The reason an improperly repaired wrecked car will "crab" down the road is because it's rear axle is a common, long axle with two immoveable wheels on each end. The two axles (front and rear) are not parallel. (It has nothing to do with toe-in or toe-out.)
Our airplanes, running on 3 wheels, the rear of which (tailwheel) is castering, ...therefore there is simply no possibility whatsoever of there being an out-of-parallel condition. It still has nothing to do with toe in or toe out, or how that may affect handling.
Having the main landing gear "square" with the fuselage and the tailwheel is a good idea, certainly. But it has no relationship to the toe in/toe out condition.
As for being on the ground during the actual measurement: Yes. LIft a wheel into the air just to chalk it and spin it to scribe it. Do the same on the other side. Then put it on the ground (either on bags/grease plates or simply roll it along the hangar floor a few feet to normalize things) before measuring the toe-in/out. The scribed chalk line is sufficiently durable to withstand short rolling around. (And being on the outer circumference of the tire rather than the brake disc, ....the measurement is more accurate.)
landing gear alignment
Moderators: GAHorn, Karl Towle, Bruce Fenstermacher
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russfarris
- Posts: 476
- Joined: Wed May 15, 2002 2:25 am
I went and did some full stall/three point landings today, and it is here the improvement was most dramatic. I always wheel land, unless it's a short strip. My three-pointers on pavement are shall we say, painful.
Even with the stall warning blaring as all three tires touched down dead ahead, I was usually rewarded with a sort of darting, crow-hopping motion(?) I always thought it was because I hardly ever practiced them. Today I did five in a row, and even royally messed one up by touching down on the mains well ahead of the tailwheel. A little porpoise action there, but it tracked dead ahead down the centerline with very little effort on my part.
A completely different airplane...
I guess I should have suspected something was not right a long time ago, but the only spring-steel gear taildraggers I have ever flown have been 170s. The tire wear problems are what led me down this road, as I never considered the handling to be that difficult. I wish I checked this out long ago.
Again, I would highly recommend checking gear alignment if you have any issues in handling and/or tire wear...Russ Farris
Even with the stall warning blaring as all three tires touched down dead ahead, I was usually rewarded with a sort of darting, crow-hopping motion(?) I always thought it was because I hardly ever practiced them. Today I did five in a row, and even royally messed one up by touching down on the mains well ahead of the tailwheel. A little porpoise action there, but it tracked dead ahead down the centerline with very little effort on my part.
A completely different airplane...
I guess I should have suspected something was not right a long time ago, but the only spring-steel gear taildraggers I have ever flown have been 170s. The tire wear problems are what led me down this road, as I never considered the handling to be that difficult. I wish I checked this out long ago.
Again, I would highly recommend checking gear alignment if you have any issues in handling and/or tire wear...Russ Farris
All glory is fleeting...
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russfarris
- Posts: 476
- Joined: Wed May 15, 2002 2:25 am
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mvivion
- Posts: 136
- Joined: Mon Aug 02, 2004 2:07 am
George,
The point was that toe in/toe out isn't the only issue regarding gear alignment. The point was to get people to get their gear aligned, and the tailwheel, if it's forced to caster, often shimmies, which is also a bad thing.
And, these are not "castering" tailwheels either. They are steerable tailwheels. There is no "range" that it doesn't caster-it's locked up by the steering pawls, the only way you can get it to caster is to break it out of the steering pawls, in which case it becomes a castering tailwheel. this is not a locking tailwheel, such as found on a Doug or a 185, with a few degrees of caster built into the steering.
Its a steerable tailwheel till it breaks out of the steering detents, then its a castering tailwheel. I would think that a landing gear so badly aligned that it causes the tailwheel to break out of its steering detent is a bad thing.
Again, the point was, get the gear aligned. Its not a small thing. But, whatever, Masta.
Mike
The point was that toe in/toe out isn't the only issue regarding gear alignment. The point was to get people to get their gear aligned, and the tailwheel, if it's forced to caster, often shimmies, which is also a bad thing.
And, these are not "castering" tailwheels either. They are steerable tailwheels. There is no "range" that it doesn't caster-it's locked up by the steering pawls, the only way you can get it to caster is to break it out of the steering pawls, in which case it becomes a castering tailwheel. this is not a locking tailwheel, such as found on a Doug or a 185, with a few degrees of caster built into the steering.
