Mowing Gyro action -- an aid to hillside mowing?

[markie61]

<font color=blue>"Note: my bike has wheels that weigh about 3 pounds in rotation and they provide enough gyro to keep me (about 150 pounds) from falling over." </font color=blue>

I thought it was those little gyroscopes in the inner ear that keep one from falling over..../w3tcompact/icons/smile.gif

New questions, but related: Let's say you are mowing sideways on a constant slope in an unimpeded, open field and you feel like you are on the edge of turning over. If you want to escape, is it best to turn up the hill or down? My instincts say turn down the hill, but I turned 40 and my brain cells aren't what they used to be.....

Mark

 

[RobS]

Hmmm, interesting topic. Hardly an expert but I'll throw out a couple of other things to consider...

Once in motion, a gyroscope does tend to resist external forces, but it takes a big gyro to resist much. The other consideration is the momentum of the body (bicycle, motorcycle, jet plane, tractor) in motion. Bike wheels do add a gyro effect but the forward momentum of the bike is there as well. Also, the human body is a terrific sensor for things going awry. A gyro in a plane doesn't actually keep it on course, it just tells the other systems via sensors when things are changing.

In the tractor world, things happen slowly (relatively speaking). Even in a rollover, the accelerations are small so changes in momentum are minimal. Not much a gyro will help with.

Now to put all that into mathematical terms... find a college kid looking for a Phd thesis topic /w3tcompact/icons/smile.gif

 

[hazmat]

<font color=blue>Now to put all that into mathematical terms... find a college kid looking for a Phd thesis topic </font color=blue>

Glad to see I'm not the only mech E afraid of the math/w3tcompact/icons/grin.gif

 

[Mosey]

<font color=blue>"Note: my bike has wheels that weigh about 3 pounds in rotation and they provide enough gyro to keep me (about 150 pounds) from falling over."</font color=blue>

<font color=red>"I thought it was those little gyroscopes in the inner ear that keep one from falling over...."</font color=red>

I agree with the "inner ear" theory on this one. I just finished teaching my 6 year old twins how to ride a bike without training wheels. They started out the first day going 6 feet and flopping over. The bike stayed the same, but they got better each day until they could finally ride all around the yard. Try taking a bike and fastening the front wheel straight, then give it a good push down a hill. I haven't tried it, but I pretty sure it won't go far.

 

[Green_in_MI]

I am surprised some of you MEs out there don't remember this stuff off the top of your heads. Gyroscopic effect is primarily measured from the angular momentum of a rigid body in plane motion. Ok, I peeked in my engineering dynamics textbooks. I also found it difficult to believe I once understood all that stuff in there. Without wading through the equations which I no longer seem to have a grasp of anyway I surmise that the small mass of the blades and its relatively slow rotation speeds will not help in preventing your rollover. Now if you had a solid spinning disk going much faster - like ten times faster- then maybe you could stabilize your tractor. Your cutting blades are nothing like propellers on an airplane which do cause a gyroscopic effect. Last time I checked my mower was not lifting the tractor.
This is not to say that rotating blades could not have a tremendous mechanical effect on your tractor. If you were operating a rotary mower on an old tractor without a slip clutch the momentun of the blades could drive you right through a fence row at the end of a run. But, it takes much less energy to push than lift.
Overall I would guess that the gyroscopic effect of the spinning blades is roughly offset by the equivalent of a full beer in your cupholder. I will say, since you have already figured out I don't know what the heck I am talking about, I still think this is a great subject.

 

[Bird]

You're right, Mark, you turn downhill; turning uphill will finish the job of rolling you over./w3tcompact/icons/laugh.gif

 

[MikeOConnor]

Heck, can't let this one go... So I asked a really smart chemist (my 80+ year old Dad) about this.

He reminded me of the "right-hand rule" from electricity days. 'Turns out you can think about gyroscopes the same way. Here's what you do;

Make like you're a little kid, making a gun out of your right hand in a game of cops and robbers -- forefinger straight out, thumb straight up, middle finger (the one you use to comment on the driving habits of others) pointing straight left. Those are the three axis we're interested in.

If you're applying force in two axis, the gyro will "resist" and try to move in whatever of those three directions you're **not** applying force.

M'Dad also used the bicycle analogy. If your gyro is a flail mower (like mine), it's like the wheels of a bike. It's rotating in the "middle finger" axis, the hill is rotating you in the "forefinger" axis and the gyro will resist in the "thumb" axis -- the only one left over. Which means that the gyro will try to make your tractor turn down the hill -- so I think Markie61 is right when he turns "downhill" to recover from a slide. That's working with the gyro, so it'll be easier.

BUT ONLY WITH A FLAIL MOWER. If you have a rotary mower, the axis are different. Now the mower is rotating in the "thumb" (vertical) axis, the hill is rotating the tractor in the "forefinger" axis and the gyro will resist in the "middle finger" axis -- ie, it will push either UP or DOWN on the nose of the tractor, depending on which way the mower is rotating.

Pretty cool, huh? That explains why I feel so glued to the hills when I'm pulling the flail mower, and why things get a little hairy when I'm dragging the rotary mower.

I asked him about weight -- he said "really? you've got 1000 pounds in rotation at 500-1000 rpm with that flail mower? It's a wonder you can do anything at all -- it's gonna be generating a heck of a lot of force." Which explains why my 3-point is so balky with the flail -- it's smooth as can be with the little (rotary -- 100 pounds in rotation) mower, but gets pretty hurky jerky sometimes picking up the flail. Especially at full speed.

I asked him the "math" question -- you know, how **much** force, how **much** aid on the hillsides... He said to get a life. <grin> I'm still curious about that though -- c'mon you ME's...