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

Raiz

Well Known Member
I know the nose gear thing has been done to death already but here is a new angle on it, which just might save your nose gear. The key takeaway is: Don?t brake on bumpy strips (if you can avoid it). Read on to understand how I came to that view.

In deciding whether to build a TW or tri-gear RV-7, the key issue for me was whether the nose gear would be able to cope with grass strips. I read everything I could find but it left me feeling that I?d be gambling with $50k and 4 years work if I operated a 7A out of a roughish strip. I became intrigued and set about analysing the gear design. What emerged is that the nose gear has relatively little vertical travel ? just 4? (1?? for the leg and 2?? for the tire).

There are two common situations that require a lot of suspension travel: Landing and bumpy ground. I?m going to assume that nobody expects to land on the nose wheel on bumpy ground and get away with it, which means we can treat nose wheel landings and bumps separately.

Van?s deals with the landing case by advising pilots against using the nose gear as a landing gear. That?s good advice, because the RV-7A has much less suspension travel than would be needed to meet the FAR 23 three point landing requirement for certificated airplanes. Put simply, it?s not designed for nose wheel landings.

Van?s suggests the 7 instead of the 7A for rough strips but how big do the bumps have to be before it counts as a rough strip? A way to think of it is that you have a maximum of 4? of suspension travel and part of that is taken by the factors that pre-load the gear (static weight, braking, residual compression from any earlier bumps and forward stick positions). Any remaining suspension travel must be greater than the bump height, if the leg is to survive. Tire pressure also plays a role, because it determines (in part) how much of the travel is taken up by the pre-load.

The graph below shows the compression in the gear, as a function of braking and tire pressure. This is for worst case loading conditions of 1800 lb all up weight, 375 lb static nose load and a 45? CG height. The travel available to absorb bumps is simply the 4? total minus the value from the graph.

graph.jpg


The gear compression with no braking is around 0.5?, leaving scope for bumps of up to 3" or so high, before the leg fails. However, the capacity to deal with bumps decreases under braking, dropping to just 1? under maximum braking (defined as when the rear wheels lock-up). The blue (25 psi) line flattens at the top, because the tire bottoms on the rim.

Pulling the stick back will reduce the load on the nose provided there is enough airspeed. However, that still leaves braking at low speed, where the elevator authority is low. Conversely, forward stick positions will increase the nose gear compression but I would argue that that represents poor piloting technique. The other source of pre-load is unrelieved compression from a previous bump, or from dropping down on to lower ground, but that?s going to have to be the subject of a future post.

In the meantime, avoiding braking on bumpy strips could be a good plan, especially at low speed.

Just getting my flame suit now?;)

Raiz
 
gear vertical travel?

Hi Raiz - interesting work, thanks for posting.
I became intrigued and set about analysing the gear design. What emerged is that the nose gear has relatively little vertical travel ? just 4? (1?? for the leg and 2?? for the tire).

Can you share your analysis on where these two numbers came from?

I think you are on to something here... In the sprit of experimental aviation I hope we can keep this going forward without too much flaming!
 
Suspension travel is the key to RV-7A nose gear collapse.

Well, no flames from me! In fact, I commend you on your analysis. But I would like a bit more explanation of a couple of your points

... the nose gear has relatively little vertical travel ? just 4? (1?? for the leg and 2?? for the tire).

How did you determine the leg travel of 1-3/4" (the tire travel is obvious)? Is this the travel at which the propeller strikes the ground?


The gear compression with no braking is around 0.5?, leaving scope for bumps of up to 3" or so high, before the leg fails.

What is your criteria of "...leg fails"? Is it the bending stress in the leg at the attachment socket in the engine mount? Does the bending stress exceed the yield stress of the spring steel leg in this condition?
 
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sounds good, i'd be interested to see where you got the number for only an inch & 3/4 for the leg.

also i think the pilot technique isn't given enough recognition, at some point in low speed travel there will be an increasing amount of weight you can take off the nose gear with proper back pressure.
 
Q: Where the 1 3/4" leg deflection number came from?
A: It's the amount of vertical deflection (measured at the axle) required to reach the yield stress in the leg. I assumed a yield stress of 168 ksi.
Raiz
 
Hi Raiz - interesting work, thanks for posting.


Can you share your analysis on where these two numbers came from?

I think you are on to something here... In the sprit of experimental aviation I hope we can keep this going forward without too much flaming!

