How to check landing gear alignment?

[McDaver63]

I'm in the process of getting my 52' PA22/20-135 ready for annual after my rebuild. I read some of the post about the handling of the Pacer and how much the gear alignment effects that. I wanted to find out how is the best way to check your gear alignment. I tried doing a search on the forum for this and found some related threads but not really anything that explains the procedure. Any information would be awesome. Thanks DC

[Steve Pierce]

Level the airplane and put two straight edges on each wheel going fore and aft. Measure between the two straight edges at the front and at the rear to see if you have toe in or toe out. Another way is to push and pull the airplane and see which way spreads the bungees out.

[walt.buskey]

DC, there's also a pretty good article, originally published by McFarlane Aviation, in this month's (Jan 2014) issue of Light Plane Maintenance. It mainly discusses nose wheel shimmy, but also describes the same procedure Steve mentioned as far as checking the toe angle. Walt

[SuperPacer]

59465947Here is a fixture that I built to help align a PA22-20 conversion. In this photo, we used it to "Check" alignment of converted Pacer from Bolder NV. It was most helpful when welding the front gear attach points to the Fuselage. As you can see, the bungee / Hydo-Sorbes were disconnected. When in the fixture, severail measurements can be made to verify the alignment, or not! of the gear. This fixture is available to anyone for loan, however is is heavy, so probably would be best to come pick it up!! It is probably over kill for just checking alignment, it is very good for a T/W conversion work.
Enjoy the day,

john
Utah Pacer

[Stephen]

John,

Great looking tool!

What angle is your alignment tool set?

[Pacer 24C]

Nice fixture - How much toe in does it establish? - and also it would allow to also make the very important measurement from the end of each axle to the tail for triangulation.

[SuperPacer]

HI Stephen,
We built the fixture to hold the gear when aligning the forwward gear attach "mounts" and so didn't put any "angle" into the tool. It holds the axel level and square to the centerline of the fuselage, so in this slightly "compressed" position (note the lower Hydro-Sorbe mounts are about an inch or so beloe the belly) the gear has "0" degrees toe in / out, and "0" camber. This would represent a "right after landing gear compression, assuming the bungees werent really stiff!! The original idea was not to replace the proper measuring of the postions for the attach bracket, but to simply hold the gear relatively square while making minor adjustments prior to welding (with the over all reference being the original forward Tri-Pacer mounts, now the rear Pacer mounts). As it turns out, the three PA22-20 conversions we have completed using the tool, all track srtaight down the runway and are well behaved Pacers. In the end, there is some "geometry" at work, because when its back on wheels, with bungees re-attached & "sucked up", we typically see an 1/16' or so toe in, as well as a bit of camber as the gear swings into the "home position". Your mileage may vary.
Enjoy the day,

john
Utah Pacer

[McDaver63]

Thanks for the replies. John, That's sure a nice fixture you made. Wish I had the resources and skills to make something like that. I ended up taking measurements after leveling out the plane by finding the centerline of the firewall between the engine mount to firewall attach bolts, dropping a plumb bob to the ground and marking a line. I then hung a plumb bob from the tailwheel springs and centered below the tail post tube and marked that on the ground. I stretched a string nice and tight from the firewall mark to the tail post mark and duct taped it tight to the ground. I dropped a straight edge vertical down to the ground from the forward and aft inside surface of the brake disk and made marks on the ground. I took measurements from the forward and aft disk marks to the centerline string on the ground. Both wheels are almost dead on equal measurements between for and aft marks. I think the alignment is very close. When I get back out to the airplane I will check the measurements between the axle ends and the tail post like Pacer24c suggested. Also I'll do Steve's measurement between the left and right wheels. Do you think it is OK to measure at the disk inside surface?
Thanks for your help.

[rocket]

I use a plumb bob and chalk lines on the floor. I have a couple sets of wheels with no tires on them that I can attach to lay my straight edge on. It's not rocket science to check but it is an art to correct!

