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A tale of two airplanes ( long post)

scsmith

Well Known Member
A Tale of Two Airplanes

This is a story about two airplanes, or rather, two stories about airplanes. At the end, I will voice some ideas for you to think about.

The first story is about an RV-8. This beautiful airplane was enjoyed by its owner for many hours; its home airport in the pacific northwest was paved, but narrow, and lined with well-manicured grass on both sides. One winter, the owner decided to sell this very nice example of what many of us regard as a fantastic airplane. I forget why – maybe he decided to build another.

Anyway, on this particular day, the owner was demo'ing the airplane to a prospective buyer. They departed the home airport and did some flying around, stopping at another airport for lunch. By the time they finished lunch, it was stormy and windy. They flew back to the home airport to find it very windy, very gusty, and it had rained, so the paved runway was soaked and the grass was slick.

The owner wasn't overly concerned about the gusty crosswind. This was his home airport, and he had lots of practice with strong cross winds there. He flew an approach with some extra speed margin for the gusts, did a careful wheel landing ( first the right, then the left). He was really attentive as the airplane slowed and he let the tail wheel down. There. All is well.

But he didn't let his guard down; there's a reason why they say you fly a taildragger until its parked. As he let the speed bleed down, he was careful to keep the airplane on centerline, straight, stick all the way back. About the time he was slowing through 25 or 30 mph, a very strong gust hit from the right. Zing! Just like that, the tailwheel broke traction, the airplane yawed violently to the right and started sliding. Both main wheels were sliding on the wet pavement until the airplane drifted off onto the grass. The right wheel slipped easily on the slick grass, but when the left wheel left the pavement, it sank into the soggy grass and dug in. Umph, the plane stopped. The left landing gear tower tore out of the floor and twisted grotesquely, the left wingtip came to rest on the grass. His beautiful airplane was now seriously damaged. The owner chided himself for perhaps being a little slow to move his feet up onto the brakes, maybe he would have been able to correct the slide in time.

I didn't make this story up – it really happened. I may have gotten some details wrong, but the salient points are true. **(edit: I'm embarrassed to have to say that I realized after I wrote this that I unintentionally have merged two separate incidences - the damage in the incident described here was not catastrophic, and apparently the gear retention bolts failed, perhaps preventing other damage- a good thing. I have examined detailed photos from two other incidents in which the gear tower was torn loose, twisted and much of the surrounding fuselage badly damaged)**
The rest of this paragraph is my speculation: it would not have mattered if the pilot was quicker on the brakes. The hard, smooth rubber tail wheel lost traction on a gust and the airplane yawed very quickly.. The runway was so wet the main wheels were sliding – brake application would have done little or nothing. It might be that a pneumatic tail wheel with some tread on it might have prevented the accident, I don't know.

The second story is about a Cessna 180 Skywagon. It happens that this particular Skywagon had been fitted with the P-Pong after-market landing gear reinforcement system. The owner was flying under IMC over the central valley of California, when the engine quit. Instantly quiet. Emergency engine-out procedures were followed, no joy. He told ATC he was descending, and turned away from the suburban area he knew he was approaching, retreating toward open farm country. He broke out of the clouds at 1500 ft. Lots of very soggy, half-flooded fields of crops, divided by a grid of muddy farm roads. He picked a road that was more-or-less into the wind, checked for wires while on downwind, turned base and noticed that the road was elevated from the fields on either side by several feet. It was narrow, but quite landable.

The pilot flew a wheel landing to maintain good visibility on the road as he touched down. He could tell immediately that the road was very slick with mud. He worked hard to keep the airplane headed straight as he slowed down. Tail wheel came down, all OK so far. At a certain point, he just could not keep it on the road, and it slid over the edge and down the slope into the drainage ditch alongside the road. It slid until the left wheel got to the bottom of the slope and stuck in the muddy drainage ditch. Umph, the plane stopped. The owner got out and was pleased, even a little surprised to find the landing gear apparently intact. The farmer showed up with a truck, pulled the plane out of the ditch and back to a dry barn to check things over.

