[TowerTalk] Wind survival + load ratings... vs,

[Jim Thomson]

Date: Wed, 1 Nov 2017 06:06:10 -0700
From: jimlux <jimlux at earthlink.net>
To: towertalk at contesting.com
Subject: Re: [TowerTalk] FW: Wind survival + load ratings... vs,
reality.


<Overall, hams are probably more comfortable running and believing an 
<electrical model than a mechanical one (more time to become familiar, 
<etc.? or just because the ham tests ask you about electrical stuff, but 
<don't ask you about mechanical stuff)

<Another factor in mechanical designs is "hidden safety margin" - 
<typically in a design, you don't claim the actual expected yield 
<strength, you design for a bit lower (or design for a bit higher loads) 
< that accounts for material properties variation, variation in 
<structure strength from the assembly process, etc.

<So you might have a design which "officially" calculates out at 70 
<mi/hr, but which actually survive 100, sometimes, on a lucky day, with 
<the wind from the right direction.  That 100 is what gets claimed as the 
<survivability.

##  Both YS and YM have been around for a loooong time, esp YS.  
They are both within 1% of a megabuck finite analysis eng program.  
Dont kid yourself.  Both  M2 + optibeam have not incorporated any
safety factor into their designs.   They are both selling smoke.  If they were
to actually incorporate  a safety factor, M2 would be rating it at  50 mph. 

##  sure, one could put the boom broadside to the windstorm. The boom
is typ a lot stronger design than the eles in most cases.  But thats a band aid
fix at best. 

##  with the changing  wx patterns, it only gets worse.    These once in 50 year
events are appearing on an almost annual basis.   We are not talking about a lot
more expense involved to build a much stronger yagi.... or modifying an existing
design. 

##  Jeff, AC0C  wanted to re-use his old 3 el 40m telrex for a new 3 el 30M   full
sized affair  + 4 els on 17M...all on the same 29 foot  oem telrex boom.  The oem
40m telrex eles were aprx 52 ft long..and the loading was done with wires at the inboard
end of each 40m ele...and along the boom, on insulators.  We thought this would be easy,
just toss the LL wires, and re-tweak for 10.125 mhz.  Not so easy.  Who ever designed the
oem Telrex had his head up his butt, damned thing is only good for 56 mph in software.   Several
folks have broken els in a 65 mph windstorm.    Difficult to re-enforce too, since they used odd ball
wall thickness.   Would require machining custom inserts etc..and even then, the max windspeed
would still not be up to any standard. . In the end, it was decided to  dump the telrex eles..and 
design from scratch. 

##  so I designed the new full sized 30M eles using YM.  What I ended up with is good for  106.5 mph
using  the no spec, wind tunnel rating..and  114 mph using the C spec.    Els start at 1.75 inch OD... and taper
down to just .375 inch OD tips.   Els weigh  just   23.6 lb each.  The price tag, new  from DX engineering 
was  $366.30  for all the tubing to build 3 full sized els for 30m band.   Then u got shipping on top of that,
plus channel AL pieces and stauff clamps etc, etc,  so they could be mounted to the  oem telrex boom. 
End result is a beauty to behold. It will take all the ice you can throw at it.  Very minimal wind sag, just 
25 inches at the tips..and thats with NO ele truss.   No vortex shedding, and no tubing wasted anywhere. 

##  while on a roll, I designed a full sized 40M  REF.  Starts at 2.25 inch OD...then tapers down to .375 inch
OD tips,   The 73.125 ft long REF weighs  61.8 lbs..and is good for  106 mph using NO spec.... and  113 mph
using C spec.   Ele sag at the tips is only 41 inches... with no ele truss at all.   No vortex shedding. 

##  Its a straightforward matter to either design from scratch, factoring in standard length  tubing available, or 
modifying an existing design, say a F12 yagi, to make it stronger, while adding no additional windload, and very
minimal weight.   BTW, the sag on the JK ants   80m REF is very minimal, no truss required on any of the 80m 
eles, and just a single truss on the 58 ft boom.   NR5M now has a stack of 2 of these yagis. 
https://www.facebook.com/jkantennas/photos/pcb.957987491024953/957983204358715/?type=3&theater

Jim   VE7RF