NACA Ducts

cajun · cajun's Garage

Does anyone know the dimensions and placement of the NACA Ducts on the Yellowbird?

I am beginning a new project (building my version of the yellowbird), and I have been contemplating adding them to my car.

Any help would be greatly appreciated.

ChkbookMechanic · ChkbookMechanic's Garage

In this thread:

Ultimate RUF Conversion Thread

they talk about the NACA ducts and apparently Mr. Ruf himself said they didn't work the way he envisioned them to ( Ultimate RUF Conversion Thread ). However, I'm sure if you want the dimensions and locations of them, that thread is the best place to start looking.

Formerly Steve Wilkinson · 03-13-2009, 06:16 AM

Yeah, that's why they call them NACA ducts rather than intuitive-mechanic ducts. the shape and dimensions of a NACA duct are easily reproduced. Where to put them is best done with a wind tunnel at hand.

cajun · cajun's Garage

Thanks for the leads.

Reaper930 · 03-13-2009, 07:21 AM

Not to mention they work by taking a layer of clinging boundry layer air on a usually flat or flat sloping surface, not from a turbulent area such as the rear quarter of a 911 turbo.

But you will see that they were very effective and useful on the 935's once they put them on large relatively flat surfaces such as on top of the boxed rear quarters and on the sides of the raise tails of the K3's...

Reaper930 · 03-13-2009, 07:30 AM

BTW not knocking your project at all.

ChrisBennet · ChrisBennet's Garage

I had the pleasure to hear Alois Ruf speak a few years ago at Tech Tactics. He mentioned how the NACA duct thing hadn't worked out (air was going out instead of in!) because they'd put them in a low pressure area. That's why you only see them on the original (the prototype). The Porsche engineer who spoke after him said something about how the ducts in the 993TT worked great in a snarky sort of way - as if Ruf had said hip ducts don't work when he was actually confessing to his own mistakes.

BTW: Ruf was great and very down to earth. He said that the last time he was in Springfield (the Tech Tactics were in Springfield. MA) he was there to see a demolition derby (!) as a young man (after college?). His dad had told him he should see America so he did.

Another anecdotes:
When they made the Yellow Bird they didn't "know" that you have to have a red car to get on the cover of a magazine. They just lucked out that it was a rainy cloudy day when they did the test and their bright yellow car really "popped" compared to the other colors.

They barely had the car done in time for the Road and Track test and unlike everyone else, they drove the car down for the test.

-Chris

Reaper930 · 03-13-2009, 08:12 AM

well you can also look at the factory 935/76 that ran with them, so its not like RUF was the only one that tried it. You can see in the second photo they moved the location more to the rear...:

Dantilla · 03-13-2009, 02:41 PM

I remember reading a technical article in an aviation magazine about NACA ducts.

The summary is they are very effective and efficient, as long as two conditions are met:

1- Very high speeds

2- Large volume of undisturbed air needed. If there was any restriction downstream, any "backpressure", their effectiveness is lost. Basically said that they are worthless on a street car, and not very effective on a race car when drafting.