OK. Let's kill that subject once for all. My point was to warn other
builders that when you order the 1.5 inch wide 4130 steel to make your
WAFs, you may end up with something slightly narrower, as I experienced
with my order from Aircraft Spruce. This might be an issue, so check what
you buy before you start drilling, and adapt your drawing accordingly to
keep a good wall thickness. End of the advice.
Now, in MY case, the problem is made slightly worse by the fact that my
WAF holes are 1/4".
As most of you know, I live in France, and am busy rebuilding my
South-African born KR2. The plan is to make it more inspectable (that's
the airworthiness authority requirement), and using the opportunity, to
cure any ageing problems (that is my personal requirement). Obviously, the
holes are already drilled in the wood, so I have to live with the current
holes sizes and location. Now, living in France, my options to rebuild the
WAFs are also limited. The material specified in the KR2 is pretty
standard, but the problem is, it is US standard. Finding material to US
specifications here is a nightmare. Europe is metric, not British
imperial. If you want steel for your WAFs, well, you will not find 3.2mm
thickness (that is what 1/8" translates into), but an integer in
millimeters (3, 4, 5?). Meaning that your BOLTS would now be wrong, and
ordering US aircraft bolts is ALSO an issue. . Not to mention the fact
that steel is not classified the same way, so I would have to find what
exactly is the nearest equivalent of 4130, then pray for it being commonly
found in the nearest equivalent thickness and width... Because, you see,
Europe is ALREADY built, so there is a much smaller market for raw
materials in general, and therefore, less outlets. Mail orders are not
well developed either, because the distribution system is so expensive...
I wanted to redo two of my WAFs, because the bolt was an oversize (I mean,
they are an AN-7 hole instead of an AN-6) , and I don't like that. Now, I
sit with a dilemma: either I put back the old WAFs, and I have to live
with the idea of having one WAF bolt that is unmatched, or I put the new
pair of WAFs instead, and then, I have to live with the idea of having two
WAFs that are slightly weaker than the 14 others.
Which one I am going to choose is an issue that concerns only me and my
airworthiness authority.
A consideration will be that my WAFs (both new and old) are pretty well
built, and are a tight fit.
Now, if you want to convince me that I must redo my WAFs or do endless
calculations, or seek professional advice from a stress engineer, or see a
shrink, forget it! Also, there is no way I am going to order, again, steel
from the US (talk about a cheap shipment bill!), with no guarantee of
what I will get... FORGET IT! ;-)
(Bob, thank you indeed for your post. It is always interesting to learn
more about procedures used in airliner construction. I was not familiar
with the existence of "Liaison engineers" in the civilian world. We do
have the equivalent in the military,).
Serge Vidal
KR2 "Kilimanjaro Cloud"
Paris, France
"Robert Morrissey" <cam...@earthlink.net>
Envoyé par : krnet-boun...@mylist.net
2005-09-17 02:37
Veuillez répondre à KRnet
Remis le : 2005-09-17 02:35
Pour : "KRnet" <kr...@mylist.net>
cc : (ccc : Serge VIDAL/DNSA/SAGEM)
Objet : Re: Réf. : RE: Réf. : KR> RE: Dual Ignition Systems, it Saved
My Bacon -
CORRECTION
Serge: As a former "Liaison Engineer' for a big aerospace company that
was bought out by Boeing I will give you the SOP (standard operating
procedure) for your aircrafts 'non conformance' that occurred during
'manufacturing' or 'depot level overhaul". (My text book answer)
FOR THE;
a. original WAFs with the oversize 3/8 inch hole---are they a standard
oversize? Let us assume they are not a standard size.
Looking at FWD WING ATTACH FITTINGS, drawing no. 18 in my RR book pg.23
dated january 1990 the WAF is 1 1/2 inch wide and has a 3/8 dia. hole
drilled 3/4 inch from the end. In my working days I would go to a stress
engineer, show him the part drawing, identify the 3/8 hole is oversize
and tell him what my method of repair was to be. The stress engineer
typically went to his books and evaluated the over size hole condition as
a
'lug analysis' with reduced edge distance (tear out) and 100% of the time
went along with the installation of a larger diameter fastener. For really
critical lugs, with really screwed up damage, I actually had inspection
record
the tear out as well as the actual part thickness and provided this data
to
the
stress engineer. The hole could be drilled to an oversize such as 1/64
oversize,
1/32 oversize or even a next full size standard size as was required by
the
part condition..
