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RV-7 N117TR

As long as I measured correctly it should work, I was surprised how far forward I was able to bring the inlets. I temporarily mounted the prop to check clearances, the inboard blade trailing edge is about 1 1/8 from the flat face of the cowling behind the spinner.

0DBF8A17-0FD6-445D-BF0E-131E4B805CCE_zpssockmahq.jpg


More clearance outboard due to the blade shape of the Catto.

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Exit area?

by enlarging the inlet are you also enlarging the exit area?
Most of the RV-s that have cooling problems do so because the exit
area of the cowl is smaller than the inlet, creating back pressure.

The appropriate ratio I believe is 1.17 to one in favor of the exit.
Thoughts?
Jim Frisbie
RV-9A, 450+ hrs.
 
by enlarging the inlet are you also enlarging the exit area?
Most of the RV-s that have cooling problems do so because the exit
area of the cowl is smaller than the inlet, creating back pressure.

The appropriate ratio I believe is 1.17 to one in favor of the exit.
Thoughts?
Jim Frisbie
RV-9A, 450+ hrs.

Well........shrinking my exit to create more pressure will be part 2.

I'm hoping the larger lower velocity round inlets in addition to the extra sealing of said inlets and the plenum will keep CHT's in the low 400's on a heat soaked takeoff at 90* and a full power 110 knot climb to altitude. At cruise I should have more pressure differential than needed to cool the engine, I'll need to shrink the exit area to create more pressure in the lower cowling to squirt the air out the exit with less cooling drag.

I measured my original inlet area at roughly 43 sq inches, 43 x 1.17 = 50.31 sq I inches. My original exit subtracting the exhaust pipe area was about 49 sq inches, I nibbled a bit off the end and made it 52 sq inches, both are in the ballpark of your ratio. I don't think cutting a larger opening would yield much of a result as my lower cowling pressures are already quite low.

After reading (several times and still not exactly understanding) NASA CR 3405, AIAA80-1242R I've gleaned these summaries and I'm hoping to gain from:

-Lower Vi/Vo (larger) more forgiving inlets, drag penalty from lower Vi/Vo is very small
-Annular inlets are more forgiving at higher Angles of Attack
-Better diffuser shape aft of the inlets
-Moving the inlets outboard and closer to the prop
-Better sealing with a plenum vs baffle material
-Better sealing of the inlets to the cowl/plenum
-Easier cowling removal and installation

But then again I'm an amateur learning by doing (messing up)!
 
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Most of the RV-s that have cooling problems do so because the exit area of the cowl is smaller than the inlet, creating back pressure.
The appropriate ratio I believe is 1.17 to one in favor of the exit.
Thoughts?

Yes.

There is no "correct" inlet-outlet ratio.

Cooling is a function of mass flow through the cylinder fins and oil cooler fins. That mass flow is directly related to pressure difference between the upper and lower cowl volumes. It doesn't matter what those pressures might be, as long as the difference exists. For example, assume a particular engine model needs 6 potatoes pressure delta across the fins to cool a particular power setting at a particular air density (i.e. altitude). Airplane A measures 10 potatoes in the upper cowl and 4 in the lower cowl. Airplane B measures 7 potatoes in the upper cowl and 1 in lower cowl. They will cool exactly the same. Airplane A will be faster, as exit velocity will be higher (less momentum loss), because it had a better intake (i.e. better conversion of the available dynamic pressure to static pressure).

Note that "backpressure" can be highly desirable.
 
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Good news is the prop flange bushing that I staked and used red loctite on seems to be holding, I torqued a socket over the bushing to 50 ft/lbs twice as a crude test.

Bad news is I didn't care for the way I originally blended the rings into the front of the cowl. I basically ran dental floss over the front of the ring and back along the cowl to find the intersection where blended in ring met cowling, the results were to sharp/square/pointy for my liking and didn't jive with the rest of the cowling. I sanded all the foam back off and ran some more tape to try to come up with a more pleasing shape, back to the mind rambling drawing board.

