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06-08-2012, 07:02 AM
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Join Date: Aug 2005
Location: Dallas/Ft Worth, TX
Posts: 5,686
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Quote:
Originally Posted by az_gila
A regular washer would still help to spread the load.
An AN3 bolt has a circular flat area under it's head of 0.360 in diam.
An AN960 washer has an outer diameter of 0.438 in diam.
Above numbers not precise due to tolerances, but you can get the idea that the washer has a much larger flat area than the bolt head.
Since a washer under the head is standard aviation practise, I would just do it anyway... 
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The first time I saw the slotted hole in this piece I was shocked that it came from the factory that way. I agree with Gil and install washers on these bolts on every airplane I inspect.
__________________
Walt Aronow, DFW, TX (52F)
EXP Aircraft Services LLC
Specializing in RV Condition Inspections, Maintenance, Avionics Upgrades
Dynamic Prop Balancing, Pitot-Static Altmeter/Transponder Certification
FAA Certified Repair Station, AP/IA/FCC GROL, EAA Technical Counselor
Authorized Garmin G3X Dealer/Installer
RV7A built 2004, 1700+ hrs, New Titan IO-370, Bendix Mags
Website: ExpAircraft.com, Email: walt@expaircraft.com, Cell: 972-746-5154
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06-08-2012, 07:19 AM
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Join Date: Oct 2005
Location: 08A
Posts: 9,500
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Rob, when you have the vertical stab off for replacement of the F981, take a hard look at the location of the two drilled holes in the VS main spar:
It is a very common builder error to locate and drill those two holes in line with the rivet rows above and below. The result is a serious loss of edge distance on the thick spar web doubler.....with potential for the same ligament failure Kurt outlined above in discussion of your front spar attach plate. The correct placement is well inboard of the rivet lines and the doubler edge.
Returning to the cracked F981, would you be kind enough examine the edges of the slot and the perimeter edge of the part after dis-assembly and report here? Kurt's ligament failure is the fundamental mechanism. The crack merely starts at the weakest point, which is almost always a surface defect or an internal inclusion. A square edge with tool marks or an edge with mechanical damage (from the underside of the bolt head) would provide the surface defect. Neither should exist in a well built airplane, but we see it all the time. I'd put poor edge preparation near the top of the "common sins" list, perhaps even before poor riveting.
For those building now, please review your Vans Construction Manual, Section 5B. A part with properly prepared edges will reach its theoretical capacity without complaint. A part with square edges and/or tool marks will fail at some lower load. This is not the designer's fault.
Speaking of design, there seems to be some debate about the slot. I would suggest that the choice of slot or hole makes no significant difference in terms of stress concentration, in this orientation to load. Here's the simple view of elliptical vs round holes. Anyone have something similar for a racetrack shaped hole?
It should be noted that the stress concentration for a perfectly sharp notch (the perfectly bad tool mark, or the beginning of a shear) is infinity. Infinity is much larger than, for example, 2, 3 or 5.
__________________
Dan Horton
RV-8 SS
Barrett IO-390
Last edited by DanH : 06-08-2012 at 09:30 AM.
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06-08-2012, 07:52 AM
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Join Date: Jun 2006
Location: 8I3
Posts: 3,564
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A couple of years ago when the F1 was still in production the Czech engineers did a FEA analysis of the tail and found one of the weak points was the fwd spar. On my F1 there is a 1/8" tapered finger plate reinforcement that extends up the spar approximately 8" and the attach plate is 1/8" thick. Very overkill but an area I would gladly take overkill with. Especially considering that this is a stressed area from pushing and pulling and particularly when vibrating at lower RPM's such as runup.
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Please don't PM me! Email only!
Bob Japundza CFI A&PIA
N9187P PA-24-260B Comanche, flying
N678X F1 Rocket, under const.
N244BJ RV-6 "victim of SNF tornado" 1200+ hrs, rebuilding
N8155F C150 flying
N7925P PA-24-250 Comanche, restoring
Not a thing I own is stock.
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06-08-2012, 09:05 AM
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Join Date: Dec 2010
Location: WASh, DC
Posts: 26
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Vertical Stab Attach Plate Crack
I sent in some of the suggestion from this forum to Van's In part this is their response to install a washer, and/or make a new piece with a wider right edge.
"Neither of these suggestions could do any harm and the increased edge
distance could make the plate more crack resistant, but it would mean
making a custom part."
For Dan H's suggestion I will look for the alignment and I have concern for the HS that attaches to F-981. I had to catch up with a few things around the house and hope top remove the tail tonight, or on Sunday.
I will reinstall with the washers, but am unsure about fabrication of a modified right edge F-981. IMHO if a crack should develop again the crack just has more room to propagate to the right. It would seem a thicker piece, or stronger alloy is what would be needed for F-981.
