lower cowling

[LargePrime]

I have been looking at a lot of lower cowlings and it seems like the curve sharply to the prop. Perhaps too sharply? I cant see airflow sticking to the cowling. Did I miss something?

[dust]

A problem with pushers behind the fuselodge is airflow to the prop, so the smaller the better, thats why a prop extension is used, the prop needs air to push de plane.

 

Also because of the prop in the vicinity, i doubt that you are getting laminar airflow, regardless of the shape, there is probably a "little" turbulence in the area, causing laminar flow separation.

[Marc Zeitlin]

Originally posted by LargePrime

I have been looking at a lot of lower cowlings and it seems like the curve sharply to the prop. Perhaps too sharply? I cant see airflow sticking to the cowling. Did I miss something?

You are correct that there are issues with the airflow over the cowling, both top and bottom. The CSA newsletter is a wonderful reference, as there have been MANY articles written over the years with people doing oil flow studies of different cowl shapes with air attachement pictures. Invaluable information - get the back issues. There is no one "answer", but there are clues as to what to avoid and what to attempt.

 

Originally posted by dust

Also because of the prop in the vicinity, i doubt that you are getting laminar airflow, regardless of the shape, there is probably a "little" turbulence in the area, causing laminar flow separation.

I think that what you are trying to say is that the air is separated from the fuselage in the cowl area (the CSA oil flow studies show this clearly in places), but I can guarantee you that there is NO laminar airflow anywhere near the cowling.

 

By the time that the airflow has reached 3 - 4 feet back from the nose of the aircraft, it has long since transitioned from laminar to turbulent flow (this is dependent upon Reynolds #, which is a function of flow distance, viscosity, and speed). This is a GOOD thing, as it ensures that there is energy in the boundary layer that will keep it attached to the fuselage - if the BL remained laminar to this distance, it's extremely unlikely that it would remain attached, and the drag would go up considerably.

 

The reason that some people need turbulators in front of the NACA scoop to get adequate cooling air is because the boundary layer is 1/2" to 1" thick there, and more energy needs to be injected into it to keep it attached. By the time the air gets back to the cowl, the boundary layer is even thicker, even as a turbulent boundary layer, and it's all that we can do to keep it close to attached.

[LargePrime]

So how would one get the CSA back issues?

 

Is there any concensus on what a good shape would look like?

 

It would seem that if the flow is turbulant already the last thing you would want is the sharp curve up to the prop there. But if this has already been solved then i would like to know.

[Marc Zeitlin]

Originally posted by LargePrime

So how would one get the CSA back issues?

 

Is there any concensus on what a good shape would look like?

a) See:

 

http://www.canard.com/~csa/

 

and contact Terry Schubert. He's the editor and brains behind the newsletter.

 

b) Not really, but there are some things NOT to do, mostly having to do with sharp corners and angles. I wouldn't say it's been "solved", but there's been a lot of effort to figure out the kinds of things that work and those that don't. Whatever Klaus Savier does, copy it :-).