Empennage
It Begins...
My tail kit came today 10/23/2006 2 days ahead of schedule! The FedEX lady goes "high priced
airplane parts...you're building an airplane in there?" to which I reply "yep". My first impressions are
that the tail kit is surprisingly heavy. I guess that’s because I’m used to balsa wood RC planes.
I’ve been working on the practice kit for about 5 hours. I can see that the majority of the
construction time is going to be spent on visualizing how the pieces go together as well as measuring
the location of the rivets. Let me sum up the process: you measure, measure, drill, cleco, disassemble,
debur, dimple, scuff, prime, reassemble, rivet, repeat 13,000 times. So this may take a while, never
the less, its incredibly fun. After only about 40 or 50 rivets I am really starting to see some
improvements in the consistency of the rivets. Its neat to think that each piece will help to support
my ass thousands of feet above the ground.
Horizontal Stabilizer
The horizontal stabilizer spar strengthening bars are not prefabricated on the RV-3. Trimming them down to size requires a substantial amount of work if you don’t have a band saw. The hinge brackets are fabricated from aluminum L channel; you can see a before and after shot of them below.
Note: There is a potential gotcha regarding the location of the rivets between the inner hinges. The HS mounting bars that tie the HS to the fuse can interfere with some of these rivets if they are not spaced correctly. Take a look at the spacing of these bars on one of the fuselage drawings.
Im starting to notice that the overarching theme of the RV-3 vs the new RV's is "fabrication". A picture below shows the completed HS skeleton and skin; Its starting to look like an airplane!! I'd estimate 85% of the skeleton rivets are set by hand with a rivet squeezer, making for clean consistent rivets.
Skinning the HS is a tricky process because you cant see where you’re drilling. I would only drill two rivets at a time before I checked from behind to see if they were drifting off centerline. I started with the inboard rib, clecoing as I drilled, then I drilled the outboard rib, forward spar, and finally the rear spar. After its all put together it is disassembled, de-burred, dimpled, sanded, primed, reassembled, and riveted. You can see a picture of my shiny new HS in the photo album below. Unfortunately, I found a slight twist in the tip of the right side. After what felt like an endless debate, I've decided to redo the HS. I just would not feel comfortable with any twist, especially when I’m at 8,000 ft going 200mph. I guess this is to be expected, after all, it has been a steep learning curve.
Vertical Stabilizer
The VS is constructed in much the same way as the HS. I started with the rear spar, then the ribs, front spar, and finally the skins. The rear spar doubler must be trimmed down to fit in between the flanges; this is not obvious from the plans. The end result is a strong and straight VS
Rudder
Construction of the rudder is much different than both the HS and VS. The rudder skin is only 0.016" thick which makes it incredibly flimsy. Skin stiffeners are used to increase rigidity. Unfortunately, I’ve read that these skin stiffeners can induce cracking in the skins. To counter this problem I'v coated the bottom of the skin stiffeners in a layer of DAP Kwik Seal. This should provide enough damping to prevent any cracking around the skin stiffeners. The rudder spar has two 0.063" spar stiffeners that double as hinge/control arm mounts. The lower control arm/hinge bracket is kind of tricky to fabricate because the plans do not show the spacer between the control arm and the spar. The spacer needs to be the same thickness as the rib in oder to ensure that the control arm is level.
Bending the skin is also an important step that needs to be done right, an improperly bent skin can cause cracking and flight control problems. I used a simple handmade wood brake to bend the skin to the point where it would touch the spar on its own. After bending the rudder skin I built a V groove jig and mounted it on the H frame jig. The skeleton is then set into the skin and riveted in place. The final step is to bend the leading edge skins and rivet them together. THIS IS NOT EASY. I had one hell of a time getting the curve right, but in the end I think it came out alright. A 3/4" steel pipe was used to create the initial bend. Be sure to maintain pressure down and away from the spar so as not to make a crease at the spar edge. Despite my best efforts I ended up creasing the skin near my rudder tip, oh well. Aside from the fiberglass tips, the HS and rudder are now complete, check them out below.
Right Elevator
Construction of the right elevator is very similar to the rudder. The skin is reenforced with about 10 stiffeners just like the rudder. The skeleton consists of a spar, two ribs, bearing brackets, and a control arm. The control arm is made of steel and comes pre welded from VANS. The control arm must be ground down slightly around the edges in order to fit between the spar flanges. Because the control arm will be hidden in the fuselage, I decided to prime the control arm with the primer I’ve been using as opposed to powder coating it. The only advise I can give an RV-3/4/ or 6 builder is to BE CAREFUL WITH THE SKINS! They are very thin and can be dented if you blow on them or look at them the wrong way.
Left Elevator
The left elevator is a bit more challenging due to the trim tab. The basic skeleton is the same as the right elevator, except for the trim tab cable support and trim tab spar. The skeleton wasn’t too tough so ill concentrate on describing the trim tab. BTW, in the far right picture in the first row, the small L stiffener shouldnt be there.
