Fly Electric!

Absence of Matter

This is a 'construction' page. The 'Absence of Matter' heading may not be immediately obvious, but the secret to successful electrics (and other planes) is often in the weight of a model. Heavy planes usually do not fly well, whereas light ones tend to be very forgiving. The trick, therefore, is to avoid having an excess of 'matter'. 'Power to weight' ratio is another way of thinking about it. With a given power source, increasing weight will usually detract from performance (manoeuverability, not speed). So how do you make the appropriate matter 'absent'? What can you omit, what can you lighten, what materials are needed? Let me take you through my process plus some of the more interesting construction techniques I use...

Weights
Depron
Foam Wheels
Fibreglass Moulding
Hot Wire Foam Cutter
Vacuum forming
Rib tape cutter
CD-Rom brushless motors
DIY milling machine from a cheap drill


WEIGHTS: To start with, you need to know the weight of raw materials. This will allow you to look at options and to choose alternative building techniques or materials to achieve lighter weight. I have developed a list which I use when I design new aircraft. Your materials will in many cases be different, but understanding the relative weights can be enlightening.

Excel spreadsheet (66kb)

WEIGHT BUDGET: Knowing the weight of various raw materials, you now need to learn how to control the weight of the finished aircraft. I started by following the technique I describe below. However, I've now learnt many new building approaches and which materials to use where, so these days I usually just get on with it and make choices as I proceed. This is how I taught myself:

* Firstly by carefully planning how I will build the model. I list every item I can think of, estimate the size and quantity of each, and use the table of weights above to predict the finished weight (the 'weight budget'). This allows me to make design or material changes BEFORE I start. It's quite quick once you've done it a few times, especially with the spreadsheet below.

* As I build I weigh components and compare them to my weight budget above. If I discover that an item is perhaps larger or heavier than anticipated, I try to make changes like using different materials, drilling lightening holes, etc. This requires having a measuring scale of reasonable accuracy. Mine is accurate to 0.1g and goes up to 4.8kg, but 1g and about 2kg would be adequate.

I have set up various Excel spreadsheets (one for each plane) to help me design and control the weight of models. If you look at the contents of the numeric cells you will see the formulae I am using. In most cases the weights come from the table of weights above, although sometimes I simply guess at a weight. Please note that some of the weights will be different from the spreadsheet above because I keep updating the weights list. My spreadsheet is also designed to experiment with different model sizes ('Model Scale' parameter) and automatically calculate model weight (within reason). Have fun! You have a choice of (1) a simplified html version (no formulae), (2) the same version in the original Excel file format and (3) how I designed Bubbles using this technique.

Design example

View layout and values (html) on screen (74kb)

Excel spreadsheet

Save file to disk (28kb) (Save Target As...)

Bubbles

Save file to disk (22kb) (Save Target As...)


FURTHER READING:

* I wrote an article for the BEFA magazine in 1999 on building light weight aircraft. It repeats much of what I have covered above, but also provides some additional information:

Part A

The principles (9kb)

Part B

Application and examples (21kb)

* Keith Shaw is a master at designing light aircraft and an outstanding article has been written based on his teachings. This document is referenced in my Further Info section under 'The Future is Electric'.


DEPRON is one of the 'wonder' materials now available! We've found 3mm, 5mm and 6mm thicknesses in the UK, in sheets ranging in size from 210x300 (A4) to 700x1000mm. It has the rigidity of balsa the same thickness but weighs almost nothing. If you look at my table of weights above, you will see that a cubic meter of light/medium Balsa weighs between 91 and 163kg. Depron only weighs 40kg, ie: a QUARTER TO HALF THE WEIGHT! It cuts well with a good blade, preferably on an undamaged cutting board, and is easily sanded. White glue or PVA adhesives work well; 'Odorless' Cyano is great but experiment with accelerators; some attack it. I've used it for my Electric 40 wing ribs, the sides of the Fly Baby (which added only 47g/1.5oz), and formers in my E40's Floats. I would have used it for the Fly Baby's ribs but at that stage I could not find large sheets. Look at 'And_Now' and 'Butterfly' on my Tiny Treasures page. See my links page for international sources of Depron.

Fly Baby sides
15 kb
Fly Baby fuz
16 kb

FOAM WHEELS: The wheels on the Fly Baby are huge but extremely light. I bought a pair of commerical 6" modelling wheels and when I discovered that they weighed 22oz, I knew I had to devise something better. This turned out to be 'kneeling pad' foam which resulted in a pair of larger, correctly sized, scale wheels (6.5") which weighed only 5oz; a saving of over a pound!

I bought my foam from a shop called Homebase in the UK. Most DIY or gardening stores should have something similar, but shop around as there are usually a few different types and you may be able to find a colour or thickness which suits you. Specialist outdoor camping shops usually stock high density foam mattresses (about 10-12mm thick) which may also be suitable. If you need to laminate layers together to form thick wheels, experiment with different adhesives. I used a flexible epoxy called 'Wonder Fix' (made by Pratley in South Africa). Click on the photos to find out more...

Basic components assembled
36 kb
1 wheel half rounded
34 kb
1 wheel coloured
32 kb

FIBREGLASS MOULDING: Fibreglass is a great material for creating durable or complex components. If moulded, they make things easy to reproduce. Cowls are a good application for this as built-up ones are a bit tricky, and it's likely to be the front end of an aircraft which needs rebuilding in the event of a 'mishap'. They are also a good place to start experimenting with moulding techniques as they are relatively simple. Although I have some photos of very ambitious projects as well, I will use a cowl to describe the principles first.