The maximum tire deflection is just when it bottoms on the rim. The leg deflection is determined by the yield stress in the leg, as per my other reply.

Keep the questions coming !

Raiz
 
sounds good, i'd be interested to see where you got the number for only an inch & 3/4 for the leg.

also i think the pilot technique isn't given enough recognition, at some point in low speed travel there will be an increasing amount of weight you can take off the nose gear with proper back pressure.

Danny 7, I think you are right with pilot technique and back pressure but the problem is that there is always a speed below which a rearwards stick position has negligable effect and, if you hit a big enough bump at that point, the leg will buckle.

Raiz
 
Interesting. I've often been keen to learn where exactly the CG was on the aircrafts that have had nose failure. Braking could further exasperate the situation in certain areas of CG.
 
there is always a speed below which a rearwards stick position has negligable effect and, if you hit a big enough bump at that point, the leg will buckle.

This would explain the low speed nose gear leg failures that have happened.

Raiz, how sensitive is your analysis to CG position? Any chance of posting results for forward, middle, and aft CG locations?
 
Interesting. I've often been keen to learn where exactly the CG was on the aircrafts that have had nose failure. Braking could further exasperate the situation in certain areas of CG.

Hi GregM,

You got me thinking, so I re-ran the analysis with a different AUW and static nose load (1600 lb and 250 lb respectively). This moves the CG back from 82.6" to 85.4". The graph in my first post changes to this:

graph2.jpg


If you compare the two graphs, you can see that these changes increased the bump capacity from 1 to 2 inches under heavy braking. Interesting !

Raiz
 
Suspension travel is the key to RV-7A nose gear collapse.

Couple of comments:
1. The nose gear leg is at 43 degrees, so the nose gear bump load has two components, vertical and horizontal. The horizontal component partially cancels the vertical component, thus resulting in a lower stress in the leg at the upper end. This is not true of the braking loads which only have a vertical component.
2. I think you have assumed that the aircraft is rigid as the nose wheel passes over a bump. Even a step bump will deflect the tire and the nose gear leg at their spring rate which is no longer a step input. Then as the vertical component of the bump load increases, the nose gear leg deflects, the load is transferred back into the engine mount and this causes the nose to rise.

And a question:
The nose gear leg failures that I am aware of were caused by the nose gear swivel digging into the ground bending the leg backwards and usually flipping the aircraft inverted. Are there nose gear leg failures caused by the leg breaking at the motor mount socket and allowing the propeller to hit the ground?
 
Q: Where the 1 3/4" leg deflection number came from?
A: It's the amount of vertical deflection (measured at the axle) required to reach the yield stress in the leg. I assumed a yield stress of 168 ksi.
Raiz

have you done any tests to confirm this?
 
I don't suppose the engineers at Van's would release their analysis on this structure. It would be nice to see how they tested (practical versus theoretical) and exactly where the failure occurs.

I'm building the wings for the 7A and this topic is very interesting to me. Although I don't plan to land on grass strips much of the time, I would like to know how "durable" this configuration is for a low time pilot (me).

Were the failures of the past gear that bent, snapped, cracked? If so, did they always fail in the same part of the structure? Did they all have prop strikes and/or the nose fork digging into the ground?

I wonder if this topic has been generalized over time into all failures being the same.

Thanks for the research and effort everyone! I would like to stay with the 7-A if it's reasonable. It appears it is.
 
terrye, thank you for introducing some of the more advanced aspects of this. To your specific points:

Q1: The nose gear leg is at 43 degrees, so the nose gear bump load has two components, vertical and horizontal. The horizontal component partially cancels the vertical component, thus resulting in a lower stress in the leg at the upper end. This is not true of the braking loads which only have a vertical component.

A1: I agree completely and I have taken this into account in the detailed analysis. The graph below shows how the stress varies along the leg as the angle of the load changes (0 deg is vertical and 90 deg is rearwards). The second graph shows how the yield load and the location of the yield point vary with load angle. Notice how the position switches from just below the engine frame for a purely vertical load to just above the fork for moderate angles and then to the thinnest point of the leg (roughly above the axle) for higher angles. I believe you can see these distinct types of failure in the photographs of failed legs.
graph3.jpg

graph4.jpg


Q2: I think you have assumed that the aircraft is rigid as the nose wheel passes over a bump. Even a step bump will deflect the tire and the nose gear leg at their spring rate which is no longer a step input. Then as the vertical component of the bump load increases, the nose gear leg deflects, the load is transferred back into the engine mount and this causes the nose to rise.

