There is lots of info and opinion on nose wheel shimmy; the more one reads the more confused the issue gets. More air in the tire, less air, positive rake, negative rake, balance the wheel, tighten the pivot for more friction, grease the pivot, un grease the pivot, put in a damper....all of these are recommended. The"real" research out there (NACA and others) often pertains to steerable rather than castering gear.
After reading A LOT, I decided to weld up a castering nose gear assembly with an adjustable rake angle, attach it to my car, and haul it down the road at speed to see what happens.
Long story short, minimal side to side friction and tipping the top of the pivot axis fwd resulted in
substantial shimmy at 35 mph while tipping the top of the pivot axis backward delayed the onset until 45 or so and the shimmy that did occur was less severe than before. Finally, tightening the pivot while it was still tipped backward at the top (still movable by hand however) eliminated all shimmy up to 55 mph, the speed limit on the test road.
I did this so that my first landing in my 0-470 powered BD-4 (if I ever get my medical back) will be less exciting. More testing will follow as I try to fine tune rake angle some more and pivot friction
Hope this helps someone else pondering this...
P.S. Upside down pics from the Ipad I used for the pictures! I'm working on fixing them, they are confusing at best, sorry.
After reading A LOT, I decided to weld up a castering nose gear assembly with an adjustable rake angle, attach it to my car, and haul it down the road at speed to see what happens.
Long story short, minimal side to side friction and tipping the top of the pivot axis fwd resulted in
substantial shimmy at 35 mph while tipping the top of the pivot axis backward delayed the onset until 45 or so and the shimmy that did occur was less severe than before. Finally, tightening the pivot while it was still tipped backward at the top (still movable by hand however) eliminated all shimmy up to 55 mph, the speed limit on the test road.
I did this so that my first landing in my 0-470 powered BD-4 (if I ever get my medical back) will be less exciting. More testing will follow as I try to fine tune rake angle some more and pivot friction
Hope this helps someone else pondering this...
P.S. Upside down pics from the Ipad I used for the pictures! I'm working on fixing them, they are confusing at best, sorry.
