What's new
Van's Air Force

Don't miss anything! Register now for full access to the definitive RV support community.

Breaking News: tests on landing gear nuts and bolts

scsmith

Well Known Member
I have been a little skeptical about the strength of the NAS679A6 nuts that Van's supplies for the main landing gear bolts on the RV-8. So, I decided to do some tests of various different nuts to see what the different strengths are.

We wanted to simulate the tension load that gets put on the bolt and nut when the landing gear gets a fore-and-aft load, which puts a twisting moment on the gear which tries to pry apart the mounting saddle ( U803) and puts the bolt and nut in tension.

I will describe the results here, then describe the test procedure in a second post.

So, we tested four different nut types, as follows:

MS21042-6 low profile nuts with 7/16 wrench flats
NAS679A6 low profile nuts ( stock style) with 9/16 wrench flats
MS21045-6 full-height metal lock nuts with 9/16 wrench flats
NAS1804-6 mid-height nuts with 12-point 7/16 wrench flats

Below is a picture that shows each of the four nuts, and the cut-open failed nuts. The legend shows the breaking strength that we found. All the nuts failed by some or all of the threads stripping. The NAS679 tended to have a couple of threads un-stripped, as if they had radially stretched enough to leap-frog over the mating thread on the bolt. The MS21045 showed significant swelling of the base of the hex, so that a wrench would no longer fit on the flats. This was also true to a lesser extent with the MS21042.

In summary, the strengths were as follows:
MS21042-6 11,600 lb
NAS679A6 12,500 lb
MS21045-6 15,800 lb
NAS1804-6 16,200 lb

The expected strength of the 3/8-24 NAS close-tol bolts is about 14,000 lb. so only the last two types of nuts are capable of developing the full tensile strength of the bolts. We used Unbrako socket-head bolts (better than grade 8) to be sure that the nuts broke and not the bolts.
nut_test2b.jpg

here's the link too: http://www.hpaircraft.net/rv8/nut_test2b.jpg

A couple of other builders in other threads have mentioned using the MS21042 nuts in place of the ones Van's supplies. An advantage is that they take a smaller size socket which helps get a socket on them in the gear tower. But as you can see, they are not as strong. The same benefit of smaller socket can be gained using the high strength NAS1804 nuts. You can order them from GAHco, they are pretty pricey but worth it for the peace of mind in my view.
 
Last edited:
More BREAKING NEWS on landing gear nuts and bolts

OK, I promised a description of how we did the tests.
Above I mentioned that I wanted to simulate the prying action on the bolts when the gear arm is pulled fore and aft.
A second issue is that our home-made tensile testing machine is only capable of about 6000 lbs of tension, so we needed a way to amplify the load with some leverage. Here's how we did it:

nut_tester1.gif


You can see that this arrangement gains a 4:1 leverage on the bolts. The bars are 3/4 x 3" hot-rolled steel bar stock. The bolts and nuts were assembled with a washer under the nut, and torqued to 16 ft-lb. (correct for AN bolts, a bit low perhaps for the NAS close tol bolts, but the preload will not affect the single-load tensile strength of a test like this significantly).

As I mentioned above, we used Unbrako socket-head bolts. Each nut style was tested twice, new bolts and washers (and obviously nuts too) were used for each test.

The home-made tensile tester is shown here,with the bolted assembly at the bottom. The large "S" shaped device hanging between large eye bolts is the electronic load cell.

nut_test1a.jpg


If you google "break-o-tron" I think you will find some U-tube videos of Bob Kuykendall ( hpaircraft.com) doing some tests on climbing gear and such.
 
Last edited:
Both!

Of course the bolts also see very large shear forces. And for that, the nuts have no effect on the overall strength. But as I described, the tension in the bolts from the twisting of the gear leg is also very significant and in that case, its all about the nuts.
 