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I also made a little progress on my center baffling piece, I made my first real airplane part with my ancient slip roll. You can see the RTV line where the stock baffling was, I tried to extend it back for a longer diffuser. Now to dream up some attach brackets and electronic ignition flywheel sensor wiring armor. I read someone else's thread on protecting the wiring, mine could be better.

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Andy, the next weekend day with decent weather around here I'm coming to OWI and taking you for a much-needed RV flight. You've been down far too long!
 
I made a center line down the cowl, it doesn't look right but the inside lines are parallel to the center line. My eye likes the lines pointing toward the outside.

914B6341-DDAD-4BF3-A642-09435E79B947_zpstuo6uvse.jpg


In my first go I used the wife's hot glue gun to glue foam to the cowl, It OK but as the foam was sanded down the little parts without any glue would lift and flex. I think the hot glue gun would still work good to hold blocks of foam together as long as the glue didn't protrude into the area being sanded.

I cut up a Coors Light box and a Dr. Pepper box and used packing tape to stick it on the cowling inside out to hold my pour foam. The Coors Light box was slightly waxier/slicker than the Dr. Pepper and released from the foam just fine, I waxed one of them but couldn't tell the difference when I pulled them off. The pour foam is something else, mix it as fast as you can and it's growing in 20-30 seconds! It's ready to sand in about 30 minutes, too bad I have to go to work!

83B71166-15B9-4E44-A567-040E789670DA_zpsggz65w5m.jpg


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Second time is going much better, sand a little here a little there and then stepping back and looking. The side profile blends much better with the side now.

0778A82D-5658-4FE5-9142-F566D70CDBFB_zpsypmfmdz1.jpg


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The top and bottom portion are starting to mirror the round portion behind the spinner.

587A2C12-0B55-48BB-8025-0928A2D57C7D_zps2gv2hq15.jpg


I'm thinking a little deeper between the inlet and the flat area behind the spinner.

26A2E021-4AC0-4835-B711-EB93D7A0DE68_zpsty5ir4jy.jpg


More filler and shaping to go, but I'm liking this shape much more.
 
Andy,

Be sure the end result removes all the pour-in place foam. I had poor results any place I left some as core material. The stuff seems to be brittle and exhibit poor crush strength; given flight time, changes in atmospheric pressure resulted in lumps and depressions in the overlying glass.
 
Wouldn't it be easier to just buy a Sam James cowl?

Possibly, the thought had crossed my mind several times but $$ for the cowl/shipping, and the inlets are lower than I would like on the SJ Cowl. I almost bought the X-Large rings from SJ, but........
-$250 for the pair
-ID is only 5 7/16, I wanted larger albeit only a 1/2ish inch
-Inlet lip radius, I wanted a larger radius on the lip of the inlet
 
Andy,

Be sure the end result removes all the pour-in place foam. I had poor results any place I left some as core material. The stuff seems to be brittle and exhibit poor crush strength; given flight time, changes in atmospheric pressure resulted in lumps and depressions in the overlying glass.

Thanks Dan, yes removing the foam is on my radar. I plan on cutting the underlying portions of the stock cowling out and probably putting a few layers of glass on the inside of the cowl. I thought about using automotive body filler instead of the foam to shape things, but I think it will be much easier to remove the foam than the body filler.
 
That cowl split, beautiful!!

[
587A2C12-0B55-48BB-8025-0928A2D57C7D_zps2gv2hq15.jpg
]

Andy,
Would you mind describing how you got the split between upper and lower cowls so perfectly uniform? Do you have hinges installed down that split?
Not trying to hijack the thread!!
Thanks.
Johan
 
It's been a while but I think I used a 3 ft oak 1x4 with sandpaper to get the cowl edges straight. When I installed the hinges I made the split line gap pretty tight and went back with a small jewelry file and filed the split line to the thickness of the file.
 
