VAF Forums - Suspension travel is the key to RV-7A nose gear collapse

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- - Suspension travel is the key to RV-7A nose gear collapse (https://vansairforce.net/community/showthread.php?t=61178)

RV-6A better than RV-7A?

Quote:

Originally Posted by hevansrv7a (Post 457423)

I lack the engineering skills to critique the discussion above, but I think I can offer evidence that there is more to it. All of the above may well be true, but it's not likely the whole story.

The RV-6A was not noted for flip-overs. It still is not if this forum is a good survey. If the 8A has the problem, I'm not aware of that, either. Thus it is reasonable to infer there is something different about the 7A and 9A which makes them more vulnerable to the problem. The recent SB for which most of us changed the forks did not change any of that but did shorten the down portion of the leg and, I think, made the entire thing a little stiffer (it feels that way to me).

Since the nose tire is the same size, we can rule that out. For the 7A and 9A, the gear legs are longer and the airplanes when not flying sit more nose-down.

When a small wheel hits something tall enough (a hole or a step) that it cannot roll over then bad stuff begins to happen. Ever try to roll your rolling luggage out of the elevator by pushing instead of pulling? The problem here is geometry. Tire inflation can play a big role, too, since a higher inflation keeps more of the size of the tire and a bigger circle rolls over a bigger step.

Since the 8A is a whole different shape (longer, etc), I'll ignore it for now. The 7A and 9A center of mass must be in a position to encourage the pole-vault effect. We know this because it happens.

Imagine that you had a Van's side-by-side with a very low tail and your nose wheel stuck but the gear did not totally fail. The center of mass would not tend to rise but might even drop and lever the wheel out. Now imagine the 7A as it is with the nose down slightly and the center of mass above the line of the top of the nose leg. A force sufficient to stop the nose wheel will force the center of mass to begin an arc upward and forward. This is easier to imagine if you picture a 7A with a taller main gear and a shorter nose gear; now you are half way to a flip over without even trying. If the nose leg collapses and the airplane nose comes down to the ground, then it may flip over it.

I'm not suggesting making the nose gear longer. I do wish the main gear and the nose gear were shorter, like the 6A. I wanted to change over, but the diameters of the legs are wrong. It would take all new parts. I would gladly give up some x-wind performance for better safety on grass and rough fields.

Hi hevansrv7a, your post got me thinking and I ran some RV-6A cases on the model. On an equal nose wheel weight basis, the RV-6A results are similar to the RV-7A. This surprised me, because I also had the impression that the RV-6A has fewer problems.

However, if the model is right in predicting that it is lack of suspension travel that is important, we should expect similar results, as these two aircraft share the same leg.

Therefore, in understanding whether the model is correct or not, it becomes important to know for sure whether the accident rate is different for the different models. Do you (or does anybody) have any definitive data on this?

Raiz

Quote:

Originally Posted by PCHunt (Post 460684)

So, Raiz-

If a relatively large "bump" causes the nose gear to fail in your model, why aren't the nose gear failures in the mode of nose gear "collapse", where the nose ends up on the ground with the nose gear failed upward and forward into the bottom of the cowl and into the prop?

Does your model predict the failure of the gear back under the plane, with the following "pogo" manuver that results in the plane flipping over on its back?

Trying to follow you engineers along on your disucssion, but not really keeping up!:o

PCHunt, this must be a misunderstanding, because the model does predict the leg will fold under in all cases when the aircraft is moving forwards. It predicts the point of initial yielding will be either just above the castor bearing or at the thinnest part of the leg roughly above the axle. In either case, the castor nut will touch the ground soon after the initial failure and the nose-over follows on from there. I haven't modeled the actual pogo movement, as the game's over once the leg yields.

Raiz

Quote:

Originally Posted by Raiz (Post 460852)

... I ran some RV-6A cases on the model. On an equal nose wheel weight basis, the RV-6A results are similar to the RV-7A. ...

Raiz, Is that a valid assumption? Because of the different main leg geometry (7A sits much higher) I believe the max static load that most 6As fly at is significantly lower than the typical 7A load - I don't have any data at the moment but will try to get some.

Pete

Quote:

Originally Posted by Raiz (Post 460852)

Raiz, I am becoming a big fan of yours, even though it took me a while. Kudos! I hope you stick with this project a while longer as we are all learning a lot about a subject that we thought we had already talked to death.

A minor correction - the 6A legs are shorter. I know the diameter of the root of the mains is smaller. I don't have the data on the nose leg. I am pretty sure it is shorter, though. I once called Van's seeking to use 6A legs on mine and was assured it could not be done for the reason of how they fit into the airplane.

Since they are shorter, one wonders what their suspension travel and strength are and how that bears on the earlier discussion about the feasibility of making the legs stronger on the 7/9 A models.

