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02-20-2014, 10:05 AM
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Join Date: Oct 2007
Location: Sacramento, CA
Posts: 3,351
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Quote:
Originally Posted by Walt
Helps to relieve the stress concentration in the corner of the web where the cracking is occuring.
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Thanks Walt. I do realize I had not explained my question right but you got it.
__________________
Mehrdad
N825SM RV7A - IO360M1B - SOLD
N825MS RV14A - IO390 - Flying
Dues paid
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02-20-2014, 10:29 AM
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Join Date: Dec 2009
Location: West Jordan, Utah
Posts: 228
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Quote:
Originally Posted by Walt
Helps to relieve the stress concentration in the corner of the web where the cracking is occuring.
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I was just about to ask this question as it seemed silly to cut them off. Your explanation clears it up. Thanks!
__________________
Jason Hess
N777JH - Reserved
RV-7 - Builders Log Tail complete, wings nearly done, starting fuselage!
RV Factory - Complete sans floor coating, Slideshow Here
Quote:
People are not excellent because they achieve great things; they achieve great things because they choose to be excellent.
Gerald G. Probst, Beloved Grandfather, WWII B-24 Pilot, Successful Businessman
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02-20-2014, 11:21 AM
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Join Date: Sep 2009
Location: Beavercreek Ohio
Posts: 5
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SB 14-01-31
HS Spar inspection showed no cracks, took some time to remove the Paint overspray and Primer, but none seen. Relief notches per drawings.
RV-7, 200 hrs, some Acro, O-360, no cracks
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02-20-2014, 11:32 PM
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Join Date: Sep 2005
Posts: 14
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RV6-A: No Cracks
RV6-A
First Flight 1996
980 Hours TT
No cracks in HS or Elevator Spars
Relief per plans
Heavy Acrobatics first 25 hours / very little thereafter
98% paved / 2% grass or dirt / 10% very poor landings 
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02-21-2014, 03:25 AM
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Join Date: Apr 2007
Location: Norway, Stj?rdal
Posts: 598
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Quote:
Originally Posted by Walt
Helps to relieve the stress concentration in the corner of the web where the cracking is occuring.
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Maybe, but in this case I can't understand how that would matter at all. The main problem is clearly an original design choice where the flange (on a structural beam) changes from being on the fwd side to the aft side with no overlap. Thus leaving a small section of the structural beam void of flange material which causes the shear web to carry too much compression/tension load. In fact, so much that it ends up cracking.
Since the main problem has not been addressed by the SB, the forces carried by the shear web is exactly the same amount as it was before. However, the added web material will of course make the stresses in the web much less than they were.
Fatigue in metals is a function of the main stress value and the stress amplitudes together with the number of cycles. Stress "peeks" as function of design means nothing as long as the critical values are controlled. In the SB, the single only thing that decreases stress is the added material on the web. Trimming of the flanges certainly help smoothing out the stresses, but has little or no effect on the max stress amplitudes. You cannot fool the basic laws how structural beams work.
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02-21-2014, 07:13 AM
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Join Date: Sep 2008
Location: Pagosa Springs CO
Posts: 243
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Inspection report
RV8
IO360
Hartzell CS prop
1650 hours
Lots of acrobatics up to 5 G
Some unpaved runways
No relief notches
Tiny (1/8") cracks on top right and bottom left (blended out by creating notches).
Installing modification per SB
Questions for Walt:
1. one rivet hole measures .147" on the bottom angle (HS 714) and I slightly damaged (oval) one hole in the upper angle (HS 714). Should these holes be drilled out to 5/32" and AN470AD5-7 rivets installed?
2. The two rivets just inside the bend line (labeled "staggered rivet is oriented up" and its counterpart) seem very close to the notch adding to the stress riser. Would it be appropriate to move this rivet down 3/8"?
__________________
Craig Taylor
RV8 flying
2020 VAF dues paid with thanks
Navy A7E Vietnam era
Ret. Corporate Pilot: G1159, IA Jet, Jetstar, Falcon 10, 20, 50
Last edited by gear1 : 02-21-2014 at 07:19 AM.
