Wing flutter
08-24-2002 | 10:29 PM
#26
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From: Baton Rouge,
LA
Wing flutter
Someone correct me if I am wrong, but I don't
thing this plane had a spar in the outer wing
panels. The owner told me it had some ribs
to keep it from smushing.
Eddie Weeks
http://www.corpcomp.com/weeks1/
thing this plane had a spar in the outer wing
panels. The owner told me it had some ribs
to keep it from smushing.
Eddie Weeks
http://www.corpcomp.com/weeks1/
08-24-2002 | 10:36 PM
#27
Wing flutter
Hi Eddie,
Your theory on the cause of flutter is pretty much what the text book will tell you. For true flutter you need 2 forces that feed each other and a some what elastic structure.
In one case the wing might bend as it loads up which makes the tip twist up, which increases the load, which makes the tip twist more, and so on until the structure can resist any further movement, at which point it acts like a spring and sends it shooting off the other way, and the whole thing repeats on the other side. This can go into a limit cycle, where it just keeps doing this with out breaking, or the structure fatigues and fails, or it can keep building until something breaks. If the structure breaks as it loads up the first time that is more of a structural divergence.
Another flutter mode that is seen on full scale light aircraft is where the inertia of the control surface causes it to lag behind the flex of the wing or tail, and this induced control deflection increases the bending or twisting of the main surface which again increases the control deflection which creates more load and so on as above.
Having the max thickness point further aft does not help, since this will move the elastic center (I think I remember my structures terms correctly) aft, and as you stated if the elastic center gets behind the center of lift the wing wants to twist the wrong way.
Since many of us have seen Raptors zooming by with out a problem I would guess that the problem in this case was a bad glue joint that your friend could never have know about. This is always a possible problem when some one else build part of you model.
Steven
Your theory on the cause of flutter is pretty much what the text book will tell you. For true flutter you need 2 forces that feed each other and a some what elastic structure.
In one case the wing might bend as it loads up which makes the tip twist up, which increases the load, which makes the tip twist more, and so on until the structure can resist any further movement, at which point it acts like a spring and sends it shooting off the other way, and the whole thing repeats on the other side. This can go into a limit cycle, where it just keeps doing this with out breaking, or the structure fatigues and fails, or it can keep building until something breaks. If the structure breaks as it loads up the first time that is more of a structural divergence.
Another flutter mode that is seen on full scale light aircraft is where the inertia of the control surface causes it to lag behind the flex of the wing or tail, and this induced control deflection increases the bending or twisting of the main surface which again increases the control deflection which creates more load and so on as above.
Having the max thickness point further aft does not help, since this will move the elastic center (I think I remember my structures terms correctly) aft, and as you stated if the elastic center gets behind the center of lift the wing wants to twist the wrong way.
Since many of us have seen Raptors zooming by with out a problem I would guess that the problem in this case was a bad glue joint that your friend could never have know about. This is always a possible problem when some one else build part of you model.
Steven
08-25-2002 | 09:33 PM
#28
My Feedback: (6)
Wing flutter
But many of us have seen Raptors zooming by successfully, but if you will remember back a few years ago, there were rumors and stories floating around about several Raptor wings that fluttered and blew apart. I suspect that when the wing is built right, it is an adequate structure. But when it is built just a bit wrong, say not good enough adhesion on one side of a spar perhaps, then it flutters at speed and BOOM! Sounds like a quality control issue.
08-26-2002 | 12:39 PM
#29
Senior Member
My Feedback: (11)
My Raptor finally flew...
Well, my Raptor finally did its maiden flight yesterday, after sitting in my garage/trailer for two years or so. So far, the wing is still intact (well, apart from the chip missing from one wing TE where one of my friends carried the aircraft into a fencepost...;-)
The beast acted pretty noseheavy, despite being balanced exactly per the instructions. Are any of you other Raptor owners using the CG from the instructions, or did you find a better one ? (Actually, since the instructons changed several times, I should probably justy ask "Where do you have the CG on your Raptor?")
We also burned up the Glennis brakes on each landing (melted the plastic), but other than that, it went better than expected thanks to the expert thumbs of my hired-gun test pilot (Chris Huhn).
Gordon
The beast acted pretty noseheavy, despite being balanced exactly per the instructions. Are any of you other Raptor owners using the CG from the instructions, or did you find a better one ? (Actually, since the instructons changed several times, I should probably justy ask "Where do you have the CG on your Raptor?")
We also burned up the Glennis brakes on each landing (melted the plastic), but other than that, it went better than expected thanks to the expert thumbs of my hired-gun test pilot (Chris Huhn).
Gordon
