Van's Air Force
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RV-6A Tip Over
- Thread starter plehrke
- Start date
mannanj
Well Known Member
Tip Over
Hey Guys:
Do a search on "jockstrap", "Plane Talk Hangar" "Ted and Phill at Lockhart, Tx."
They designed a steel plate that covers the nut and screw thread on the bottom front of the fork. It is designed to skid along the ground instead of digging in. As you can see, once that nut breaks through the wheel pant (spat), it digs into the dirt, and the nose gear will pole vault you onto your back.
I think the 6 in. wheel and a heftier nose gear leg are good ideas too, but you have to prevent the big nut and threads from contacting the dirt during any ground operation.
My Dos Centavos.
Hey Guys:
Do a search on "jockstrap", "Plane Talk Hangar" "Ted and Phill at Lockhart, Tx."
They designed a steel plate that covers the nut and screw thread on the bottom front of the fork. It is designed to skid along the ground instead of digging in. As you can see, once that nut breaks through the wheel pant (spat), it digs into the dirt, and the nose gear will pole vault you onto your back.
I think the 6 in. wheel and a heftier nose gear leg are good ideas too, but you have to prevent the big nut and threads from contacting the dirt during any ground operation.
My Dos Centavos.
Not the wheel or forks fault
This is just MHO. I'm not going to post any further responses.
Based on the length of the strut, lack of shock absorption and other factors, I conclude that the strut is adequate only as long as no significant side load or longitudinal load is imposed.
Side load we can control (mostly) by flying a straight landing and not making drastic high speed turns. Still, runway imperfection can bite us (witness the 9A that folded on a paved runway a short time back).
Longitudinal force (front to back) is controlled only by exposing the nosewheel to "low friction" runways, as well as having a properly inflated and installed nosewheel.
The lack of any shock dissipation in the strut compounds the problem, since any force from the runway is transmitted directly into vertical motion of the aircraft. If you are exercising "proper" technique (stick full aft on rollout), this translates into nose popping off the ground. When you come back down, if there is any more forward resistance, the additional force will fail the nose gear, since it cannot (and is not designed to) take much in the way of a longitudinal event.
Finally, the "icing" on the failed event cake. Failed nosegear can easily cause flipover, even at relatively slow speeds. The A models are high CG. With the nose gear folded back, the aircraft is quite literally teetering on falling over with very little assistance. I know, because as I righted my 7A model after a flip, IT DID NOT WANT TO STAY RIGHTED with the nose gear folded back. It wanted to stand on the NG and prop.
If my RV had not flipped, the damage would have been substantially (on the order of $40K) less. Oh well.
I love RVs. Great planes. Took 6+ years to build mine (and coming up on a year to rebuild). The nose gear appears (to me) UNSAFE for unpaved runways (and even some paved ones).
Based on the numbers (5 or 6 thousand flying, how many are A models?) we are approaching a 1 % failure rate. That seems to me a bit high.
Just MHO.
I will now don my flame retardant suit and "repair to my lair".
Later.
This is just MHO. I'm not going to post any further responses.
Based on the length of the strut, lack of shock absorption and other factors, I conclude that the strut is adequate only as long as no significant side load or longitudinal load is imposed.
Side load we can control (mostly) by flying a straight landing and not making drastic high speed turns. Still, runway imperfection can bite us (witness the 9A that folded on a paved runway a short time back).
Longitudinal force (front to back) is controlled only by exposing the nosewheel to "low friction" runways, as well as having a properly inflated and installed nosewheel.
The lack of any shock dissipation in the strut compounds the problem, since any force from the runway is transmitted directly into vertical motion of the aircraft. If you are exercising "proper" technique (stick full aft on rollout), this translates into nose popping off the ground. When you come back down, if there is any more forward resistance, the additional force will fail the nose gear, since it cannot (and is not designed to) take much in the way of a longitudinal event.
Finally, the "icing" on the failed event cake. Failed nosegear can easily cause flipover, even at relatively slow speeds. The A models are high CG. With the nose gear folded back, the aircraft is quite literally teetering on falling over with very little assistance. I know, because as I righted my 7A model after a flip, IT DID NOT WANT TO STAY RIGHTED with the nose gear folded back. It wanted to stand on the NG and prop.
If my RV had not flipped, the damage would have been substantially (on the order of $40K) less. Oh well.
I love RVs. Great planes. Took 6+ years to build mine (and coming up on a year to rebuild). The nose gear appears (to me) UNSAFE for unpaved runways (and even some paved ones).