A2: Again, absolutely true and taken into account in the detailed analysis. In practice, though, the aircraft has considerable inertia, so the gear is subjected to virtually all of the compression before the fuselage has time to lift up unless the bump rises very slowly or the aircraft speed is very low. Running some typical examples over a smooth ramp shaped bump suggests the speed (mph) divided by the ramp length (ft) needs to be below about 5 before there is any significant reduction in gear load and below about 0.5 before the leg compression is negligible. In other words, if the aircraft is doing 20 mph and the ramp is 4 feet long, the gear sees virtually all of the compression. If the speed were 2 mph and the ramp 4 ft long the aircraft would ride up the ramp without significant compression of the gear. I will include an example of this in another post, as this one is already getting very long.

Q3: The nose gear leg failures that I am aware of were caused by the nose gear swivel digging into the ground bending the leg backwards and usually flipping the aircraft inverted. Are there nose gear leg failures caused by the leg breaking at the motor mount socket and allowing the propeller to hit the ground?

A3: The 2nd graph (above) indicates that the leg fails upwards if the load is purely vertical but, as you pointed out, this is probably not a common situation, as there will be a rearwards component of load from the bump. Once there is a rearwards component of load, the leg folds under and the model predicts that this happens before the castor nut touches the ground in all but the most aggressive of bumps (very high and or steep bumps). My view is therefore that the leg yields first and that the nut touches because the leg has yielded. Again, I?ll post an example of this later.

Raiz
 
have you done any tests to confirm this?

Danny7, the calculations on the leg are a straight forward application of engineer?s bending theory, which is widely accepted as being pretty accurate. However, in terms of a practical test, nobody has volunteered to taxi their RV-7A over some bumps until the leg fails. What I have done is an independent calculation as a cross check.

Raiz
 
Danny7, the calculations on the leg are a straight forward application of engineer?s bending theory, which is widely accepted as being pretty accurate. However, in terms of a practical test, nobody has volunteered to taxi their RV-7A over some bumps until the leg fails. What I have done is an independent calculation as a cross check.

Raiz

So if this is the case, how would you solve this problem and build a better nose gear leg?

This is a very common design on many aircraft....... Skycatcher etc.
 
Good but not satisfying

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

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

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

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

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

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

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

This is a very common design on many aircraft....... Skycatcher etc.

Probably already been said before, but make a smaller version of the RV-10 nose gear assembly for the RV-a's?
 
Well i believe I have seen my first Oleo Strut on a 6A. Nose gear - YES Several weeks ago. I think you'll hear more about it this year. Has anybody else seen it?
 
Test for the 6A

As far as testing, to where a break would occur........

About 10 to 12 years ago, Van's set up a large diameter high speed cam wheel, that litterly beat the c-r-a-p out of nose gear assembly's. This is how they tested a new & improved gear leg for existing RV6s. The originals failed rather quickly, while the new model stood up to the test.

L.Adamson --- RV6A
 
the leg fails before the nut touches !?!?!

Once there is a rearwards component of load, the leg folds under and the model predicts that this happens before the castor nut touches the ground in all but the most aggressive of bumps (very high and or steep bumps). My view is therefore that the leg yields first and that the nut touches because the leg has yielded.
If your model (and your conclusion) are accurate, this represents a pretty signficant shift in thinking about these nosegear incidents. I think most people (and the NTSB in their structures report) have been thinking that the nut touches before the failure, not after a failure. Of course, you may be correct here - in no way am I trying to tell you that you are wrong.

I believe that the correct answer to this question will be critical to the next nose gear leg design improvement. It's always good to solve the correct problem!

Whether the model and it's conclusions are accurate or not: Raiz - as an engineer I commend you. Keep the good work coming!
 
This is an interesting thread...

Raiz, thanks for putting the time in on this.

But...

If the gear is failing before the nut is hitting the ground, why won't we see the gear bent up and not under? The only force on the gear at point of failure would be an upward force. Maybe some backward force, but with a 43 degree slope to the gear it sounds improbable that the gear would fail with a tuck under.

note: I am not a mechanical engineer, just thinking here (which can be dangerous).

Kent
 
So if this is the case, how would you solve this problem and build a better nose gear leg?

This is a very common design on many aircraft....... Skycatcher etc.

