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Re: Corked Nose Wheel
Posted: Sun Jun 09, 2019 11:27 am
by greg
Pilot55 wrote: ↑Fri Mar 29, 2019 8:09 pm
yes i can. I make them with glass filled resin. PM me and let me know you want one.
Thanks
Bret
Can you make them with depleted uranium instead?
I might be able to solve my rear C of G problem at the same time!
Sadly, I doubt I'll be able install a fin on my bird; I don't think our regulator will take kindly to such a modification.
Re: Corked Nose Wheel
Posted: Tue Jun 11, 2019 7:19 pm
by Rich
For the longest time I've been looking for the cause of the nosewheel not being stable in a position more aligned with the axis of the plane and I think I've found the main culprit - namely the fact that the lower end of the nosewheel pivot axis angles rearward. By my measurement, with the plane sitting on the gear, it's about 5 degrees. When the weight is off the gear after takeoff, the arm pivots downward and this angle actually increases.
Here is the side view of the nosewheel:
Since almost all the weight (certainly the CG) of the nosewheel arm, wheel, and even the fairing is
above the rotation of the axis, the structure is unstable. I simulated this with a hinge and vice-grips set at a 5-degree angle from vertical. Here it is sitting in a line:
Looks OK, right? But disturb it even a little bit and the whole shebang flops around until something stops it. It can go to either side, depending on the disturbance. And remember it's actually also moving downward, due to the angle. Here's where it went of it's own accord:
Depending on how the preload (i.e. friction) has been set, it takes more or less force to get it moving, but from there it keeps going until it either hits the stop on that side or there's enough relative airflow force to arrest it. Once it sets up, any correcting airflow force has to produce enough lateral force to actually lift the assembly, rotating it back in place
and overcome the preload. Clearly the stock fairing isn't up to this.
Re: Corked Nose Wheel
Posted: Tue Jun 11, 2019 8:13 pm
by Steve
Rich:
The degree to which the pivot axis angles rearward is dependent on the height of the shock absorber stack. Mine is pretty much vertical with weight on, but it does indeed pivot rearward with weight off the gear. Check out this photo with the nose gear off from 2015:
The blue tape is to protect the paint from overspray prior to doing the dye penetrant inspection for nose gear cracks.
Steve
Re: Corked Nose Wheel
Posted: Wed Jun 12, 2019 2:30 am
by BRS
Rich, That's a very good observation. After all these years, yours is the first post that I've seen to point this out. Probably in climb the angle is not such an issue but just after takeoff and accelerating from a slow lift-off then pitching the nose down to gain speed in ground affect it seems that it would, apart for the slipstream, want to flop to one side or the other.
Re: Corked Nose Wheel
Posted: Wed Jun 12, 2019 6:17 am
by Don
Rich wrote: ↑Tue Jun 11, 2019 7:19 pm
For the longest time I've been looking for the cause of the nosewheel not being stable in a position more aligned with the axis of the plane and I think I've found the main culprit - namely the fact that the lower end of the nosewheel pivot axis angles rearward. By my measurement, with the plane sitting on the gear, it's about 5 degrees. When the weight is off the gear after takeoff, the arm pivots downward and this angle actually increases.
Here is the side view of the nosewheel:
Nosewheel.jpg
Since almost all the weight (certainly the CG) of the nosewheel arm, wheel, and even the fairing is
above the rotation of the axis, the structure is unstable. I simulated this with a hinge and vice-grips set at a 5-degree angle from vertical. Here it is sitting in a line:
Sim1.jpg
Looks OK, right? But disturb it even a little bit and the whole shebang flops around until something stops it. It can go to either side, depending on the disturbance. And remember it's actually also moving downward, due to the angle. Here's where it went of it's own accord:
Sim2.jpg
Depending on how the preload (i.e. friction) has been set, it takes more or less force to get it moving, but from there it keeps going until it either hits the stop on that side or there's enough relative airflow force to arrest it. Once it sets up, any correcting airflow force has to produce enough lateral force to actually lift the assembly, rotating it back in place
and overcome the preload. Clearly the stock fairing isn't up to this.
Bingo! I think you are right on Rich. Excellent analysis and proof of concept photos.
A few years back, I was videoing my wife dong six touch and goes because she had not flown in a while. Thinking about what you said above , I went back and studied this video. I believe it supports your theory. Each approach , landing, then ground roll and rotation is uncut so one can study what the gear is doing the whole time when you maximize the screen size on you tube. Right each of the six rotations, the very back of the nose gear pant has immediately rotated 3 to 4 inches towards the right side of the plane. Also, the wind was pretty much straight down the runway that day so there was very little crosswind factor to affect the noise wheel alignment.
https://www.youtube.com/watch?v=qe0P4PSJRPk
Re: Corked Nose Wheel
Posted: Wed Jun 12, 2019 3:44 pm
by BRS
Seems some sort of 'V' notch with a ball-bearing and a spring would be able to center the nose wheel. I think this sort of thing is done on some tail-wheel'd acft.
Re: Corked Nose Wheel
Posted: Wed Jun 12, 2019 5:00 pm
by Roxi5m9
Bret,
Any progress on drafting one for later models? 2004 and newer?
Re: Corked Nose Wheel
Posted: Wed Jun 12, 2019 5:21 pm
by Rich
Roxi5m9 wrote: ↑Wed Jun 12, 2019 5:00 pm
Bret,
Any progress on drafting one for later models? 2004 and newer?
No good deed goes unpunished, eh?
I would guess uncertainties lie mostly with the "speed gear" and possibly the NG (there is an OSB for nose fairing replacement in conjunction with different versions of the nose gear).
Re: Corked Nose Wheel
Posted: Thu Jun 13, 2019 1:08 pm
by Roxi5m9
Rich wrote: ↑Wed Jun 12, 2019 5:21 pm
Roxi5m9 wrote: ↑Wed Jun 12, 2019 5:00 pm
Bret,
Any progress on drafting one for later models? 2004 and newer?
No good deed goes unpunished, eh?
I would guess uncertainties lie mostly with the "speed gear" and possibly the NG (there is an OSB for nose fairing replacement in conjunction with different versions of the nose gear).
Haha. For sure. Just excited and hopeful. Bret sent me a fin for a trial run on my 2004 and it was quite a bit different in curvature of the fairing and mounting screw placement. I would have offered up my fairing for drafting but I have a partner in the plane and we fly a lot and didn’t want to go without it for several long cross countries. So I’ve been hoping he found an available one located close to him. But I will say what he has so far is a solid design and looks sharp.
Re: Corked Nose Wheel
Posted: Thu Jun 13, 2019 1:32 pm
by Pilot55
Nick, I am trying to locate a fairing to laser scan. I thought I had one but not yet. As soon as I can scan the 3d profile I will model the fin on that wheel pant and do CFD check and make up a mold. If I can get a wheel pant I can tooling ready within a week.
Thanks
Bret