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1. 05:17 AM - Re: LAA enginering (chris Sinfield)
2. 01:44 PM - Re: LAA enginering ()
3. 02:04 PM - Re: LAA enginering (Jim Belcher)
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| Subject: | Re: LAA enginering |
great review of their reports..
Thanks..
I have hingless ailerons so I guess it does not mean much to me in the end.. But
what of the elevator trim tab horn change? why was that done??
Chris
Read this topic online here:
http://forums.matronics.com/viewtopic.php?p=261174#261174
Message 2
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| Subject: | Re: LAA enginering |
Jim, well reasoned and well written. It does seem like a solution in
search of a problem. I'd like to have seen some supporting reference to
a particular fix, or reinforcement for the carry-through and the spar
caps. Seems to me if something breaks, even if we don't know why, we can
look hard at making it stronger, unless we know King Kong was at the
controls. But I note you didn't rule out my theory of bad juju. Before
first flight I shall sacrifice a live chicken, wear a pyramid around my
left wrist, and obey the flight parameters.
Paul R
----- Original Message -----
From: Jim Belcher<mailto:z601a@anemicaardvark.com>
To: zenith601-list@matronics.com<mailto:zenith601-list@matronics.com>
Sent: Wednesday, September 02, 2009 2:34 PM
Subject: Zenith601-List: LAA enginering
I'm lazy, so rather than write two emails, one for the person who
asked these questions, and one for the list as a whole, I'm combining
them into one document. If that individual wishes to identify themselves
to the list, I'll leave that up to them. He certainly did not make any
statements for which they should be ashamed, or have regrets.
On Wednesday 02 September 2009 12:16, you wrote:
> I thought I'd write to you offline as it appears everyone else is
quietly
> digesting the LAA modifications. As you have an engineering
background, I
> think you might have some insight to the spar carry through that I
fail to
> see. I emailed to the engineer at LAA requesting they run the
airplane
> through the tests before and after the modifications so the effect
of the
> modifications could be quantified. The engineer refused, saying that
the
> admission of flutter by Zenith obviated the need to do before and
after
> tests.
My initial question here would be "what admission of flutter by
Zenith?" I don't recall seeing this.
> I admittedly find this reasoning specious. However, my review of
> crash reports and photos leads me to believe the wings were pulled
off by
> excessive g's, whatever the initiating event. These failures were at
the
> wing spar caps which bent and and then broke as the g's increased.
Perhaps
> I'm missing something(s), but the modifications to the spar carry
through
> do not seem to address this, or any other identified problem.
I agree.
> What do you think?
I think we have a set of solutions in search of a set of problems.
This reads like a document written by someone under government or big
company influence. They've opened their mouths, and grounded the
aircraft, probably under political pressure.
Now, there is a cry to unground the aircraft, and they, being the
experts, must come up with some approach to allow this.
========
I have reviewed the four documents issued by the UK LAA. I question
how generally applicable the solutions may be, and to which aircraft, if
any, they may apply. As best as I can tell, we are addressing possibly
four distinct aircraft design variants, all of which seem to be loosely
referred to as 601XLs:
A) The 601XL version sold in the U.S. up until last year.
B) The version sold by CZAW in Europe, while their agreement with
Zenith was in effect.
C) The version sold by CZAW globally, after termination of their
agreement with Zenith, which has a substantial difference in external
appearance.
D) The version sold by Zenair Europe, after the termination of their
agreement with CZAW. This may be identical to the version described in
"B;" but it may not.
The question one might ask is, "does the test aircraft correspond to
A) or C) above," since these are the only versions normally found in the
U.S. If not, these documents are largely not applicable to U.S.
aircraft.
I'm going to focus rather heavily on ground vibration testing (GVT),
since that's the testing area with which I have testing concerns. My
other concern is that we don't have a good analysis of what the problems
were that were being fixed, nor do we really have a structural analysis
of the impact of the changes. They just didn't break in flight, a
time-honored approach originated, I believe, by the Wright Brothers.
A) What was tested?
B) How was it tested?
C) Data and conclusions before any modifications.
D) Data and conclusions after any modifications.
I'm going to reference each of the four documents issued several
times, and the all have long names. Therefore, within this email, I will
reference them in the following manner:
Report 1: "FLIGHT TESTING OF ZENAIR XL G-EXLL WITH AILERON MASS
BALANCES AND STRUCTURAL REINFORCEMENTS"
Report 2: "ZENAIR CH601XL G-EXLL FLUTTER FLIGHT TESTING"
Report 3: "Zenair Flutter Testing - Analysis of the Results"
Report 4: "MANDATORY MODIFICATION MOD/162B/004 ZENAIR CH601 XL"
What Was Tested
On page 3 of the Report 1, we are told this aircraft was built from a
kit supplied by CZAW, in 2004. The photo on page 16 of the Report 3,
plus the date of construction shows this aircraft to be one sold by
CZAW, as described in "B" above.
Whatever else may be the case, it certainly isn't the version
described in A) or C) above. The external appearance, and the discussion
of the "Y" yoke mentioned various places in the text rule out
possibility C).
Possibility A) is ruled out by the reference to "rudder centering
springs," at the bottom of page 7 of Report 1.
First conclusion: These tests are not likely to have direct
applicability to aircraft in the U.S. The results are more applicable
than tests run on, say, a Boeing 777, but not applicable enough cause me
to implement their changes.
In fairness, LAA UK is concerned about aircraft in their country, not
ours.
How Was It Tested
Most of the test program appears to be reasonable, with the exception
of the ground vibration testing (GVT). I confess I had never heard of
conducting GVT in this manner. I regret that, after we both retired,
I've lost contact with the structural dynamicist with whom I once
worked. I would be very interested to hear his comments.