That's a difficult one. The RV-7A has many great qualities - performance, handling, simplicity, low cost and, with good technique and a good paved strip, an adequate nose gear. What I'm trying to say is that building a more robust gear is not necessarily what everyone wants, because it will be heavier and more complex. Having said that, sure, it can be done.

I'm not familiar with the Skycatcher 's gear but the DA40, SR20/22 and AA-5 have all met FAR 23 requirements with a similar gear except they all have a spring element that increases the suspension travel. If there is room inside the cowling, it might be possible to do this with a 7A but I expect it would involve a new engine mounting. One other difference (apart from the SRs) is that the leg is closer to horizontal, which makes the gear less likely to dig in.

Raiz
 
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Raiz, thanks for putting the time in on this.

But...

If the gear is failing before the nut is hitting the ground, why won't we see the gear bent up and not under? The only force on the gear at point of failure would be an upward force. Maybe some backward force, but with a 43 degree slope to the gear it sounds improbable that the gear would fail with a tuck under.

note: I am not a mechanical engineer, just thinking here (which can be dangerous).

Kent

Kent, you leave me with nowhere to go when you talk about thinking being dangerous ! The answer to your question is terrye's point (post #11, I think) but I'll try and explain it in the most "engineering-free" way I can.

First of all, the leg does bend upwards due to an upwards (or upwards with a little rearwards) load. However, that is not to say that all parts of the leg are being bent upwards. In fact, the part of the leg behind the axle is being bet up but the part in front is being bent in the opposite (rotational) sense and it is here that the failures occur. I know that is probably conceptually difficult if you are not an ME but try it like this: Imagine you were holding the leg at the engine mount end and looking from the starboard side of the plane when somebody pushed the wheel up. You've effectively already said that you'd expect the leg to bend in an anticlockwise sense (or upwards). Compare that to the situation where, instead of the leg) you are holding the fork at the castor bearing end, when I think you will see that the fork would rotate clockwise i.e. in the opposite direction.

I hope that helps.

Raiz
 
Raiz,
How did you come up with the 168ksi figure? I do not think that is a valid figure to assume.

There is a spreadsheet available on the EAA website that is available to the general public as a gear leg design tool/aid. It starts the process at 200ksi. In practice the legs and designs have all been different, but they pretty much start at 200ksi vary a little from that point.

Since this nose gear design is not mine, (It is Van's of course) it is not my place to be publishing their details, but I can tell you I have never made a gear leg with a tensile strength lower than 190ksi at a minimum Most are in the 200-205 range and a few even higher.
 
Nice explanation

Raiz, very nice explanation to Kent's question. I tried to explain this several months ago in a thread on the effect of making the fork longer. I don't think I explained it well because no one believed me.

I believe it is exactly the reversed-moment portion ahead of the axle that yields, allowing the castor nut to touch, and flips the plane. stiffening/strengthening the forward part of the axle where the moment reverses might be enough to minimize the failures even without adding more suspension travel, but surely this, combined with adding some suspension travel at the motor mount would solve the problem.
 
Raiz,
How did you come up with the 168ksi figure? I do not think that is a valid figure to assume.

There is a spreadsheet available on the EAA website that is available to the general public as a gear leg design tool/aid. It starts the process at 200ksi. In practice the legs and designs have all been different, but they pretty much start at 200ksi vary a little from that point.

Since this nose gear design is not mine, (It is Van's of course) it is not my place to be publishing their details, but I can tell you I have never made a gear leg with a tensile strength lower than 190ksi at a minimum Most are in the 200-205 range and a few even higher.

Richard, that's really helpful information, thank you. The 168 ksi is for 6150 steel and came from matweb http://www.matweb.com/search/datasheet.aspx?MatGUID=6fbcc44fd0124994b46587a1aa905ef0
but, the difference may be that you are talking about tensile strength and I am talking about yield strenth.

Raiz
 
Richard, that's really helpful information, thank you. The 168 ksi is for 6150 steel and came from matweb http://www.matweb.com/search/datasheet.aspx?MatGUID=6fbcc44fd0124994b46587a1aa905ef0
but, the difference may be that you are talking about tensile strength and I am talking about yield strenth.

Raiz

The specifications on that page are a little lower than what gear are usually made too. The number I was referring to is the yield tensile.

