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02-21-2011, 05:43 PM
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Join Date: Jan 2005
Location: Las Vegas, NV
Posts: 488
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On second thought,,,
My RVr friend has a very capable machine shop in his hangar (Mill, Lathe, Plasma cutter, etc.). If there is anything you need that I might be able to fab up on his machine tools, please don't hesitate to ask. KHND to KTUS is only 2 hours  .
__________________
Terry F.
RV 7A N457RV
250 hours and lovin it! 
Southern Nevada EAA Chapter 1300 - www.eaa1300.org
Paid VAF 03/17
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02-21-2011, 06:29 PM
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Join Date: Feb 2010
Location: Vancouver, BC, Canada
Posts: 860
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Glad to see this.
I thought about building a rig like yours, but am still building the fuselage, so don't really have the time at this point in my project. However, please let me offer a few thoughts in defining the tests themselves.
Having studied most of the taxiing videos on YouTube, it looks like there is a lot of fore/aft motion of the nosewheel on fairly smooth pavement. I think replicating this motion should be your first test, that is find out whether it is low tire pressure or loose bearings, or a lack of a spacer between the bearings. All tests should be repeated at three or four speeds up to the stall speed of the aircraft, say 50 mph, or let's say at 12, 24, 36 and 48 mph.
Once the cause of this behaviour has been established and a remedy found, then progress on to some bumps. As another post mentioned, this should be a bump that only the nose wheel sees, not the trailer wheels. And it should be a well defined "calibrated" bump. For example, a 2x10 or 2x12 plank with a 1:4 taper ramp on one end. You might start with a single "bump" and then progress to a series of them. A "hole" of a similar size would be nice, but I suspect not a practical idea. Something to watch for is whether the nose gear reacts to the bump by a vertical movement or a horizontal movement (probably some of each). I think the horizontal movement is the most dangerous reaction as it is in the direction of folding the nose gear under. Raiz' analysis seems to indicate that the reaction is mainly vertical and the failure mode is tensile stress failure at the nose gear rod socket (if I interpret his results correctly). I suspect the "pole vault" tuck under failure is a combination negative bending moment (caused by aft movement of the nose tire) coupled with a column buckling tendency.
If the nose gear has not yet failed, you can consider testing on some "rough ground", but the problem is defining just how "rough" it should be.
I spoke with Van's about this issue, and in addition to the eccentric test rig (RVator Fifth Issue 1998), they did instrument an aircraft and landed/taxiied it on a grass strip. But I'm not sure how rough it was or the results of the instrumetation.
Well, you're getting lots of advice and encouragement, I look forward to some results.
Last edited by terrye : 02-21-2011 at 06:32 PM.
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02-23-2011, 02:30 PM
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Join Date: Aug 2006
Location: Battle Ground, WA
Posts: 426
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Calibrated bump
Barry, my hat is off to you for undertaking this project!
I am in agreement with the "calibrated bump" suggestion. In order to have useful test information, it is important to make the test simple enough and the test protocol rigorous enough that the test is repeatable (the basis for the scientific method, i.e., some one else can independently replicate your test and get the same result).
This generally means that the test will be "oversimplified" to a lay observer. The best example I can think of is automobile barrier crash testing. Does the test represent a real world collision scenario? Of course not. However, because the test is so simple to conduct, results are repeatable. Over years of testing, much has been learned and the state of automotive crashworthiness has been greatly improved. If you want proof, go to the IIHS website and watch the video of an offset front-end collision between and 1957 Chevrolet and a 2007 Chevrolet.
Back to the calibrated bump. By varying the shape and height of the bump and comparing the response, a tremendous amount of information will be learned. I personally like the idea of the grid and video camera because it will capture the complex wheel motion. In automobiles, the natural frequency of the suspension is about 1 Hz. Standard video cameras run at 30 frames/second. Perhaps it would not take a really high speed video to produce good results. The vertical deflection is cantilever bending, so the moment arm is the distance from the engine mount anchor to the axle. The "tuck under" is the local moment from the rearward directed force just above the ground about the attachment to the end of the gear leg superimposed on the static moment (moment arm = trailing arm length; F= nose wheel weight).
It will be exceptionally valuable to vary tire size in addition to tire pressure. Speaking from personal career experience, auto manufacturers spend tons of money trying to address "impact harshness" issues, defined as the ability of the suspension not to isolate the "impact" of striking a step in the pavement from the vehicle's occupants.
This might (probably is) be more information than most forum members want. If you would like some additional input regarding testing and analyzing data, I would sure be willing to help. Please contact me by PM because the it will probably take something more interactive, like telephone conferences, to explore the possibilities.
LarryT
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02-23-2011, 02:38 PM
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Join Date: Aug 2006
Location: Battle Ground, WA
Posts: 426
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Impact Harshness
I am clearly linguistically challenged (I am not a cunning linguist). What I was trying to say was:
Good impact harshness = good isolation from occupants
Bad impact harshness = bumps are transmitted to the occupant compartment.
Examples:
MacPherson struts have poor impact harshness inherent in the design.
SLA suspensions (upper and lower control arms) have the capability for good isolation
LarryT
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02-23-2011, 02:49 PM
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Join Date: May 2007
Location: Winder Ga
Posts: 914
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Gear Leg
This may sound far out , but why not contact Myth Busters out in California for some ideas on what they might use for test monitoring, measuring stresses, etc/ They seem to exhibit a vast knowledge of physics.
Just a thought
Jerry 
__________________
Jerry Fischer KW4F
VAF# 1646
USS Randolph CVS-15 V-6 Div. AT2
RV7 N364SJ Miss Sandy sold 
RV1 N1583 "Mockingbird 1" SOLD
EAA volunteer SNF
HBC volunteer/co-chairman OSH
Tech counselor
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02-23-2011, 03:35 PM
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Join Date: May 2005
Location: Canby, Oregon
Posts: 1,786
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Larry, I couldn't find the video
Quote:
Originally Posted by LarryT
Barry, my hat is off to you for undertaking this project!
