New Way to Compare Cowlings

[hevansrv7a]

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.

[AX-O]

Few questions. What are the X and Y axis? What are the units? Is "0" the center line?

[hevansrv7a]

Originally Posted by AX-O

Few questions. What are the X and Y axis? What are the units? Is "0" the center line?

Said the first time:
"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."

Restated and expanded:
The Y axis (vertical) is the radius of the circle of which the area equals the area of the cowl cross section at the station. The X axis (horizontal) is the station, measuring aft from the forward-most point on the cowl. Measurements are in inches. Picture this as half a blunted bullet, pointing left. the 0-0 point is front, center. The firewall would be toward the right at around 36" depending on model. The relative wind is thus blowing from left to right.

[AX-O]

Thanks. Was confused with the units. Did not know if the Y axis was in inches or inches^2 (since you also refered to area).