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Year long discussion of "Another nosegear failure" -09/19/2010

local news article

http://bit.ly/9maRSU

My buddy Leon, myself, and Frank are in the picture to the left. At the right of the picture you can see the bare low spot in the grass. This is the spot where the nosegear made its first destructive impact.

The fireman laying down prone was snapping pics with his camera phone I presume to show off to his mother or whoever.
 
David-aviator said:
With engine running and full aft stick, the NG may be on the ground but it is mighty light. If you land nose high with nearly full aft stick and taxi that way, it probably will never flip on you.
Click to expand...

Yup. ;-)
Some folks even retract the flaps to give the tail more authority after landing... It allows them to hold the nose up longer into the rollout... Works great.
 
Here is my nose wheelpant nose section reinforcement. I laid up multiple layers of carbon fiber. I think I had left of center pieces, right of center and center pieces. I stayed away from where the back section would mate to the front.

I also went almost to the top of the wheel pant but ended perhaps 1.5" - 2" from the place where the gear goes through the wheelpant.

The thickness of the unmodified wheelpant is about 0.063" at the edge. At the bottom, which is in line with where the nut is, is about 0.163" thick so 0.1" thicker due to the carbon fiber layups.

This is not engineered except using TLAR bubba analysis. I would like to have Bob look at it and give his opinion if it "may" have prevented this flipover under otherwise identical conditions.

NoseWheelNose1Small.jpg
 
The BD-4 was notorious for having a similar problem with nose gear failures for much the same reason. When I built mine I put a C-172 nose strut instead of the stock gear which is similar to the RV-XA's. Performance was on par with other BD-4s and I was able to handle some pretty rough conditions without any trouble with over 1000 hours. I had to work the W&B a bit since the C-172 gear was significantly heavier but it wasn't hard at all.

The biggest issue with this modification was ground handling as this decreased the distance from nose gear to CG significantly due to the steep angle of the C-172 gear off the firewall. A C-152 gear would work well for an RV.
 
These airplanes do not seem to have a noseover problem. Why not just design the gear like this.


84588_6.jpg

80386_3.jpg

85225_4.jpg


83739_3.jpg
 
N395V said:
These airplanes do not seem to have a noseover problem. Why not just design the gear like this.
Click to expand...

Milt, to a novice, that gear looks very similar to mine (RV-6A). What is different about it that leads you to conclude that it does not have a nose over problem? Do those planes go into grass strips?

Is the gear leg itself thicker? Is there something about the orientation of the various parts that works towards the solution that Bob suggested earlier?
 
Another Hypothesis?

My RV-6A has been flying for more than 17 years with no nose wheel tuck-under incidents. I haven't flown off grass much but some of that has been pretty punishing due to pilot ineptitude. Some of the reason for my apparent success with the nose gear may be for the following reasons:

1. I have wood stiffeners on all of my gear legs. I like to think of them as dampers since some sliding probably occurs between wood and steel. This probably reduces the amplitude of oscillations when near a gear leg resonant frequency.

2. My nose gear wheel assembly has an extremely stiff axle. It's an early design and uses a through bolt passing through a 1.25" diameter shaft. This is much stiffer (in bending) than more recent designs which rely mostly on the bending stiffness of the bolt and, to a degree, the wheel. The reason this resistance to bending is important is that the tapered roller bearings don't like to have the shaft misaligned. (Axis of the shaft not in line with the axis of the bearing). The tapered roller bearings have tremendous capacity but only if properly aligned. Misalignment would result in very high internal friction in the bearings and act as a brake on the nose wheel, aggravating the nose wheel gear leg bending problem.

If Rocket Bob has the opportunity to examine the nose wheel bearings of the damaged aircraft, he should look for markings on the bearing races that might indicate skidding of the rollers on the races or indenting of the races.