I made some lower attach brackets for the curved center baffling, I used some heavier paper to make templates and get a basic shape. I'm still not sure how this is all going to fit together with the inlets etc so I'm leaving things long so I can trim to fit later.

56CA74B0-638F-4138-873A-936DF0EA309F_zps4lphw1sz.jpg


I filed a slight radius on a 2x4 and used it on my el cheapo harbor freight bending brake but with the radius of the curved part I found a clamp and my fingers did the job for .032.

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The lower 2 brackets fit on top of the existing Van's baffling, I plan on nutplates and screws I can hopefully reach with a long screwdriver behind the ring gear support.

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I must've got lucky on the first 2 brackets because I've already tossed 2 top brackets in the scrap bin and am dreaming up version 3. Good news is the plenum seems to fit well and is trimmed to the front baffle. I want to verify the clearance from the top cowling before I start drilling holes, to bad the top cowl is "tied up" at the moment.

7B951CDB-5B4F-45CA-821D-E58925C53DAB_zpsowlqa0by.jpg
 
I'm finally satisfied with the new shape of the cowling, I think. I bought a gallon of auto body filler to do the final shaping mostly because I could spread it and be sanding in 25 minutes, it was also cheaper but I missed the workability of the west systems. I did add some West Systems and flox immediately aft of the inlet rings. All the foam, filler etc is getting removed after the glass is on and cured.
My reference lines have mostly been sanded out, but they helped me keep things uniform as far as I can tell.

79A3E15F-3395-425A-8069-968EBF928B33_zpsligl210i.jpg


I tried to shape a bump aft of the new inlets to mirror the bump after the spinner as well as fitting into the lower cowling shape.

77002F0D-CCD6-4A60-9519-D0F498BA45B6_zpsj9vgoj0s.jpg


You can see the left inlet is taller than the right, I don't think it will be obvious on the airplane.

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Front shot, as another friend said "Those are kinda big...." Yes, yes they are.

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My latest version of the top bracket for the center baffling turned out decent, but I really should've drilled it on the drill press after bending. The predrilled holes are slightly of after bending, hopefully the next one will be good.

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//i1137.photobucket.com/albums/n516/crabandy/airplane/E3770EBC-496F-4DA4-9E60-66099B3637FA_zpsgelwqosi.jpg

I sprayed the filler/foam with some leftover spray paint and several coats of PVA and then laid up 2 layers of 8.9 oz glass.

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My flannel clad helper is finally old enough to start helping out! I drilled and clecoed an old yardstick as a saw guide and used a flush cut trim saw to cut the split line.

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Now to cut the old parts/foam/etc out.

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I really should've used more than 2 plys, the split line seemed a little weak so I added a couple layers of glass before I dig all the foam etc out.

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Way back when, Steve Smith (a highly qualified source) was good enough to point out that exterior shape around the inlet was critical to drag. A sharp edged pitot-type shape may result in a flow separation starting at the outer edge of the inlet ring radius. The above photo illustrates to point very well. Look closely at the exterior curvature of the original Vans inlet vs your new shape.

At the time, Steve said to look at the shape of modern fanjet engine nacelle inlets for guidance. I had seen similar advice previously, thus the exterior curvature below. Is mine optimum? Heck, I dunno, but the point is valid.



See posts 15, 18, 30, and 35 here:

http://www.vansairforce.com/community/showthread.php?t=68241
 
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Well, I was going for lips like Angelina Jolie.....

It's easier to see the differences in radius now things are split, I think the aforementioned picture really makes it look pointy and sharp. Here's a pic of the same inlet from another angle, it is a sharper radius than Van's but not by much.

B2E63610-268E-4BC0-9986-5FB8B926FF52_zpsp2ykfvdj.jpg


It did turn out flatter than I wanted and will massage the curves a bit. Besides it's just fiberglass, I can always grind it off and start on version 3!
 
Andy, had a opportunity to shoot a few detail photos this weekend. Some of the other photos you've seen may have fooled you; it can be hard to judge curvature. These were shot at right angles.

