Setting the MLG Width

[jaredyates]

I'm to the point of adjusting the threads on the rod end bearing of the shock strut. I have in my notes that I'm supposed to set it to 74" center to center at gross weight. The trouble is that I'm working with an empty airplane. It seems overly complicated to load the airplane up to 2500 pounds, especially considering that I'm going to have to take the weight off of one wheel at a time to make the adjustment. I'd rather be jacking up/propping up with as little weight a possible. How is everyone else doing it?

[Enewton57]

Hi Jared, I set mine up according to drawing #23 where it says the axels should be level and 72" center-to-center with the shock strut extended 1 3/8". To hold the shock strut at 1 3/8" I removed the springs and made some spacers out of 7/8" (I think) tubing. That said, I was talking with Bob the other day and he asked me if I had set the shock struts on my Patrol. BTW - the Patrol and the Bearhawk both have 72" as the gross weight spread from center of tread to center of tread. Anyway, Bob said to get all the weight off the landing gear and let them hang down. Now set the spacing at 67" center of tread to center of tread. Bob wasn't quite sure of the distance but thinks that would be just about right. Now thats for the Patrol with a gross weight of 2000 lbs. I am going to try it that way and see what happens with the airplane fully loaded to gross. If works out, I will post it here. One other option is to use some bags of concrete or sand and load up your airplane to gross. I used bags of concrete for my flight testing program on the Bearhawk while under phase 1. See pictures here: http://mybearhawk.com/flying/25hours.html

[X'N]

Plastic tubs, buckets, garbage cans filled with water is an easy way to add weight too.

[Mark Goldberg]

Jared, I believe the guidance is "not to exceed 74" at gross". A little less would be better. FYI - there is no difference in ground handling (that I could feel) with my BH set up for 69"-71" with just me in it. The nose is certainly higher with the gear in tighter. But no ground handling difference. You will probably have to make an adjustment or two as you get gas in the tanks and all the weight that is associated with flight. Mark

[jaredyates]

Thanks Guys, I also spoke with Bob to see what he thought. He mentioned that 68" with the gear unloaded would be a good starting point. When I put the shock struts on I threaded them all the way in (with a stop nut) so I'm a little narrower at 65 or so. I was able to get things set up to make an adjustment today but ran out of time. The temporary hangar at the municipal airport is nice, but it doesn't have any lights to speak of and the days are getting shorter. Bob recommended using a 1/2" rope tied around the axles in such a way to keep them from slipping, then a come-a-long attached to those ropes. He uses this method to be able to unhook the bottom of the shock struts (one at a time) to rotate them for adjustment. Mark, that's good advice, I'll err on the side of narrow.

[S Lathrop]

Jared:

Your question inspired me to calculate the wheel rate for the main gear. Bob's suggestion is spot on, as one would expect.

I calculated a load on the wheels of 1056 lbs. will change the track 2". This assumes a 3000 lbs. per inch spring in the strut. Or to change the track by 1" you must compress the springs by 0.21 inches each.

The other side of this calculation is the wheel rate of the gear in compression. The gear is reasonably consistent from 70 to 74 inches wide at close to 900 lbs. per inch of height change. But the rate drops to 650 at 76".

[jaredyates]

To follow up after the actual adjustment, for now I have the rod end screwed out so that about half of the threads are exposed, which os 3/4". Bob said he's comfortable with 5/8" engagement, or maybe 1/2" if the threads are really tight. He pointed out that since the threads are pitched at 20 per inch, then one can also count the turns to tell the engagement. The issue with being much narrower than required is that the gear starts toeing in at 72", and the narrower it gets, the more it toes in. I'll have at least 30 hours or so of lightweight testing where I can keep an eye on things. When it becomes time to test at max gross I'll measure to see if another adjustment is required.

[Battson]

The toe allowable is 0.0 to 0.5 a degree INWARDS, right? Obviously more toe giving you more positive handling and stopping action in the roll out. Our kit has 0.4 degrees of toe-in at basic empty weight, so the jigs are obviously quite good. Hopefully yours are the same. My other question was, you measure the width to the centreline of the WHEEL including the kit-standard wheel spacers, correct?

[marcusofcotton]

Jared:

Your question inspired me to calculate the wheel rate for the main gear. Bob's suggestion is spot on, as one would expect.

