sdthatcher
Joined: 13 Mar 2008
Posts: 91
Location: Port Saint Lucie
|
 Posted: Fri Apr 17, 2009 6:03 am Post subject: Thoughts regarding Flutter |
 |
|
I believe the problems that we are experiencing with the 601XL are not the result of flutter or any other single issue but instead the result of 4 specific and simultaneous factors that have caused the wing failures of our aircraft.
First, why not flutter? Because in the instances that have been reported to be flutter, the time that was required to identify the vibration and to escape from its destructiveness has been too long (more than a few seconds). My opinion is that we are seeing a simple harmonic vibration of the ailerons that, depending on the tension in the cables, can be considered as scary enough to cause real concern.
My first theory is that the problem we are seeing with the ailerons is harmonic vibration, not flutter, and is dependent on the tension of the cables. In my flight, the cables had loosened to the point that I could feel a 1-2 inch movement of the stick without any corresponding movement of the ailerons. As I went over the Valley River Mountains near Andrews, NC, I could see the ailerons start to vibrate, so I decreased the throttle and the vibration stopped. In another case locally, the pilot had loose cables and, during flights above Va, experienced severe vibration. He tested this by purposely flying into that portion of the envelope repeatedly (observing the same type of vibration each time) and then to test it again after tightening his cables. In his case, the vibration stopped above Va. I believe that depending on the combination of engine, aerodynamics and cable tension, the vibration of the ailerons could be seen as mild or severe... but not flutter. Another possible instance of severe vibration was a 601 in Georgia that experienced vibration after flying over a power plant and which required a dive to eliminate the vibration. It is known that to reduce flutter, you need to reduce airpseed and pray. But in this case the means of reducing the aileron vibration was to dive at speeds that were close to Vne. Not the usual way to stop flutter and in any case, the resulting actions took considerably longer than required to avoid desctruction from flutter. In two of the three cases mentioned, the vibration was severe enough to apparently cause movement of the wing, as stated by the pilots.
So, if the problem is just vibration how does that cause destruction of the wings? My second theory is that the wings of every aircraft that has failed to date was structurally compromised at some point in the history of the aircraft. Recently, I was talking to a gentleman here in Andrews, NC that was in the Lakeland bound 601 just a week or so before it lost its wings and was destroyed. He stated that while flying with the pilot over the mountains near here, they experienced what he described as severe turbulance. He mentioned to the pilot how really bad the turbulance was and the pilot stated that "this was nothing compared to a week earlier". Apparently the turbulance was so bad that the pilot called the experience "bone crushing". As for the Yuba City 601, the owner of that aircraft had stated that there was an indication of "smoking rivets" on the bottom of the wings. This condition usually is the result of rivets becoming loose and creating a condition that results in a decrease in structural integrity. The Yuba City aircraft also (I believe) was flying near or in mountainess areas during its history.
The third condition that must be met is looseness of the aileron cables. I believe that vibration will not occur while the cables are tight.
And the fourth criteria for these wing failures is flying above Va (maneuvering speed).
So to summarize, if these 4 conditions are met, the probability is high for the aircraft to be lost even in mild turbulance.
Once again, they are:
1. Aileron Vibrations
2. Previous structural damage to the wings.
3. Loose Aileron Cables
4. Speeds above Va
I had originally thought that the only way we could explain the loss of wings on the 601XL was from flutter, however, since 98% of the fleet has not lost its wings and yet reports continue to surface of severe vibration, loose cables, etc., it seems to me that there must be a combination of problems that occur simultaneously that result in destruction of the wings.
Anyone who has experienced severe turbulance should carefully look over their wing roots for damage and avoid flight, especially if they also have loose cables!
Scott Thatcher, N601EL
601XL with WW Corvair, 25 Hours
Just finishing first Condition Inspection
[quote][b]
| | - The Matronics Zenith-List Email Forum - | | | Use the List Feature Navigator to browse the many List utilities available such as the Email Subscriptions page, Archive Search & Download, 7-Day Browse, Chat, FAQ, Photoshare, and much more:
http://www.matronics.com/Navigator?Zenith-List |
|
_________________
Scott Thatcher, Port Saint Lucie, FL
601XL with Corvair, Registered as E-LSA
N601EL, EAA203 140 hours and not flying currently. |
|
Get Email Distribution Too!
FAQ
Search
Memberlist
Usergroups
Register
Profile
Log in to check your private messages
Log in
