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For those of you not familiar with the latest controversy (date of this update is 12/16/97), I'm devoting this page to the dissemination of all the information I could gather regarding the flutter situation identified by Martin Hollmann which he believes exists with the Lancair 360 outfitted with the large (MK-II) horizontal tail. Unlike control surface flutter, which is typically managed by counterweighting the surfaces and/or modifying their control actuation systems, the flutter identifed here is with the fuselage itself, and is caused by a coupling of the forces imposed by the MK-II tail on the aft section of the fuselage. Rather than try to put it into my own words, I've gotten permission from Martin to reprint the shortened version of his findings here, and he was kind enough to supply me with copies of his computer generated drawings which illustrate the actual flutter modes being discussed. Additionally he provided me with his sketch of the proposed fix, and that drawing is included here as well.

In response to Martin's report, Lance Neibauer published a reply to the issue on Compuserve, a copy of which was included in the Dec '97/Jan '98 issue of the Lancair Network News. That document is also included here, along with Martin's subsequent reply to same. I sincerely hope that everyone interested in this issue will give the proper consideration to all information provided here and make whatever decision they see fit as to its possible impact on their projects. The data provided herein is strictly informational in nature and not intended as an endorsement by this Web author of any view one way or the other. I am not an aeronautical engineer and do not suggest either the inclusion or exclusion of Martin's modifications... that is a decision you will have to make for yourself.






Lancair 360 Big Tail Flutter
by M. Hollmann
Reprinted from The Stallion News volume 4, number 2


About a year ago two Lancair 360 builders (Tom Replogle and Dick Reichel) contacted me to perform a flutter analysis on their aircraft. Tom Replogle is using the larger horizontal tail that has now become a standard on the Lancair 360. This analysis had not been performed by Neico since I am the person that has performed all the flutter analysis on their aircraft.

I set up a very detailed flutter model. The model showed me that the Lancair 360 with the big tail had a critical flutter speed of 145 kts. at S.L. and 141 kts (194 kts TAS) at 20,000 feet. Fuselage twisting and bending were coupling as shown on the figures below.

flutvert.jpg - 67360 BytesFuselage bendingfluttwis.jpg - 68615 BytesFuselage twisting
(Click on the images above to view them full-size.)
The fuselage bending occurs at 9.43 Hz and the fuselage twisting occurs at 13.03 Hz. To verify these frequencies we hired Sandy Freizner to perform a ground vibration test (GVT). We asked Dave Morss to fly his aircraft to Camarillo since Dave has a large horizontal tail on his Lancair 360. Camarillo was a central place for everyone to meet. We loaded the computer into the Stallion and flew down to Camarillo and performed the GVT test on four Lancairs, including Dave Morss's Lancair 360. The GVT test showed that the fuselage bending frequency was 9.27 Hz with 6% damping and 13.08 Hz for fuselage twisting. This was in very close agreement to our analysis and as such we felt very confident of our alarming results. With this flutter speed the Lancair 360 with the large tail should not be flown above 145/1.2 = 120 kts. By the way, the Lancair 360 with the smaller horizontal tail did not show this problem.

To increase the flutter speed for the big tail, I added three strips of 18 inches wide, style 7781 fiberglass cloth to the top and cottom of the fuselage and reran the flutter analysis. No flutter is observed for speed up to 600 kts for all altitudes. The fiberglass should be oriented at 0 (zero) degrees to the longitudinal axis of the fuselage.

I should mention that even though many of the Lancairs with the big tail have flown thousands of hours with no flutter, it does not mean that there is no flutter problem. Records show that some airplanes have flown 10 years with no flutter problem and suddenly it happens. And it only needs to happen once. Steve Wittman's fatal accident is a good example of that. He had flown his Tailwind over 10 years before he encountered a tail flutter problem. He and his wife were killed in their Tailwind and the tail of his aircraft has never been found.

If you own a Lancair 320 or 360 with the big tail, I strongly recommend making this simple modification. Fly safe!






Before getting on with Lance's response to the above report, below you'll find the sketch that Martin provided regarding the placement and BID schedule of the required fiberglass layers to eliminate the flutter problem outlined above. Please notice that while in the above article Martin recommended the use of 18" wide 3-BIDs on both the top and bottom of the fuselage, the drawing he provided at a later date shows a staggered BID schedule, with the innermost BID being 18" wide, the middle BID at 12", and the outer BID at 8". He also recommends that the wet layup be applied using the Shell Epon 862/Teta epoxy system, I can only assume that the layups will also work using Jeffco's epoxy system, or whatever system you have used for the construction of your aircraft. You might verify this fact with Martin before going ahead with the modification, should you decide to do so.


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Once again, just click the image to see it full size (891 X 696)






Reply to above report by Lance Neibauer, November 1997

Having read some of the... stuff ...on the web recently I can at least say that it, for the most part, ended well with statements suggesting the historical integrity of the Lancair company. I at least say thanks for that! But of course there are always those who enjoy embellishing controversy. It's that mushroom characteristic - one that seems to stimulate growth in dark musty fertile hangars.