Reason: misspelling
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02-21-2014, 07:23 AM
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Join Date: Aug 2005
Location: Dallas/Ft Worth, TX
Posts: 5,687
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Quote:
Originally Posted by gear1
Installing modification per SB
Questions for Walt:
1. one rivet hole measures .147" on the bottom angle (HS 714) and I slightly damaged (oval) one hole in the upper angle (HS 714). Should these holes be drilled out to 5/32" and AN470AD5-7 rivets installed?
2. The two rivets just inside the bend line (labeled "staggered rivets is oriented up" and its counterpart) seem very close to the notch adding to the stress riser. Would it be appropriate to move this rivet down 3/8"?
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I appreciate your confidence in me, but, I know my limitations... when it comes to deviating from an approved spar repair I think the engineer would be the best place to go for that info.
It's "probably" ok as most engineers will build in some buffer for mistakes, but I don't think probably is good enough in this case, get the answer from "the man".
__________________
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
Last edited by Walt : 02-21-2014 at 07:25 AM.
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02-21-2014, 10:56 AM
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Join Date: Jul 2005
Location: Hubbard Oregon
Posts: 9,035
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Quote:
Originally Posted by SvingenB
Since the main problem has not been addressed by the SB, the forces carried by the shear web is exactly the same amount as it was before. However, the added web material will of course make the stresses in the web much less than they were.
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???
Not sure how forces/stresses can be the same and be less at the same time...
Quote:
Originally Posted by SvingenB
Fatigue in metals is a function of the main stress value and the stress amplitudes together with the number of cycles. Stress "peeks" as function of design means nothing as long as the critical values are controlled. In the SB, the single only thing that decreases stress is the added material on the web. Trimming of the flanges certainly help smoothing out the stresses, but has little or no effect on the max stress amplitudes. You cannot fool the basic laws how structural beams work.
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As mentioned previously in the thread, this mod is not for making the structure stronger. Previous static testing has already proven that it meets the design requirements for limit and ultimate loads.
The SB is to a localized stress area that after lots of load cycles can possibly develop a crack. Tapering the flange of the fwd. spar, and confirming that there is a tapper in the top flange of the top angle does help further distribute the load through this area. This was substantiated with FEA and is also a fundamental engineering practice.
Please refrain from comments that might make people take shortcuts while doing the SB modification, without having personally done a full analysis of the structure.
__________________
Opinions, information and comments are my own unless stated otherwise. They do not necessarily represent the direction/opinions of my employer.
Scott McDaniels
Van's Aircraft Engineering Prototype Shop Manager
Hubbard, Oregon
RV-6A (aka "Junkyard Special ")
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02-21-2014, 11:48 AM
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Join Date: Apr 2007
Location: Norway, Stj?rdal
Posts: 598
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Quote:
Originally Posted by rvbuilder2002
Not sure how forces/stresses can be the same and be less at the same time...
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Stress = Force/Area
The forces are the same. Your airplane weighs the same, you pull the same amount of G etc. Adding more material obviously decreases the stress in the material. ABC in engineering.
Tapering the flanges weakens the structure further. It does however, smooth out the stress distribution. But to what gain? Absolutely none stress wise. The stress at the weakest point does not change if you weaken the area around it. The strains could benefit from the tapering however, making the HS flex in somewhat the same way as before. This could in fact be important regarding vibration and also for the other SB, the cracks in the elevator.
Static loads and fatigue loads are different things. But stress and strain does not differentiate between the name of the load. To decrease stress, you increase material dimensions for a given load (force). This is the same for static loads and dynamic loads.
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02-21-2014, 12:55 PM
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Join Date: Jul 2005
Location: Hubbard Oregon
Posts: 9,035
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I strongly recommend that people installing the doubler parts prescribed in the SB do not take short cuts and skip any of the steps.
__________________
Opinions, information and comments are my own unless stated otherwise. They do not necessarily represent the direction/opinions of my employer.
Scott McDaniels
Van's Aircraft Engineering Prototype Shop Manager
Hubbard, Oregon
RV-6A (aka "Junkyard Special ")
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