The best example I can think of is automobile barrier crash testing. Does the test represent a real world collision scenario? Of course not. However, because the test is so simple to conduct, results are repeatable. Over years of testing, much has been learned and the state of automotive crashworthiness has been greatly improved. If you want proof, go to the IIHS website and watch the video of an offset front-end collision between and 1957 Chevrolet and a 2007 Chevrolet.
LarryT
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Maybe this is the one you were looking at:
http://www.youtube.com/watch?v=joMK1WZjP7g
kent
__________________
Kent Byerley
RV9A N94KJ - IO320, CS, tipup
AFS 3500, TT AP, FLYING....
Canby, Or
Last edited by kentb : 02-24-2011 at 09:35 AM.
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02-23-2011, 05:31 PM
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Join Date: Oct 2005
Location: Melbourne, Australia
Posts: 1,868
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Serious reservations about test method
One of the really overlooked factors in the great RV nosewheel failure debate is the effect of torsional moments on the nose gear.
When a two seat RV nose gear yields and fails it results from the combined effect of all applied loads...typically compression force, bending moment, and torsional moment.
Torsional moment (twisting of the gear leg) occurs when there are lateral (sideways) forces applied to the nose of the aircraft. For example, push sideways on the engine of an RV7A and the nose of the aircraft will readily rock from side to side (like a great big jelly in fact). That sideways movement induces twisting in the gear leg....lots of it.... due to the fact that 1) the gear leg is a very long spindly rod 2) the nose wheel is not concentric with the axis of the gear leg.
Unless the test rig can simulate all major loads on the gear leg then it cannot hope to simulate the real world.
I cannot see how the test rig as it stands will enable the simulation of torsional moment in the gear leg because there is no possible lateral (sideways) force being applied to the upper gear mount as there would be in an RV aircraft. On the test rig the nose gear is effectively located centrally between four road wheels ( 2 on the trailer and 2 on the pulling vehicle) and therefore is is not subjected to any lateral forces.
That alone suggest to me that any results from the proposed tests are unlikely to be valid unless the test rig is seriously modified.
__________________
You’re only as good as your last landing 
Bob Barrow
RV7A
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02-23-2011, 06:32 PM
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Join Date: Dec 2009
Location: Boulder, CO
Posts: 4,435
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What I don't understand yet is what this information is going to be used for.
Are you going to find a set of controlled conditions that cause this leg to fail and then use that as an acceptable criteria for a redesign? Because if that's the plan, how will you know that the criteria is adequate? All you'll know is that the old leg failed with it and the eventual new one didn't. You won't know how it relates to flying an airplane.
Information is primarily used to make decisions. What decisions will be made using this information?
Not trying to ruffle feathers here, just trying to find out the purpose of the exercise.
Dave
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02-23-2011, 06:45 PM
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Join Date: Feb 2010
Location: Vancouver, BC, Canada
Posts: 860
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Torsional Moment
Sorry to disagree with you Bob, I do not think the the torsional moment contributes to a primary failure mode. You are right about pushing sideways on the nose of an aircraft, but in this condition, the machine is at rest. So the weight on the nose tire contact patch prevents (unless you push really hard) the tire from rotating around the castor axis which is about a foot in front of the contact patch. If you put a ball bearing under the nose wheel and tried again, then all you would have to overcome is the breakout force on the castor axis preload springs.
Watching the taxiing videos, when the aircraft is moving there appears to be no noticeable torsional moment in the gear leg, that is no deflection, as the tire castors in response to the turning of the aircraft.
Again from watching the taxiing videos, it appears that the primary failure mode of "pole vault/tuck under" failures is longitudinal load through the gear leg plus a negative bending load from either a bump resisting the forward motion, a harmonic vibration, or from the gear leg nut digging into the surface. The combination of longitudinal (compressive) load plus negative bending moment can produce a column buckling failure consistent with the "pole vault" pictures we have seen.
In any case, "a test is worth a thousand opinions". Even if this test fails to identify what the culprit is, it will probably identify what the culprit is not. And this will probably lead to another test program. You always learn something from a test program. Sometimes it is not what you expected to learn.
So as before, I await the test results with great interest.
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02-23-2011, 07:30 PM
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Join Date: Dec 2005
Location: San Pedro
Posts: 1,013
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Donations???
I certainly am not any kind of expert pilot or even a judge of good/bad landings and only speak for myself here. I have set down beside the landing zone of several runways at fly ins and watched the RV-As as they landed. Many RV pilots appeared to land hot and immediately put the nose wheel on the runway - (in order to begin hard breaking?) - thus causing a fore and aft oscillation of the nose gear that was clearly visible. Thus, Van's admonition not to land too fast and not to put the front wheel down until there is insufficient elevator authority to keep it off of the runway.
I watched Van himself land his RV-10 as he came to work at the factory last year and his landing roll out I guess was about 600'.
Some pilots have suffered the nose wheel tuck under while taxing at low speeds = bad luck? I gotta line up with those who say that that particular failure should not happen. So how is it possible that a well engineered nose landing gear could suffer a collapse at a low taxi speed? Yeah - I know that RVs are not Cessnas or Pipers but I'll bet that most of us 'graduated' from Cessnas or Pipers and none or few of us flipped one of them over.
I am glad that you guys are trying to do something about what is a problem (in my opinion). Keep up the good work and I hope that you will find an answer that will reduce the accident rate.
I am happy to send you guys some $$ in order to help defray expenses. Just tell me how to send my $20 donation to you. Hope that will help.
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