Light aircraft generally are more forgiving and perform better than heavy ones. In my first article I suggested that an aircraft needed to be ‘absent of matter’ to achieve light weight, and listed the weights of most of the materials that I use. I would now like to describe how I use this information to choose the ‘matter’ to either use… or make ‘absent’. This approach is not unique; I know that builders of large model aircraft use it. However, I think it has relevant to any size where there is a need to control weight. It probably is more useful for scratch building, but may also help you modify kits or plans.
You obviously need to first decide what you want to build, its size and desired flying characteristics. I’m going to continue to ignore the power source for now as you have to know the aircraft’s weight before choosing a motor. To do this, you need to be able to visualise and plan ahead because it requires setting up a ‘weight budget’.
My first step is to identify how I will build the model, and therefore what all the components will be. If you are following plans, this is fairly easy. If you are developing your own, looking at other plans can be very helpful. Using my list of weights, I make comparisons and choose sizes and materials that will give me adequate strength. I list every significant item I can think of, decide on the materials, and multiply these out using my standard list of material weights.
Simple, right? Well, yes and no! If you already have a motor which you want to use, you may have to follow an iterative approach until you arrive at the correct size/weight. I’ve ignored the choice of power but in reality you have to decide this at the same time as designing the plane. You may start estimating weights before you have drawings, and you are probably going to change your mind about structures and materials as you build! However, this method sets a target to strive for, and has helped me develop better building techniques.
Most planes turn out to be heavier than we would like. This approach has helped change this for me. When you realise before you start that all the components add up to more than intended, you have the opportunity then to change things. Even if you have forgotten some items, you will usually have a list with too much on it and can easily see where the heavy parts are. You then target these areas and search for lighter techniques. For instance, I would normally always use 1/16” balsa to sheet the leading edge of wings (approx. 195g/m2). However, 1/32” may be adequate (134g/m2), as might 1/8” square strips (multi-spar approach/‘turbulators’). You should examine every item and decide whether it can be eliminated, made thinner, smaller or different in any way to save weight. Remember also that a number of small savings will add up to a larger overall benefit so don’t just accept the weight of small components.
So what’s involved in setting a ‘weight budget’ and deciding what ‘matter’ you intend using? The following are a few examples of how I estimate the weight of a fuselage before building:
2 sides x 1m long x 6mm square x 567kg/m3 (spruce) = 2 x 1 x .006 x .006 x 567 x 1000 = 41g
2 sides x .7m long x 10mm wide x 797g/m2 (1/8” hard balsa) = 2 x .7 x .010 x 797 = 11g
Lite Ply diagonals
2 sides x 3 pieces x 100mm long x 10mm wide x 1227g/m2 = 2 x 3 x .100 x .010 x 1227 = 7g
500mm long x 1/8” diameter x 7817kg/m3 = .500 x ( (.003175 / 2)2 x 22/7) x 7817 x 1000 [area of a circle = radius2 x pi] = 31g
1m x 70mm high x 50mm wide (ave) x 58g/m2 + 10% overlap = ( (Sides: 2 x 1 x .070)+(Top/Bottom: 2 x 1 x .050) ) x 58 x 1.1 = 15g
To give you a feel for the level of detail I go to, I normally identify about 15-25 items for each of the three main areas (fuselage, wing, and tail surfaces). A really simple airframe would have less and a complex structure more. Others are needed for motor, batteries, radio, etc. This may seem a bit long-winded, but if you can set it up on a PC spreadsheet its actually very easy and quite fun planning your project. It is then usually a very simple exercise to change the weight of a component for an alternative material or size. A little contingency for forgotten items can be a good idea at this stage, and basic guesses will be needed for some items (eg: glues). I usually update the list a few times once I start building as it helps plan future models if you keep track of the weight of components as built. This approach does not work perfectly for me, but I can say that I am very pleased with the weight of my models built using this technique.
I have not tried to help you decide whether to use 6mm or 10mm square longerons for your fuselage sides, but this method will highlight the weight implications of your choice. Achieving a suitable strength to weight ratio is a black art for most of us, but hopefully it will contribute to choosing the ‘matter’ for your next project.