Beware old and new aero data

[Hersbird]

I'd be curious what do you think about the Cybertruck's potential aerodynamics Julian?

I personally think the prototype as shown with 35" mud tires, and big wide fender flairs is never going to get a .3 Cd, not even close. Some aeroshell on the bed of a pickup isn't going to suddenly result in the lowest drag pickup design ever.

I still think it was just a cool thing somebody sketched up that Musk liked and now everything is being back engineered to try and fit that, especially aerodynamics.

I'm sure you don't have any insight on that aera, but just generally, is this (the prototype at least, who knows what actually is final) better aerodynamically than a new Ford or Ram?

[aerohead]

Originally Posted by aardvarcus

Julian,
I don't know how much surface pressure or lift/downforce data if any Phil collected when he took his T100 to the Darko wind tunnel for testing in 2014 and 2017. But I doubt he dismisses all measurement data seeing as how he spends time and money on money on wind tunnel testing.

I know from experience all measurements require a grain of salt, and there are things i have seen in print i remain doubtful of based on my past experiences. Thus it doesnt suprise me that the two of you are lending varying levels of credence to different sources based on your past experiences. Thus you dont agree.

I don't actually know either of you, but realize my motivations in this conversation are selfish, in that i enjoy seeing both of your content, and I personally wouldnt want either of you to not post. Both of you have far more experience than I do.

I personally feel that if we were all sitting in a living room and not on the semi anonymous internet removed from broader context we would get along better.

There were $144,000 worth of Honeywell strain gauge load cells and air bearings under Spirit at DARKO. At an equivalent 135-mph, Spirit generated 30-pounds downforce on the front and 22-pounds lift at the rear,for a vehicle with a static loading of 4,220-pounds.With 50/50 weight distribution,that's 1,055-pounds per wheel.At 135-mph Spirit weighed 4,228-pounds.I must presume that this would be defined as 'zero-lift,' for all intents and purposes.
The shape was pre-tested in 1935.I never saw any justification attempting to reproduce the pressure profile testing.What I'd done consumed 23-months worth of savings as it was.

[aerohead]

Originally Posted by JulianEdgar

We now seemed to have progressed(?) to rationalising incorrect statements as better than nothing.

I am sorry; I don't agree.

Rules of thumb that aren't stated with qualifiers but instead as absolutes, are deceptive. It's pretty odd that people then turn around and say "But I am sure most people realise that they're not absolutes". Why not just say that at the time? It's not hard:

Very approximately (it depends on the car, engine and other factors), at 55 mph, a 10% drag reduction translates to a 5% increase in fuel economy. At 70mph,a 10% drag reduction translates into a 4% increase.

There we are, a useful rule of thumb that won't lead anyone astray.

A template that is presented as the absolute best approach, when there's very little evidence that it actually is, and the shape isn't even mentioned (as far as I am aware) in any current aerodynamic technical literature, is deceptive. That is, when it is presented without a great deal of qualification.

When I started reading this site I was surprised by:

(1) The way pat numbers were thrown around as if, of course, just following these will give you the best results. (Then I realised most of that information seemed to be coming from one source.)

(2) The complete ignoring of vertical aerodynamic forces, as if lift (and induced drag) didn't exist. (Then I realised most of the misinformation on that topic was coming from one source.)

Someone said that I should expect my statements to be scrutinised. Well, fine. If it's something theoretical, let's compare references, with obviously greater weighting given to most recent research. If it's something practical, let's compare on-road test results.

I only take exception to criticism when it's from a keyboard warrior - someone who hasn't actually done what I am describing, but just knows I am wrong (eg in my test results)... without their presenting any evidence to support their statements whatsoever. (That is, it's all just what they think.)

As I have said, the content of my book was vetted by four top automotive aerodynamicists. I am pretty confident that a lot (but certainly not all) of the advice on this forum would never have made it past them.

I just want accurate, useful and effective advice being given to those who wish to modify the aerodynamics of their cars!

And, in my opinion, bad advice is worse than no advice.

*You're putting words in my mouth.
*The rule of thumb had nothing to do with any variable other than pressure drag.The caveats were clearly identified.
*And it's 6% per 10% at 70-mph,not 4%.Please check your work!
*If you'll read all the entries at the 'Template' thread you'll find all the qualifiers. You won't understand it yet. That's not my problem.
*There's no guarantee that your aerodynamicists have command of the data.If so,it would be reflected in the products of their clients.

[MeteorGray]

Originally Posted by JulianEdgar

I am not sure you should bring up coronavirus...

Deaths in Australia from coronavirus: 98 people. That's with a population of 25M

Deaths in the US from coronavirus: 85,000. That's with a population of 328M.

So US deaths from this virus is 867 times that of Australia, despite having a population only 13 times that of Australia.

So US death rate over 66 times as high per capita as Australia...

I know it is off-topic, but I think this will go down for the US as one of the greatest fails of government in history.

Julian, you must have written another book that you forgot to list:

Gratuitous Insults for Dummies

[freebeard]

Originally Posted by Hersbird

I'd be curious what do you think about the Cybertruck's potential aerodynamics Julian?

I personally think the prototype as shown with 35" mud tires, and big wide fender flairs is never going to get a .3 Cd, not even close. Some aeroshell on the bed of a pickup isn't going to suddenly result in the lowest drag pickup design ever.

I still think it was just a cool thing somebody sketched up that Musk liked and now everything is being back engineered to try and fit that, especially aerodynamics.

