New plenum build - RV7

[agent4573]

Original build was a Sam James long cowl with their plenum. It was slightly rushed to finish the plane, and I never liked the way it fit. I also didn't use enough fasteners and I think installed it way to low over the cylinders. Results were fine in cruise, 75% ROP would run around 400F, LOP would drop to 320-350 depending on OAT. Climbing out on hot days was done at 120-130 KIAS which limited the ceiling to about 11,000 ft at full gross. I had 2 weeks off while I'm changing jobs and decided to tackle rebuilding the plenum. I tore off all the old baffling and started with a new baffle kit from vans, modified for the ECI tapered cylinders. The plenum was created by vacuum bagging 3 layers of carbon to the inside of my cowl. I laid up pieces that were ~33 inches wide and 17" long. This wasn't enough depth, but was where the James cowl would allow for a mostly flat part between the oil door and where the round inlets started to become prominent. I then scarfed on 3" to the front to have enough depth to make all the connections. Once everything is fitted and riveted, I'll go back and add a 4th layer as an aesthetic top layer.

The new baffles got -4 piano hinge along the top and the carbon piece got -3. I cut up some random wing ribs to reuse the flanges. During final install, I'll tape one side up and form an in place gasket out of RTV to fully seal the piano hinge. The front piece has 5 holes per inlet nozzle for the top seal and 3 holes on each side of both inlets that will eventually get nut plates and a formed in place gasket. The hump over the top has a flange that will also get nut plates to hold it in place. Clearance between top of baffles and cowl was set at 3/4".

I carved the foam molds and wet laid 3 layers of carbon to form the round to square adapters. While carving. I used .065" spacers on each side of the molds to simulate the thickness of the carbon. This worked really well and it fit better than I expected.

I've probably got just about 100-120 hours into this so far, and I still need to do the oil cooler line, fuel lines, and injector wire harness path throughs. I wanted to go 4" on the oil cooler line but just can't fit that around everything else under the cowl. I ordered some 3.5" sceet tubing today and should be able to make that work. I'm also waiting on nut plates and some more carbon for the new oil cooler shroud to arrive, plus I start my new job tomorrow, but will hopefully have an update and some finished pics in 2-3 weeks.

 

[mstrauss]

Very cool, thanks for sharing. Please post a final shot of the plenum.

 

[Freemasm]

Original build was a Sam James long cowl with their plenum. It was slightly rushed to finish the plane, and I never liked the way it fit. I also didn't use enough fasteners and I think installed it way to low over the cylinders. Results were fine in cruise, 75% ROP would run around 400F, LOP would drop to 320-350 depending on OAT. Climbing out on hot days was done at 120-130 KIAS which limited the ceiling to about 11,000 ft at full gross. I had 2 weeks off while I'm changing jobs and decided to tackle rebuilding the plenum. I tore off all the old baffling and started with a new baffle kit from vans, modified for the ECI tapered cylinders. The plenum was created by vacuum bagging 3 layers of carbon to the inside of my cowl. I laid up pieces that were ~33 inches wide and 17" long. This wasn't enough depth, but was where the James cowl would allow for a mostly flat part between the oil door and where the round inlets started to become prominent. I then scarfed on 3" to the front to have enough depth to make all the connections. Once everything is fitted and riveted, I'll go back and add a 4th layer as an aesthetic top layer.

The new baffles got -4 piano hinge along the top and the carbon piece got -3. I cut up some random wing ribs to reuse the flanges. During final install, I'll tape one side up and form an in place gasket out of RTV to fully seal the piano hinge. The front piece has 5 holes per inlet nozzle for the top seal and 3 holes on each side of both inlets that will eventually get nut plates and a formed in place gasket. The hump over the top has a flange that will also get nut plates to hold it in place. Clearance between top of baffles and cowl was set at 3/4".

I carved the foam molds and wet laid 3 layers of carbon to form the round to square adapters. While carving. I used .065" spacers on each side of the molds to simulate the thickness of the carbon. This worked really well and it fit better than I expected.

I've probably got just about 100-120 hours into this so far, and I still need to do the oil cooler line, fuel lines, and injector wire harness path throughs. I wanted to go 4" on the oil cooler line but just can't fit that around everything else under the cowl. I ordered some 3.5" sceet tubing today and should be able to make that work. I'm also waiting on nut plates and some more carbon for the new oil cooler shroud to arrive, plus I start my new job tomorrow, but will hopefully have an update and some finished pics in 2-3 weeks.

