Scratch Building :- (Building a model from plans or from your own design.)
For many building a model from scratch is what it’s all about. The satisfaction of creating a model that looks good and flyes well from nothing more than a set of plans or an idea is an important part of this hobby.
To help you get started we have set out the following to explain some of the important differences in the basic model building materials. Balsa wood, Spruce, and Plywood & Adhesives.
It’s not complicated, but choosing the correct materials for the various areas of a model is important. We have an extensive range of building materials as well as tools, and hardware. The following should help, But if you want we are quite happy to select the most suitable wood sizes and types, as well as the appropriate hardware for your project.
Simply call in or fax, email or phone a list of the materials you need along with a description of the intended use and we can recommend or select the items most suited.
Balsa Wood. A fast growing, light weight timber with generally straight grain. A wood that has been the primary timber for model construction for many years.
Because of it’s quick growth it can and does vary considerably in weight, and in strength.
Balsa is available in either 915mm or 1220mm lengths, and in sheets either 75, or 100mm wide (We only stock the 100mm wide sheets as I can’t see the point in skinny little bits. The odds are you will find the wider more practical (less joins) and off cuts can and will be used up on your current or next project) Thickness range from 1, 1.5, 2, 2.5, 3, 4, 5, 6.5, 8, 9.5, 12, 16, 19, and 25mm Then into blocks up to 100mm
As well as sheets, balsa is available in square, rectangular and triangular sections, as well as special Leading Edge (LE) and Trailing Edge (TE) profiles,
You can divide balsa sheets into 3 main types. Standard grain ( more correctly A grain, & B grain), Contest Grade, & Quarter Grain. We make a point of stocking al three types. Contest and Quarter grain sheets are more expensive than standard, largly because of having to have quarter grain specially cut, and the contest grade hand graded.
Quarter Grain. This is a cut of balsa that can easily be identified by the mottled grain pattern (see oposite). It is much stiffer across the sheet than standard grain balsa.
It’s main use would be anywere were you need stiffness, wing ribs or some formers, you would not use it for sheeting the leading edge of a wing for example, as it’s not going to be happy bending.
Contest Grade. Because balsa is fast growing it’s weight does vary greatly, Contest Grade is hand selected to be about 6lb density, it is the lighter weight sheets within a sheet size. It will not have the stiffness across the sheet that quarter grain has, nor will it be as strong as standard grain, but it will be lighter.
It’s main use is anywhere were weight is going to be an issue, and maximum strength per size is not required. The tail end of a model is a good example. Keep the tail light, remember one extra ounce in the tail will take about five ounces added to the nose of the model to achieve balance. That’s a total of six ounces (170 grams for the un educated) wasted in non productive weight. Heavy models don’t fly, and weight is not strength !!
Standard Grain. For simplicity we will call anything that is not quarter grain or contest grade standard grain. It is a mixture of A&B grain types.
A Grain has long fibers that show up as long grain lines. It is very flexible across the sheet and bends around curves easily. Also warps easily. Sometimes called “tangent cut.” Use for sheet covering rounded fuselages and wing leading edges, planking fuselages, forming tubes, Don’t use for sheet balsa wings or tail surfaces, flat fuselage sides, ribs, or formers.
B Grain has some of the qualities of both type A and type C. Grain lines are shorter than type A, and it feels stiffer across the sheet. It is a general puropse sheet and can be used for many jobs. Sometimes called “random cut.” Use for flat fuselage sides, trailing edges, second choice for wing ribs, formers, planking gradual curves, wing leading edge sheeting.
Spruce. Heavier than balsa wood but far stronger, it’s an ideal wood for load carrying jobs such as wing spars and fusalage longerons. The best spruce is “Sitka”. Typically spruce has very straight very closely spaced grain. Generally is it’s in square or rectangular sections ranging from 3mm square to sizes like 25 by 6mm Used correctly it can add a lot of strength without a significant gain in weight.
Plywood. No, we are not talking about the cheap, low grade ply’s found in the local cut price hardware store. What we are talking about is high quality multi layer plywood, typically having 5 or more layers even in sheets as thin as 2mm.
Ply should be used in any high strength areas ( Engine mounts, Wing Joiners(Dihedral braces) landing gear mounting area, Servo mounting plates. etc. Ply is available in a wide range of thickness from under 1mm to over 8mm.
Lite-Ply is a typically 3mm in thickness and is lighter than the ply we have just been talking about, it’s lighter but not as strong, and is more flexable. It has it’s uses, fusalage doublers for example. however you would not use it in high strength areas such as the main firewall, landing gear mounting, or dihedral joiners.
Adhesives. As the glue stays in the model when it’s finished we’re classing adhesives as building materials. ( the glue on your finger we class as annoying)
There are four main types of adhesives used in constructing models. Epoxy, Cyanoacrylate (Cyno, or CA) Aliphatic Resin, and Contact cement.
Epoxy. A two part very high strength adhesive that is very heavy as it does not evaporate when drying. It is ideal to use in high strength areas such as around engine mounts, main firewall’s, landing gear mounts, and wing joining. Curing times range from 5 minutes to over 3 hours. While fast might be good slow is better, the longer the setting time the stronger the bond and the joint will not be as brittle. Think of it this way, if your going to use epoxy because you want maximum strength, can’t you wait just a little bit longer to achieve it.
Aliphatic Resin. Can be considered a great general woodworking glue, similar to PVA, but much easier to sand with a shorter drying time than PVA. Ideal for attaching sheeting over ribs as it gives you adequate working time. The best alternative for anyone not wanting to use CA’s for general construction.
Contact Cement. Can be used for gluing doublers to fuselage sides. You apply a coating to both surfaces and wait a few minutes and then bring them together. Once they touch they stick, no second chances. Epoxy can also be used for doublers, but contact cement works well and is lighter due to evaporation.
Cyanoacrylate (CA) Fast drying and very strong, this group of glues has speeded up model construction considerably. There are three main types Super thin, Thin and medium.
Super Thin (Hot Flash my favorite) lists a bonding time of one second, it will penetrate into the wood giving a very strong joints. However you must make sure there are no gaps in your joints.
Thin, is similar to the above, with a bonding time of one to three seconds
Medium, Also known as gap filling has a bonding time of five to fifteen seconds It will fill gaps, but gaps don’t make good joints, and only good joints make good models.