I usually don't like using tri-fold mounts in smaller tubes like a BT-5, 20 or 50. But on this older kit I'll (almost) follow the instructions
Here's how the instructions show how to glue up a tri-fold mount.
Three folds and three bends of the shock cord.
Older kits included a mount printed on 65 lb. cardstock.
Current kits have the mount printed on the 20 lb. stock instruction sheet.
Over the years tri-folds have evolved to two folds of rubber.
The original tri-fold mount had you glue the shock cord at an angle. This makes a flatter finished mount inside the body tube.
I run a light line of glue at an angle down the cardstock. The elastic shock cord is pressed into the glue and allowed to dry.
Apply a skin of glue over Section 1. Note I wrote "skin" of glue. When gluing paper to paper you don't need a heavy glue coat. Be sure the glue coat covers all of the section area - side to side.
On the left I'm burnishing the first section.
On the right you can see how much glue is placed before folding it over onto section 3.
Shape the mount into an arc to better fit the inside curve of the body tube.
On the right - you can see how the angled shock cord ends up in the folds. The finished mount is flatter than it might have been otherwise.
Hi Chris,
ReplyDeleteYou mentioned that you normally don't like using tri-fold mounts in small body tubes such as the BT-5. How would you do it otherwise? I just built a BT-5 kit called Star Trooper and installed the trifold mount per instructions. Very tight, even for a small streamer! Have yet to fly it, but hope the recovery doesn't get hung up at ejection.
Hi Anonymous,
DeleteYep, it surprises me that a tri-fold mount is used in a small diameter. Recovery is a streamer so it should eject pretty easily. I would have probably tied some Kevlar to a notched engine block.
Check out the older Centuri instructions (the Estes kit was originally from Centuri) They used curved cardstock for a mount.
https://plans.rocketshoppe.com/centuri/cenKM-3/Centuri_Star_Trooper.pdf
One idea is to thread the shock cord through two slits cut through the body tube as on the Screamer.
ReplyDeletehttps://www.spacemodeling.org/jimz/estes/est0802.pdf
The first time I'd seen the shock cord threaded through two slit was on the Astron X-Ray.
https://www.spacemodeling.org/jimz/estes/k-18.pdf
The methods are similar (shock cord threaded through two slits in body tube), but also a bit different (on the BT-5 sized Screamer, a knot is used to help secure the cord, while on the BT-20 sized X-Ray the cord is "reverse-threaded").
Obvious downside would be the visible marring of the body tube, as well as being possibly fiddly (especially if you've got big fingers).
I've not found many problems with tri-fold mount in BT-50 and larger, a bit fiddly on BT-20, and a bit of a squeeze with BT-5 (mostly determined by how easily my fingers fit in the body tube).
ReplyDeleteIndeed the pre-forming of the curve in the tri-fold mount (before the glue has dried) makes quite a difference in terms how well you'll be able to get it to stick. Just noticed something about the direction of the curve you put in the mount (with folded section toward inside) -- on the instructions in some kits it seems to be implied that the folds should be toward the outside.
Hi Naoto,
DeleteOver the years, I've seen the folded side facing inside or outside. It can go in either way. I usually go with what seems flatter.