Rocket

[Semper Fi]

Level the airplane and put two straight edges on each wheel going fore and aft. Measure between the two straight edges at the front and at the rear to see if you have toe in or toe out. Another way is to push and pull the airplane and see which way spreads the bungees out.

I dont quite understand what you are saying here.. can someone explain a little better exactly where the strait edges are placed.. im not understanding how to measure the toe in/out

[Gilbert Pierce]

a long straight edge clamped midway to both wheel rims right at the axel. Measure the distance between the two straight edges as they project both forward and behind the airplane. The longer the straight edges the more accurate the measurements.

If distance is not equal between the front and rear porjection then you need to drop a plumb bob and find the center of the fuselage at the front and rear and draw a line on the floor connecting theses points. Then measure to your extended straight edges to see which wheel or wheels is off and by how much.

There are other ways to do it. That is how I do it.

After you get the numbers a little high school trig will tell you how many degrees you are off. You really don't need that number. Best to have both wheels point the same direction.

Toe in is better than toe out IMO. Some people believe in in Toe-IN. I don't. Just because automobiles have toe-in doesn't mean I want it on my airplane. Personal preference. Supposedly toe-in makes it less likely to wander. As the airplane starts to turn the wheel opposite the direction of turn has more scrubbing force and wheel in the direction of the turn has less so it is self correcting. That is why cars have toe-in. Less wander.

[walt.buskey]

70659: here's my "thousand words."7699

[Semper Fi]

thank you gentleman and YES a picture is worth a thousand words!

[tloes]

The picture is exactly what I imagined. But I think the "best result" would be achieved with "tires off" (I'm imagining some "bulge" over the rim). Possibly with axles on jacks, it could be done simply with the tires deflated?

[Brian]

I always thought a little toe in was good because it keeps the bungees pulled up tight when rolling forward. Toe out would be bad since that would extend and stretch the cords. Neutral may be ideal but I would suggest a little Toe In as a starting reference, knowing the alignment will change over time and you could easily end up Toed Out, due to worn bushings, hard landings, etc, when starting perfectly neutral. With that said, I can't suggest how much Toe In, degrees wise would be good, so I'm no expert on the subject. Just an Opinion and we've all heard what Opinions are like and worth. :~)

[Semper Fi]

The picture is exactly what I imagined. But I think the "best result" would be achieved with "tires off" (I'm imagining some "bulge" over the rim). Possibly with axles on jacks, it could be done simply with the tires deflated?
yes the picture is what I imagined to, I couldnt understand how that would make an accurate measurement though because the tires are going to be out a little farther than the wheel.. so I thought what I was picturing in my mind was not correct... so now im picturing the strait edge clamped so it snuggly touches the tire sidewalls.. I will do this on top and bottom of tire since I have an extended cap on the axle right now for mounting wheel pants

[Jim]

Hi,

There's a picture above in comment #4 that shows the axels trapped on a bar. The bungees are removed and the tubes for them are hanging a little low. So, when things are reinstalled the tires will pull together a little creating some camber angle, out of plumb in the front view. This makes me think toe-in and out will be affected by the tail being raised or lowered. I'd think a tail wheel airplane would measure a different toe-in or out if the fuselage rear was raised to flying attitute if the wheels aren't plumb when viewed from the front. As the bungees wear out the camber will change a lot, start looking like the old VWs.

Should alignment on tail draggers only be done with the fuselage in flying attitude?

[bigjohnnie]

From experience, I know I like a little toe in. My tripe was a handful ( footull ? ) on almost every takeoff / landing rollout when she was toed out --- even in calm winds . Now with a little toe in, no issues. I can't tell you the exact degrees because my mechanic fixed it ..,, and I just watched.

[Semper Fi]

i dont have a strait edge right now.. and i am at the plane going to take the belly panel off and look around a bit.. i will just measure the distance between the front of the wheels and the back of the wheels and hope its not too different

[Semper Fi]

I didn't measure from the wheel I measured from the middle fatest part of the tire front left tire to front right tire and again back left tire to back right tire, the back distance was more by 1/4 to 3/8 inch, thats definitely toe-in.