Frank R.
RV-6A, 1400+ hours
 
All:

There are a minimum of four factors that can cause an accident:

1.) Weather conditions - CAVU that day
2.) Pilot experience and inputs - I don't know the individual but wish him and his passenger well.
3.) Capability of the aircraft - Good discussion here
4.) Landing surface - not discussed

In my 40 years of flying, I have never landed on such a rough surface as that at 8I3. In my humble opinion, that airport should be closed until the undulations are corrected. The undulation appears to be an old fence row.

I was with Frank that morning with his RV6 and felt his aircraft get launched back into the air at a point where there was no elevator control. His aircraft could have just as easily nosed-over. I also twice witnessed a Cherokee 180 launched into the air at the same place.

Watching the subject aircraft taxi in early in the morning, I did observe the action of the nose gear and agree with the contributors in this thread that an improvement in the "A"s nose gear must be made. At the time, there was also a 7A and 9A parked on the field.

However, I disagree with the premise that the accident was completely a defect in the aircraft, or the experience of the pilot. Instead I propose all three, aircraft, airport, and I believe to a lesser extent the pilot, were contributors to the accident.

Granted there have been very many successful operations at the airport, but a serious accident, and resulting legal issues, may be in its future as the experiment movement continues to grow compared to that of the certified aircraft.

If the owner of the airport and tenants promote a fly-in, they should insure an acceptable landing surface, for all types of aircraft, that may attend.

Paul
N277PM
LAF
RV7A
 
Grumman AA-1 gear; thoughts on a fix

I've flown the little Grummans off grass for a couple years, including the AA-1C with an O320 on the nose. The nose gear on these planes will fail or deform at the firewall, -vs- bending the gear leg like the RV's do.

The Grumman gear is so close in design to the Vans gear, looks to me like Van copied it, but didn't account for the dynamic loads the leg would see on uneven surfaces. And the RV gear fork design leads to high bending loads on the gear leg.

I think RocketBob is on the right track with wanting to address the design issue. The gear needs a fix. The similarity with the Grumman gear helps to bring clarity to where the RV gear is deficient. The RV gear leg appears to be way too soft and cannot deal with dynamic loads. If the airplane gets into a pitch phugoid on the ground - which seems easy to do with this much flex in the MLG and NG - it can be excited at the right frequency and with enough amplitude to cause the NG fork to tuck under. Looks to me like the NG leg needs to be replaced with a stiffer one, and make changes to the fork like RocketBob mentioned.

This is a great market opportunity - anyone out there who could fab up a better leg and fork would have a smoking hot little product to sell! Probably every RV-XA owner out there would be a buyer.
 
Not really similar...

Bill Wightman said:
...
The Grumman gear is so close in design to the Vans gear, looks to me like Van copied it, but didn't account for the dynamic loads the leg would see on uneven surfaces. And the RV gear fork design leads to high bending loads on the gear leg.
.....
..
Click to expand...

...except in an external view.

The Grumman leg is much stiffer and the "spring" in it comes from a torque tube that spans the cockpit area just aft of the firewall.

Even that "spring" caused some problems in bounces, and the later models have shock absorbers at the top of the gear leg.
 
Some days I wish I could do math.

What happens to Grummans when the nose gear fails, as a rule? Do they flip over, or land on their chin?

If a nose gear can't be fail-proof, I wonder if engineers could make the gear fail in a controlled manner, like crumple zones in a car, so that when it happens, the plane falls flat on its chin? It's still a prop strike and a bad day, but much less traumatic to people and airplane.

(Glad the "A" in RV-3A is a spar mod, not a nose gear! :p )
 
Pbr47906 said:
All:

There are a minimum of four factors that can cause an accident:

1.) Weather conditions - CAVU that day
2.) Pilot experience and inputs - I don't know the individual but wish him and his passenger well.
3.) Capability of the aircraft - Good discussion here
4.) Landing surface - not discussed

In my 40 years of flying, I have never landed on such a rough surface as that at 8I3. In my humble opinion, that airport should be closed until the undulations are corrected. The undulation appears to be an old fence row.

I was with Frank that morning with his RV6 and felt his aircraft get launched back into the air at a point where there was no elevator control. His aircraft could have just as easily nosed-over. I also twice witnessed a Cherokee 180 launched into the air at the same place.