Actually rather minimal bending

The bars we used were pretty stiff. The tension is obviously reacted by the contact pressure right at the edge of the bars, but they did not bend much, nor gap very much. The gap at the far edge of the bars was less than 1/4 inch. None of the bolts were bent so much after the test that they could not be pulled through the holes in the bars.

Bolt bending is actually another serious question about the RV-8 gear attachment. The U803 saddles are pretty thin, about 0.25" thick, where they transition into the end blocks. And remember, the end blocks are supposed to have a 0.030" gap to the wear plate when the bolts are torqued. So already the saddle is bent slightly. Under the prying load I am concerned about, that saddle will try to bend even more, causing a combination of double-shear and bending in the bolt shank.

I believe my load fixture put substantially less bending into the bolt than the actual RV-8 assembly. But none of that is relevant for the question of the nut strength.
 
Steve:

So what conclusion am I to draw from this? Are you saying that the nuts Van's supplies are not adequate or does the test show they will do just fine? What are the loads on the nuts in real-life flying? I need a reference point to draw a conclusion.

Chris
 
Conclusions? well that depends....

Well, I could analyze a typical, although hypothetical load case on the gear assembly to show what the loads are that the attachment hardware is expected to carry. I have not done that at this point.

I can not say that the current assembly is inadequate - from the ground-loop damage I have seen, the gear tower seems to tear out of the fuselage with the gear leg still bolted on. I have not examined any of those to see if there have been any failures of the nuts.

I can only say that from my engineering experience I was rather skeptical about the supplied nuts, which is why I did the tests. I can say that it is typical engineering practice to use hardware that allows you to develop the full tensile strength of the bolts, when possible. ( of course you would not bother to do that in a pure shear loaded situation). But in a situation where gear loads can go from normal levels to catastrophic levels easily in a ground loop or wheel falling in a squirrel hole on landing, I think I want all the strength I can get.

My OPINION is that I think it is better to use a higher strength nut, and I will. It may be that the existing nuts are fully adequate, I don't have any evidence that says otherwise. It is likely that there are other weak links in the assembly that would usually fail first. It is also possible that a particular scenario would lead to failure of the nut before any other part failed. I might as well go for all the strength I can just in case....
 
The design strength...

......
My OPINION is that I think it is better to use a higher strength nut, and I will. It may be that the existing nuts are fully adequate, I don't have any evidence that says otherwise. It is likely that there are other weak links in the assembly that would usually fail first. It is also possible that a particular scenario would lead to failure of the nut before any other part failed. I might as well go for all the strength I can just in case....

...used by the designer should be from MIL-Handbook-5.

The applicable page is here, but I don't know which bolts are specified, so I can't give the design ultimate tensile strength.

How do these numbers fit in with your tests?

Bolt-strength.jpg


Note "d" certainly fits in with your conclusions.
 
Very interesting!

Very interesting indeed.

Seems you have come up with a solution that has evaded a problem!

Unless I've missed something, I haven't heard of one incident where these nuts have failed. Is this something no one is speaking of?
 
bolts exceed mil-specs by quite a bit

Thanks Gil for printing the mil-spec chart for the bolts.

I'm fairly sure that the NAS bolts are 125 ksi, so that would say that they are required to achieve 10,300 lbs. from the chart.

Because of good bolt design, including rolled threads, 2-3 thread run-out at root, and radius under heads, most bolts FAR exceed the specs. So, although the rated strength for design purposes is 10,300, the actual strength is probably much higher. My GUESS is about 14,000 lb. When we get a chance, we'll break one and let you know.
 
Steve:

So what conclusion am I to draw from this? Are you saying that the nuts Van's supplies are not adequate or does the test show they will do just fine?

Chris
IMHO as long as the nut's strenght is withing design limit I'd rather have weaker nut than bolt. In case it breaks it's much easier to just put new nut on and go home than get stuck with broken bolt. In many cases elements are chosen to act as fuses.
 
