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LAA enginering

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z601a(at)anemicaardvark.c
Guest

Posted: Wed Sep 02, 2009 11:42 am Post subject: 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: 

 	  Quote: 	
		   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. 

 

 	  Quote: 	
		   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. 

 

 	  Quote: 	
		   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 

=============================================  [quote][b]

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Gig Giacona

Joined: 10 Jan 2006
Posts: 1416
Location: El Dorado Arkansas USA

Posted: Wed Sep 02, 2009 1:00 pm Post subject: Re: LAA enginering

Jim,



Thank you. That was a very well thought out and written post.

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_________________
W.R. "Gig" Giacona
601XL Under Construction
See my progress at www.peoamerica.net/N601WR

chris Sinfield

Joined: 28 Nov 2006
Posts: 270
Location: Sydney Australia

Posted: Thu Sep 03, 2009 4:17 am Post 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

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paulrod36(at)msn.com
Guest

Posted: Thu Sep 03, 2009 12:44 pm Post subject: LAA enginering

<?xml:namespace prefix="v" /><?xml:namespace prefix="o" /><![endif]-->  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

 [quote]   ---

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z601a(at)anemicaardvark.c
Guest

Posted: Thu Sep 03, 2009 1:04 pm Post subject: LAA enginering

On Thursday 03 September 2009 15:44, paulrod36(at)msn.com wrote:

 	  Quote: 	
		   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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Thruster87

Joined: 16 Apr 2008
Posts: 193
Location: Australia

Posted: Thu Sep 03, 2009 11:00 pm Post subject: Re: LAA enginering

Any rumors out there with respect to the following from Zenith;Regarding the 600 KG Zodiac, Zenair has indeed recommended reduced speeds and weights



to catch people's attention (we really, really need to stop these accidents due to over-loading



the aircraft). We will come up with an officially sanctioned (unlike the UK one) "Zenair



modification" in order for pilots to operate the aircraft up to the full LSA limits (i.e. MTOW



of 1,320 lbs with a safety factor of 1.5+)

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