landing gear alignment

[russfarris]

Eric, I measured from the datum (the ten foot hat section in front of the tires) to the brake disc at exactly at the 1/2 diameter of the disc. I considered this to be truer than the Cessna method, which uses the flange of the wheel. The actual distance from the disc is meaningless; the difference between the front of the disc and the back is the value we are interested in...or in my five beer stupor did I miss something??? Russ Farris

[russfarris]

On my Cleveland brake airplane, the rotor is the same diameter as the outer flange of the wheel, so I considered it to be as accurate as checking the wheel flange...both check out exactly the same toe-in/out. Russ Farris

[N170BP]

Bela's post brings up another interesting aspect of the alignment question. Where do you measure the toe-in/out? It sounds like he measured by extending a line from the wheel ( perhaps by holding a straight-edge against the brake disc) back to the horizontal. It wouldn't take much toe-in/out at the wheel to indicate 3/8" way back there.
Or do you measure at the wheel itself? A sixteenth measured at the wheel would equate to an inch or more measured at the horizontal.

Eric

That's exactly what we did. First, we hung (hanged? never can
get those words right) a plumb bob off of the tailwheel spring bolt
to get a centerline. Then we placed some white tape on the leading
edges of the horizontal stabs in about the right place (TLAR method)
and using some basic trig, measured/marked equal distances from
the plum bob to the leading edges of each horizontal stab which matched
the centerline distance between the brake disks. Then we used a long
straight edge placed against the brake discs and "sighted" onto the same
white tape we stuck onto the leading edges. I made a mental note at the
time that a cheap hardware store laser sighting tool would be fantastic for
this task. This is where it's easy to do on a C-180 'cause I was told zero
toe in on the main gear using the brake disks as a datum plane would
sight right onto the skin joints/laps on the horizontal stab.

Anyway, in my case, the left main was almost perfect / dead-on (zero
toe-in or out). The right main had substantial toe-out, which was
corroborated by the "weirdness" I felt when the airplane got light on the
mains during takeoff roll (she wanted to seem to jerk to the right).

We tried several combinations of shims my IA had on hand, but simply
turning the existing shim around 180 degrees brought the right side
in so that measured at the horizontal stab "datum point", it was 3/8"
toed-in. You could do some basic trig and figure out the degrees
of toe-in that would be (wouldn't be much, considering the arm/distance
from the main gear to the leading edge of the horizontal stab....).

[johneeb]

Eric,
I risk a guess that during cross wind landings a little toe in would be good. With the airplane landing wing low into the wind and the tendency of to weather vane into the wind, tow in on the upwind gear would tend to turn the airframe downwind.
If this does not make any sense its because I am only a pilot [8)] not an engineer.
Johneb

[GAHorn]

It seems to me that all the effort getting things "triangulated" with the tail and the airframe to be a lot to do about nothing.
If we were talking about a vehicle with fixed rear axles and wheels then it would be imp[rtant. But one that has a castering tailwheel (which will automatically trail regardless of whether or not the main legs are "square" with the fuselage) ....not to mention the pilot that is dealing with a quartering wind, etc. etc.
In short, I don't think that matters. What matters is whether or not the wheels (with respect to EACH OTHER) have toe-in or out and how much.

[N170BP]

Be that as it may, if you're just using the diameter of the
brake discs to measure toe-in and toe out, your measurements
need to be fairly accurate to get your toe-in set just right.

If you add the distance from the main gear to the tail, as
Eric said, a small deviation in toe-in angle results in a
relatively large deviation measured at the tail.

I for one found it easier to measure set things this way.
YRMV.

[zero.one.victor]

It seems to me that all the effort getting things "triangulated" with the tail and the airframe to be a lot to do about nothing.
If we were talking about a vehicle with fixed rear axles and wheels then it would be imp[rtant. But one that has a castering tailwheel (which will automatically trail regardless of whether or not the main legs are "square" with the fuselage) ....not to mention the pilot that is dealing with a quartering wind, etc. etc.
In short, I don't think that matters. What matters is whether or not the wheels (with respect to EACH OTHER) have toe-in or out and how much.

George, let's take this example to an extreme. There was a 1953 180 for sale in the Puget Sound area a couple years ago that both Bela & me looked into ( Marlin's 180,Bela, remember?). Bela's friend Mike was familiar with the airplane's history, and told me that it had two different p/n gear legs on it. Now if it had a 1953 gear leg on one side, and a 1955 gear leg on the other, there is a difference of about 3" more forward rake on the later gear. Now are you telling me that fore/aft difference in the main wheels will not make a difference in handling? I think that airplane is gonna handle squirrelly no matter if the toe-in is the same on both LH & RH wheels.
Plus, if you square off of a straight-edge laid across the front of the mains ( like lots of people do) to check your toe-in/out, you've got a built-in error. If the airplane described above was checked with that straight-edge method, and toe-in/out set at neutral, you'd actually have extreme toe-in on one side (the side with the later gear) and extreme toe-out on the other (the side with the early gear). Get what I'm saying?
So triangulating to make sure the main gear is square to each other & the airframe is important. IMHO.

Eric

[N170BP]

Yes, I remember Marlin's 180. Heck of a nice guy. The
airplane had "issues" as far as I was concerned. Mike
did say it "flew nice"

Here's another thought. If you're using the main gear tires to "square
off" the front end, it's well known (?) that there is some variation
in overall tire size, even with the same manufacturer, same air pressure
and same tire size (McCreary, I'm told, is notorious for this).

