I apologize in advance for the wrong-number-of-wings content, but you guys are far more likely to give me useful advice than the Aeronca groups I frequent.
On landing yesterday, in a fairly smooth but slightly fast 3-pointer, the Champ developed a violent tailwheel shimmy, which it has never done before. I was carrying a 180 lb passenger (putting us full gross at 18.99" CG, with an acceptable range of 10.2-19.2"), which was the most unusual thing about the conditions of the landing. The runway was asphalt, in mediocre condition. The plane just came out of annual the day before, where I suspect but don't yet know that the Scott 8" pneumatic tailwheel would have been inspected and probably inflated to a higher pressure than I'd been running. I noticed on the first preflight that the chains were looser than normal, but not by a lot.
My final flight was to my home base, where I did a wheel landing and dropped the tail at about 25 mph. The shimmy started again briefly, but full forward pressure on the stick stopped it. Full forward pressure on the bad landing didn't change anything, but I also didn't try it until we were pretty slow. No problem taxiing back dead slow after the final landing.
Post-flight inspection after the bad landing revealed that the chains were noticeably looser than earlier in the day, though both compression springs we still springing and not obviously broken. The only place I could have lost length is in the springs, so I'm assuming for the moment that they're wearing out and need to be replaced. The shimmy was violent enough to bend the lower perimeter of the rudder, which I pushed back into place to make the flight home (the plane is grounded until the rudder is repaired). The rudder frame felt as strong as a wet noodle, but that's a separate issue.
The tailwheel's axis of pivot at rest appears to be essentially vertical. The angle of the wheel spring hasn't changed that I can tell since I got the plane in February. While loaded, particularly landing at full gross, it's reasonable to think the axis of pivot would tilt forward (the bad direction). I've had a few harder landings since I got the plane, but nothing unusual or that made me worry about damage.
The only thing which definitely changed between no-shimmy and shimmy is the control spring tension. The tire pressure might have changed, but I haven't checked that yet. The release mechanism still seems to be working correctly, and I felt the shimmy clearly in the pedals, so I think it was still locked. I did five landings (four three-point, one wheel) before the bad one, and all were normal without the slightest indication of shimmy.
Is it likely that with a vertical axis of pivot, loosening chains or higher pressure would be enough to tip the wheel over into this kind of shimmy?
On landing yesterday, in a fairly smooth but slightly fast 3-pointer, the Champ developed a violent tailwheel shimmy, which it has never done before. I was carrying a 180 lb passenger (putting us full gross at 18.99" CG, with an acceptable range of 10.2-19.2"), which was the most unusual thing about the conditions of the landing. The runway was asphalt, in mediocre condition. The plane just came out of annual the day before, where I suspect but don't yet know that the Scott 8" pneumatic tailwheel would have been inspected and probably inflated to a higher pressure than I'd been running. I noticed on the first preflight that the chains were looser than normal, but not by a lot.
My final flight was to my home base, where I did a wheel landing and dropped the tail at about 25 mph. The shimmy started again briefly, but full forward pressure on the stick stopped it. Full forward pressure on the bad landing didn't change anything, but I also didn't try it until we were pretty slow. No problem taxiing back dead slow after the final landing.
Post-flight inspection after the bad landing revealed that the chains were noticeably looser than earlier in the day, though both compression springs we still springing and not obviously broken. The only place I could have lost length is in the springs, so I'm assuming for the moment that they're wearing out and need to be replaced. The shimmy was violent enough to bend the lower perimeter of the rudder, which I pushed back into place to make the flight home (the plane is grounded until the rudder is repaired). The rudder frame felt as strong as a wet noodle, but that's a separate issue.
The tailwheel's axis of pivot at rest appears to be essentially vertical. The angle of the wheel spring hasn't changed that I can tell since I got the plane in February. While loaded, particularly landing at full gross, it's reasonable to think the axis of pivot would tilt forward (the bad direction). I've had a few harder landings since I got the plane, but nothing unusual or that made me worry about damage.
The only thing which definitely changed between no-shimmy and shimmy is the control spring tension. The tire pressure might have changed, but I haven't checked that yet. The release mechanism still seems to be working correctly, and I felt the shimmy clearly in the pedals, so I think it was still locked. I did five landings (four three-point, one wheel) before the bad one, and all were normal without the slightest indication of shimmy.
Is it likely that with a vertical axis of pivot, loosening chains or higher pressure would be enough to tip the wheel over into this kind of shimmy?
