A Personal Perspective
A further, personal perspective/explanation (opinion!):
I personally consider Van’s nosegear/nosewheel design to be “marginal” for two reasons.
One is geometry. I agree with Steve that the geometry is not optimum from a load perspective although I wouldn’t go as far as saying the design is deficient. Both Steve and I would like to see an RV-10-style system or some other design on the 2-place RVs which does a better job of transferring the nosewheel load upward rather than aft. I understand that Van designed the tapered rod strut to save weight, though. An alternate geometry would probably be moderately heavier unless expensive materials are employed to keep the weight down. Certainly, Van’s existing design has proved to be generally reliable in normal use with 2,500 nosewheel RVs flying. As a result, nosegear geometry is not number 1 on my personal “marginal” list.
The main reason I personally consider Van’s nosegear design to be “marginal” is that it is very sensitive to proper adjustment of the nosewheel bearing preload (tapered bearings) and the fork bushing frictional loads.
A good example of what happens when the nosewheel bearing preload is improperly adjusted, or the bearing itself is failing for some reason, is the video posted by Mark (mcattell) on page 5 the “Anti-Splat” thread below:
http://www.vansairforce.com/community/showthread.php?t=78237&page=5
Mark shot the video to show the effect of the Anti-Splat “Nose Job.” The reason why the video works so well in showing the Nose Job’s action is because the nosewheel itself is seriously dragging most likely due to a wheel bearing problem. (This is just my personal opinion from watching the video. I think Allan Nimmo of Anti-Splat came to the same conclusion. Pete Hunt noted that the nosewheel could be a badly out-of-balance as well.) Maybe Mark will post his post-flight inspection/test results.
Mark purposely landed on a smooth runway as his initial test (start smooth and progressively go rougher to see what happens . . . a good idea!). What we see is that a dragging (or seriously out-of-balance) nosewheel is fairly good at simulating a rough runway. In other words, Mark actually started “rough” although he probably did not intend to do so. We are all indebted to Mark and the other RVers who have recorded and posted nosegear videos to show the nosewheel/nosegear dynamic action. Thanks, Mark!
Numerous solutions to the tapered wheel bearing problem have been employed: adjustable axles, spacers, aftermarket wheels/axles, and replacement with ball bearings (what Allan/Anti-Splat is suggesting). The goal of all of these solutions is to properly set the bearing preload to hold the nosewheel firmly in-place relative to the axle while allowing the wheel to rotate freely with a slight amount of drag to help counteract shimmy and check by feel that the tapered bearing’s integrated rubber seals are not rotating with the wheel. The slight drag is usually set to allow the wheel to rotate a few turns (3-to-4 turns seems to be good) after the wheel is spun at slow speed by hand. I personally don’t know if anyone has found an accurate, repeatable, and quantifiable way to set the nosewheel drag statically, because the required breakout force is very low. If anyone out there has a good, quantifiable method, please post or refer us to the proper thread; Thanks!
As Alex Peterson has posted in this thread (see “Nose wheel bearing – elephant in the room?”), the sensitive thrust bearings, affected by preload, axle configuration/adjustment, and possibly yoke and axle distortion/loads, may be the main problem with Van’s nosegear design; not the strut. I would personally agree with Alex. Certainly, the strut needs help with rough surfaces (Anti-Splat “Nose Job”), but the nosewheel can certainly generate a rough ride all by itself. Heaven help the RV nosewheel pilot who attempts to land on a rough surface with a seriously-dragging or out-of-balance nosewheel!
To set the fork bushing frictional loads (anti-shimmy), Van’s specifies that the strut nut should be torqued and cotter-pinned to set the normal force on the bearing surfaces (normally the bottom of the fork bushing and top of the upper Belleville washer) such that a lateral, static breakout force of 22 pounds is maintained at the wheel axle.
Watching a 9A land, I saw what happens when the lateral breakout force has become too low. The nosewheel/fork rapidly oscillates around the nosegear strut; banging stop-to-stop; with wheelpant damage! Unfortunately, I didn’t have a video camera to record what happened. Certainly, the video would have been a strong motivator for frequently checking the breakout force!
Fortunately, the 9A pilot realized what was happening (he inadvertently touched the nosewheel first), performed a go-around, and then executed one of the nicest soft-field-style, nose-high, slow landings I have ever seen! That 9A is impressive, and increased pilot awareness and motivation works! The nosewheel did shimmy again, but wasn’t banging stop-to-stop. Afterward, we checked his nosewheel. As I recall, the lateral breakout force at the axle was less than 2 pounds. The wheel bearing preload was low enough that nosewheel would spin freely by hand with no drag at all.
After witnessing the 9A incident, I’m a firm believer in frequent breakout force checks!
In summary, I’m “marginal” on Van’s 2-place nosegear design due mainly to the sensitive, frequent adjustments required to maintain the proper wheel bearing preload and fork bushing frictional loads. The strut could be better, too. It’s hard to argue with Van’s success, though. Van’s Aircraft really “took-off” as a business with the introduction of the RV-6A.
My Personal Bottom Lines:
1. I purchased Van’s new fork and shortened my nosegear strut by an inch.
2. I purchased the Matco adjustable axle to accurately set the wheel bearing preload. I think the Grove and Beringer options are nice, but more expensive. Allan Nimmo (Anti-Splat) thinks the stock tapered bearings should be replaced by ball bearings; maybe Allan will post an explanation. I’ll listen, but the tapered bearings seem fine to me as long as they are preloaded properly.
3. I will religiously set and monitor both the nosewheel drag (wheel bearing preload) and the axle breakout force (fork bushing frictional load); frequently!
4. To provide some margin for ham-fisted moments (extremely rare, of course!) or a rougher-than-expected surface (more likely . . . I would like to try some grass runways), I’m going to purchase and install Anti-Splat’s “Nose Job.”
5. I’m going to set my nose tire’s pressure at 45-50 pounds and periodically balance my nosewheel/tire combination.
6. I’m trying to limit the maximum weight on my 8A’s nosewheel to about 300 pounds.
Given the above, I am quite confident that I will have a very robust, benign nosegear system that I don’t need to worry about on any reasonable paved surface. I am also confident that I will have a practical, safe margin for “somewhat rough” runways (paved, gravel, or grass with no big potholes, bumps, or dips) as long as my soft-field proficiency is good. Rough, uncertain strips with rocks, undulations, bushes, holes, mounds, and that sort of thing? Those are for the tailwheel folks to try!
Well, that’s it for me. Thanks to all the great posters on these forums and the efforts of Van’s, Anti-Splat, Grove, Beringer, Matco, etc. who have enabled us nosewheel folks to reach a reasonable understanding of, and have confident control of, Van’s nosegear/nosewheel system. Again, Happy Holidays to All!
Bill Palmer
RV-8A Finishing Kit