The Ultimate Tensile strength is much higher. Closer to 280ksi if my memory is correct. (Upper 50's on the Rockwell 'C' chart)

This might be why your analysis shows the gear leg to fail in an upward direction before the nut touches, and also why there is such a small amount of travel provided by the gear leg before it is compromised.

From what I have seen working with this material it is pretty surprising how much deflection a gear leg can withstand without being deformed.

Not trying to start a flame war here, just trying to make sure the data you are basing this analysis on is as accurate as possible.

Richard
 
I see - said the blind man....

Kent, you leave me with nowhere to go when you talk about thinking being dangerous ! The answer to your question is terrye's point (post #11, I think) but I'll try and explain it in the most "engineering-free" way I can.

First of all, the leg does bend upwards due to an upwards (or upwards with a little rearwards) load. However, that is not to say that all parts of the leg are being bent upwards. In fact, the part of the leg behind the axle is being bet up but the part in front is being bent in the opposite (rotational) sense and it is here that the failures occur. I know that is probably conceptually difficult if you are not an ME but try it like this: Imagine you were holding the leg at the engine mount end and looking from the starboard side of the plane when somebody pushed the wheel up. You've effectively already said that you'd expect the leg to bend in an anticlockwise sense (or upwards). Compare that to the situation where, instead of the leg) you are holding the fork at the castor bearing end, when I think you will see that the fork would rotate clockwise i.e. in the opposite direction.

I hope that helps.

Raiz

I needed to stare at a picture of the gear before it was clear.

So the weakest part (next to the fork) will receive the largest down force when the wheel hits a bump. Another factor that would play into the amount of forced received would be the friction preload of the axle. This would contribute to more reward force.

Kent
 
Vans did publish...

....
Since this nose gear design is not mine, (It is Van's of course) it is not my place to be publishing their details, but I can tell you I have never made a gear leg with a tensile strength lower than 190ksi at a minimum Most are in the 200-205 range and a few even higher.

...the nose gear (same as the main gear) leg specifications for the RV-6x plans on sheet 45 -

"6150 steel, bend and then heat treat and temper to 42-44 Rockwell "C" hardness"
 
Richard,

Could you make titanium legs for the 7? Something like the rocket gear...round tube, with a machined flat. Saving weight?

Also, a titanium rear stinger for the tail wheel models, if it could save a pound or two, would help me out immensely.

Any interest?
 
I have considered building an RV-7A and my home base is grass. Should I rethink this at all or just use extra caution. I have also considered the RV-10 and still contemplating the need for the extra seats.
 
...the nose gear (same as the main gear) leg specifications for the RV-6x plans on sheet 45 -

"6150 steel, bend and then heat treat and temper to 42-44 Rockwell "C" hardness"

az_gila, thank you for this. I cross checked in "Tool materials" by Joseph R. Davis, ASM International. Table 39 (6150 steel) gives a yield strength of 175 ksi (197 ksi ultimate). Here is the table:

Table1.gif

401 Brinell is equivalent to 43 Rockwell C - see http://en.wikipedia.org/wiki/Hardness_comparison

So, it looks as though my 168 ksi may have been a bit pessimistic compared to the 175 ksi but, even if I take Richard's 200 ksi, the gear deflection at yield only increases from 4" to 4 1/4". That's about half what would be needed to meet the FAR 23 requirements.

Raiz
 
The specifications on that page are a little lower than what gear are usually made too. The number I was referring to is the yield tensile.

The Ultimate Tensile strength is much higher. Closer to 280ksi if my memory is correct. (Upper 50's on the Rockwell 'C' chart)

This might be why your analysis shows the gear leg to fail in an upward direction before the nut touches, and also why there is such a small amount of travel provided by the gear leg before it is compromised.

From what I have seen working with this material it is pretty surprising how much deflection a gear leg can withstand without being deformed.

Not trying to start a flame war here, just trying to make sure the data you are basing this analysis on is as accurate as possible.

Richard

Richard, thanks for avoiding the flames! I re-ran the model with 200 ksi and it only increased the gear deflection to 4 1/4 ", so not enough to change my view about the limited travel. However, the best information would be data from a real test. Would you have anything that you could share? Any leg would do, as I could analyse that for comparison purposes, as long as I know its geometry and material properties.

The predicted failure in an upwards direction with a purely vertical load remains unchanged with the higher yield strength and I think this is a valid aspect of the model. The LAA (UK EAA) have told me that they have seen one such falure on an RV. Nevertheless, I expect it to be very rare in practice, because there will generally be some rearwards force from the rolling resistance. Even a very small rearwards force is enough to move the predicted failure point to the front of the leg.

