This is about what I figured out - I didn't see any set percentages or specific numbers behind this:
A damaged R&D will not repair if the plane being researched is more than 2 years out. If it's between 1 and 2 years out, the point has a small chance of repairing. If the plane is less than 1 year out, it has a better chance of repairing that broken point.
Here is what I had in my notes...
Once fully repaired, a size 30 factory produces one research point per day, two if you have the engine bonus. 30 is the size you want : over 30, they do not contribute more, below, they contribute much less. 100 research points advance the model by one month.
Damaged factories have a probability of repairing every day. If N is the number of days left until the model is available, the probability is (about?) 1/N. Up to statistical fluctuations, this is independent of the number of factories you repair. Repairing 30 factories takes as much time as repairing 10.
On average, if the model is N days away, the factories should be repaired by 0.63 N, and you have 0.37N days left to advance the model. This means the sooner you begin repair, the better.
You can summarize all this into one formula. If N is the number of days before the model is available, and U is the number of size 30 (damaged) factories you assign to this model, you should get a 0.37NU/(50+30U) month advance with the engine bonus, and 0.37NU/(100+30U) without.
One important way to speed research is to use upgrade paths. A repaired factory can be upgraded to the next model in the research path at no damage. This mean that over long paths (zero, oscar...), you should research the earliest model, so that your factories repair faster, and then upgrade to the later ones, which you will advance significantly. As a result, there is a trade-off between brand new designs, that you have to research early, and won't advance a lot, and late models of old designs, where a lot can be gained.
All of this is constrained by the number of research factories : you only have so many sites, and once you update one to production, you can't go back (but you get the new model produced at once). Ideally, you should plan very early in the game what you want to research, and when you turn those factories to actual production.
And of course, it also depends on whether you're playing PDU on or off. In the latter case, it is quite possible for a model to be available, but but you have to wait for the squadrons to arrive.
Hope this helps,
< Message edited by fcharton -- 9/3/2017 3:54:53 PM >