mrfish

I think we can end this one quite easily. I was never much good at the theoretical bits of Engineering, but I did like practicals. Here is one for the Zipp engineers or those rich enough to buy 2.

Equipment:
2 Zipp wonderwheels
Metal fixing rod
Parking lot
Wind
Anemometer
Safety goggles

Method:
Fix 2 wonderwheels together connecting the LHS of one hub to the RHS of the other hub and place them in a parking lot such that the wind hits them at 15 degrees.

Measurement:
Do the wheels move into the wind or away from the wind?
At what wind speed to the wheels start moving?

Andrew Salmon
M.Eng Oxford 1999.

pino pomo

Can these wheels get me laid?

asgelle

Other than less precision and control and greater noise, how does your proposal differ from what they did in the wind tunnel?

mollusk

You are correct. Post number 1 claims a negative drag, but not a negative drag coefficient. I suppose you could have negative drag with a positive drag coefficient if your area or density is negative (but not both negative) or your speed is imaginary. Please forgive my mistake.

asgelle

As recommended earlier, perhaps a review of vector arithmetic is in order.

SpongeDad

Okay they're being sloppy with terms here - they really mean that at certain wind angles there is "lift" that creates the vector equivalent (as asgelle might say) of negative drag (i.e., thrust).

The question isn't word choice, it's whether the phenomenon exists.

mollusk

Do you know what drag and drag coefficients are? I will give you a refresher course. The drag coefficient is the dimensionless parameter that is the ratio of the drag force to the dynamic pressure times a characteristic area. Usually the characteristic area is the projected area perpendicular to the flow direction, but sometimes a planform area is used instead. The dynamic pressure is ((freestream density) x (freestream speed)^2)/2. In equation form you get (drag coefficient) = (drag force)/(area x dynamic pressure). No vector arithmetic is involved.

There are special cases where the projected area is infinite so a modification to this definition is required. An example is an infinitely long circular cylinder. In such cases a drag coefficients for a unit length of the body is employed. In the literature the first type of drag coefficient is a capital C with a capital D subscript. The second kind has a lower case d subscript.

In any event to get a negative drag force you must have negative drag coefficient because area and dynamic pressure are non-negative quantities. Again no "vector arithmetic" is required if you use the term "drag" how it is being used in fluid mechanics and aerodynamics.

raptor3x

Wow, you really have no idea what you're talking about. Dimples most certainly do not generate lift, their purpose is to energize the boundary layer close to the surface in order to delay separation, thus reducing pressure drag at the cost of an increase in viscous drag. I've included a little cartoon to help clarify this. As far as the negative drag comment goes, the important thing to point out is that if there is a small yaw angle, then all disc wheels will produce a forward thrust. See my second attachment. What ZIPP is claiming is that their pressure and viscous drag combined is less than that forward thrust.

Edit: It seems someone else drew almost the exact same diagram.

asgelle

Thanks, but you really should be sure in your own mind before teaching others. To cut through a lot of words. Force is a vector, speed is a scaler. Check your work.

nerobro

Only with someone of the same sex with who's as desperate to prove their superiority by being associtated with high end components.

prendrefeu

I used to hate this thread, mostly because I was being bashed around a bit.... but now I love this thread, it's the first time I've seen BF go into an actual discussion in questioning marketing hype.

+subscription

ranger5oh

I think what everyone here has missed is that there are 2 separate forces acting on the wheel.
#1 - The torque applied at the hub
#2 - The aero drag
(I am neglecting hub friction, and tire friction.. etc.)

The wheel CAN produce "negative drag", it is spinning from an outside force which is not responsible for the rotation. If the wheel is completely still, not rotating, then no... no way for it to produce "negative drag." The measurements are taken in two different directions... they are just choosing to ignore one.

Think of it this way... a helocopter rotor. Sure, each blade has drag associated with it, BUT since it has a torque applied to the center, and it rotates, it produces more lift than drag. Which, is what I assume they mean by negative drag. The copter has a "negative drag" in the upward direction, and similarly, the wheel has a "negative drag" in the forward direction... Im guessing... this is what they are talking about.

Zipp is only measuring the drag though... and not the force going in to keep the wheel in motion. The wheel isnt "adding force" it is merely reducing the effort you need to put into it to keep going forward. The total force to keep the wheel going forward will still be positive, otherwise Zipp has created a perpetual motion machine.

prendrefeu

No, but apparently the negative drag of the wheels will **** you when you take them out of the box.

and oh yeah, shipping is cheaper, these wheels propel themselves when moving through air or with the angle of wind that's *just right* - just give 'em a light spin to start off. Careful though, once you've given a little spin, they'll **** you.

asgelle

That is not at all what is going on. See post #88.

mollusk

The direction of the drag force is understood to be aligned with the freestream velocity. Much like weight (a force) is in a known direction in everyday applications so one need not explain the direction of the weight of an object on the surface of the Earth. It is understood. Since the direction of the drag force is known the specification of a single scalar quantity will define the force vector. The communication problem that we are having is that you want to make the drag force go in a direction other than in the direction of the freestream velocity. You want to align the drag force with the wheel instead and that is not the correct direction for the drag because the wheel is yawed and not aligned with the freestream. You are talking about a component of force, but it isn't the drag.

So, it is theoretically possible that there is an force in the plane of the wheel that pushes it forward if the wheel is yawed with respect to the freestream? Sure. Is it due to negative drag? No way. That is my main complaint. If it is true it must be due to lift.

waterrockets 

Are stiffer frames actually faster? Discuss.

prendrefeu

yeah, but that thread started before I took some active part of BF.

DrPete 

This is getting fun to watch--I think someone just whittled their slide rule into a shiv.

pino pomo

Damn. I'm usually not picky, but equipment dorks are one of my very few exceptions.

ericcox

The joy of semantics. Though I am not a rocket scientist, physicist, or any one else who should have any say on this thread, in great BF tradition I'll post anyway. It seems to me, based on my lay knowledge, that the language used in the articles on the wheels to describe what is going on is imprecise and probably a bit of a result of trying make it sound good to the reader. It is, however, possible, that these wheels may act in a manner that (as I read it) causes something other than rider input to contribute to forward motion (forgive my imprecise language). Simply described, at certain yaw angles above a certain speed, the interaction of the wheel and the air contributes slightly to forward motion. My actual vision is for a human powered airplane. In a very loose way, a more appropriate analogy than a sail is that Zipp's wheel is acting like a propeller, not a perpetual motion machine, generating lift, which acts against drag, but is not itself negative drag. Unless, of course, it is a sail, transmitting wind energy into forward motion.

But then again, I am just a social scientist. I'm not even sure what I just wrote, but I am happy to say that both Mollusk and asgelle are right. But not DocRay. Never DocRay.

lawlasaurus

ElJamoquio

Understood by whom?

I guess I must've missed that law in fluid flow. It certainly would make the data given by Zipp much less useful, though; I grant you that.

ElJamoquio

Whittle my slide rule into a shiv?


Get with the 21st century, Doc. I know technology hasn't progressed that far in medicine; but in engineering, these days, I use a 5-Axis CNC to machine my slide rule into a shiv.

I find the 1/8" single-flute ball mill to be quite adept at carving in some dimples. My shivs thrust themselves into other people's bodies. When it's windy out, my shivs thrust themselves into lesser engineers' bodies.

DrPete 

Nice.

mista_chewey

i thoughts sails produce thrust not create negative drag.....
im confused....too many pseudoscientists in this thread!!!