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  #11  
Old 04-21-2013, 04:14 PM
RijkersJ RijkersJ is offline
 
Join Date: Mar 2010
Location: Netherlands
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Default Nose job (2)

Further to my attempt to generate a discussion on the subject I looked at my RV6A, took some measurements, and did some brief calculations.
My conclusion is this;
1. VAN?S documents indicate a maximum nose wheel weight with forward CoG of 375 lbs. (?flight prohibited?) This is 23% of gross and as such higher than established design criteria. Furthermore some pilots/builders may not be aware of this limit or ignore it.
2. The steel of the leg is 6150 that has been hardened/normalized. I do not know the exact final temper in this area of the leg but the steel properties are 400Mpa annealed and 600Mpa hardened. Assuming a yield strength of 500Mpa in the bent section the moment generated by the arm of the trailing link at VANS figure of 375 lbs results in a stationary stress approaching 50% of this figure (taking into account curvature factor 1.5). If you add to this the deceleration ? regardless how it is generated, i.e. without braking on soft grass ? of 0.1 G resulting in an additional load of 100lbs. Add some dynamic loading from ground roughness, wheel bearing friction, harmonics, etc. and the stress quickly approaches the yield limit in the bottom part of the leg. That is with a good landing. Clearly a bouncy landing or significant braking (0.3 G?) could multiply the load significantly. The aluminium fork appears to be more than strong enough so that is not a factor. The failure mode therefore appears to be bending of the leg just above the swivel resulting in lock-up of the wheel and/or digging in of the nose of the fork. Subsequently this then translates into bending of the centre portion of the leg and possibly nose-over.
3. The solution of Antisplat addresses the bending of the centre section but does not have an effect on the bottom part where the problem originates. The positive aspect is that their bracket only affects the bending back and not the flexibility from vertical loading. At a cost penalty in $$ and weight I must conclude that it does not address the core of the problem. Similarly the installation of a wood/epoxy gear-leg stiffener does not reinforce the bottom part of the leg and actually increases the dynamic loading as the leg becomes stiff.
4. The bottom line is that I do not have a simple solution other than to rely on pilot skill and avoiding the brakes. Short of a redesign away from the trailing link, the solution should in my opinion be sought in a reduction of the nosewheel load, e.g. pilot/builder controlled weight distribution/Gross/CoG, and moving the mains slightly further forward. The diameter of the bottom part of the leg (bend section) could be increased without a material effect on flexibility and weight. It is presently tapered to 1? at the swivel socket and increasing the bottom end to 1-1/8 would increase the strength by 42%. Other than that a free turning and balanced wheel and some other form than the wood stiffener for damping of harmonic vibration of the leg might have a positive influence.
5. A comprehensive improvement is not a simple problem that can be addressed by the builder individually and in my opinion should be considered as a further development by VANS. Alternatively you could order a custom leg from Langair and design your own profile, with a double reverse taper, on the basis of weight and CoG criteria of your particular airplane.
Hope I did not bore you all with this long story
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  #12  
Old 04-21-2013, 07:11 PM
flyvans.com flyvans.com is offline
 
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Location: Zurich, Switzerland
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Default

your engineering analysis matches what we had to find out/had to have calculated during the registration process over here. theoretically, the weak link should be just above the bend on the lower gear leg.
problem is, the dynamics of reality seem to clearly proof theory wrong.
practically all folded gear legs i have seen, kept their original bend/lower shape, yet had severely bent where the antisplat brace tries to picks up.
i have to admit we got the brace a while ago but because of a mix between not wanting downtime, not having a workshop currently with most tools stored away required for fitting the new fairing and having to go through a paperwork modification process, we have so far held off the installation.
i tend to agree with the "fly the aircraft properly and you're fine camp." however, the margin is clearly thin, we keep off of any grass runways and any level of piloting is useless in case of an excursion due to brake failure or an off airport landing.
this alone had me going for a -14 were i to start over today...

bernie
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  #13  
Old 04-22-2013, 02:26 AM
Neil Neil is offline
 
Join Date: Feb 2006
Location: Hong Kong/Tasmania
Posts: 30
Default Nose Job

Have read with interest the attempts to analyse the failures of the nose gear in 7A and 9A aircraft.

RijkersJ - you throw maths at the problem... but have you the correct physical model you are looking at ? Certainly some of the forces you talk about are present.
flyvans.com - sorry but reality, in my opinion, proves the theory correct if you use the correct physical model and apply basic simple engineering principles.
The maximum moment, due to forces at the nose wheel, occurs at the point of effective fixity of the strut. That happens to be at, or close to, the strut entering the engine bracket. (exactly where failures and bending are occurring.)

