brad walton
Well Known Member
A dab of RTV was recommended in the trailing edge at the end of each stiffener in my plans in 1998.
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RV-8 Elevator Failure
- Thread starter Ron RV8
- Start date
+1 from 1997. And I put a more than a dab. I squeezed in quite a bit knowing I didn't want the skins moving/vibrating. No cracks or issues after 17 years and 1925 hrs.
RV10inOz
Well Known Member
Alternate view of things.
I have watched the video multiple times and looked at the failure, and I am very much agreeing with Scott McDanials from VANS. This is highly unlikely to be flutter from a considerable but small overweight balance horn.
The pilot maintains there was no damage done while at the fuel pump and the aircraft was in his sights the who time..........this may be true, but so what! I believe this is not when the initiating damage has occurred.
There are two fatigue cracks (easily explained) and one impact point on the underside of the elevator. If I am not mistaken, the line of which is in line +/- a negligible margin with the main gear.
The impact strike that has weakened the elevator would appear to be from a small hard and high speed object. Think running over a bolt or nut, and it firing into the skin, most likely on landing but equally on take off.
Scott.........what do you think?
I have watched the video multiple times and looked at the failure, and I am very much agreeing with Scott McDanials from VANS. This is highly unlikely to be flutter from a considerable but small overweight balance horn.
The pilot maintains there was no damage done while at the fuel pump and the aircraft was in his sights the who time..........this may be true, but so what! I believe this is not when the initiating damage has occurred.
There are two fatigue cracks (easily explained) and one impact point on the underside of the elevator. If I am not mistaken, the line of which is in line +/- a negligible margin with the main gear.
The impact strike that has weakened the elevator would appear to be from a small hard and high speed object. Think running over a bolt or nut, and it firing into the skin, most likely on landing but equally on take off.
Scott.........what do you think?
snopercod
Well Known Member
That's probably why the resale value of fiberglass airplanes is MUCH less than RVs. OTOH, we don't really know what's inside that failed elevator, do we? For either fiberglass or aluminum construction, the skill of the builder is critical, I think. I made my choice a long time ago to go with fiberglass, but I certainly do admire the RV series.
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David,
After watching the video again, it looks like the edge broke. Is that what you are seeing? It is hard to tell if the damage to the edge is from flexing or from some other force. My first thought was the ribs ripped from the spar, but in the video it looks like the edge just broke. Perhaps from flexing in flutter or from previous unknown damage or weak spot on the edge. I will be real interested in what it looks like when it is opened up.
After watching the video again, it looks like the edge broke. Is that what you are seeing? It is hard to tell if the damage to the edge is from flexing or from some other force. My first thought was the ribs ripped from the spar, but in the video it looks like the edge just broke. Perhaps from flexing in flutter or from previous unknown damage or weak spot on the edge. I will be real interested in what it looks like when it is opened up.
grubbat
Well Known Member
Interesting
If the trailing folded edge was compromised from a strike, that would be a very interesting development. Couple that with a stress area and now you have a perfect storm. I wonder what are the odds that a random rock, bird, or whatever could compromise the folded trailing edge? If it was a zippered trailing edge, would we get the same compromised structure assuming it was some sort of UFO strike?
If the trailing folded edge was compromised from a strike, that would be a very interesting development. Couple that with a stress area and now you have a perfect storm. I wonder what are the odds that a random rock, bird, or whatever could compromise the folded trailing edge? If it was a zippered trailing edge, would we get the same compromised structure assuming it was some sort of UFO strike?
1989 plans did not call for RTV or anything else. Just properly bent trailing edges. My -6 has been flying for over 24 years with .016" skin, no RTV, and no cracks. First 300 hrs. with 150 hp. Remainder with 175 hp.
Just want to clarify that on the stiffeners on the bottom of the elevator that are cracked, that it really does not appear to be caused by FOD. The cracks are similar in that they all start near a stiffener. No dents on bottom of elevator at all from impact that I can notice. Just the cracks. And also for clarification the trailing edge is compromised in two spots both having a crack going forward on top and bottom making it no longer rigid. This is just my opinion of what I see. Sure wish we could say that is was just Foreign Debris that caused this as my mind would be at ease as well believe you me, but not convinced.
scsmith
Well Known Member
It really does look to me like some kind of impact damage right at the trailing edge in two spots.
I confess there is an element of speculation in that, but I don't see an alternative way of causing those dent/kinks at the trailing edge.
My take-away is to examine my control surface trailing edges, upper and lower, more carefully on pre-flight than I have been in the past. Thanks.
I confess there is an element of speculation in that, but I don't see an alternative way of causing those dent/kinks at the trailing edge.
My take-away is to examine my control surface trailing edges, upper and lower, more carefully on pre-flight than I have been in the past. Thanks.
what if someone would model the elevator using finite element analysis and apply a gust load to determine the failure points? I don't have the time now but maybe over the Winter. seems like it would be a good project.
may also want to look at the dynamic response.
may also want to look at the dynamic response.
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A possible theory
I referred back to my builders manual to check this. Part of the process to construct the elevators is to squeeze the trailing edge to the required radius.
The elevator trailing edge is essentially the "rear spar" of the elevator. It takes the bending loads across the rear of the elevator due to control application, vibration, and aerodynamic buffeting. If during construction the trailing edge radius is squeezed down too much, or if it is repeatedly squeezed and opened up again to get the right radius, it may be weakened or even cracked. If that has occurred, it seems possible that the weakest spot along the trailing edge will eventually fail locally. If that happens, bending along the trailing edge will be focused at that point, resulting in rapid longitudinal cracking of the upper and lower elevator surfaces and associated increasing bending deflection from further weakening. This looks like it might fit what is seen in the video.
Stripping the paint off of the trailing edge area of the elevator to get a look at the metal may help in understanding whether or not this is actually the case.
I referred back to my builders manual to check this. Part of the process to construct the elevators is to squeeze the trailing edge to the required radius.
The elevator trailing edge is essentially the "rear spar" of the elevator. It takes the bending loads across the rear of the elevator due to control application, vibration, and aerodynamic buffeting. If during construction the trailing edge radius is squeezed down too much, or if it is repeatedly squeezed and opened up again to get the right radius, it may be weakened or even cracked. If that has occurred, it seems possible that the weakest spot along the trailing edge will eventually fail locally. If that happens, bending along the trailing edge will be focused at that point, resulting in rapid longitudinal cracking of the upper and lower elevator surfaces and associated increasing bending deflection from further weakening. This looks like it might fit what is seen in the video.
Stripping the paint off of the trailing edge area of the elevator to get a look at the metal may help in understanding whether or not this is actually the case.
