Saturday, May 14, 2011

Windshield Frame & Heat Valve

On Monday they took the blog site down to "read-only" for an hour. It's now Saturday and it's finally back working. You've got to love computers.

 The heat valve was sticking and of course the steel parts were rusty.  I've cleaned it up and painted it with engine enamel.  It works great now.  It was riveted on but the IPC shows it attached with 4 #8 screws and 12 #6 screws with flat Tinnerman nuts.  I need to find out if there is a reason it needs to be riveted.  The screws weigh more but allow it to be removed and cleaned, etc. to keep it working well.  Right now my plan is to screw it on when my young apprentice is visiting for a week this summer.  It takes someone on the inside to hold the nuts.  She reminded me the other day that I'm supposed to build a plane for her and one for her brother when my planes are done.

The lower attachment for the windshield is made of 2 strips of aluminum. The inner strip (shown here) is riveted to the boot cowl area with about 12 3/32" countersunk rivets. The outer strip is attached with screws and nuts with the windshield sandwiched between the 2.  The outer strip was probably originally riveted on but the holes are to messed up to allow riveting it on.  I wouldn't have removed the inner strip but it was all bent up and caked with clear RTV.  By carefully working with a hammer and dolly it's all straight and fits very well.  There was a lot of crud under it such as old paint stripper, paint residue and the remains of some black rubber as well as RTV.  I thought it was odd that this part was dimpled for the rivets but the fuselage was not dimpled to fit.  Back to the parts manual (IPC).  It says there was a strip of 3/4" electrical tape under this piece 60 " long.  Clearly the cheap vinyl electrical tape we use today was not going to fill the gap from the rivet dimples nor would it do anything else of value.  The brain finally started working and I remembered the rubber electrical tape we used at the phone company.  We never used vinyl tape.  The rubber tape was used on battery cable terminals and splices.  After wrapping with the rubber tape, overlapping 1/2 width, the rubber was wrapped with friction tape and shellacked.  The rubber tape was about 3/4" wide and 1/32" thick, perfect to fill the gap under the windshield strip, provide protection from chafing, and seal out wind and rain.  After calling the nearest electrical supply houses to make sure such tape was available I did some checking on the web and found 2 tapes made by 3M which should work.  The first is Temflex 2155 Rubber Splicing Tape.  It's 3/4" wide and .030" thick, used for exactly what we did at the phone company and even recommended to be wrapped with friction tape to protect the splice.  OK, I think this is what Cessna intended.  While I'm impressed with how much I was able to straighten the aluminum strip I don't think it is even close to how well it fit when new.  So, do they have anything which might work better?  I also found Scotch 2228 Rubber Mastic Tape, the name seemed closer to what I need.  It comes in 1" width is 1/16" thick and has a mastic layer on both side to help it stick in place.  It's also a softer rubber so the added thickness should allow it to better seal the used strip than just the rubber tape.  I'll use the 2228 unless it doesn't work for some reason.  I'll have to trim it a little but I think the extra work should be worth it.  Now where to get it.  It turns out both tapes are available at Lowes, Hi-Ho Hi-Ho it's off to Lowes we go.  They're in the electrical department with the cheap vinyl tape.  They only have the 2228 ( called Moisture Sealing Electrical Tape) in a 4 ft. roll ( about $5) so it takes 2 to do this.  It's still cheaper than ordering a 10 ft. roll on line.  Before I can finish this I need to paint the glare shield area and prime the underside of the strip, when the rain stops.
To remove the RTV I went to the hardware store looking for some kind of stripper claiming to work on Silicone Sealants. I found some. The one I chose, Pro-Strength De-Sov-it, because of the big label which says "Removes Silicone Lightning Fast!" comes in a spray bottle for easy application.  Well, in the small print it says it takes 1 - 7 hours, so much for Lightning Fast.  I've long since quit being in a hurry so my plan was spray it on the strip and the fuselage, go back to work in the shop while it does it's magic, then repeat every hour until the RTV could be removed.  At the end of the first hour it was clearly working it's way into the RTV.  The outer layer was crumbly and really thin areas rubbed off.  Three passes of spray set rub off and repeat were needed to remove the RTV residue.  I started by removing as much of the RTV as I could with a pen knife so there would only be a thin layer to dissolve.  It is a citrus based stripper which seams to remove the plasticizers from the RTV so you can easily scrub it off with a wood or plastic scraper.  I won't know until I wash it with prep-sol and MEK whether I really have all the Silicone removed so the primer will stick, but the RTV is gone.