Its a steerable tailwheel till it breaks out of the steering detents, then its a castering tailwheel. I would think that a landing gear so badly aligned that it causes the tailwheel to break out of its steering detent is a bad thing.
Again, the point was, get the gear aligned. Its not a small thing. But, whatever, Masta.
Mike
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FredM
- Posts: 83
- Joined: Sun Feb 09, 2003 7:24 am
When the steering pawls are released the tailwheel becomes FREE castoring. The rest of the time during steering the tailwheel can still castor regardless of steering input if the castoring force applied over comes the tailwheel spring tension. That is the whole purpose of the spring, to allow castoring of the tailwheel even with steering input. If you were to land with a significant amount of rudder input and the tailwheel were not allowed to castor, it would be ripped off by the castoring force encountered at touchdown. I'm not taking sides or trying to start arguments here. Everyone seems to have a different definition of castoring and free castoring.
Fred L. Mahan
51 C170A N1289D
51 C170A N1289D
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GAHorn
- Posts: 21308
- Joined: Fri Apr 12, 2002 8:45 pm
Castoring is what your grandmother did to you when she thought you hadn't pooped sufficiently in recent history. 
Castering is what that little wheel in the back of the airplane does when it aligns itself to roll in the direction it's travelling. (Which the Scott 3200 tailwheel most certainly does or you'd go off the side of the runway regardless of the friction plates/pawls or what the wind did.)
Tailwheel caster itself does not cause "shimmy" unless the angle of caster is incorrect. Shimmy can be caused by several combinations of issues but correct castering is not one of them. It's most likely causes are: Poor friction plate condition/pressure, Poor attach bracket angle (or mainspring angle), loose control chains/springs, worn tailwheel bearings/axle/spindle, or incorrect mounting angle resulting in negative caster.
As I mentioned earlier, there's no reason to allow an airplane to be assembled incorrectly or with incorrect/mismatched parts. Taking measurements to correct assembly errors is not disqualifying at all.
Only that it has little/nothing to do with toe in or toe out measurements, which are solely with respect to each main wheel's relationship to it's partner and regardless of any other airframe measurement. Proper toe in/toe out condition will not correct an egregious assembly error, nor will an airplane's slightly out-of-square alignment affect it's ground-handling characteristics with regard to toe in/toe out. Since the two items have no relationship to each other, there's no need to "zero" the main wheel's toe in/toe out with the airframe.....only with each other.
Castering is what that little wheel in the back of the airplane does when it aligns itself to roll in the direction it's travelling. (Which the Scott 3200 tailwheel most certainly does or you'd go off the side of the runway regardless of the friction plates/pawls or what the wind did.)
Tailwheel caster itself does not cause "shimmy" unless the angle of caster is incorrect. Shimmy can be caused by several combinations of issues but correct castering is not one of them. It's most likely causes are: Poor friction plate condition/pressure, Poor attach bracket angle (or mainspring angle), loose control chains/springs, worn tailwheel bearings/axle/spindle, or incorrect mounting angle resulting in negative caster.
As I mentioned earlier, there's no reason to allow an airplane to be assembled incorrectly or with incorrect/mismatched parts. Taking measurements to correct assembly errors is not disqualifying at all.
Only that it has little/nothing to do with toe in or toe out measurements, which are solely with respect to each main wheel's relationship to it's partner and regardless of any other airframe measurement. Proper toe in/toe out condition will not correct an egregious assembly error, nor will an airplane's slightly out-of-square alignment affect it's ground-handling characteristics with regard to toe in/toe out. Since the two items have no relationship to each other, there's no need to "zero" the main wheel's toe in/toe out with the airframe.....only with each other.
Last edited by GAHorn on Sun Jun 19, 2005 4:41 pm, edited 1 time in total.
'53 B-model N146YS SN:25713
50th Anniversary of Flight Model. Winner-Best Original 170B, 100th Anniversary of Flight Convention.
An originality nut (mostly) for the right reasons.
50th Anniversary of Flight Model. Winner-Best Original 170B, 100th Anniversary of Flight Convention.
