Quote:
Originally Posted by JulianEdgar
I can see you might think that if you've just skimmed stuff. But unfortunately a great deal of what Aerohead writes here - and especially when he gives advice or theorizes - is simply outright wrong.
Whether that's because he hasn't understood the theory or is just mis-applying it, I don't know. Sometimes, he takes accepted (and correct) theory and then just extrapolates it into complete weirdness.
And then we have to add that he hasn't bothered keeping up with aero developments of the last 20-30 years.
The result is that he is grossly in error on many aero topics.
Just off the top off my head here are some of things he constantly gives incorrect advice on:
- attached and separated flow patterns
- pressure distributions on cars
- the relative importance of lift forces
- the lowest drag wheel designs
- how rear spoilers work on current cars
- best angles for rear diffusers
- sharpest rear angles that will retain attached flow
- flow patterns on current notchback cars
Then overlay all that misinformation with some truly weird conspiracy theories about how car makers develop cars, write in a pseudo-sophisticated mumbo jumbo that is often impenetrable, and we have a fascinating situation that has obviously developed over a long time.
Unfortunately, by far the number 1 source of misinformation on aerodynamics on this forum is Aerohead.
I used to think about half of what he wrote was wrong / misleading / irrelevant, but as he has now been nominating references in an attempt to support his misleading statements (and where I have them, I have been checking those references), I now realise it's even higher than that.
I don't like misinformation about car modification being spread: it costs too many people money and time - people who should be benefitting from the best info available, not wrong and/or outdated advice and strange theories.
|
I haven't bought any SAE Papers for the duration. At $ 30 a pop, that's too rich for my budget. The internet although, has been a source for some contemporary investigations. Like rolling road tunnels and wheel drag.
EcoModder members have been very good about scanning for, and sharing links to online research papers.
* I keep waiting for you to correct my understanding of flow separation. I can't seem to find any counterfactual evidence, only validation.
* The pressure distributions aren't of the same value as axle loading vs static wheel loading, with passengers and luggage onboard, at full GVWR, at 'normal' passenger car velocities.
* As vehicle speeds have increased, so have vehicle mass. Tire technology has kept pace. Aquaplaning risk may not be any different than 30-years go. The popular press doesn't mention it.
* The ' WS12 Aerodynamic Performance' report spoke of 'uncertainty.'
For the rear wheel fairing data, depending on fairing depth, 'wheel stationary' vs ' wheel rotating' demonstrated no discernable drag difference, depending on fairing depth. Alan Pope talked about 'rotating' wheels vs 'stationary ' wheels amounting to a 'few percent' difference in drag. A locomotive example showed a 5.263% delta. General Motors published that the difference was around 5%, was easy to compensate for, and not worth the expense.
* As to rear spoilers, we're not through the discussion yet, so you might hold your opinion.
* Carr and FIAT got identical values for diffusers. If you have some value-added material it would be much appreciated.
* The sharpest rear angles are from Buchheim et al. Cars aren't built like that any more. As to 'curved-roof' rear contours, the 'template' is scientifically close to 'ideal / optimum' ( Hucho uses the two terms interchangeably ).
* The 'template' can be found at the bottom of Figure 2.1, page 61.
* The streamlined body of revolution from which the 'template' is derived is 3rd from the bottom of Figure 2.1. It is the 'shortest' streamline body which respects Mair's 22-degree cut-off threshold for rear slope angle for preventing separation.
* Any shorter and we suffer pressure drag.
* Any longer and we suffer friction drag.
* It is 'optimum.'
* For flow patterns on contemporary notchback cars, the Cd will indicate the amount of separation. Drag is a function of separation. Nothing can be done about skin friction.
* The days I come to town, I experience Sentras, Accents, Rios, Lancers, Civics, etc.. which appear incapable of flow reattachment. Technical drawings compared to the 'template' indicate separation with no reattachment.
* ' The problem of 'streamlining' is to devise shapes such that the boundary layer will not break away and the wake will remain inconsiderable. It is found that to delay the breaking away of the boundary layer the region where the fluid is moving against increasing pressure should curve as gradually as possible, i.e. should have a large radius of curvature. Good streamline shapes should be such that the breaking away point is as near as possible to the trailing edge.' L. M Milne-Thompson, C.B.E., Professor Emeritus,' THEORETICAL AERODYNAMICS, page 22, 4th-Edition, Dover Publications, Inc., N.Y., N.Y..