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Cowl & plenum
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Copyright 1999-2005 by Randy Lervold, Romeo Lima Consulting. All rights reserved.

On this page:
The Concept
The Project Begins
The Plenum
The Cowl
Cowl fasteners
Finishing touches & mods
Heat damage
Final thoughts
Supplier contact info
Until this point in the project I have made only minor deviations from the Van's plans. I decided however to stick my neck out (gulp!) and go with an aftermarket cowling and engine cooling plenum. These pieces are from Sam James who is well known in the RV aftermarket for both his fiberglass parts, and his painting and fiberglass videos. The plenum and cowl are integral to each other, so I consider them one system. My interest in going this route is the inherent efficiency increase of the molded plenum design: it should cool the engine more efficiently on less air, put less stress on the cowl by not using it as a pressurized part of the plenum, and provide a speed increase in the process. We'll see if this is actually achievable. As you will read below, the story also includes an alternative cowl fastening system as well.

(click on a pic to see a larger version)

The Concept
Cowl&Plenum1.jpg (433392 bytes) Cowl&Plenum3.jpg (498029 bytes)
Cowl&Plenum2.jpg (522691 bytes) These are pictures of a prototype cowl and plenum installed on Dick Martin's RV-8 and pictured at AirVenture '99.
Cowl&Plenum4.jpg (86187 bytes) Cowl&Plenum5.jpg (89587 bytes)
The Project Begins
After further investigation I have learned that this cowl as pictured will not work on my plane. I have an O-360 A1A which is carbureted. Dick Martin's plane depicted here, and apparently every other plane that has used this "holy cowl" style of cowl has also been injected. The carburetor, with the Van's filtered air box (FAB) or any other sort of air box, protrudes down below the level of the fuselage. This requires a scoop such as the standard Van's cowl has to house the FAB. The carburetor protrudes down so far that there is less than an inch of clearance between the bottom of the carburetor and the cowl... not enough room for ANY sort of air box or elbow. Therefore Sam will make the cowl with no induction inlet at all. I have arranged to buy the intake scoop portion of the Van's cowl separately which I will then graft onto Sam's cowl.
CowlParts1.jpg (83471 bytes) The cowl and plenum arrived on time from Sam. He sends them air freight so that they don't get damaged. In the upper right is the customized bottom half with the injection intake blanked out. In the lower left is the induction scoop for the standard RV-8 scoop that M&W fiberglass made special for me (great people!). Then in the lower left is the James plenum. He provides you with plans for aluminum side pieces to go with this, or you can just use the Van's Baffle Kit.
CowlParts2.jpg (73814 bytes) Note the rough surface where he modified the mold. This will be easily smoothed out and is not a problem. Sam makes the cowl with vinylester resin. This cowl is actually slightly lighter than the Van's S cowl which is epoxy pre-preg. I will need to reinforce the edges because it is fairly thin, but the surface will be much less work to get paint-ready than the very porous standard units.
CowlParts3.jpg (69034 bytes) You're looking at one of the tasks to be accomplished... making the stock induction intake scoop fit the lower cowl. The problem is that about the forward third of the cowl has an entirely different contour than the stock cowl, which the induction scoop is made to fit. Making this two work together will be challenging.
The Plenum
Plenum1.jpg (123730 bytes) My plan for the plenum is to use the standard Van's baffle kit for the sides and back. Then Sam's trick plenum piece will be formed to fit the front of the engine and attach to the aluminum sides and back with nutplates and screws. The assembly sequence will be to completely fit and assemble the baffling, plenum, and oil cooler, then mount the prop, and finally begin on the cowl.
Plenum2.jpg (142235 bytes) You can see how much lower Sam's plenum sits - it will barely clear the spark plug wires. I have already cut quite a bit of height off the Van's baffle, more to come. Fitting this thing has already required lots of trial and error.
OilCooler1.jpg (106173 bytes) Let's see, the oil cooler should go right about here. Sam James reports that a 15 degree angle seems to flow the most air.
Mvc-152l.jpg (115173 bytes) Ok, brackets fabricated and everything in position. The only problem is that the upper corner of the oil cooler will now come very close to hitting the cowl, as will the AN822 fittings. In this position the cooler cannot be reversed and have the fittings clear the engine mount. Back to the drawing board.
Mvc-424x.jpg (189185 bytes) New design I used two battery brackets to fabricate a non-tilted design that provides plenty of cowl clearance and also allows the oil line fittings to clear the engine mount coming off the inside of the cooler. Problem solved.
