ptxman
Posts: 351
Joined: 7/31/2002
From: Calgary, AB, CANADA
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[QUOTE]Originally posted by flianbrian
I'm curious about the f/g fuses especially popular with pattern planes. What weight/style of cloth, how many plies, what resin? (And any other tid bits you may think of).
Thanks
flianbrian [/QUOTE]
Composite pattern/aerobatic models have unique & challenging requirements. The layup sequence is usually a compromise of many (& sometimes conflicting) parameters: cost, manual effort, target weight, rigidity, vibration dampening, aesthetic finish, service life, material availability… the list goes on. Basically you have to get in there, try a layup, fly the hell out of it, observe the results & iterate from there. Start by examining successful analogous models but also recognize what works for model A may not be quite right for model B.
Here are some pictures of my PTX Vector (2mx2m F3A model) to get you going as an example. The ordinary all-glass fuse was 3 oz outer + 6 oz inner reinforcement, both cloth layers run continuous through the mold, laminating epoxy used. The removable canopy & belly pan was reduced slightly to a 3 + 4 oz sequence. Removable sections are desirable features for functionality, but they also take big chunks out of the structure. You can reduce them to a point - if they are good fitting, they become part of the structure. If you make them too flimsy, the fuse has to compensate. Belly pans in particular must withstand a lot of heat & usually get cut up to accommodate inlet cooling, exhaust & engine accessories. I used simple CF tow stringers in the aft fuse section because this is a tall slabby section requiring reinforcement for the torsional rigidity. I’m also a big believer in interconnected lightened cross bracing cleverly disguised as servo trays, tank plates hold downs etc.
From the plain-Jane glass layups it basically turns into how much effort & dollars you want to put into the project. The glass models were coming in acceptably within the target weight range (at the time) of 9.50-9.75 lb with component weights: canopy = 66g (2.3 oz), b/pan = 134g (4.7 oz), fuse = 580g (20.4 oz), total = 780g (27.5 oz). Later on, I replaced the 6 oz glass with 4 oz carbon on the fuse resulting in about 15% weight saving & better than expected rigidity. The next step would be cut back on the 3 oz glass & replace it with lighter/different material. Like most things it becomes one of those diminishing return type deals: 40$ worth of carbon fiber buys you 2 oz. Or, a 1000$ paint booth, 30$ of Kevlar, 40$ of carbon + 60$ of bagging consumables buys you 5 oz. Fictional numbers but you get the idea. Fortunately, there are lots of glass styles and weights to choose from. Unfortunately, the same cannot be said about carbon & Kevlar. Try and find a 2 oz plain weave carbon cloth or a 3 oz satin weave Kevlar cloth. They might be perfect for our models but they don’t exist from weavers or at a price you are willing to pay. Happy layups!
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