Watching the subject aircraft taxi in early in the morning, I did observe the action of the nose gear and agree with the contributors in this thread that an improvement in the "A"s nose gear must be made. At the time, there was also a 7A and 9A parked on the field.

However, I disagree with the premise that the accident was completely a defect in the aircraft, or the experience of the pilot. Instead I propose all three, aircraft, airport, and I believe to a lesser extent the pilot, were contributors to the accident.

Granted there have been very many successful operations at the airport, but a serious accident, and resulting legal issues, may be in its future as the experiment movement continues to grow compared to that of the certified aircraft.

If the owner of the airport and tenants promote a fly-in, they should insure an acceptable landing surface, for all types of aircraft, that may attend.

Paul
N277PM
LAF
RV7A
Click to expand...

There is much that I would like state in my opinion of your "humble opinion, I will keep it brief, as this thread is particular to the discussion of the -xA N/G.
Please refrain from making public statements that an airport should be closed! You are a developers dream!
I have no knowledge of the runway at 8I3, and it could be entirely possible that that runway is not suited for operations in a RV-xA, and questionable for some other aircraft. Possibly some modifications/repairs should be made to improve the surface. So, lets close the airport!! Great idea! Sod/Dirt runway conditions are continuously changing, due to use, surface vehicle traffic, and even ground squirrels! However....I submit that when you choose to operate your plane into an airport such as this one, you need to retain some responsibility for the operation.
Not all runways are "acceptable" (using your words) to all planes and all pilots. And, to blame owner/operator of the airport is misplaced finger pointing!
Sorry for the rant, folks
 
Skid Plate

This type of failure occurs when the nose-strut plants itself into soft ground and a pole-vaulting action follows. Vans has improved the situation by increasing the ground clearance beneath the strut. However, it is impossible to prevent bottoming out under all circumstances.

A second line of defense might be to prevent digging/planting when it bottoms out. Off-road vehicles all have skid plates to help them slide when they bottom out. I believe a small skid plate under the nose-strut might make the difference sliding and planting.
 
Mark, Paul is right in that if there are obvious problem areas, then they should be fixed.

My opinion, stated before, is to just stay off of grass strips. If you know every inch of the landing surface and know that it is good...then do as you wish.

A Cessna 180 pilot with huge balloon tires cannot evaluate what a suitable surface is for me.

I recall a Christmas time 7A flip over in Texas. Recent rains and what appeared to me to be gopher activity. The surface was not suitable...or it was a gamble on any given landing/take-off.
 
David Shelton said:
This type of failure occurs when the nose-strut plants itself into soft ground and a pole-vaulting action follows. Vans has improved the situation by increasing the ground clearance beneath the strut. However, it is impossible to prevent bottoming out under all circumstances.

A second line of defense might be to prevent digging/planting when it bottoms out. Off-road vehicles all have skid plates to help them slide when they bottom out. I believe a small skid plate under the nose-strut might make the difference sliding and planting.
Click to expand...

You mean like this?

normal_MVC-001S.JPG
 
rocketbob said:
Landed and rolled out over a spot on the runway with a bit of a hump. Following the hump is a low spot where grass does not grow well. The airplane took a bounce over the hump and the nosewheel came down in the bare spot. It was clear to see where the front wheelpant broke and the nut started digging in. There is paint worn off the fork and mud in the nut. It slid approximately 90 feet when the nosegear leg finally yielded and flipped the airplane over. He first heard a bang and felt the nose drop, as soon as that happened he pulled all the way back on the stick and heard another bang, then flipped.

There is a groove dug into the runway 90 feet long where the fork dug in.
Click to expand...

Bob, most (all?) of your report here makes sense. What I do not understand is how the nut makes contact with the ground initially. What forces, deformation, bending, etc of any component in the system, allows the nut to contact the ground?

I can see it using the originally posted picture IF the gear bends backward far enough to allow the nut (once the wheel pant forward section is gone) to contact the ground. But what causes the gear to bend that way?

You may not know the answer today, but maybe those questions will help.