Not sure what you're really trying to test here, but what you've really got is an apples and oranges situation. First, an ideal fastened joint is really one where you put the bolt or male fastener into a state of maximum elastic deformation. That is, by stretching the bolt to a point just before the plastic deformation point, you are creating max clamp force. Think of a bolt as a rubber band - no tension or stretch, no clamping force. That is why torque is used to "set" a joint. Even better, is to use torque angle, which really measures linear displacement of the nut as it moves around the bolt's helix. Sooo, if a joint does not require clamping force and the bolt merely serves as a clevice, than the only important number is bold shear strength. In a clamping joint, the important numbers are proof strength and tensile strength. These numbers are derived from cross sectional area and strength of material calculations and are properly tested by SAE and ASTM standards. Home-made tests don't mean much unless they simulate the actual application, and then only when the joint design and actual loading are properly understood. The only important number for the nut is it's proof strength, which is really a measure of the strength of the overall thread area. Finally, for those that see test numbers for locking or prevailing torque fasteners, the test numbers are typically:first on torque, first off torque, and fifth off torque. These are again done using standards from SAE or ASTM and are just to measure the performance of the lock-nothing more.
Terry, CFI
RV9A N323TP
 
Great discussion. Another great question is do I want the bolt to stay in one piece while the tower rips out of the fuse, or would it be preferable to have the bolt fail and save the tower? I'm not sure anyone could design it that closely though because of variability of materials, construction, and loads.

As a 26 year engineer I learned long ago to define design intent before defining the design. I'm not sure what I would choose for design intent for this one.
 
Perhaps you should...

...re-run the tests, but using the same type of bolt as called out in the plane?

The MIL-Handbook does say that bolt and nut fit specifications both need to be considered.

Thanks Gil for printing the mil-spec chart for the bolts.

I'm fairly sure that the NAS bolts are 125 ksi, so that would say that they are required to achieve 10,300 lbs. from the chart.

Because of good bolt design, including rolled threads, 2-3 thread run-out at root, and radius under heads, most bolts FAR exceed the specs. So, although the rated strength for design purposes is 10,300, the actual strength is probably much higher. My GUESS is about 14,000 lb. When we get a chance, we'll break one and let you know.
 
Interesting results and full marks for doing it. I have not seen the landing gear bolt setup on the RV 8, but I am not sure how relevant this test is. In this test, given the large forces involved, there will be some stretching of the bolt and some bending of the bars. Both these actions will conspire to put most of the forces on the one side of the nut nearest where the force is being applied. In effect you now have a force trying to rip the nut apart in addition to trying to pull the bolt through the nut. In this situation, as you would expect, the nut with the smallest cross sectional area (the MS21042) would likely fail first. I have built two different aircraft and I can't think of any fixture that would put so much of the forces involved on the one side of the nut. The closest I can think of is where a stub axle is bolted to a leaf spring. In an extreme heavy landing there may be a tendency to load the lower bolt/nut in a similar manner, however this is really only likely if all the nuts on the stub axle were loose. As far as I am aware, the MS21042 develops the same tensile strength as the MS21045. It would be interesting to see a test where the nut is under tension without a significant force being applied to one side of the nut. I suspect the MS21042 would be up there with the MS21045.

Fin
9A
 
Last edited:
If the nut is torqued onto the bolt, the bolt is already in tension and the tensile load should be close to the proof strength. Unless the impact of landing exceeds this number, it doesn't matter - the forces are NOT additive. For example, if the strength of the bolt is 10,000 pounds and there is only one bolt per gear leg and the plane is landed on one gear, the impact has to be at least 6G's to overload the bolt. I suspect the gear leg will collapse before joint failure. In a properly designed joint, the nut is ALWAYS stronger than the bolt. That's how you generate elasticity. As a side note, the threads on the nuts are cut rather than rolled, yet they still exceed bolt strength. Many of these observations and tests previously cited are not relevant to the actual joint.
Terry
 
Fin: I assure you, the asymmetric loading onto the nut is MUCH MUCH smaller than you imagine. Three reasons.