Based on the numbers (5 or 6 thousand flying, how many are A models?) we are approaching a 1 % failure rate. That seems to me a bit high.
Just MHO.
I will now don my flame retardant suit and "repair to my lair".
Later.
I pretty much agree with Joe here. You can do whatever you want to the fork and wheel. This will slightly reduce the chances of a tip over on some undulating grass fields but the strut is the biggie. Dig the nut in, bend or twist this the wrong way and it's over (sorry for the pun).
Land on grass at your own peril, pretty hard to know how bumpy it is or if the rodents have dug a new hole somewhere on that runway. Is it worth wrecking your RV just to land on grass?
Heavy engine/ prop combos increase gear loading and your chances of a NG failure on uneven surfaces.
Land on grass at your own peril, pretty hard to know how bumpy it is or if the rodents have dug a new hole somewhere on that runway. Is it worth wrecking your RV just to land on grass?
Heavy engine/ prop combos increase gear loading and your chances of a NG failure on uneven surfaces.
Geico266
Well Known Member
Like I have said before, the nose wheel on RV-XA models is just a kick stand to keep the prop off the ground. If you can't do a main gear landing and plant the nose wheel hard you are going to be in trouble.
osxuser
Well Known Member
A couple of interesting notes on this one (interesting to me at least):
1. First time I can recall seeing an -6A with the low sitting tail flip. The -7A's and 9A's seem to have been much more susceptible to the problem. As their landing gear geometry keeps the tail higher. Which also means more weight on the nose.
2. Tire clearance seems to have almost no bearing on the situation, and is simply a stop-gap fix. (I've gotten in trouble for saying this before but with this one... it bears repeating)
3. Having worked on a -6A recently, and retrofitted a Grove nosewheel, I can easily say that with the rubber seals (Matco wheel), if the wheel is torqued properly, the Nosegear Leg will start porpoising by itself at higher speeds. There also seems to be very little stability laterally in the gear leg. All this was observed while the aircraft was in tow behind a golf cart.
I reintroduce at this time the idea of a -10 gear leg cut down, with the 5.00x5 wheel and fork complete, and put in with the same rubber doughnut absorbsion system the -10 uses.
I'm glad we still retain our no-fatalities record with these. Praying for a speedy recovery.
1. First time I can recall seeing an -6A with the low sitting tail flip. The -7A's and 9A's seem to have been much more susceptible to the problem. As their landing gear geometry keeps the tail higher. Which also means more weight on the nose.
2. Tire clearance seems to have almost no bearing on the situation, and is simply a stop-gap fix. (I've gotten in trouble for saying this before but with this one... it bears repeating)
3. Having worked on a -6A recently, and retrofitted a Grove nosewheel, I can easily say that with the rubber seals (Matco wheel), if the wheel is torqued properly, the Nosegear Leg will start porpoising by itself at higher speeds. There also seems to be very little stability laterally in the gear leg. All this was observed while the aircraft was in tow behind a golf cart.
I reintroduce at this time the idea of a -10 gear leg cut down, with the 5.00x5 wheel and fork complete, and put in with the same rubber doughnut absorbsion system the -10 uses.
I'm glad we still retain our no-fatalities record with these. Praying for a speedy recovery.
AlexPeterson
Well Known Member
Mike, thanks for the report. It would really, really be valuable to determine the exact nose wheel bearing setup. Specifically, does it have the 1.25" round bar going all the way across from fork to fork, or does it have the newer style "mushroom" that some of the later model 6A's have. If you are able to find out with certainty, it would be great.
Ron Lee
Well Known Member
When I changed the fork and nose gear leg (shortened) I also reinforced the bottom and side of the front part of the nose wheel pant with carbon fiber to act as a skid. Hopefully this may prevent the nut area from digging it and acting as a pole vault.
I avoid grass strips/taxiways/parking areas.
I avoid grass strips/taxiways/parking areas.
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Let's not forget pilot skills, you can do whatever to the nose wheel, if you ain't good on the pilot skills, what you did will mean nothing. I'm reminded of an europa that started out as a mono wheel, he had trouble landing it so he went to the standard tri gear. A year later he put it in the ground at OSH on landing.
I don't think the axle/ bearing setup has any significant impact on the flipping mechanism unless the tire actually locked somehow. I would agree that the stock setup is pretty crappy though.
Stiffening the leg with carbon fiber (lots of layers) would seem like a better idea. This should decrease the amplitude of any pogo effect.