The landing gear was fine. The engine stoppage was determined to be from a plugged carburetor jet, from a small flake of orange RTV that somehow got inside the carburetor. When the carburetor came back from the rebuilder, it was bolted back on and the airplane took off from the same road it had landed on the previous week, and flew home.

I didn't make this story up – it really happened. I may have gotten some details wrong, but the salient points are true. The rest of this paragraph is my speculation: The P-pong landing gear reinforcement may have saved the airplane. Perhaps the original stock Cessna landing gear would have survived too, I don't know. Skywagons are pretty tough birds. And with the availability of some well-thought-out modifications, they can be even better than new.

Some thoughts:
First of all, I do NOT understand the view expressed by some that Van's airplanes are perfect, and you are an idiot if you suggest changing it to perhaps make it better. I have nothing but respect and appreciation for Van and his team for developing such successful kit airplanes that are superior to many production airplanes. Special kudo's to Ken Kruger, who I believe did the majority of the detailed design work on the –8. It is a magnificent airplane, to be sure. But that doesn't mean there aren't some places for improvement.

What if some simple reinforcement was available for the RV-8 landing gear that substantially increased its strength in a couple of critical areas, and added just one pound to the empty weight? Its not possible to make any landing gear failure-proof. Subjected to enough violence, anything will break. But what if the reinforcement was just enough extra strength to save the poor fellow in the first story from the despair of breaking his beautiful airplane?
Some people would say this is a solution without a problem because every failed gear on an RV-8 is from pilot error. Some would say that Van knows best and it would be foolish to try to make the –8 better. I say that I have studied more than one gear failure, and I have studied the structural design, and I think a simple parts substitution during construction can make a big difference to the strength of the landing gear, at the cost of just about one pound of weight.

I started from a quick-build kit, and the assembly was far enough along that I was not able to incorporate my ideas into my airplane. When I build the next one, I will do it. If you think I'm wasting my time and breath, fine. I don't need to hear from you. If you are building an RV-8 fuselage and you have not yet attached the subfloor parts to the inner floor and bottom of the gear towers, and you think you would like to add some strength, send me a PM.
 
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If i was building an RV 8, I'd be contacting you for the parts.

I'd consider a pound (or two) to be well worth it for a substantial increase in the landing gear strength. It's all the other possible unintended consequences that need to be looked into that make this a challenge, but I would definitely take a look at what you've done.
 
Steve... Whilst the +1lb = stronger aircraft would be nice, I suspect life is never that simple.

Whatever the reason, the eventual problem all came from the fact the aircraft went off the runway. If they hadn't gone off the runway = no damage. Aviation would normally tend to address the issue in this area - either different flying techniques / limits to prevent the excursion, or design changes to give more directional control.

IIRC the aircraft was repairable - in fact quite easily? This would suggest the gear leg design was near perfect ;) The gear leg "broke off"... had the gear leg / bolts / attachments been stronger, then the damage would have transferred to fuselage and distorted the whole structure, or worse, the gear leg dug in and held and flipped the aircraft. Either = more damage and hazard to the occupants.

I am aware of an RV-8 landing that resulted in the gear legs bending. 4'g' on the meter. New gear legs fitted and they were flying again. They queried Van's and said they did not feel the landing was that bad, and why were the legs so "weak"? Reply - legs are designed to bend at 3? 4? g - if the load transferred to the fuselage is higher than that, then the fuselage will distort = effectively written off aircraft.

Another example is the "slot" cut into the forward wing attachment - designed to encourage the wings to snap off rather than stay attached whatever.

Just my 2ps worth ;)

Andy
RV-8 G-HILZ
 
I want to say first that I don't disagree in principle regarding using proper nuts, but -

I have been working for allmost 20 years as an engineer/scientist, and I know that everything can be improved, that is my job - to make things better, find new solutions. I have also learned that while things may look inadequate on the surface, they are more often than not extremely well thought out. Other times when things that looks and feel "high tech", they have missed some basic functionality.

