mat
i,m just back in c/l flying after about a 20 yr. hiatus thats all i ever fly is profile ive not had that problum with my planes. always think (bullet proof) these planes are not the rubber band planes they take a lot of abuse up there in landing and stunt operation. if your useing rubber bands to hold a tank on use good hooks screwed into the plywood doublers they should not come undone.                                                 

   roger                                                              BULLETT PROOF*                                                                             

Matt, pretty hard from a distance to diagnose something like that,, several things to look at though,
first is it really vibrating, or perhaps were the tank brackets not soldered on well?

Mark,  the tank bracket itself broke, not the soldering job.  I guessed it was vibration, because isn't that what would make a tank bracket split right atthe bend???

Can you feel pronounced vibration?

Not until the tank bracket broke.  It has been awhile since I balanced that prop...well I should anyway rebalance it.

This hasn't been the first time the brackets have broken loose, all those times were from the solder joint.  I did originally have a McCoy .35 on it with a 4 ounce tank.  The brackets were positioned long ways on the tank.  I switched to an LA to get more power.  When I switched to this engine, I didn't get a lot of run time, I still don't, but switched to a 4 1/2 ounce tank.  The tank brackets were moved to the top and bottom of the tank and that is when I started getting troubles with the tank brackets coming loose.

Could it be all the holes drilled through the fuse that could be putting stress on the nose be causing it to break loose???

Matt,
How long are the brackets? if they are fairly short, then I would say its probably a result of the configuration that is giving you grief. Especially since you say the problem didnt start till after you switched the setup. what LA are you using,, 40? 46?
On my profiles, I use a 1/16 ply plate that fits behind the whole tank, its epoxied to the back of the tank and has slots milled in it for adjustment. Of course I epoxied it before the tank had any fuel in it so that is not as good foryou since its really hard to get all the oil off. I also put a piece of 1/4 foam behind the tank before I screw it down. I want my tank to be completely supported. If there is any way that it can move, it will and that movement translates to vibration which translates to fatigue at some point in the attachment system. Rubber bands do work, especially if you get creative and setup some sort of "shelf" that you can shim the tank consitantly on, otherwise I find the tank moves around or isnt adjustable both can be problems as you I am sure are aware.
Perhaps one solution would be to solder 4 tabs on the tank, ( yeah I know what a pain right?)  I have also made straps that solder to the back of the tank so they have no bends in them say a peice of thin brass 1/2 inch wide . Perhaps by now you know where the tank needs to be for consitant settings and you could attach it more permantly without needing to adjust it? that would simplify your situation a lot.
Keep up the good work, sounds likeyou are understanding the process and thats a huge part of the solution!

I have had the exact thing happen to me on planes where I had fabricated the bracket out of K&S brass stock.  For a while I went back to eyelets and rubber bands but have since switched to the pre-fabricated Brodak tank brackets.  They are inexpensive, well made, adjustable and I have never had one fail due to vibration.  You may want to give them a try.  FWIW I also use a pad of somekind between the fuselage and tank - foam, cork, duct tape, piece of soft balsa, all will work.

I have broken tank brackets also. What I do is cut a piece of steel 1" long by 1/4" wide by 1/8" thick, and drill a hole in the middle for the hold down screws. By placing the steel over the brass slotted brackets I can still adjust the tank up or down but have had no problem with the brackets breaking as they clamp the brackets to the fuse. I use the brass threaded inserts for the tank hold down bolts. I also like my tanks mounted as solid as I can get them as I get the best runs this way.

 

Matt –

From this post and others, it is clear you are a very logical problem solver.  Yes, mounting a larger tank on a relatively short nosed profile means that there is no room for end brackets. Using top and bottom brackets is a viable way to go, and there is no reason to abandon that approach. (There are other good solutions, as mentioned above.)
   You didn’t mention how the brackets were fastened to the fuselage. The inherent vibration in a profile nose means that the fasteners need to be darned tight, essentially engine bolt mounting tight. Wood screws or sheet metal screws can work, but often cannot be tightened enough to prevent some loosening due to vibration, which will in turn eventually cause the bracket itself, the tank, or the solder joint to fail. That does not mean that any of those parts themselves are not strong enough.
   As Jim Kraft mentioned above, threaded inserts, although marginal for engine mounting, are quite solid enough for tank mounting in my and others experience. A dab of white glue around the threads and / or at the bolt head / bracket interface also acts somewhat like Loctite (without the permanence) and can serve as a micro vibration dampener. A solid mounting / attachment system, however you choose to accomplish it, is the key to successfully using soldered tank brackets.
   Any of the padding choices mentioned above work. One not mentioned is two sided foam tape, about 1/16” thick. Stick it to the (very clean, lacquer thinner clean) tank, and leave the paper on the fuselage side so it doesn’t stick to the fuselage – leaving the desired adjustment capability intact. (Pics below) With multiple (thicker) layers, remove the protective paper between layers, of course. The tape adhesive is not 100% fuel proof, can get gummy, but stays in place just fine. I like the tape because (a) I have some, and (b) it is easy to use.

Larry Fulwider