Landing gear adjustment

How to keep the Cessna 170 flying and airworthy.

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Re: Landing gear adjustment

Postby blueldr » Mon Feb 24, 2014 7:47 pm

Thats a very long but very complete and accurate writeup. Cutting corners on that job is just asking for trouble. A nice smooth and perfectly level hangar floor makes the job much easier too.
One of the slickest tools I've ever seen for this job was a pair of ball bearing bar stool rotators used instead of grease plates under the wheels. They were mounted between the two metal plates instead of using grease.
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Re: Landing gear adjustment

Postby n2582d » Mon Feb 24, 2014 11:04 pm

Lopez wrote: The installation should have the shim(s) pushed inboard as far as possible and then the wedged tapped in to where it gets tight. DO NOT use the bolts to draw the wedge in to place, they are there just to hold the wedge once it is installed. I usually put the bolt in place just to maintain alignment of the wedge/shim while I tap the wedge into place on either side of the bolt. Once it is seated, I tighten the bolt to the appropriate torque spec.
Nice write up. Should find a place in the Maintenance Library for it.

MAF used a longer bolt with a spring between the bolt head and the wedge to keep the wedge from coming loose.
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Re: Landing gear adjustment

Postby Lopez » Tue Feb 25, 2014 5:25 am

blueldr wrote:Thats a very long but very complete and accurate writeup. Cutting corners on that job is just asking for trouble. A nice smooth and perfectly level hangar floor makes the job much easier too.
One of the slickest tools I've ever seen for this job was a pair of ball bearing bar stool rotators used instead of grease plates under the wheels. They were mounted between the two metal plates instead of using grease.


Many moons ago, when my dad and I installed the pponk gear kit in his airplane, he built a set of ball bearing grease plates. They worked TOO well. The damn airplane kept rolling downhill. We use 18" square 3/16" steel plates, way heavy duty and when slathered with bearing grease they slide over each other really well.
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Re: Landing gear adjustment

Postby minton » Mon Mar 03, 2014 1:56 am

You shold be a "Tech" writer!!! Very good job. I'm downloading it for my A/C records
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Re: Landing gear adjustment

Postby robrien » Sun Mar 20, 2016 11:40 pm

Thought I'd post a couple images of the wear on my main wheels. The pilot's side wheel has excessive wear on the inside of the tire. I have a set of shims, and wonder which one to use? (.058, .073, .110, .126)
Attachments
IMG_1696.JPG
right gear
IMG_1696.JPG (51.47 KiB) Viewed 8301 times
IMG_1695.JPG
left gear
IMG_1695.JPG (52.24 KiB) Viewed 8301 times
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Re: Landing gear adjustment

Postby WJRem » Wed Jan 22, 2020 2:41 am

Lopez wrote:Here are the best instructions I've ever seen on alignment of Cessna tail draggers.


I've been working on these airplanes for a long time and one thing that I believe causes more ground loops than anything else (besides pilots) is bad wheel alignment. Any of us who have flown multiple 170s, 180s, or 185s have experienced different ground handling characteristics between seemingly identical aircraft. Some are just squirrelly on the ground. Whenever I fly one that is like this, I look 3 places, the first of which is the main gear alignment. Second and third, just so you don't have to ask are tailwheel steering and rudder rigging, but that's for another day. I sat down to write this so hopefully more folks can keep from wrinkling up their airplane and their ego. I hope it helps.

The first thing I do when leveling an airplane is lock the gear leg in the gear box. This is done with a pair of shim/wedge assemblies on top of the gear at the outer gear leg bracket. These 2 wedges are loose on about 20-25% of the airplanes that I work on. You'll notice them being loose when your taxiing around and you hear a clunking noise coming from the gearbox. Sometimes you can hear it after lift off with an unbalanced tire/wheel shaking the gear leg as they slow down. The way to check for a loose gear leg is to have your helper shine a light from outside UNDER the gear leg into the gear box. You take a mirror and look UNDER the gearleg from inside the gearbox. If you see light coming through between the gear leg and the outer bracket, it’s not tight enough.

To tighten the gear leg in the bracket, you need to lift that side of the airplane somewhere besides the gear leg since you need the gear leg dangling in the breeze. There are a few ways to do this. You can use a cherry picker and lift on one side of the engine mount where it meets the firewall, don’t use a chain, you’ll screw up your engine mount and who knows what else. The other easy way is a screwjack/stiffleg under the wing just outside of the strut. I suppose you could use the float lifting rings if you have them as well.

