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Dale, Al, Richard, Ernest: You're all mostly correct, BUT it's a very difficult concept to get your head around.
I just finished assembling my 20B, and spent quite some time turning a rotor with the eccentric shaft to get a good picture of the four cycle process and the geometry of the rotor movement into my head.
The rotor's internal tooth gear follows the housing's external tooth fixed gear around in a perfect circle. Since the rotor's gear is larger than the fixed gear, it causes the rotor to spin, but at a different speed than the eccentric shaft. The rotor spins on its axis, but the axis is traveling in a circle at one third the speed of the shaft.
This all results in a 'slinging' movement of the rotor, both up and down and sideways, but it is all smoothly circular; no starting and stopping like a piston.
This 'slinging' is why the rotary requires the counter-balance weight on the outside. To counter-act the slight imbalance of the rotors' movement within the engine.
I find the video on Tracy's site somewhat helpful, but it moves too fast for my old brain to fully absorb all that is happening simultaneously. I am regularly amazed to remember that Wankel figured out all this geometry over 75 years ago!
Phil White (20B gonna be joined to an RV-10 airframe, one'a these days)
Dale Rogers: Umm, guys? None of the above ... or both?
The rotor does in fact rotate around its own center, which is also the center of the eccentric (offset part of the shaft)
Since the eccentric shaft is also rotating, the center of the rotor is also in constant motion. That is why the shape traced by the apex seals is called a "trochoid ".
Al Gietzen:
Unless I'm mistaken, or misunderstand what you're saying; the rotor does not
rotate around its own center. And the CM of the rotor does not rotate
around the center of the e-shaft, but rather follows the center of the
e-shaft lobe. Watch the animation on Tracy's web site.
Richard Sohn:
the rotor is not sliding "back and forth" , it only makes a clean circular motion around its own center. The weight only is significant for the centrifugal force on the e-shaft lobe.
This is why the rotary can be perfectly balanced. Just trying to keep the rotary straight.
Ernest Christley
OK. You caught me.
I always thought there was some back and forth motion in that rotary rotation. But I've just spend the last few minutes staring at http://www.keveney.com/Wankel.html
There has to be some recipricating motion. If there wasn't, the apex seals would all trace a perfect circle.
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