My recent experiments with building electric powered planes have emphasised a very basic principle for me, namely the 'power to weight' ratio of an aircraft. There are many ways of manipulating the ‘power’ part of the equation in both electric and IC aircraft, but I thought I would put pen to paper and start to share my experiences controlling an aircraft’s weight.
I would like to start by defining a ‘vacuum’. We usually think of a vacuum as being ‘suction’ or perhaps ‘nothingness’. However it was once better defined for me as being ‘the absence of matter'. Even something as light as air has millions of particles per square inch. As you remove these particles, perhaps using suction, it starts to approach a vacuum. Light weight aircraft are very similar: they need to be 'absent of matter'. What you don't add cannot add weight. What you do add should be as small and as light as you can get away with.
Having determined the principle needed for building light aircraft, I found I needed to understand the purpose of each individual component. If you look at ‘old timer’ aircraft, you will often see that a great deal of attention has been paid to structural design. These techniques are very useful in designing or modifying planes to keep the weight down. The secret is achieving strength with the ‘absence of matter’. For instance, you don’t need solid fuselage sides if you have a sensible built up structure using longerons, diagonals, etc. Every component should be critically reviewed as to its purpose and therefore the strength required.
When it comes to actually building, an accurate measuring scale is a big help (1g accuracy is about right). The first step is to weigh all the materials that you use, and the list which follows contains those which I have. Obviously these weights are a very rough guide only; they can vary greatly between batches, cuts, humidity, etc. If you do not have a scale, this list should give you a good start. They are what I use to start a project even having a scale. Note that some materials need to be considered in terms of their surface area weight (i.e.: g/m2) or volume (i.e.: kg/m3). For instance, a soft 1/4” balsa fuselage side would be half the weight of 3mm lite ply (606 vs 1227g/m2), whereas it would make little difference whether I used my soft 1/4” or 1/16” balsa for a solid wing tip (93 vs 91 kg/m3).
I’ve found that covering can be a significant weight. On my recent planes, this ranged from 2,4% of flying weight using Litespan, 4% using Solarfilm and 6,5% with Solartex. I wanted the fabric texture and strength on the Solartex covered aircraft, but clearly this carried a significant weight penalty. Again these ratios will probably vary somewhat from one plane to another but they may guide you on future projects.
Glue can also important. In weight terms, I now use almost no glue on my planes. I always use CyA where I can as it weighs nothing and sets quickly. I occasionally use PVA which adds very little as all the moisture evaporates. Epoxy now goes a long way as I only use it for a few high stress joins and not as a filler as I did in the past!
I trust that these initial notes and the list of material weights will be useful to you and will stimulate more ideas. I hope to have another article in which I will explain how I use this information to save weight, but in the meantime I’m sure you will agree that Matter matters!