The Cowl
CowlBottom1.jpg (61585 bytes) Cowl construction begins. Here I've taped the top half in place, and positioned it vertically relative to the spinner, for a preliminary look.
CowlBottom3.jpg (63478 bytes) Now you see why I have to graft on the standard carb/FAB scoop... the carburetor is so low it won't even clear the cowl much less allow an airbox to be attached. I had to cut this hole just to do the preliminary fitting.
CowlSpinnerGap.jpg (70733 bytes) If you look closely you can see that the preliminary gap between the cowl face and the spinner is not at all parallel. I'll need to do quite a bit of adjusting here. It turns out that Sam's mold for the RV-8/O-360 cowl was warped. he shipped five of them this way including mine. It was some time before this was discovered and I went to quite a bit of trouble to fix it as you will see below. To his credit, Sam replaced all of the other cowls, but mine was fixed and finished before he made the same offer to me. He reports that all molds are now straight with good clean parts coming off.
Mvc-219l.jpg (64523 bytes) Mounting this cowl has been especially challenging compared to the stock Van's cowl because there are really no surfaces to use as a reference in fitting all the others. First off I had to cut off the flange behind the spinner on the lower cowl half because it was too far forward and also warped so as to not be parallel (see above explanation regarding warped mold). I will glass another one on along with some support flanges for the joining area between the inlet holes and the prop hole. You may note that I have decided to go with hinges on the horizontal joining line rather than Camlocks... simpler, and no concern with the glass scalloping between fasteners. I will exit the hinge pins to the front with some sort of tab (haven't figure that out yet).
Mvc-225l.jpg (59342 bytes) You'd think that mounting the intake scoop would be easy, but the contour of the new cowl is different than the flange on the cowl. Therefore quite a bit of work went into cutting and fitting and heating the cowl to get the two pieces to match contours. Then they must be attached permanently...
Builder's tip: Be aware that your carburetor is NOT centered on the engine, but rather offset to the aircraft's left. therefore the FAB has a tendency to hit the left side of the intake scoop. Since I had to mount mine I offset it to the left approximately 3/8" to compensate for this. If you have an older cowl check the alignment of the FAB with the scoop from both the front and the back and adjust accordingly.
Mvc-226l.jpg (61071 bytes) The first step was to glue them together with West Systems epoxy and cotton fiber (flox) while clecoed and fitted in place. Both surfaces will be sanded smooth and then 9 oz. glass laid up to form a permanent (and hopefully strong) joint. I then used a disc grinder to feather the edges both inside and out, then laid a single layer of cloth about 4" wide over all the edges. Over this goes Poly Fil and LOTS of sanding.
Mvc-234l.jpg (90839 bytes) Now I need to create a flange behind the spinner to replace the one that had to be cut off. I also created a flange behind the solid area between the intake scoops and prop hole that will eventually have platenuts riveted on so the upper and lower halves can be screwed together in this area. Sam does not provide for this, and I wonder how the cowling would stay together without some such method. 
Mvc-235l.jpg (69460 bytes) Here's a closeup of the prop hole flange layups. I'm no fiberglass wizard, but if you take your time and think carefully about the structure it can be done. Here I'm using clear packing tape as a form to build the flange up. Even though the glass/resin is on the adhesive side it peels right off when done.
Tech tip: You may wonder about using epoxy to substantially modify a vinylester structure. Not only did Sam James tell me this was ok, but I called Gougeon Brothers, makers of the West System, and talked to their tech staff just to be sure. Since epoxy and vinyl ester are of the same chemical family they have similar properties with regard to expansion/contraction. They said that when grafting onto a cured vinyl ester structure, that epoxy adheres better than using vinylester resin. Therefore use the West System when glassing anything other than polyester, and even then minor repairs are ok.
Mvc-260l.jpg (71175 bytes) Progress! Here I've just fitted the aluminum cooling inlet pieces... lookin' pretty cool. At this point they are not supported so I'll need to figure out some support from the top and bottom to lock them in place. 
Mvc-263l.jpg (78942 bytes) Note the new lower prop hole flange... nice and parallel with the top one. Did I mention that there's lots of sanding to do? 
Mvc-270l.jpg (58214 bytes) Intake scoop now getting fairly smooth, front mounting flanges in place. Time to start priming soon.
Mvc-362x.jpg (76981 bytes)

