Replies to This Discussion

Permalink Reply by Phillip Stacy on February 28, 2010 at 1:19pm

Doug, I have an off-line question about your certificate of compliance photo. I've sent you a friend requst...

Permalink Reply by Terry Phillips on February 28, 2010 at 10:49am

Regarding future testing of the 601XL/650:

I do not see that the FAA is “requiring” testing. First of all, the FAA is primarily addressing the S-LSA aircraft, not the E-AB aircraft. My understanding of the LSA concept is that it was designed to be a “self policing” process. That is, the manufacturer asserts in documentation to the ASTM or the FAA or someone(?) that the aircraft they want to sell as an S-LSA is compliant with ASTM F2245. Regarding the 601XL/650 situation, two F2245 requirements need to be addressed.

Flutter:

“Vibrations—Flight testing shall not reveal, by pilot observation, heavy buffeting (except as associated with a stall), excessive airframe or control vibrations, flutter (with proper attempts to induce it), or control divergence, at any speed from VSO to VDF,”

And Structural Strength:

“The structure must be able to support limit loads without permanent deformation. At any load up to limit loads, the deformation may not interfere with safe operation. The structure must be able to support ultimate loads without failure for at least 3-s. Proof of Structure—Each design requirement must be verified by means of conservative analysis or test (static, component, or flight), or both.”

I interpret the FAA position as, “The upgraded 601XL/650 is essentially a new aircraft design, and, before selling this new design as an S-LSA, the manufacturer must assert that this new design is compliant with F2245.” If I am correct, then testing is required before resuming S-LSA 601XL/650 sales.

I do not believe that there is any provision in F2245 to avoid the flutter flight test. On that subject, in the fall of 2009, Zenair announced that they were considering a flutter flight test on the 601XL/650. In December 2009, ZBAG submitted a report by Stephen Mitchell to Zenair giving his understanding of the requirements for a valid flutter test under F2245. The most difficult part of the requirement lies in the statement “with proper attempts to induce it.” Stephen’s report included documentation that simply rapping the stick with a mallet would not reliably excite the vibrational frequencies and modes that were predicted by flutter analysis of the 601XL/650. In consequence, some sort of remotely controlled, electro-mechanical device would be needed to induce the vibrations in the appropriate frequencies and modes.

F2245 allows substitution of “conservative analysis” for load testing. However, Zenair (apparently) relied on “conservative analysis” in lieu of testing in their original submission of the 601XL/650 as an S-LSA. It appears that the first load tests of the 601XL/650 at +G loads by Zenair were done in 2009. According to the FAA, those tests revealed that the “conservative analysis” was, in fact, optimistic and non-conservative. In consequence, I’m not convinced that Zenair’s assertion of compliance by “conservative analysis” would be credible with the audience that really matters, the airplane buying public.

I believe that there is danger here to the whole “self-policing” S-LSA concept. It is not up to the FAA to force Zenair to do the testing. But, if Zenair chooses to go forward without testing to prove compliance of the upgraded 601XL/650 with F2245, then the powers that be might well question whether the concept of a self-policing S-LSA industry is compatible with the public safety. I don’t know where that might lead, but unhelpful outcomes are definitely possible.

Personally, I believe that the upgrades will eliminate the structural failure accidents. But I also believe that, unless Zenair is proactive in proving that the upgraded 601XL/650 meets the requirements of F2245, then the future of the design is bleak. I am moving forward (slowly) with building my own 601XL. My airplane is an E-AB. I do not anticipate problems in obtaining an airworthiness certificate for the upgraded airplane. The real problem is with the future S-LSA version and Zenair’s credibility in the market place.

I support testing the upgraded 601XL/650 to F2245 standards. I believe that Zenair’s survival is in my interest. With that in mind, to support additional testing, I mailed my $100 check to Zenair on February 24.

