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ricksked(at)embarqmail.co
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 Posted: Mon Feb 25, 2008 1:03 pm Post subject: Fitting the cowl tips and the 1/4 turn fastner |
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Fitting the cowl..
some of this is cart before horse but I tried to keep it in some semblance of order
I hope to help others who have not come this far...I have spent the last three weeks getting the cowl to fit properly...maybe I'm being to anal but this is a very cosmetic piece so I hope to help everyone achieve the same fit, maybe you already have. First off I am using mil spec 1/4 turn fasteners on the cowl sides, and along the top cowl firewall side. They are spaced at 4 inches around the firewall, 3.5" at the top to bottom cowl seam. I used hinges along the firewall the firewall for the cowl sides, only because I had already mounted the hinges prior to bolting on the engine mount.
TIP #1 if you are mounting ether hinges or flanges for 1/4 turn fasteners, do so prior to mounting the engine mount...not enough readily accessible space to buck the rivet prior to installing the mount.
OK....I can not emphasis this point. You need to get the forward part of the cowl fitted first...the inlets, the spinner diameter all that stuff you need to get right first. You may need to trim the top cowl to allow getting the nose ring to fit and the bottom to slide up over the top. The bottom cowl over laps the top cowl on the sides. Sand and make a very straight line on the bottom cowl now....it will be the guide for marking and cutting the top cowl when that time comes and the material seemed thin on the very edge, you will need to straight, blunt edge. Fit your top cowl as best as you can before even bringing the bottom cowl, left right, up down get the top as close to perfect before going any further, including the spinner gap. Get two sanding bars/blocks. I used a 12" and a 30" The 30 was fitted with 80 grit paper and the 12 has a coarse Carborundum metal backed abrasive plate. The edges sand very easily...too easily if you get aggressive. Once you mount the cowl bottom to the top on the aircraft you will notice that the back over hangs the firewall quite a bit. Trim the bottom to allow it to fit flush with the bottom. Beware of the flange interfering with the flush fit. I had to cut a portion of my bottom mounting flange back about 3/8, just make sure it does not hit the honeycomb. The masking tape method previously posted worked well for cutting the initial line on the top, bottom and sides. I still did not end up with a perfect flat nose ring once the cowl was fitted. To fix that I shorted the cowl about 3/16 to allow me to build up the nose ring with micro/epoxy. Then sand it for a perfect gap. Me engine has been mounted for 6 months so I don't fear sag to much at this point.
OK...1/4 turn fasteners. I recommend using .063 for the flanges. Make sure they are at least 1-5/8 wide...Once you scallop the flange, it bends fairly easily into position. The reason I don't recommend anything lighter is the holes that must be made to mount the receptacle. The receptacles are mounted with -4 rivets, which just barely make edge distance from the middle receptacle hole...that hole is 11/16. The distance for the edge of that hole is very close to the edge of the flange, so don't go with too thin of a flange. I had all my flanges clecoed in place for fitting the cowl. Once the cowl was fit very tightly to the firewall I drilled all the fastener locations with a #40 and clecoed, checking fit after each hole. Did the bottom first, using #8 screws there but drilled and clecoed non the less. Then did the top cowl, firewall side then taped and strapped the two halves to mark the side cuts. Marked the top cowl using the bottom as a guide and cut near the line then sanded to the line with the long sanding block. Made up the side flanges with 1" -3 rivet spacing, matched drilled them to the bottom cowl, clecoed and reinstalled the cowl...side seam was perfect but tight, that's a good thing. Now I drilled the top cowl/side flanges while the assembly was secured to the aircraft with tape, straps and the neighbors small kid who worked underneath  . The trick is to get that whole shebang drilled and clecoed to the flanges. OK...tip coming! remove your flanges (mark them) then go over to your trusty drill press because your gonna drill a boat load of holes. Open the #40's to #30 and use the drill jig provided by Milspec to drill the rivet holes for the receptacles. Then open the center receptacle holes with a unibit to 11/16" that's a big hole and lot of chips. Then countersink the rivet holes for a flush -4.
TIP!!! prime and install the receptacles now, don't wait till the strips are riveted to the aircraft. Once the receptacles are on, run them in on a best guess depth, use one of the fasteners to get close, all of mine were to shallow. Do this before mounting the strip...just by attaching to the cowl while it's on the bench. Then rivet the flanges to the aircraft, mount your cowl and see how it all fits..you will have to adjust (read bend down) each scallop to allow the outer edge of the cowl to fit flush...actually do it as soon as you do your first fitting of the flanges and cowl.Then final adjust your depths. I used the snap ring tool and the pliers from Milspec...right tool, right job...they worked perfect. Don't skimp on the clecos...use them in every hole.
Final fitting...now that everything is installed with the fasteners, I removed the cowls and using masking tape as a guide (OK, anal here 3/64" material showing to be trimmed off, maybe a little more) I removed the small amount of material from all the edges to allow for that small line that will be occupied by paint. By waiting to do the final trimming now, you just push each part to the firewall snug each and every time during the fitting process.
Would I use them again? Yup...Cons..they ain't cheap, took me about 60 hours to totally fit the cowl and install the fasteners, not including the oil door, louvers etc.. Once finished I am going to remove the metal grommets in prep for paint.
Put your power tools up high and ignore them while doing this, the one exception is the Dremel, I used it to cut all of my cowl exclusively with there quick change made for plastic cut off wheels. Easy to control and very straight smooth cut. I hope I helped all of you thinking about this method. Thanks to pics of Vic Syaracuse, Randy's RV-3 works and a close examination of Wayne Edgerton's for helping me through this whole thing. Now on to the wheel pants & fairings...they are easy right?
Rick Sked
40185
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acs(at)acspropeller.com.a
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| Posted: Mon Feb 25, 2008 7:38 pm Post subject: Fitting the cowl tips and the 1/4 turn fastner |
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Rick fantastic write up.
Valuable stuff indeed!
Any photos?
John 40315 (cowls)
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