Nice work. I suck at composites but unlike the pix in the attached, mine finally came out OK. Hinge connection sounds like a better option. I was never completely comfortable with the stresses at the baffle/plenum connection. If you want a belt with your suspenders, here's an idea stolen from Mooney.

Plenum Center Support

 

[Toobuilder]

Hinge connection sounds like a better option. I was never completely comfortable with the stresses at the baffle/plenum connection.

If my experience is any indication, there is not too much to worry about regarding the “strength” of the plenum lid. The lid on my Rocket is nothing more than a sheet of .016 aluminum. The entire perimeter is retained by piano hinge so the point loads are very low. There are no ribs, center supports, or any other bracing other than the doublers shown (to cover some misdrilled holes). It’s just an aluminum diaphragm, and it holds up fine at Rocket speeds (for the last 200 or so hours).

To the OP - I have broken an eye or two on my hinges over the time in service and found that clearancing the eyes with a file a few thou keeps them from locking together from thermal expansion and individual cylinder movement. It’s a bit of a pain, but if you take .010 off each eye you will be happier down the road. Also suggest stainless or extruded hinge stock for long term durability. The hinges I am using are those light duty folded aluminum seen on the trim tabs and gear fairings. I honestly didn’t think this scheme would work, so I just went with the materials I had on the shelf.

 

[Freemasm]

....If my experience is any indication, there is not too much to worry about regarding the “strength” of the plenum lid. The lid on my Rocket is nothing more than a sheet of .016 aluminum. The entire perimeter is retained by piano hinge so the point loads are very low. ..

Wasn't the strength of the plenum material I was concerned about; was the strength of the connection.to the baffle. As I mentioned, hinge was probably a better choice than #6 screws. The center post/connection significantly redistributes that stress.

Every OEM has their strengths and weaknesses. As an engineer, I always had respect for the old Mooney engineers (Bill Wheat was damn impressive human) and the Beech designs. Some other OEMs, quite a bit less.

Would be interesting to know the shape of your plenum and baffle at 200mph.

 

[Toobuilder]

Yeah…. Piano hinge is the key to making this scheme work. The point loads of screws would drive hefty doublers at every connection and would probably still crack over time. I’m sure my lid balloons up a bit, but the aluminum certainly isn’t going to stretch. It has yet to hit the upper cowl anywhere, and it’s within a 1/2 inch in most places.

 

[agent4573]

If my experience is any indication, there is not too much to worry about regarding the “strength” of the plenum lid. The lid on my Rocket is nothing more than a sheet of .016 aluminum. The entire perimeter is retained by piano hinge so the point loads are very low. There are no ribs, center supports, or any other bracing other than the doublers shown (to cover some misdrilled holes). It’s just an aluminum diaphragm, and it holds up fine at Rocket speeds (for the last 200 or so hours).

To the OP - I have broken an eye or two on my hinges over the time in service and found that clearancing the eyes with a file a few thou keeps them from locking together from thermal expansion and individual cylinder movement. It’s a bit of a pain, but if you take .010 off each eye you will be happier down the road. Also suggest stainless or extruded hinge stock for long term durability. The hinges I am using are those light duty folded aluminum seen on the trim tabs and gear fairings. I honestly didn’t think this scheme would work, so I just went with the materials I had on the shelf. View attachment 56529

The hinges are already locking together due to the curving nature of the install. I've filed the ones that needed it in order to get the mock up done, but I'll definitely go back and file all of them when I'm prepping for the final layer of carbon. 3 layers of carbon already feels over built but I've committed to at least one more layer. The lid doesn't haven't to be super strong if it's held well around the perimeter.

 

[agent4573]

Some progress from this last week. We got the first round of RTV down and got the oil cooler line in. Next week is formed in place gaskets for the plenum lid and diffuser nozzles and securing the SKEET tube so it doesn't rub the motor mount, then we should be ready to fly again. Just in time to take it back down for condition inspection....

 

[DanH]

The lid load isn't small, so good distribution is important. Figure dynamic pressure in psi or psf for some speed in excess of VNE, x .85 (if your pressure recovery is any good), x area.
Also, the cylinders vibrate independently. That's why the side baffle walls have a slip joint. A really stiff lid (carbon) will stress the fasteners near that joint.

 

[DanNiendorff]

Nice work. I can say from experience that this a significant undertaking. Kudos to you for taking it on.