Permalink Reply by Paul Mulwitz on February 28, 2010 at 12:13pm

Hi Terry,

While your analysis is probably valid, it doesn't really meet the needs of the whole Zodiac community well. I think those of us building and flying E-AB versions of the XL actually benefit from the more restrictive requirements of the S-LSA versions.

Rather than trying to be a "Lawyer" and interpret the fine details of the requirements, I would rather see the most conservative ideas regarding validation of the XL design satisfied. The whole LSA concept is very young and the details of the rules and regulations probably still need fine tuning. I know there are many issues in the S-LSA maintenance area that need further refinement, and the accident record of the XL shows a real need for further engineering design and validation.

I also want to see Zenith/Zenair survive this whole mess. That means I am willing to help pay for the additional effort even though I am not "Legally" required to do that. I don't think these small companies have unlimited funds, and I think all owners and builders have a stake in proving the soundness of the final design. I hope all the companies, builders, and fliers of the Zodiac XL and its nearly identical twin the 650 survive and thrive in the future.

Paul
Just starting installation of upgrade.

Permalink Reply by Matt Heintz on February 28, 2010 at 6:20pm

Terry, Paul and Doug including other comments before yours.

The comments are very good and positive in nature. Thank you.

Once we have the FAA letter and know exactly what is required, we can start focusing on the testing.

If we are going to do testing, I will want the FAA's input as it will be important to make sure that they take the test we do seriously.

Stephen Mitchell's report on how to perform a flutter flight test is very complex and I am not sure if the FAA will agree that such a test will in fact prove that the aircraft has no flutter. However, it is very detailed and does require special equipment that we do not have.

The argument that some have about "raps on the stick" is that its hard to simulate all the different possible frequencies etc. with this method. However, like Terry pointed out, there is not much about flutter in F2245. Do we really believe that most of the SLSA manufacturers actually took their aircraft through a detailed flutter program?

However, that is no reason for us to not do something more substantial, especially since the NTSB is sticking to flutter as being the cause of the accidents.

Another option is to get the German flutter expert to complete a brand new flutter analysis with the upgrade and to see from this report if there is any potential for flutter. If there is, we can do more tests.

Permalink Reply by Bob Pustell on February 28, 2010 at 7:58pm

Hopefully, when Mathieu and Company get firmer guidance from the Feds, it will be for a simple and affordable test regime that they can easily afford to do on their own. If it turns out to be a more expensive solution/suggestion/requirement, then I think the idea several people have presented of the owners kicking in to help with the cost of the testing is excellent. We all will benefit from it. I, for one, would be willing to kick in to a fund. There are a lot of owners and builders out there. If we all kick in, it would help a lot. Let us know when you know what you are up against, Mathieu.

Permalink Reply by Terry Phillips on February 28, 2010 at 11:24pm

Doug

I'm sorry, but I cannot give you Stephen's report. That report is his property, and he supplied it to Zenair (as far as I know, I am the only other person who has a copy). Also, sorry to disappoint you about my engineering training. I am a chemical engineer, and, to be honest, I never really understood the control guys when they started talking about Laplacian space, etc. If you want to get into about chemical kinetics, heat and mass transfer, fluid mechanics, etc., I can do better :-)

I believe that Stephen's understanding of flutter is on solid ground. For the basic background on flutter, see AC 23.629-1B in general, and for flight testing, see appx 2, in particular. Read the whole thing. It sounds like you have the background to understand it. I cannot claim that I do. But I do recognize that 8 hz is different from 12 Hz, and if I need 12 Hz, but I can only excite to 8 Hz with a mallet, I have a problem. Of course, I don't know for sure that I need 12 Hz until I do the GVT.

Actually, this is one area where I think that the FAA was right on. Hire a "noted flutter expert," don't fetter him with a restrictive NDA, let him do what he believes is necessary, then let him tell the world what he learned. Finally, bust up the airframe in a load test (and please publish the load analysis so that those who understand such things can decide whether Zenair or the FAA was right!) and show the world how strong the (upgraded) airframe is. I'm betting it will pass with flying colors.

