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Air Box to cowling connection

Vansconvert

Well Known Member
I'm looking for ideas on how to connect the front of the air box on a vertical induction IO- 360 to the air scoop on the cowling. This is the air scoop below the propeller, directly in front of the Vans air box - not the snorkel setup.
 
I've previously suggested a one-piece formed rubber sock, but no one has reported trying the idea.

The optimum seal criteria would be ...

1. Highly flexible.

2. Airtight, and driven tighter by air pressure.

3. Doesn't interfere with removing and installing lower cowl.

I'd suggest a tubular sock, molded in place for fit, made from a single ply of 9oz glass fabric impregnated with room-temperature cured urethane rubber. It would be a female fit over the cowl spigot, and a male fit into the airbox. The gap between the end of the airbox and the cowl inlet might be increased to 1-1/2". The sock is simply pulled inside out, back through the inlet, for cowl installation. When in place, it would extend a few inches into the airbox. Use a few rivets with backing washers to ensure it will never move on the cowl spigot.

Objections focus on the possibility of the male end closing due to low pressure. I doubt it, but an experimenter could easily video it, and test fly after insuring the operation of the alternate air source.

The current flap seals don't seal very well. Lack of sealing probably adds lower cowl pressure, decreasing cooling deltaP. Using a small gap with stiff seals places a structural load on the airbox top plate during engine shake.
 
Are you describing the typical setup you'd expect to find with an O-320?

We recently replaced the FAB inlet seals and repaired the lower cowling scoop (assuming I've got you on this one) on an RV9A equipped with an O-320. Pop rivets, washers and 2 inch wide black rubber strips (baffle seal material).

By the way, this as well as a multitude of other small improvements has resulted in almost dead even cylinder head temps on the ground, on climb and in cruise. Typically 3.0 deg C (5.4 deg F) CHT split between cylinders.

The latest Van's USB stick doesn't give up much in the way of pictures but on purchase the aircraft above arrived with a bunch of docs from 2003 which happened to have the assembly instructions for the FAB, as well as instructions for forming the interface between the FAB and lower cowl with rubber sheet.

I don't have access to those doc's anymore but perhaps someone here might upload them for you.
 
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Airbox to Cowling Connections

Since my plans to be doing first flight this year are taking a back seat to quarantine and not taking a risk of becoming a Covid statistic I took the opportunity to redo a couple of areas of the design called out in the plans. The cowling to airbox interface is one of them. I am building a RV-7A with a IO360B1B (vertical induction), constant speed Hartzell Prop. I considered a number of alternate approaches including the sock approach suggested by Dan Horton. The considerations were:
1. Good seal for the ram air inlet to get the maximum pressure recovery and minimize the air leaks into the lower cowling to provide the maximum cooling airflow through the engine.
2. Mechanically decouple the airbox from the cowling to minimize airbox failures and stress on the cowling. (a known problem on engine start and stopping)
3. Minimize the impact of the ram air inlet interfering with airbox while installing the lower cowling. The lip seals on the lower cowling cooling air ducts are almost impossible to get up over the baffling lip with the extension to the ram air inlet glassed and the rubber ducting installed on the airbox in in my build.
After a lot of sketching and mocking up trial configurations I finally settled on a solution. I chose to cut off the front of the airbox and glass in a fitting that is equivalent size and shape as the extension on the cowling ram air inlet. I cut off the front of the ram air inlet and then glassed in a lip on the bottom of the aperture on the cowling and glassed in a lip on the top of the removed section of the ram air inlet. The airbox/ram air inlet are joined by a 3.25 dia Neoprene scat tube. An alternate connection to be investigated is using a Dorman part n# 696-083 automotive connector hose. The hose connection is the critical part. This is still a work in progress with prototype parts, so the workmanship is a bit scruffy.

Keith T
 

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I have done what Dan suggested, although not quite as spiffy as his suggestion. I have a piece of 3" scat tubing that I pulled the wire out of that fits well (i.e., pretty airtight, needs stretched to get over the fiberglass) over the interior part of the snorkel. I cut a few slits in the end, and use a zip tie to tighten it on the forward side of the snorkel, which is smaller diameter than the aft part of the snorkel. This keeps the tubing from possibly coming off. I then modified the FAB with an oval inlet that is the same circumference as the 3" scat tube. The scat then fits inside the inlet and air pressure keeps it reasonably tight against the inlet.

Definitely an advantage over the standard setup, in that I can just reach inside and pull the tubing out (forward) before removing the cowl. Easy to look inside and check for security on preflight, and cheap and easy to replace when the scat starts showing signs of wear. It is also flexible for accommodating engine shake during startup or shutdown.

Been working fine for several years.
 
Cowl/airbox interface

Here is my interpretation. In the process the cowl inlet got reworked from oval to 3” round and moved forward about 2.5”.