It's probable that the fender flair/wheel-tire combination is the mot malleable part of the design. What might a low-drag optimized solution be like?

I'm curious about Julian Edgar's opinion as well.

I've been looking for a place to throw this in. Ignoring the cab overlap and open wheels, is this a reasonable solution for a pickup bed? Fences to prevent vortex generation and as a tropfenwagen style fender?


JustaCarGal: Bugorama Irvine 2011, Custom
I'd like to see a mirror-image difusser with fences and skegs.

[aerohead]

Originally Posted by freebeard

It's probable that the fender flair/wheel-tire combination is the mot malleable part of the design. What might a low-drag optimized solution be like?

I'm curious about Julian Edgar's opinion as well.

I've been looking for a place to throw this in. Ignoring the cab overlap and open wheels, is this a reasonable solution for a pickup bed? Fences to prevent vortex generation and as a tropfenwagen style fender?


JustaCarGal: Bugorama Irvine 2011, Custom
I'd like to see a mirror-image difusser with fences and skegs.

There's nothing about the vehicle embodying low-drag potential. About Cd 0.85 as she sits. 'Low-drag' begins around Cd 0.25. The Jaray body would give you 0.25. Hoerner did this with the 1935,Cd 0.85, DKW Meisterclass convertible,according to his own measurements.It would be a pickup with an aeroshell though.

[freebeard]

https://ecomodder.com/forum/showthre...tml#post624339 would fit this thread as well

Originally Posted by aerohead

It's conceivable that the edge vortices as drawn in in red won't exist.The early prismatic bodies from which this phenomena do exist,in no way represent Cybertruck's form.
Tesla provides the following features lacking in the prismatic models:
*nose chamfer (Prandtl surface of discontinuity will steer inviscid flow around a traveling stagnation bubble as if it were an ideal nose)
*chamfer-to-cowl-to to roof apex curvature (there's not a straight line anywhere)
*the pre-apex-to post- apex angle intersection is a fraction of that on the prismatics
*post-apex roofline is curved,not straight
*Tesla's greenhouse/beltline is inclined,continuously attacking the flow field
*Tesla's greenhouse has tumblehome
*the Tesla has plan body camber (it has 6-widths)
*The Tesla has boat-tailing,beginning between the doors,nothing seen like this since Impact/EV1
*Tesla has 8-inches of vertical suspension travel,fore and aft.We know from research conducted on the Ahmed body,that all attached longitudinal vorticity can be totally eliminated via roof,sides,and belly inclination tuning. Tumblehome alone can help mitigate vorticity in some circumstances.
*The wheel eruptions could actually be introduced to tune pressure spikes along the flanks as in Whitcomb-waisting/Coke-bottling/area-rule/sectional density
*As depicted,she could be as low as Cd 0.28
The entire forebody was tested in the late 1960s,with Nuccio Bertone's Carabo concept car.The tuft flow is perfect,with zero vorticity.

"Ignoring the cab overlap and open wheels, is this a reasonable solution for a"[n Ahmed body]?

Would Batmobile fins make the Cybertruck better or worse? I think half the rational for the Cybertruck is the four monocoque butresses at the four corner.

[aerohead]

Originally Posted by freebeard

https://ecomodder.com/forum/showthre...tml#post624339 would fit this thread as well

"Ignoring the cab overlap and open wheels, is this a reasonable solution for a"[n Ahmed body]?

Would Batmobile fins make the Cybertruck better or worse? I think half the rational for the Cybertruck is the four monocoque butresses at the four corner.

The only time we'd use a fin(s) would be if there were a center-of-pressure/yaw moment issue vs center of gravity at high speed, and a wind gust could turn the nose of the vehicle even more 'across' the flow,increasing the yawing moment,potentially putting the vehicle out of control.
The 1985 Ford Probe-V concept,of Cd 0.137 had a degree of cross-control issue,and it was alleviated with a very small,vestigial fin behind the backlight,which was enough to 'weather-vane' the Probe back into the wind,cancelling the torque.
For Cybertruck we ought to wait until it's final iteration,and see how she handles before we apply any palliatives. There's been a lot of investigation into high speed stability and I doubt that it's been lost on Tesla designers. Remember, she's designed by rocket scientists. I'd be surprised if any significant shortcoming fell through the cracks. We'll see.

[JulianEdgar]

Originally Posted by Hersbird

I'd be curious what do you think about the Cybertruck's potential aerodynamics Julian?

I personally think the prototype as shown with 35" mud tires, and big wide fender flairs is never going to get a .3 Cd, not even close. Some aeroshell on the bed of a pickup isn't going to suddenly result in the lowest drag pickup design ever.

I still think it was just a cool thing somebody sketched up that Musk liked and now everything is being back engineered to try and fit that, especially aerodynamics.

I'm sure you don't have any insight on that aera, but just generally, is this (the prototype at least, who knows what actually is final) better aerodynamically than a new Ford or Ram?

I have no idea - I simply don't speculate without evidence.

[JulianEdgar]

Originally Posted by freebeard

It's probable that the fender flair/wheel-tire combination is the mot malleable part of the design. What might a low-drag optimized solution be like?

I'm curious about Julian Edgar's opinion as well.

I've been looking for a place to throw this in. Ignoring the cab overlap and open wheels, is this a reasonable solution for a pickup bed? Fences to prevent vortex generation and as a tropfenwagen style fender?


JustaCarGal: Bugorama Irvine 2011, Custom
I'd like to see a mirror-image difusser with fences and skegs.

I have no idea - I simply don't speculate without evidence. I just wish others did the same.