Low drag shapes and lift

[JulianEdgar]

Here's a topic where complete rubbish has (very unfortunately) been disseminated on this forum over many years.

And what's that?

It's that coupe (curved fastback) shapes have low lift - but they don't. Here's a good comparison.

https://www.youtube.com/watch?v=PoEGHtZB8TA

So... typically, these shapes have low drag but high lift! One very important reason that car manufacturers add rear spoilers (etc) to most low drag shapes.

[aerohead]

Originally Posted by JulianEdgar

Here's a topic where complete rubbish has (very unfortunately) been disseminated on this forum over many years.

And what's that?

It's that coupe (curved fastback) shapes have low lift - but they don't. Here's a good comparison.

So... typically, these shapes have low drag but high lift! One very important reason that car manufacturers add rear spoilers (etc) to most low drag shapes.

I believe that any 2-door ( and now 4-door BMWs and AUDIs ) with frameless door glass constitute a 'coupe'. whether fastback, sportback, or notchback.
The image you've chosen for a 'fastback' isn't actually a fastback, as a true fastback has an unbroken roof contour. What's shown would constitute what is currently referred to as a 'Sportback', as Audi and Mercedes-Benz refer to them.
Also, the term 'fastback' could be taken as a true, streamlined contour, as with the 1922 Jaray Cd 0.13 pumpkin seed, or a 'pseudo-Jaray' contour, high-drag contour, as with the original Porsche Cd 0.40, 911/ 912.
Also, also, streamlined fastbacks, as the Nissan LEAF, will not generate as much rear axle lift as Mercedes-Benz CLA 250, or Tesla Model S notchbacks.
Any blanket comment about 'fastbacks' and propensity for rear lift would fail in light of available evidence.

[Vman455]

Originally Posted by aerohead

Also, also, streamlined fastbacks, as the Nissan LEAF, will not generate as much rear axle lift as Mercedes-Benz CLA 250, or Tesla Model S notchbacks.

In what world is this a fastback:



...and this a notchback?



The Leaf's low lift is probably due to the fact that it is quite clearly a squareback, and squarebacks--as Hucho and many, many other authors note--typically develop less lift than fastbacks or notchbacks.

[COcyclist]

I seem to remember many years ago an EM member being flamed and banned for suggesting that “template” shaped cars could get light and could be dangerous at high speeds on dirt roads. Does anyone recall this video of an Insight crashing at 190 mph at El Mirage? It sure looks to me like the back end got light and tried to pass the front despite this being front wheel drive with the weight of the motor and trans ahead of the front wheels.

https://youtu.be/hbNlR-fMlOw

[aerohead]

Originally Posted by Vman455

In what world is this a fastback:



...and this a notchback?



The Leaf's low lift is probably due to the fact that it is quite clearly a squareback, and squarebacks--as Hucho and many, many other authors note--typically develop less lift than fastbacks or notchbacks.

We may have to invent conventions for identification.
* Aerodynamically, a fastback is an uninterrupted contour which is streamlined, and leads to low drag. Such is that on the LEAF. The LEAF roofline, however, has been cut short, losing additional pressure regain, losing additional drag reduction potential from the roof truncation, and opportunity for even lower rear lift.
If the LEAF roof wasn't cut short, it would be a 'Kammback.' And a 'Kammback' is simply a truncated long-tail ( lang-heck ).
In spite of the roof chop on the 'Drag Queens' LEAF, it had lower rear lift than the Tesla Model S.
--------------------------------------------------------------------------------------
* As to the Tesla Model S, I would describe it as a fastback-type form, perhaps compromised for rear visibility, with a spoiler incorporated into the contour to create the short deck, allowing for flow reattachment, and perhaps an alcove, from which an active rear pop-up spoiler might emerge. As on the original Model X.

