'98C Gets a New Windshield

[cessna170bdriver]

Well, it just can't be put off more: '98C needs a new windshield:

My AI offered to "stitch up" the crack with safety wire to get a little more time out of it, but when I showed him where the stress was coming from, he changed his mind . From the looks of the break in the strap attachment, it's been that way quite a while.

To document the replacement, I'm starting a separate thread here. The following is quoted from the tail-end of a previous thread.

… Can anyone describe the Cee Baileys STC installation? Couldn't find any information on their site...

I've had a Cee Bailey 1-pc in my '53 since about '83 without any problems. It does require the three thru-bolts in the overhead…
Scotty

Any more data points on Cee Bailey Installation? The installation of the Great Lakes and the LP Aero windshields are well-documented on their respective websites, but nothing on Cee Bailey. I tried to call them this afternoon and their "expert" had gone home for the day. The guy who answered the phone said he'd try to find installation instructions and fax them to me, but nothing so far.

I'm curious if the Cee Bailey and the LP Aero are actually the same, as the Cee Bailey part number for the "strapless" windshield is CB-312/CON, and the LP Aero part number is 312/CON.

I'm leaning toward the Great Lakes based on good experience with my current 26-year-old (strapped) windshield, and the apparent ease of installaton of their "strapless" model 2055.

In this case I've decided to forego originality for functionality and go with a thicker windshield to eliminate the center strap, and hopefully quiet the cabin a bit. (I've taken no noise data so the results in that area will be purely subjective.) Well, it's after noon and I still haven't heard from Cee Bailey on how their "strapless" windshield gets installed, so I'm about to pull the trigger on an order to Great Lakes. I decided against LP Aero due to the necessity drill out the front row of rivets in the center section carry through spar in order to install a new windshiled retainer strip. The Great Lakes installation uses the existing retainer and adds 4 screws through the cabin roof, windshield, and retainer. The holes in the windshileld come pre-drilled, and will be fitted with elastomeric bushings to preclude the plastic riding directly on the screw threads. They send a drill template for locating the screw holes, so the installaton looks fairly straightforward.

There was some discussion in the earlier thread about sealants. One of the sealants recommended in the 1962 and prior single engine service manual, Presstite 579.6, is avialiable from Chief Aircraft at a reasonable price. Presstite appears very similar if not identical to the putty I used before and it never allowed a leak, so I'm going that way again. The remnants in the photo below are still quite pliable even after 26 years.

It's not often one gets to stand up in a 170, so this photo op was quite irresistible. (You can also see I'm in the process of tearing out the interior for replacement. I'll cover that in another thread.)

[lowNslow]

Miles, have you decided if your going with solid rivets, cherry rivets, screws with self locking nuts or screws with rivnuts decisions, decisions. I went with screws with self locking nuts but wished now I had gone with the rivnuts. I got approval for the thicker windshield with the center strap using the same profile. LP made the windshield for me and provided all the paperwork I would need for the 337. I did not notice any change in noise level from the thinner plastic.

[cessna170bdriver]

Karl,

I assume you're talking about the screws that hold the lower retaining strip. Mine had already had the rivets drilled out and replaced with screws and locknuts when I bought the airplane. When I replaced the windshield in 1984 I put it back in the same way. This time, just laying on the floor and looking up I thought I could get a socket on the nuts, but that proved to not be the case for a lot of them.

After depopulating the instrument panel for the second time just to be able to reach the nuts to change the windshield, I started thinking about nutplates or rivnuts. I asked my AI which he would prefer, and he suggested type-B sheet metal screws and tinnerman nuts. As seldom as these fasteners have to come out, that might be an option worth considering, especially if I use some sort of adhesive when I put them back in. When I removed the nuts this time, not a single washer fell off, and I had to pick them all out of the putty that had squeezed through the holes on installation, so it may be that that would be enough to hold a flat tinnerman in place for next time. I'm still mulling it over.