So, regarding flutter, Iíll provide a brief history to describe this non-event. As we all know, the 320 has never ever exhibited any indications of flutter whatsoever. But ok, this is not proof that flutter canít occur. Not being an expert on flutter myself, I have spoken to many "experts" in the field. One trait rings true, "ask ten experts (although youíll be pressed to find ten flutter experts on this planet) and youíll surely get ten different answers" - and each will say the other is of course wrong.

Last year, we contracted with one of the best respected flutter experts in all of Europe (Norbert Niedbal of Vibration Engineering Inc.) who has specialized in composite airframe flutter analysis for all the major makers in Europe. Europe, by the way, leads the world in composte airframe manufacturing.

We outfitted a 360 MKII (large tail) with 38 accelerometers to measure normal mode deformations. Phase resonance methods were used for the survey. Mr. Niedbal states that although this method may be slow, "you see the mode shapes and know better what you do". Thirty six different normal modes were measured. Flutter calculations were made according to the P-K Method. For testing to 1.2 Vd, the frequency range was enlarged by a factor of 1.2 to 58 Hz. Actually some vibration tests were performed for a frequency range up to 65 Hz.

And the 132 page survey says: None of the control surfaces showed any instability. There was no fuselage tail section coupling found either. In fact everything came up excellent with one finding of possible interest: The flap rotation / torsion mode coupled with the wing bending and the wing torsion modes. Short story is that the flap, being essentially unsupported at the outbd end showed some potential for coupling with the wing. This case, according to the engineer, "has a very shallow curve and no aggressive slope". With no service history to suggest any problem any service bulletin would be of the precautionary nature, however we are about to issue one since a very simple 4lb preload on the outbd flap (in the up position) fully cured the case on the computer modeling. With this preload, the overall aircraft flutter free speed exceeded 300 KTAS.

We could undoubtedly contract with another flutter engineer and perhaps receive yet another set of data and flutter free speeds. Why all the differences? It seems there are lots of possible reasons such as, proper methods of excitation and monitoring, number of accelerometers and interpretation and accurate recognition of exactly what your measuring. Mr. Neidbal saw some "strong contradictions" in the one existing report that we sent him. With those contradictions, one would have had to "fix" the FE model which would require a few "tricks". Tricks such as varying shear modulus or densities for example but such "fixes" can cause added problems with subsequent interpretation.

To summarize a bit, we feel that Vibration Engineering Inc. is clearly a leader in flutter examination for composite structures, in fact we know of no one better qualified. Their report is quite detailed and reaches satisfactory conclusions for the MKII with and without wingtip extensions. He (Mr. Niedbal) recommends the modification to preload the outbd flap as a precautionary step particularly since itís of little effort to install. We will shortly be issuing a service bulletin regarding a simple means of attaining this small preload. With this addition, the flutter free speed calculates to exceed 300 KTAS and history, although through coincidence some will say, happens to fully support these findings.

Sincerely,

Lance Neibauer
President
Lancair International, Inc.






And finally, Martin's letter to Lance
To: Lance Neibauer


From: Martin Hollmann


Subject: Flutter Analysis on the Lancair 360 with the Big Tail


Date: 3 November 1997


I read your response to my flutter analysis on your aircraft and I was more than surprised about your comments on "tricks and fixes." You should read my book on flutter analysis which tells explicitly how a flutter analysis is performed today. The method described is the latest method that is used by most large aerospace firms in the USA and it is recommended by the Flight Dynamics Laboratory at Wright Patterson Air Force Base in Dayton, Ohio. It is the method I have successfully used on your aircraft and on over 20 others.

Many years ago, maybe 50 years or so, before the days of finite element analysis, a ground vibration test was performed and the data read from accelerometers was used to determine the mode shapes and frequencies of a structure. These mode shapes and frequencies were then used to perform a flutter analysis. The flutter analysis was performed using charts and calculations and later using computer codes.

This method worked if all mode shapes could be recognized and the mode shapes that coupled were compared. Often mode shapes were not identified and often improper results were obtained.

Today we perform a finite element analysis and compare the results to a ground vibration test. Using both methods we compare results and make certain that a mode has not been overlooked. Often one method will find a mode that was not found by the other method.

That is exactly what happened with the BD-10 jet on which I performed the "post crash" flutter analysis. The initial analysis was performed by an ex-Douglas Aircraft engineer who [was] only comparing 2nd bending and twisting of control surfaces. He never checked vibration mode coupling with other structures such as the fuselage or coupling of other surfaces.