Anyway, I agree with you. These airplanes are 50+ years old,
and precious few of us know the complete history of the airframe
dating back to day 1 (and I'm not talking about just what's in
the log books). I think it's a good Idea to measure everything
to see that it's square to start with, and then make adjustments
to the main gear axles from a known base / starting point.

[N170BP]

PS: I know of a 170B that sold locally here a few years ago
that had a "lady" gear leg on one side, and an early
(like A or even a ragwing) gear leg on the other!

The airplane sat kinda weird on the ground, and also acted
weird (I was told) during takeoff and landing. Maybe
you've seen it around. It's maroon and silver and the
tailcone / empennage has been "grafted together" with
the fuselage via a giant scab / splice patch right about
where the tail pull handles would go.

[mvivion]

One of the reasons that I won't buy McCreary tires is the significant variability in their tire diameters, and yes, it does make a difference in handling.

George, you call the 170 tailwheel a "full castering" tailwheel, but this is true only if the tailwheel breaks out of its steering detent.

Misalignment of the type noted is quite likely to produce shimmy in a tailwheel.

Personally, I want everything as straight as possible on any tailwheel airplane I fly. I have no desire to be a heroic tailwheel hero, thank you.

I've flown enough squirrelly tailwheel Cessnas over time that I don't need to go back there, and I've been involved in re-rigging several, including mine, and getting everything straight really does help.

Maybe the procedure described is taking this to a bit of an extreme, but there are a lot of cobbled up airplanes out there, so it's not a bad idea.

Mike V

[zero.one.victor]

PS: I know of a 170B that sold locally here a few years ago
that had a "lady" gear leg on one side, and an early
(like A or even a ragwing) gear leg on the other!

The airplane sat kinda weird on the ground, and also acted
weird (I was told) during takeoff and landing. Maybe
you've seen it around. It's maroon and silver and the
tailcone / empennage has been "grafted together" with
the fuselage via a giant scab / splice patch right about
where the tail pull handles would go.

Yeah, I remember that one, a guy from Crest owned it, a gal in California owns it now. She showed up with at at the McMinnville fly-in last year. I remember one of the bruise brothers pointing out that "graft" line. I never eyeballed the gear legs, if I ever see it again I will.
You call yours "Powerline", they oughta call tha one Frankenstein....

Eric

[russfarris]

The results are in on The Great Gear Alignment project.

Taxiing was easier and more positive...the improvement is small but definitely there. Directional stability on both take-off and landing is noticeably better; it is less "darty", for lack of a better term. I was working the rudder less, that's for sure.

Another change, which I'm sure will invite debate, is that lift-off at 60 MPH was smooth as glass. Before, with the excessive toe-in it almost leaped into the air as the gear un-loaded. I'm guessing that at 60 MPH, the toe-in was, well, doing weird things to the gear position. When back pressure was applied I was being literally sprung off of the ground. With the caster now set at zero, that effect is entirely gone. It's easily the most dramatic change, whatever the reason.

While I'm waiting for the laughter to subside on that one, I want to add that I'm aware that after any project like this, you hope to see ANY improvement, and wishful thinking can color the results. I've got over 500 hours in this 170, and the changes are REAL. I would recommend to anyone experiencing handling or tire wear issues to CHECK THE GEAR ALIGNMENT! The Cessna service manual method worked well for me. Thanks for all the input guys...Russ Farris

[mvivion]

Russ,

Thanks for the real life/real time validation. There is no doubt that gear alignment can be an issue in these aircraft.

Mike V

[GAHorn]

Mike, I did not say "full castering". Don't change my words please. I make enough mistakes on my own.
The tailwheel casters sufficiently to overcome any gearleg alignment...EXCEPT a mismatched gear such as Eric discusses. Certainly any combination of MISMATCHED airplane parts will throw the whole excercise into futility mode. I'd dare say that main-gear-to-tailwheel alignment is NOT the problem with those two airplanes. Heh?
Certainly it's a good idea to have your airplane "square". MOst of the fleet has been wrecked...whether or not there's a "history" of it. Using any good method of determining a "square" airframe is always a good idea. I just don't believe it will have any effect on performance of the airplane with regard to toe-In or Out.
Toe-In or Toe-Out is only a measurement with respect to the relationship of each main WHEEL and nothing else. Using a more fixed item such as a brake disc ...over a rubber tire....may be easier. But an adequate method is to simply jack each wheel, place a piece of chalk on the center of the tire, and spin the tire...thereby marking a line around the circumference of the tire. (It will matter not whether the tire is perfectly round or not when this method is used.) Next, using a board with a nail driven thru it as a scribe (some shops use a nail at each end of the board, spaced the same distance apart as the two chalk lines, but that is not a necessity) spin each tire so as to scribe a line in the chalk. This will establish two, fine lines ....marked in the chalk. Measure the distance between the scribe- lines both at the FRONT of the tire,...and at the BACK of the tire....and you'll have a very accurate reading of toe-in or toe-out.
It makes no difference if the wheels are exactly lined up with the tailwheel, the wingtip or anything else. They roll along the ground with respect to each other.

[zero.one.victor]

George, with your description of the "fine-line-in-the-chalkline" method, it sounds like you are measuring toe-in/out with the wheels still jacked up off the ground. That isn't the case,is it? After marking, do you set the wheels on the grounds atop greased plates or garbage bags, as to let them slip around & find their own location? I would have to agree that this appears to be a fine method of checking toe-in/out, provided your chalk/scribe lines are accurately centered or otherwise symetrical.
I still think that the mains should be triangulated from a point at the aft of the airplane, if nothing else than to avoid that "catty-whampus" configuration that you sometimes see whan following a rebuilt wrecked car. That condition can't help but handle funny, and cause uneven tire wear & stress on the airframe.

Eric