Here's what I had to do to get to many of the nuts. It's not a totally wasted effort, as I was kind of looking for an excuse to fabricate a new panel anyway. The only instrument I actually had to disconnect was the oil pressure gauge, as it is front-mounted, and there is no slot in the panel through which to remove the line.

[15A]

Miles,

I hope the new windshield cures your headache, but by the looks of that center strap breaking like it did, I think there is more to this than what you see.
My question is why did the windshield crack, and what made that strip of metal pull itself apart?
Could full flaps at too high a speed create that kind of stress? I'm not a structural engineer, but I think there is something going on there...
Please, prove me wrong!

Joe

[BeeMan]

You might consider a V brace while you have the windshield out. My 170 and most in Alaska have a V brace from the top of the door posts to the top of the firewall to resist the twisting of the windshield when taxiing over rough ground or operating on skis. Besides strengthening the forward part of the fuselage, the V brace makes a great hand hold for pulling your seat forward and for hanging up your headset.

[15A]

BeeMan,
Great point!
Surely that 'V' brace adds alot of strength in that area. I've been to Alaska and have seen what you guys up there have to deal with. It's ROUGH, ROUGH, and more ROUGH. My hat's off to you. You're in a class of your own!
But for us 'flat-landers', most of that 'bridge-work' isn't necessary. For us, staying light is good!
I'm just curious as to the 'why' things happen...

[GAHorn]

Beware....steel V-braces can influence the compass.

[lowNslow]

Miles, if you need to drill your windshield I have found that Unibits work great. They step up the hole size without having to change bits and cut clean burr free holes. With really thick material you may have to come in from both sides.

[cessna170bdriver]

Miles,

I hope the new windshield cures your headache, but by the looks of that center strap breaking like it did, I think there is more to this than what you see.
My question is why did the windshield crack, and what made that strip of metal pull itself apart?
Could full flaps at too high a speed create that kind of stress? I'm not a structural engineer, but I think there is something going on there...
Please, prove me wrong!
Joe

I very rarely use full flaps, and I'm just barely strong enough to pull them in at 80, much less than 100, so I don't think that's it. Another member PM'd me and said his center strap is joggled where it goes over the edge of the roof panel. Mine isn't made that way, so he's thinking there could have been some added stress where the strap transitions into the area that's riveted.

Miles, if you need to drill your windshield I have found that Unibits work great. They step up the hole size without having to change bits and cut clean burr free holes. With really thick material you may have to come in from both sides.

Fortunately, even the windshield from Great Lakes that gets the screws come pre-drilled. I will do a visual QC on them, though.

[wingnut]

After you get the windshield installed, but before you drill the 4 holes up top, grab a wing tip and rock the aircraft real good. The glass floats, and you'll see it slip side to side at the cabin top. Rocking the plane lets it settle into that spot where it likes to be. After you've done this, check clearance between the cuffed area and the wing leading edges. There should be at least 1/8", but I like to shoot for a 1/4". I can't remember if I read, or was told, acrylic sheet expands .1" per foot, between a temp range of 30-100 degrees.

[cessna170bdriver]

Thanks for the tip, Del. I haven't seen that in any of the several sets of installation instructions I've read recently. I sure wish I lived closer to Mena!

[wingnut]

Thanks for the tip, Del. I haven't seen that in any of the several sets of installation instructions I've read recently. I sure wish I lived closer to Mena!

Your welcome. I just realized I didn't state the reason for the "tip". If the glass is contacting, or very close to contacting the wing leading edges, it wouldn't float, and could lead to cracking.
Also, this process of installation is even more important on the strapless, because the 4 added screws don't permit as much movement (even with the rubber bushings).

[cessna170bdriver]

Yeah, I kind of thought that might be why. I just checked and couldn't find any pictures, but I'm quite sure the old one had at least enough room to get my fingers between the windshield and wing root, so that should rule out that reason for the crack in that one. I would also think that an intact center strip would prevent a lot of side-to-side motion too.