Since I am fixing a nosewheel flat, now is a good time to see if the clearance around the tire (when inflated) is adequate. What is the consensus on tire spacing to the wheelpant?
 
Hey guys,

I really don't have a hat in the ring since I'm building a bearhawk (I live in Alaska so most of my operations will be on grass or worse), but I did want to make a few comments from an engineering perspective as that part of this problem is interesting to me.

First, I didn't know much about the RV-6A nose wheel since I never considered a nose dragger, but after looking at the video (http://www.youtube.com/watch?v=YYCerJM_Qww) I must conclude that the nosewheel was a complete after-thought that feels like an engineering hack.

If you look at how much that thing moves around even on a relatively smooth surface then it's easy to see how it is completely intolerant of anything shy of perfect. I feel that this negates the entire point of a nose dragger which is to make the airplane more tolerable of mistakes.

While thinking about how to solve the problem I have a few thoughts:

1. I would try to dedicate a spring to the work of absorbing impact and try to separate this from the work of the gear which is to hold up the airplane. One way to do this would be to make the front gear much stiffer, then use a leaf spring where the arch is. Another way would be to shim the top half of the gear leg to make the top much stiffer than the bottom. Having the entire gear leg move is just too much leverage.

2. I would move the vertical shaft to the tire side of the for instead of outside the fork, then make it 3/4" shorter so that the nut is hidden up under the fork. This will make it so that if the gear fails you have a flat spot hitting the ground that gets even shallower the more the gear fails. That should prevent the airplane from flipping even if the gear fails.

3. Obviously the best solution would be to use a pogo-stick type suspension like a cessna, which really wouldn't be hard to make. All you need is some tubing that holds a coil spring, with a plunger that has a seal around it, with a hole in the plunger that allows oil to escape from one side to another. You would need to experiment with the spring and hole to get the dampening correct but it shouldn't be that hard.

If I had an RV-A I would probably start working on this problem because it wouldn't be terrible to solve and would probably make some money, but then again, I would never consider a nose dragger given the conditions up here.

Anyway, I hope my ideas are helpful to someone.....

schu
 
I believe that the nosewheel "skid" is closing the barn door after the horse got out. IMO, things are past the point of no return when that skid hits the ground.

I think the nose gear airplanes would be well served with a larger diameter nose wheel. That would greatly reduce the forces involved when the nosewheel hits a rut or a bump.
 
Ron Lee said:
Bob, most (all?) of your report here makes sense. What I do not understand is how the nut makes contact with the ground initially. What forces, deformation, bending, etc of any component in the system, allows the nut to contact the ground?

I can see it using the originally posted picture IF the gear bends backward far enough to allow the nut (once the wheel pant forward section is gone) to contact the ground. But what causes the gear to bend that way?

You may not know the answer today, but maybe those questions will help.

Since I am fixing a nosewheel flat, now is a good time to see if the clearance around the tire (when inflated) is adequate. What is the consensus on tire spacing to the wheelpant?
Click to expand...

Go back and re-read post 19. I think the idea there is on the right track for some of the accidents, specially the ones where the surface is seemingly too smooth to be a factor.......
 
Kyle, what if the skid makes the difference between a pole vault and just a skid where the airplane remains belly down until it stops moving?

Thanks Mike. Maybe that is it. Hence my question on separation between the tire and the wheelpant. At least I can minimize or eliminate that as a possible causal factor.
 
Last edited:
az_gila said:
...except in an external view.

The Grumman leg is much stiffer and the "spring" in it comes from a torque tube that spans the cockpit area just aft of the firewall.

Even that "spring" caused some problems in bounces, and the later models have shock absorbers at the top of the gear leg.
Click to expand...

Yes, I should have added more clarity to my remarks to make the difference clear.

The Grumman gear leg itself is much stiffer. It doesn't get much spring from the gear leg flexing. Its weak point is at the firewall, versus the gear leg itself being the weak point. I've never seen an AA-1 go over on its nose but like everything else, its probably happened somewhere.

