hevansrv7a
Well Known Member
In Jack Norris's book, the "SSSS" appendix, he explains how the effect of the cowl on the prop is calculated. A preliminary step is to convert the shape of the cowl to a "body of rotation". This means determining the cross section square inches of the cowl at several points, working from front to firewall or even the LE of the wing. You convert each section from square inches to a radius of a circle with the same area. He gives the example of an RV-8 done from Van's actual engineering drawings. The vertical axis is radius and the horizontal is station. The zero station is the spinner which is a radius of 6.5" for both.
When viewed graphically, it's a new kind of visual representation of how the cowl interacts with the air stream.
I did my -7A with SJ cowl and compared them. I'll cover why and how in another post, sometime.
Anyhow, the results were odd and surprising so I thought I'd share them for what it's worth. The details are such that the comparison is not precise. For example, each of these is "corrected" for the air that enters the holes. In my case, I allowed 2/3 of the air for cooling to go through and 1/2 of the air for combustion. Again, the rationale for that can be covered some other time, but the correction for the holes was not necessarily done the same for the -8. Another possible source for difference is that the -8 stations were 0", 9", 18"... while mine were in 3" increments at the front and then 9" when the curve got flatter.
When viewed graphically, it's a new kind of visual representation of how the cowl interacts with the air stream.
I did my -7A with SJ cowl and compared them. I'll cover why and how in another post, sometime.
Anyhow, the results were odd and surprising so I thought I'd share them for what it's worth. The details are such that the comparison is not precise. For example, each of these is "corrected" for the air that enters the holes. In my case, I allowed 2/3 of the air for cooling to go through and 1/2 of the air for combustion. Again, the rationale for that can be covered some other time, but the correction for the holes was not necessarily done the same for the -8. Another possible source for difference is that the -8 stations were 0", 9", 18"... while mine were in 3" increments at the front and then 9" when the curve got flatter.
