Today's Message Index:
----------------------
1. 12:42 PM - LAA enginering (Jim Belcher)
2. 01:56 PM - Shielded wire for LED strobe? (dalemed)
3. 02:00 PM - Re: LAA enginering (Gig Giacona)
4. 02:14 PM - Re: Shielded wire for LED strobe? (Paul Mulwitz)
5. 03:25 PM - Re: Shielded wire for LED strobe? (Carlos Sa)
6. 03:55 PM - Re: Shielded wire for LED strobe? (Jim Belcher)
7. 05:09 PM - Re: Shielded wire for LED strobe? (Rick Lindstrom)
8. 05:44 PM - Re: Shielded wire for LED strobe? (Paul Mulwitz)
9. 06:43 PM - Re: Shielded wire for LED strobe? (dalemed)
Message 1
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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
============================================
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| Subject: | Shielded wire for LED strobe? |
I'm in the process of installing an AveoFlash combination LED position light and
strobe. I'm getting mixed signals on whether or not I should use shielded wire
on the strobe line.
Most "experts" say that shielding won't help because if there's a problem its conducted,
not radiated. Initially, the factory said shielded wire isn't needed
but they later came back and said that there are grounding issues with Zenith
airplanes and therefore I should use shielded wire.
I've asked them how the shield connection should be done. Do both ends need to
be attached to the airframe or just one end? If one, which end?
Does anyone have experience with the Aveoflash strobe?
Thanks!
--------
Dale
Flying Cessna 170B
Building Zenith CH650
Read this topic online here:
http://forums.matronics.com/viewtopic.php?p=261095#261095
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| Subject: | Re: LAA enginering |
Jim,
Thank you. That was a very well thought out and written post.
--------
W.R. "Gig" Giacona
601XL Under Construction
See my progress at www.peoamerica.net/N601WR
Read this topic online here:
http://forums.matronics.com/viewtopic.php?p=261096#261096
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| Subject: | Re: Shielded wire for LED strobe? |
Hi Dale,
I don't have experience with those units, but I was an electrical
engineer for more years than I have fingers. Let me try to clear up
the shielding and grounding issue.
There are two different wired connections in a strobe unit. One is
the 12 volt DC power wiring that goes from the battery to the strobe
power supply. The other goes from the strobe box to the flash tube
mounted on the wing.
The regular 12 volt power wire probably doesn't need any special
attention. The ground path completing the circuit from the strobe
box to the battery must be at least as big as the power wire, but
this isn't a difficult thing to do with either wire or the metal skin
of the plane. You should include a switch and fuse or circuit
breaker in this power circuit - just like any other device on the plane.
The wire from the box to the flash tube is a different kettle of
fish. It has high voltage (thousands of volts) and switches
completely on and off as often as the tube lights up - possibly a lot
more if it has an A/C signal. This path should be kept as short as
possible and probably shielded with one of two methods. Either it
should be shielded wire or twisted pair wire should be used between
the tube and the box. There will be two wires going to the tube -
these are the ones to twist. The problem is different if you have a
central power supply for the tubes and long high voltage lines or
separate supplies mounted near the flash tubes. In my plane (XL with
Zenith supplied strobes) the power supply is mounted in the wing tip
and the high voltage wires are only a foot or two long. Even if
these are not twisted the wing skin will shield the RF noise anyway.
All of this assumes you have a good electrical connection from the
wing skin to the fuselage. This will happen automatically if you
don't insulate all the metal to metal joints with zinc chromate or
some other paint. If you do that then just include some bonding
wires (usually braided copper) between the big wing and fuselage
parts to make a solid electrical connection. I recommend you do this
whether you use the skin as a ground for the strobes or not.
Good luck,
Paul
XL awaiting engineering changes.
At 01:55 PM 9/2/2009, you wrote:
>I'm in the process of installing an AveoFlash combination LED
>position light and strobe. I'm getting mixed signals on whether or
>not I should use shielded wire on the strobe line.
>
>Most "experts" say that shielding won't help because if there's a
>problem its conducted, not radiated. Initially, the factory said
>shielded wire isn't needed but they later came back and said that
>there are grounding issues with Zenith airplanes and therefore I
>should use shielded wire.
>
>I've asked them how the shield connection should be done. Do both
>ends need to be attached to the airframe or just one end? If one, which end?
