Part 1
I read through this thread and would like to put down my thoughts here. I have only become aware of this issue recently because I am looking at an RV-8a to buy here locally – I would prefer a TD, but sometimes you have to look at what’s availble. I must say when I was told by a local RV-4 owner to beware of the tricycle RVs because the nose gear has a tendency to collapse, that got my attention. At least the 8a that I am looking at has the Anti-splat mod, which I believe will lower the chances of a collapse/flip, but still...
I really appreciate the video that this unfortunate pilot was able to provide. I have done my own personal analysis of it (I am an analyst) and come to some of my own personal conclusions. I have also been on three accident investigations so have some experience in the matter. Hopefully I don’t offend anyone because my personal opinion, based on my analysis (though of course limited to watching videos and studying pictures of damaged -XA nose gear, and looking at the nose gear on my prospective plane) is that the RV-xA nose gear is a very flawed design. I find this surprising since I so admire Van’s ability to create simple, safe, efficient and high performance aircraft designs.
First, there was a lot of talk about “the pilot was too fast, the landing was too hard, he should have held the nose wheel off longer,” etc. These are probably all true but mostly irrelevant in my opinion, because (again, only based on watching the video - multiple times, at multiple speeds) the landing appeared to be within normal limits that the average pilot flying an RV should expect to occasionally deviate to – part of the safety factor that all planes and their parts are generally designed to. No pilot is expected to fly perfectly all the time and one shouldn’t have to worry about the nose gear collapsing and flipping your airplane over because you deviate outside of nearly perfect parameters. And no I am not saying a pilot should be able to drive his plane into the runway nose gear first and expect it to take the abuse, but again, the video showed a landing that I considered to be within the parameters of what landing gear should be able to easily survive.
As far as operating on dirt/grass/unimproved runways, one of the advertised advantages of Van’s aircraft is their ability to land and take off in short distances. So, logically, those aircraft should be designed to operate on said unimproved runways since there aren’t a lot of perfectly paved 1000’ runways.
OK, now I’ll get into the specifics of the design. It’s clear why the nose gear is angled forward – to get the mounting point at the fire/wall engine mount point and bring the wheel as far forward as possible for ground stability. Lots of planes utilize this basic design. The idea being that the elastic steel pipe will flex up and down (at the 45 or so degree angle so slightly forward and back as well as up and down) as it encounters bumps, ideally, when the wheel encounters a disturbance, we would expect the primary flex to be up, away from the disturbance. Also, the flex should be distributed along the strut more towards the end and less towards the top (why most gear that flexes is tapered). However, if we look at other videos of -xA nose gear in operation, and especially this video, we see that the gear does not flex along its length, but mostly pivots at the top rear of the assembly, very near the mounting point. You can see this clearly because you can see the nose strut fairing moving considerably at the cowling and the entire strut below that point appears to remain stiff. This causes a (more than normal) negative dynamic in the flexing process – the gear encounters an irregularity in the runway (or anything to apply a rearward force to the nose gear) and this flexes the entire gear tube back and down, which puts more force on the gear, which bends it back and down further, etc.
It would seem this happens every time a -xA nose gear touches down, but the majority of the time, the stress is relieved before plasticity deforms the gear, with a combination of the tire flexing, the fuselage rotating up in pitch, and/or the runway obstruction being passed. You can see this in the video – both times the front wheel touched, the flex is initially back and down – the first time the entire plane bounced a little so that relived the pressure on the nose gear and no deformation occurred. The second time, the plane had downward momentum as the nose wheel touched, and the back and down force reached the plastic deformation stage before the increasing force applied could be relieved by some other factor. You can see this on a normal (not catastrophic) landing:
https://www.youtube.com/watch?v=QpS3JzUIgTE – nose gear touchdown occurs around 4:40, and, as is usual, the first flex is back and down. The tire absorbs some of this and the fuselage rotates up slightly in pitch. The tire pressure aspect of this issue has me thinking too. I saw several propose increasing the tire pressure to make sure the gear assembly is kept well away from the ground. But this also lessens the tire’s ability to absorb some of those bumps. That would be a whole ‘nother case for analysis right there.
Here’s an SR-22 with similar gear.:
https://www.youtube.com/watch?v=jT4fOehZKNs -Watch at 0:45 as the nose wheel touches down, even with severe shimmy, the initial and most flex is up, not back and down. It would appear that the -xA nose gear strut is at the worst angle – and probably has another factor contributing that I do not have enough information to figure out yet – that causes that consistent back and downward flex.
There was a lot of discussion about what causes the backwards force besides runway deformities – like a seized or tight wheel bearing – but those are conditions that are ‘normal’ deviations and one should not expect a tight wheel bearing, or a slightly off tire pressure, to lead to your plane flipping on its back.
To me it’s obvious that a back and down action of a nose gear is exactly the opposite of what should be happening. We want a back and UP action that is naturally dynamically stable in absorbing runway irregularities. The wheel encounters a bump, the force is applied backwards and the tire rotates backwards and upwards. Or in the case of a straight (or slightly angled forward nose strut - think Cessna) the runway bump applies a backwards and upwards force, and the backwards force is mainly re-directed up along the strut, which retracts, relieving the force. It’s the round tire that rotates and re-directs a rearward force from a bump up along these struts. It appears the tire on the -xA does NOT do that most of the time in the videos I’ve seen. My guess as to why is that too much flex is inherent at the top of the strut and that long moment arm contributes to all the problems I’ve been discussing.
If nothing else, the gear strut should be redesigned so the flexing happens at or near the wheel, not at the mounting point of the strut, and so the flexing is up, not back and down. Ideally, a trailing link should be used. This is relatively simple and would integrate very well with the current nose strut configuration. It also works well with the pivoting nose wheel steering. When a bump is encountered, the wheel fork pivots backwards and upwards, decreasing, not increasing, the force applied to the strut. A simple rubber dampened spring would absorb the force and prevent bounce. Even a design such as on the Pulsar -
http://www.eaa1246.org/projectsnplanesdisplay.asp?id=23&pic=42 - would work much better than the current Van’s design, and most likely add little extra cost.
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