I've ridden through some good bumps in the little AA-1C (with the shock strut) *with* an O320 on the nose which loads the NG more heavily than the O235. Its a pretty tough little bird!

Thx again Gil.
 
Pbr47906 said:
All:
In my 40 years of flying, I have never landed on such a rough surface as that at 8I3. In my humble opinion, that airport should be closed until the undulations are corrected. The undulation appears to be an old fence row.

Watching the subject aircraft taxi in early in the morning, I did observe the action of the nose gear and agree with the contributors in this thread that an improvement in the "A"s nose gear must be made. At the time, there was also a 7A and 9A parked on the field.

If the owner of the airport and tenants promote a fly-in, they should insure an acceptable landing surface, for all types of aircraft, that may attend.
RV7A
Click to expand...

Its been generally rough this summer because its been a dry summer, so there haven't been any opportunities to roll the runway. It takes the right amount of saturation for the roller to be effective.

Yes you are correct there used to be a fence row right there 30+ years ago.

I have been beating the owner of the airport in the head for YEARS to fix the rough spots. The trouble is, is only the RV guys complain about it. I routinely land the Saratoga over the same spot and it just doesn't care...you barely notice any roughness. In the RV I've learned to avoid said spots so its not much of an issue for me. But if you're not one that knows the airport well, it does become a problem. But he finally has agreed to fix these spots. There is a lot of training going on there so its difficult to justify shutting the airport down so the RV guys dont bitch.

David Shelton said:
This type of failure occurs when the nose-strut plants itself into soft ground and a pole-vaulting action follows. Vans has improved the situation by increasing the ground clearance beneath the strut. However, it is impossible to prevent bottoming out under all circumstances.

A second line of defense might be to prevent digging/planting when it bottoms out. Off-road vehicles all have skid plates to help them slide when they bottom out. I believe a small skid plate under the nose-strut might make the difference sliding and planting.
Click to expand...

Skid plate won't work because there is too little area to displace the weight of the nose of the airplane when you're talking about a dynamic load when the nosegear leg is bent. I don't think it would do any good, considering the gear is already bent to get to that point.

Ron Lee said:
Bob, most (all?) of your report here makes sense. What I do not understand is how the nut makes contact with the ground initially. What forces, deformation, bending, etc of any component in the system, allows the nut to contact the ground?

I can see it using the originally posted picture IF the gear bends backward far enough to allow the nut (once the wheel pant forward section is gone) to contact the ground. But what causes the gear to bend that way?

You may not know the answer today, but maybe those questions will help.

Since I am fixing a nosewheel flat, now is a good time to see if the clearance around the tire (when inflated) is adequate. What is the consensus on tire spacing to the wheelpant?
Click to expand...

I do know the answer, and its simple. Think of the fork as a lever. Lets say the lever is five feet long, think about how easy it would be to bend the gear if you lifted on the end of the lever. The nosegear leg will start bending at its thinnest and weakest point, where it has a bend in it already. Bend it far and then the nut contacts the ground. Once it contacts the ground, another force is applied by friction due to ground contact which accelerates the bending. The way to reduce this arm is to shorten it by reducing the overall length of the fork, and moving the pivot as close as possible to the axle centerline, without introducing shimmy. If one looks at the Grumman's, or the RV-10, or the RV-12, you'll see a longer nosegear leg that has a constant diameter. That certainly helps prevent loads from being concentrated at the fork/gearleg intersection.

Today I spoke to a friend of mine and we know of five A models that have collapsed the gear, and that is just within a 100 mile radius of Indianapolis. One airplane I know of has been on its back twice, and both times it was on pavement! Yes I understand piloting technique is a major consideration here but regardless, how often do we hear of other airplanes going over on a routine basis?
 
i also don't buy into the skid plate theory... at least not until it has been proven / demonstrated to be effective.
the forces are simply too strong and the angles too blunt.
maybe a slight chance of success on grass, but my guess is it would be too little too late. (after the unwelcome chain of events has started and the plate is not powerful enough to break the chain.)
of course this is an opinion and little more than and educated guess (as are the theories of the proponents).
if there is a fix to come along, it needs to tackle the root causes.

rgds bernie
 
Older RV6A

I have the older 6A I ordered the new gear, but as i watched what was going on, I have a few opinions. On my old ng I have the wood bonded to the leg, i think it adds stiffness. I have landed on maybe 30 or 40 grass strips, I am careful of where I go. But have been a rough strips,dont make a habit of it. I asked Vans about adding the wood to the new gear, they were most insistant to not do it. I think they are wrong. Hence I havent completed service bulletin.
 
