Today's Message Index:
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1. 05:48 AM - gyroscopic precession moments (Gary Casey)
2. 09:10 AM - Re: gyroscopic precession moments (Tedd McHenry)
Message 1
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| Subject: | gyroscopic precession moments |
--> Engines-List message posted by: "Gary Casey" <glcasey@adelphia.net>
<<Right on, Jerry. I expect it would be an interesting exercise to
compute the radius of gyration of both the typical prop and the typical
flywheel.>>
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
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| Subject: | Re: gyroscopic precession moments |
--> Engines-List message posted by: Tedd McHenry <tedd@vansairforce.org>
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
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