The Fastback bloc

[JulianEdgar]

Originally Posted by Taylor95

That is a pretty good looking spoiler. Do the lateral fins make a noticeable difference?

Is that tape on the back or a light strip?

This is giving me ideas for a future Jeep project.

On the Insight the fins make a major difference to stability in crosswinds. The ability of fins to make this difference does, though, depend on the shape of the car. Fastbacks (and sedans) are particularly prone to crosswind wander as their lateral centre of pressure is often ahead of the centre of gravity. By moving area rearwards (as seen from the side) the centre of pressure is moved backwards, so that it coincides (or is even behind) the lateral centre of pressure.

On the Insight the presence of fins appears to lower drag - this is what my throttle-stop testing has shown. I think that this is because the trailing vortex development off the inclined rear pillars is altered - but that's just my guess. (That is, the fins alone - no spoiler).

The spoiler measurably increases pressures on the rear hatch (ie reduces lift) and does not increase drag. I was careful to make the spoiler so that the wake area is not increased over the standard car. (That is, the trailing edge of the spoiler is at the same height as the original trailing edge of the hatch.)

Lots of testing was done with different rear spoilers (including one that also operated pneumatically as an airbrake) and different rear fins - all just mocked-up from plywood, tape and plastic. The shape of the rear fins was found to be relatively unimportant but the shape of the spoiler was very important to getting best results.

The strip across the back is a full-width LED brake light.

I covered the development of the fins and spoiler in a series of videos on my YouTube channel - Part 7 is the summary:

https://www.youtube.com/watch?v=D8QvOukLjes

[aerohead]

Originally Posted by JulianEdgar

Just having a quick look at the forum.

(My emphasis added to above quote.)

This is absolute rubbish. Just go and measure some car profiles and see for yourself. Honestly, saying something again and again doesn't make it true!

A fastback contour, compared to say a squareback, has far more lift.

Researchers that are WAY above your pay grade have done the measurements and published before we were even born. It would be STUPID on my part to duplicate such an expensive undertaking, only to arrive at the same result.
CAR and DRIVER tested a 'template' car at the A2 Wind Tunnel. It produced zero-lift. My 'template' pickup truck measured zero-lift.
You've never experienced a 'streamlined' car before, so you wouldn't have a first-order-reality experience with them. That, however, does not prove their inexistence.
Exceptio probat regulam.

[freebeard]

Originally Posted by DDG

Including results for exception probat regulam.
Search only for "Exceptio" probat regulam?

[pointless filler to lengthen message for vBulletin's Procrustean Bed.]

[aerohead]

Originally Posted by JulianEdgar

Sorry, but I have this really weird idea that misinformation being spread isn't beneficial to people, and should be called out.

Strange, I know, that I don't want to see people being continually mislead.

I'm forever the student, and I value actionable information, and correction, if handled factually, tactfully and diplomatically.
Having read your book, I take technical issue with specific aspects of your handling of lift.
After providing, literally, chapter and verse counterfactual evidence to your view, from Hucho, you continue to cling to 'folk knowledge' for why, I do not know.
And since you obviously do not have a complete command of the topic, I can only laugh at at your reactive behavior when dosed with reality.
Operating within an information void, I can see why the real world would seem foreign to you. What's most troubling, is that you're obviously not interested in the complete narrative on lift, attacking anything outside your myopic understanding as heresy. Your aerodynamicist friends didn't do you any favors when they signed off on your book.

[aerohead]

Originally Posted by JulianEdgar

1. I have covered how fastback shapes cause lift many times here before.

2. I suggested that 'Aerohead' (and of course anyone else who wants to see what really happens) measure aerodynamic pressures on some cars - easy to do. Best not to take my word for it, but to see for yourself.

3. When a discredited idea is trumpeted again and again by the same person, I'd call that idea rubbish.

* The streamlined body represented by the 'template' is a fastback.
* A streamline body, taken to its conclusion, possesses zero flow separation.
* A streamline body recovers all static pressure, less what's lost to surface friction drag.
* The static pressure acting on a streamline body is nearly identical at nose and tail. It's incapable of generating lift.
* You've never analyzed a streamline body.
* None of your measurements are germane to streamline bodies.
* Hucho is a champion of streamline bodies, the only path to really low drag. Germany thought enough of his work that it's in their national museum.

[aerohead]

Originally Posted by JulianEdgar

Incidentally, for anyone who is actually interested in facts rather than supposition and erroneous theories, take the Porsche Taycan as an example of a modern fastback - and let's look at the topic of aero lift.

In profile:



CLr (coefficient of lift, rear) : 0.16

(That figure is from a presentation Dr Thomas Wolf of Porsche was kind enough to send me - rear spoiler not raised.)

Frontal area 2.33m^2.

Calculated rear lift at 100 km/h - 17kg

At 200 km/h - 70kg

At 260 km/h - 119kg (that's 262lb!)

No wonder they fitted an automatically deploying rear spoiler....