Frank Smidler said:
It hit a rough spot, the nose popped up and came down hard then a second later it was standing on its nose, tail straight up.
Click to expand...
Seems the nose popping up could have been the start of the sequence. Having the nose pop-up like this has happened to me a few times but never with disastrous consequences.

The accepted advice of having the stick full back on roll-out when the nose wheel is on the ground will increase the likelihood of the nose popping up. At this stage of the roll-out the elevator at full back-stick has insufficient power to lift the nose but enough power to make the nose light so the right (wrong) bump can launch the nose high and without sufficient elevator authority it will come back to earth with a thump. So, what to do?

I think it would be impractical to control the launch with elevator as I would have to be mighty quick off the mark to reduce back pressure as the nose lifts and then ease in back stick as it descends. Also there is a risk of messing up the control inputs and possibly making the situation worse.

Less back stick in the roll-out could decrease the likelihood of the nose launch but that would put more weight and stress on the strut when the nose wheel initially hits the bump.

I think the best I can do is have the aircraft at the slowest possible speed in the roll-out when the nose finally comes to ground due to loss of airspeed and elevator power so if I do subsequently hit a nose launch type bump my speed will be low when the nose drops back down.

I operate out of a short grass strip and my normal technique is flaps up as soon as the mains are planted as this allows the nose to be held up to a slower speed. At the same time, braking if required/possible while keeping the nose wheel in the air with increasing back pressure. Then lower the nose gently as the elevator loses power then little or no braking once the nose wheel comes down and in the subsequent roll-out.

Thoughts, suggestions or criticism welcome.

Fin
9A
 
My 0.02

I'm not a mechanical engineer so take this observation for what is worth. Watching videos of the A models' nose gear on grass, it becomes apparent that any roughness in the terrain pushed the wheel and gear backwards. I think that is the root of the problem. I would think that a properly designed nose gear "should" move up and down when it hits a rough spot. If that was the case, the more deflection of the gear, the farther away the nut (and other non-rolling surfaces) would be.
As I said, I have no sufficient knowledge to design anything properly, but if the curvature of the leg were changed so a bump would push the leg up, most of these incidents would be minimized.

Comments? I would appreciate your thoughts on this possible solution.
 
What effect would tire size have?

Having the small wheel as the nose wheel, what effect would going to a larger wheel have on the overall geometry?
 
billpilot1 said:
Having the small wheel as the nose wheel, what effect would going to a larger wheel have on the overall geometry?
Click to expand...

First, I *think* you'd need a new fork or gear leg design to provide adequate clearance between the wheel/wheelpant and the gear leg. But from a practical standpoint, the larger wheel would roll more easily across bumps and greatly reduce the forces on the nosewheel for a given size bump or depression.

But, there are also wheel spin-up forces in play. Taking a small wheel from 0 mph to `~50 mph instantly (like when a nosegear touches down) requires a fair amount of force. That force is transmitted to the nosegear and would tend to bend it back. Spinning up a larger wheel takes more force than a smaller one.
 
what about welding a strip of steel about 1" wide, 1/4" thick and 3/4 the length of the gear to the backside of the gear leg just above the fork oriented so the thin edge is against the tube.

Just looking at the photos its obvious this thing is weakest at the lower section of the gear leg and is rolling up on itself once the "nut" contacts the ground. Stiffen this lower section where the gear is weakest to prevent the initial roll up and maybe it wont "pogo" so easily.


No analysis here just intuitive.
 
Heat treated alloy steel..