When I performed the analysis I set up a very detailed FEA model of the tail assembly and input the data into my flutter program (SAF). The results showed that the fuselage bending up and down was coupling with the horizontal tail bending. A critical flutter speed of 385 kts was determined and that is exactly at what speed the tail came apart on the BD-10. The CEO of Peregrine International was killed in that accident. After the crash, we also made a ground vibration test to determine the actual control surface responses. During those tests we did not find the fuselage bending frequency. however, from the FEA and flutter analysis, we knew that fuselage bending was the culprit. I knew that earlier ground vibration tests had been performed. I looked through the data and "Bingo", there it was, "the fuselage bending frequency" that the FEA model had shown. Had I just used the control surface responses from the last ground vibration test I would not have found the correct answer. The finite element model data had shown us what to look for. I have also found cases where the vibration test came up with modes (assymetric modes) that the FEA model did not. It is therefore important to recognize that finite element modeling and vibration testing go hand-in-hand. There is no flutter analyst that will refute this. No one except Mr. Niedbal. Mr. Niedbal's analysis of your Lancair 360 tail suggests that only a vibration test was performed.

The flutter results of your aircraft are very similar to those of the BD-10. My analysis of your aircraft shows that the aft fuselage bending couples with fuselage torsion. It takes a large shaker to excite the aft fuselage for a ground vibration test. Did Mr. Niedbal find the fuselage bending and torsional frequencies? Did he include these in his flutter analysis? If you send me a copy of his report I will be happy to look it over and give you my opinion of what has been done.

One comment that you make really bothers me. You state that Mr. Niedbal quotes: (saw "strong contradictions" in the existing report(I assume that this is my report) With those contradictions, one would have had to "fix" the FE model which would require a few "trickes".) This statement clearly indicates that Mr. Niedbal is not familiar with finite element modeling. Certainly he is not using it as we do in the USA today.

I might add that I worked for Messerschmidt Bölkow Blohm and other companies in Germany for several years, and that Germany is far behind the USA in the development and use of computer programs. I have also worked in most major US aerospace companies where the real progress and development of composite materials is being made. If it were not for those developments, your aircraft would not have been made out of those materials. The honeycomb, the glue sheets, woven graphite prepregs and vacuum bagging methods were developed by such companies as DuPont, Hercules, Fiberite, Hexcel, Convair, Lockheed, Northrop, and many others. I know because I was there when it happened. But that, again, is another story.

I hope that you take my analysis seriously and take the correct steps to make your aircraft safe. Good luck.


Sincerely yours,


Martin Hollmann, President, Aircraft Designs, Inc.






An additional comment was sent to me by a Wittman Tailwind builder which addressed Martin's comments regarding the unfortunate breakup of Steve Wittman's airplane, and rather than paraphrasing anything he had to say I'd like to present his post to you here:


I am a Wittman Tailwind builder...and as such belive that Mr. Martin Hollman is incorrect in his assesment of Steve Wittman experiencing empennage flutter which led to his and Paula's death in the Wittman Tailwind O&O Special. I have attached the NTSB report on this to help clear things up.
Thanks,
Stan Julian (N1068WT)

NTSB Identification: ATL95FA092. The docket is stored in the (offline) NTSB Imaging System.

Accident occurred APR-27-95 at STEVENSON, AL Aircraft: WITTMAN O&O, registration: N41SW Injuries: 2 Fatal.

REPORTS FROM GROUND WITNESSES, NONE OF WHOM ACTUALLY SAW THE AIRPLANE, VARIED FROM HEARING A HIGH REVVING ENGINE TO AN EXPLOSION. EXAMINATION OF THE WRECKAGE REVEALED THAT THE AIRPLANE EXPERIENCED AN IN-FLIGHT BREAKUP. DAMAGE AND STRUCTURAL DEFORMATION WAS INDICATIVE OF AILERON-WING FLUTTER. WING FABRIC DOPE WAS DISTRESSED OR MISSING ON THE AFT INBOARD PORTION OF THE LEFT WING UPPER SURFACE AND ALONG THE ENTIRE LENGTH OF THE TOP OF THE MAIN SPAR. LARGE AREAS OF DOPE WERE ALSO MISSING FROM THE LEFT WING UNDERSURFACE. THE ENTIRE FABRIC COVERING ON THE UPPER AND LOWER SURFACES OF THE RIGHT WING HAD DELAMINATED FROM THE WING PLYWOOD SKIN. THE DOPED FINISH WAS SEVERELY DISTRESSED AND MOTTLED. THE FABRIC COVERING HAD NOT BEEN INSTALLED IN ACCORDANCE WITH THE POLY-FIBER COVERING AND PAINT MANUAL; THE PLYWOOD WAS NOT TREATED WITH THE POLY-BRUSH COMPOUND.

Probable Cause

AILERON-WING FLUTTER INDUCED BY SEPARATION AT THE TRAILING EDGE OF AN UNBONDED PORTION OF WING FABRIC AT AN AILERON WING STATION. THE DEBONDING OF THE WING FABRIC WAS A RESULT OF IMPROPER INSTALLATION.





Well, there you have it. I haven't heard if there's been another response from Lance regarding this last letter from Martin, but should something further come of this I'll be sure to report it here. If you have any questions about the above I suggest you contact either Martin Hollmann at Aircraft Designs (408)-649-6212, or Lance Neibauer at Lancair Aviation (541)-923-2244.

In case you're wondering, I will be incorporating Martin's suggested modification into my aircraft. Details of the process will be included here on the site for all to see, of course.






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