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  #41  
Old 01-27-2012, 02:29 PM
Mark12A's Avatar
Mark12A Mark12A is offline
 
Join Date: Jun 2011
Location: Huntsville, AL
Posts: 89
Default Service Life

What I'm getting from this is that, failing flight into cumulo granite, I'll reach my expiration date long before my airplane will.
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  #42  
Old 01-30-2012, 11:21 AM
rgrigson rgrigson is offline
 
Join Date: Feb 2008
Location: Mill Creek, WA
Posts: 19
Default Service Life

The question of service life is an excellent one, and one which I reckon is not helped by the war stories of guys flying aircraft with 50 000 hr+ airframes. Fact is aluminum cracks under conditions of cyclic loading, and therefore our aluminum RV airframes have areas that we should be paying attention to during annual condition inspections (particularly high cycle and high time RV?s).

Service life is a difficult question. There are no safe-life estimates for an RV that I?m aware of. Vans may have tested a wing spar to 12000 hrs. This means little without knowing the details such as spar type, spar material, load spectrum, workmanship, etc. Also, the wing spar is by no means the only item one should be concerned about.

The service life of an RV from a metal fatigue perspective will be dependent on:
A) Basic design and strength of the aircraft (materials, layout, joint configurations, sizing etc)
B) Construction workmanship (maintaining edge distance, hole spacing, drilling or reaming, deburring, sharp edge removal, surface finish, surface treatment, and flaws created by bucking bar mishaps, dropped tools, mis-drilled holes, etc )
C) Loading spectrum (did you fatten your RV with luxuries, do you mostly fly A to B, fly over mountains regularly, do aerobatics, dog-fighting, do you operate on rough fields, is your prop dynamically balanced, is your engine smooth? etc)
D) Ability of the RV community to identify problems, spread the word, and implement on-going repairs.

We will assume Vans did a tremendous job getting A) right with an adequate service life in mind (however nothing is documented, remember these are amateur build ?experimental? aircraft where we as the builders are taking responsibility for our airframes). Now, assuming everybody stuck to the plans, it is likely A) will be similar between similar RV?s. However B) and C) will not. In the amateur world, workmanship and loading spectrum are big variables. Just because someone has reached 4000 hours with no identified problems is not a reason for everybody else to ignore service life, drink scotch, and go on pulling 6G.

Regarding D), as the RV fleet gathers more cycles and hours, there are going to be some issues that arise. There are several key locations around the airframe that undergo cyclic loading every flight (sometimes many times a flight) such as wing spars, wing carry-through box, horizontal tail spars, vertical tail spars, fuselage longerons, engine mount, landing gear, fittings & back-up fittings etc. Hopefully the trail blazes in our community will take time during annuals and investigate some of these areas. VAF (and Vans hopefully) will be pivotal in making sure the message gets out if something comes up. Waiting for the NTSB and then the FAA to do it is not the way to go. Be vigilant during your annuals.

I for one will perform some basic airframe inspections at annuals, and during any other time when some key locations become more accessible (ie wings come off etc). Also, as the builder I know of some locations where there were some accidental flaws. I will look at those too. I look forward to an airframe that will hopefully deliver thousands of hours of fun.

In response to DanH, yes, commercial aircraft tend to refer to ?cycles? when they design for safe-life (a cycle is usually a Ground to Air to Ground (GAG) mission). A fatigue mission profile is developed whereby the aircraft is at its typical weight, relevant fuel load, doing a typical mission. The structure is then analyzed using this fatigue mission to show it good for its safe-life of XXXXX GAG cycles. In some cases, the structure may also be subjected to ?high cycle? loadings, and in those cases the additional cycles would be included in the calculations.

Robert Grigson
RV-8 FB with IO-375
266 hours and loving it
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  #43  
Old 01-30-2012, 12:38 PM
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Andrew M Andrew M is offline
 
Join Date: Aug 2006
Location: Secluded Lake,Alaska (AK49)
Posts: 359
Default Data point

I know I am just a builder, I hope my experience as a maintainer is of value to someone.
Cessna requires eddy current inspections at different thresholds for the C208B depending on if it is flown in "normal" or "severe" conditions. Structures for the engine, landing gear, wing attach, and stabilizer attach all get looked at. Some inspections require complete removal (stabilizer and strut attach), others require on bolt at a time. These inspections are designed to find cracks using either bolt hole probes or surface probes. One uses a ring probe to look for second layer cracks. Out of all these inspections, the only thing we found was chafing on the vertical stabilizer attach (14,000hrs, 18,000 cycles if I remember right). This required replacing both vertical stab spar webs, and a bulkhead. We found the chafing because it had to come apart for the inspection, not the eddy current inspection itself.
I intend to use close tolerance bolts in reamed holes for the vertical stabilizer attach and start removing to look at it at 3 or 4 thousand hours. This attach area has AD's against it for the C206, C207. Seems to me Bonanza had some trouble in this area to, Can any one chime in? The loading is not symmetrical and doesn't carry all the way through like the horizontal stab.
Doing the one bolt at a time for the wing attach is not so good from a labor, quality of work perspective. If I do (special) wing attach inspections, I would consider x-ray, if it can be done for a couple hundred bucks. A surface probe eddy current might work O.K. (any Level 2 techs have an opinion?), again starting at 3000, or 4000 hrs.
Landing gear and it's mounting seem to let go without catastrophic results, an are being found visually before anything really serious occurs. The interior part of this assembly will likely not be covered in my airplane so I can keep an eye on it. I would like to see a pole on how this area fails, but am not sure how to present the questions. Hours, cycles if tracked, and home airport surface material would be nice to know.
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A&P, IA
-9 empennage
Wings arrived 12 JAN 13
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