with the recent passing of a beautiful Curare and the cause of the crash being on everyone's mind i wanted to start a separate thread on the cause (the CF DEPS failure). i thought the problem was the titanium end threaded adapters pushing away from the CF rods during or after assembly. apparently the CF tube and it's adapter is also a problem? i modified the DEPS system from Central Hobbies to where the tube isn't a problem but the adapters over the rods is a different story. you can't drill a hole in the rod and let air escape. see photo. i know there are other ways of building a DEPS but this system is incredibly light and strong.

david

David,

Just saw this thread after reading of the passing of the Curare. It looks to me like your photo describes pretty much the idea that came to mind when reading about the DEPS failure (see post in that thread). I'm not sure whether you are saying you like or don't like the system in your picture but it looks good to me! The air issue problem would be addressed by making a hollow coupler of sorts - a small orifice through the center of the coupler. We would just need someone to make them industrially .

Cheers, David.

David, and others of the Curare Thread

Some extra explanation, first physical.

First picture
The static (barometric) pressure in the tube is about 15 psi, 1 bara, 10 m water column. When starting the mounting of the second adaptor
.
I will use 15 PSI as static pressure and the temperature is the same in start and end situation.
See picture 1

Pressure rises in the situation of a volume reduction: because the product of pressure and volume is constant (1)
The amount of rising depends on the start volume and volume after compression (2)
So when the second adaptor shortens the length of the tube with 1 inch and the length of the tube is 20 inch the maximum volume reduction can be, when no leakage 20 / 21 = 0,952 is 4,8 %
Pressure will rise 0,75 psi ,(21 / 20 x 15 psi = 15,75 psi (3))


This 0,75 PSI is enough to press an important amount of the glue out of the adaptor .
This volume is max the volume of the adaptor when the glue is fluid.!
In the explanation I did not calculate the wall thickness, to make it easy for us.
Also the temperature rise during this kind of compression is negligible and not noticeable.

Second picture

When you close the end of the tube, you simple make the tube massive.
During mounting no air can leave the tube.

But more important
All the glue that was in the adaptor has to leave through the narrow space between the connector and the closed end tube.
No glue will disappear in the CF tube.

When you close the CF tube on the way I show on the picture, you can use a lot of cement types someone of us can tell us the best to use.
It is important to mount the adaptor vertical. CF tube in top position, adaptor in the bottom position see picture 3.
Maybe it is a plus to use for a moment a hair dryer to make the glue fluid and get it in the bottom of the adaptortube before mounting the CF tube.

BTW when it is possible to make a little hole in the end of the connector, also then you have to close this hole a moment during mounting to get the glue in the narrow space between closed end tube an adaptor and to prevent the glue escapes through the hole!!!!
But, what has to be the mounting position????? Vertical, with adaptor in bottom position? No, the glue dissapears through the hole and you cannot use the hair dryer.

When it is no clear, please ask,

Cees

How about heating the CF tube with the hair dryer to expand the air inside/move some out/reduce the pressure before gluing the second end on? The reduced air pressure in the tube will compress less and even create negative pressure when it cools to help hold the ends on.

Terry in LP

Terry in LP,

Your result can be picture 2 last situation of my last post.
Glue in the tube, plane in the ground.

I think it is important the glue does not moves during the hardening process.

That is for me the primary reason to close the tube and keep the temperature of the tube constant.


Cees

I think there is some cross-discussion on the topic between using rods and tubes and using inserted fittings (as with tubes from CH) and external couplers.

Cees, the plug idea is a good one when using couplers and tubes as it provides a solid end and, as you say, it prevent glue from entering the tube rather than staying in between the coupler and the tube's outer surface. However, I believe David's (dhal22) idea is to use couplers with rods - hence, the plug is not necessary.

Anuthabubba, the idea of reducing pressure inside a tube is conceptually sound but, not having done any calculations, I'm not sure to what temperature you'd have to heat the air inside to produce a vacuum of substance upon coupler insertion. Also, for the idea to work, the surrounding environment has to be kept at a lower temperature so that the partial pressure inside the tube is lower than that outside - otherwise one is just, as it were, blowing a lot of hot air . This is probably tricky to achieve with a hair dryer and tube in hand but..., I think Atlanta 60 on the Curare thread was discussing this idea too. There might be something to increasing the temperature inside the tube by repeatedly compressing the air inside the tube using a plunger of sorts - much like would happen in a scuba tank say.

Back to the compressing volume issue - whether we use sealed couplers and rods or fittings and tubes, the pressure increase inside the rod/coupler or tube/fitting combination remains. Plugs or no plugs the 1.0x factor is still there. In fact, the P=k/V would be higher in the case of rods since dV, the volume differential, would be higher than when using hollow tubes resulting in most of the glue being expelled from the junction. In short, this concept is believed to be at the crux of the elevator failure on the Curare - the clevis attachment to the pushrod coming apart due to positive pressure inside the hollow structure.

My tendency in building the rod/coupler combination would be to apply a gel type glue (rather than a less viscous form) on the tip of the rod and around the end and hold the rod vertically on a table with glue and second fitting up (reverse direction from your drawing Cees). This prevents the glue from flowing and keeps it at the tip and, as much as possible, in the junction between the surfaces. It also prevents glue from clogging up the coupler orifice, allows air to escape when pulling the coupler over the rod and achieves a bond between the rod and the coupler on the inside at the tip (for what it's worth). Further, given that this orifice, through the center of the threaded end of the coupler, would be the equivalent of a "pin hole" glue would only enter it with much difficulty as it would have to be highly compressed (particularly with a gel) and, as we know, things look for the path of least resistance forcing the glue to stay in the coupler/rod junction area.

Of course this idea would work equally well with tubes and fittings provided the glue is applied to the circumference of the fitting and the fitting is made with the required orifice to keep zero pressure differential (dP=0).

...sorry for the long winded explanation of a simple idea.

Are we having fun yet? I think I'll go back to building now...

David.

anything to prevent what happened recently.

david