From: Lehanover@aol.com
Message-ID: <16d.26286b42.2d2312b7@aol.com>
Date: Tue, 30 Dec 2003 12:41:11 EST
Subject: Re: [FlyRotary] Re: aluminum hose bungs/fittings, evap core mounting
To: flyrotary@lancaironline.net
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In a message dated 12/30/2003 9:22:28 AM Central Standard Time, 
13brv3@bellsouth.net writes:

<< Thanks again for the tips.  What if I filled the core with water prior to
 welding?  Maybe I could change the water periodically as it heats up?  Would
 the water vapor be a problem with TIG welding?  At least I'd know if I
 reached the kit stage :-)


Well, it would work fine so long as the water is not very close to the area 
to be welded. Any hint of steam blows away the curtain gas (argon) and ruins 
the weld. 
I have never tried that idea. So between welds, the area must be obsoletely 
dry. 
With a wet rag, you can be cooling the brazed joints while having a dry weld 
area.
It sounds like a big deal, but once you try it, it goes real fast. You nail 
the power, weld for a few seconds, maybe 3/4" long bead, and stop. Let it cool 
down, and go again. It is just too easy.

  
 > You are about to rigidly attach a heavy aluminum cooler to (in effect)
 > an engine that can ring it's own case bolts hard enough to fatigue them to
 > death. The brackets are what? 
 
 The brackets are 6061-T6, and they could be steel if needed.  Weren't the
 broken case bolts a matter of harmonic vibration at certain RPM?  That's
 not likely to be a problem with these brackets, unless I'm really unlucky. 


Nobody that ever held one of those fat bolts in his or her hand could ever 
believe that you could break it by just shaking it. I sure didn't. Above 8,000 
RPM it is common. 
And as Tracy has discovered, below 8,000 RPM it is possible. The bolts will 
break in different places depending on the offensive engine RPM that is being 
used most often. Just as a piano string is tuned to one frequency, it will be 
sympathetic to a number of frequencies that are multiples of, or divisible with 
factors of 4. Like 8, or Octave? or 16 or 32 and so on.  

What???

The fourth and sixteenth harmonics of a base frequency are also very strong. 
So the case bolt will ring in sympathy, at several RPM ranges. Damping the 
very center of the bolt as Mazda does now, damps out the low (RPM) frequency that 
is the strongest for breaking the bolt. But the fourth harmonic of the base 
frequency would have a null at the center of the bolt anyway, so damping only 
the center does little at a much higher frequency.

So, the cure is to wind s big messy string of silicone sealant (GE tub and 
tile caulk 100% silicone) around the length of the bolt. Now there is no 
sympathetic frequency that can excite the bolt and all is well. So your bolts will 
break toward the center, and my bolts pop the heads off or snap off right at the 
first thread out of the front cast iron. Because we excite the bolts with 
different frequencies. 

Am I far enough into the weeds yet?

Every system has a natural frequency based on about 1,200 different things, 
but they have one. And they will be excited by frequencies above and below that 
frequency as above. The fourth and sixteenth harmonics being the strongest. 

So things like elevators and rudders are systems that have a natural 
frequency, and must be designed so that the "system" is never exposed to that 
frequency in service. Otherwise it flutters at it's base frequency, thus creating 
enormous drag for a second, and is ripped off of the wing. Or rips the wing off. 

Typically, these surfaces are 100% balanced. The control system must be free 
of slack or slop. The hinge system must also be free of slop. Control rods 
must be stiff in bending, and have a natural frequency that does not compliment 
(not a harmonic of) the moveable surface. The fixed surface that the control 
surface is attached to, must also be rigid to the extent that it cannot be 
excited by pulsation's from the propwash. This goes on to include everything on 
earth, but you get the idea. 


The rotary, I admit, does not shake like a big flat 4 (real airplane engine). 
Ray Charles can see the whole plane shaking. But, the rotary does shake. With 
great authority at engine speed, at a smaller amplitude (doesn't move very 
far). It must be, because the case bolts break. So your cooler, and attach 
bracket, and hoses form a system.

And, as above, all of Gods systems have their own frequency based on 
everything, as above. So your engine will provide the excitement with a big primary 
and a long list of secondary harmonics, and this energy will seek out you cooler 
system's primary or, a long list of secondary sympathetic frequencies, and 
will buzz the hell out of it. 

Maybe it will just be for a second while you go through this or that RPM. Or 
(with my luck) it will be for hours on end while you try to make it to 
Sun&Fun. The amount of movement induced into the cooler may be nearly undetectable. 
Just holding on lightly and revving the engine may show it off. Maybe not. Some 
airplane engines cannot use some props because of this. 

So you mount the source, and, the victim of the vibration in a damping 
material that will not pass the troubling frequency (Motor mounts and weather strip 
foam) and check often to be sure nothing has changed with time, and there is 
no metal to metal contact anywhere. Lest mother nature find and punish you.  

Lynn E. Hanover   

 
 OK, there are a number of you out there that have cores mounted to the
 engine.  Do you have them solidly mounted, or are they on an effective
 cushion of some type?  Can you post some pics?  Has vibration been an issue
 with any of your mounting methods?    
 
 Thanks,
 Rusty (shaken, not stirred)
 
 
  >>