Alternative method of doing the landing gear UNI Wrap

[chasingmars]

Hello all. I got one of those "bright ideas" so before I go mess something up, I figured I'd post and see if there's good reason not to do it...

 

While I'm still plodding throught chapter 7, I got my LG components the other day, so I've been reading ahead a bit in the plans to see whats involved. I've read that chapter nine involves a huge number of hours, and I'm looking to find ways to change things to take longe... um... I mean, save some labour.

 

Anyhow, I was looking at the first part, where one lays offset cut UNI, 8 layers in all, from two directions, in pieces, without getting the fibres bent This is to provide additional torsional strength to the gear.

 

I'm not entirely sure why 35 degrees was chosen, and why the angle is biased towards the directionality of the S-glass hoop, given that for a tubular skin in torsion, the principle stresses would be at +/- 45 degrees (skin in shear), and the addition of the bending of the hoop would offset this in different directions on top and bottom as the top's in combined shear/compression with the bottom in combined shear/tension. Maybe I'm missing something and Marc or someone else with a better stuctures background than I can fill me in.

 

Anyhow, the reason this is on my mind is that I noticed that the gear circumference tailors fairly well to that of 4" diameter braided sleeve, which is pretty commonly available these days. At the fattest parts, the gear is about 12-1/2 to 12-3/4 around, which would have a 4" sleeve at just about 45 degree bias, then as it tapers it would pass through 35 degrees about 2/3s to the wheel (10.2" circumference) tapering to about 27 degrees at the strut end and on the other side about 24 degrees through the centre where it necks down.

 

So what I was thinking is, since I figure it's probably a lot easier to get straight consistant fibres by smoothing down a braided tube over the landing gear, is there any glaring reason why this shouldn't work if I run the numbers to account for the glass strength of the braid and account for the variation in angle? I haven't done all the math yet, but something like 3 layers of 24oz braid should be easier that the 13 pieces of UNI?

 

What do the folks that did it the plans way think of the plans process, is it as much of a bear as I'm thinking it will be, or does it just sound complicated and is easier in practice?

 

Has anyone worked with braid before, from a manufacturing standpoint, is what I'm suggesting workable or is there something I haven't considered? Are there advantages to more layer of lighter braid? Should I put a finishing layer or interlayers of lighter braid between heavier keep the resin weight down?

 

And from a stuctural standpoint, am I missing anything significant here? From a weight perspective, I don't mind a *little* extra glass to make up for the change in approach, as I'm much more easily able to vac bag a braided LG hoop than a plans done one (the supports make it more complicated) and a good vac bag here can probably save 1-2 pounds given that there's about 6 sq yards of glass in this wrap.

 

Anyhow, thoughts? I want to have a nice warm fuzzy before trying it as it's an awfully expensive piece to mess up. your feedback is appreciated!

[Marc Zeitlin]

I'm not entirely sure why 35 degrees was chosen.... Maybe I'm missing something and Marc or someone else with a better stuctures background than I can fill me in.

I had the same thought regarding that angle, and others have in the past as well, as searching the COZY mailing list archives will indicate.

 

The only thing I can come up with is that there's some combination of torsional stresses, shear stresses and bending stresses that make the 35 degrees better for the combination of stresses than a 45 degree layup for pure torsion would be.

 

Or maybe the designer plucked the number out of a hat, or somewhere even darker :-). Doubtful, but would hardly be the first time that the thing commonly called "intuition" provided an answer in these aircraft :-).

 

...but something like 3 layers of 24oz braid should be easier that the 13 pieces of UNI?

I dunno - maybe. The braid's non-trivial, and the plans method is really not hard. I'm guessing 6 of one, 1/2 dozen of the other, but it's just a guess.

 

...What do the folks that did it the plans way think of the plans process, is it as much of a bear as I'm thinking it will be, or does it just sound complicated and is easier in practice?

The latter, I think.

 

Has anyone worked with braid before, from a manufacturing standpoint, is what I'm suggesting workable or is there something I haven't considered?

See above.

 

Since I know that you're vac. bagging, I'll say this for the benefit of others - I would NOT suggest using the braid for anything unless you're bagging it - it's very stiff, and difficult to get to stay where you put it without some constant force on it.

[Jason Heath]

I've read that chapter nine involves a huge number of hours......

But very few of them have anything to do with the torsional layups. You may be adding to the hours??

 

What do the folks that did it the plans way think of the plans process, is it as much of a bear as I'm thinking it will be, or does it just sound complicated and is easier in practice?.

Much easier, It took me three hours for each set of 4 plys which included cleaning up etc.

 

Anyhow, thoughts? I want to have a nice warm fuzzy before trying it as it's an awfully expensive piece to mess up.

I too had my worries so I just went for it, When I was done I laughed about how EZ it was and kicked myself for waiting as long as I did to do it. IF you think you've screwed up either way, you can still remove the glass from the strut and start over without to much headache.

[Marc Zeitlin]

I dunno - maybe. The braid's non-trivial, and the plans method is really not hard. I'm guessing 6 of one, 1/2 dozen of the other, but it's just a guess.

Bad form to followup my own posts, I know...

 

After some discussion with the fabricators who made the part that we tested braid on, and then went away from it :-), I'm going to say that the braid is good for things that have relatively constant radii, but might be difficult for things that have wildly varying radii and/or shapes. I'd vote for the plans method here.

[Phil Kriley]

I painted the gear with epoxy, then wetted out 4 layers of cloth on plastic flat on my building board, alternating the weave per plans. I laid the whole thing on the strut and played with it until it laid down nicely. I repeated for the other side, then put it under my heat box.

 

It turned out great, and did not take anywhere near the 6 hours that others have reported - closer to 1.5, not counting cutting out the cloth, which I did the night before.

 

I used aluminum flashing to back up the 2 BID over the "straws", and that worked well too.

 

I've sanded the work again, have flipped it over and attached it to taller screws (lag bolts screwed into 2x4 pieces) and am ready to do the second 4-ply layup, and I will to it the same way I did the first 4 layers.

[chasingmars]

Looks like the plans method is fairly straightforward. While the idea of Feining off an experiment gone wrong doesn't appeal (and that's an understatement!) I may try it anyhow, just out of curiousity, once I can completely assure myself that I'd be getting the same strength at no more weight.

 

One thing I was thinking about with going with a braid is that because it's continuous, it should be stronger (plans is only 4 plies thick at nose and trailing edge) but this is probably a pretty moot distinction as it's clearly a layup designed to improve stiffness characteristics and strength is not the driving factor here.

 

Marc, when you mention substantial variation in cross section, it varies from about 2/3 nominal to nominal braid diameter, I wouldn't have thought that too bad... I think next time I'm at my supplier I'll pick up a small amount to play with and see how it wets out and lays over some scrap PU or something before making up my mind. Better to get an idea of what I'm working with before trying something like this I think.

 

Thanks all for the feedback so far.

[Big Steve]

I just finished chapter 9. The torsion lay-ups are really no big deal at all. what is the big deal is all the odds and ends you have to do. It takes a while and Make sure you check out a lot of websites so you can figure out the orientation of the matco brakes the plans have nothing on them so you are on your own. Working with the s glass is also a very itchy situation wear long sleeves and a dust mask or you will hate yourself the night you sand that stuff. STeve