Originally Posted by
JohnBurke
Actually, the concept is the same, but apparently it's still not well understood.
For donkey's years, ignorant instructors told students that maneuvering speed was a safety speed; any maneuvering under that speed would cause a "stall" before it would hurt the airplane. This, of course, was wrong (still is), but it was passed on in ignorance, and many plots were actually surprised when AA587 shed a tail.
The control input at or below maneuvering speed is a one-time maneuver, and only to a given acceleration loading. It's not a blank check, and it does not account for reversal.
What the rudder does on the ground is irrelevant. The problem isn't the rudder. It's the structure to which it is attached.
I grew up in cubs, slipping airplanes with abandon. It was second nature. I flew large, four engine bombers down canyons, in full slips, regularly. It was part keeping speed under control in a steep descent to a fire. Then I began doing heavy maintenance on them; C checks, and D checks, and began finding fractured attach fittings and hardware for the vertical stab...on multiple aircraft. I quit slipping big airplanes. Mostly small ones, too, as I found numerous cracked vertical stab attach brackets on small aircraft too. None of these airplanes were being operated at high speeds when the sideloads on the vertical stab occurred, but if one considers the load on that stab, given the size, repeatedly, it shouldn't be too hard to understand where those stresses are concentrated, and to imagine the cumulative effect over time.
Reversing direction with the rudder on the ground is irrelevant, as it's not causing bending and torsion on the vertical stab. In flight with an air load, it is. That's where the problem occurs.
With a control jam, pressing on the control to free the jam and a subsequent rapid movement, isn't really the concern. Reversing that input is the structural one-two punch that can lead to damage.
Rolling out? No issue at all, especially given the availability of a tiller and nosewheel steering.
I think the issue is yaw oscillations on the ground and keeping the plane out of the mud more than structural failure due to aerodynamic loads. If you're breaking a sheer pin, say with frozen controls, you're pushing until you get proper control movement and response not full deflection.
Tangent about light planes: I slip light planes and fly aerobatics as do a lot of folks here. I think the stresses that break planes are occasionally aerodynamic but more often from poor ground handling. Pushing on vertical stabs to steer taildraggers around, for example.
Not saying you are wrong. Snap maneuvers for example are hard on tails. Slips to landing, even fast ones, I'm not so concerned about. By all means watch your planes for wear and tear. I'm always asking dumb questions so I know what to look at and what to replace/monitor. I'm just saying I wouldn't worry about slipping to land in your Cessna 120. The Drop Zone that used to fly 185s and 182s in Omro, WI used to do full slips from altitude to land, and they did it at high speed for thousands of hours. Tail bulkheads held up.
Swept empennage would make me think twice. T tail too.
I guess, after all this typing, I've actually said nothing at all.