Outboard quadrant:



Above the inlet. Without a width constraint, I worked in a tad more curve:



I can't claim either one is perfect, but note the similarity to the Vans cowl, and to various low drag radial cowls:



My humble understanding is that your current sharp pitot shape will promote separation as the freestream tries to flow out around the cowl.

 
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My My My, it looks like I will also have some work to do on the SJ inlets. I have a little extra clearance to work with, but the original shape is sharper than Andy's.

Do we know how big the correct shape needs to be? Surely all scales of the same profile are not equal. Dan - I see yours is not as large as the original Vans shape. Any guidance for Andy (and others . . :D).
 
Does anyone happen to have a 3D CAD model of these various inlet shapes? I happen to have an unused CFD workstation here at home and may be able to play around with it a bit. It may take me a while to remember how to run it (and get the mesh set up) but I might give it a shot.

Come to think of it, a 2D mockup might be a better way to start...
 
Do we know how big the correct shape needs to be? Surely all scales of the same profile are not equal. Dan - I see yours is not as large as the original Vans shape. Any guidance for Andy

Nope. I just try to follow along when the smart people talk. The goal is to keep the flow attached, but as stated, I have no idea if my own is an optimum shape. It would be great if Dave (above) fired up that CFD tool.

Recall that we talked about stream tube diameter in a previous post. At 175 knots the stream tube would be about 3.2" D, and the rest of the air approaching the 6" inlet must flow outboard and around the inlet lip.

 
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DanH sent this to me in a separate email, it illustrated the point nicely for myself. I need to lower the AOA of the inlet lip.

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I re-foamed one side, that's all the pour foam I had. The good thing about the pour foam is it is ready to sand in less than 30 minutes and it sands easily. The bad part is it is easy to sand past the desired shape, and acetone doesn't dissolve the cured foam like the pink/blue insulation foam. It's as far as I'm going to sand for now, I want to split the cowl and see the curve from that view. I don't care for the aesthetics as much, we'll see. The outside of the cowl by the split line is gonna take some work to blend in the larger curve.

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The other inlet I stuck 1/4 foam window insulation to the cowl and then wiped a lip of epoxy flox/filler with a piece of bent/curved cardboard. I just did a couple inches in case I end up sanding it all off.

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I think the foam strip guided epoxy/flox/filler lip may do the trick, the lip radius is looking better and I think I can fair it into the cowl a couple inches back. I think using the foam as a guide will help to make a more consistent shape around the lip.

F4EA1645-5159-47F6-99D8-3B532E1241BD_zpsewqvmtsy.jpg


The fatter foam shape on the other hand retained the more pointy and less curvy shape, probably just me visualizing and sanding the same mistakes over again.

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I test fit the cowl to the airplane, the prop has 3/8 inch plus clearance on both sides from the front of the inlets. The new cowling shape opened up more room along the front of the plenum/cowl area, I'm still trying to visualize which path to take.

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Bill, I dug around on my hard drive, as I had forgotten what I was looking at circa 2008. My project prior to the RV-8 involved radial engine development, and I got hooked on the old NACA papers for info. Try this one for fundamental drag information relating to shape. As previously noted, a radial engine cowl is merely a low Vi/Vo inlet, and as such, a lot of the information transfers nicely.

http://naca.central.cranfield.ac.uk/reports/1942/naca-report-745.pdf



The focus is on drag rise due to compressibility, which is of no interest with our RVs. However, there is some good stuff starting on page 297....





....with more on 298.

Andy, there are ordinates, Table 1, page 289.
 
Bill, I dug around on my hard drive, as I had forgotten what I was looking at circa 2008. My project prior to the RV-8 involved radial engine development, and I got hooked on the old NACA papers for info. Try this one for fundamental drag information relating to shape. As previously noted, a radial engine cowl is merely a low Vi/Vo inlet, and as such, a lot of the information transfers nicely.
<snip>
....with more on 298.

