Elliptical Bell Mouth Template.

[BernardNaish]

If you want to make a bell mouth for the outlet pipe of a top hat you will need a template to check the shape.
The diameter of the end of an elliptical bell needs to be twice the pipe diameter and the end of the bell set into the chamber by a distance equal to the pipe diameter. This needs to be balanced against the Thien requirement to set the outlet half the pipe diameter from the baffle.

The example here is for a 5" pipe (4" pipe in brackets).

Draw a rectangle 10" wide by 5" high. (8" x 4"). Divide it in half to give two 5" (4") squares.
Label the bottom right corner "A" and the lower left corner "B".
On the top line mark each half inch and number these marks from the right 0 to 20. (0 to 16)
On the right hand vertical line, mark each quarter inch and from "A" up to the top number these 0 to 20. (0 to 16)
Connect "A" to the top left corner and "B" to the top right corner. These two lines will cross in the centre and represents the inside edge of the pipe. Mark this point with a dot.
Draw a line from "B" to 19 (15) on the right vertical. Place a ruler from "A" to 19 (15) on the top line. Mark where these cross with a dot. Repeat with "B" to 18 (14) on right vertical "A" to 18  (14) on top line and "B" to 17 (14) and "A" to 17 (14)  etc. until you have 21 (17) dots.

Connect the dots with a line faired into a curve. You now have an elliptical shape in the bottom right rectangle. Glue it to stiff card and cut it out ALONG THE INSIDE OF THE CURVE and you have a template.