Friday, May 6, 2011

Firewall Cleaned & Painted

I decided to paint the engine side of the firewall and treat the inside with a light coating of Corrosion-X.  I didn't want paint on the inside of the firewall in case of fire in the engine compartment.  The possibility of paint in the cabin smoking or flaming while trying to land in an emergency is more than I want to consider.  For the engine side I mostly wanted something which could take as much heat as possible while keeping rust to a minimum.  Since the rust after all these years was minimal I chose to clean the firewall lightly with Scotcbrite to scuff up the surface and MEK to remove any residual oil.  I primed it with 2 light coats of Dupli-color 500 degree engine enamel primer.  This was the highest temp primer I could find which did not require curing at 600 degrees.  The light coats were intended to just get the advantage of better adhesion by using a primer under the finish coat.  For the finish I used one light coat and one medium coat of GPM High Heat Aluminum paint.  It's rated for up to 1000 degrees for use on outdoor grills, etc.  Do I have a clue whether either of these will hold up or handle high heat, nope.  What I do know is that most paints, including my much loved epoxy primer can not handle much over 200 degrees.  Only time will tell how this mix works.  Hopefully it will never be tested by fire.

The large hole is for the cabin heat valve.  I've got it cleaned up and painted with 500 degree engine enamel.  I need to find out why it was riveted on since the parts manual shows it screwed on.  Screws would have made keeping the thing working well a lot easier.  I'm planning to put it back with screws unless I can find why it should be riveted.  I'm tired of things which do not work because they were not maintained properly.  The valve was stuck with rust and paint which had gotten into it.

Wednesday, May 4, 2011

Tail Wheel Patent 2,396,277

If you would like to know how the designer of the Scott tailwheel thought it worked here is his patent 2396277 by Richard Ledwin filed 18 August 1943.  Don't read this when you're sleepy.


I've realized I can't put all the controls back into the cabin until I clean up the light rust on the firewall.

Also, all the Tinnerman nuts are rusted and some are broken.  Since they're riveted through the firewall it would be tough to get at them once the rudder pedals, etc. are in the way.  I've ordered new Tinnermans.  One thing I realized is Cessna used plated nut plates and all you can get today are primed.  I think I'll add some paint, probably silver, to give them a better chance against rusting.  I'd like to think I've learned form all the rusty steel parts I've been cleaning which were only primed. 

The Tinnerman nut strips (A6221-8Z-8-D) used on the edges of the firewall for attaching the cowl are no longer available.  I read a posting at the club site where the person carefully riveted new nut plates to the strips in places where the little spring tabs had died.  I'm just going to make new strips from aluminum and rivet the nut plates to them.  Using Matched hole construction, remember the Thorp T-18, I can make the new strips faster than fixing the old ones.

The other thing I expected to find at the club site was a good recommendation for painting or treating the firewall to stop the rust, no such luck.  The firewall is Ternplate, think steel coated with 90/10 lead/tin solder.  They don't make it anymore because of the lead thing and cancer.  It's what was used to make the gas tank on my WACO NINE, 83 pounds of steel, rivets, and soldered joints.  Originally I thought it was galvanized steel so I was going to clean it and lightly treat it with Corrosion-X.  Galvanized is coated with Zinc and resists rust even at minor scratches.  It has to do with the galvanic action of the Zinc.  Ternplate has no such galvanic action, scratch it and it rusts.  I suppose you could re-solder the affected areas. That sounds like to much work except where you need a ground strap attached. If I were doing a restoration I might replace the firewall, but I want to fly this thing before the grandkids turn into teenagers.  So, I'm going to lightly clean it up and paint it with some Rust-Oleum probably a high temp version.  I don't like the idea of painting a firewall in case of a fire.  They make a paint which can be used on exhaust pipes but it has to be cured at those high temps, not practical for a firewall.

Tuesday, May 3, 2011

Tailwheel Details

This simple little Scott 2000 tail wheel which has followed me in several airplanes, Fly Baby, Taylorcraft, Champ, Cub, etc. has been quite a bundle of learning.  I started with 2 complete (?) assemblies and assumed I should end with at least one good one and one re-buildable one.  I have one good assembly now and a few parts.  This exploded assembly drawing does not show all the parts and several parts it shows I didn't have.
Let's start with the wheel.  The part at the left (12) is a felt grease seal.  It has a piece of felt about 1/8" thick trapped between a washer and a dish shaped piece which is rolled on the edge to trap the felt and washer.  The felt needs to be oiled.  I'll get to that later.  The ones I had were both so warn that there was no felt sticking out into the hole for the shaft.  As a result neither would provide any seal to keep the grease in, and keep out dirt or water.  They had probably been that way for a while since all the bearings were shot.  The rolling surfaces of the races were furrowed like a plowed field.  I'm shocked the wheels didn't shake to high heaven.
The part (11) to the right of the seal is just a short piece of tubing 1-3/8" O.D. x .050" wall and 5/32" tall.  It's a spacer to keep the grease retainer/seal in the right place on the step on the the fork shaft.  This spacer is pressed against the bearing outer race and the grease seal is pressed in against the spacer. Next are the bearing inner race and rollers and then the outer race.  Between the 4 bearing sets I had 3 Timken A4050 inner assemblies and 1 from Bower.  For the outer races I had 3 from Bower and one Timken A4138.  For what it's worth both of these part numbers, made by National, are available in from Advance Auto.  They can get them in 2 days, no shipping and a cheaper price.  The wheel half for this side has the grease fitting and hex shaped sockets for the nuts.  The tires tend to dry out and become lose in the wheel.  New ones are about $60, if the tire is lose get rid of it.  The outer half of the wheel has counter bores for the screw heads and threaded holes at the edge for the 4-40 screws which hold the hub cap on.  The drawing does not show the spacer on this wheel and it would serve no purpose but both my wheels have them, go figure.  The 3 screws, washer, nut, and cotter pin are all standard AN hardware. 
The next part has no call out but is a felt grease seal under the hub cap. I made mine with 3 layers of 1/16" felt glued together. I made it that thick because the boss on the wheel for holding the seal centered is that tall. I made the O.D. 2-1/2" to clear the screws for the hub cap.  To get it to seal out dirt and water I worked wheel bearing grease into it.  Just push it down over the boss and the hub cap squashes it down into a nice seal.  Speaking of wheel bearing grease.  The grease called out for the tail wheel is AN-G-15 which is a general purpose grease not wheel bearing grease.