Even though everything can be improved, this is not the same as saying the old stuff is not good enough. What you have showed us is only part of a solution to a problem that as of yet has not materialized. What you have showed us is that some nuts are stronger than other nuts, that is basically it. You have not showed any estimates of forces, no estimates of how the forces are distributed within the structure, and no estimates of what causes these forces. The story of the bad landing, does not give any evidence whatsoever that the structure is not good enough, only that the structure obviously is not designed to withstand that kind of punishment. But again, exactly what kind of punishment did it endure in terms of forces? and should the gear really be able to withstand that kind of punishment?

Making a mod for the gear assembly, a rough field mod or something, is one thing, and it requires much more work than replacing som nuts to be sure that the rest of the aircraft structure is not comprimized. But, you say you are an experienced aeronautical engineer, and in my opinion you should know better than trying to convince people that the current design is not good enough, based purely on your instincts (as far as I can see). I mean, replacing the nuts is OK, but it seems to me that your main reason for doing this is to adhere to good engineering practice because your instincts tells you so (If I had an -8 I would probably do the same thing for that particular reason). This is fine, but it is no evidence that this will improve anything at all regarding the overall strength of the gear assambly.
 
I have been on this board quite a bit. I have never seen anyone claim Vans designs are perfect. The chorus is usually along the theme: the designs are an optimal compromise. In fact I think I have read and heard Van-the-Man himself say that.

Do whatever you like; one pound or thirty. If you come up with something good, your mailbox (or email) will be full of orders. :D Come up with something bad and we will say "Oh I remember the guy that tried to...." :eek:
 
I agree!

I agree with most all of the points made by SvingenB. I have not presented any loads analysis or strength analysis. One can pick some representative loads to design to, but landing gear can take a wide variety of different loads in various mishaps that are not well represented by anyone's design cases. That's why they often fail in mishaps. The strength analysis is actually very complex once you get past the gear leg and the bolts. The way the load is distributed into the monocoque structure is difficult to analyze without detailed FEM.

What I am trying to do on mine is raise the threshold of how much punishment may be endured without damage.

By how much, I will not be able to demonstrate since I don't currently have access to a FEM analysis. But i do understand the loadpaths, and I well understand how to avoid the pitfalls of adding stiffness in the wrong places in a statically indeterminate structure.

No one else has to do this, or care about it if they don't want to. I wrote the post to see if others might be interested.
 
Correction

A Tale of Two Airplanes.......

......The second story is about a Cessna 180 Skywagon. It happens that this particular Skywagon had been fitted with the P-Pong after-market landing gear reinforcement system. .........

............The rest of this paragraph is my speculation: The P-pong landing gear reinforcement may have saved the airplane.......

I believe the correct spelling of the STC Cessna mod is:

P. Ponk

Details matter.
 
Spend your hours building, Steve....

.....instead of agonizing over a problem that has not yet manifested itself on any -8.

Regards,
 
<<the damage in the incident described here was not catastrophic, and apparently the gear retention bolts failed, perhaps preventing other damage- a good thing. I have examined detailed photos from two other incidents in which the gear tower was torn loose, twisted and much of the surrounding fuselage badly damaged>>

Steve,
Any chance you can post some of the gear-tower-torn-loose photos?

Randy's incident (per his own account on his website) seems to suggest a fold-under while sliding sideways, ie applied force primarily perpendicular to the fuselage centerline. Note the bend in the gear leg just above the axle. Such loading would be close to a best case for the gear tower, so the bolt(s) failed. The website does not detail which nuts were in use. Fuselage damage appears to be minimal. Perhaps Randy (or the new owner who did the repairs) could comment.

Force applied parallel to centerline (like dropping a tire in a really bad hole) would result in a very different set of loads....which is why I ask about the pictures.