An originality nut (mostly) for the right reasons.
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N170BP
- Posts: 552
- Joined: Mon Jul 22, 2002 7:24 pm
I'm having difficulty agreeing with that last sentence. Let's take an
extreme example, and say the toe in is perfect when you compare
one wheel/tire to the other, but at the same time, you could have each
axle pointing to the right (let's say), so that the centerline of each
wheel/tire is offset from the centerline of the airframe to the right.
You go to land the airplane with the airframe aligned with the relative
wind (no-wind situation for example) but the tires are pointed to the
right when you touch down. Are you saying the aircraft won't try to
"dart" to the right? Or at least exhibit "quirky" handling?
When Cessna built the airplane, I'm willing to bet a box of your
favorite beer they measured everything near the completion of
final assembly, and they chose the exact shims (various thickness /
angles being available) based upon those measurements / math that
would get the gear alignment as close to perfect as possible.
My other aircraft/project has several pages in the Erection & Maintenance
manual devoted to this procedure.
extreme example, and say the toe in is perfect when you compare
one wheel/tire to the other, but at the same time, you could have each
axle pointing to the right (let's say), so that the centerline of each
wheel/tire is offset from the centerline of the airframe to the right.
You go to land the airplane with the airframe aligned with the relative
wind (no-wind situation for example) but the tires are pointed to the
right when you touch down. Are you saying the aircraft won't try to
"dart" to the right? Or at least exhibit "quirky" handling?
When Cessna built the airplane, I'm willing to bet a box of your
favorite beer they measured everything near the completion of
final assembly, and they chose the exact shims (various thickness /
angles being available) based upon those measurements / math that
would get the gear alignment as close to perfect as possible.
My other aircraft/project has several pages in the Erection & Maintenance
manual devoted to this procedure.
Bela P. Havasreti
'54 C-180
'54 C-180
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lowNslow
- Posts: 1535
- Joined: Mon Apr 22, 2002 4:20 pm
I've got to agree with Bela on this. George is right in that the toe-in is a relational measurement between only the two main wheels. But if both wheels are aligned say 10 degrees to the right, I think we would agree this is not a desirable situation even tho the toe-in is correct. When landing and taking off, we align the longitudinal axis of the aircraft with the direction of flight and not the main gear. If your going to take the time to adjust the toe-in, you might as well take the time to align the mains with the center line of the aircraft.
Karl
'53 170B N3158B SN:25400
ASW-20BL
'53 170B N3158B SN:25400
ASW-20BL
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GAHorn
- Posts: 21308
- Joined: Fri Apr 12, 2002 8:45 pm
I think we're all chasing the tiger all around the same tree.
I've already said airframe issues are important. They've just got nothing to do with toe in/toe out.
If your landing gear legs are bent sufficiently to cause the airplane to "dart" to the right any appreciable (read that: noticeable) amount then you'll be able to see that misalignment with the naked eye. You won't need to take the sort of micro-measurements talked about in this thread to discern that.
Think about it. The average pilot will land his airplane on a concrete runway many times over and will not be in perfect alignment with the runway. In fact, he'll be well outside of several degrees of mis-alignment before he ever notices it. That sort of mis-alignment of landing gear legs (both main wheel pointed to the same mis-aligned direction to the right) will be noticeable to even the casual observer walking around the airplane.
In the other case of mis-assembled/mis-matched landing gear legs,...even a walk-around to the wingtip and looking across the main wheels will reveal several inches of main wheels being one-before-the-other. (And even THEN, if they are ZERO toe in/toe out they will still roll the same direction!) (Look at any Hawker 125 and you'll notice that of the dual mainwheels on each main gear...one IS actually a couple inches forward of the other! ...and they have zero toe in/toe out ....and they both roll the same direction! Every time.)
Toe in/toe out has nothing to do with the axles being square with the airframe (such as being equi-distant from the tailwheel as previously described in this thread.) If the main gear legs are bent/misaligned/etc. then the fix is not toe in/toe out adjustments. It's a major repair of the gearlegs/gearbox.
Toe in/toe out is a separate issue. If such were not the case, then Cessna would have insisted that identical and equal numbers of shims be placed in exactly the same orientation on each axle.
