Composite F3P model building tipsŽilvinas Lapačinskas
There are several approaches to build composite F3P model out of carbon fiber rods:
- To build model with straight lined contours. It doesn’t look great so is ignored in most of a cases.
- To take straight carbon fiber rods as a material and bend them into desired shape under tension by compensating this tension with constructive elements. This approach allows to build models with nice curved contours by using standard materials. Downside of this approach is that design is restricted by flexibility of a material. The smaller desired bending radius, the more tension it is needed to compensate thus more reinforcement elements is needed. One of a solutions is to use carbon fiber tube (3 or 5 mm) as a backbone to compensate tension of rounded contours. Of course any additional reinforcement means additional weight and complexity. Especially for beginners necessity to compensate tensions creates a lot of problems during build process. Especially annoying is situation when part under tension of curved rods changes its shape after taking it out of assembling board.
- To use Shaped carbon parts for Your composite F3P model contours. It allows to make any desired form and reduce weight by throwing away unnecessary reinforcements and backbone tube.
While I and my colleagues gain experience in model building using shaped carbon parts, we would like to share few tips about it.
- Considering to build composite F3P model according to tested foamy prototype note, that center of gravity of composite version will appear much more forward than in foamy version due to lightness of an airframe. To solve CG problem it is not enough to move some servos to the tail. In most of a cases it is necessary to move wing area forward and use much lighter front side equipment (motor, battery, esc).
- Model should be designed without backbone tube. Vertical part of fuselage should be divided into two separate parts horizontally across central line (the same as foamy).
- Do not restrict your imagination by designing contours. Shape could be any You like.
- Design corners as a part of shaped carbon rod piece. So corners will be stronger and more rigid.
- Avoid sharp corners of a contour. Mylar film tend to wrinkle near sharp corners.
- Trim tips of shaped parts by putting them on 1:1 drawing.
- Lie shaped parts on assembling board without tension and fix
- Do not twist shaped parts, or do not try to remake form of a part by applying tension.
- Because there is significantly less stress on joins between rods, there is possible to fix them with resin and milled carbon fiber mixture, avoiding necessity to bind knots with Kevlar thread on every joint. So saving time.
- Critical part of a process is covering with Mylar film. Because no backbone tube was used and whole parts are with minimal amount of reinforcements it is absolutely necessary to make sure that parts keep their initial shape during covering process. It is very easy to slip 1-2 mm from initial shape. There are several solutions:
- put a drawing of a part under Mylar film, so it will guide you when you apply frame on film;
- use temporary reinforcement for frames. In fact it is enough to reinforce parts along horizontal axis line ensuring that cross section line of fuselage is absolutely straight.
- Assemble model in order similar to foamy. Start by putting horizontal part of fuselage upside down on flat surface and assemble it with bottom part of fuselage.
- Your model will need less bracing. Leave tips of ailerons, ruder and elevator without bracing. They will work just fine.