AlexPeterson
Well Known Member
My comments are in no way intended to be anything but constructive. However, with all due respect to the report's author, there seem to be some glaring omissions in the NTSB report. I am also in no way suggesting that the nose wheel fork modification doesn't make sense to do - I am doing it to my 6A. That being said, I feel there are some things worth discussing.
First, please reference:
http://www.ntsb.gov/publictn/2006/RV_Photos.pdf
http://www.ntsb.gov/publictn/2006/RV_Study.pdf
http://www.ntsb.gov/ntsb/brief2.asp?ev_id=20051006X01588&ntsbno=ANC05LA123&akey=1
There is a table at the end of the first document, which is very interesting. The raw numbers indicate, by model type, the following accidents/incidents:
6A - 4
7A - 9
8A - 4
9A - 6
and, flying aircraft from Van's website:
RV-6/6A 2238
RV-7/7A 528
RV-8/8A 752
RV-9/9A 351
There is not a breakdown of trike vs taildraggers on Van's website, but it is clear that the 6A's are underrepresented, by a long ways, in the accident report table referenced above. Some generalizations can be made. For example, the majority of 8's are likely taildraggers, and conversely most 9's are trikes. In any case, one could estimate that somewhere around 3 times as many 6A's are flying than the combined 7, 8 and 9A's. We should be curious as to why the large discrepancy.
If one pares down the 23 acc/inc. in the table, as the NTSB does in the text, by removing bad landings, off airport landings, running into obvious potholes, the list is eight acc/inc (the NTSB got nine, I get eight). None of these eight are 6A's.
The question that is begged is why? Is it coincidence? I suspect not. There are two distinct, and potentially very important, differences (excluding the 8A) between the 6A's and the 7/9A's.
First, many, if not most, of the 6A's would seem to have the older wheel bearing design, which includes a 1" bar all the way from fork to fork. Another thread deals with that:
http://www.vansairforce.com/community/showthread.php?t=18885
I believe all 7/8/9A's have the newer design wheel bearing setup, which doesn't have the large, fork to fork, axle other than the 3/8" bolt. Finish kits for the 6A's shipped after about '97, give or take, also had the new design.
So what? Well, it appears that the analysis that was done may not have contemplated vibrations or dynamic resonance considerations. There was a comment in the report stating that the analysis showed that the drag of the nose wheel when locked would not cause the gear to fail. That may be true if one simply locked the axle and dragged the plane (static situation). However, there have been many who have written here that they had a fore/aft or up/down pitching vibration on their nose gear. Clearly, if the bearing design is such that there can be variable drag on the nose wheel, one could imagine a resonance run away until the nut drags on the ground, at which point the party is over. If bearing drag contributes to this, it could be a culprit, particularly if bearing drag increases with wheel load.
Secondly, the longer main gear on the 7/9A's aren't helping the situation. The nose gear has further to come down when the elevator runs out of authority to keep it up. I do not know if this is a factor or not. But, the cg of the airplane is higher above the ground, so clearly less pitching moment is needed to start the flip over process. Are these differences enough to explain the discrepancy? I don't know.
The third item of interest is contained in the third link, which is the full narrative of the Palmer AK accident report. This portion is interesting:
"He said his touchdown speed was about 58 mph, and he touched down about 20 feet from the end of the runway. He said that several seconds later, he felt what seemed to be the nose wheel dragging, and shortly thereafter, the nose wheel seemed to drag again, but much harder. The airplane subsequently nosed over, and received structural damage to the fuselage, the left elevator, the left wingtip, the rudder, and the vertical stabilizer."
The portion which really caught my eye was the pilot's report of an initial feel of something dragging, followed by a much harder dragging. What was this initial dragging? One could imagine that the harder dragging was when the gear nut started plowing.
Another interesting finding in that report:
"Slight, uneven surface undulations were noted in the area where the nose wheel fork assembly began to scuff the tops of the undulations, producing slight gouge marks that began about 500 feet from the approach end of the runway. The gouge marks increased in depth toward the end of the runway, and ended where the nose fork dug into the ground."