It looks like this piece just magically got finished and painted. Not! Many months, lots of sanding, priming, and more sanding were needed to get to this point. Frankly, I never thought I'd get all the pinholes and irregularities out. Shooting the paint took so little time compared to the rest of the work that it was kind of anti-climatic. The PPG K38 filler/primer that I used worked really well, I highly recommend it... couldn't have gotten a decent surface without it.



Cowl fasteners
In assessing the RV-8 cowl before I started I just never liked Van's attachment system which uses hinges and requires the removal of several pins to get the cowl off the firewall. After talking with lots of RV builders, watching the difficulty in removing the pins, especially on the firewall, and reading lots of stores of failed hinges due to wear and tear, I looked for alternatives. Certified aircraft generally use some sort of "quarter turn fastener" of which there are several types. As I studied the options it looked like the Camlock 4002 series was the best of the bunch. A company in Florida, Skybolt, turned out to not only be a good source for these fasteners, but they had developed their own version of the 4002 fasteners that had some real improvements. Still, using hinges had some benefits: no visible fasteners, cheap, and a continual attachment. In the end I went with a hybrid which I feel provides the best of both worlds... camlock type fasteners around the entire firewall perimeter, and hinges on the horizontal cowl-half seam.

. In the final analysis I'm very glad I went this route and recommend it highly. Skybolt has a very informative catalog, as well as a great product. I recommend them as a resource. (see Supplier contact info for more information on how to contact them)

Several builder's have asked for the specific part numbers, so here's what I used for my application:

Grommets: #SK-018S
Studs: #40S5-1S
Receptacles: #SK245-4
Grommet retainer clips: #R4G

Then you'll also need the installation tools and the drill templates. Get their catalog and check their web site and you'll understand what these parts are and how they work. Suffice it to say, they work very well.


Finishing touches & mods
MVC-397X.JPG (123136 bytes)
MVC-398X.JPG (121112 bytes)
First you get the plenum installed, then you get the cowling fixed up finally, then you put them together... and they don't line up! Great. I called Sam and an hour later he called me back after checking the 180 hp plenum against the cowl mold... "yea, the frickin thing doesn't line up!"  Sam volunteered to modify his mold if I would just give him the measurements on which way the inlets needed to move to be centered. I sent him the measurements and also some clear pictures of it so he could see for himself. He layed up a quick & dirty temporary plenum out of polyester and sent it to me. The left side was very close but the right side was still off. I phoned these changes to him and he made up a new hi-temp epoxy version for me. 
MVC-499X.JPG (134197 bytes)
Mvc-500x.jpg (131046 bytes)
When this new unit arrived I eventually installed it and discovered that while the spacing was correct at 24" center to center, both inlets were shifted to the right as you can see in this photo. Of course the problem with this is that the air will catch on the edge of the inlet adding unnecessary drag and possibly dislodging the rubber connector. The plane was now ready to fly so will fly it with this plenum and we'll see how it works. I sent photo enlargements and a letter to Sam. He made yet another modification to the mold and made yet another plenum. This one should line up perfectly... 3rd time's the charm.
Mvc-515x.jpg (89101 bytes) Just to make sure I don't get any damage to my cowl I put some of this adhesive-backed thick aluminum foil that another Home Winger got at Boeing Surplus (thanks Gary!). Many builders have asked Gary for the source so he finally tracked it down. The product is now stocked by Van's Aircraft..
Heat damage

Well, in spite of thinking I was adequately protecting my cowl from heat damage I incurred some damage due to heat. If you look closely at the picture with my initial shielding you'll see that I didn't extend it back all the way to the aft edge thinking that all that air rushing out the exit would keep things cool. Wrong. The joint where the induction scoop and cowl were joint incurred heat damage at the aft edge. Even though the paint on the inside of the cowl showed now signs of damage, the resin and filler in the joint were burned and the shape of the joint was changed just as if it had been treated with a heat gun. The repair was significant: grind out the burned area, re-glass the joint, sand/prime/paint lower cowl. Lesson learned: thoroughly protect the inside of your cowl!



Final thoughts
Mvc-521x.jpg (144142 bytes)There has been quite a bit of interest in this cowling and plenum, and many builders have asked me "was it worth it?". Tough question. Let's start by looking at possible motivations for this endeavor:
  • Reduction of cooling drag (due to the increased efficiency of smaller openings with less air volume traveling through the cooling system, and more even distribution over the cylinders).
  • Reduction of wear & tear on cowl due to it not being pressurized and not mechanically coupled via the baffle seals.
  • Possible reduction of drag from the exterior cowling shape (who knows, it looks sleeker).
  • Distinctive look.

In the final analysis I cannot answer this with a definitive answer, rather I have mixed thoughts/feelings on it. Since my cowl was warped and I had to rebuild it, and since my plenum(s) didn't fit, and since I had to graft on the intake scoop, and since Sam doesn't provide any mounting flanges, I had lots of extra work to do, not to mention the consternation and headscratching required to figure it out. When I think back to all the glassing and sanding and filling this required it makes me think it's not worth it. However when I now look at how good it looks (at least to me!) and how well it seems to work (plane is 4 mph faster than Van's numbers in top speed and cruise crosschecked with GPS groundspeed, my cylinders cool fairly evenly, and the cowling now fits very well and is relatively easy to remove/install) I'm kind of glad I went to the effort. That said, I estimate it added at least four to five months to my build time. It should be noted that Sam has since corrected his molds and you should not have the fitment problems I did. Sam will definitely stand behind his products. Even though I had the problems I did Sam was understanding and helpful along the way, although it's tough when you're over 3,000 miles away from him. If I had bought my parts six months later than I did I would not have had several of the problems.

Performance? Since I did not fly the plane with the standard cowl I have no way to know what is responsible for the apparent small performance increase. Who knows, maybe it's the engine Bart Lalonde builds a very nice engine as many other builders will document. In the final analysis everyone needs to make their own decision here. I have tried to accurately and objectively record my experience with the hope it will be of some help to you. Bottom line, if you're willing to take on a bit of extra work, if you like the concept of a closed plenum, and if you like the look, then you should probably do it. If you're just looking to get into the air quickly then no question the standard cowl is the way to go.


Supplier contact info
For anyone interested in contacting the suppliers I used:

James Aircraft
12185 Schooner Lane
Moore Haven, FL 33471
(863) 675-4493
Web site: www.jamesaircraft.com
E-mail: will@jamesaircraft.com

Web site:  www.skybolt.com
Phone: 800-223-1963
E-mail:  info@skybolt.com.