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Ask HN: Flying Car Practical Tech

1 points by NarcisMirandes ↗ HN
In this presentation, Elon Musk briefly mentioned a practical benefit of the new flying car, at minute 8:45: https://www.youtube.com/live/qt5p-_tE8d0?t=525

"The actual benefit ... where the downforce of the car ... because the car sticks to the ground so hard."

Could anyone who understands the technology expand on that and explain it in simple terms for non-experts?

11 comments

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PaulHoule
See https://en.wikipedia.org/wiki/Downforce
NarcisMirandes
Thank you for the reference. After reading it I get the impression that the connection between the Tesla presentation and a technology that could be applied in real life is not close, to say it mildly. Do I understand?
kingkongjaffa
yes. as https://news.ycombinator.com/item?id=50122639 put it already, active aerodynamics including wings and diffusers has been a thing in automotive design for decades. A skirted active fan is not practical on anything but a smooth road surface especially at speed.

The actual concept is to reduce the pressure under the car to create downforce along the underside of the car (often ducted and pass through a rear diffuser for more efficiency.

The purpose of this is cornering speed. Rubber wheels have limited grip to translate lateral forces and avoid skidding or understeer, so downforce is used to provide a momentary vertical load to keep the wheel pressed into the ground to avoid slip.

We can easily generate downforce to go around corners, but it comes at a cost of adding drag, so straightline high speed is reduced. and the car is less efficient.

underside aero does not incur nearly as much drag penalty so is attactive on F1 and other race cars.

sejje
When the fans and fins press you down you have more traction, so you can turn harder and faster without rolling or sliding
NarcisMirandes
So, the Tesla demonstration is just to show that they can make the car more "stick" to the road and then go faster. Do I understand?
MSKJ
That's the practical purpose, faster cornering, faster takeoff. It's been tried before as well. For example a version of this car from 1978 Formula 1:

https://en.wikipedia.org/wiki/Brabham_BT46

"The "B" variant of the car, also known as the "fan car", was introduced at the 1978 Swedish Grand Prix as a counter to the dominant ground effect Lotus 79. The BT46B generated an immense amount of downforce utilizing a fan, claimed to be for increased cooling, but which also extracted air from beneath the car."

linksnapzz
A car acts like an (imperfect) airplane wing at speed-the pressure difference between the air flowing around the top of the car and the air going underneath can lift the car up. This is why you have airflow spoilers on the lip of the trunk of certain cars, and actual upside-down wings on struts on higher-performance ones.

But...those create drag, so there's diminishing returns in using them. Another option is to disrupt or remove the air flowing under the car. This can be done by passive airflow redirection using an undercarriage tray, or actively via fans/blowers, or both.

F1 designer Gordon Murray famously developed a racecar using a fan duct (the '78 Brabham BT46) which did so well it was immediately banned. He later went on to do the McLaren F1 road car, as well as various recent GMA sports cars with this feature.

NarcisMirandes
Now I understand more. But it surprises me that they did not explain it. Why do they not make a clear connection between the "show" and the real possible application in the real car?

Is this only a big Ad with a very far connection to reality?

linksnapzz
It depends on the audience.

For 99% of drivers, serious active aerodynamic features are irrelevant. You'll never be driving fast enough for it make a difference.

For racers, it's a big deal.

mikewarot
To get the downforce a set of retractable skirts comes down to touch the ground around the body of the car. They're consumables.

The debris that would get thrown out the back going down the road would be a serious hazard.

NishanStepak
It might be useful on Mars where there is lower gravity, especially with thrusters. You might be able to go much farther. It is after all where Musk might go.