Cessna 120-140 Association

Howdy All,
Just received new skins from Northwest Aero and am building the fixture to hold the rudder as I reskin it. There seems to be very little information on the web. Any advice appreciated. Will take pocs as I progress. Thanks! Jim

My C140 project came with the rudder already re-skinned, but I had to re-skin every other control surface (flaps, ailerons, elevators) due to numerous patches, excessive corrosion, and even elevator spar cracks. As you did, I got my skins from Northwest Aircraft, and Northwest even fabricated new aileron spars for me. I got my elevator spars and hinge reinforcements from Univair, which at the time were some of the more affordable parts.

I think some of the experiences I adopted in re-skinning control surfaces may be transferable to the re-skinning the rudder, and I enclose some photos to illustrate.

1. Make fixtures to match drill and hold your parts
2. You may be able to match drill your old skins to the new skins. Match drilling has worked very well in every instance that I have used it. If you are reusing the rudder leading edge spar, you will need to match drill those holes to the new skins, and the best way to do it is to use the old skins to get alignment. If the skins your got from Northwest are oversize a bit, then trim to match your new skins. A Dremel tool can work if you are careful and it won't deform the corrugations. On straight skins, we cut them to length with an aluminum blade in our table saw.
3. If you can't match drill for some reason, then you are going to need to fixture the skins to align the right and left trailing edges being careful to align the corrugations. We always drilled the mating skins together at the trailing edge first. This worked well to ensure the leading edge would line up. In your case, if you are reusing your rudder leading edge spar, if you set the trailing edge holes first to align the corrugations, then you are going to have to transfer the LE spar holes to the skins using back markers or a strap duplicator. I like the back markers you can buy from Aircraft Spruce, and because the rudder LE spar is almost enclosed, I doubt you can get a strap duplicator in there. AC Spruce used to sell back markers for 3/32 and 1/8 rivets, but the fellow who made them passed on I think. Now, there are limited sizes. PNs 04-00124 and 04-00129 might be close enough to modify the shanks to fit 3/32 and 1/8 rivet holes if you can grind them down a bit. Back markers are used in other industries too, and I found some threaded ones for #6-32 screws on Amazon.
4. When transferring the leading edge holes from the spar to the skins, make sure the edge of the skin is parallel to the spar and clamp it to secure before you tap on the skin with a mallet to set the back markers. Keeping everything straight is essential to a good end result. Spend some time with your fixturing to ensure this.
5. Although we drilled the trailing edges of all of our skins first to ensure corrugation alignment, we riveted the trailing edges last because often their were internal ribs that we had to rivet, and the only way to get access was to roll the skin from one end to the other to make it possible to get a bucking bar inside.
6. Plan ahead on your riveting to ensure that you end up using the minimum number of 1/8 blind rivets, either NAS1398 or Cherrymax. I like the NAS1398 "A" code, single action, blind rivets because they are all aluminum, and the universal heads are not as big as Cherrymax. You won't find NAS1398 rivets at Aircraft Spruce, but you can get some limited lengths from Rich Industries, Inc. in Arizona. Cherry also makes these rivets as CherryLock A. It is important to get the A code rivet because the puller is less complicated. The non-A code rivet uses an expensive double action puller. Even for the NAS1398 A code rivets, you'll need a special air-driven puller because the stem lock is external rather than internal like the Cherrymax type. Olympic makes the pullers as Allfast RV50GB with puller head RV812-4. I bought mine surplus on eBay for $50, but since then prices have gone up dramatically for these surplus tools that typical come from aircraft companies who auction off their used tools. The cost of new tools is prohibitive, and I recommend the single action A code rivet tool since it is much less expensive than the non-A code rivet, albeit the A code rivets are harder to find than the non-A code rivets. If it is too much effort to get tooled up for NAS1398 rivets, then stay with Cherrymax if you end up having to use some because you can't figure out a way to buck the last few solid rivets.
7. I like to epoxy prime the shop heads of my rivets after bucking, so before I close up the skin and lose access, I take an artist's brush dipped in Alodine 1132 pen fluid, coat the rivet, then after the Alodine dries, coat the rivet shop head with Polyfiber EP-420/430 epoxy to seal it from water and the potential for corrosion. You need to do this as you rivet, so pre-mix your epoxy primer and keep it in the freezer until you need to use it. I can stay uncured in the freezer for many days. I've seen this technique used at the company I used to work for that made military and commercial helicopters.

I hope this helps.

More photos

More elevator photos

Elevator - rivet the trailing edge last (although you match drilled it first). Also, the fixtures I made our of scrap skins and metal were useful to getting a constant 2 diameter setback from the curved trailing edge to drill the rivet holes. The scrap corrugated skin template used 3 holes to get constant spacing between the rivets. This sped up the measuring process quite a bit. I used Talcum powder to keep from scratching the aluminum during the measuring process.

Flap alignment and drilling. Note the fixture I built to hold the TE of the flap skins square before drilling. This kept the corrugations aligned too.

More flap rebuild photos. The flap was easier to rivet because there are only tip ribs, unlike the aileron, which has mid-span components that need to be riveted to the skins.

Forgot a photo of the finished elevator.

Here are aileron rebuild photos showing the need to roll the skin from end to end to allow bucking bar access to the interior.

Also, note the use of a heavy aluminum angle to hold the trailing edge straight during riveting. The trailing edge was riveted last, but it was match drilled first.

Aileron photos