Andy, there are ordinates, Table 1, page 289.

Thanks Dan, very timely, I was just searching my drives for that paper. I could have sworn that I saved it the last time you posted. This adds some dimensions, sorry for you having to do my homework.
 
Kuchemann and Weber

The first several chapters of Aerodynamics of Propulsion by Kuchemann and Weber covers intake design theory and experimental results and is the best guide I've found on the subject (in addition to the NACA and Cranfield papers). A smaller radius has less drag at a particular design point, but the larger radius has a larger range of effective operating angles (both AC aoa and Vi/Vo). My guess is that the drag saved from an optimally sharp radius over a fatter radius is minimal, but that the cooling drag/problems from an excessively sharp radius causing separation/turbulence at the head of the cooling system can be significant. For an RV-type airplane, I would err on the side of a fat LE radius lip, and then use adjustable exhaust outlet flaps a la Dan Horton to recover a bit of velocity.
The book is out of print but you should be able to find one at a good engineering school to make a copy.
 
I've been gone for a couple days, just so happened I was in a window seat just forward of the engine. When takeoff power was applied fog/vapor was forming around the inboard edge of the inlet until approximately 60 knts. I'm guessing the engine was sucking air from behind the lip until airspeed was delivering the correct amount of air, at that point it was spilling over the sides. Unfortunately I didn't get my phone out fast enough to take a pic of the vapor/fog.

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The NACA report DanH mentioned in a previous post used a 10.4 inch diameter cowling, my inlet diameter is 5.75 inches. My 6th grade math is solid, after that I have a general idea but I usually need the answer to work my way backwards through the math problem.
To get a properly sized curved lip for my size of inlets should I set up a ratio like: 10.4/5.75=1.81 inches to correctly size the curve to my diameter inlet?
If the diameter of the inlet is the "y" plane, and the distance behind the inlet is the "x" plane should I divide both the x and y distances by 1.81 to correctly size the curve for my inlet? I remembered this little conundrum only after ploting out the ordinates in the charts on some foam graph paper. Ordinate Chart from the NACA report (Wikipedia has a good explanation of "ordinates":)):
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Here's the ordinates from the chart graphed out on some cheap foam graph paper (Walmart), I graphed inlet curve 1, 2, B and C and cut out C since it was the lowest drag shape. Looks like my original shape was close to 1, and my newer lip shape fits C nicely up front. Being the ordinates are for a radial engined cowling that tapers much more than my cowling I have a little "customizing" to do. I think I can disregard the aft 1/3 of the ordinates and blend it into the existing cowling shape.

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Lip radius as originally shaped.

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Of course things seemed to have shifted slightly after making the cut, I tweaked things back in shape and laid up 5-6 plies to make a flange on the inside of the cowling to attach the hinge to. I'm still trying to figure out the best way to build more curve into my lips.

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In the spirit of progress and getting at least something done I switched to making "Man Glitter." Version ## (i forget how many brackets I threw away) is on I think will serve it's purpose. I added a doubler to the middle of the center baffle and used a bracket on each side of the middle.

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By some miracle the rear baffle bracket version 1 seems to fit well.

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I drilled and cleco'ed the plenum to the baffles and laid up the front flange of the plenum. I was pretty happy with how the front flange turned out, until I started putting my fiberglass supplies away and realized I used West Systems instead of AeroPoxy. Sigh, I was planning on adding more plies on the inside of the flange of the plenum anyway, I'll add a ply or two on the outside both made with AeroPoxy (if I don't grab my trusty West Systems again). It may turn out like a hot Ice-Cream sandwich, we'll see.

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I'm still dreaming up some protection for my EFII crank sensors.

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Hi Andy,

FYI - I plotted the same dimensions and noticed that was for a diameter of 10 or so inches. I was thinking about scaling the curve down proportionally to the diameter of the inlet. Once I did that it is not much different than the SJ original shape, a flat spot and progressive curve.