First, the total elastic elongation of the bolt at the nut-failure load is .005 inches. About 1/3 of that was achieved through pre-load torque, and so only about .0035" of elongation is caused by the parts gaping.
Because of local yielding in the threads as the nut seats, and nut-embedding into the washer, the actual parts gap was probably closer to 0.007 - 0.010". So that is about 1/2 degree of rotation of the bar at the bolt.

Second, each half of the bolt is constrained in the close clearance hole in the bar, and can not rotate with respect to the bar - the 1/2 degree rotation produces bolt bending in the zone close to the slightly gapped joint, and where the bolt exits the bar to engage the nut, it exits very nearly perpendicular to the bar.

Third, local crushing of the washer under the nut will easily adjust and redistribute the load fairly well onto the nut.

For those familiar with the RV-8 attachment hardware, I guarantee you that the asymmetric loading of the nut in the actual assembly is MUCH MUCH BIGGER than the asymmetric loading in my test. This is because the saddle is rather thin ( average 1/4 inch) and there is an intended gap between the end blocks of the saddle and the wear plate. The springyness of the saddle makes it difficult to get a good preload in the bolt, and when the end block flexes, the bolt and nut see much more asymmetric contact than I have simulated. But in part, it was this asymmetric prying load that I wanted to approximate.

---------

I think many of you are expecting much more out of this test than was intended. I'm not trying to replicate all the loads that a landing gear assembly can be subjected to - in fact, that is impossible, and the design criteria used to establish design loads do not anticipate large unexpected loads during mishaps.

I AM trying to replicate one particular loading condition for which my engineering instinct suggests the hardware is a little bit under-designed. I have found that strength values for nuts is very hard to find. Generally the tacit assumption is that the nut is capable of developing the full strength of the bolt. Not the rated strength, or the proof strength, but the actual full bolt strength. I have found this assumption is sometimes wrong.

But big picture, if we are not hearing about nut failures, they must be ok. I would NOT recommend using a 21042 nut in this application. I will be operating from austere fields a lot, and I want more margin. I will use the NAS1804 nuts.

I'm trying hard not to be defensive here - with 29+yrs experience as an aerospace research engineer, I certainly DO NOT need a lecture on bolted joint design, behavior, and analysis. I'm resisting the temptation to give one!
Much of what has been stated here by others, especially by Terry Kohler, is either incorrect or only partially correct. Industry standards for bolt pre-load vary depending on application, but for general-purpose joints, the preload is typically selected to achieve 60-80% of bolt yield stress. The AN bolt torque charts are much less - closer to 35% of yield stress, and unless the added friction torque is accounted for when torquing lock nuts, do not even achieve that level of preload.
If any of you would like to discuss generally the topic of bolt torque and preloads, that is a separate topic that we can take off line.
 
1200 lb drag load produces 10,000 lb at bolt!

For real: if you apply a 1200 lb load directly aft, at the axle, you will produce 10,000 lb tension in the front outboard gear attach bolt. That is over and above the 5200 lb of shear force that is shared between the two outboard attach bolts.

The distance from the rear edge contact point of the gear strut to the center of the forward attach bolt is 3.57". The vertical height from the axle to the gear attach is 30". ( assumed - I have not measured)
Moment equilibrium about the rear edge of the gear strut contact point gives:
1190 lb x 30" = 10,000 lb x 3.57"

I don't think it would take much of a pothole, or a big rock, on landing, to exceed that.
 
Possible mod to U-803 attach bracket

Steve, great effort and great writeup!

The thing I've looked a bit sideways at is the saddle (U-803 attach bracket) and how it constrains the gear leg from twisting due to drag loads applied at the wheel. That saddle thins down to around 1/8" or so where it spans the width of the gear leg... eek! So I'm a bit surprised to hear the 8's have had gear failures where the gear box was ripped loose. Maybe the gear box fails in torsion before the saddle bolts tear loose in tension.