 

[Toobuilder]

Recommend adding a bellmouth inlet to the SKEET tube at the engine side. That sharp transition is going to create some local turbulence that will effectively reduce the size of the tube. And with that much tube in use, you are going to suffer duct losses anyway. You might consider extending the oil cooler side fabricated duct up high enough to clear the engine mount tubes AND turn the corner. Then feed it with a much shorter and straighter piece of SKEET. The extra height of the duct would enable you to have a very nice divergence angle, you could massage a smooth direction change over the mount tubes, and the SKEET duct losses would be reduced. Oh, and you would not have to worry about the SKEET rubbing the mount. All good things.

I’m curious to see how the FIP seal on the plenum lid works out in practice. I’m completely dry with mine, relying entirely on the continuous piano hinge as the barrier to leaks. Props to you for going the extra mile. Hope it does not hinder maintenance.

 

[agent4573]

Thanks for the input. I'll try to address some of the concerns about why I did things the way I did.

The lid load isn't small, so good distribution is important. Figure dynamic pressure in psi or psf for some speed in excess of VNE, x .85 (if your pressure recovery is any good), x area.
Also, the cylinders vibrate independently. That's why the side baffle walls have a slip joint. A really stiff lid (carbon) will stress the fasteners near that joint.

I did separate the piano hinge between the metal baffles and the gap will be filled with RTV to allow the cylinders to be independent. If the carbon starts cracking in that area, I plane to cut a 1-2" relief notch in the carbon lid and fill with RTV so that at least the vertical portion of the vibration is independent. Not sure how anyone with a plenum can completely decouple the cylinders for vibration purpose, but carbon is extremely flexible, so hopefully this doesn't propagate cracks.

Recommend adding a bellmouth inlet to the SKEET tube at the engine side. That sharp transition is going to create some local turbulence that will effectively reduce the size of the tube. And with that much tube in use, you are going to suffer duct losses anyway. You might consider extending the oil cooler side fabricated duct up high enough to clear the engine mount tubes AND turn the corner. Then feed it with a much shorter and straighter piece of SKEET. The extra height of the duct would enable you to have a very nice divergence angle, you could massage a smooth direction change over the mount tubes, and the SKEET duct losses would be reduced. Oh, and you would not have to worry about the SKEET rubbing the mount. All good things.

I’m curious to see how the FIP seal on the plenum lid works out in practice. I’m completely dry with mine, relying entirely on the continuous piano hinge as the barrier to leaks. Props to you for going the extra mile. Hope it does not hinder maintenance.

The carbon diffuser going into the oil cooler can't be any taller or I would have to remove the engine mount in order to install it. I made it as tall as possible while still being able to twist and rotate it into position. I'm not willing to redesign it to be a multi piece part that's assembled in place, so I'll have to accept the skeet tube being as long as it is. The bellmouth is a good idea I didn't think about. I put a radius on the flange, but not a large one because it is edge distance limited to fit under the plenum lid while still being high enough to let the skeet tube clear the engine mount. I think I have a sticky vernatherm that's driving my strange oil temps, but if this doesn't cool adequately, I'll layup a carbon bellmouth and install it to see about increasing air flow through the tube.

 

[Toobuilder]

... I think I have a sticky vernatherm that's driving my strange oil temps...

Suggest you throw a viscosity valve in there and bypass the vernatherm function. That will identify if the vernatherm is the issue.

I started running the viscosity valve as my hot weather season prep, but now I run one year round.

 

[agent4573]

Final update. 99% done. I ended up moving the oil cooler again and going with a 4" tube to feed it instead of the 3.5". Turns out my mental picture of where the dipstick went was wrong, and it ran straight thru first 3.5" hose routing. This new run is a lot shorter as well. The baffles probably need one more round of RTV sealing to get every last hole, but I think this is a huge improvement over the same James setup. There's 2 pics down the inlet to show the difference between the SJ plenum inlet and the new inlet. The new one is about 3" taller as well, and it makes a huge difference in the volume for expansion. Haven't flown yet, but taxied it around to check for leaks and it's all good. The one oil line is pulled fairly tight, and I ordered a 2" extension fitting, but ordered a -10 instead of a -8 by accident so I'm stuck one more week waiting for that to come in. Overall, I think I have about 200 hours into this plenum and baffle.

 

[DanNiendorff]

taxied it around to check for leaks

Great work!

Another test method: Leaf blower…. you can feel everywhere for airflow and tighten things up if needed.