RV8R999 said:
what about welding a strip of steel about 1" wide, 1/4" thick and 3/4 the length of the gear to the backside of the gear leg just above the fork oriented so the thin edge is against the tube.

Just looking at the photos its obvious this thing is weakest at the lower section of the gear leg and is rolling up on itself once the "nut" contacts the ground. Stiffen this lower section where the gear is weakest to prevent the initial roll up and maybe it wont "pogo" so easily.


No analysis here just intuitive.
Click to expand...

...does not take kindly to welding operations.

LarryT
 
If the moment around the fork . . .

If the moment around the fork is a big contributor, welding reinforcement isn't realistic without some other redesign. Looking at it, it's not a garage weld hack. The bending moment isn't like a beam load. You put the two together . . . bad karma on the way.

So my ME degree kicks in - along with my personal instinct for self preservation. I'm not sure what I'm seeing, to be honest. Surely a lot of undamped spring/mass activity. I don't think you can tell the contribution of the fork moment from grainy video. Remember - dealing with load isn't the same as dealing with inertia. The inertia of a porpoise is related to the cg rotating around the mains contact patch (not the axle) - and it's the whole mass (do the FBD) moving. I'd love to get some strain gauges on the leg. To get a decent model, Van's would have to offer the design drawing, or we'd have to cut up a good part.

Anybody have a copy of the NTSB analysis? Not just the final report, but the data?

I can see alternate designs, lots of them, but there are plenty of risks there too. There are, what, 4000 A models out there? Sure, there are a lot of folds, but a HUGE number of airplanes(relatively) and landings - but unless we can get to folds per thousand (or hundred thousand) landings, 'how many we've heard about' isn't really a moving 'statistic'.

Just my $0.02, worth what you paid.

Rick 90432
9A on the gear, looking at spring 2011. O320, fixed Sen, Grove wheels. BSME :)
 
Us -6A builders have the drawings for the nose gear leg (well, the old leg at least). I think having a slightly stiffer leg custom made would reduce the flex of the gear. More shock loads would be tranfered to the mount also, so some additional support members may be required.
 
When you look at the photos of the legs that have failed they all show the same thing. It has rolled up from the bottom. Seems that the lower section around the yoke point "knuckles" over and then the leg continues to bend back as the fork and nut contact the ground.

The focus should be on the bottom as that is where it all starts and the rest of the banana shape is just a result of the weight bending the whole thing back. I am somewhat amazed that Van's continue to blame pilot error and while that contributes to the failure to a degree there is just not enough margin in the design to cope. The bottom needs to be stiffer and shift the spring action higher up the leg to stop the action starting.

Tapering the leg up from the bottom would stiffen the bottom and shift the bending moment higher on the leg.

Van's need to take this seriously before someone is badly hurt or worse. I sure don't want my insurance company to see this thread :eek:
 
I am an engineer, so I should know what I am talking about, right? Wrong, an IT engineer should know not to offer any opinion when he knows nothing about mechanical/engineering, but I can't resist.

Anyhow, as we are re-engineering this, please keep in mind that the leg is tapered in the middle to allow for shock absorption. The reason they bent at the location they do, is due to the taper portion which starts right around there. By using materials to stiffen the gear leg will only cause more force to be applied to the engine mount at the point the leg is fixed to and due to the greater arm, the force would be much greater then currently it is design to take, so one can not [reasonable] stiffen one part without paying attention to other part.

Perhaps if it could be designed in such way that once it starts bending, lets say after a certain degree of bent or force, it could collapse so the bird does not tip over, rather fall on its chin. It should also be designed to collapse only in the direction of the way they bend and not the opposite (a heavy force such as nose wheel landing) not to cause it to collapse.
 
Mehrdad I am no engineer either but when I look at that photo at the start of this thread I see a skinny steel rod that just looks inadequate for the job. Given the thickness of the rod and the length it just has to flex all over the place.

Like other photos I have seen there appears to be a significant bend at the yoke part of the leg. I agree with you that stopping it bending backwards is the key to part of solving the problem.
 