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Don Broussard
RV9 Rebuild in Progress
57 Pacer
 
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+1 on the engine moves around a lot - side to side mostly.
Parts falling off in the path of the induction air and going through the engine generally results in a very bad day.
KT
 
Inlet “sock” solution

Its an elegant solution with the moulded sock. The moulding needs to be closely contoured to the inside of the airbox and stiff enough to not flap at the inboard lip and flexible enough to decouple the airbox from the cowling. Making the gap correct between the airbox and the inlet extension on the inside of the ram air inlet to match the stiffness of the sock is likely to need some iterations. As always the devil is in the details.
Dan, if I had your skills to make the neoprene sock and to do excellent glass work I might have been more tempted to give this approach a try. Cutting the cowling ram air nose off and using a off the shelf hose solution has more chance of success with my modest composite skill.

KT
 
Cowl/airbox interface

My IO-320 vertical induction installation only had about 1.5” gap between the fiberglass cowling duct and the airbox. After observing engine shake at startup and shutdown it appears the most of the motion is rotational around the crankshaft. I saw the valve covers moving up and down about 3/4”. Since the airbox is a little further from the crank it’s probably safe to assume the airbox will move 1” left & right from its static position. I could not find a off the shelf hose with enough flexibility to handle this movement over a 1.5” run. The scat hose with the wire removed sounds pretty good as does Dan’s super flexible sock. I went a different route with the airbox, but will mold a duct for the remote mount oil cooler.

Don Broussard
RV9 Rebuild in Progress
57 Pacer
 
Don,

I cut back the airbox so it is further from the snorkel, allowing more distance over which the hose can flex.

Greg
 
Revisit on the airbox sock

Still trying to optimize the airbox to ram air inlet interface. Tried the Dorman inlet hose (696-083) which fit perfectly but the bellow diameter interferes with the bottom of the cowling and it is stiffer than I would like - so back to the neoprene or silicon tubing. Revisited the sock solution and did a mock-up with 3 inch tubing with the coiled wire removed. With the sock attached to the removable ram air inlet and the just pushed into the airbox inlet before attaching the ram air inlet nose. Only major issue with that approach is ensuring that the tubing doesnt come off the ram air inlet or get shredded and get sucked into the airbox causing a blockage around the filter. If the tubing is on the outside of the airbox this issue is resolved, but moves the problem to one of how to clamp it to the front of the airbox. I feel like I am close to a workable solution but not there yet.

KT
 
Keith,

Over many hours of operation, I have seen no tendency of the hose to shred. It does wear in certain places and requires occasional replacement (like every couple year). Having the zip tie around the snorkel inlet seems to work fine - I've seen no potential tears or other signs that it may come off of the snorkel.

My experience, anyway.

Greg
 
Inlet hose sock

Thanks Greg,
I had come to the conclusion that a flexible clamp (such as zip ties) is required around the ram air inlet extension to secure the sock. Now working on a solution with two layers of 3 inch scat tubing with the wire removed. Cutting non overlapping slits in the two layers of scat tubing and gluing the two layers together at the ram air inlet before installing may be part of the solution. Will continue the experiment and report findings. Developinga simple effective, convenient solution that is easy to replace is the goal.
KT
 
What I do for vertical injection. Note the fiberglass transition the connects to a 3” SCAT hose. The cowl gets an aluminum inlet ring glassed in (available from James Aircraft). The SCAT hose is just a touch longer than the distance between the two, and is attached to the air box with a hose clamp. The hose slides over the cowl inlet ring under compression (because it is a little long).

Do not use SKEET hose - too rigid and you will end up cracking the air box top plate.

Carl
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My inlet sock

What I did was make a neoprene tube out of an old mouse pad. The circumference at my cowl inlet tube is slightly bigger than the intake tube on the FAB, So the flat pattern is tapered a little. Then I glue the edge with neoprene wet suit repair adhesive. In addition, I glue a 1" wide reinforcing strip across the outside of the seam.

The sock is a female fit over both the FAB inlet and the cowl inlet tube, with a worm-type hose clamp on the FAB side, and just rely on the elastic fit on the cowl inlet tube.

When I install the lower cowl, it is pretty easy to reach into the inlet and guide most of the sock around the outside of the cowl inlet tube. But usually a bit of it is pinched between. Once the cowl is fully on, it is easy to just run my finger around the inner tube edge and push the remaining part of the sock out into place.

It is true that with an unclamped female fit over the cowl inlet tube, that ram air may be bulging the sock out and leaking some air into the lower cowl. I don't really know, although the elastic tension seems to keep it nicely tight.

My neoprene sock lasts about 5--6 years. I am on my second one and it is still doing well. I got two socks out of a mouse pad. Next time I need to make one, I hope it is not too much of a hunt to find a mouse pad. They used to give them away free with advertising graphics on them. Now, nobody even hardly remembers what a mouse pad was. I suppose I could cut up an old wet suit.
 
