My aerodynamic car for today
 

I am going to post some aero cars - not every day, as the thread name implies, but whenever I feel like it. (Unlike the Random Aero Cars thread, I will always try to provide some measured data.)

All the cars are covered in my book - A Century Of Car Aerodynamics (link below).

Today's car - the 1983 Fiat Uno.

https://i.postimg.cc/5tGxtB1J/1-3-941.jpg

The Uno was a low-cost car made in huge numbers – over 8 million were built. Most interesting from an aerodynamics perspective, it was a small hatch. The smaller the car, the harder it is to achieve a low Cd. And since a hatch will always have a large wake, with that shape we have another aerodynamic negative. But Fiat took great care to optimise the aerodynamics – especially drag – at a time when few manufacturers were bothering much about the aerodynamics of their cheapest cars.

Cd was 0.34 (standard car), 0.33 (economy special) and 0.30 (Turbo). Lift figures were also good - and across the whole range of yaw angles.

https://i.postimg.cc/wMjFbmyt/Uno-graph.png

May be it is more useful to post CdA instead of Cd?
The real fuel efficiency comes from the product of the drag coefficient (Cd) to the frontal area (S or A:confused:) = CdA - this is what really interesting (IMHO :rolleyes:).

Sure. It's a pretty small car, so you can do that research and post the figure. It's certain to be very good for the era.

I have found 2 good sources:
1. https://books.google.com.ua/books?id...20drag&f=false

2. Aerodynamics - Racing Cars

Both mention real area equal S=1.83 sq.m. and max drag area CdA=0.62 sq.m. (standard trim).

Close enough - the tech paper (and so my book) states 1.81m2 for the std car.

Great idea for a thread! What were the differences that reduced the drag? Not much jumps out at me when I look at photos of the Turbo Uno. Is all of the improvement from the air dam?

Compared to the standard car, the Turbo had the following changes:

- Rear spoiler
- Different front bumper
- Flush glass on front doors
- Improved under-hood airflow
- Modified rocker panels

Another one from the Uno - front spoiler depth. (No smooth underfloor in a car of this age.)

https://i.postimg.cc/2ygXzTMZ/Uno-front-spoiler.png

My aerodynamic car for this week.

Uno - Cd versus downward slope angle of the roof.

https://i.postimg.cc/cHYyM1C8/Uno-roof-angle.png

I can see how they could test the front spoiler easily. But, were the five data points modifications to a complete test shell? Full scale?

Yes, they were testing full scale with the cars made from polystyrene and having interchangeable bits.

Thanks.

It always bothers me when they reduce a compound curved surface to a single angle. You can do a lot with three or four points on a Spline Curve.

'smaller'
 

One might want to provide some context with respect to a qualifying adjective as 'smaller'.
We know from verisimilitude that there exists no prohibition from 'shrinking' the proportions of a very low drag vehicle, creating a 'smaller' vehicle of identical Cd.
An example would be Morelli, who took a small, 1976 FIAT of Cd 0.44, and turned it into Cd 0.23.

Most people understand what a smaller car is. I don't know of any smaller production car that looks exactly like a big one but is just shrunk proportionally in all directions.

1975 Porsche 924

https://i.postimg.cc/TwBzjNBk/924-cropped.jpg

Aerodynamic development of the 924 began using one-fifth scale models that were tested in Paris Laboratoires Eiffel. The results gained from these tests were then applied to full-size prototype that was optimised in the Volkswagen wind tunnel.

With the headlights retracted, the Cd was 0.36. With headlights up, it increased to 0.38. Projected frontal area was 1.76m2, giving a CDA of 0.634m2 with the headlights down, and 0.669m2 with the lights raised.

I don't know of........
 

That's not even germane to the topic.
If you're going to make a simplistic, overgeneralized comment about 'small', then It's my opinion that you haven't properly served the reader.
The 1987 Renault Vesta-II remains one of the lowest drag 'cars' ever created, in spite of it's 139-inch overall length.

Is there enough data to make the 924 the next car of the week?

Very interesting ad for the 924:

https://i.postimg.cc/FRXdF65j/stream...int-ads-13.jpg

Re the above ad. It's interesting on a bunch of levels.

1. It gives the power to propel the car at 55 mph - 15hp.

2. It gives the lift forces at front and rear at 100 mph - 46 and 105(!)lbs, respectively.

3. From the above, and knowing the projected frontal area, we can work out the front and rear lift coefficients, and so work out lift forces at all speeds.

4. It states that the lateral centre of pressure is behind the centre of gravity, so we know it has decent straightline stability

5. It shows the centreline surface body pressure measurements, including in the wake (but not, unfortunately, under the car) so we can see how much of the upper surface of the car is experiencing low pressure (ie lift - see #2 above.)

Good ad!

Fiat Uno II 1989-2004 Europe and Africa
5-door 0.30 Cd
3-door Turbo 0.29 Cd

Porsche 924. With smoke injected into the wake, it shows that there is attached flow right to the end of the hatch.

https://i.postimg.cc/nzdy6F8F/Porsch...1456793938.jpg

That can be correlated with the upper body pressures shown in the advertisement.

The above pic is also interesting because it shows how smoke streamlines can be a bit deceptive. Here it could be thought that there is flow separation occurring over the rear hatch, but if that were the case, the wake area would obviously be bigger. In fact, the smoke injected into the wake, and its very clear upper edge, better shows the actual flow pattern.

https://i.postimg.cc/q7wv1tGz/924-lift.png

You can see why they needed to add the rear spoiler for the faster Turbo model.