Quote:
Originally Posted by JulianEdgar
Aerohead has a belief, that has unfortunately been apparently adopted by many people here, which is quite wrong.
That belief is that The Template (and roughly similar shapes) have zero lift. Of course, these shapes have in fact very high lift.
Aerohead often quotes references to support this incorrect belief, but the clearest and most accessible he has quoted is Figure 2.4, page 51 of Hucho (second edition).
So let's look at that diagram. (Note that I not going to (mis)quote or paraphrase, as Aerohead does so often. Instead, I will reproduce the diagrams and text.)
The diagram Aerohead references:
Now, what does Aerohead say about it? He said:
Note how there are no caveats or qualifications in what Aerohead says - he's accepting the diagram as being applicable to real cars. But look at the caption. The diagram is for inviscid fluid - that is, an imaginary fluid without viscosity!
And does that matter? Let's look at what is written on the next page:
Note from the passage:
On the rear part of the vehicle's upper surface a steep pressure rise occurs, and it is in this region where considerable differences exist between the real flow of a viscous fluid and the inviscid flow shown here.
and
If all X-components of the pressure distribution on the vehicle surface are integrated, the result for the drag will be D=0.
and
In the real, viscous flow there exists a drag force, but it cannot be explained by considering an ideal, inviscid fluid.
None of this is much of a surprise to me - if you measure real stuff on real cars on real roads, you quickly find where people have misunderstood (or mis-applied) theory.
The idea that Aerohead constantly pushes that low drag = low lift is simply rubbish. It's one of many misconceptions he has, most of which are very easily shown by getting away from the keyboard and doing some real-world measurements on the road.
It's not hard - and it might prevent silly mistakes like referencing a diagram for inviscid fluid as if it applies to real cars. |
If I can get into my photobucket account some how, I have the schematic Sawatzki came up with for a Jaray car, which has identical flow features as the 2-D image. It appears in Hoerner's AERODYNAMIC DRAG, page 160. The significant lift occurs at the A-pillar.
The car has quite a rear overhang, and with a positive pressure acting over a 23 % body-length moment arm, going beyond the rear axle it generates significant downforce.
Diffusers can also provide a counterbalance with respect to overall lift.
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As to low drag / low lift, don't get obsessive about the upper body pressure profile without investigating under the vehicle.
Spirit generated 22-pounds rear lift at 135-mph. And 30-pounds downforce on the front axle.
Her travel weight was over 4,200 - pounds. My travel speed was 65-mph. Any lift at that velocity wouldn't be on the radar screen at all. Just take the squares of each velocity and compare them as a percentage.