Besides the already mentioned thread on torsional behaviour, you might find this one useful;
The whole topic of PSRU's and drive shafts and torsional vibration has been talked around and I keep seeing the same difficulty in the thinking of the involved folks. Since we on HBA tend to be an intelligent bunch, I got to thinking about how much I had to work to get my arms around the whole...
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Next, I can tell you that I helped some folks with a Jabiru four cylinder engine, a 44" long shaft, and prop, all in direct drive, tame their vibration. An elastomeric coupling at the engine helped with the shaft alignment issue and allowed the engine to deflect and vibrate on its mounts without overloading the shaft. This also allowed the system to operate without any lash in the system. The shaft was the primary torsional compliance having an adequately low spring rate that, when combined with a substantial increase in engine flywheel inertia, drove resonance down near engine idle rpm, and tested adequately. I modeled their design and adjusted inertia using SolidWorks and was also able to find all other nearby modes were significantly above operating input vibrations. The system tested well, and we verified no hazardous vibrations using Rotec Munich measurement system and laser tachometers pointed at targets applied to engine and both ends of the prop shaft.
Your system with a high revving outboard head will require a prop reduction system somewhere with considerable attention to torsional vibration management, particularly toward driving :
- If you run the shaft at engine speed and make it a lashless system, you may need a substantial flywheel at the engine end, allow the shaft to be small diameter (engine torque) and relatively torsionally flexible, then the gearbox at the prop hub can operate in a vibrationally clean area. This is the Allison/Airacobra system, and it worked. This system must have the shaft designed for a specific spring rate to put resonance above cranking speed and below idle speed;
- The alternative is to run the gearbox at the engine end. UGH. First you will need a significant flywheel on the engine, then a soft element such as a clutch disc center spring set or a rubber giubo or other compliant element to prevent the gearbox seeing all of the engine vibe. Then you need a U-joint of some sort to the shaft, then the shaft has to be sturdy enough to carry prop level torque and speed.
Both systems require a bunch of design and analysis work to be even close to being right when you build the first copy. The first will be lighter and simpler, and thus easier to to make work. The second will be more difficult to make work as you must isolate torsional vibration of the engine in a small package, then the speed reducer, then a beefier shaft, then the prop. Oh, besides Hertog's book or other ME vibration text, you should look up critical speed calcs for shafts. Whirl mode must be prevented, with several ways of preventing it, including increased shaft diameter (and concurrent increased torsional stiffness), breaking the shaft into more pieces with U-joints and supports (like are done with many trucks), and a system of support bearings as is seen in helo tail rotor shafts.
You did not pick an easy one...
Billski
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