For the seal on the other side of the wheel the part comes from Univair without any oil.  There is only about 1/16" of felt sticking out from the metal housing so working grease into it didn't seam a practical idea.  What I really wanted was for it to be well oiled.  After all grease is just a carrier with oil added.  It's the oil that does the work of lubricating.  I thought of just soaking it in oil but wasn't sure it would get further in than just the little bit of felt sticking out.  Instead I soaked it in 200 degree 5W30 motor oil like an Oilite bearing.  The coolest thing happened when I added the bearing to the hot oil.  The air trapped in the felt expanded and blew out so after soaking it about 15 minutes I just removed the heat and let it cool to room temperature to draw in as much oil as it could.
I didn't find and obvious wear or play in the Bracket (1) bushing (4) for the Fork (5) spindle so not much to say there.  I have no Idea what the acceptable limits for this are.
The next part at the top of the tailwheel is the magical Arm and Cam assembly (6).  It is riveted together so normally no one opens it.  One of mine had been opened and reassembled with screws by threading the lower plate.  The other assembly snagged when moving the arms so both were taken apart.  I'm glad I did because the screwed together one had the better parts but some were missing, never good.
The missing parts were the 2 little spacers I made in an earlier posting.  The first picture shows all the parts inside with the lower bushing installed as well as the 2 spacers. Inside the housing are 3 bronze bushings, 2 spacers and 2 arms.  The bushings are all identical and the arms are left and right based on the bend tipping the arm up slightly.  The stack-up of parts is: bushing, left arm, bushing, right arm, bushing.  The spacers which hold the covers solid also act as stops for the 2 arms.
When the arms are aft, straight out, the shaft hole is open and the tailwheel swivels freely.  Without the springs holding 25-30 pounds on the end of the arms there is no steering.
When the arms are held forward by the springs the hole becomes sort of triangular shaped which matches the angled slots cut in the spindle of the fork.  That little clamping force on the spindle is what allows the steering.  If you push the tail to the side the spring load on one side releases and the arm moves to open it's half of the hole and the tailwheel starts to swivel freely.
The arms in the assembly have to line up with the slots in the spindle so they made shims to put between the bushing in the bracket and the face on the fork as the bushing wears down.  I have one shim but it was not needed to line all this up correctly.  The last piece which I just figured out, and have none of, is a spacer which goes on top of the end of the spindle.  It allows the nut to be properly torqued while maintaining the needed clearance for everything to move.  It also provides a bearing surface for the top of the Arm and Cam assembly.  I'll see if I have one on the Fly Baby which I can copy or if I need to make 2 of them.
The tail wheel is back on but I still can't sign it off until I solve this last problem.

Monday, May 2, 2011

Tail Wheel Spring Back Off

After downloading and reading Neal Wright's article on tailwheel springs I took the spring back off.  At least I have the correct springs.  There were 2 parts missing, the aluminum steering spring guard and the steel piece inside the abrasion strip.  Those have been fabricated and installed.  Also the pin on the shackle assembly which goes in the 1/4" hole in the 3rd spring was ground off years ago when someone installed a spring without the hole.  The hole keeps the shackle from sliding off the spring stack.  Rather than machine a new riveted pin I used an 8-32 socket head cap screw and AN365-832 nut.  The 8-32 screw fit the 5/32" rivet hole in the shackle piece and by just turning the knurle off the head of the screw it fit a 17/64" hole in the spring.  Since there was no hole I decided it was as easy to dirll it to to have the same clearance with a hole 1/64" larger.  If the screw wears it is easy to replace it.  As suggested in Neal's article, I also replaced the main 5/16" bolt with a 3/8" bolt.  Since the hole in the support was 3/8" it seamed better to have a snug fit.  All this required longer bolts to fit properly.  The cross bolt in the shackle is now an AN4-22A.  The main bolt is an AN6-22A and the bolts in the rear support are AN3-18A with the heads down. It's all back on the plane.  On to the tailwheel.