[JulianEdgar]

Originally Posted by aerohead

We may have to invent conventions for identification.
* Aerodynamically, a fastback is an uninterrupted contour which is streamlined, and leads to low drag. Such is that on the LEAF. The LEAF roofline, however, has been cut short, losing additional pressure regain, losing additional drag reduction potential from the roof truncation, and opportunity for even lower rear lift.
If the LEAF roof wasn't cut short, it would be a 'Kammback.' And a 'Kammback' is simply a truncated long-tail ( lang-heck ).
In spite of the roof chop on the 'Drag Queens' LEAF, it had lower rear lift than the Tesla Model S.
--------------------------------------------------------------------------------------
* As to the Tesla Model S, I would describe it as a fastback-type form, perhaps compromised for rear visibility, with a spoiler incorporated into the contour to create the short deck, allowing for flow reattachment, and perhaps an alcove, from which an active rear pop-up spoiler might emerge. As on the original Model X.

When Aerohead is wrong, he simply changes the definitions of words. We've seen it before (attached flow isn't really attached flow if it doesn't match the template) and now a Nissan Leaf is fastback.

It's a fascinating world where reality can be altered to fit belief.

[aerohead]

Originally Posted by COcyclist

I seem to remember many years ago an EM member being flamed and banned for suggesting that “template” shaped cars could get light and could be dangerous at high speeds on dirt roads. Does anyone recall this video of an Insight crashing at 190 mph at El Mirage? It sure looks to me like the back end got light and tried to pass the front despite this being front wheel drive with the weight of the motor and trans ahead of the front wheels.

https://youtu.be/hbNlR-fMlOw

I thought the crash was at Bonneville. Now, not sure.
Like the Racing Beat' Mazda RX-7, the HONDATA lost traction, yawed, became a crude wing section, then lifted off, initiating the series of barrel rolls.
If speed limits are to be raised to 190-mph, then we have a problem.
At 135-mph, Spirit generated negative 30-pounds at the front axle, and positive 22-pounds on the rear axle. On a truck which weighed 4200-pounds.
With a 'template' body. I don't believe it to be an issue.
The 1992 Corvette LT1, which was two cars ahead of me at DARKO went airborne for the same reason as HONDATA and Racing Beat. The car lifted around eleven feet vertically, landing on its; roof at 220-mph. Guess we should ban Corvettes. The owner was given the recommendation to install air fences. Which never happened.

[aerohead]

Originally Posted by JulianEdgar

When Aerohead is wrong, he simply changes the definitions of words. We've seen it before (attached flow isn't really attached low if it doesn't match the template) and now a Nissan Leaf is fastback.

It's a fascinating world where reality can be altered to fit belief.

So, why don't we work it out. You have Hucho's 2nd-Edition. You know that he basically said that 'fastbacks' are the only real path to low drag.
If that's the qualifier, then the LEAF is a roof-truncated fastback. Adding the 'elongation' to achieve the lower drag would visually drive the message home.
Like the 1978 Daimler-Benz, Mercedes-Benz, C-111, III, in three flavors: Cd 0.237 to Cd 0.178. Depending on length.

[JulianEdgar]

Originally Posted by aerohead

So, why don't we work it out. You have Hucho's 2nd-Edition. You know that he basically said that 'fastbacks' are the only real path to low drag.
If that's the qualifier, then the LEAF is a roof-truncated fastback.

That is a pretty good example of Aerohead 'logic'.

1. You have Hucho's 2nd-Edition. You know that he basically said that 'fastbacks' are the only real path to low drag.

2. If that's the qualifier, then the LEAF is a roof-truncated fastback.

Same logic as:

1. Dogs have four legs.

2. This animal (a cat) has four legs, therefore it is really a dog.

[aerohead]

Originally Posted by JulianEdgar

That is a pretty good example of Aerohead 'logic'.

1. You have Hucho's 2nd-Edition. You know that he basically said that 'fastbacks' are the only real path to low drag.

2. If that's the qualifier, then the LEAF is a roof-truncated fastback.

Same logic as:

1. Dogs have four legs.

2. This animal (a cat) has four legs, therefore it is really a dog.

1) The LEAF has a pseudo-Jaray roofline failure, just as Kamm/ Fachsenfeld investigated at the FKFS.
2) The solution was to extend the roofline, following an extrapolation of the initial contour, to the very end of the vehicle, filling in the turbulent void.
3) This is the reason your Insight has the roofline that it has. If it were mutilated as with the LEAF, you'd experience increased drag and lift.
4) If carried to it's conclusion, the LEAF becomes a pumpkin seed, just as Kamm, Lay, and Jaray arrived at. True 'fastbacks.'