I calculated a load on the wheels of 1056 lbs. will change the track 2". This assumes a 3000 lbs. per inch spring in the strut. Or to change the track by 1" you must compress the springs by 0.21 inches each.

The other side of this calculation is the wheel rate of the gear in compression. The gear is reasonably consistent from 70 to 74 inches wide at close to 900 lbs. per inch of height change. But the rate drops to 650 at 76".

You mention assuming a 3000 lbs./inch spring. The Dayton chart shows the EH200-800 spring as 1185 lbs./inch. Hmmmm. I'm a bit rusty on geometry/math end of this problem and feeling like I need to freshen up a bit to get it through my skull.

Mark J

[Mark Goldberg]

72" tire center to tire center at normal flying weights is what Bob is looking for Jonathan. Not to exceed 74" at gross. Mark

[S Lathrop]

You mention assuming a 3000 lbs./inch spring. The Dayton chart shows the EH200-800 spring as 1185 lbs./inch. Hmmmm. I'm a bit rusty on geometry/math end of this problem and feeling like I need to freshen up a bit to get it through my skull.

Mark J

I took the spring rate from drawing #25, "Die spring 3000#". This morning I looked up the numbers for the springs and your number is correct.

The motion ratio, spring/wheel, averages 0.42 to 1 for track changes. The vertical movement ranges from 0.730 to 0.428 over 6" of vertical travel. The vertical range is for track widths of 66 to 78 inches. A spring of 1185 gives a wheel rate of 209 lbs. per inch for track movements and 361 for vertical movement. The vertical rate is only at 72" track.

I stand corrected.

[marcusofcotton]

I see where the 3000# came from the print, that's an interesting tidbit. I appreciate you filling in more of the numbers above, helpful when one might be considering subtle variations - a person could get in trouble in a hurry. Interesting how fast the load vectors change as the suspension travels or any dimensions might be modified.

Thanks!
​Mark J

[Bcone1381]

Check my thgouts on this topic please.

It seems to me like the rigging the gear has two things going on. One is the gear spread, and the second is the alignment of the wheels. The Patrol Plans P.26 call out 72" with 1.25" shock strut extension no toe in.

But in Post #2 Eric Newton said that the Patrol's gear spread between the center of the tire tread is 67 inches with no weight on the gear. I can lift my Patrol fuselage off the ground easily right so I will set it to 67" now.

Alignment... a Q1 2009 Beartracks talks about wheel alignment. Generally it applies to the Patrol, but specifically the measurements apply to what we today call the Model A four place. It says

1) put the Fuselage in 3 point attitude,
2) get the tread width to 68-70" wide center of tread to center of tread.
3) clamps angle iron to the brake discs and check the tracking of there gear 40 inches ahead of the wheel.
4) tolerances are 0 to 1/2 degree of toe in.


Does anyone know if these Beartracks numbers apply to the Patrol?
If I have it aligned with the gear at 67" spread is that good enough?
Does the toe in change as the gear spreads out?
I cant assume that spreading it just another inch is good enough, because I have a Patrol, not a Four Place.

[Bcone1381]

Bob wants the Patrol main gear to be set at 72" to check the gear alignment. To obtain 72" spread with and empty aircraft its okay to support the fuselage, remove the shock struts, and set the width of the gear.

[PaulSA]

I've just finished covering and painting the fuselage and have her back on her wheels. I'm now trying to adjust the track width (Bravo). I understand that weight off, it should be around 68" "centre of tyre to centre of tyre". Presumably, that is measured at the front of the wheel because the angle makes a large difference top or bottom, as does the size of the tyre. With only 3/4" of top rod end engaged, I currently have only about 65" measured from front centre to front centre. If I need 5/8" engaged, it doesn't seem that I'm going to be able to get anywhere near 68"

Can someone help/explain?

[AKKen07]

Hey Paul, I'm right behind you (so no help I'm afraid) but I'm hoping that while your in the midst of this process you could share how you figure it all out? I had a hell of a time getting the gear and struts into position without measuring anything this last time - far fewer places to prop up the fuselage now with the paint on...I imagine repeating rod end adjustments and reattaching struts might be a nightmare if I don't mooch someone else's procedure! If you can, thanks!

[PaulSA]

I have wound out the rod-end bearings as far as I dare - leaving 5/8" wound in. My track is now 66.5". I have discussed with Mark and he says to wait until the engine and prop are on and all the FWF is finished and then check again. If it is in the order of 69-70", then should be good to go .....