BTW gang, each individual builder gets to decide which mods, if any, are worthwhile. Some are smart, many are not, and a few are really dumb. Even if you think the structure perfectly adequate and the subject moot, please remember many of us find discussion of loads and structure to be very educational.
 
I am not that familiar with the RV-8 concerning the subject of this thread but I do know what else can happen when the airplane ground loops and goes off the runway.

On April 30, 2008 a good friend going back for many years was involved in a landing mishap at Spruce Creek. I spoke with him about what happened and after everything stopped moving there was much fuel leaking across his legs. Evidently, a fuel line passes near the leg tower? I do believe the left gear leg collapsed in that event.

The canopy was somewhat jammed and getting out of the airplane took some time much to the consternation of pilot and passenger. Fortunately, very fortunately, there was no source of ignition near the leaking fuel line.
 
I personally love to see work like this - carefully thought out engineering analysis of potential or real issues with current designs is a lot of what I do for a living. Every design deserves scrutiny for items that might have been missed by the designer. At the very least, good analysis is better than "that looks about right!" modifications.....

I personally am not that worried about the particular bolts that Steve is looking at being a failure point, but then, I haven't looked at detailed pictures of the aircraft that i am aware of that have had gear failures to see what condition these parts were in after the event. I have always been much more concerned over the gear tower damage - that looks to me (a quick builder) to be a horrendously difficult repair! I really did appreciate the nut testing that Steve presented in his previous thread because it presents an alternative nut that might be much easier to get a wrench on when it comes time to re-torque. (Sorry builders - if you haven't done this "in the field" with a completed airplane, you really haven't gotten the full experience.)

I'd make two suggestions for further pursuit Steve - first, you might give Ken Kruger a call or chat off-line. All designs are compromises, and all designers know that. I usually try and pick the brains of the designer to understand the decisions that he/she made in developing the device - sometime,s you find that they have gone down a different decision path for a reason, sometime,s the light bulb goes off over their head and they go charging down a new path with you. But a good place to start is the original designer's intent.

Second, I'd be interested to know if you have looked at the one-piece Grove gear design, to see if this might make the system more rugged. I heard about it after I was already committed to my stock van's set-up, and don't know if it is still available or not - but I always thought it looked pretty neat.

Paul
 
Good reading for non engineer pilot

Good point Paul. My gut feeling, is that the RV-8 design is adequate for the application. Time spent on flying skills would be more productive. There is no aircraft that can't be destroyed. RV "A" models have nose gears that are much weaker that trainer aircraft like Cessna 150's , 172's and must be flown accordingly. That doesn't mean that the "A" models are not designed ok. They just can't be abused like a C-150.

I have enjoyed the discussion here, and take no offence. The Doll is stock and will remain that way. If I ever build Doll II, she will be stock as well. Like they say: Don't fix what isn't broken!
 
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<<Evidently, a fuel line passes near the leg tower? I do believe the left gear leg collapsed in that event.>>

Through holes in the gear tower, the left tower to be precise.
 
OT - I used a flex hose for the fuel line through the tower

I was worried about that, so I used a braided stainless flex hose for the fuel line that runs through the tower from the selector valve to the boost pump. I made it a little long so it has a shallow S bend inside the tower.

I have not analyzed what the loads would be on the fuel line in a ground loop, so I don't have any evidence that the standard aluminum fuel line tube isn't strong enough. I can not show how much stronger the flex line is - but I think the flex hose will be stronger, and can flex some without breaking if the structure deforms.

Sorry, couldn't resist.:D

Speaking of fuel exposure in a ground loop though - a couple of folks have mentioned the slot in the forward wing attach bracket as an example of an intentionally designed break-away point. The reason this was done was that the root rib is closing out the fuel tank, and if you put a big drag load on the wing (like in a ground loop, or hit a tree) you do not want to have that attach bracket rip the fuel tank open. So it is slotted to allow the wing to move without tearing out the root rib.
 
Since this slot...