His approach (and the only one I'd ever encountered for GVT) was to
place the aircraft in a hangar, suspended from the ground. In flight, an
aircraft cannot transfer energy to the ground, and thus does not have
resonance dampened by ground contact.
One then excites the aircraft with a transducer - something like a
loudspeaker, but more sophisticated - over the entire range of
frequencies in which resonance might occur. Simultaneous with this, one
measures vibration in the aircraft with sensors. For mathametically and
physically minded readers, the measurement of frequency is being made
directly, in the frequency domain.
This appears to be the approach used by the outside consultants hired
by Chris Heintz for the tests in Europe. I take those tests seriously. I
really question the GVT conducted by the LAA. I'm not saying they are
wrong, just questionable.
First, we aren't told where the aircraft was located or how in was
suspended during GVT. There is a broad hint at the top of page 2 of
Report 3: "....but below that is another vibration at 2 Hz. This turned
out to be the frequency that the aircraft rocks from side to side on its
tyres."
It's very hard to rock from side to side on tires if the aircraft is
suspended and isolated from the ground. It sounds as though the aircraft
was resting on the ground during GVT. If so, I would be forced to
conclude that the GVT conducted was invalid.
We are told how the aircraft was excited in an attempt to find
resonance. Rather than use a transducer of some sort to sweep the
frequencies of interest, the wing was hit with a steel bar (page one,
paragraph 2, Report 3). This makes the aircraft ring, just like hitting,
say, a bell, with a hammer. The ringing may be too low in frequency to
be audible, but it is there, nonetheless. The amplitude variations were
recorded versus time, that is, a record in the time domain, not the
frequency domain.
Theory wise, this can be a valid measurement, if all other precautions
were taken (but as noted, they may not have been). There is a process,
referenced in the report, called the Fourier Transform, which can
convert the amplitude vs time into the frequency domain. That's the end
result needed, because it show the frequencies of resonance.
I have some concerns about this method of GVT testing. I suspect it
isn't is wide spread use because it likely has limitations. It is
dependent on giving a clean impulse to the aircraft. That is, hit the
thing, and get whatever you hit it with away from the surface
immediately, so it does not dampen the ringing,
I've done measurements of this type on other things (not, repeat not,
on GVT), and the results can vary widely depending on how hard and
consistently the impulse is given, from impulse to impulse, and how good
the measurement was. The question I would ask (and I think I know the
answer) is: "why a conventional GVT wasn't conducted?" The likely answer
is "lack of proper equipment." I'm forced to wonder (assuming this is
true) how much it may have colored the ability to properly conduct other
tests.
I have three conclusions about GVT:
1) Zenith's GVT tests were conducted in a more conventional manner. I
accept their results.
2) There is too little information about how the LAA tests were
conducted, and the information which is provided calls them into
question. I question the validity of their results.
3) I don't think I would care to lend an aircraft to the LAA for
testing.
I don't have any real issues with the flight testing, although I'll
admit my review in this area was cursory. It's a fair guess that the
report is going to say the plane flew fine with the modifications. Which
brings me to the next two things I feel we need to know.
Data and conclusions before any modifications
This whole area is missing from the report. We have no idea what the
resonant frequencies may have been before modification. There is no
record of any stress tests or structural analysis. Put rather abruptly,
we have no formal test or analysis of whether any problem or problems
actually existed. It's just assumed they did, or the aircraft wouldn't
have been grounded.
We do not know that it performed any better or any worse after
modification. We may very easily be fixing something that is not broken.
Data and conclusions after any modifications
The reports actually focus heavily on this area. It would indeed be a
surprise if a government influenced organization modified something, and
produced test results saying the modification did not work.
But, as noted previously, the value of most of this information is
limited, since lacking premodification test data, we don't know if the
changes helped or hurt the aircraft
We are given a number of modifications to be made, What we are not
given is anything that really substantiates the need for these changes.
Overall Conclusions
There was insufficient commonality of accidents to conclude why
problems were occurring with aircraft sharing some similarity of design
and construction. A politically acceptable approach was taken, and the
aircraft were grounded. A politically acceptable method needed to be
found to unground the aircraft.
Conducting pre-modification testing of the aircraft would likely not
have shown any problem or problems that needed correction. Modifications
were made anyway, and testing done to show that there were, indeed, no
problems on the modified aircraft. Everyone is happy, except possibly
the people who must pay to have their aircraft modified.
--
===================
Do not archive.
===================
Jim B Belcher
BS, MS Physics, Math, Computer Science
A&P/IA
Retired aerospace technical manager
===================
http://www.matronics.com/Navigator?Zenith601-List<http://www.matronics.co
m/Navigator?Zenith601-List>
http://www.matronics.com/contribution<http://www.matronics.com/contributi
on>
Message 3
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| Subject: | Re: LAA enginering |
On Thursday 03 September 2009 15:44, paulrod36@msn.com wrote:
> Jim, well reasoned and well written. It does seem like a solution in search
> of a problem. I'd like to have seen some supporting reference to a
> particular fix, or reinforcement for the carry-through and the spar caps.
> Seems to me if something breaks, even if we don't know why, we can look
> hard at making it stronger, unless we know King Kong was at the controls.
> But I note you didn't rule out my theory of bad juju. Before first flight
> I shall sacrifice a live chicken, wear a pyramid around my left wrist, and
> obey the flight parameters.
Don't forget to burn candles at each point of a pentagram. :-)
--
============================================
Do not archive.
============================================
Jim B Belcher
BS, MS Physics, Math, Computer Science
A&P/IA
Retired aerospace technical manager
Happiness is a belt-fed weapon
============================================
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