The Concept |
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These are pictures of a prototype cowl and plenum installed on Dick Martin's
RV-8 and pictured at AirVenture '99. |
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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. |
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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. |
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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. |
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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. |
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The
Plenum |
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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. |
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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. |
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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. |
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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. |
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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. |
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The
Cowl |
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Cowl construction begins. Here
I've taped the top half in place, and positioned it vertically relative to the spinner,
for a preliminary look. |
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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. |
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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. |
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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). |
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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. |
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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. |
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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. |
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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. |
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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. |
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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? |
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Intake scoop now getting fairly
smooth, front mounting flanges in place. Time to start priming soon. |
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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. |
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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.
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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.
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Finishing
touches & mods
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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. |

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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. |
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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.. |
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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! |
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Final
thoughts |
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.
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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
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Skybolt
Web site: www.skybolt.com
Phone: 800-223-1963
E-mail: info@skybolt.com.
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