Is this a dynamic vibration situation? I Don't know.
I plan to continue to contemplate this issue, and welcome any meaningful inputs to this discussion.
First, please reference:
http://www.ntsb.gov/publictn/2006/RV_Photos.pdf
http://www.ntsb.gov/publictn/2006/RV_Study.pdf
http://www.ntsb.gov/ntsb/brief2.asp?ev_id=20051006X01588&ntsbno=ANC05LA123&akey=1
There is a table at the end of the first document, which is very interesting. The raw numbers indicate, by model type, the following accidents/incidents:
6A - 4
7A - 9
8A - 4
9A - 6
and, flying aircraft from Van's website:
RV-6/6A 2238
RV-7/7A 528
RV-8/8A 752
RV-9/9A 351
There is not a breakdown of trike vs taildraggers on Van's website, but it is clear that the 6A's are underrepresented, by a long ways, in the accident report table referenced above. Some generalizations can be made. For example, the majority of 8's are likely taildraggers, and conversely most 9's are trikes. In any case, one could estimate that somewhere around 3 times as many 6A's are flying than the combined 7, 8 and 9A's. We should be curious as to why the large discrepancy.
If one pares down the 23 acc/inc. in the table, as the NTSB does in the text, by removing bad landings, off airport landings, running into obvious potholes, the list is eight acc/inc (the NTSB got nine, I get eight). None of these eight are 6A's.
The question that is begged is why? Is it coincidence? I suspect not. There are two distinct, and potentially very important, differences (excluding the 8A) between the 6A's and the 7/9A's.
First, many, if not most, of the 6A's would seem to have the older wheel bearing design, which includes a 1" bar all the way from fork to fork. Another thread deals with that:
http://www.vansairforce.com/community/showthread.php?t=18885
I believe all 7/8/9A's have the newer design wheel bearing setup, which doesn't have the large, fork to fork, axle other than the 3/8" bolt. Finish kits for the 6A's shipped after about '97, give or take, also had the new design.
So what? Well, it appears that the analysis that was done may not have contemplated vibrations or dynamic resonance considerations. There was a comment in the report stating that the analysis showed that the drag of the nose wheel when locked would not cause the gear to fail. That may be true if one simply locked the axle and dragged the plane (static situation). However, there have been many who have written here that they had a fore/aft or up/down pitching vibration on their nose gear. Clearly, if the bearing design is such that there can be variable drag on the nose wheel, one could imagine a resonance run away until the nut drags on the ground, at which point the party is over. If bearing drag contributes to this, it could be a culprit, particularly if bearing drag increases with wheel load.
Secondly, the longer main gear on the 7/9A's aren't helping the situation. The nose gear has further to come down when the elevator runs out of authority to keep it up. I do not know if this is a factor or not. But, the cg of the airplane is higher above the ground, so clearly less pitching moment is needed to start the flip over process. Are these differences enough to explain the discrepancy? I don't know.
The third item of interest is contained in the third link, which is the full narrative of the Palmer AK accident report. This portion is interesting:
"He said his touchdown speed was about 58 mph, and he touched down about 20 feet from the end of the runway. He said that several seconds later, he felt what seemed to be the nose wheel dragging, and shortly thereafter, the nose wheel seemed to drag again, but much harder. The airplane subsequently nosed over, and received structural damage to the fuselage, the left elevator, the left wingtip, the rudder, and the vertical stabilizer."
The portion which really caught my eye was the pilot's report of an initial feel of something dragging, followed by a much harder dragging. What was this initial dragging? One could imagine that the harder dragging was when the gear nut started plowing.
Another interesting finding in that report:
"Slight, uneven surface undulations were noted in the area where the nose wheel fork assembly began to scuff the tops of the undulations, producing slight gouge marks that began about 500 feet from the approach end of the runway. The gouge marks increased in depth toward the end of the runway, and ended where the nose fork dug into the ground."
Is this a dynamic vibration situation? I Don't know.
I plan to continue to contemplate this issue, and welcome any meaningful inputs to this discussion.
