Hi Bob,
I think your on the right track.
1. Before you reinvent the wheel you may want to contact Jerry Silver of the Valley Circle Burners in SoCal. I think it is Jerry who designed and built the two almost perfect pull test machines that he lets us use at our big contests. They are just like you described in your post. I am sending you Jerry's email via your mailbox. He may have some photos, or drawings that may be of use to you. Jerry is a great guy and the CD of last weeks great Cal Champs contest!
2. I think the horizontal leg is about 2X the vertical leg. This cuts your weight required in half, a good thing.
3. Your slider idea is perfect. A few weeks ago I made some drawings to modify the ones (Jerry's) we use at contests. Like you, I also want to use a slider for two reasons. One is what you already said, mark the horizontal leg in "model weights in oz" then slide the weight to that point and pull.
The 2nd reason is that when the pull is made, the horizontal leg is tilted toward the plane. If the slider is smooth enough it will move the weight toward the plane and will LESSON the amount of force, thus preventing ANY chance for an "over pull". ....... At least that is my theory?
I plan on using a TRAVERLER CAR (with the weight on it) and TRAVERLER CAR TRACK from the sailing world for the "slider" assembly. This car has excellent bearings and will slide easily, both for adjustment of weight, and to relieve the force. Maybe too easily??? But it can be adjusted to have the right amount of friction. (BTW, there are many cheaper/easier ways to make this slider and I'm sure you will come up with one! :-)
4. The only other mod I would make is to have a RED LIGHT connected to a micro sw under the horizontal leg. This would allow the pilot (who is pulling the plane) to instantly see when the pull is complete. (placing a hood over the tail light may help).
5. I think when you see Jerry's excellent system you will save yourself a lot of work and time by copying his design, with a slight mod for the new weight/pull rules.
6. As someone who pulled TWO line breaks at last weeks contest, (one at 10 lbs. and one at 15 lbs.), I would like to have a system that ONLY has the pilot involved in the actual pull and NOT the pit boss. (we used the old fashioned fish scales to pull, we did not have Jerry's system at the contest) This would require the test unit to be staked down in grass, or fixed somehow on pavement. This would be fast, safe, smooth, and less stress on the pit boss when something breaks!
Thanks for making your post. It will be nice to see how different people/clubs solve this problem. We have many smart guys on this list, the other ideas should be awesome!!!!
Regards,