Piloting skills pretty much end being useful once the tire is on the ground and speed is below 30-40 knots assuming you use your brain and don't do sharp, high speed turns with the brakes on. Holding up elevator below 30 knots will have minimal effect on NG loading and at taxi speeds is essentially useless. It's a good idea but mainly psychological.
I don't get any porpoising of the gear leg on takeoff from 0-60 knots, all stock, old style setup. A bump on the runway is about the only way this motion starts on my 6A and this can get pretty bad as there is little damping in the system
Stiffening the leg with carbon fiber (lots of layers) would seem like a better idea. This should decrease the amplitude of any pogo effect.
Piloting skills pretty much end being useful once the tire is on the ground and speed is below 30-40 knots assuming you use your brain and don't do sharp, high speed turns with the brakes on. Holding up elevator below 30 knots will have minimal effect on NG loading and at taxi speeds is essentially useless. It's a good idea but mainly psychological.
I don't get any porpoising of the gear leg on takeoff from 0-60 knots, all stock, old style setup. A bump on the runway is about the only way this motion starts on my 6A and this can get pretty bad as there is little damping in the system
lorne green
Well Known Member
Sorry Stephen, you would be wrong on this point.
Last summer the distinguished and admired Dr. Ron Tuttle of Salmon Arm B.C.
Tuttle, Ron (2007-07-19)
TUTTLE - Ron passed away on Thursday July 19, 2007 at the age of 71. Survived by his loving wife of 43 years, Arlene, son Jeff, wife Becky Tuttle of Escondido, CA, three grandchildren, Lauren, Emily, Andrew and mother-in-law Lillian Casselman. Ron was born July 10, 1936 in Colorado Springs, CO. He graduated from Colorado Springs/Palmer High School in Colorado Springs in June, 1955 (named to Palmer Alumni Hall of Fame, October, 2000). He received a BS in zoology from Colorado College in 1960 and went on to achieve a doctorate in pharmacology from the University of Manitoba, Winnipeg, in 1966. Ron returned to the US to continue his scientific career, where he went on to pioneer many aspects of pharmaceutical development. He holds several US and European patents for life-saving drugs, most notably dobutamine. In 2001 Ron retired to the Okanagan Valley in British Columbia, Canada where he pursued his lifelong passion of flying small aircraft. A memorial service will be held on Wednesday July 25, 2007 at 4 pm at the Chapel of Springfield Funeral Home, 2020 Springfield Road, Kelowna, BC.
Regards
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L.Adamson
Well Known Member
I was introduced to the 6A's in 1995. In all of these years, I haven't heard of 6A's flipping either. There has probably been a few, since I know of several "6" taildraggers that have when landing in fields. But just no where the "flips" I see these days.
L.Adamson ---- RV6A
kentb
Well Known Member
RV's are not Cessna's.
This last week I flew the C172 that I am selling to an airport 40 miles away so that the buyers mechanic could do some work on it. Winds were calm when I took of but when I was landing they were gusting from the 90 degrees about 15 kts. I made one of the worst landing in the past 10 years. I did land on the main gear, bounced around a bit and then lowered the nose to the ground.
A nice guy at the FBO gave me a ride home in his C172. When he landed had my home airport, he did even worse than I did. Bounced on the mains and then banged the nose into the ground. Bounced a couple more times and then got it under control.
Now I didn't inspect his plane for firewall damage, but the plane did hold together. If he had made that landing with an RV-xA, I would have expected the nose gear to had damage.
These planes are not as strong as a Cessna. RV are not the correct plane to learn your landing skills in. If you don't keep your landing skills fresh, then you better expect some problem. You need to be able to land with more skill then the average Cessna driver.
Kent
This last week I flew the C172 that I am selling to an airport 40 miles away so that the buyers mechanic could do some work on it. Winds were calm when I took of but when I was landing they were gusting from the 90 degrees about 15 kts. I made one of the worst landing in the past 10 years. I did land on the main gear, bounced around a bit and then lowered the nose to the ground.
A nice guy at the FBO gave me a ride home in his C172. When he landed had my home airport, he did even worse than I did. Bounced on the mains and then banged the nose into the ground. Bounced a couple more times and then got it under control.
Now I didn't inspect his plane for firewall damage, but the plane did hold together. If he had made that landing with an RV-xA, I would have expected the nose gear to had damage.
These planes are not as strong as a Cessna. RV are not the correct plane to learn your landing skills in. If you don't keep your landing skills fresh, then you better expect some problem. You need to be able to land with more skill then the average Cessna driver.