....
Speaking of fuel exposure in a ground loop though - a couple of folks have mentioned the slot in the forward wing attach bracket as an example of an intentionally designed break-away point. The reason this was done was that the root rib is closing out the fuel tank, and if you put a big drag load on the wing (like in a ground loop, or hit a tree) you do not want to have that attach bracket rip the fuel tank open. So it is slotted to allow the wing to move without tearing out the root rib.

...was not in the original -4 and -6 designs, it probably was added as a result of some unintentional customer stress testing beyond the original design limits....
 
Randy's incident (per his own account on his website) seems to suggest a fold-under while sliding sideways, ie applied force primarily perpendicular to the fuselage centerline. Note the bend in the gear leg just above the axle. Such loading would be close to a best case for the gear tower, so the bolt(s) failed. The website does not detail which nuts were in use. Fuselage damage appears to be minimal. Perhaps Randy (or the new owner who did the repairs) could comment.

Dan, you are exactly correct. The load was directly perpendictular to the centerline, the rim caught the pavement and folded the gear leg under. The gear box tower structure was undamaged, repair to the fuse consisted of replacing the inner weldment (forget the part number) and the side fuselage skin which had some minor distortion from the stress. BTW, in my case it was the bolts that failed, not the nuts.

In the final analysis I thought the design did well where the bolts (all of them) sheared as they should just before the tower incurred damage. Ken Krueger came out for a look and concluded the same thing. Your question about how the structure would cope with a force parallel to the centerline, such as a pothole, is a good one and I don't know the answer.

When contemplating repairs I was going to convert to the Grove one piece gear had I done the repairs myself, but upon further reflection I'm not sure that would've been a good idea because then you're just transferring the load somewhere else and might end up with more fuselage damage in an off field landing.

Bottom line: I think the current gear box structure is adequately designed other than being a bit more difficult to build that it should be. Someone did a mod where they made the inside face of the gear towers removable -- good mod, if I were building one again that's what I'd do.
 
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Primary premise of this thread - lost?

I've read the commentary here, from the nut stripping test to the story at the top of this thread, and it seems we've lost sight of the original intent.

The idea here is to present a modification or improvement to the original RV8 landing gear box and/or leg attach hardware. The mod would only cost about ONE pound weight.

I'm having a real hard time arguing with that. Of course, the proposed mod would have to be tested and proven to provide value commensurate with price and install effort. But that's true of anything.

Many of you express total confidence in the RV8 gear design "as is". I have the same plane as you do, and I agree the gear is "adequate" - BUT I know like everything else in this world it can be improved. Why discourage the effort? Some of you think you know better, that there's no room for improvement or that it can't be done easily. Now you're pitting your engineering prowess against someone who does this kind of work at the highest levels of the profession.

If Steve does develop, test and validate a mod that increases gear strength substantially for only about a pound of weight, would you turn your nose up at it? I sure wouldn't!
 
I wont fly with the Van's Nuts

At minimum I'm upgrading the nuts on my gear before the upcoming first flight. Debating the issue now, as are others.

There is a tale of another RV8, in Colorado.

It's left gear tore out on takeoff with a competent pilot at the controls. Yes takeoff. Its legs were torqued not that long before the incident, there were no strange landings or events, and the nuts stripped out with plastic deformation. The washers deformed and one even cracked. The nuts have pulled into the washers and they are now permanently attached. I think a harder washer with closer tolerance to the bolt is appropriate in addition to a stronger nut, but my mechanical engineering degree is a bit old at this point.

Its up to the local pilots there (if they desire) to post the details, I'll respect their privacy and willingness to discuss the event or not. They are investing some time into a different nut configuration.

I like stacking the odds in my favor and where there is smoke there may be fire. Could there be others that people have written off as 'pilot error'? I'm not pointing fingers, but I'm not sticking my hand in the sand either. I have a lot of respect for what Steve is sniffing out.