>
>Does anyone have experience with the Aveoflash strobe?
>
>Thanks!
>
>--------
>Dale
>Flying Cessna 170B
>Building Zenith CH650
Message 5
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| Subject: | Re: Shielded wire for LED strobe? |
Hey, Paul, didn't you miss the point of the question?
(If you didn't then I did...)
My interpretation:
The Aveo strobe is solid state based, so there is no high voltage at all -
therefore the question is, does one STILL need shielding, given the low
power of this type of strobe?
Cheers
Carlos
2009/9/2 Paul Mulwitz <psm@att.net>
>
> Hi Dale,
>
> I don't have experience with those units, but I was an electrical engineer
> for more years than I have fingers. Let me try to clear up the shielding
> and grounding issue.
>
> There are two different wired connections in a strobe unit. One is the 12
> volt DC power wiring that goes from the battery to the strobe power supply.
> The other goes from the strobe box to the flash tube mounted on the wing.
>
> The regular 12 volt power wire probably doesn't need any special attention.
> The ground path completing the circuit from the strobe box to the battery
> must be at least as big as the power wire, but this isn't a difficult thing
> to do with either wire or the metal skin of the plane. You should include a
> switch and fuse or circuit breaker in this power circuit - just like any
> other device on the plane.
>
> The wire from the box to the flash tube is a different kettle of fish. It
> has high voltage (thousands of volts) and switches completely on and off as
> often as the tube lights up - possibly a lot more if it has an A/C signal.
> This path should be kept as short as possible and probably shielded with
> one of two methods. Either it should be shielded wire or twisted pair wire
> should be used between the tube and the box. There will be two wires going
> to the tube - these are the ones to twist. The problem is different if you
> have a central power supply for the tubes and long high voltage lines or
> separate supplies mounted near the flash tubes. In my plane (XL with Zenith
> supplied strobes) the power supply is mounted in the wing tip and the high
> voltage wires are only a foot or two long. Even if these are not twisted
> the wing skin will shield the RF noise anyway.
>
> All of this assumes you have a good electrical connection from the wing
> skin to the fuselage. This will happen automatically if you don't insulate
> all the metal to metal joints with zinc chromate or some other paint. If
> you do that then just include some bonding wires (usually braided copper)
> between the big wing and fuselage parts to make a solid electrical
> connection. I recommend you do this whether you use the skin as a ground
> for the strobes or not.
>
> Good luck,
>
> Paul
> XL awaiting engineering changes.
>
Message 6
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| Subject: | Re: Shielded wire for LED strobe? |
On Wednesday 02 September 2009 15:55, dalemed wrote:
>
> I'm in the process of installing an AveoFlash combination LED position
> light and strobe. I'm getting mixed signals on whether or not I should use
> shielded wire on the strobe line.
>
> Most "experts" say that shielding won't help because if there's a problem
> its conducted, not radiated. Initially, the factory said shielded wire
> isn't needed but they later came back and said that there are grounding
> issues with Zenith airplanes and therefore I should use shielded wire.
>
> I've asked them how the shield connection should be done. Do both ends need
> to be attached to the airframe or just one end? If one, which end?
A shield should be used as exactly that: a shield. It should not be used to
carry current. You should have a separate ground wire (unless, of course,
you are using the airframe as as a current-carrying ground for the strobe).
If I were wiring this, I would (assuming it is possible) not connect the
strobe assembly to the aircraft frame electrically. I would connect the
strobe power supply to the airframe, and run a separate, current carrying
ground wire out to the strobe, not connected to the airframe at the strobe
end. I would carry the strobe signal on a shielded cable, with the shield
connected to the strobe power supply at the strobe power supply, and nowhere
else.
Failing to do this can create a ground loop. Grounding the shield at both ends
causes it to carry current, and fail to act as a shield.
--
============================================
Do not archive.
============================================
Jim B Belcher
BS, MS Physics, Math, Computer Science
A&P/IA
Retired aerospace technical manager
============================================
Message 7
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| Subject: | Re: Shielded wire for LED strobe? |
Hi, Dale. I'm reasonably familiar with the AveoFlash units, since we sell Aveo
products at my shop.