Since the U-803 bracket is 4130 cold rolled, its soft and has fairly low tensile strength (90-95 ksi?), would it make sense to take those off and have them hardened up to around RC 30? That would increase the tensile strength of U-803 to around 140 ksi, or a 40% increase and it still wouldn't be too brittle to worry about fracturing.

edit: The distance on my 8 from the pavement to the center of the leg at the attach point is 32". I figured real quick that the bolts would reach 14000# with applied load of 1800# at the pavement. This looks to be in your ballpark.
 
Last edited:
NAS Realities.

Thanks Gil for printing the mil-spec chart for the bolts.

I'm fairly sure that the NAS bolts are 125 ksi, so that would say that they are required to achieve 10,300 lbs. from the chart.

Because of good bolt design, including rolled threads, 2-3 thread run-out at root, and radius under heads, most bolts FAR exceed the specs. So, although the rated strength for design purposes is 10,300, the actual strength is probably much higher. My GUESS is about 14,000 lb. When we get a chance, we'll break one and let you know.

Your basic NAS Bolt is 160 to 180 Ksi Tensile, and a min of 95Ksi Shear.

Reference Book Page,

http://www.gen-aircraft-hardware.com/images/pdf/nashhb.pdf

:)If I saw more action with the 12 point nuts I could get the prices down.:rolleyes:
They are primarily used on larger aircraft wheels and critical mounts under tension. Most things on RV see little tension, except for the engine mount and propeller attachment.
 
Last edited:
Most Important

What type of primer was used to prevent bolt corrosion and could these bolts not be replaced with Sikaflex?
 
about those U-803's...

Bill Wightman's observations about the saddles are right on. I have actually started machining a re-designed saddle that maintains more than 7/16" thickness across the gear leg, and then has counterbores for the bolt heads so the total height is the same as original. ( I wasn't going to share about those, but it is uncanny how often Bill and I think alike;))

Bill's suggestion to heat treat them would increase the strength of the saddle, which would be nice, but not the stiffness. Too bad we can't change the modulus with heat treatment, wouldn't that be cool!:D The stiffness is relevant because it influences how much flexibility there is in the assembly, and how much preload torque we could put on the bolts before the saddles bend.

Thanks GAHco for setting me straight on the NAS bolt strength. The fact that they are 160ksi+ means they deserve even more preload. Oh, if only Terry were right and we could get a preload to a large fraction of yield, rather than the dismal preload we get now, but the saddles just bend and then start bending bolts, causing non-uniform contact at the nuts, etc.
Thus my (until now secret) beefed up saddles.

The bolt strength shown in GAHco's document further supports that the NAS679 nuts are inadequate based on Mil-5 spec's, since they can't match the 13,150 lb required.(at least under my admittedly less than ideal tensile test conditions)
 
Last edited:
This is experimental aviation after all...but since there has been no demonstrated history of the gear leg attachment on an RV-8 to be inadequate, I feel quite strongly that all the increasing and strengthening you are doing will have only one result...

Even worse damage to the fuselage structure should your airplane ever be involved in an incident or accident.

Just one opinion...
 
Last edited:
This was very interressting, but I am not sure I understand the premises for the discussion (I don't have any drawings). If the bolts are loaded to the point where the nuts start popping off, then clarely the joint needs more bolts or larger diameter bolts. On the other hand, there has been no reports of this happening, so either something else yelds first, or the antissipated forces acting on the bolts are much larger than they really are.

But then again, proper nuts never hurts, unless the real problem here is related to corrosion.
 
For me, the take home message is that I can substitute the 12 point nuts for better wrenching and maintain equal or better strength. Thanks for running these tests!
 
I agree!

Very interesting indeed.

Seems you have come up with a solution that has evaded a problem!

Unless I've missed something, I haven't heard of one incident where these nuts have failed. Is this something no one is speaking of?

I've owned an 8, have friends with 8s and followed 8s for years. The only thing on an 8 main gear that needs regular attention is checking the attach bolt torque. I am not aware of any other issue with this area of the airframe, Vans or Grove. Am I too missing something?