Its heartbreaking to look at the failed parts and damaged airplane. It will probably require most of a new fuselage, engine, prop, etc.

I hope that if there is indeed a weakness here that it is properly flushed out and addressed with objective facts and data, as not all builders are engineers.

Tim
 
If Steve does develop, test and validate a mod that increases gear strength substantially for only about a pound of weight, would you turn your nose up at it? I sure wouldn't!
I still state, I am afraid, that without a lot more info, I would not deviate from the manufacturers plans, in such a safety critical area.

The MD11 is an example of where an "overstrong landing gear" has caused multiple wings to come off, aircraft invert, and fatalities. Most aircraft have the gear collapse / detach first which seems a somewhat better idea :eek:

So as with the gear leg design above (designed to "bend" before fuselage damage occurs) I would check with Van's as to whether it is by design that this percieved weakness occurs... Randy's
In the final analysis I thought the design did well where the bolts (all of them) sheared as they should just before the tower incurred damage. Ken Krueger came out for a look and concluded the same thing.
seems very relevant ;)

Or put it another way... If you can substantially "improve" the design at a cost of 1lb in weight, I am sure Van's would adopt it overnight.
 
<<..and the nuts stripped out with plastic deformation. The washers deformed and one even cracked. The nuts have pulled into the washers and they are now permanently attached. I think a harder washer with closer tolerance to the bolt is appropriate in addition to a stronger nut..>>

Appreciate your report Tim. One thing struck me as odd in the above account, and you may wish to pass it back up the chain. The hardware call for the outer pair of NAS attach bolts specifies a 062 25783054 washer. A quick search doesn't turn up any spec for that number, but a Van's part list calls it a hardened washer, apparently Grade 8. It seems odd that a pulled nut would embed in a Grade 8 washer. Perhaps possible (I don't know), but could there have been a washer substitution?

On the flip side, I don't understand the reason for a hardened washer in this application, unless the purpose is merely to get a close tolerance fit on the bolt diameter. Usually a hardened washer is specified where the bolted joint requires bolt preload to minimize stress cycles. A soft washer might deform and give up some preload in a single minor overstress. Can't preload this joint to any significant degree. Maybe it has something to do with minimizing distortion of the funky NAS679 nut.

Note to all....I'm hearing a lot of opinion. Please allow me to offer one in the form of a quote from my own (very highly qualified) engineering mentor. He taught me "you do not fully understand a thing until you quantify that thing". No numbers, no real truth. Think about it; there's a message there for both the "good enough" and "maybe not" camps.

Randy, thanks for the note, and may I say you provide an fine example of how to handle a difficult situation for the benefit of all...a measure of character.
 
Andy's example is perfect!

The MD-11 situation that Andy Hill mentioned is a perfect analogy. There was an MD-11 crash just a couple of weeks ago that exhibited exactly what Andy said. Rather than the gear strut tearing off, it tore the wing off, the other wing kept lifting so it rolled over, big fire, nasty.

The analogy here is that what I am suggesting to do is the equivalent of strengthening the wing of the MD-11 up to the point where the gear would break before the wing. In our case, I would like the gear structure in the fuselage to survive to the point where the gear bends or separates from bolt shear. One difference is that it can be done with very very little added weight.

In Randy's case, it essentially did - it was the best-case scenario in that it didn't apply any significant yaw-torque to the gear tower. Others have been much less fortunate -with some drag load on the wheel, the fuselage gets ripped up and the gear strut is relatively intact.

By the way, I should say something about the Grove gear - in a couple of places I've speculated that maybe it would bend before damaging the fuselage. BUT, I have not analyzed it - I know it is a thicker section, and it is high strength aluminum instead of low/medium strength steel. It may be that it is just as strong or stronger than the steel gear strut. So, sorry if my speculation made the Grove guys mad - they have a beautiful product. By the way, zero hits on a one-piece, so I'm growing to believe there is not one.

Stay tuned - this summer I may build a couple of structural mock-ups and show a comparison load test.
 
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