The trim tab is first cut from the elevator skin. The skin becomes really flexible after removing the trim tab, so be careful. The trim tab wedge needs to be trimmed and bent into shape. I used some thin poster board to create a template; it only took two tries until I was satisfied with the fit. Be sure to account for the bend radius at the end of the trim tab. I used a hand seamer to make all of the bends on the trim tab. The trim tab spar is pop riveted to the bottom flange and flush riveted to the top skin and hinge. I wasn’t able pop rivet a few rivets on the flange/spar joint because the pop riveter wouldn’t fit into the small area, so used a few nuts to effectively extend the length of the pop riveter head. The tab hinge must be cut to length, but before you cut it be sure to leave some extra hinge pin length so you can attach it to some sort of safety device. You wouldn’t want the hinge pin to pop out in flight. Also, the rear elevator spar flanges may need to be bent a bit more to match the bend in the elevator skin. In other words, the spar flanges don’t sit flush with the elevator skin towards the tip of the elevator. I found this out after I riveted the rear elevator spar to the skin.
After finishing the elevator and mounting it on the HS I noticed I wasn’t getting enough down elevator deflection. To solve this problem I trimmed the HS spar flange and the control arm flange. The control arm is made of steel so this wont significantly effect the strength. Everything else should be pretty straight forward. Well thats about it for the empennage, aside from the fiberglass tips.
Fiberglass Tips
Im going to suck it up and blend the fiberglass tips into the aluminum skin with fiberglass and epoxy filler on my whole plane. I got a chance to see one RV with the fiberglass simply riveted on and one with the tips blended and feathered into the aluminum and I found that the feathered version looks much nicer. However, it is much much more work. OK here goes, I'm going to outline some of the tricks I used to mount the fiberglass tips.
I decided to reenforce the fiberglass around the rivet holes because the fiberglass does not look strong enough to prevent the rivet from pulling through. I cut some 5/8" wide strips to the length of the fiberglass tip out of 0.063" aluminum. Then I roughed up both the aluminum strip and fiberglass surface to create a nice bonding surface for the Epoxy. Finally, I cleaned everything with acetone and used two part Epoxy to glue the aluminum strip to the fiberglass. I think this should provide enough strength to support the rivets
I ordered a load of fiberglass stuff from AircraftSpruce.com.
1080-50 LT.WEIGHT 1.45 OZ INDUS CLOTH
01-00642 BID FIBERGLASS 38" RA7725
01-37600 STYROFOAM 2 LB 4" X 8" X 16"
12-00880 AVERY #30 X 3/8' CSNK CUTTER (NOTE: You need a spare countersink to use
strictly for fiberglass.)
01-24904 SINGLE-EDGE RAZOR BLADES
01-25008 11" ALUMINUM TEE-BAR
01-00299 RTY-2/G ROTARY CUTTER W/BLADE
01-14600 3M GLASS BUBBLES 1 LB.
01-14800 FLOCKED COTTON FIBER 1 LB.
09-15100 SURFACE TAPE DACRON 2" SMOOTH (AKA peelply)
01-00318 WEST SYSTEM 301/303 PUMP PACK
09-28250 POLY-FIBER SUPERFIL EPOXY A&B
01-08400 WEST SYSTEM EPOXY KIT B-2 SLOW ($100 a gallon!!)
Composites
Before I glass the empennage tips I’m going to practice a few techniques on my botched HS. Im going to perform 3 mini experiments to get a feel for the capabilities of SUPERFIL and fiberglass. I want to know how well SUPERFIL and fiberglass bond to aluminum. I scuffed the aluminum under the SUPERFIL with 220 grit sandpaper and the two fiberglass strips with 220 and 180 respectively. After everything dried I found that the 180 grit scuffed aluminum is the way to go.
I plan on blending the fiberglass tips with a 2 inch strip of fiberglass followed by some SUPERFIL. Im going through all of this trouble because I don’t want the paint to crack along the tip/aluminum joint. The fist step in the fiberglass process is to lay down some wax paper on your workbench. Next, you lay out a large patch of glass cloth. Then you mix the resin with the hardener and pour it over the cloth. Getting the appropriate ratio of epoxy to glass fibers can be quite tricky without pre-preg glass. Its easy to use too much epoxy, which is just as bad as using too little. This is where the dacron peelply comes in handy because the peelply absorbs the excess epoxy from the glass, theoretically leaving a decent ratio of glass to epoxy. The peelply goes on just before you sandwich this mess together with another layer of wax paper. The final step is to squeegee the epoxy evenly throughout the fiberglass, making sure to squeegee the excess epoxy off to the side. The composite is now ready to be cut out and applied. I used a pizza cutter style knife to cut out rectangles of this messy sandwich. Simply peel the bottom layer of wax paper off and apply to the aluminum.
Change of plans. I read somewhere that flocked cotton fiber and epoxy works well as a structural filler. Im going to use a moderately thick mixture of flocked cotton and epoxy as the primary structural filler, followed by SUPERFIL to clean it up. This should be much easier. The following pictures are pretty self explanatory. The final result looks amazing.
The end is near with respect to feathering the tips. I wont lye, its a pretty tedious job. Each tip required on coat of epoxy/cotton fiber and 3 or 4 coats of SUPERFIL. Right when I think its over I blow it off with the air compressor and a micro hole that was filled with dust shows up. This process has all but eliminated my desire to build a Lancair...on second thought.