---------------------------------------------------------- Engines-List Digest Archive --- Total Messages Posted Fri 08/22/03: 2 ---------------------------------------------------------- Today's Message Index: ---------------------- 1. 05:48 AM - gyroscopic precession moments (Gary Casey) 2. 09:10 AM - Re: gyroscopic precession moments (Tedd McHenry) ________________________________ Message 1 _____________________________________ Time: 05:48:37 AM PST US From: "Gary Casey" Subject: Engines-List: gyroscopic precession moments --> Engines-List message posted by: "Gary Casey" <> Okay, here's some very rough data. Don't count it as gospel, but it might be a place to start. The only propeller data I have from 5 minutes of looking is that the Hoffman 2-blade has a polar moment of 2700 lb-in 2. I would expect that one could use a number of maybe 4,000 to cover most 3-blade composite or wood props. Aluminum blades might be twice that again?? Anyway, the average flywheel/pressure plate/driven plate combination on a V-8 car engine would weigh something like 30 pounds and I'm guessing would have a radius of gyration of 4 inches, maybe 4.5. Using 4 inches the polar moment comes out to be 480 lb-in 2. A race engine flywheel/clutch might have half that. Gyroscopic precession forces increase as the square of the rpm and let's say that the car engine turns twice as fast. To make it "equivalent" the car engine flywheel then has a gyroscopic moment equivalent to about 1,000 lb-in 2, compared to the 4,000 of a prop. Use the above numbers for a race car and taking into account the lighter flywheel and higher rpm and it's not much different. Precession moments are directly proportional to yaw rate, so does a car experience 4 times the yaw rate of a plane? A passenger car might not, but I'll bet a dune buggy or race car probably do. Remember, roll rates don't count, only pitch and yaw rates. Would the rate be as high as 360 degrees/second? Maybe. But then consider a race car as it hits the wall at 8,000 rpm - would it change direction by 20 degrees in 10 milliseconds? That's a yaw rate of 1800 degrees per second, 5 times what the aircraft will see. Most crashes only take about 10 milliseconds, maybe 20. And all we need is peak yaw rate, not any sustained rotation, to break the crank. When I toss all these numbers around, put them in the blender and see what comes out I don't get a big difference between cars and planes, at least assuming composite blades. I think the calculations above are only good to an order of magnitude and I certainly wouldn't bet my life on them being within a factor of 2. Gary Casey ________________________________ Message 2 _____________________________________ Time: 09:10:42 AM PST US From: Tedd McHenry Subject: Re: Engines-List: gyroscopic precession moments --> Engines-List message posted by: Tedd McHenry Gary: Good analysis, and I agree with your comments. I'll be meeting this weekend with friend who's an engineer for a CART team. I'll ask him his thoughts on yaw and pitch rates. The only place I disagree with you is the significance of roll rates. If the car has a transverse engine installation the roll rates are also a factor. Tedd McHenry Surrey, BC DO NOT ARCHIVE