd suggest a hard look at Ron Moring's RV8, or Darwin Barrie's Grand Champion RV-7.

Ron's inlet and plenum below. I'm sure he would never post these pictures himself, given he is such a bashful fellow....
.

Yes, Vans conventional.
Wow! Thanks. Those are worth the 2,000 words. Explains a lot.
 
Cowl mod

This one's for Dan.
I did it. It's not pretty yet but it fits. Inlets are part of the bottom cowl.
Thanks for all your helpful tips.
20220105_093234.jpg

20220105_093434.jpg

20220105_093548.jpg
 
Cowl mod

If you do this mod, the standard Skybolts don't work above the inlets. The material is much thicker.
I think these are the correct part numbers.
SK-O14S Grommet
SK40S5-8S Stud
A few floating receptaclea will make your life easier too.
SK245A161
 
Hi Larry,

Good on you for tackling such and ambitious project! Your rough out looks great.

I have just one caution as you go forward, and you have probably already thought of this, but I'll say it just in case. Be mindful of how you will remove the bottom cowl.

There is only a small amount of clearance between the front lip of the bottom cowl and the back of your prop. Be sure you engineer the joint between your inlets and your plenum/baffles so that it disengages with only a fraction of an inch of forward movement.

I don't have a plenum. All I have to do is wiggle a piece of baffle seal up over the lower ramp of my forward baffle and it is still a PITA to remove and reinstall.

Good luck, and I hope you are flying soon!
-Ivan
 
Baffle

Hi Larry,

Good on you for tackling such and ambitious project! Your rough out looks great.

I have just one caution as you go forward, and you have probably already thought of this, but I'll say it just in case. Be mindful of how you will remove the bottom cowl.

There is only a small amount of clearance between the front lip of the bottom cowl and the back of your prop. Be sure you engineer the joint between your inlets and your plenum/baffles so that it disengages with only a fraction of an inch of forward movement.

I don't have a plenum. All I have to do is wiggle a piece of baffle seal up over the lower ramp of my forward baffle and it is still a PITA to remove and reinstall.

Good luck, and I hope you are flying soon!
-Ivan

Thanks
I'm not there yet but the plan is neoprene sock sleeves to connect the inlets to the plenum. Remove the top cowl, pull the socks back, drop the bottom cowl.

It shouldn't need to move forward. I also checked it with the Whirlwind prop spinner simulator. It seems ok.
 
Good! That sounds like a reasonable approach. I figured you'd thought it through.
Good luck!
 
Gaps

Follow up. This may help someone working on the cowl.
If you have a gap along the firewall or split line, try this tip.
Grab that cheap black electrical tape. Harbor Freight 10 rolls.
Lay down a strip over the Skybolt flanges or the hinge then lay another strip with 1/2 the tape width on the skin. Use a credit card to force the tape down against the firewall edge so it forms a nice square edge and flatten any remaining so it overlaps the previous tape line. After doing all the edges go back and lay a strip of packing tape overlapping the black tape on then skin so any filler won't stick. Now wax the tape throughly.
Mix some floxed epoxy. Transfer it to a plastic baggie. Nip off the tip. Run a bead right in the crease. Doesn't take much. Maybe 1/8" diameter. Install the cowls. Use a credit card to spread any excess or fill gaps. Walk away. Cure.
Remove the cowl. Not easy, but it should pop off. Remove the tape. File or sand the edge. You should have a gap exactly the thickness of the tape and just about perfect for a paint gap.
FYI, I prefer flox because mico can sometimes break away. Flox will make a nice hard edge.
20220108_125400.jpg
 
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