The actual weight on the nose wheel (it is a vertical force) is not such an important parameter... if it was so important the strut would bend forward into the propeller. Clearly the major effect is the horizontal force created when the nose wheel is significantly retarded by digging into a hole or similar cause. With the wheel being stopped from moving forward and the momentum of the forward moving aircraft - the force is horizontal in the wheel strut. The moment on the strut at the point of fixity is too great and it bends backwards from a point close to the body of the aircraft (as it should) and .. oops Over I go ! The point of effective bending of the strut depends on the strength of the strut - being tapered a little - it may not be in the cowling.

The antisplat brace tends to address this problem .. and is certainly on the right path...

The speed of the aircraft, when the wheel is effectively retarded, will be the main consideration in determining if the strut bends backwards.
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  #14  
Old 04-22-2013, 09:33 AM
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Buggsy2 Buggsy2 is offline
 
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Quote:
Originally Posted by RijkersJ View Post
Another opinion by a high time RV owner was: "If you break the nose gear you can't fly properly - don't worry about it"
Comments?
Quote:
Originally Posted by RVnoob View Post
Haven't heard of anyone flipping with no reason.
Quote:
Originally Posted by Camillo View Post
Van's says front gear is a taxi gear only.
I still have in mind the image of a short video clip, I think posted here on VAF, of an -A model nose-over caused by merely taxiing on asphalt and rolling over a lip in the pavement...something no one would question in any other aircraft. I've applied all 3 Anti-Splat "fixes" for the nose gear, but still wonder what risk I will have when I fly it (working on Firewall-forward parts). Thinking that it's always caused by some incorrect pilot technique, thus, can't happen to me, is unsubstantiated IMO.
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  #15  
Old 04-22-2013, 11:47 AM
gasman gasman is offline
 
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Quote:
Originally Posted by Buggsy2 View Post
I still have in mind the image of a short video clip, I think posted here on VAF, of an -A model nose-over caused by merely taxiing on asphalt and rolling over a lip in the pavement...something no one would question in any other aircraft. I've applied all 3 Anti-Splat "fixes" for the nose gear, but still wonder what risk I will have when I fly it (working on Firewall-forward parts). Thinking that it's always caused by some incorrect pilot technique, thus, can't happen to me, is unsubstantiated IMO.
If you install the nose faring too low and close to the tire, the fiberglass can catch on a raised surface and be pushed under the tire causing the tire to drag (roll up on the inside of the faring) and if the drag is enough, the fork will tilt enough to dig in and cause failure.

Keep the nose wheel rolling and you almost eliminate the problem.

1. Understand the bearings and axle past problems, and correct them.

2. Maintain air pressure..... I keep mine at 40#.

3. Allow more tire exposed under the faring so it has a chance to roll over an one inch lip.

4. Fit the faring with at least a finger gap between it and the tire sides. A little more front and rear. You should never see any scuffing under the nose of the faring.

5. Learn to fly (maintain altitude and do turns) at 65 mph..... so you can touch down at stall speed... or JUST above. All to reduce ground roll speed and the need to brake.
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Last edited by gasman : 04-22-2013 at 04:43 PM.
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  #16  
Old 04-22-2013, 01:45 PM
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LettersFromFlyoverCountry LettersFromFlyoverCountry is offline
 
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I believe the pilot in this case blamed "a problem with flaps"

?He was telling people there was something going wrong with flight controls. He had issues with his flaps. He said either they stuck or he had some problems with them,? the sheriff said.

That tells me there's a much bigger factor in this accident than any perceived nose gear flaw.
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  #17  
Old 04-22-2013, 02:18 PM
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Buggsy2 Buggsy2 is offline
 
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Quote:
Originally Posted by LettersFromFlyoverCountry View Post
I believe the pilot in this case blamed "a problem with flaps"

?He was telling people there was something going wrong with flight controls. He had issues with his flaps. He said either they stuck or he had some problems with them,? the sheriff said.

That tells me there's a much bigger factor in this accident than any perceived nose gear flaw.
I take it you mean he allowed himself to become distracted by the flaps? Because as long as the flaps were equally stuck (no differential flaps) they shouldn't have caused the subsequent poor landing and nose-over.
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  #18  
Old 04-22-2013, 03:46 PM
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ChiefPilot ChiefPilot is online now
 
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Quote:
Originally Posted by Buggsy2 View Post
I still have in mind the image of a short video clip, I think posted here on VAF, of an -A model nose-over caused by merely taxiing on asphalt and rolling over a lip in the pavement...something no one would question in any other aircraft.
Must have been a pretty fast taxi....
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  #19  
Old 04-22-2013, 04:40 PM
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rzbill rzbill is offline
 
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Default I can't stand it

This was not a nose gear failure.

Thats all I'm sayin'
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  #20  
Old 04-22-2013, 05:06 PM
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mikehoover mikehoover is offline
 
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Quote:
Originally Posted by rzbill View Post
This was not a nose gear failure.

Thats all I'm sayin'
+1

Thank you!
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