I don't have real data on the 6A rate of failure or flip over, but when the 7A was a new model, there were literally thousands of 6/6A's and the problem was not being discussed yet. My strong inference is that the 6A is much more immune for some yet to be defined reason. I still can't recall hearing about a 6A failure but I'm sure someone can help with that. I also don't recall any 8A failures and can't come up with any reason that does not involve the much different geometry of the airplane vs. the legs. Maybe I'm just using selective data-recall?

I think that it's a much bigger deal to note that when you analyzed the hole scenario as compared to the bump scenario the problem looked significantly worse. It's equally a big deal that the nut catching the ground was only a problem in the simulation AFTER the failure. That implies that the expensive and inconvenient "fix" that most of us did was perhaps time and money not well spent.

If the SB that had us install different nose wheel forks does eventually prove to have beneficially affected the problem, I will suspect it has more to do with the change in the bending forces at the point where the leg goes vertical (down) from slanted (down and forward). The vertical arm is shorter by a significant percentage to compensate for the higher top of the fork (the plane sits at the original angle to the ground. Subjectively, it feels stiffer to me when I taxi than the original did. My non-engineer visualization of this suggests that the forces on the upper leg my thus come from a different angle, too.

The FAA study of this issue covered 23 aircraft.

4 - RV6As
9 - RV7As
4 - RV8As
6 - RV9As

The pictures included in the report do not seem to indicate failure near the lower part of the gear.

Reading the remarks about each accident is highly instructive.

See http://www.ntsb.gov/publictn/2006/RV_Photos.pdf

Numbers vs. Pcts

Quote:

Originally Posted by chunt0 (Post 460908)

Even today there are 2.5 times a many 6's as 7's. (see Van's Hobbs meter) It's a reasonable guess that the average 6 has been flying longer than the average 7. Unfortunately, we don't know how many are the A model in any series. Given that, the percentage of these events for the 6A would seem to be considerably lower than for the 7 or 9 A's Thanks for the very helpful data.

I admit that I did not study every post in this thread, but is there a solution being recommended here? Is it more complicated than switching to a larger nose wheel?

As for technique - no one believes that a tricycle gear airplane can be landed on the nose wheel, but sometimes we goof and at the very least land on all three wheels at the same time. I'd like to know that my airplane can survive the occasional botched landing (within reason of course). That's called a forgiving design. Any solution needs to be retrofitable to existing RV7A's or we risk severely devaluing the current fleet. Adding a spring (a la the RV10) is not an option in my opinion since that would probably require a new engine mount as already noted.

So what's the recommendation here?
Stronger steel?
Different material/metal?
Larger nose wheel?
Shorter main legs?
All of the above?

Thanks,

Tom

Figures 19-23 at http://www.ntsb.gov/publictn/2006/RV_Photos.pdf show in a simple way that aircraft with tall landing gear are more prone to nose over than aircraft with shorter gear following a "tripping" event. This is somewhat analogous to an SUV being more likely to roll over than a sports car. A casual reading of the data says that 6As are less likely to tip over, and center of gravity height difference provides a possible explanation. All designs being a compromise, I will not be rushing to shorten my landing gear. On the other hand, I won't be installing larger tires either.

Quote:

Originally Posted by Av8torTom (Post 460958)

Tom, this topic is covered frequently in these forums, please read the first post in this thread. I stand behind what I wrote then, and since this discussion almost three years ago, there have been very few tip overs on-airport. I know many replaced their nose wheel axle/bearing setups after this (and other similar threads of the time). Many reported noticeably different taxi behaviors.

Somewhere there is a video of a first landing in a -10, during which the nose wheel skidded the plane to a stop after a perfect landing on pavement. The plane then taxied back with only a huge flat spot on that tire to show for it. This should make believers out of anyone as to the problems with a non-rigid axle.

Quote:

Originally Posted by penguin (Post 460876)

Pete, I have that impression too, i.e. that RV-6As typically operate at lower nose load but I also don't have any good data, so anything you can dig up would be good.

This diagram illustrates your point about the geometries. Each color represents a different plane Green=9, Magenta=8, Black=7, Red=6.

The short horizontal bars represent the allowable CG range and the fact that the RV6 bar is further back than the RV7 bar (relative to the rear wheel) indicates that, all other things being equal, the load on the nose of the RV6 will be lower than on the 7 (but practice may differ). The lower MAUW of the 6 and the lower CG position will also help. Notice the 8 with the wheel further forward. This also reduces the nose load but the higher CG will tend to count against that under braking. I plan to post a comparison of all 4 aircraft over the same bump but I have a bit more work to do on the model before that is possible.

Raiz