Nose gear Theory..

[AlexPeterson]

As I've written in previous posts, I have concerns with the current nose wheel bearing/axle setup. My understanding of the current nose wheel setup is that there are two axle adapters, which are against the roller bearings. The bearing preload is determined by how tight the bolt is/isn't.

It would seem that this system is not overly rigid. Under loads, the bolt will attempt to flex somewhat. When this flexing occurs, there is an increase in the bearing load on the load side, and an unloading on the opposite side. The deflections could be very, very small, yet still have a large effect.

Is it possible that runaway bearing drag is contributing to the nose-over incidents? It is not hard to imagine that there is a whole range of states of bearing pre-load out there, from sloppy to tight.

There are two things which could contribute positive feedback (forces which feed on themselves) in this system. One is that as the nose wheel hits a bump, the axle bolt flexes, increasing the rolling drag of the bearings. This in turn pushes the nose gear aft, which tends to increase the load even more and so forth and so on (the pole vault). Of course, once into pole vault mode, it is up and over.

I will work on some range finding force estimates over the next days.

BTW, my nose wheel bearing setup seems to be an anomaly, received in about 1997 or so. Here is a picture of the basic components:



Sorry about the crummy cell phone picture, but you can see that there is a large, about 1" diameter, hollow bar which goes all the way through. The bearing retainers are simply hollow cylinders. The lengths of the various parts needs to be correct for bearing preload, or shims need be used to set the preload. The important feature is that the bolt is torqued to nominal values, and the stack becomes very rigid compared to just a bolt. Does anyone else have this setup? It is simply what came from Van's with my kit.

I would welcome others' thoughts about this possible explanation for nose gear problems. Like many others, I was very, very humbled by that video of the 7A in England.

Something just does not make sense, perhaps this could explain it?

As an aside, I do not believe that a wheel fairing would have had any effect on the outcome, as once things have deflected enough for the nut to drag, it would seem to be all over.

Due to the serious nature of this, it would be very helpful to this thread if we could keep it on the engineering topic discussed above - thanks.

[Mike S]

Alex, your setup is just about exactly what is supplied in a -10.

Mike

[plehrke]

The nose wheel set up that came with my RV-6A quickbuilt in 1998 was as you orginally described with the axial bolt torgue setting the bearing pre-load. After 5-10 hours of flight I noticed that the bearing adapter/spacers (do not have my drawing here to have the part number) provided where rotating in the fork. I decided to modify the set-up to include a clamp up bushing I made so the bearing pre-load is set by the bushing length and the axial bolt can be torqued so nothing spins but the bearing in the bearing race. Took one or two tries to make the bushing the correct length but started long and trimmed until the bearing pre-load was what I wanted. This is how all bearing joints should be designed to use clamp-up to ensure the roatation is on the bearing.

I agree with you that there is still a lot unknow about why the flip over happened in England (along with others). I agree that the bearing load could contribute as it can create a drag force. I am also not convinced that the nose gear fearing has much effect (but that is another thread).

[AlexPeterson]

Philip,

Thanks for the reminder about the axle adapters rotating - that might be a very important piece of the puzzle. I recall another lengthy thread about that issue, but I don't think it was implicated as a possible cause of front wheel lockup.

[n5lp]

Originally Posted by AlexPeterson

Philip,

Thanks for the reminder about the axle adapters rotating - that might be a very important piece of the puzzle. I recall another lengthy thread about that issue...

The Roberta Hegy staking mod. Here is one of the threads http://www.vansairforce.com/communit...hlight=staking

[Finley Atherton]

Tightening the axle nut to the correct torque on the current style nose wheel bearing system to obtain the correct bearing preload seems to be a rather imprecise method to me. The legs on my nose fork (new style) had to be forcibly sprung open somewhat to allow the wheel assembly to slide into position. Due to the spring-back, clamping action of the fork, there was considerable rotational resistance on the wheel without the nut even installed. Tightening the nut up to the specified torque increased the rotational drag even more. Maybe this is how it was designed so that the wheel assembly becomes a more snug fit between the fork legs once the bearings bed in. What have others experienced here? Do the legs of the fork have to be forced apart to allow the wheel assembly to slide into place?

Fin 9A

[plehrke]

Originally Posted by Finley Atherton

Do the legs of the fork have to be forced apart to allow the wheel assembly to slide into place?
Fin 9A

Mine had extra clearance (maybe 0.06") and therefore the axial had to be tightened just to spring them together. The fork is a weldment and therefore will never be very percise (that whole heat thing). That is why I made a clamp-up bushing to pull together the forks, torque the nut, and still not over load the bearings.

[GrayHawk]

Here is my nose gear summary list:

PREVENTATIVE:
1. Immediately upgrade to the new nose wheel fork (I already have it)
2. Pull Out set to 25#
3. Nose tire pressure at 35psi (set for pavement)
4. Stake 'biscuits' to nose wheel fork (ala Roberta & others)
5. Take off & land from smooth paved surfaces (my personal choice for now, as a low hours RV-A pilot)
6. Treat every takeoff & landing as soft field (nose on runway ONLY if there is not enough elevator authority & unless there is a good reason for otherwise)
7. Correctly preload the nose wheel bearings
8. Land as slow as I can without stalling
9. Minimize use of brakes
10. Any screw up like appreciable bounce on main gear is a GO AROUND
11. Make sure wheel pant is solidly mounted with thumb+ clearance all the wall around (fiberglass to tire)

CORRECTIVE:
1. Consider either a Vern Little or Jock Strap skid design. Right now the Vern Little looks easier to implement and may have the advantage of holding the nose cone together better. Possibly also do the modified clearance wheel pants at the same time.

PERSONAL:
If, after doing all of the above to the best of my ability, bad luck still hits; that's all it is, bad luck & an accident. It's no one elses fault & for that matter, not really my fault.

[Mike S]

Here is a link to a post I put up a while back, although this is for a 10, the issue is the same, and the parts are vary similar.

http://www.vansairforce.com/communit...ad.php?t=17878

Mike

[Leland]

Originally Posted by GrayHawk

8. Land as slow as I can

Last Thanksgiving I landed too slow at an airport I was unfamiliar with. My -9A stalled a foot or so above the runway, hit hard on the mains, and then immediately banged the nosewheel so that it "twanged". My best landings are done coming over the fence at 60 knots and rounding out until the mains kiss the runway, holding the nose off until elevator authority is nearly gone, and then lowering it. Full stall landings are not the way to go here.
Leland