GregP
Major
I was asked to clarify the landing gear geometry issue in the Bf 109 by a visitor to the museum last weekend. Attached is a front view drawing of a Bf 109.
When the tail is up, the tires are pointing parallel to eash other and straight down the runaway. No problem,
Now imagine for a second that the aircraft is pointing straight up, but is sliding forward in the direction of the belly. See the layout of the landing gear below the aircraft drawing. It is easy to see that if the gear is moving in the direction of the center arrow, the tires would be trying to slide outward from each side.
The aircraft cannot do this physically, but as the aircraft tips backward, the alignment track of the tires becomes increasingly toe out and, by the time the tailwheel is on the ground, both tires are toed out to a significant degree. The leanback angle is about 13°. If one tire or the other gets significantly more pressure (or weight on it) than the other, as in a crosswind where one wing lifts a bit, then the main gear will move in the direction of the wheel with the greater weight on it. If the pilot isn't very quick with the rudder or brake or both, a groundloop can easily result.
Fitting a tailwheel lock helped but did not eliminate the problem.
When the tail is up, the tires are pointing parallel to eash other and straight down the runaway. No problem,
Now imagine for a second that the aircraft is pointing straight up, but is sliding forward in the direction of the belly. See the layout of the landing gear below the aircraft drawing. It is easy to see that if the gear is moving in the direction of the center arrow, the tires would be trying to slide outward from each side.
The aircraft cannot do this physically, but as the aircraft tips backward, the alignment track of the tires becomes increasingly toe out and, by the time the tailwheel is on the ground, both tires are toed out to a significant degree. The leanback angle is about 13°. If one tire or the other gets significantly more pressure (or weight on it) than the other, as in a crosswind where one wing lifts a bit, then the main gear will move in the direction of the wheel with the greater weight on it. If the pilot isn't very quick with the rudder or brake or both, a groundloop can easily result.
Fitting a tailwheel lock helped but did not eliminate the problem.
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