Raiz
 
Raiz, please accept my compliments.

Neal Willford's landing gear design spreadsheet (a terrific tool) assumes 220,000 psi ultimate for steels.

The spreadsheet will predict higher capacity before failure than you'll predict if using a strict "outermost fiber" approach. Neal's approach is valid.
 
Just checking in, and lured again by this topic

Hi all, I was just checking in to see what's up in the RV arena and saw this thread and just like old times the hook was set in my lip. I've managed to check in and read once in a while, but it's been almost a year since I've posted. First let me say its fun to catch up and see so many new people that are now active and fully into this hobby.

This thread has been intersesting and certainly the theory behind the post might be a factor in the nose gear issues. The main issue remains the same however and it still seems to be widely misunderstood. For those unfamiliar, my old airplane was used in the NTSB study on this, or better stated my incident involving running over a tie down ring while parking was. Because of this I spent hours several times on the phone with an NTSB member more than once regarding the nose gear leg issue.

Simply put so that I could understand the issue without a math degree it was explained to me that the steel used in the nose gear leg when put into a press and having rearward force applied showed an interesting characteristic. At around 1,500-1,700lbs of force the integrity of the metal changed and the gear leg effectively became a noodle. With this force applied they could move it around at will as if it had no resistance to offer. As the force was reduced the strength returned and the gear leg acted as intended. This translates in the real world a 1G event for an RV that is moving on the ground being capable of removing all the designed strength from the gear leg. So at whatever speed you are moving if anything causes the nosewheel to want to stop turning or if the gear leg strikes the ground and digs in and 1 G's worth of force is met or exceeded the gear is going to flex. This was alluded to in the study, but with a more complicated way of stating it. So say that you are rolling along and have a 1 G interuption and the gear flexes back on you. At some point the forward force from inertia becomes less than one G and the gear leg is once again able to overcome the force being applied. It now has that 1G of energy stored in the bent leg that unloads and hence, the flip over. It's really this simple. What is hard is getting people to believe it. The NTSB went all in during this study.
 
SPRING GEAR

The late Steve Wittman designed and patented the flat spring gear in the early thirties and round gear in the late fourties/early fifties. He had an agreement with Cessna for their use of the gear. The patents have long since expired. It is not Vans gear, it is not a Cessna gear, it is a Wittman gear.
The statement---"is not mine(its Vans of course)" is absolutely wrong. Wittman put a spring nose gear on his third Tailwind in 1958. He foresaw the issues with the gear digging in and put a small skid in front of the nosewheel. He flew the airplane in this configuration for about five years, and then converted it to a tailwheel. An article on the tri gear Tailwind can be found in Sport Aviation from 58/59. There have been a number of Tailwinds, as well as other homebuilts that used a spring nose gear many years prior to the first Vans A model.
Just wanted to set the record straight. The spring gears are public information and have been for many years.
 
dynamics of the flip

ok, lots of great minds at work here, and fascinating reading. My engineering is from many years ago, but enough remains to make me question all the load simulations.
We have some flips on film, right? What I'd like to know, is if the roller-coaster nature of many surfaces is an issue. The aircraft, rollling along at 1g, experiences a rise & fall and rise in the surface, changing the load in significant ways. Driving your car thru a little dip in the road can go unnoticed, or bottom out the suspension, or leave metal or blown tires in it's wake!
I think good pilots, using good technique, are all subject to the loads imparted by these varying surfaces, and the attendant loading; up down rearward etc.
is triggering the pogo calamity.
I appreciate the great analysis, but it still causes major sphincter tension when landing on a new runway, or crossing an unknown taxiway.
Hoping some kind of 'fix' comes of all this.
 
RV-7A This...

thread is the too funny... silly... crazy etc... Fly the deadgum plane like it was designed and get over it. THE NOSE WHEEL IS NOT A LANDING GEAR. Wow... that's to simple I guess. :) Look at the numbers, just a guess 90% of pilots LAND these planes to hot... coming across the #'s at 85 MPH, heck they stall at 52. JMHO... learn to fly the plane as designed. :) I see this with RV's everyday.
 
thread is the too funny... silly... crazy etc... Fly the deadgum plane like it was designed and get over it. THE NOSE WHEEL IS NOT A LANDING GEAR. Wow... that's to simple I guess. :) Look at the numbers, just a guess 90% of pilots LAND these planes to hot... coming across the #'s at 85 MPH, heck they stall at 52. JMHO... learn to fly the plane as designed. :) I see this with RV's everyday.