By far the best solution is an oleo gear.

Works well as you can see in this video: http://www.youtube.com/watch?v=_lO3nAB5Zz8&feature=related

Redesigning the existing rod is just shifting the deck chairs around on the titanic. Someone suggested the 152 unit; good idea, the aeroplane weight is in the ball park. Surely a non-certified copy of that is the way to go and won't cost too much. The 152 unit mounts on the engine mount, so there is a good basis for reverse engineering, as opposed to the 172 unit which is mounted on the firewall.

A bonus will be that it is steerable.

Once that's sorted out, we can then move onto the mains and come up with a better system there too.

Cheers,
Andrew.
 
kymjon said:
The bottom needs to be stiffer and shift the spring action higher up the leg to stop the action starting.:
Click to expand...

Current legs have the flex location "taper" just behind the wheel. The pre-2000 legs had the flex location located about 6" (guess) from the socket.
I have one of each. Did any of the OLD STYLE pre-2000 legs ever fold up like this one did?
 
The -10 has a good solution...

....in that the leg is stiff but hinged at the firewall and 4 rubber doughnuts provide the spring. Seems that the idea could fairly easily be adapted to the two seaters.

Best,
 
I'm with you Pierre. The RV10 leg is a far better option. Much heavier aircraft and none of them have ended up on their back sunning themselves :D

If there was an option provided from Van's to fit that on my RV6A count me in.
 
pierre smith said:
....in that the leg is stiff but hinged at the firewall and 4 rubber doughnuts provide the spring. Seems that the idea could fairly easily be adapted to the two seaters.

Best,
Click to expand...


I think that is the ideal solution. The weight increase would be minimal and it keeps the leg from flexing aft. It would take some measuring, but we might be able to barrow the gear leg from some existing aircraft and adapt it to the RV. There are a number of models that use the rubber doughnuts and pivoting gear.

Modifications would have to be made to the engine mount to accomodate the pivot point and shock doughnuts. Also, the leg fairing and cowling interface would have to be changed slightly to accomodate the additional movment at the top of the gear leg.
 
gasman said:
Current legs have the flex location "taper" just behind the wheel. The pre-2000 legs had the flex location located about 6" (guess) from the socket.
I have one of each. Did any of the OLD STYLE pre-2000 legs ever fold up like this one did?
Click to expand...

I don't know, but they experienced fatigue failures at or near the engine mount. That was the purpose of the redesign.

I do know one thing for certain - engineering is a much more humbling profession than it appears to those not in it. Things with this nose gear design are much more subtle than they appear, with trade offs being unavoidable.

It will be valuable to see which nose wheel bearing design this airplane has (no need to post requests for this - it is underway).

For additional thoughts please see this thread and this one.
 
"Staked"?

I'm wondering if this accident aircraft (or any other noseover accident aircraft) had the Roberta H. "staking mod"? Perhaps RocketBob or others can chime in? That seems like an awfully good idea to me.

I totally agree with her (and others) who have said that landing with a preloaded bearing (NW "drag") is is like landing with a NW brake partially engaged. That can't be good.

I know this is also commonly used to alleviate NW shimmy--maybe not a good solution for that either.

My .02--worth what it cost you :D
 
The Gruman photos show a plane that sits lower and just looks more stable than the A models which are higher. Here is something to consider, shorten the main gear. It will shift the CG when on the ground to the rear, increase prop clearance, increase the NW nut clearnce (albeit small), improve ease of entry, look better and perhaps help.
 
There has been some recent full-tilt engineering in this area. It will be interesting to see if students put Skycatchers on their backs on a regular basis.
 
could a -12 NLG work?

I love the idea of adapting the -10 nose gear.... but I was also wondering what type of nose gear the -12 ended up with?

Looks like it had an oleo at one point:
vans_aircraft_rv_12_n912va.jpg


But now it has a familiar looking steel rod:
vans-rv12-for-dave-blog-medium.jpg


Does anyone know how the nose gear attaches on the -12? Rigid to the mount, or rubber donuts?
 
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