I think you are absolutely right about the rears being shorter on the 6 but the nose leg is the same part number U-603-3 on the 6A, 7A and 9A. The 8A has a different leg U-809-2 but I've no idea what the difference is. Here is the source: http://www.vansaircraft.com/pdf/sb07-11-9.pdf. I've just posted a diagram that shows the side view of all three aircraft.

Raiz

This all makes sense to me (sort of anyway, not being an engineer.) It would seem to me that this is another good argument for building light, especially FWF. I would love to have a CS prop, but that would take up more of this precious gear travel dimension. This validates some of my building choices, and no, I have never had aft-CG problems.

The 9A gear is the same, and in fact my whole FWF, including the gear leg, came from a 6A that had a hangar fall on it. The axle (circa 1994) in the nose gear was IMHO FAR superior to what comes with today's A's.

Bob Kelly

I've got a C/S prop, the old heavy starter motor, and heavier six pac instruments. But I can still walk back to the tail, push down and raise the nose without too much effort. Or two people standing on the steps at the same time will throw the tail to the ground. It's because I distributed other weighty parts behind the mains and CG. But don't get the wrong idea........... because my specs work out great CG wise for fore & aft........yet there is not a signicant amount of extra weight on the nose, when the aircraft is rolling on the mains.

L.Adamson --- RV6A

RVator references to nose gear
 

I did a little checking into references in the RVator newsletter about nose wheel/leg since I remembered a nose wheel test rig article. Here's what I found:

1. Fifth Issue 2007. P9 setting of nose wheel rotating torque in Matco nosewheels. (I think this torque could be reduced with a proper spacer and pin as already discussed in this forum). P19 Service letter November 9, 2007 shows gross weight and weight on nosewheel for various models. P25 Mandatory Service Bulletin 07-11-09 Upgrade of nose gear leg and fork.
2. Sixth Issue 2004. P8 Nose Gear Design by Van. Discusses the loading components, failure modes, contact of fairing/swivel nut, low tire pressure and binding of tire on fairing. Closes with a request for users to share their experiences with Van's Aircraft.
3. Fifth Issue 1998 (and this is the one I was really looking for). P3 Improving the Breed - Van's Develops a New Nose Gear Leg by Van. This discusses the early issues of RV-6A nose gear legs, both fatigue failures and overload failures. The interesting things about this article are i) Harmon Lange's "rotating runway" test rig for cyclic bump loading and ii) Ken Krueger instrumented the nose gear of the factory airplane and recorded the stress levels on asphalt and turf runways. This data was then used to refine or calibrate the "rotating runway" test rig. This resulted in a new nose gear design U-603-2. It also emphasized that the nose gear leg must act as both a spring to absorb irregularities as well as a strut to accurately locate the nose wheel.

So, while the current discussion is interesting, I am wondering:
1. Is Raiz' model "bump" more severe than will actually be encountered in service?
2. Is there a plan to do rig testing or aircraft instrumentation to verify the model?
3. What changes to the leg can be made to improve it's strut function without detracting from its spring function?

Comparison of 6A, 7A, 8A and 9A
 

Ok, so in an earlier post, I promised to look at a comparison of the 6A, 7A, 8A and 9A. I've assumed the RV8A leg is the same as the others (but I don't know this for sure). I used a 10 ft long, 1.5" deep pothole and 10 knots as the reference case.

For the first comparison, I set the AUW to 1650 lb and the nose load to 325 lb (worst case that they can all cope with and still be within Van's recommendations). This is intended to highlight any differences due to geometry and gave the following "% strength remaining" results:

Model ____ No braking ____ 0.15g braking
6A ________ 50% __________ 29%
7A ________ 49% __________ 25%
8A ________ 49% __________ 26%
9A ________ 49% __________ 26%

Although the 6A is better, the differences are very small compared to the effect of braking.

Next, I considered the worst recommended loading case for each model. For the 6A that is 1650/375 (AUW/nose weight). For the 7A and 8A 1800/375 and for the 9A 1750/325. This gives:

Model ____ No braking ____ 0.15g braking
6A ________ 42% __________ 23%
7A ________ 40% __________ 17%
8A ________ 40% __________ 17%
9A ________ 49% __________ 25%

In this case, the 6A and 9A come out slightly ahead (less likely to fail) compared to the 7A and 8A due to the lower all up weight or nose load but the differences between models are still small compared to the influence of braking.