Any hole increased in size must have the same fastener/hole assembly
tolerances
as the rest of the WAFs. If there was a possibility of the oversize bolt
being removed in the
future the area was marked to denote the hole and special size fastener.
For those parts that would undergo constant bolt removal and replacement
then a minimum 0.016 inch wall thickness bushing would be pressed fit into
the discrepant part. The bushing would have a 0.0005 to 0.001 inch press
fit
and
be of the same material. This would bring the discrepant part back to
blueprint.
b. for your new WAFS-- my drawing referenced above shows a series of 3/16
inch diameter holes located 1/4 inch from the long edge of the part. If
I
take half of the 3/16 inch and subtract it from the 1/4 inch we have 5/32
(0.0156inch) of an inch for tear out. That is not much edge distance.
As an intelligent guess, if you are looking at
tear out of less than 0.140 inch I would not use them.
I am going to ignore the fact that the WAF drawings used are not to
aircraft standards. I have never seen fractions used on such a critical
machined part.
Standard sheet metal tolerances are +- 0.030 inch. Can you see what would
happen
on the WAF if that tolerance was used indiscriminately during manufacture?
How about checking your WAF holes to see what kind of assembly tolerance
your fittings have.
I never did get involved in reliability engineering during my work career
but I do love redundancy. I hope the above info gives you a feel for what
you have on your hands
Do you have any friends that are stress engineers that you can pass this
by?
Regards
Bob Morrissey, New Bern NC
----- Original Message -----
From: "Serge VIDAL" <serge.vi...@sagem.com>
To: "KRnet" <kr...@mylist.net>
Sent: Wednesday, September 14, 2005 5:16 AM
Subject: Réf. : RE: Réf. : KR> RE: Dual Ignition Systems, it Saved My
Bacon - CORRECTION
Well, as a former reliability expert, I can give you the text book answer:
- Reliability is risk management;
- The universally accepted method of managing that risk is what gives
birth to aircraft safety standards;
- The authorities (like ICAO) are supposed to define what is your
acceptable level of risk for the activity (roughly, basically, deaths per
X flight hours).
- At designer level, you then find what is likely to cause these risks,
and define "unwanted events" (example: power loss, flight controls
failure, etc.)
- If failure of one component that is likely to cause an unwanted event is
so unlikely that it is less than the acceptable risk level, then you don't
have to do anything about it. You assess that by estimating the
probability of failure (occurrence) and its consequences (severity), to
define the level of CRITICITY. A part can be non critical because it is so
unlikely to fail, or because .. If it is not the case, then you have to
improve the reliability. You have many ways to do that, (can be better
technology, better part design, monitoring, maintenance, you name it.). Of
course, one method is redundancy. It is seldom the correct answer, but it
is generally the easiest, and that's why it is so popular in aviation.
Now, this is the rule for expensive designs (airliners, jet fighters,
nuclear power plants, space shuttles or whatever). In general aviation,
the trouble is it is not affordable to calculate exactly all the risks
attached to all the parts and equipment. So, we apply a rule of thumb,
which is: no single failure may lead to an unwanted event. That is what
you do with your ignition or fuel system.
But we also apply criticity, through return of experience of 100 years of
designing and flying. We know that single engine is an acceptable
solution, so we don't make the engine redundant. But we make the engine's
most critical components redundant. The ignition, but not the carb...
Likewise, we know that control cables are unlikely to fail, provided you
rig them properly and inspect them regularly, so we don't make them
redundant either.
In my opinion, the statistics of engine failure in aviation are an
absolute shame, and the ignition is the main culprit. So, I go for a
better technology, and ultimately, as soon as it will become practical, I
will go for an engine technology without ignition: the Diesel engine.
Serge Vidal
KR2 "Kilimanjaro Cloud"
Paris, FranceSent: Tuesday, September 13, 2005 9:13 AM
To: Corvair engines for homebuilt aircraft
Subject: Re: CorvAircraft> Dual Ignition Systems, it Saved My Bacon
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