[tloes]

I didn't measure from the wheel I measured from the middle fatest part of the tire front left tire to front right tire and again back left tire to back right tire, the back distance was more by 1/4 to 3/8 inch, thats definitely toe-in.

Maybe. It means that at least one side has toe-in. Hypothetically the other side could be "straight" to even "slightly toe-out" and your measurement wouldn't know the difference.

Clamping the straight edges directly to the rims and also having a "centerline of fuselage" line snapped on the floor would give you a more comprehensive picture.

Those aren't the comments of an experienced aviator or mechanic. Rather, they are the comments of an engineer who works through "what could go wrong" exercises every day. Take them or leave them (comments).

[Jim]

Hi again,

As tloes wrote, the tires aren't the best to use for this. How far out from the front and rear of things were your measurements taken?

I went back and read a little more about alignment and found out a couple of things. Cars can have either toe-in or out depending on if they're driven tires or not. Rear wheel drive cars, like our planes, have toe-in. One of the reasons is because of the tires rolling resistance. They are being pulled back by drag, friction, bearing play, braking etc. When those forces are in play the wheels are tracking nearly straight. Front wheel drive cars often have toe-out because those wheels are trying to pull ahead of the car while also pulling all the slack out of the system. Cars have a lot more play in their steering components, but our planes probably have some of the same things happening, especially pulling back during braking.

[Semper Fi]

Maybe. It means that at least one side has toe-in. Hypothetically the other side could be "straight" to even "slightly toe-out" and your measurement wouldn't know the difference.

Clamping the straight edges directly to the rims and also having a "centerline of fuselage" line snapped on the floor would give you a more comprehensive picture.

Those aren't the comments of an experienced aviator or mechanic. Rather, they are the comments of an engineer who works through "what could go wrong" exercises every day. Take them or leave them (comments).

I like engineers keep the good stuff coming. I know that it doesn't tell which wheel is in or even or out, I was just happy the distance was greater in rear not front.. im not an engineer and at least I know that tells me i may be ok in the alignment :)

I didn't plan on doing this tonight or I would have brought the straight edge I just happen to be here thought I would take a ballpark figure anyway I could.. I do appreciate all the help!

[Semper Fi]

Jim, the measurements were takin right on the tires the center of the back tire to the center of the other back, and again center of the front to the center of the other front

[Semper Fi]

This is the reason Im checking alignment, last week I noticed my bungees were sagging so I decided to put new ones on and noticed the left is not ligned up where it should be. I just opened the belly and took this picture. Something is causing the left main gear to pull forward at the bungees. I dont notice bad behavior on taxi takeoff or landing.. I wonder if someone bent the fuselage mounts to get the toe correct.. and that could explain the gear not ligning up at the bungees?

Im gonna take the gear off and look at it this weekend and see if I can see any abnormalities, and put new bungees on.

[59pacer]

My engineer introduced me to his 'grease plates' to make sure that all the side stresses are off the gear before doing alignment checks.
Get 4 pieces of flat iron sheet about 1/8" thick and about 12-15" square. Cover one piece liberally with grease and put another piece on top of that like a sandwich. Then make up another pair. Place the sandwiches in front of the main wheels and roll the a/c on to the plates. The top plates will slide on the bottom plates and the wheels move to where they want to be without the friction of the wheels on the floor. It's surprising how much the gear slides laterally when they roll onto the grease plates.
He then got out his 'straight edge' (an old PA25 spar), and blocked it up in front of the main wheels. Making sure it was the same distance forward from the axles on each side, it just required the use of squares/adjustable squares off the straight edge against the side if the tire (or rim if you have spacers and want to be a purist!) to determine toe-in/out.

[Semper Fi]

Thanks for the tips

[Steve Pierce]

You have an early gear and a late gear by looking at the fairing mounts. I would suspect that the airplane was ground looped sometime in it's life.

[Semper Fi]

You have an early gear and a late gear by looking at the fairing mounts. I would suspect that the airplane was ground looped sometime in it's life.