Kent
I'm reminded of my training I did about a month ago in a 6A. It was real fun, the landing I figured out was like a wheel landing in a tail wheel plane, which can be very intense, the consentration that is, but the thing that I thought was supprising was the comment that I landed square to the runway every time. What was meant by this is the fella that was flying with me(Brian) said that most people land crooked(yawed to one side). I would emagine along with wearing the left tire, the nose wheel would get some side stress as well, expecially if you flat land, or three point it.
I did once prang the nosegear of my RV-6A. I entered a PIO at touchdown and banged it hard several times at (seemingly) extreme angles before I figured out the landing couldn't be salvaged. I trashed the front bottom of the pant and had to rebuild it, which given the geometry of the gear meant I must have fully depressed the tire on the rim. The nosegear was more than "just a kickstand" that day! I had a local shop perform NDT on the gear and determined there was no damage.
This is relevant to the discussion only because my personal experience with the gear seems so at odds with the numerous failures that have happened: "How could I beat the heck out of my gear with no apparent damage, and other guys are just rolling out and having the gear fold."
I would like to address the issue but as I approach the annual the problem I have is where to spend my money. I can invest in Van's fix, but the rollover in San Diego makes me question if the new geometry hasn't introduced another failure mode. I guess getting a bigger tire isn't going to work based on this accident. The theory that this only happens to the taller legged models goes out the window now as well. The Grove hardware looks promising. . .at least there's a sensible line of reasoning behind it.
Perhaps I should just keep the gear that has already survived my ham-handedness?
John Allen
RV-6A
This is relevant to the discussion only because my personal experience with the gear seems so at odds with the numerous failures that have happened: "How could I beat the heck out of my gear with no apparent damage, and other guys are just rolling out and having the gear fold."
I would like to address the issue but as I approach the annual the problem I have is where to spend my money. I can invest in Van's fix, but the rollover in San Diego makes me question if the new geometry hasn't introduced another failure mode. I guess getting a bigger tire isn't going to work based on this accident. The theory that this only happens to the taller legged models goes out the window now as well. The Grove hardware looks promising. . .at least there's a sensible line of reasoning behind it.
Perhaps I should just keep the gear that has already survived my ham-handedness?
John Allen
RV-6A
AlexPeterson
Well Known Member
Well, you may indeed be right. However, the evidence is substantial that bearing drag/lockup is a player, perhaps the main player. It either just locks up or gets into a sympathetic vibrational system in a porpoise mode (of the fork/wheel assembly) until the nose strut digs in. This has been quite extensively discussed in the other threads. There was even a -10 on a first flight that had the nosewheel lock up on landing. Ground a flat spot in it. The dynamics of what is deflecting and how it interacts with bearing drag is not a simple thing. Some discuss the seal drag - I don't believe it is a player, as it simply is not that much. Many have reported seeing galling between the fork and the "mushroom" parts indicative of bearings locking up causing the mushrooms to spin. Many have staked or otherwise prevented this mis-behaving rotation of the mushrooms. There is video evidence of porpoising nose wheel fork assemblies during taxiing. Something is exciting this resonance. The tire cannot excite this resonance unless there is bearing drag. It is not hard to imagine that if this resonance gets too wide, the whole system goes to heck.
I submit that the main issue is the newer style (late model 6A's, all 7A's and 9A's and probably 8A's) mushroom, no solid axle design. I don't say this casually - a lot of thought and observation has gone into this. I don't believe that the higher cg on the non-6A models can explain the very large discrepancy in reported flips between the much more numerous 6A's as compared to all the other trikes. Hence my earlier (in this thread) question about the nose axle design on the 6A in England.
A very dynamic situation
Gents,
I am glad to hear someone tell about a bad PIO and minor damage to the wheel pant but no damage to the nose gear leg.
I have personally met a few who have bent their nose gear. We discussed their accidents. Two incidents were extreme conditions that just about any aircraft would have been severely damaged. One is believed to be a deficient nose gear design. In fact I have the bent nose gear leg sitting right next to me as I write this post.
I too am building and RV-7A and have been following all the posts about the nose gear issues. I just installed my gear legs and my RV-7A is looking mighty proud in my shop.
I believe several comments on this post are rather accurate. As a Mechanical design engineer by profession I believe the following
1. Increasing the size of the tire and increasing the length of the fork arm may have contributed to the failure more then one would like to believe. As someone mentioned increasing the fork arm length increases the moment (torque) on the gear leg when a load is applied to the tire. Considering the history of the gear leg increasing the moment isn't a good thing at all.