Impressive work, but I prefer to explore the shear strength of a good top shelf margarita or a cold premium beer. Southern engineering at its best!

Thanks for the info.
 
I say weld in some 1/2" steel plates all across the fuselage and increase the gear leg to 2" thick titanium. Go to 3/4" grade 8 bolts. Put chains on the tires also for good winter traction. Sorry, couldn't help myself! :D

Second guessing some designers comes with risk - especially this one who has an amazingly good track record. Scott said it well - it may just serve to transfer the failure to a more difficult to repair area. The original analysis of the bolts was good stuff - analyzing the surrounding structure's strength is much more difficult. For example, what happens to the surrounding structure when the bolt(s) in question hit 10,000 pounds?
 
Come on guys....read between the lines.

Steve is a professional aero engineer and no doubt aware of the "weak link" concept, ie simply moving the failure elsewhere. He started the thread with some nut strength numbers. Later he disclosed a review of the clamp saddles. His stated use for his airplane includes rough surfaces. Think about it.

I conclude he has already looked at the gear tower, or intends to do so. I suspect we're not going to hear much about it unless (a) he finds some previously unknown horror story (unlikely), or (b) some easy mod which will improve strength, maybe even prevent tearing out a gear tower in a groundloop.

In the meantime, hard data supports an easy mod as well as a caution. A particular full height nut offers more tensile strength than the stock nut. A particular reduced wrench size nut offers less strength than stock. A particular specialty nut offers more strength and reduced wrench size. Good stuff.
 
Last edited:
Burt Rutan introduced canard airplane builders to the MS21042 nut (the weakest link in the test) a long time ago because of his fanatic compulsion to keep weight to a minimum in an airplane - if you tossed it up and it came down, it was too heavy, I believe he invented that saying, although I think if you tossed a hand full of nuts up they would all come down at the same time.

The MS21042 nuts do weigh less than any of the other nuts and are quite strong for their size. I believe they came out of NASA space age technology.
 
..and they would be...

.....
The MS21042 nuts do weigh less than any of the other nuts and are quite strong for their size. I believe they came out of NASA space age technology.

...great in a shear application - but poor for this particular tension application.

It's relevant test with interesting results.
 
Dan H is correct on all counts

What Dan said is right. See my new post called "A Tale of Two Airplanes" if you care to know what I am up to.
 
Bracket gaps - Landing Gear

OK guys, I figure this is as good a place as any to post this question. I just switched over to the 12 point NAS1804-5 nuts, and also the NAS6606-27 bolts, which are slightly longer, thus giving a few threads through the new taller nuts. These nuts are certainly easier to get on with a socket to torque, but I have another issue. My gear both had no gap in the front but about the 0.035 recommended in the rear between the bracket and plate. By being careful in the torquing process, going back and forth I ended up with about .010 gap, both front and rear on the left, but on the right .003 front and .014 rear, and all gaps varied between the gap at the inside, closest to the gear leg, and on the outside of the bolts, away from the gear legs. I've read about removing them and taking material away, but is shimming a possible solution? I don't know about other RV-8's, but getting the bracket completely out requires the removal of my vent and fuel fittings. Hardened shim stock in the recessed area of the bracket, between the leg and bracket, would increase the gaps, but I am not sure the same quality of results.
 
Better Bolts, Nuts and maybe washers!

Gahco has 12 point bolts (MS21250-06028) 180Ksi plus.

We also have 12 point nuts that are rated to 220 ksi and they are nylon locking, this means little if any thread galling.

We have washers flat and counterbored hardened to 260 ksi.

I will get exact item #,s as I am at home and not all of thes items are on the webstore.

The 12 point bolts do have a taller head than the Vans bolt.

I am building a 6, so I am open to suggestions about the RV8,s

GAHco will consider additional discounts for RV customers. Let us know what your thoughts are?
 
