Yesterday I reached the 300 flight milestone with my modified RC Powers Su-27 V5 :)
In the meantime since building the Su-27 V5, I have also built three other planes primarily from MPF, the Parkflyers International Su-35, RC Powers F-22 V5 and the RC Powers Eurofighter V5. All of these planes have ended up in the 20-21.5 oz (567-610 gr) range.
I have accumulated 118 flights on the PI Su-35, 171 on the F-22 V5 and 51 flights on the Eurofighter V5, so altogether I have 640 flights on score and fold planes built with MPF. I will focus primarily on the Su-27 in this article, but also mention some things I learned from these other three planes as well as an RC Powers Mig-29 V1 which I built quite awhile back with MPF.
What I found happening was the Depron was starting to change consistency, almost a more crystalline feel to it. It didn't seem to be any weaker, but it appeared that long term use and exposure to the sun while at the field was definitely changing the consistency of the foam. So hopefully my Su-27 will still be around in another six months and I will be able to evaluate any long term change in the foam :).
What I found was happening is the layer of epoxy I put between the wooden motor mount and the flat surface of the shim would eventually pull a thin layer, perhaps the outer "skin" of the foam off and the motor would be loose. With the more popular "+" style motor mounting, there are several more seams to be reinforced where the wood meets the foam. I have never had this issue with the PI Su-35 which does not have a round "shim" behind the wooden motor mount. Fortunately, it never cost me a plane and with a rather liberal application of expanding Gorilla Glue on all the V5 motor mounts, I have not had a repeat of the motor mount cracking.
I can't really speak at length how "repairable" MPF is after any crashes as I have been pretty lucky in this department :). I did have one major crash with my F-22 on it's initial maiden flight, but it was significant enough that simply building a new fuselage was a better option than trying to repair what was done.
On all four of the score and fold planes I have built with MPF, I have used the same glues and tapes and have had no issues at all with tape peeling or glue joints not holding.
I used fiberglass strapping tape on the inside of all score lines and transparent duct tape for all my control surface hinges and have had no issue even after 300 flights on the Su-27 with either of these tapes peeling off the foam. Still holding strong after 300 flights, a little extra vegetation in the intakes for good measure :)
I have found that 4mm carbon rod has worked very well for main wing spars on all the MPF planes I have built, although I also use the same with Depron planes I have built in the past, 3mm carbon tube for main wing spars is a bit too flexible for my liking. With MPF planes, I have added extra reinforcement to vertical stabilizers, elevons and back plates to prevent flex and torsional twist. Compared to Depron planes of the same size, on average I found I have added about 0.7 oz/20 gr to get the MPF surfaces to be stiff enough for how I like my planes to perform for my style of flying. So this might not seem like a lot of weight, but it will finishing like filling and painting. I have only done filling on the PI Su-35, but kept the paint scheme to a minimum, so I think in future there are a couple of things I will do to keep the weight down and allow me to have fully painted planes.
Since I still have quite a bit of scrap and full sheets of Depron, I will use it to make elevons, vertical stabilizers, rudders and perhaps back plates whenever possible to keep things stiff without needing extra reinforcement. I used Depron for the vertical stabilizers on the F-22 and Eurofighter V-5s, so that gave me good strength without needing extra reinforcement weight. Also, I will not fill the gaps on the noses of my planes like I have in the past, rather sand them as smooth as I can without jeopardizing any glue joints. Fortunately, Foam Cure glue sands well, so I should still have a very sleek plane without adding a few extra grams in spackle.
So overall, I'm very impressed with how MPF has held up over hundreds of flights, with the right build techniques and using small amounts of Depron where and when I can, I think I can keep my planes light yet strong and enjoy many more flights to come :)