In actual fact, even in the case of a bent or excessively sprung main gear leg, the need for micro-measurements are negated, as Cessna only requres a visual inspection. No measurements are even called for in the manual. (See pg. 5-2 of the 100 Series Service Manual.)
Lastly, in the most wildly exaggerated example I can imagine as regards this issue....think about the crosswind gear. Even in a strong crosswind, as the gear unlocks and both main wheels point decidedly in the same direction off-center as it compensates for the crosswind....the tailwheel neatly casters and follows the main gear. The airplane still goes down the runway just fine. It's got nothing to do with toe in/toe out.
I've already said airframe issues are important. They've just got nothing to do with toe in/toe out.
If your landing gear legs are bent sufficiently to cause the airplane to "dart" to the right any appreciable (read that: noticeable) amount then you'll be able to see that misalignment with the naked eye. You won't need to take the sort of micro-measurements talked about in this thread to discern that.
Think about it. The average pilot will land his airplane on a concrete runway many times over and will not be in perfect alignment with the runway. In fact, he'll be well outside of several degrees of mis-alignment before he ever notices it. That sort of mis-alignment of landing gear legs (both main wheel pointed to the same mis-aligned direction to the right) will be noticeable to even the casual observer walking around the airplane.
In the other case of mis-assembled/mis-matched landing gear legs,...even a walk-around to the wingtip and looking across the main wheels will reveal several inches of main wheels being one-before-the-other. (And even THEN, if they are ZERO toe in/toe out they will still roll the same direction!) (Look at any Hawker 125 and you'll notice that of the dual mainwheels on each main gear...one IS actually a couple inches forward of the other! ...and they have zero toe in/toe out ....and they both roll the same direction! Every time.)
Toe in/toe out has nothing to do with the axles being square with the airframe (such as being equi-distant from the tailwheel as previously described in this thread.) If the main gear legs are bent/misaligned/etc. then the fix is not toe in/toe out adjustments. It's a major repair of the gearlegs/gearbox.
Toe in/toe out is a separate issue. If such were not the case, then Cessna would have insisted that identical and equal numbers of shims be placed in exactly the same orientation on each axle.
In actual fact, even in the case of a bent or excessively sprung main gear leg, the need for micro-measurements are negated, as Cessna only requres a visual inspection. No measurements are even called for in the manual. (See pg. 5-2 of the 100 Series Service Manual.)
Lastly, in the most wildly exaggerated example I can imagine as regards this issue....think about the crosswind gear. Even in a strong crosswind, as the gear unlocks and both main wheels point decidedly in the same direction off-center as it compensates for the crosswind....the tailwheel neatly casters and follows the main gear. The airplane still goes down the runway just fine. It's got nothing to do with toe in/toe out.
Last edited by GAHorn on Tue Nov 30, 2004 6:26 pm, edited 3 times in total.
'53 B-model N146YS SN:25713
50th Anniversary of Flight Model. Winner-Best Original 170B, 100th Anniversary of Flight Convention.
An originality nut (mostly) for the right reasons.
50th Anniversary of Flight Model. Winner-Best Original 170B, 100th Anniversary of Flight Convention.
An originality nut (mostly) for the right reasons.
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zero.one.victor
- Posts: 2271
- Joined: Tue Apr 23, 2002 12:11 am
Another example is my 170. The main wheels are equi-distance from the tail ("triangulated") and equi-distance from the airplane centerline, therefore square with the airframe. I have 1/8" toe-in on the LH side and 1/16" toe-out on the RH side, for a net of 1/16" toe-in (measured at wheel diameter).
So George, this is acceptable? I haven't gone to the trouble to change it, cuz the handling & tire wear are OK, but isn't it better to have the same toe-in on each side? Any "darting" tendencies would be different from left to right when on only one main wheel, such as during a wing-low X-wind landing or takeoff. Like I said, it handles OK, but if the toe-in/out measurements were extreme ( such as the early/late 180 leg example I cited earlier), that probably wouldn't be the case.
