View Full Version : lots of cowl making questions (experimental)

03-03-2012, 01:12 PM
Hi everyone, looking for yet more advice:

I am starting to make the cowl for my experimental Vagabond. I am contemplating a few things.....

1) adding an oil door: My oil filler tube ends up at a perfect depth under the cowl for an oil door to be very useful. I like the convenience of the oil door idea as well as the reduced wear/tear on the main cowl door hinges/fasteners, especially on a windy day.

2) assuming I have an oil door; I am considering using camlocs/skybolts all the way around the "doors" (they would now be panels I guess!). some reasons: I would like to avoid the seemingly inevitable paint chipping off the hinges, wind catching the door/hitting me in the head, etc. unless i skip the oil door, my cowl doors would be quite large as the oil door location forces the c-channels/cowl door hinge line to be further inboard than usual... adding to the "flimsy factor" of the doors when opened. Part of this thinking is also tied to the next question:

3) we all put lots of effort into making beatifully sealed baffles... but the cowl door hinge lines would allow lots of air leakage out the top of the cowl. I'm assuming that the interior of the (upper) cowl is at a higher pressure than the topside (exterior) of the cowl. If that's not true, then there is no issue. Otherwise, i suppose this area could be sealed with a baffle strip attached to the c-channel/overlapping the hinge line... or eliminated by using idea #2 (above).

4) whichever way I go (doors vs panels) considering butt joints vs overlap joints at the bottom of the panel/door where it meets the lower cowl. takes care of a transition issue at the fore/aft ends & might prevent some paint scratching/scuffing. yes, this is extraordinarily nit-picky stuff, but that's how I build!

5) chafe strips- i'm thinking about using some UMHW Polyethylene tape (mcmaster carr, etc) instead of the usual fabric type chape strips... it's much thinner (many thickness options), comes with an acrylic adhesive and is also translucent in color. the "step" in my nosebowl which makes the lip to receive the cowl panels is pretty small.. the usual fabric chafe strip would result in the sheetmetal being "proud" of flush w the nosebowl. Any pros/cons of this (or similar) chafe material?

Wow, lots of rambling and overthink! All input appreciated.
Thanks everyone!

03-03-2012, 02:26 PM
I like the idea of a oil door. High end airplanes (jets) get
around paint chipping at the hinge by polishing the hinge rather
than painting. Stainless polished fasteners round out the look.
Hinge forward if possible in case it comes open.

Steve Pierce
03-04-2012, 09:51 AM
I like the stock Clipper cowl design with a door on both sides. I use Dzus fasteners but Camlocks would work well. I line the sheet metal with the edge og the hinge halves and haven'y noticed the chipping problem. The UMHW tape works very well. McMaster Carr has a lot of choices in widths and thicknesses.

03-28-2012, 01:58 PM
any advice on forming the lower cowl where it meets the boot cowl? it's a fairly tigt radius; to get the cowl to lay flat on the boot cowl would require some shrinking. All I have for a shrinker is the usual harbor freight variety.. works fine but would straighten out the bend (the jaws are 1.5" or so wide).

03-28-2012, 06:05 PM
I don't think you need a shrinker, the cowl on these have no compound curves, should be able roll that section over a large tube slowly bending it to get the radius of the bend around that lower firewall/bootcowl area. If you shrink the material you will be creating a compound curved surface and I believe you will run into more fit problems. That's how I did mine.

03-28-2012, 06:25 PM
at the bottom rear corners of the cowl, the intersection of the cowl and boot cowl create an angle. yes, at the forward edge of the boot cowl, the engine cowl will be in contact. However, as the cowl continues back for some overlap (1.125" for me), it creates a gap of roughly 1/8" to 3/16" at the greatest point. Look at lots of shortwing cowl pics.. lots of them show the gap I'm talking about; some do not. I'm hoping to end up in the "does not" catagory.

03-28-2012, 06:30 PM
Understand now, I guess mine has that small gap due to the angle you mentioned, never looked at it that close, next time I am under the fuselage will examine how much of a gap I see. A shrinker in that specific area should correct the gap but you might wind up having to do that piece several times if it doesn't go just right.

Steve Pierce
03-31-2012, 06:50 AM
I roll them on a slip roller and don't do any shrinking. To get it to fit absolutely perfect I guess you could do some shrinking but I have never seen a need since they look good to me as is. Might be good practice for me to make a compound cowl using my English wheel.

04-04-2012, 06:27 AM
Baffle time!
The cowl is now made; no shrinking, and I'd say it looks pretty good! Next on the list of things to think too hard about are the baffles. I've been reading a lot on the RV site about the front baffles and the importance/influence of sealing around the front of the engine (the "doghouse" baffle); but I see many many Lycoming powered planes without the "doghouse". Is it worth the time to do this, or are the more typical ramps, which end somewhere in the starter ring/crankcase vicinity just fine for our slower planes? Also learned from the RV site, ideally the baffle seal material for the leading edge of the baffle ramps would be attached to the cowl & lay on top of the ramp, allowing air pressure to seal the two together. But most of what I see on the ramp has the seal material attached to the ramp, making it possible for airflow to try to seperate the seal from the nosebowl. Any thoughts? I read lots about how slight baffle leaks can really impact temps, but then I see huge gaps around the starter ring, front inlets, etc.... Being my usual picky self & need some advice please!! FWIW I'm using a bearhawk nosebowl, Van's spinner, and firewall mounted oil cooler.

Steve Pierce
04-04-2012, 06:35 AM
I would like to install the dog house baffle on my Dad's Clipper and see what difference it makes since he knows the engine well and has an engine monitor. I wonder if the speeds of the RV makes gives more advantage.

Gilbert Pierce
04-04-2012, 08:23 AM
I would like to install the dog house baffle on my Dad's Clipper and see what difference it makes since he knows the engine well and has an engine monitor. I wonder if the speeds of the RV makes gives more advantage.

I've got the pieces, all I need is a sheet metal expert to put it together. Shall I send them to you?

Steve Pierce
04-04-2012, 09:06 AM
We need to do it next time you are here I guess since I would probably need to fit it to your existing cowling and baffles.

04-04-2012, 09:42 AM
We need to do it next time you are here I guess since I would probably need to fit it to your existing cowling and baffles.

Be interesting to see the temperature differences when you get there.

04-04-2012, 03:54 PM
Gilbert / Steve,
When you say "Dog House" baffel are you talking to just the front portion that blocks air leakage around the chank shaft / hub area? I was looking at my friend's RV-6 the other day, and he has the front blocked as well as a carbon fiber "Cover" that seals the entire upper portion of the baffel system, in effect, making a high presure plennum. The only areas for the air to get out is down through the cylinders, out the rear mounted Oil Cooler or the bleed air to the magnitos. The only cooling issue he has reported is while long taxi on warm days. The input ports to this plennum are maybe a third smaller that our typical nose bowl openings. It would seem to support the reduced drag / fast airplane concept, rather than just better cooling. Still, would like to see if you get better cooling on Gilbert's Clipper this summer, maybe Gilbert could report out on the success to the SWPC Convention here in Ogden in June!!
Enjoy the day,

Steve Pierce
04-05-2012, 06:43 AM
John, Yes it is just behind the flywheel/crank flange. Same thing the RV baffle kits have.