Quote:
Originally Posted by aerohead
Looking down from above (plan view) the section would be 4X as long as it is wide.If you look at the aft portion of this section,you will see that this tail is actually longer than that of the 'Template.'
Fachsenfeld used this in his patent application illustrations,whereas most of the streamlining was done along the sides.The body began to narrow at the A-pillar,a bit more radical than EV1,which began its taper midway down the door.
A number of VERY low drag cars have used this approach.
No matter what you do,the roofline or body sides should never exceed 22-23 degrees.A turbulent boundary layer cannot support attached flow with a steeper curvature unless you provide blown or suctioned slots.
If you have a notchback car go ahead and use VGs as on the Lancer.
For large trucks and vans there is little evidence (none that I personally know of)that anything which might help an airplane would help a road vehicle,except slots.Hucho had no evidence that VGs or turning vanes could help lower drag of a truck.
In free flight,the aft-body of an aircraft fuselage,or bullet enjoys a jet-pumping action which doesn't occur in ground proximity.On the ground,the flow is asymmetric and causes a doubling of drag.
The reason for the 'Template' was that it was developed from a separation-free form.Since aerodynamic streamlining is predicated on reduction or elimination of flow separation,it is a 'natural.'
And the curvature is already as steep as it can be without triggering separation.If you make the tail steeper you will increase pressure drag (the big no-no!) If you make the tail longer you're adding additional skin friction ( the lesser of two evils).
And don't be looking at 'laminar' sections either.They would only help you alone on a closed course,with zero wind,at around 20 mph.If the wind came up,or you got into traffic,or went any faster,the section would transition right into a turbulent boundary layer and skin friction increase.  |
Ok,
As I understand it you may be suggesting a taller vehicle may need an even longer proportional taper profile.
Would this be the same whether that vehicle had a longer body or shorter one with the same frontal area?
This then also contradicts the use the narrower section to govern the taper as has been mentioned on numerous posts in regard to boattail design.
Got the 22-23° rule, never break it under any circumstances.
Got the Laminar thing, not relevant to road going vehicles.
Got the VG thing, many questions still remain unanswered and no conclusive evidence of benefits shown except in rare specialised cases.
But things still don't add up:
The Dryden van boattail, now they reported significant benefits and yes they did get seperation further down the on the original tail before they truncated it, but they most likely went way past the 22° point then as well.
As I understand the template theory, they should have had seperation on that tail in the first 6", yet flow remained attached for a good 4'.
from this thread:
http://ecomodder.com/forum/showthrea...n-21952-7.html
And then there is all the designs, patents and fitted boattails to Semi Trailers, none of these are anywhere near the template, and most are in the same range as the Dryden Van 10-15°, I assume they have had some proven testing over the last 10years, otherwise we would not be seeing an increase in their application.
I don't dispute the effectiveness of the template profile in boattail design, but am just trying to answer some of the contradictions.
I do know now that in fluid motion the rules change with size, materials and conditions, infact it seems we really don't know what the rules are.
We have a set of rules that can effectively describe an outcome under a fixed set of conditions, then as conditions change we need to change the rules.
Doesn't seem to be far from quantum mechanics, everything is just a model for an observed effect, but actual governing laws are ever elusive.