I swear guys, it would be easy to make a custom fitted glass and urethane sock (first sketch, post 10). The stuff has proved to be a whole lot tougher than I imagined 12 years ago.

Kicker is, I'm pretty sure it could just as easily be made using Loctite 598 and 9oz glass.

https://vansairforce.net/community/showthread.php?p=1325681&highlight=soft#post1325681

Yes it does. Loctite 598 in a tube - used about half a tube per sock. Made a clay mould and fashioned a two layer sock from 9 oz glass. Loctite 598 is horrible stuff to work with to get the glass properly wetted out - changed gloves twice, but the final result looked presentable for a female sock over the removable cowling inlet and a male end into the airbox. The clay mould needs another iteration to get a better fit but the sock is flexible and strong. Another solution I have been considering is to use the cut off top of an old wetsuit bootie that I use for kayaking. I was getting ready to throw them out and though that the ankle diameter was about right. Sure enough it is. The neoprene is glued and stitched. The airbox offset angle to match up with the ram air inlet works perfectly with the ankle angle of the bootie. This is a female fit on both the ram air inlet and the airbox, a configuration I am more comfortable with when considering getting the sock sucked into the airbox. The bootie is a comfortably tight fit over the ram air inlet and glued onto the removable ram air inlet extension. Reports of continuing development work and pictures to follow.

KT
 
my so far failed attempt

I've been trying to implement a leak free connection between front cowl and airbox using some of the ideas in this thread. My lower cowl snout is separately removable so it can be slid on to the cowl using a horizontal front-to-back motion. My thought was to attach a flexible hose to the lower cowl snout and it would fit into a closely shaped duct on the front of the FAB.

My first attempt used a 2.75" SCAT tube (wire removed for flexibility) that was clamped onto the snout. No pictures because cause it is too rough, and I had trouble getting good alignment between the two parts which would cause the tube to be wrinkled.

My question is - is 2.75" too small? The cowl snout inlet is about 4.25 x 2.2 inches, but with large radius corners, so something under 9 in2, maybe 8 in2? A 2.75" SCAT is only about 6 in2. Going up to 3" SCAT increases the area to 7 in2.

So my question is: does it matter? I don't want to go thru all this work and make a restrictive inlet.

I was wondering if going round was a dumb idea... maybe stick with a more rectangular shape on the flexible duct/sock.
 
I made my snorkel removable and extended the snout about 3", making a 3" round opening with an embedded 3" round aluminum tubing inside to accept 3" Scat. I also have a highly modified standard Van's FAB to accept the other end of the Scat. I don't have a photo of my FAB, but in the second picture, you can see the 3" aluminum tube in the snorkel snout.
 

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Rubber induction moldings

Edit: Should title this rubber molding of induction path. This wasn't induction molded.

My challenge was with a FM-200 on a Superior sump. Using the K&N filter yields very little room for anything. So my answer was to lay-up a kevlar/urethane filter body connecting the FM-200 to the inlet ring.

I suspect that there may be some collapse at high power and low airspeed. This seems evident by some oil imprints from the K&N on the inside of the cone. This could also be happening at/caused by high engine torque.

I did some estimates of airflow at high engine power and low DA then used a shop vac to try to produce that flow x 1.5. I did not see any cone collapse doing that test so I cannot be sure where the soft filter body-filter contact may be coming from.

If I were to be doing it again I would definitely lay some reinforcing rings into the circumference of the cone to stiffen against collapse without affecting lateral flexibility.
 

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Would using the 90 degree elbow airflow performance sells beteeen the sump and the servo be an option?
 
I've been trying to implement a leak free connection between front cowl and airbox using some of the ideas in this thread. My lower cowl snout is separately removable so it can be slid on to the cowl using a horizontal front-to-back motion. My thought was to attach a flexible hose to the lower cowl snout and it would fit into a closely shaped duct on the front of the FAB.

My first attempt used a 2.75" SCAT tube (wire removed for flexibility) that was clamped onto the snout. No pictures because cause it is too rough, and I had trouble getting good alignment between the two parts which would cause the tube to be wrinkled.

My question is - is 2.75" too small? The cowl snout inlet is about 4.25 x 2.2 inches, but with large radius corners, so something under 9 in2, maybe 8 in2? A 2.75" SCAT is only about 6 in2. Going up to 3" SCAT increases the area to 7 in2.

So my question is: does it matter? I don't want to go thru all this work and make a restrictive inlet.

I was wondering if going round was a dumb idea... maybe stick with a more rectangular shape on the flexible duct/sock.



The Rod Bower set up I have uses a ring of roughly 1/4 inch neoprene (I think wet suit material) sewn together to make the flexible connection between an AL transition from the filter inlet to the front ring on the removable lower cowl inlet. It works great.
 

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