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  #1  
Old 04-25-2009, 07:29 PM
scsmith scsmith is offline
 
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Location: Ashland, OR
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Default A tale of two airplanes ( long post)

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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Last edited by scsmith : 04-26-2009 at 03:01 AM. Reason: mia culpa - merged two different events
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  #2  
Old 04-25-2009, 08:52 PM
Danny7 Danny7 is offline
 
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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.
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  #3  
Old 04-26-2009, 12:49 AM
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Andy Hill Andy Hill is offline
 
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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
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  #4  
Old 04-26-2009, 03:03 AM
SvingenB SvingenB is offline
 
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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.
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  #5  
Old 04-26-2009, 03:13 AM
asav8tor asav8tor is offline
 
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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. Come up with something bad and we will say "Oh I remember the guy that tried to...."
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  #6  
Old 04-26-2009, 03:31 AM
scsmith scsmith is offline
 
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Default 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.
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RV-8 N825RV
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WW 200RV
"The Magic Carpet" Flying since Sept. 2009
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also
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LS6-15/18W sailplane SOLD
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  #7  
Old 04-26-2009, 03:54 AM
PCHunt PCHunt is offline
 
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Default Correction

Quote:
Originally Posted by scsmith View Post
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.
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  #8  
Old 04-26-2009, 06:25 AM
pierre smith's Avatar
pierre smith pierre smith is offline
 
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Default Spend your hours building, Steve....

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

Regards,
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  #9  
Old 04-26-2009, 07:51 AM
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DanH DanH is offline
 
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<<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.
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  #10  
Old 04-26-2009, 08:40 AM
David-aviator David-aviator is offline
 
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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.
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