Finally, I looked at nose laod on a 7A at 1800 AUW:

Nose load ____ No braking ____ 0.15g braking
__ 275 ________ 56% __________ 26%
__ 325 ________ 49% __________ 20%
__ 375 ________ 40% __________ 17%

In summary, the model predicts that braking is the most important factor, followed by nose load, then all up weight and (only) then the differences between models. Only if there were systematic differences in braking or nose load would I expect to see differences between models. For example, if braking on the roll-out were less common in the 9A (because of the lower landing speed) we might expect fewer failures. Alternatively, if the 7A tends to be nose heavy compared to the 6A, we might expect more 7A failures. I think both of these examples have some grounding in reality but I don't have any definitive information. However, overall, the model suggests the likelihood of failure should be similar for all models with maybe the 6A and the 9A somewhat ahead of the 7A and 8A. I know that's not in line with the general perception that the 7A and 9A are generally worst than the 6A and probably the 8A as well.

The next step will be a look at the accident statistics.

Raiz

You're doing some fantastic analysis work on this Raiz, really - but until you can do some testing to prove the models, or find test data that has already been taken, you can't really make the leap to a re-design with any sort of confidence. I don't trust unverified models, because I have too often seen them fail due to unexpected variables that hadn't been taken in to account. But what you have done is very valuable - have you thought of contacting the engineering guys at Van's to see if any of it fits with what they have already done, and the testing they have performed.

I am not being critical, just suggesting the logical next "engineering" step. Analyze, test, then design - then test the results to see if the model works in the real world.

Paul

Paul, I agree entirely. I have no intention of proposing a change to the fleet without proper validation but I do intend to extract whatever useful information I can from the analysis. I'd be happy to support Vans or anyone else with the facilities and expertise to develop a better gear.

I have tried to engage with Vans. I sent them a summary a month or two before the first post on this forum but only radio silence from then on. If anyone can engineer a connection for me, that would be great.

Raiz

Nose leg failure statistics
 

The following results come from the NTSB database and include all accidents in the period 1/1/2000 to 12/31/2009. Actually, there is one exception (N448GM), which is in the FAA study but apparently not in the NTSB database. I have only counted those nose leg failure accidents where there was no previous problem (such as engine failure or a stall) preceding the nose leg failure incident.

Model ___________ 6A __ 7A __ 8A __ 9A
# nose leg fails ___ 24 ___ 8 ___ 3 ___ 5
total # accidents _ 78 __ 20 ___ 9 ___ 13

To compare the different models in terms of their relative likelihood of experiencing leg failure, we need to know how many of each model were flown in this 10 year period. By analysing the FAA register, I got these figures for the number of aircraft-years of each type with an airworthiness certificate (as an approximation of the # flying):

Model ____________ 6A ___ 7A __ 8A ___ 9A
# aircraft-years ___ 6770 _ 1404 _ 905 _ 1266

100 aircraft-years could mean 10 aircraft for the full 10 year period or 20 aircraft for an average of 5 years etc. It provides a way of dealing with the fact that 6As have been around longer than 7As.

If we assume that all models are flown the same amount per year on average and exposed to the same types of strip and piloting technique, we can compare the relative likelihood of a nose gear failure in failures per 1000 aircraft-years:

Model _________________________ 6A __ 7A __ 8A __ 9A
# leg fails per 1000 aircraft-years __ 3.5 _ 5.7 __ 3.3 __ 3.9

Thus, on the face of it, the 6A, 8A and 9A are very similar but the 7A is apparently more susceptible to leg failure. A word of warning though. A couple of accidents either way would change the picture significantly.

So, does this invalidate my model? At the overall level of lack of suspension travel being the issue, I think the model remains intact. However, it apparently lacks the ability to predict the greater likelihood of failure in the 7A unless the 7A generally has a heavier nose load than the others. If anyone has access to general information on nose loading for the different models, I'd love to know about it.

Raiz

Nose gear failures much more likely in the UK
 

Now here's one to get you thinking. The equivalent anaysis of leg failure rates for the UK Vans fleet shows they are much more likely to suffer nose leg failure than the US fleet:

Model ___________________________ 6A __ 7A __ 8A __ 9A
US # failures per 1000 aircraft-years _ 3.5 __ 5.7 _ 3.3 __ 3.9
UK # failures per 1000 aircraft-years __ 28 __ 44 __ N/A __31

The N/A is because there have been no nose leg failures in the UK on 8As (there are only 2 flying).