Oh - Really nice looking progress on that front plenum flange. Hot ice cream sandwich - I like that visual description. ;)
 
Bill, I dug around on my hard drive, as I had forgotten what I was looking at circa 2008. My project prior to the RV-8 involved radial engine development, and I got hooked on the old NACA papers for info. Try this one for fundamental drag information relating to shape. As previously noted, a radial engine cowl is merely a low Vi/Vo inlet, and as such, a lot of the information transfers nicely.

http://naca.central.cranfield.ac.uk/reports/1942/naca-report-745.pdf



The focus is on drag rise due to compressibility, which is of no interest with our RVs. However, there is some good stuff starting on page 297....





....with more on 298.

Andy, there are ordinates, Table 1, page 289.

Yes the study did use a 10.4 inch opening, I thought about scaling the curve down as well but not sure that it needs to be rescaled.....but I can make cases for both in my uneducated opinion.

I think the radius 1 is very similar to my original shape and that of SJ's cowl lips, maybe 25% ish more drag mirroring the same flat curve as lip B and C.

I'm going to let the ideas marinate for a few more days.
 
Crabandy - "I'm still trying to figure out the best way to build more curve into my lips.".

Botox???

Sorry I couldn't help myself......
 
I plotted the ordinates "x" and "C" on a piece of cardboard, I then took C/2 and plotted the curve underneath. I averaged/drew a curve between the two and made a template for my lip curvature. The new curve seems doable to shape into my existing cowling. I made a short and a long template, the short template shows the inner lip next to the spinner was already close to the right shape.

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My final version of the paper template for the ignition pick-up armor.

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I forgot my die grinder at home so the final curve was cut with a #30 30 times over.

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It seems pretty stout, the top bracket doubles as an attachment for an adel clamp holding the wires on the aft side of the baffling with a solid 1/8 inch clearance from the rear of the ring gear support.

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I think My fiberglass skills have progressed to the next level, I now do lay ups and spread epoxy/micro/flox in my regular clothes. I'll save the pants and shoes spattered with little crispy spots and dallops of micro for the next pro-seal session.
I made a couple of "custom" spreaders for my epoxy/cabosil/flox/micro mix, I bent a Popsicle stick under warm water and taped it to some cardboard to make a larger slightly curved spreader. I've found pop/beer boxes to be useful impromptu spreaders, the cardboard shapes nicely and epoxy doesn't stick to the outside painted/waxy surface.

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A couple of regular plastic spreaders cut/sanded to shape.

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Wow, Andy, it almost looks like it would be easier to make it in clay and make a mold. But - I see molds everywhere.:eek:

Great progress!
 
Wow, Andy, it almost looks like it would be easier to make it in clay and make a mold. But - I see molds everywhere.:eek:

Great progress!

Yeah, Dunno. I've thought about it but the inlets are in the right spot and the back half of the cowling is correct it's just the bits in the middle. I probably should've built a mold around the existing cowling while I was still flying with the old cowling. Sometimes I find my direction by wandering around.
 
I've spent mornings drinking coffee and evenings drinking beer, just sipping and staring at my cowling. I couldn't pass the garage without stealing a glance at my cowling peering at different angles, more filler here and sand more filler off there. After thinking several times there was no way to blend things together I think I about have it. My youngest helpers getting dirty with me.

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Top of the inlet showing the fatter lips, the template helped a lot to keep things uniform and determine filler addition/sanding.

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I think my next step is to layup the inside of the cowling, the new shape is just 2 plys of 8.9 oz glass sanded thin in spots and too weak to do the finish sanding on. Thanks to BillL's lead I ordered some lantor soric from Fiberglast, it's kinda like a cross between honeycomb and cloth (thinner and cheaper than honeycomb). It is like a dense thin mattress topper or dense quilting batting. It is made for vacuum bagging applications but the sales rep agreed it should work fine having epoxy brushed on and squeegeed out between a couple pieces of plastic. I made 2 test pieces, 1 with a layer of glass on each side of the soric and 1 with just the soric.