The beauty of these units is that the "power supply" (such as it is), is molded
into the wingtip unit itself. All you need to do is ground the units to the airframe
with the black wire, provide power (red wire = nav lights, yellow = strobe),
and connect the blue wires to each other if your want the strobes to be
synchronized. If you're nervous about having a good electrical ground through
your airframe, you could always run another conductor out to the wingtips from
the battery ground, but I can't imagine this really being necessary on any metal
Zenith that has good electrical continuity through the airframe.
When compared to traditional strobe power supplies that send high voltage pulses
to a xenon flash tube from a multivibrator type power supply, there's virtually
no RF interference generated by the Aveo units. However, if you were to mount
a wingtip or beacon unit right next door to your comm antenna or a microphone
cable, you might pick up some noise from the internal flash power supply.
These units use so little current and are so voltage tolerant that the amount
of spurious RF energy generated is damn little. But you might be able to induce
a bit into an antenna or mic cable if you really tried by putting the cable
very close to the base of the Aveo unit.
I just went out to the shop with my handheld and performed a little experiment.
With both the strobes firing and the nav lights on, and tuned to the local tower
freq, I could only pick up radiated noise (that coincided with the strobe
pulse) when the handheld's whip antenna was within an inch of the base of the
AveoFlash unit (where the circuit board dwells), or when the antenna was wrapped
up in the power leads. It wouldn't break squelch on its own, but you can hear
a faint "swish swish swish" in the background of other tower transmissions.
When the antenna was moved a foot or more away, I could no longer detect any interference.
Hardly scientific, I know, but it suggests that the RF noise generated is very
minor and might only be an issue if you had your comm antenna mounted very close
to the AveoFlash unit itself. Do you have any antennas in composite wingtips?
Or an Aveo fuselage beacon near an antenna? I can't think of other scenarios
where it would be an issue, other than very near a mic cable or power lead to
an intercom or audio panel. But running a shielded power cable certainly wont
hurt anything. If you ground only one end of the shield, it should be the one
closest to battery ground.
Just my $0.02. Hope it helps.
Rick Lindstrom
ZenVair N42KP
-----Original Message-----
>From: dalemed <dalemed@gmail.com>
>Sent: Sep 2, 2009 4:55 PM
>To: zenith601-list@matronics.com
>Subject: Zenith601-List: Shielded wire for LED strobe?
>
>
>I'm in the process of installing an AveoFlash combination LED position light and
strobe. I'm getting mixed signals on whether or not I should use shielded
wire on the strobe line.
>
>Most "experts" say that shielding won't help because if there's a problem its
conducted, not radiated. Initially, the factory said shielded wire isn't needed
but they later came back and said that there are grounding issues with Zenith
airplanes and therefore I should use shielded wire.
>
>I've asked them how the shield connection should be done. Do both ends need to
be attached to the airframe or just one end? If one, which end?
>
>Does anyone have experience with the Aveoflash strobe?
>
>Thanks!
>
>--------
>Dale
>Flying Cessna 170B
>Building Zenith CH650
>
>
>Read this topic online here:
>
>http://forums.matronics.com/viewtopic.php?p=261095#261095
>
>
Message 8
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| Subject: | Re: Shielded wire for LED strobe? |
Hi Carlos,
Apparently I did miss the point. If the device in question is just a
bunch of LEDs powered by 12 volts then it seems unlikely that any
special actions will be needed to prevent noise in the radio
receiver. If anything is needed, a small filter on the power leads
at the device would probably do just fine. It could be as simple as
a small ceramic capacitor - probably 50 cents at Radio Shack.
It all depends on whether the primary light "Bulb" is a flash tube or not.
Best regards,
Paul
At 03:18 PM 9/2/2009, you wrote:
>Hey, Paul, didn't you miss the point of the question?
>(If you didn't then I did...)
>
>My interpretation:
>The Aveo strobe is solid state based, so there is no high voltage at
>all - therefore the question is, does one STILL need shielding,
>given the low power of this type of strobe?
>
>Cheers
>
>
>Carlos
Message 9
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| Subject: | Re: Shielded wire for LED strobe? |
Thank you all very much!
Since the factory is recommending shielded wire (probably just CYA) and because
I have some, I'll just tolerate the added few ounces. The factory, like Rick,
suggested connecting the shield as close to the battery negative as possible.
--------
Dale
Flying Cessna 170B
Building Zenith CH650
Read this topic online here:
http://forums.matronics.com/viewtopic.php?p=261138#261138
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