Last edited:
An interesting read and excellent discussion. One thing I'm fairly certain of is that I sure don't want to be in the aircraft when either the nut, bolt or anything surrounding it lets go!
 
Mark, how many hours on on the plane? The small gap may be a result of wear on the wear plate. It should not be that hard to change if that is the problem, if I remember correctlly, it is held on with two 1/4 in bolts and nuts. Also, I don't think shaving a Few thousandths off of the u bracket would Not hurt.


Bird
 
Last edited:
No shimstock

Any kind of shim between the saddle bracket and the gear strut is likely to walk/work its way out over time.

Just remove the saddle and grind 0.010 or so off the ends. Not sure why you need to get the vent and fuel lines out to get the saddle out -- I don't think you should have them that close to the saddle. The brake line does run by pretty close, but I think you can squirm the saddle bracket out without taking anything else off. Probably don't even need to jack the plane up, as long as you don't move it with the bracket off.

And one other point for emphasis - reading back through the old thread, there were a number of people who said that there were no known failures so why fix it -- when in fact, at that time there were at least two known failures. Since that time, a few more cases have also emerged. Some failed on landings, some were found loose on inspections.
An interesting thing about the failure mode of the horrible NAS679A6 nuts is that they have inadequate hoop strength around the thread, so when loaded in tension, they stretch circumferencially, climbing up the ramps of the threads. At a certain point, they actually leapfrog over a thread, and are now loose by a full thread, without ever turning. Some folks have looked at them after finding them loose and say the threads look OK so it must have just loosened (by turning) but I have witnessed the leapfrog-mode and I really think those old nuts are trouble.
I happened to be looking at my old preview plans, which date from a time about 8 years before I bought my kit. It was interesting to note that as of that point, the call-out for the gear nuts was for AN365 - which would be a somewhat better choice, but still not as good as the strong NAS 1804-6 nuts.


OK guys, I figure this is as good a place as any to post this question. I just switched over to the 12 point NAS1804-5 nuts, and also the NAS6606-27 bolts, which are slightly longer, thus giving a few threads through the new taller nuts. These nuts are certainly easier to get on with a socket to torque, but I have another issue. My gear both had no gap in the front but about the 0.035 recommended in the rear between the bracket and plate. By being careful in the torquing process, going back and forth I ended up with about .010 gap, both front and rear on the left, but on the right .003 front and .014 rear, and all gaps varied between the gap at the inside, closest to the gear leg, and on the outside of the bolts, away from the gear legs. I've read about removing them and taking material away, but is shimming a possible solution? I don't know about other RV-8's, but getting the bracket completely out requires the removal of my vent and fuel fittings. Hardened shim stock in the recessed area of the bracket, between the leg and bracket, would increase the gaps, but I am not sure the same quality of results.
 
Last edited:
Hi Steve...

Interesting thread / research.

I know structures / loads to a basic level, but nowhere near yourself, so it is a god education ;)

Just fitting a Grove Gear, and it was supplied with:
  1. NAS 6206-36 Bolt (longer than Vans plan due thicker leg (?) and maybe longer nuts)
  2. MS14183L-6 Washers - I have no idea of the merits between various washer types, I am sure you will let us know [Vans plans use 062-25783054]
  3. NAS 1804-6 Nuts as you are tending to...
If you are looking to up the strength for soft field, I would think the first line of defence is the larger tyre (380-150-5)?

Whilst I can see a broken bolt or nut is preferably to a damaged gear tower, it misses the point that I would think a load that breaks the bolt or nut is very likely to see this followed by the leg folding with associated wing/fuselage/propr/engine damage.

Vans do give some detailed explanation of how to set this up, but seems more concerned with wear aspects than configuring the components for Ultimate Strength.
 
Thanks

Thanks Steve for your post. Very informative.

We used a lot of those NAS nuts building jet engines way back when and I had to update my tool chest with 12 point sockets and wrenches when I went to the engine shop. Now I know why those nuts were preferred.
Fj
 
NAS1804-6

Resurrecting an old, but ongoing thread.

