A quiz for people

[JulianEdgar]

And finally, for people who like the idea of 'ultimate low drag shapes':



Note: the final column is not Cd but instead CdA. However, as you can see, the nominated frontal areas are all pretty close to 1 square metre so in turn you can read these as close to the Cd.

These drag values are much lower than the oft-quoted 'ultimate shapes' of the 1930s (and so also of course to any 'template' shape derived from those old shapes).

[California98Civic]

Very interesting case, but your last line in post 29 does seem like a straw man argument. Nobody is arguing that absolutely any road vehicle body that someone classifies as "streamlined" is of zero lift shape. "Properly streamlined" I am sure I have seen claimed, but "properly" is a big qualifier. Seems to me Honda probably knew this shape created lift and that they may have tuned this car for a little lift to reduce rolling resistance. A question I would have would be whether this lift is dangerous. I wonder what degree of lift under yaw will "lift" the car off the road. Were there race rules that would stop the race under certain wind conditions. I would think there might have been given the lighweight and wing like shape of solar racers.

I gotta add one edit: up to 100 kph, lift is pretty modest and the car ran at about 90 kph. And I definitely have not seen anyone argue that a streamlined body under yaw would produce no lift. It is an interesting study but it proves less than you seem to think.

[JulianEdgar]

Originally Posted by California98Civic

Very interesting case, but your last line in post 29 does seem like a straw man argument. Nobody is arguing that absolutely any road vehicle body that someone classifies as "streamlined" is of zero lift shape. "Properly streamlined" I am sure I have seen claimed, but "properly" is a big qualifier. Seems to me Honda probably knew this shape created lift and that they may have tuned this car for a little lift to reduce rolling resistance. A question I would have would be whether this lift is dangerous. I wonder what degree of lift under yaw will "lift" the car off the road. Were there race rules that would stop the race under certain wind conditions. I would think there might have been given the lighweight and wing like shape of solar racers.

I gotta add one edit: up to 100 kph, lift is pretty modest and the car ran at about 90 kph. And I definitely have not seen anyone argue that a streamlined body under yaw would produce no lift. It is an interesting study but it proves less than you seem to think.

1. It is good evidence that one of the slipperiest cars ever made (and so surely by any normal definition, the most streamlined) had a lot of lift. (Or are you suggesting this car is 'improperly' streamlined?!)

2. This idea that for lift to discombobulate the car it has to lift off the ground is complete rubbish, and I have never seen that argument used anywhere except here. Refer to SAE papers 1999-01-0651 and 2009-01-0004 to see how quite small amounts of lift can cause problems in car stability.

3. I'll repeat the quote from Honda: "Future research and development will be required to refine the suspension and/or front lift coefficient in order to further increase the cruising speed." To draw the obvious inference, the aero lift (and suspension softness) were limiting their cruising speed.

4. I don't know of any rules to stop the race in windy condition. As far as I know, the cars just went slower - with these figures, you can certainly see why.

5. If anyone has the measured coefficients of lift for any other really low drag cars, I am very interested.

[Vman455]

Originally Posted by California98Civic

I gotta add one edit: up to 100 kph, lift is pretty modest and the car ran at about 90 kph. And I definitely have not seen anyone argue that a streamlined body under yaw would produce no lift. It is an interesting study but it proves less than you seem to think.

One of the qualifying runs was apparently a top-speed competition, for which the engineers lowered the nose in an attempt to "suppress CLF increase, in order to achieve the highest possible speed."

Regarding lift and stability, the engineers wrote:

A solar car on the road has to run through natural wind and thus receives its disturbances. The '96 "Dream" was thought to be prone to crosswind disturbances because it was considered lightweight, at approximately 300 kg, with a relatively large body. Fig. 7 shows the crosswind sensibility of the '96 "Dream" measured in wind tunnel tests. It can be seen from the figure that increasing the yaw angle from 0º to 12º (corresponding to receiving side wind of 6 m/s from the lateral direction, while running at 100 km/h) reduces front and rear wheel load by approximately 2.1 and 21.4 kg, respectively. The reduction cannot be neglected because it translates into a 1.4% and 14.2% reduction in front and rear wheel load, respectively. Disturbances induced on vehicle behaviour by lift force resulting from crosswind, as well as other disturbances by side force and yaw moment, must be suppressed to a level low enough not to disturb vehicle driving.

[GreenTDI]

To quote myself:

Originally Posted by GreenTDI

It looks like a wing, so I would say 3.
If the part under the car (between those wheels) is flat, straight and not curved, I would even say 4.

Originally Posted by GreenTDI

My guess is that it will take off at highway speed

OK, did I win something?

It's quite an interesting case.

[JulianEdgar]

Originally Posted by GreenTDI

To quote myself:

OK, did I win something?

It's quite an interesting case.

No prizes... excepting for knowing you were right!

[mort]

Originally Posted by JulianEdgar

Here is the first part of the answer:

Is there some explanation of how it can increase lift without increasing drag? Either they fudged the "graph" or it breaks the laws of physics.
-mort

[AeroMcAeroFace]

"Is there some explanation of how it can increase lift without increasing drag? Either they fudged the "graph" or it breaks the laws of physics."

Maybe the lift causes the front to lift more than the back thereby increasing the angle of attack of the body shape. As the centre of lift is probably more towards the front than the back.

Correct me if I am wrong but that is what happens in many older cars.

[JulianEdgar]

Originally Posted by mort

Is there some explanation of how it can increase lift without increasing drag? Either they fudged the "graph" or it breaks the laws of physics.
-mort

Yes, that's an interesting question. You're talking about induced drag - ie drag caused by the lift?

Porsche's Taycan records its lowest drag in its highest lift configuration. (I did ask Dr Thomas Wolf of Porsche about that but he chose not to reply to that question.)

[JulianEdgar]

Originally Posted by AeroMcAeroFace

"Is there some explanation of how it can increase lift without increasing drag? Either they fudged the "graph" or it breaks the laws of physics."

Maybe the lift causes the front to lift more than the back thereby increasing the angle of attack of the body shape. As the centre of lift is probably more towards the front than the back.

Correct me if I am wrong but that is what happens in many older cars.

With its CLf much greater than its CLr (at least at zero yaw) you'd certainly think the centre of pressure was well forward.

But I am not sure that answers Mort's question, does it?