Just to lighten it up a tad. You mean the one with a "C" in the N number at T74 practicing intentional no flaps, max gross, aft CG, in 100 deg heat, no power, spot landing? Yep, she was hot the first few times, but I assure you she wasn't measuring it in MPH :). We did 13 laps in a row that afternoon. (just poking fun. That's how we spend our entertainment avgas)

This is how you get over the nose gear, by practicing in every possible loading condition that you can think of:
2njj2nt.jpg
 
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Just to lighten it up a tad. You mean the one with a "C" in the N number at T74 practicing intentional no flaps, max gross, aft CG, in 100 deg heat? Yep, she was hot the first few times, but I assure she wasn't measuring it in MPH :). We did 13 laps in a row that afternoon.

No... I will never judge your landings... :) nor your wife's landing. Both are outstanding PILOTS!
 
That I guess is when you never even said HELLLO over the radio...? Maybe? You hang in your CLICK, and we do our own thing with the rest of the group.
 
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That I guess is when you never even said HELLLO over the radio...? Maybe?

Yeah, we tend to get busy in the cockpit on the first part of downwind resolving the last approach and hatching an improvement for the one coming up when in training mode (most weekends). This is a nose gear thread, so, practice makes perfect to fly it as designed.
 
Speed the issue?

Some RV's may land too fast but I don't think this is the issue with the flipping. If I recall, they seem to be flipping at faster taxi or slowed rollout speeds. I stand to be corrected by those who have been directly involved. I'm sure it's all in the archives.

Bevan
 
375 Lbs. static nose gear?, why not just move the main gear forward to decrease the nose weight. I think 200 Lbs would be enough for dynamic stability.
 
375 Lbs. static nose gear?, why not just move the main gear forward to decrease the nose weight. I think 200 Lbs would be enough for dynamic stability.

Would the plane be difficult to taxi with the tail dragging the runway because two 230lb folks are in the cabin (and sitting behind the main gear axis)?
 
Simulation of impact with a bump

On a couple of the previous posts, I promised a more detailed model of the nose gear. The graph below is an example of the output from that model, which attempts to predict the leg, tire and fuselage motion on impact with a bump. It is based on the elements described earlier in this post but it should be treated as unvalidated. Part of the reason for posting it is to ask for details of nose gear collapse incidents that could be used for validation.
graph5.jpg

This example is for an RV-7A, 1800 lb AUW, on the runout at 10 knots with no braking. It assumes the rear wheels are in a flat surface but the nose wheel hits a rounded 2" high bump.

The blue line represents the ground - in this case a mound 50.8mm (2") high and 3 m across. It looks much steeper than it actually is, because the vertical axis is magnified. The steepness of the ground reaches a maximum of just 3 degrees, so it is more like a little launch ramp than a bump.

The green line shows how much compression (in mm) is left in the tire before it bottoms. The purple line shows how much of the leg strength remains in % and the red line is the height of the castor nut (in mm). The orange line represents the fuselage. It starts at -14mm, which is the compression in the gear and tire due to the static nose weight (375 lb).

When the aircraft hits the bump, the tire and leg compress (shown by dips in the green and purple curves). After a short delay, this compression forces the fuselage upwards and it bounces clear of the ground where the orange line crosses the blue line at about 1.8m. After reaching a height of about 70mm, it falls back to impact the ground again at about 5m from the start of the bump. The tire bottoms out on this second impact and the remaining leg strength drops to about 40%. The castor nut stays well clear of the ground throughout.

It is possible to explore the bounds of survivability varying the nose load, all up weight, braking, speed, tire pressure and bump geometry. As I reported before, nose load and braking are key parameters but I would like to check the validity of the model by running real accident cases through it. If there is a good correlation, it should be possible to generate guidance on the situations to avoid. If anyone can share this information (nose load, all up weight, braking, speed, tire pressure, bump geometry) and outcome I'll run it through the model and publish the results.

Raiz
 
i'm not an engineer, so you can explain this to me if its a bit off base, but isn't this just a bunch of theory? what if there is a variable that you do not know about and don't have plugged into your model? You will never know without running actual ground tests with recording equipment, right?
 
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