I have often read that the foam we use to build our planes has a "grain", similar to what you find in wood, so orienting certain parts with the grain being stronger than against the grain. Building with Depron, I admit I didn't pay much attention to that, I wanted to get as many pieces out of each sheet of foam as I could since on average I was paying about $9 (Cdn) per sheet.
With MPF, I pay about half that price and not that I am going to be purposely wasting any, but since most folks claim it is only about 80% as rigid as Depron, perhaps it was time to start paying attention to the direction of the "grain" in the foam.
I am using XPS or the equivalent of MPF Grade B, which has a very wavy side as you can see in these comparison pictures. Smooth side on the left, wavy side on the right. These waves run lengthwise on the 24" x 48" sheet of foam (ie along the longest part of the foam) similar to the grain on a board you would get at the lumber store. I'm not sure if it is noticeable on MPF Grade A as both sides are smooth, but you can see a slight grain even on the smooth side of the MPF B that runs the same direction as the waves. You can find links to these foams on our parts list. Unfortunately, I think these products are currently only available in North America.
Here are the differences or how much each piece flexed with the weight of the quarters applied.
Cross grain - 7/16" or 11.1 mm
With the grain - 5/16" or 7.9 mm
45 degrees to the grain - 6/16" (3/8") or 9.5 mm
So this is over essentially a 2" by 4" area. I know that the test is not exactly overly scientific, but does show measurable results that I think can be applied to how we lay out our parts on a sheet of foam for maximum strength before we start to reinforce. 1/8" difference in flex over a 4 inch length of foam is pretty significant I think and makes me think even more how important it will be to orient the parts in relation to the grain of the foam for maximum strength. Over a much longer area like wings, this amount of flex could make a big difference in performance if the piece was cut the wrong way with respect to the grain and might require significant weight be added to the plane to have a strong enough wing.
I took these two pictures which are how the pieces for the NAMC Mig-35B are laid out on the MPF plans. Stephan has been producing plans for both Depron and MPF since our first plane the Mig-35A, the first park jet company that I know of to do so. This was in recognition of how many folks were building with MPF even over a year ago before the availability of Depron changed.
In this first picture, you can see the wing plate is oriented so that from wing tip to wing tip where you want maximum strength to prevent flex, it runs with the grain, same with the back plate, elevons and vertical stabilizers/rudders. I think it important for the vert stabs to be oriented with the grain as they get a lot of side force from wind and when the plane is rolling and turning. I'm not sure if the grain is overly important for the nacelles as once they are glued in place, they form a fairly solid box.
For the smaller internal pieces, I'm also not sure if grain direction is too important as they will all be glued together to form a strong structure.
I know this sounds like too much detail, but the canopy being oriented across the grain should make it easier to curve and shape :)
The elevons should have the strength where they need as they are pushed by the prop wash and airflow and should have the best strength when deflected to move the plane. For torsional strength, this will be an area we will explore in future prototypes, the back plate and elevons might need some reinforcement across the grain to help prevent twisting in aggressive turns and rolls when building completely of MPF.
For my next round of tests, I am going to cut out some elevons and test several popular reinforcement techniques to see if there is a difference in strength and weight. Here are some of the methods I am thinking of testing.
- Carbon rod and bamboo skewers embedded in the foam with epoxy.
- "Glassing" or using drywall tape and glue to form a poor man's fibreglass. I will be testing with both epoxy/Foam Cure and Aleene's Original Tacky Glue (Stephan has used this glue with success).
- Pre stressing with duct tape where you curve the foam slightly, apply duct tape to that inside curve, then pull the foam straight again and tape the other side, the tape then causing tension and rigidity in the foam.
I will be comparing these to a non-reinforced piece of Depron to see the difference. Currently, I don't use any reinforcement in the elevons of my Mig-35Bs which are made of Depron and have never seen any sort of flex or twisting that worries me.
More testing with MPF to come :)