A quiz for people
 

So here's a good example of a really low drag vehicle - incredibly well streamlined with a Cd of 0.101 - yes, 0.101.

https://sunwindsolar.com/wp-content/...1/SOLARCAR.jpg

https://live.staticflickr.com/4055/4...425ea6dc_b.jpg

It's the 1996 Honda Dream solar race car.

Now, who wants to guess whether, overall and at zero yaw, it has:

(1) Downforce
(2) Neither lift nor downforce
(3) A small amount of lift
(4) A lot of lift

Just putting down the number that corresponds with your guess would be good.

5

;)

3, though the question is subjective.

Wondering why the wheel cowlings weren't combined?

3.5

Pi

I'll take that as #3!

4

I won't guess since I just downloaded and read the paper, but I will say: not what I expected.

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 say 4.

Not an informed guess but if I was piloting that through the Oz-tralian Outback, I'd want downforce, just in case. :eek:

Zero yaw is a momentary condition.

FYI: EcoModder has a polling function.

So that's a (1)?

Must I say it? Would that make me right?

I vote for

1 or 2?

No other entries? I'll give it a few more days.

Whenever you post the answer I'll just edit my guess.

Summary so far (no edits)!

(1) Downforce
(2) Neither lift nor downforce
(3) A small amount of lift
(4) A lot of lift


California98Civic - 5 (I assume therefore 4)

Redpoint5 - 3

Jakobnev - 3.5

Oil pan 4 - 3

Jimhs - 4

Green TDI - 3-4

Freebeard - 1

Ksa8907 - 1-2

Can we see a picture of the underside? Or is that irrelevant?
My guess is that it will take off at highway speed :D

Thank goodness for email notifications. I will have forgotten by then.

1 or 2, bu if I had to pick...2

To expand on my guess, without editing my post...

I would assume that downforce would work out to mean more rolling resistance, as the vehicle would behave as if it's heavier the faster you go...

Therefore neutral would be better for FE, presumably. And no loss of traction at speed. Of course, if it only weights 50lbs, you might want downforce at speed to keep you grounded. Bad things might happen when you catch air, though...

If it's heavy, they might have designed it for a bit of lift, to reduce RR the faster it goes.

Too much lift sounds like a bad idea in general.

Of course, if it only goes 30km/hr, lift and the Cd are both pretty moot points...

YMMV.

I haven't been able to find a pic but I assume it's fairly flat.

Car had calculated 160 km/h (~100 mph) top speed and averaged 89.76 km/h (55.77 mph) in the actual race.

Without seeing the underbody, I'd assume 1: Downforce.

It has a slightly concave underside:
https://ecomodder.com/forum/member-v...onda-dream.jpg

It looks like they didn't do anything to increase downforce, but I'd expect some.

The wheel covers create a tunnel under the car so anything is possible. I'll say 2 - neutral just to have a vote.

I would expect the area around the canopy and the canopy itself to produce lift, but that would be compensated slightly on the front and the long tail. The underbody should provide downforce.

So final guesses are:


(1) Downforce
(2) Neither lift nor downforce
(3) A small amount of lift
(4) A lot of lift


California98Civic - 5 (I assume therefore 4)

Redpoint5 - 3

Jakobnev - 3.5

Oil pan 4 - 3

Jimhs - 4

Green TDI - 3-4

Freebeard - 1

Ksa8907 - 1-2

Stubby79 - 1-2

Fat Charlie - 1

RedDevil - 2

Average score - 2.5, so somewhere between 'neither lift nor downforce' to 'a small amount of lift'.

Here is the first part of the answer:

https://i.postimg.cc/XJw5dJHN/Honda-...t-zero-yaw.png

The line with red boxes shows overall lift coefficient (CL). It's at around 0.05 (low lift) until about 100 km/h, whereupon it starts to rise, reaching about 0.16 at 140 km/h. That's a fair bit of lift.

The line with yellow diamonds show front lift coefficient (CLf). It is about 0.13 (a fair amount of lift) until 100 km/h, and then rises to 0.25 at 140 km/h. That is a lot of lift.

The line with the black diamonds shows the rear coefficient of lift (CLr) having small/medium downforce (about -0.09) at all speeds.

The amount of aero lift the car experienced in zero yaw was enough to increase the height of the front of the car, causing the further worsening of the front lift values. Note: this was not desired by Honda - they said: "Future research and development will be required to refine the suspension and/or front lift coefficient in order to further increase the cruising speed."

So overall, this most streamlined of cars has, at zero yaw, quite a lot of lift. As Honda said: "The overall body shape was designed like the cross section of an airplane wing, resulting in a large CLf (front lift coefficient). The front part of the body would therefore receive more upward force at a higher speed."

Interesting, and the story actually gets considerably worse in crosswinds (next post).

Here is the second part of the answer:

https://i.postimg.cc/2Sk3XGHD/Honda-...t-with-yaw.png

So what happens when there is a yaw (crosswind) component to the airflow? Not good things!

The CLf (yellow diamonds) doesn't change much - in fact, it actually improves (gets lower).

But the CLr (black diamonds) increases massively, going positive (into lift) at only about 4-5 degrees of yaw, and then reaching a stunning 0.53 at 15 degrees of yaw. That's the highest CLr I've ever seen.

As a result of that, the overall CL (red squares) goes to well over 0.6(!!) at 15 degrees yaw.

Frontal area of the car was 0.999 square metres and mass was 330kg - I'll let someone else do the maths for the effective weight of the car at 60 km/h - let alone at 15 degrees of yaw and 140 km/h (obviously, in those conditions the car would have been impossible to drive).

So, next time you read someone saying confidently that, well of course, streamlined cars have low lift, you can say: "You're dreaming!".

And if you're wondering at the incredible amount of rear lift with yawed airflow, look at the shape presented to the partly sideways airflow by the rear half of the car:

https://live.staticflickr.com/4055/4...425ea6dc_b.jpg

A huge area for the airflow to wrap around that upper curve, while lateral undercar airflow is largely blocked.

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

https://i.postimg.cc/d3JmHdpS/Different-shapes.png

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).

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.

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:

To quote myself:


OK, did I win something? ;)

It's quite an interesting case.

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

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

Increased angle of attack
 

"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.

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.)

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?