Good information. I understand better what torsional vibration is, but I’m still trying to get my head around all that causes it in a piston engine.
In a straight-4 engine every power stroke at the piston, especially farthest away from the output shaft, will create a flex perpendicular to the shaft, or cause a rocking tendency, correct? With a boxer engine the force of one power stroke on the shaft will be countered by a force 180 degrees, opposite side of the shaft and therefore counter balance the forces on the shaft, correct? Does the fact the boxer engine has a shorter shaft than the equivalent cylindered straight-4 have a significant effect? If so then it follows that a straight-3 engine would be less prone to TV than a straight-4?
Also, I’ve noticed that the boxer aircraft engines, like the Lycomings, have a much larger bore to stroke, the large bore give it the torque needed at lower rpm, though that could have also been achieved with a larger stroke. With a shorter stroke the power pulse on the shaft will be more evenly distributed over the half turn of the crank than a larger stroke of the same torque, which seems intuitively to be a way to decrease TV, is this true?
Forget the power pulses, they don't affect the resonant frequency, they just excite things.
Take a piece of coathangar wire, bend the ends 90 degrees so you can grip them.
Twist, that's a torsional spring. Go up to crankshaft size: IT STILL TWISTS, it's just a helluva lot stiffer.
Put a weight on each end. you have two masses connected by a spring. excite those at the right frequency, they resonate in a torsional version of the video a few posts up. In that video, there are two springs and one mass. The end result is still a large amplitude but torsional instead of linear when the particular resonant frequency of an undamped system is reached. The damping in the video is probably mostly air resistance. if a piece of card, increasing air resistance, had been attached to the weight, the amplitude would have been much lower.
The crank only appears to have one big mass (the prop) on one end. But the crank is not infinitely light, the con rods and other stuff attached to it all weigh something.
Find a broom, rotate along the axis of the shaft, it can be twisted quite easily. That's low rotational inertia, the torsional version of mass. Now hold it at 90 degrees to your arm and twist your wrist again, way harder to change direction! It's got much more mass at a large diameter and a higher rotational inertia, although it's still the same broom.
Crank stiffness depends on length. It's not hard to work out that a 1/2 length crank is twice as stiff as full length crank. Bigger diameter increases stiffness, that actually varies with the 4th power of diameter.
Straight shaft are fairly easy, but calculating the stiffness of something shaped like a crankshaft is the kind of headache I prefer to solve with actual experiment...
HTH