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Good process of experimentation, Andy. I was just joking about a mold, you are very close. Meaning, lots of tweaking and sanding, but close.

I, too, tested the Soric for layup with some test strips. Making several test strips (2" X 8") with different resin application methods showed that if both cloth and Soric are wet, then the shear bond is very good. One strip got the edges removed to reveal the core and rely on just the shear strength. It was very strong and stiff with a 1-core-1 test piece. I was concerned about debond or shear failure in the core, but it held nicely. No quantified test results though :(.

Use a roller and a final layer of peel ply to get an even resin content. Your body working skills will make short work of the final plenum piece. You will be in the plenum business in no time.

Spring is on the way!
 
Bill, I was sure hoping you were joking!!

I couldn't believe how stiff/strong the soric sandwiched between 2 layers of glass was, not sure how to quantify it.
My first full half day I've had to work on the airplane in over 3 months summed up in 1 photo. The existing cowl/honeycomb was cut/sanded at an angle, a fiberglass soric sandwich was placed in the newly shaped areas with an epoxy/flox butter evening out the seams and an extra layer of glass over the seams topped with peel ply.

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soric (aeromat)

Great stuff. I made my plenum out of it as well. Nothing else I tried worked as well on compound curves like this. It may be a bit heavier than traditional honeycomb but the ease of working with it and the strength of the finished part is worth it.
 
The ol' yard stick straight line sawing jig and flush trim saw did another decent job making the split.

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The top of the lip seems to be fine, fat enough and blends in with the rest of the cowl.

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The outboard side of he cowling on the other hand has the fat lip than quick transition to a "pitot" tube.

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I think inlet is placed fine vertically but needs to be moved inboard 3/8-1/2 inch, moving the inlet inboard would give me the geometry to work more curve on the outboard side like this:

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As it sits now I think it's too flat to easily insert the horizontal hinge pin holding the cowling halts together. It's also a little heavier due to some extra glass and filler. I'm going to sit on it and ponder.

BillL or DanH do you happen to have a dimension between the center of your round inlets?
 
SJ long cowl, RV7, 23 15/16" or 1/32" less center to center distance for inlets. The design dimension could have been a 24.000, but my glassing of the inlet rings is off that much.
 
Thanks Bill, mine are pretty close at 23 5/8. Hmmmm, more coffee sipping staring and thinking.

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If I had it to do over again I probably would move each inlet about 3/8" inch inboard for more outside curve, but I'm moving on with the existing inlets. I carved out a spot to nestle in the forward section of the new hinge, it helped with the angle for inserting the hinge from the front. I followed the Van's instructions on drilling extra holes and aggressively sanding the hinge flange and epoxying it to the cowl, the old hinges were not letting go virtually welded to the cowl. I used the hinge pin and an extra piece of hinge to position the new hinge with the existing hinge. After the epoxy dried I riveted the new sections.

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I really should've waited to make my plenum until after re-shaping the cowl, oh well it's just fiberglass. The new cowling shape allowed for a slightly larger rounded opening up front, I thought about laying up a new plenum in the cowl but would be left with the flanges fitting the baffling problem again. I decided to shape some pour foam on the top to sand to shape. I laid the existing plenum in the cowl and measured a rough arc prior to pouring the foam.

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Sanded to shape laying in the top cowling, about 1/8 shy for extra wiggle room.

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I laid 2 plys of glass over the newly shaped foam. I poured more foam over the top of the plenum as reinforcement to hold the shape as I cut out the old fiberglass and foam.

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Inside of the plenum sanded and ready for more glass, I cut the soric and glass but I'm waiting on more epoxy to finish it.

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I also spent some more time sanding things to shape on the outside of the cowling at the juncture of old vs new cowling, a little more filler here and sand more off there....I think my main problem was trying to shape the curve with my larger 12" sanding block, it was sanding all of the curve out and I needed a smaller 4-6" block to shape the curve.
 
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