Landing Gear shock strut alignment for cutting boot cowling extension.

[FFTravism]

I am working on the boot cowling extension where the shock struts penetrate. Following Collins manual and he talks about installing the landing gear legs and shock struts to take measurements for cutting the holes. Do I need to put the plane on the wheels and set the width to 72" and then figure out where the shock structs need to go through the boot cowling extension? That would have no real weight load on the shocks. I couldn't find anything in the Tips area or forums.
Thanks

[jaredyates]

Naturally the hole will need to be large enough to accommodate the strut at empty and heavy weights. There is a template for the shape of the hole on the 4-place here which may help get you close:

https://bearhawk.tips/772

[FFTravism]

Yes I saw the sealing plates and that gives me a start on the movement of the shock struts but not where to start for placement of the templates.

[rkennell]

Jared, A while back you posted that you were having problems with the silicone seal that you used as a strut seal, breaking up after a few landings. Did you find a solution?
I found some high strength neoprene sheets at Graingers that meet the specs in BH tips which I plan to use unless I find a better solution.

[jaredyates]

I haven't got the opening sealed yet on our current plane. The cabin is a little drafty and they aren't at the top of the list for the next leaks to fix. I think the biggest threat is petroleum exposure. Let us know how it goes!

[rkennell]

https://www.grainger.com/product/GRA...Strength-1DUK9 This is the spec sheet of what I plan to use. I will let you know how it goes.

[Paul Johnston]

That stuff will be fine if it stretches- I cut up an inner-tube like you would use to go swimming. The trick I found was to cut the hole only half the size of the shock- strut.

[spinningwrench]

I made a “fake” shock strut using adjustable turnbuckles which quickly allowed me to dial in the minimum and maximum thread widths without having to remove the shock struts each time.

[Nev]

One way to seal is to put the material of your choice around the small shaft just below the rod end (which only moves side to side a small amount) rather than the outer large tubing (which moves up/down as well as side to side a greater amount).

[Bcone1381]

I like data. My assumption has been the big risk to an unsealed gear strut is co2 and exhaust getting into the cabin during flight. I have no data, so my experience is nil.

In this thread I see a number of Industry Best Practice VS a BCCE (Brooks Cone Common Error.)

Paul's Post #6 suggestion on hole size. Do this. My rubber hole is larger than Paul's suggestion and my material is from a rubber automobile inner tube. My application seals up when the gear is relaxed with no weight on it. When weight is applied the shock strut movement stretches out/enlarges my hole and it opens up a cresent shape leak on the inboard side. This is not an industry best installation but I think when I fly someday my gear will seal.

rkennel's post #5: The spec sheet material has elogation of 380%. I bet my rubber is 50% which data suggest is not an industry best material to use.

NEV's post #8 idea to seal the rod end bearing seems like out of the box brilliant design idea to me.

I wonder if a Material with better Elongation characteristics than inner tube rubber, then sealing by attaching it to the strut is better than what I have. Using better material with the smaller hole secured tight to the shock strut with a zip tie or hose clamp might be the best practice, as will Nev's rod end bearing idea.

[jaredyates]

Here is some baseline data based on not having the struts sealed. The green line represents airspeed and the orange line represents CO in parts per million. I would say that generally as long as there is a little airspeed, any gases that do enter through the shock strut area are sufficiently constrained to the area under the floor. There is a nice little air outlet behind the cabin which I think helps keep the tail section pressure low. The CO does climb to a peak of around 40ppm on the taxi-in, probably because of losing that low pressure source. Sometimes the ppm will creep up a little during high AOA/low speed maneuvers like stall entries and slow flight.
brooks.jpg

[Steve W]

AFA8EA5C-483A-494F-B35B-9EE98C2861A6.jpg 011EF359-38D5-4091-ABE1-E1C0CA3A0848.jpg I’ve previously posted what Nev described about sealing the small piston rod rather than the outer tube. I was concerned about stressing the bottom skin. Used a cheap shifter boot from Amazon. Nothing moves other than the shock itself, free from any contact with the bottom skin.
Steve P203

[Bcone1381]

I am working on the boot cowling extension where the shock struts penetrate. Following Collins manual and he talks about installing the landing gear legs and shock struts to take measurements for cutting the holes. Do I need to put the plane on the wheels and set the width to 72" and then figure out where the shock structs need to go through the boot cowling extension? That would have no real weight load on the shocks. I couldn't find anything in the Tips area or forums.
Thanks

Your post is about positioning the template. I fabricated the bottom boot cowl from posterboard. Think like a 5 year old when I do this Tape & scissors are my friend. Posterboard sheets from the dollar store are a bit flimsy. Get some 3m spray glue and past two of them together. Now you have material properties similar to aluminum sheet.

Using scissors cut the template to fit. Drilling and clecos are ok to hold it in place. Cutting a hole that is larger than needed into this template to accomodate the shock struts is ok.....just use more posterboard and tape to final size it. My templates normally look too ugly when I get the shape just right right, so then I make a second template from the first. I looks great! By now I have practiced on the part twice and making to from Aluminum goes pretty well.

My panel has a large oversize oval hole that accommodates each strut penetration. The square sealing plates (that Jared and Spinning Wrench shared) cover up these penetrations. To get the sealing plates into the right position (This is the goal of this thread) I made templates of these sealing plates and positioned & match drilled them to my Aluminum panel.

Now you want to know about there position......Get your shock strut fully relaxed...no weight on wheels. Now, positioned the plates so the strut is at the inboard end of the opening. So then when I prang on a landing and the gear absorbs this shock the strut will move outboard then back to the inboard as I spring back into the air.

I Hope this makes sense.

[FFTravism]

Thank you. That was what I was looking for. It goes along with what I was thinking.

[kestrel]

I'm curious why it seems to be trendy to seal below the upper ball links when there is no movement above them. A box over them would seal out CO without parts that stretch and slide. I can't help feeling that the most common way to address this is the most difficult.

My experience with CO in the cockpit does not mirror Jarred's. I was getting CO in the cockpit in flight. I tried a few quick hacks to seal things up with duct tape and didn't succeed without doing it right. However, I bought a couple of down turned exhaust tips. That solved the CO problem and I stopped working on it.

I don't know who's plane this if from, but I have squirreled away this image that shows in interesting solution. It seals in flight and has the possibility of being lower drag if properly fared with fillets.

strut_fuse_fairing.jpg

[Nev]

I'm curious why it seems to be trendy to seal below the upper ball links when there is no movement above them. A box over them would seal out CO without parts that stretch and slide. I can't help feeling that the most common way to address this is the most difficult.

Thats a very good idea. If we could find a solution along those lines that still allowed sufficient access to the rod ends it might well be the perfect solution.

I experienced the same issue with CO as Kestrel above. When I lowered the exhaust pipes another inch or two away from the fuselage the problem went away. It was most noticeable at slower speeds, high AOA and rich of peak (LOP doesn't generate sufficient CO).

[Bcone1381]

I would say that generally as long as there is a little airspeed, any gases that do enter through the shock strut area are sufficiently constrained to the area under the floor. There is a nice little air outlet behind the cabin which I think helps keep the tail section pressure low.

Can we see a photo or description of this outlet?