[cessna170bdriver]

Well, I got the lion’s share of trimming done this weekend. I would have done more, but I’m paranoid about working with expensive acrylic parts when the OAT is under 80˚ F. Tehachapi didn’t get that warm 'till mid to late afternoon Sunday, and I did the initial fit with the sun shining in my west-facing hangar. I thought I might have a problem when it looked like the leading edge shape on the top left corner appeared to have been molded too wide, as the acrylic was standing proud of the wing leading edge shape on the top and bottom. The bottom also appeared to be a bit long, but I already knew how to deal with that. Fortunately, the problem turned out to be that I hadn’t noticed that the template for drilling the attach holes in the lip of the cabin roof was taped to the top of the windshield inside the protective paper, and wasn’t allowing the top edge to fully seat in the channel.

Initial Fit, Left Side

Out of the Box Fit, Front

Monday, after removing the template, the prognosis looked much better. The top left wing root shape fit much better, but will require about 1/16 or so to be beveled off the bottom to releive a slight interference with the wing root fairing. Next, I had to trim the aft edges where they ride in the channel in front of the door posts. Even with the edges fully seated in the channels, I couldn’t get the bottom to fit snugly all the way around the inner retaining strip, and the windshield wanted to "rock" in place, apparently pivoting on the lower aft corners inside the channels. In two passes, I ended up trimming a total of 1/8 inch at the top and 7/16 at the bottom of both aft edges. I removed the first 1/8 inch with a Vixen file, which took about 30 minutes per side. The next fitting indicated trimming 5/16 at the bottom and zero at the top, so I broke out my new RotoZip with 90˚ cutoff attachment (early birthday present ) and made quick work of it. A 1/8” thick wood strip clamped to the edge gave me a nice straight line to follow. Clean up with the Vixen file and 150, 220, and 320 sandpaper left nice smooth edges.

Setup for Trimming

The length of the windshield top to bottom was near perfect out of the box, except for about a 6-inch long strip where the windshield comes over the corner of the boot cowl in front of the copilot side. That area required about a 1/8” arc to be trimmed out of the bottom of the windshield to nest nicely in the radius of the inner retaining strip. I took care of that with a small 100-grit sanding wheel in a 90˚ die grinder. I did a trial fit on the retaining strip, and the black line at the bottom represents the top edge the strip.

Final Fit, Bottom Right

Even though this windshield replacement is going much better and quicker (so far) than the last one (1984), I’m still amazed at how tedious this job is. Not difficult at all, just tedious. I’ve already had the windshield in and out at least five times, and I’m sure there will be several more by the time I get done. I still want to trim the wing root areas a bit per Del’s suggestion, and smooth the edges all the way around (one more in and out cycle).

Left Wing Root Needs Trim.

After that, what's left is to apply the felt all the way around, mark and drill the holes in the cabin roof (yet ANOTHER in and out cycle ). I’ll hold off on final installation until after shooting paint in the interior (see ‘98C Gets a New Interior"), but at least all the prep will be done while the weather’s still warm.

[cessna170bdriver]

Not as much progress as I'd hoped (too much socializing around the airport, which is not really a bad thing... ), but I did get the wing root areas of the new windshield trimmed another 1/8 inch on each side with the RotoZip with cutoff attachment, all the edges smoothed and slightly radiused with vixen file, 150, 220, then 320 wet sanding.

Yet another temporary windshield install with all the retainer fairings this time (every 3rd hole in the lower one to save a little time) set the final position for marking the screw holes. After trying the instructions' method of placing the drill template without much success, I came up with my own much more repeatable method that I'll detail as soon as I can remember to pop the memory card out of the camera. After drilling and checking the screw holes, I had my AI come over and drill out the old broken end of the center strip attach point and helped him use his two-man squeezer to replace the two #5 rivets (necessitated by an earlier, less than acceptable repair) through the main spar carry-through.

Pictures tonight or tommorrow, hopefully.