Once the gear leg is dangling in the breeze, you can tighten it up. This is done by tapping the wedges in a touch further or adding shims. You can only push the wedges in so far before a thicker or additional shim is needed. You don’t want the shim riding up into the radius of the bend in the wedge. If it gets this far, take it apart and add a thicker or second shim. The installation should have the shim(s) pushed inboard as far as possible and then the wedged tapped in to where it gets tight. DO NOT use the bolts to draw the wedge in to place, they are there just to hold the wedge once it is installed. I usually put the bolt in place just to maintain alignment of the wedge/shim while I tap the wedge into place on either side of the bolt. Once it is seated, I tighten the bolt to the appropriate torque spec.

A couple notes on this step.

You don’t pound the wedge in with a 2 lb. sledge hammer and your biggest swing. I use my smallest ball peen and a brass punch with medium tap taps. When it seats, you’ll feel it get solid.

The gear leg probably has some room to pivot fore and aft in the slot of the outer bracket. One of the most reputable shops in the rebuild business usually sets the gear at the front limit of the travel. I have no idea why or why not. I like to set it at the aft end for 2 reasons. The first is the majority of the loads the gear sees will be up (landing) or aft (hitting bumps and braking). If the gear is already as far aft as it can go, these forces won’t move it. The second reason I like to set it at the aft end is, if the gear moves, it will move forward and change the alignment to more toe in. Toe out is bad news and unexpected toe out is real bad news. Toe in leads to tire wear, toe out leads to insurance claims.

The shims that you use with the wedge should be steel. I like to use 4130 since I usually have that around the hangar. I’ve heard that the ones you buy from Cessna are aluminum, but I don’t know because I usually don’t buy what I can make in 5-10 minutes. I’ve removed aluminum shims from a loose gear box and found them deformed and smashed, so I prefer steel.

On to leveling...

Now that both gear legs are locked in place in the gear box, we can level the airframe. Cessna specifies 3” as the max difference between the left and right wing tips. With a little effort, you can get much closer than that.

The first thing you do is level the axles. 99% of the hangar floors out there are not level, so it’s worthless to use that as a reference. To do this, I start with rolling both mains on to a pair of grease plates. You may or may not need to do this until you start aligning, but I’m a picky bastard and it can’t hurt.

I am quite fond of a water level. This can be crafted at your local home improvement store (blue or orange store, it doesn’t matter unless you have a preference who is going to frustrate you that particular day). Buy yourself 30 ft of 1/2-3/4” ID clear plastic hose and fill it with water. The beauty of water is it is damn near always level when left alone. Accuracy can be improved with the use of “Organic Leveling Water”, but it is hard to find a good source and I’m not sharing mine.

Take the ends of the hose and hold them up by the inside of the gear legs where the axle bolts are. Using a matching pair of left and right bolts (top front, bottom rear, it doesn’t matter, just use the same on both sides) in comparison to the water in the hose, you can see which side is higher. Pick the high side and let some air out of the tire. Keep bleeding air until the two bolts you picked are level according to your water level. Check your water level for kinks because they can throw a wrench in the works. Then grab a strut and shake the airplane side to side 3-4 times to make sure it is settled. Now check the axles and make sure they are level again. Once you are confident that they are level, we can move on.

Move the ends of the hose to the tie down rings at each strut. You can wire them in place if you are working alone. Measure from the bottom of the wing to the water level in the hose on both sides. If you want to be real picky, you can buy 40-50 feet of tubing and do this same trick at the wingtips, but unless your wings are tweaked, we can assume that they are straight between the strut and the tip, right?

Now that you’ve measured between the bottom of the wing and the water level, measure again. Once you are confident that your numbers are right, you can decide which wing needs to be raised, if any. Whichever side has a smaller measurement is lower, so we need to raise it. We raise it by adding shims under the inboard end of the gear leg. I don’t remember how much shim will lead to how much change at the tip/strut so I just experiment. This is another shim I prefer to make out of steel. I usually have some .025” and .040” 4130 around, so I just cut it in the shape of the end of the gear leg, drill a 7/16” hole, and slide it into place. Then check the tightness of the outer wedges again. Set the airplane down and repeat everything above. Repeat until the measurements are within whatever tolerance you set for yourself. If you’re happy with Cessna’s 3” at the tip, go for it. My dad taught me to fly and he was a hardass, so if I couldn’t maintain 1/2 of PTS standards, I did it again. I guess that carries over to my wrenching most days. The 170B I did last night I had within 1/16” at the strut. For what it’s worth, most airplanes I work on are pretty close with NO shims. Unless 1 gear leg has been changed somewhere along the line, they tend to be equally sprung.

