Galt, I’m sorry if you’re put off by the technical jargon. It’s an unintended consequence from 20 years as a military and commercial aviator.
By the way, I tried your experiment. I held my bicycle straight up, then tilted it a little. And sure enough, the wheel turned, just like you said. Then I let go.
Darndest thing, would you believe it fell over? I can’t figure why, shouldn’t the trail have saved it?
Then I picked it up and pushed it. It rolled straight and true for a few feet, slowed a bit, began turning left, and then fell over again. So what dynamics is at work that allows trail to function at speed but diminishes as the bicycle slows? I’m so confused!
If you still don’t get it, let Professor Bloomfield explain it to you:
http://howthingswork.virginia.edu/
As for ruts, they don’t change the laws of physics. In my experience, a rider who looses control in ruts does so because he fails to steer straight down them. He turns the front wheel causing the tire to contact the walls of the rut, slowing the wheel’s rotation and diminishing gyroscopic stability. Not to mention that the knobbie now wants to climb up the side of the rut, wresting control from the rider.
As for the track stand, I have seen track riders come to a complete stop – no back-and-forth – for several seconds. I have seen MTBers remain absolutely motionless for the better part of a minute (fat tires simplify the matter). They might twitch the handlebars, and they might shift their weight about, but the tires remain fixed to the same location on the ground. Oh, and you can find information elsewhere on the Internet stating that on 25 November, 1977, one David Steed kept a bicycle upright and stationary for 9 hours and 15 minutes. Stationary. Motionless. Without rolling. Which sort of refutes the contention that rolling is at all necessary.
As for the link you provided, it refers to “sum angular momentum of the system,” also known as gyroscopic precession. Although precession always occurs with it, it is not synonymous with the basic gyroscopic force.
Considered as a system, when an outside force is applied to any spinning gyroscope, the change manifests itself 90 degrees in the direction of rotation. This is gyroscopic precession, a.k.a. “sum angular momentum.” This is why a bicycle wheel feels like it’s twisting funny if you hold it between your hands and have someone spin it up, then try to turn it. It isn’t turning in the direction in which you pushed, but another 90 degrees around in the direction of rotation. This is also why a top falls over in a gradually decaying spiral when it looses its momentum (before it hits the ground).
If you mount two wheels on the same axle and spin them in oppsite directions, it is the gyroscopic precession that cancels. Precession is directional because the input was directional. Gyroscopic force is_not directional; it acts in a plane and cannot be cancelled.
And, by the way, any definition of torque will include some phrase akin to “twisting force.” Sitting on your bicycle does_not apply torque until you start turning something.
Chronos, you’re looking at my finger and not at the moon. I mentioned coning of rotor blades because it is part and parcel to the physics that is necessary to preventing the rotor from doing a veg-a-matic routine on the fuselage. You can’t create centrifugal force without also creating gyroscopic force; they both are manifestations of inertia being forced to travel in a circle. Like gyroscopic force and gyroscopic precesion, they are joined at the hip.
This is lapsing into a long-winded aerodynamics lecture, but here goes. All winged aircraft owe their stability to a relative equilibrium of aerodynamic forces. Consider a simple airplane flying straight and level. What makes it turn, for instance, is causing one wing to generate more lift than the other. This tilts the vertical component of lift, disrupting the equilibrium that existed when you were straight and level. The weight that had been in direct opposition to lift now needs a counterbalance. Equilibrium is restored when the airplane turns, its now-angular lift component balanced by an equally-angular centrifugal force.
A helicopter has wings too, they just happen to rotate. Although pilots sometimes refer to it as a “rotor disk”, it obviously is not a disk. Neither do these wings create lift in some magical, uniform disk-shape; they obey the same physics as airplane wings and only create lift directly above each individual rotor.
All aircraft – airplanes, gliders, helicopters, gyrocopters, balloons, blimps, dirigibles, zeppelins, hang gliders, parasails and parachutes alike (with the exceptions of military fighters and aerobatic airplanes) – are designed to seek this equilibrium. With the particular exceptions noted, positive dynamic stability is a good thing.
Particularly at a hover a helicopter has little aerodynamic equilibrium apart from that created by the big fan on top. In a manner of speaking, the fuselage is ‘borrowing’ its stability from that generated by the rotor system. In forward flight it gets considerable slipstreaming but even then it still is just tagging along with that oh-so-stable rotor system.
You probably have seen amphibious helicopters, usually with pontoons mounted on the skids. Have you noticed that you never see one shut down the engine(s) while on the water? They can’t. Or start it either. The dynamic forces generated by the slow-turning rotors would cause them to rock so hard they’d tip over.
All helicopters rock about when starting or stopping their rotors, particularly those with 2-bladed rotor systems (Hueys, Cobras, Jet Rangers, etc.). But when the rotors are up to speed they create a gyroscopic force that resists that wobbling and, in resisting, (somewhat) dampens it. And the wobbling is violent enough that it could capsize any amphibious helicopter (including the enormous Chinook).
Land-based helicopters aren’t immune to this transitory imbalance, either. Some helicopters – such as the Hughes/Schweitzer 269 & 300 – suspend their skids from shock absorber-like devices known as an Oleo struts. As their rotors are being engaged, these “shock absorbers” can amplify the rotor-induced rocking in a harmonic phenomenon known as ‘ground resonance.’ If allowed to proceed unchecked, ground resonance will shake the aircraft apart before the rotor reaches operational speed. The pilot’s one consolation is that when the rotor blades come loose, he’ll be safe in the eye of the hurricane.
The pilot relies on that gyroscope for bigger reasons, too. He counts on it being a nice, stable flying disk that he can, through manipulation of aerodynamic forces, cause to change velocity and direction. Fortunately for him, because of its gyroscopic force, it wants to remain horizontal, just like a well-thrown Frisbee®.
All gyroscopes resist change; that is their fundamental property (and the reason they are an essential component in guidance systems). The helicopter pilot uses to his advantage the fact that his rotor system – owing to gyroscopic forces – wants remain horizontal.
If the rotor system did not have its own nominal “steady state,” the whole wretched aircraft would be uncontrollable. And that goes for Chinooks and Kamans and Kamovs with their counter-rotating rotor systems as well. In fact, their rotor systems, like those of all large helicopters, have such a powerful gyroscopic force that the pilot requires hydraulically-boosted controls to be able to budge it.
Sure, a helicopter can fly without gyroscopic force. So can a Frisbee®. Oh, sorry, that’s not called flying, it’s called FALLING.
I apologize for the helicopter tirade but the issue at hand was gyroscopes, and I can think of no more powerful a gyroscope on earth than the spinning rotor system on a CH-53.
Razor blades and ski bikes? You’ve left out roller blades and slalom water skis and ice skates and the Beatles’ skiing chairs. These devices keep the user so close to the ground that no extraordinary dynamics is required; the normal human sense of balance does the trick. The moment arm from the ground to the bottom or the user’s feet is very short. And the moment arm to the user’s center of gravity is, in comparison, extremely long. Not that far from the forces present when you’re walking, actually.