The rate of nose leg failures is about 8 times higher (across all models) in the UK than the US. Why? If it were piloting technique, we would expect to see a higher accident rate for the fleet overall but this is not the case. The # accidents per 1000 aircraft-years for all 6, 7, 8 and 9 (TD and nose-gear) combined for all accidents except nose leg failures is 6.6 for the UK and 8.0 for the US.

The only thing that comes to mind is that a much larger proportion of UK RVs are operated from grass strips.

Raiz

Great work, one should take care when on a grass runway.

Raiz, I had a -9a the second in the UK I think which I operated off grass in N Yorkshire. I sold it because I saw an accident waiting to happen when on soft grass.

The point I wanted to add, after having not seen it in the previous 80 odd posts is that soft grass compounds the problem that your analysis highlights. As the weight of the plane settles on the grass it moves the mains back a little. The result being full up elevator is unable to hold the nose off very long after touch down. Since the mains are now further aft than normal the loading on the nose wheel is further increased which only exacerbates the situation which you have analysed.

I suspect it is not an insignificant increase though.

With its shorter main gear legs the -6a is less susceptible to this.

Soft ground
 

Steve I think you make a very good point. The soft ground not only moves the rears back but it also creates a braking effect on the mains, which also tends to load up the nose gear even more.

Soft ground also plays a role on the nose wheel loads directly, because it increases the rearward component of the load on the nose leg, encouraging it to fold under.

Ray

Maybe a stupid question BUT what are the efects after landing and the flaps are quickly removed and the stick in an aft pos, does that not lift the nose weight better than all the way on the rough runway with flaps down.

Thsis a a great concern to me having made my choice on a RV7A and now this reading.

Here in SA some of the strips are very much called a bush strips.

Agent Cooper,

I'm not sure it would make that much difference - once you're on the ground the balance of forces is now about the main wheels, rather than the CofG. Also the ground attitude is at a low angle of attack, so the lift produced will be low, and the speed is dropping. I find that landing on bumpy strips the inertia effects predominate - the aeroplane pitches and I bounce over bumps without the elevator authority to do much about it, especially with an aft cofg. I try to minimize the nose wheel load and lower it to the ground smoothly before I run out of elevator authority.

Pete

I mostly operate out of a short grass strip and always immediately raise the flaps at touchdown. Although I have not done accurate testing, my seat of the pants impression is that raising the flaps allows the nose wheel to to kept off the ground to a slower speed, allows more control when lowering the nose to the ground and helps keep the nose in the air while applying brakes.

Opinions vary but I find average Australian grass and gravel strips OK in my 9A. I would keep away from soft, badly potholed and strips with short/sharp undulations.
Read all the suggestions on correct pilot technique and modifications and incorporate those you think are sensible. One extra modification I did that is rarely discussed is to alter the nose fairing cone to increase the fairing clearance fwd of the tire.

Fin
9A

I just saw this for the first time today. I have a question about the suspension value for the nose tire.

Does that imply that the tire compresses until the bottom rim almost touches the ground? If so, where does the tire go? In other words, does the tire bulge outward possibly contacting the wheel pant? If so, what does that do to the scenario?

I also saw three psi values: 25, 30, 35. I assume that is the nose tire pressure. Interesting that you seem to get less suspension as the psi goes up. I recently upped my nose tire pressure to 40 psi thinking that was the way to go. True or not on a 6A?

Interesting......
 

Ron, that is an interesting thought!

I let the air out of my front tire just to check that it would not catch on the fairing, if I would ever encounter a "flat" during flight or landing. But..... what you are saying is something else!..... if the tire is flattened by the load of the gear, it will not just be "flat" but it will bulge out!... a lot! And then.... it will catch on the fairing for sure! (unless you have more than the additional clearance that I already have, I guess)

I will be following this discussion!

Regards, Tonny.

Another RV Nosewheel
 

I am curious why the RV-12 has not been considered in this discussion. Is it due to the fact that the nose wheel tire is a 500 X 5" just like the mains, or the fact that it has less weight to support. As far as I know, there have been no RV-12 nose gear failures or roll overs. True though that there are less than 50 flying. Also, many of the ones that are flying don't have wheel pants yet. There is a youtube video showing a landing on a pretty rough island sod strip. Can't see the nose wheel, but you can tell from the cabin oscillations that it ain't the smoothest!!
Hope the below link works.
Tom

http://www.youtube.com/watch?v=nOKvd...e=mfu_in_order