A quiz for people

[JulianEdgar]

Originally Posted by GreenTDI

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

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

[JulianEdgar]

Originally Posted by Stubby79

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.

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

[Fat Charlie]

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

[Vman455]

Originally Posted by GreenTDI

Can we see a picture of the underside? Or is that irrelevant?

It has a slightly concave underside:

[Fat Charlie]

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

[RedDevil]

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.

[JulianEdgar]

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'.

[JulianEdgar]

Here is the first part of the answer:



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).

[JulianEdgar]

Here is the second part of the answer:



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!".

[JulianEdgar]

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:



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