Now that the airframe is level in relation to the axles, we can align it. Personally I like to load the airplane to whatever the typical load is for that particular user. For my airplanes, I load them with full tanks and 2 people up front. This is a good middle ground for my operations. Some folks say the change in toe is so slight that loading it is unnecessary so last night I checked the alignment on a 170B with 180 legs before and after I loaded it with 400 lbs in the front seats. The toe on each gear leg changed around 1/16” so a total of 1/8” difference loaded vs. unloaded. Factor in 240 lbs of gas and the change could have been even more. If I recall correctly, the Cessna Manual even states that loading it to “typical” weight is necessary prior to alignment.

So now it is loaded to a normal operating weight and back on the grease plates. Give the strut a couple of good shakes to let the gear “settle” in to a happy place. Now take an 8’ straightedge and set it on a couple of coffee cans in front of the main gear tires. You want the straightedge to touch the tires. Take your friendly square and place it up against the straightedge just inboard of the tires. The perpendicular edge should be placed against the brake disc. It may (probably will) touch either the front or aft edge of the disc.

It should be noted here, or maybe earlier, that the wheel bearings must be snug and the brake discs should have minimal runout. It either is in questions, I’ll address that when the wheel is in the air back during the tightening of the gear leg in the box.

Make note of where there is a gap between the edge of the square and the disc. If the gap is behind the axle, the wheel is toed in. If the gap is ahead of the axle, the wheel is toed out and the aircraft has likely scared you at some point. Now that you’ve noted the toe, take a look at the camber. The camber is a secondary priority to toe for me when I set up an airplane, but you can usually get it very close to book specs as well. Now that you have an idea what needs to be changed (or not messed with), you can adjust as necessary. Between the axle and the gear leg are the shims for adjusting the toe/camber. They are tapered on a diagonal so they adjust toe and camber at the same time. You may need to simply adjust the position of one of the existing shims, or change or add shims. I think the Cessna manual allows for up to 3 shims, but you rarely need more than 2. My manual is at the hangar as I write this, so forgive the ambiguity in referencing it. Now the manuals (parts and service) both give a real neat chart that tells which part number shim affects camber and toe and in what way. I’ve never found this chart to be useful because I have never had a new shim with a part number on it so I could tell which one I had in my hand. I’ve had much better luck looking at what change needs to be made and then saying “I need more camber and a touch of toe in, which of these shims in what position will give me that?” Then I bolt it together and repeat the measuring step. You may need several tries to find the sweet spot. Once you find it, take it all apart and replace the axle bolts and nuts. Since they have probably been in there since Clyde put it all together back in Wichita.

One note on the axle bolts, The 170 and 180 originally specified AN5 and AN6 bolts, with NAS145 and NAS146 as optional for ski applications. The NAS are significantly stronger, but I’m not sure you need it. Cleveland’s double puck, 6 bolt wheel STC allows for the NAS bolts on 185s to be replaced with AN bolts. Personally, I think on the average airplane is probably fine with AN bolts, and since the NAS bolts are spendy (~$20 each), plus you need the special washers, most airplanes seem to end up with AN. Now you know, make your own decision.

Now Cessna lists the toe in spec as 0-.12” per wheel measure at the wheel flange, which is pretty close to the diameter of the disc, so I like the disc. Like I said before, I like to get it down to half that, so 0-.06”. If I have to settle with the camber at the extremes of the spec, but get the toe in nailed, I’ll take that over the other way around.

Once you get it all bolted back together, you’ll notice immediately that the airplane is easier to push in and out of the hangar. It will taxi with less power and your tires will last longer. It will be easier to handle on takeoff and landing roll and be less likely to jump up and bite you, but don’t get too comfortable...
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Re: Landing gear adjustment

Postby DaveF » Wed Jan 22, 2020 2:05 pm

Yep, Tony knows what he’s talking about.
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Re: Landing gear adjustment

Postby GAHorn » Wed Jan 22, 2020 3:29 pm

If you find you need to shim the inner/upper ends of your gear legs you don’t need to make thsoe shims. (Tony likes making things.)
Cessna recommends the use of AN960 (flat) washers. Thin washers (AN960L) can be used for minute adjustments.
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