2. While installing my nose wheel bearing (fully lubed) it wasn't very difficult to "freeze" the bearings while applying a minor preload on the wheel. I found that even loosening the axel nut the drag was still present. After removing the bearings, playing with the roller ring I was able to get the bearing to spin freely again. I repeated the installation and minor preload again only to have to remove the bearing and repeat the freeing up process again. However once I made a spacer to prevent "overloading" the bearing I didn't have any issues. So with the current design, I believe it is possible to get high drag on the nose wheel. This too can be a problem.
To highlight this point I recently saw a show on TV called Shockwave. It is a show that captures really horrific accidents on video. In this case it was a guy on a high performance motorcycle doing wheelie's on an stretch of highway. Upon letting the front wheel down (not to fast) the wheel started a nasty vibration/oscillation which intern cased the motorcycle to crash. The video was taken from another motorcycle traveling next to the stunt bike. Upon further review of the tape it was concluded that the increase drag from the new brake pads dust cause drag on the front wheel causing the oscillation.
Food for thought.
Paul
Gents,
I am glad to hear someone tell about a bad PIO and minor damage to the wheel pant but no damage to the nose gear leg.
I have personally met a few who have bent their nose gear. We discussed their accidents. Two incidents were extreme conditions that just about any aircraft would have been severely damaged. One is believed to be a deficient nose gear design. In fact I have the bent nose gear leg sitting right next to me as I write this post.
I too am building and RV-7A and have been following all the posts about the nose gear issues. I just installed my gear legs and my RV-7A is looking mighty proud in my shop.
I believe several comments on this post are rather accurate. As a Mechanical design engineer by profession I believe the following
1. Increasing the size of the tire and increasing the length of the fork arm may have contributed to the failure more then one would like to believe. As someone mentioned increasing the fork arm length increases the moment (torque) on the gear leg when a load is applied to the tire. Considering the history of the gear leg increasing the moment isn't a good thing at all.
2. While installing my nose wheel bearing (fully lubed) it wasn't very difficult to "freeze" the bearings while applying a minor preload on the wheel. I found that even loosening the axel nut the drag was still present. After removing the bearings, playing with the roller ring I was able to get the bearing to spin freely again. I repeated the installation and minor preload again only to have to remove the bearing and repeat the freeing up process again. However once I made a spacer to prevent "overloading" the bearing I didn't have any issues. So with the current design, I believe it is possible to get high drag on the nose wheel. This too can be a problem.
To highlight this point I recently saw a show on TV called Shockwave. It is a show that captures really horrific accidents on video. In this case it was a guy on a high performance motorcycle doing wheelie's on an stretch of highway. Upon letting the front wheel down (not to fast) the wheel started a nasty vibration/oscillation which intern cased the motorcycle to crash. The video was taken from another motorcycle traveling next to the stunt bike. Upon further review of the tape it was concluded that the increase drag from the new brake pads dust cause drag on the front wheel causing the oscillation.
Food for thought.
Paul
I'm thinking we know basically what happened- the gear leg folded under and the aircraft flipped over. Fortunately nobody was seriously injured or worse.
The point of discussion is to inform other A model pilots about likely causes, fixes etc. so that they may not have to experience a similar accident, whether this is achieved by not flying onto grass strips, being more careful with technique or with hardware modification, I think it is a valuable discussion and gets people thinking.
The point of discussion is to inform other A model pilots about likely causes, fixes etc. so that they may not have to experience a similar accident, whether this is achieved by not flying onto grass strips, being more careful with technique or with hardware modification, I think it is a valuable discussion and gets people thinking.
Little_Richard
Active Member
If when you say Old Style Setup you are referring to the original design (603-1) that is 1 inch in diameter just below where it goes into the engine mount (instead of 1.125 inches), I would be VERY cautious about using that gear leg. There have been more than one that have broken from fatigue stress just below the engine mount. There was a service bulletin many years ago about this and how to have it inspected regurlarly for cracks.
It is perfectly good to use until it cracks, but who knows when it will crack and break off!!!!!!!
I imagine there are still a lot of people using this old original gear leg that are unaware of the previous service bulletin.
L.Adamson
Well Known Member
I'd consider my RV6A setup the "old style", but it is the new and improved gear leg from around the year 2000. I sent the original back for the replacement. The fork is the "old style", and the wheel is Cleveland, with the axle that runs through the full wheel. I'm going to leave it this way. I know of too many 6A's with this setup.................that haven't flipped!
L.Adamson --- RV6A
