Originally Posted by
Racer Ex
Which is why it's not a substitute, which was my original statement.
And I didn't dispute that. I've said many times in many places that field testing is not a substitute wind tunnel testing; I've said it's a complement. The part I was disagreeing with was the claim that field testing *always* has poorer precision than wind tunnel testing, and that field testing *can't* handle non-zero yaw. I've shown that field testing *can* have very good precision, and that it *is* possible to measure non-zero yaw.
You're at nature's whims and while you'd now like to describe noise as a good thing, we both know it's not.
Evidently we both don't know that. In my day job I look at a lot of "noisy" data. The history of this field is that sometimes the data that we think of as "noisy" can reveal systematic variation in addition to the random component. Being able to tell one from the other can often lead to better estimates. That's why my field tests turn out to have much better precision than traditional field tests -- traditional field tests couldn't differentiate between the two.
I'm pretty OK with my protocol BTW. If you'd like to peer review it I'd be happy to forward it.
Sure. Maybe start a new thread.
Don't need [an estimate of the confidence bound on estimates]. My confidence comes from repeatability on a controlled set up during the tests. But I'm using the tunnel for comparative analysis, not for the aero equivalent of the Ewang FTP chart.
I've got zero concern with the absolute accuracy of the derived number which is probably where and why we're parting company on this discussion. The fish I'm frying is winning bike races.
Actually, I don't think this is where we disagree. If the tare on a wind tunnel balance is off then the absolute drag is off but most of us are knowledgeable enough that we don't care about that. Most of us care about comparing equipment or positions so even if the tare is off the relative difference is still good. That's what happens with well-performed field tests. The absolute CdA can be off but the relative comparison between equipment or positions depends on precision.
BTW, this is what I was referring to when I wrote about "by time you build it, you could have gone to the tunnel a bunch", not the zero yaw tests. Given that you're probably the most noted advocate and resource for field testing and you don't have the equipment or even an controlled estimate of time/cost/effectiveness of non zero yaw equipment...
Um, what makes you think that? I haven't made any specific comment on what I do or don't have available to me.
Originally Posted by
Nate552
Are you talking about force measurements? The Oran Nicks Tunnel lists the specs of the tunnel on their website. I don't know about the San Diego or A2 specs.
Nope, not exactly. I've known for a while what the claimed accuracy and resolution of the force measurements are. I'm interested in the measured precision, not the accuracy.
I'm having a tough time with the "thumb nail" CdA. A CdA diffrence of .001 m^2 works out to be, what, a watt? So your powertap, is reliable down to the 1 watt? Impressive. Have you published this test? I'd like to see your testing protocol, as it must have been really good.
Of course I've written this up. As I pointed out above, you don't even need to use a power meter -- it's just faster and easier (much easier) that way. If you don't have a power meter then you have to do the tests at zero power -- reliably at zero power. You think that's impressive? But by your questions I'm guessing you're not trained as a statistician. For example, we can talk about average global warming as measured in hundredths of a degree even though any individual thermometer may not have precision of 0.01 degrees. The Central Limit Theorem is a pretty neat thing. As for the protocols, they depend on the type of venue and situation you have available. Even if you have the right venue using the wrong protocol can give you poor results -- just as using the right protocol but in the wrong situation. I've never claimed that a good protocol magically makes everything work (I've claimed that a good protocol makes things better than a poor protocol).
Originally Posted by
Nate552
This. When it comes down to it, we're trying to figure out what is fastest at various wind conditions. A wind tunnel is superior than field testing for this purpose. I'm not saying field testing is bad, or worthless just that it has it's limitations.
Of course field testing has limitations. Who has said it doesn't? Y'all have been making blanket statements. I've been making nuanced ones: I've been saying that dismissing field testing because "it's not precise enough" or "you can't do non-zero yaw" is incorrect.