Here are scanned copies of how I approach VP spring design that I posted back on the Indoor FF mailing list around 2005, at least that is the date of these scans. Somewhere I have the originals I hand wrote and maybe can rescan for clarity.
Here are the individual pages in larger pixel format:
spring1
spring2
spring3
spring4

This spring design process is the way I approach spring design. I have only made a few VP hubs over the years. But this process is how I designed my spring for my Intermediate Stick that I won USIC in 1999 at Johnson City. That model did 38:12 and the spring shifted over a long period of time due to the high spring rate I used. It has been 26 years so I do not recall exactly how long but I think about 13 minutes of shifting. I seem to recall peaking around 12 minutes then the prop slowly started shifting until around 25 minutes then another 13 to descend. The flight got right under the girders at Johnson City and parked there. I think somewhere I have some video tape of the flight. Anyhow, here are 4 pages explainting the spring stress, soring rate, and how to calulate it. This is the same method I will be using on my new F1M and Biplane Pennyplane.

Page 1 is in reference to a spring being overloaded. This must have been someones VP prop spring dimensions posted on the mailing list and I ran the calculation and found that the spring was overstressed.

Page 2 shows an alternate spring design within allowable stress limits

Page 3 shows another alternate spring design as well as some guides on stress limits

Page 4 shows the method of how I pick the values to calculate the spring rate (in-oz/deg) I wanted a long shift range based on rubber curve, so designed a stiffer spring than what others were using at the time. Bernard Hunts VP on his Int Stick was using lots of coils (9 I think) and his prop shifted quickly to lower pitch once it shifted late in the flight.
 

Don Slusarczyk