I intend to use NAS1804-6 nuts and NAS6206 bolts. The kit comes with NAS6206-27 bolts. It looks like the ideal bolt length when using the NAS1804-6 nuts would be -29, but these do not seem to be available. Has anyone used NAS6206-28? If so, did you have a thread showing after install.

Thanks in advance, and thanks to Steve Smith for this excellent thread.
 
Nuts

I payed the money and installed those 1804 nuts on my existing bolts. Given their extra strength, the minimal cost was worth the piece of mind and they were not too bad to install.
 
Red,

Straight Flight at KAPA carries the NAS 1804-6's in stock and charged me $2.50ea. When I questioned if he was giving me the right nut, telling him the price I was quoted by the bad website guys.... he laughed and said they are only NAS nuts...not unobtanium. Said he charged me 2.50ea because I only wanted 4 of them and they have a $10 minimum. Still, I made him show me the box...just to be sure. Piece of cake to install with the gear tower mod.
 
way off track!

You guys are way off track! Where do you get Fore and aft movement of the landing gear? Are you kidding me!

There should not be any appreciable amount of aft movement on takeoff and if your getting it on landing maybe you need to grease you wheel bearings of lay off of the brakes. I don't think this is an issue whatsoever!

Dave
 
Loading vs. movement. OK, where is the Fore and Aft Loading coming from? It appears that there should not be any. That gear is designed for a downward loading. I don't think you should be having any Fore and aft loading if the airplane is flown properly.
 
I don't think you should be having any Fore and aft loading if the airplane is flown properly.

What about holding the brakes during run up. You do perform a run up before flight, right? Or using the brakes on a short field landing? Or an aborted takeoff? Our planes have brakes for a reason. Sure, they're not needed on most runways longer than 2000', but it's nice to know that our gear can take the aft loading under hard braking should it be needed.

Skylor
RV-8 N808SJ
 
Dave,

I am puzzled by your expression of anger regarding this issue. It is almost as if you are projecting your anger about something else onto this topic.

Steve Smith and I (mostly Steve) put a lot of thought into these tests. Steve is an experienced aeronautic engineer, and I know that he considered a wide variety of loading conditions, including but not limited to braking application, wheel spin-up, three-point touchdown, and obstacle encounters.

Even when operated perfectly, the landing gear legs of an airplane are subjected to substantial loadings in a variety of directions besides straight up and down. And the reality of the situation is that airplanes are operated by less-than-perfect people. So well-designed airplanes like Van's RVs have structural margins that accommodate this troubling fact of life.

The service history of the RV-8 landing gear includes several incidents in which the nuts securing the saddles at the outboard landing gear attachments failed in tension. So far as I know, none of these airplanes had been subjected to any particular abuse. Our tests demonstrated pretty convincingly that a simple hardware substitution could substantially improve the amount of structural margin at this attachment.

Thanks, Bob K.

You guys are way off track! Where do you get Fore and aft movement of the landing gear? Are you kidding me!

There should not be any appreciable amount of aft movement on takeoff and if your getting it on landing maybe you need to grease you wheel bearings of lay off of the brakes. I don't think this is an issue whatsoever!

Dave
 
Speaking Generally

....the "weak link" concept, ie simply moving the failure elsewhere.....

Remember that when this happens, the failure doesn't simply move somewhere else. The next failure, somewhere else, requires a higher load to fail. That's why it's the "next" failure. So when the strength is increased this way, it's a true increase in strength.

Depending upon what does fail next, it might be a modest increase or it might be a big increase. But we don't necessarily know that without a thorough analysis of the overall structure. All we know is that this one mode of failure has been made stronger. And that, at least, is something we do know.

Dave
 
Hey, how did I get to be the whipping boy? ;)

This is simple enough...the original nut doesn't equal the bolt strength, so use one that does.
 
Back
Top