Thanks Steve, gonna lift the gear off the ground today and see what happens with both gear, I want to get the left gear to line up correctly, not sure if I will be able to, but going to see what it does with weight off wheels. If it still wont line up I'm going to take the bungee cords off and swing the gear and see if I can see if something is bent, I've got under it and looked it over, comparing both sides and I cant see anything standing out. I wonder if the gear mounts on the fuselage are bent?

[Stephen]

The Owner of the Raven "Pacer gear" STC, Rick, says,

To get Pacer gear perfectly aligned, "measure the distance between the bolt holes in the front gear attachments and then measure the distance between the rear attachment holes". The front two attachment holes must be slightly (a few thousands of an inch) closer together than the rear. He does this by sliding a bolt into each attachment bracket. He has a four foot caliper and measures the distance from the outside of the bolt across to the opposite side. A trammel bar with adjustable pointers might also work. Rick says this is the most critical alignment measurement to make when setting up new gear.

Toe out causes a squirrelly landing airplane!

[violinmaker]

My own invention to do this..... pretty easy to make. Go buy a dollar store laser pointer.
Get a piece of 1 inch angle iron about 10 inches long. Take a piece of tubing that the laser will just slide into. Cut the tubing to make a socket for the laser.
Weld the tubing on the corner vertex of the angle iron as square as you cab get it. Use some teflon tape to wrap the laser so you get a snug wiggle free fit into the tube

Use a close pin to push the button down on the laser. once the laser is firmly imbedded in the tube socket--- take off your wheels and clamp the angle iron to the
axle with hose clamps. Swing the laser forward and allow it to project forward about 50 feet to a paper target. mark the spot. NOW--- flip the angle iron over 180 degrees
on the same axle. repeat and mark 2-nd spot. The wheel is pointing to a spot exactly half way between the to marked spots. Then do the other side. That will tell you
very accurately what your toe angles are. I used it on a non-tapered spindle--- (maybe easier than a stepped spindle-- )
Tim

[violinmaker]

BTW---- how much toe in angle are we supposed to have on each side for optimal ground loop protection ? I prob. need to measure mine -- in case it needs attn.

[Steve Pierce]

Zero or a tad in, can't remember the numbers but remember finding it in the drawings. I set them up at zero.

[Gilbert Pierce]

Pictures would help.

[Stephen]

My own invention to do this..... pretty easy to make. Go buy a dollar store laser pointer.
Get a piece of 1 inch angle iron about 10 inches long. Take a piece of tubing that the laser will just slide into. Cut the tubing to make a socket for the laser.
Weld the tubing on the corner vertex of the angle iron as square as you cab get it. Use some teflon tape to wrap the laser so you get a snug wiggle free fit into the tube

Use a close pin to push the button down on the laser. once the laser is firmly imbedded in the tube socket--- take off your wheels and clamp the angle iron to the
axle with hose clamps. Swing the laser forward and allow it to project forward about 50 feet to a paper target. mark the spot. NOW--- flip the angle iron over 180 degrees
on the same axle. repeat and mark 2-nd spot. The wheel is pointing to a spot exactly half way between the to marked spots. Then do the other side. That will tell you
very accurately what your toe angles are. I used it on a non-tapered spindle--- (maybe easier than a stepped spindle-- )
Tim

Good idea! But, a picture would help. Could you use a 10" piece of tubbing that slides on to the axel with the laser holder fastened at right angle at mid point? I'm not sure how the 1" angle iron works.

[Jim]

Hi,

...... I'm not sure how the 1" angle iron works.

I think his plan uses the open side of the angle strattling the axel length, while the laser aims forward. The laser shoots once from above the axel, then again from the bottom of the axel. Half way between those two laser hits is perpendicular/square off the axel.

[violinmaker]

Yes Jim---- exactly. One couldnt ever get the laser exactly square on the angle iron --- so you just take 2 shots and the real center point is in between those.
You compare the projected laser dots with a chaulk line on the floor which is the aircraft centerline. Then calculate the angle from the displacement.
My cousin has the tool where we used it in his fairchild. I will retrieve it and take a pic. Its a very simple thing----

Tim