All fastback shapes with attached flow develop lots of low pressure over the upper curves. That can only be offset by:

1) rear spoiler (usually only a partial offset of lift), and/or
2) very effective underfloor aero (can completely cancel lift)

But as I keep saying, there's absolutely no need to guess or use only factory coefficients. Just measure the actual, real, aerodynamic panel pressures on your car on the road.

My Insight's measured pressures (my car runs very effective undertrays) - Pa at 80 km/h:



Note the low pressures all the way across those upper curves. It's just what happens with attached flow and these shapes.

So the next time that 'Aerohead' states his completely wrong notion about fastback shapes having no lift, can we actually ask for some evidence - real evidence - and not just pretend the emperor has clothes... and that we should all be politely agreeing with claptrap?

The Taycan, while certainly a 'fastback,' is not a streamline body. It's a composite, with the greenhouse independent of, and glommed onto the main body.
The roofline of the Taycan is a bit too aggressive, there's too much pressure regain for the length of the roof, compromising the boundary layer. The spoiler helps reach out to the streamline contour. It's not as good as simply lofting the end of the body.
Upper surfaces of the body, flanking the greenhouse represent very little deceleration, with high velocity, low pressure. Nothing like a streamline body, in which the body would boat-tail from the same point as the roof apex, building pressure the entire length of the aft-body.
I believe that a close reading of Doctor Wolf supports everything I mention. It's at home, not at hand.

[aerohead]

Originally Posted by JulianEdgar

So all fastback shapes have a lot of lift - but does that actually matter?

After all, even in the case of the relatively high rear lift Porsche Taycan described above (well, high lift before the rear spoiler deploys), the lift is still only a small proportion of the car's mass - especially at the speed limits in most countries.

But my experience with the Insight, and also based on some of the SAE paper research I did for my aero book, is that yes, lift does matter. In both cases, there seems to be evidence that the aero lift is disportionately felt.

I quote some papers in the book where lift - especially rear lift - appears to be oscillatory in nature. (This would not be shown in a wind tunnel that uses averaged Cl figures.) Oscillatory forces (ie they fluctuate rapidly) have the real potential to excite resonances in car suspension. Pretty well all car suspension have natural frequencies in the range of 1-2Hz (it's easy to measure suspension natural frequency with a smartphone and an app, but off topic here) and it appears that the oscillatory aerodynamic forces are within this range. (Perhaps they are caused by Karman vortex shedding?) Thus this appears to be a mechanism by which the car may become disproportionately upset by aero lift - especially at the back.

Another (anecdotal) example is a guy who follows my YouTube channel - Nate - who put a really good undertray on his Passat wagon and immediately commented that the car felt "just planted to the road and incredibly smooth" - see https://youtu.be/jKRrhhk5zj4.

That's also my experience with the Insight (that now develops measurable downforce) - the car is just so different from 80 km/h (50 mph) and upwards. In my car, the ride quality is literally smoother (as the effective weight of the car increases) and stability is much better.

But what if you're chasing just low drag, or you drive only at low speeds (sub 80 km/h - 50 mph)? In those cases I don't think that lift matters much. I would have once said that because lift causes induced drag, you don't want any lift at all for lowest drag, but I note the lowest drag production cars in the world (eg the Taycan) achieve that with lots of lift (although the solar race cars, still on another planet in terms of drag, chase zero lift).

So, TL;DR:

• Fastback cars develop lots of lift
• Lift - and especially rear lift - really matters
• But if you drive really slowly all the time, it doesn't matter much

From a reading of Goro Tamai's book, 'The Leading Edge, many teams have chosen 'laminar' profiles, obsessed with skin friction ,as the fineness ratios of the cars make separation virtually impossible, as with aircraft.
By definition, a laminar profile is laminar, only up to the location of minimum pressure ( maximum body cross-section [ Bernoulli Theorem] ), afterwards, immediately transitioning over to a turbulent boundary layer and higher surface friction drag. By minimizing 'lift', the laminar boundary layer is maintained. A necessary strategy.

[California98Civic]

Aerohead, how might the distinction between Coefficient of Lift and the Coefficient of Pressure affect this discussion? It seems to me, aerohead, your observation about fastback bodies was about pressure, not lift. How are they different? I have been reading this really interesting paper that reports pressure and lift as part of an aerodynamic comparison between fastback and notchback body types:
https://www.researchgate.net/publica...DrivAer_Models

[freebeard]

The Taycan, while certainly a 'fastback,' is not a streamline body. It's a composite, with the greenhouse independent of, and glommed onto the main body.

https://retrorides.proboards.com/thr...ts-front-wings

The 'bubble-top coupe'? 1964½ Mustang and M-B 300SL are purer examples. Of course my favorite is the 1961 Chevy Bel Air.


https://cdn1.mecum.com/auctions/ca08...19817_2@2x.jpg

[California98Civic]

Originally Posted by freebeard

... Of course my favorite is the 1961 Chevy Bel Air.


https://cdn1.mecum.com/auctions/ca08...19817_2@2x.jpg

That's proof that sometimes lowering and bigger rims make a great design even cooler and sleeker.