However, I also think the washer suggestion with a cleaning and deburring of the holes, ensuring proper placement and alignment the VS and HS attach points.
My airplane has accumulated 815 hours and my concern is for other 9's and 9A's. If my airplane is just a one off incident then that is good for the fleet.
I will post our findings and how we proceed with the repair. I hope to have it all done within the next to weeks.
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06-08-2012, 09:21 AM
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Join Date: Jan 2007
Location: Battleground
Posts: 4,348
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Quote:
Originally Posted by rocketbob
A couple of years ago when the F1 was still in production the Czech engineers did a FEA analysis of the tail and found one of the weak points was the fwd spar. On my F1 there is a 1/8" tapered finger plate reinforcement that extends up the spar approximately 8" and the attach plate is 1/8" thick. Very overkill but an area I would gladly take overkill with. Especially considering that this is a stressed area from pushing and pulling and particularly when vibrating at lower RPM's such as runup.
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Bob - which model tail did they analyze?
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Smart People do Stupid things all the time. I know, I've seen me do'em.
RV6 - Builder/Flying
Bucker Jungmann
Fiat G.46 -(restoration in progress, if I have enough life left in me)
RV1 - Proud Pilot.
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06-08-2012, 10:25 AM
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Join Date: Jan 2005
Location: 57AZ - NW Tucson area
Posts: 10,011
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As I look more at this picture...
I notice that the location of the upper bend is very critical. If this bend is not formed right (does the builder bend it?) there is a high probablity of a lot of residual stress on the top bolt holes - the cracked one.
A lot of things should line up carefully with no fasteners for a good structural attachment. Torquing the bolts down or riveting the bracket in place "to get things to sit flat" could be bad.
__________________
Gil Alexander
EAA Technical Counselor, Airframe Mechanic
Half completed RV-10 QB purchased
RV-6A N61GX - finally flying
Grumman Tiger N12GA - flying
La Cholla Airpark (57AZ) Tucson AZ
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06-08-2012, 11:10 AM
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Join Date: Jan 2006
Location: Ehprata, WA
Posts: 318
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Quote:
Originally Posted by az_gila
As I look more at this picture...
I notice that the location of the upper bend is very critical. If this bend is not formed right (does the builder bend it?) there is a high probablity of a lot of residual stress on the top bolt holes - the cracked one.
A lot of things should line up carefully with no fasteners for a good structural attachment. Torquing the bolts down or riveting the bracket in place "to get things to sit flat" could be bad.
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Gil brings up and important point. The part is prebent from vans, however, and this was discussed in this thread, The bracket should sit behind the VS forward spar, not in front as depicted in this picture and the crack photo in question. If the bracket was attached on the front side of the forward spar, and the spar length after trimming was too long... the bracket would be under stress when bolted flat against the HS spar.
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Nate Benson
Ephrata, WA
RV9A/Slider N608MA
Flying
KitProject.com
"If you think you can do a thing, or think you can't do a thing, you're right." -- Henry Ford
Last edited by cln1owner : 06-08-2012 at 11:14 AM.
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06-08-2012, 11:14 AM
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Join Date: Jun 2006
Location: 8I3
Posts: 3,564
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Quote:
Originally Posted by JonJay
Bob - which model tail did they analyze?
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The EVO tail. The stock F1 kit can be easily modded to use the EVO reinforcements.
__________________
Please don't PM me! Email only!
Bob Japundza CFI A&PIA
N9187P PA-24-260B Comanche, flying
N678X F1 Rocket, under const.
N244BJ RV-6 "victim of SNF tornado" 1200+ hrs, rebuilding
N8155F C150 flying
N7925P PA-24-250 Comanche, restoring
Not a thing I own is stock.
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06-08-2012, 11:27 AM
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Join Date: Jan 2007
Location: Battleground
Posts: 4,348
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Quote:
Originally Posted by rocketbob
The EVO tail. The stock F1 kit can be easily modded to use the EVO reinforcements.
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Sorry, should have been more specific. I assume the analysis was done on the stock unmodified RV-4 VS?
__________________
Smart People do Stupid things all the time. I know, I've seen me do'em.
RV6 - Builder/Flying
Bucker Jungmann
Fiat G.46 -(restoration in progress, if I have enough life left in me)
RV1 - Proud Pilot.
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06-08-2012, 11:56 AM
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Join Date: May 2006
Location: Ankeny, Iowa
Posts: 434
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Just inspected my VS attach bracket and after 170 tach hours, no sign of cracks. Assembly is installed per plans with attach bracket behind VS spar.
__________________
Joe Condon
Ankeny, Iowa
RV9A - 647JC - 300 hrs - SOLD 6/23/15 
OneX - Under Construction - For Sale
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