Eric
Eric
So George, this is acceptable? I haven't gone to the trouble to change it, cuz the handling & tire wear are OK, but isn't it better to have the same toe-in on each side? Any "darting" tendencies would be different from left to right when on only one main wheel, such as during a wing-low X-wind landing or takeoff. Like I said, it handles OK, but if the toe-in/out measurements were extreme ( such as the early/late 180 leg example I cited earlier), that probably wouldn't be the case.
Eric
Eric
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zero.one.victor
- Posts: 2271
- Joined: Tue Apr 23, 2002 12:11 am
HEY! That crack certainly doesn't apply to any of the ace pilots on THIS board!gahorn wrote:.....................
Think about it. The average pilot will land his airplane on a concrete runway many times over and will not be in perfect alignment with the runway. In fact, he'll be well outside of several degrees of mis-alignment before he ever notices it................
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zero.one.victor
- Posts: 2271
- Joined: Tue Apr 23, 2002 12:11 am
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N170BP
- Posts: 552
- Joined: Mon Jul 22, 2002 7:24 pm
The shims Cessna originally put behind the axles were an
allowment for manufacturing tolerances (much like
the eccentric bolts on the wings are), and their way of
squaring up the axles with the airframe. To say the shim's
sole purpose is to adjust toe-in + camber isn't (I think) quite
telling the whole story.
Once again, the airplanes are 50+ years old. Cessna
shimmed them to square everything up when they
built them new (Boeing still shims everything everywhere).
The gear legs on our birds may have been replaced. Shims may
have been re-installed incorrectly during maintenance, something
could be bent, the list goes on. We're not talking about micro
measuring anything, and I honestly couldn't tell anything was
wrong by eyeballing my airplane prior to measuring everything.
The handling was just a bit "weird" at times compared to the '53B
I used to fly. Now it's at least as good if not better than that '53.
The measuring procedure isn't that big of a deal. I suppose anyone
who thinks it's not crucial is welcome to that opinion.
If anyone needs help measuring theirs, I'd be glad to help in any way
I can.
allowment for manufacturing tolerances (much like
the eccentric bolts on the wings are), and their way of
squaring up the axles with the airframe. To say the shim's
sole purpose is to adjust toe-in + camber isn't (I think) quite
telling the whole story.
Once again, the airplanes are 50+ years old. Cessna
shimmed them to square everything up when they
built them new (Boeing still shims everything everywhere).
The gear legs on our birds may have been replaced. Shims may
have been re-installed incorrectly during maintenance, something
could be bent, the list goes on. We're not talking about micro
measuring anything, and I honestly couldn't tell anything was
wrong by eyeballing my airplane prior to measuring everything.
The handling was just a bit "weird" at times compared to the '53B
I used to fly. Now it's at least as good if not better than that '53.
The measuring procedure isn't that big of a deal. I suppose anyone
who thinks it's not crucial is welcome to that opinion.
If anyone needs help measuring theirs, I'd be glad to help in any way
I can.
Bela P. Havasreti
'54 C-180
'54 C-180
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GAHorn
- Posts: 21308
- Joined: Fri Apr 12, 2002 8:45 pm
Certainly NOT!zero.one.victor wrote:HEY! That crack certainly doesn't apply to any of the ace pilots on THIS board!gahorn wrote:.....................
Think about it. The average pilot will land his airplane on a concrete runway many times over and will not be in perfect alignment with the runway. In fact, he'll be well outside of several degrees of mis-alignment before he ever notices it................
(Can't think of anyone here who is exactly average! Even Ol' GAR is below that level!)
Well, Eric, as regards your airplane, I'd leave it alone. It handles OK. Your tire wear is acceptable. It's within the specs. I think you're good to go. If you ever have some reason to remove/replace your axles you could attempt to identify which shims are present and perhaps relocate one from one side to the other (left to right) and perhaps zero it out, but I'd not make any special effort to do this, considering the good tire wear you're experiencing. (1/16" is only 0.006". That's hardly enough to measure and there's no individual shim that comes even close to making that small of a correction.)
'53 B-model N146YS SN:25713
50th Anniversary of Flight Model. Winner-Best Original 170B, 100th Anniversary of Flight Convention.
An originality nut (mostly) for the right reasons.
50th Anniversary of Flight Model. Winner-Best Original 170B, 100th Anniversary of Flight Convention.
An originality nut (mostly) for the right reasons.
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