Regarding airworthiness, I believe you are right. Do the mods and you are OK to fly. The SAIB is pretty clear on that. Which is why I believe that (legally) we are only talking about future S-LSA sales.

But, what the heck is an ASI?

Terry

Permalink Reply by Dave Miller on March 1, 2010 at 1:42am

Hi Mathieu,
I just received the upgrade kit and as soon as I finish fitting the canopy I'll start on it. Tho it looks like a lot of work, I'm looking forward to flying my plane in solid confidence when it's completed.

I have a related question to the upgraded demonstrator tho, if I may... what did the fuel lines and tanks after many years look like and did you repair/replace any? Have you had any trouble with them in the past? I'm at the point of installing them and am interested to know.
Thanks, and thank you for this discussion blog.
Dave Miller

Permalink Reply by Matt Heintz on March 2, 2010 at 9:31am

Dave,
It is my understanding that we have not had problems with the fuel tanks or fittings or lines. So if nothing is leaking, I would not change things.

Here is Rogers answer from Zenith.
The fuel tanks and fuel lines were in great shape, but I replaced the fuel lines and cleaned the finger screen in the tank.

On another note, I got a comment from a customer who is flying again after the upgrade.
"...after take off one has to press on the rudder petals quite hard to center the ball..."

My answer to him was:

Take a look at the 55 page pre-flight check list, Nose wheel area.

Line 107. "Inspect nose gear self centering at full deflection left and
right. Nose wheel must snap back to center by itself."

Also see line 190.

http://zenithair.com/zodiac/xl/data/601XL-PRE-FLIGHT-INSPECTION-Feb...

Also, if the rudder self centers, one can use the rudder as a trim tab. Adjust the cable lengths at the rudder so that the rudder is not centered on the ground (1 to 2 deg to the left or right depending on the rotation of the engine). When flying, the offset rudder will make the aircraft fly straight. This technique was used on the CH2000 very successfully (and approved by the authorities). You may have to adjust and fly the aircraft a few times before getting it just right. Adjustment should be made for cruise and not take-off, as engine torque in cruise is more but for a short period of time.

So ideally, when in cruse, the ball is centered. Kick the rudder peddle and the aircraft should quickly center itself.

Note that before anyone flies again, please go over the updated 55 page pre-flight inspection list. Ideally, you go over the list with someone else.

Permalink Reply by Dave Miller on March 4, 2010 at 9:03pm

Thanks fellas, Keep up the good work.
Dave

Permalink Reply by Doug Dugger on March 6, 2010 at 9:06pm

Just a thought on the hard rudder movement after take off. Go back and look at your rudder cable tension with the nose wheel off the ground. You may have way to much tension on the cables if you adjusted them with out raising the wheel off the ground, it slides forward when leaving the runway to settle in the cradle. As I said just a thought.

Permalink Reply by Bob Pustell on March 6, 2010 at 10:51pm

Am I correct is assuming that the taildragger version does not have a rudder cable tension setting? Unless you build in some sort of left and right pedal interconnection bellcrank (which is not in the plans), the two cables are not connected to each other in any way, typical of most taildraggers. My Stinson (and many other taildraggers) just has springs to hold the pedals forward against the cables, so the only cable tension is the tension of the springs. I intend to put springs on my 601XL pedals, even though there are none in the plans (that I have seen yet, at least, have not looked in great detail because I am not that far into the build).

Permalink Reply by Andy Elliott on March 7, 2010 at 1:09am

My TD 601XL has centering springs to keep the pedals from flopping around. They are not sufficient to apply any significant tension to the cables, maybe 3#. Yes, it takes a lot of rudder after takeoff to keep the ball centered, and any time I'm in a high-power climb attitude. The aircraft trim, in general, is very sensitive to power and attitude changes. Welcome to the world of light aircraft!

I use large trim tabs on the rudder to center the ball for cruise conditions.

There is one other TD 601XL and he also has springs, and trim tabs.

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