I decided to make a two-part mould for the Fly Baby cowl and the following photos show the sequence (almost every mould will need to have two or more components to allow it to be opened and for the finished product to be released/removed without damage). I sanded the plug from very soft insulation foam. This stuff is easy to shape, but being so soft, you have to be careful not to damage it before producing the mould. The first photo shows how I ended up making two plugs for the Fly Baby! Click on each photo to see detailed guidelines and tips...

Cowl plug
27 kb
Ready to do 1st half of mould
37 kb
Now for 2nd half
38 kb
Finished cowl emerging
36 kb
Optica canopy and shroud moulds
Optica (38 kb)
Stearman moulds
Stearman (11 kb)
Heli Blade moulds
Heli Blades (27 kb)
Miscellaneous moulds
Miscellaneous (31 kb)

HOT WIRE FOAM CUTTER: It is easy to make a hot wire foam cutter, particularly if you fly electrics. This is because you need a power supply, and nicads are so convenient. I used to use a 4A 16v transformer but working with mains power has its risks. I now use a 12 cell 2000mAh nicad pack. My setup only draws 2 amps. Other cell counts and combinations should work as well.

As you can just see from the first photo, I have made an 'I' shaped frame with a length of 25x25x1150mm wood in the centre and two pieces of 25x25x320mm at each end secured with a single screw each. Two lengths of single strand fishing trace wire (ie: uncoated solid wire) on each side allow you to pull the wire very taught and remain perfectly balanced. Each wire is attached to a headless 6mm steel bolt so that a nut can be used to tighten things up.

Bear in mind that the strands of wire represent resistors. They dissipate heat when current passes through them (ie: when you effectively short-circuit them across your battery or transformer). The thicker they are, the lower the resistance, the more current can flow and the more heat is generated. On my setup, one wire is 0.3mm and the other is 0.4mm (0.4mm = 0.0157") to allow experimentation with currents and temperatures. In practice I only use the 0.4mm wire with the 12 cells or 16v transformer (both have a similar voltage). However, if your dimensions and operating voltage are similar to mine, I would recommend 0.4 and 0.5mm strands because some extra heat would be nice sometimes for a faster cut. Power is supplied through ordinary two core insulated wire which should not be too light or they will add too much resistance. These wires have crocodile clips which attach to each end of the chosen cutting wire. In theory I can move the clips in closer to the centre to increase the current and heat, but have not ever actually used this feature. The crocodile clips make it easy to physically connect the power to the cutting wire, and are also a very easy method for disconnecting a battery supply between cuts to save battery power.

Full length
27 kb
One end
37 kb
Close-up
33 kb
Wire ends
18 kb

VACUUM FORMING: If you've ever wanted to make windows for a model, have a look at those on the Optica. Most of them are compound curves, but they are easy to make with suitable plastic. All you need is a household heater and the 'form' over which the window will be pulled. Observant readers will notice that no 'vacuum' is actually required with this technique!

My current stock of plastic is 0.25mm thick and is vacuum forming plastic; other types should work. I have used plastic up to 0.4mm but found 0.5mm to be too thick. 0.25 to 0.35mm is ideal for most models. The photos below are probably reasonably self-explanatory. Cut a piece of plastic quite a bit larger than the shape required. Use a staple gun to attach it to two pieces of wood (more if the compound curves are severe). Heat up the plastic by moving it over and close to the hot bars, then quickly pull it hard (to stretch it) over the plug. If you don't get it right, simply re-heat the plastic and try again; it is quite normal to make many attempts with compound curves. The pictures below show the top window being made for the Optica. Notice how the plastic is resting unsupported on the heater while I took the photo; you can damage it but it does not melt easily.

Staple plastic to wooden strips
23 kb
Ordinary household heater
29 kb
Pull heated plastic over plug
22 kb
Re-heat plastic if shape is not perfect
26 kb
Close-up of Optica canopy
24 kb

RIB TAPE CUTTER: Some years ago when I started building my 1/5th Stearman, I wanted to add scale rib tapes. The plans and resulting tool are shown below. However, I have to say that I now use dressmaking 'pinking' shears. They have a pitch of about 5mm so two 'offset' cuts produces an excellent result for large models. Read on if you want to do it 'the proper way'!

For my Stearman I was able to obtain some original covering from a damaged full size aircraft which gave me the correct proportions and attachment methods. Some photos of this covering are below. I set about making a machine to cut tape to the correct size. While I was at it I made the device capable of cutting almost any width and pitch. It can cut 10 layers of heat shrink fabric covering at one time (eg: Solartex or Coverite). It cuts both sides of the tape to simultaneously create the zigzag pitch and width. It requires a little engineering to make, and each and every seration (in pairs, one each side) requires a manual cutting stroke and a manual advance stroke (1mm for the Stearman). A little tiresome (hence the pinking shears now!), but sometimes we like to do things 'right'! Here are the results and building instructions in a choice of two file formats:

Rib tapes on Stearman model
13 kb
Rib tapes on Stearman model
13 kb
Illustrated design
documents:
Adobe 'PFD' file (25kb)
Zipped 'RTF' file (61kb)

One the good things about this approach is that you simply iron the tape in place after covering the wings. For full scale effect you will need to simulate the thread which binds the covering to the underlying structure. After covering but before adding the tape, apply little dabs of white glue along the ribs with the appropriate spacing. Iron on the tape once dry and you will get the perfect effect with the covering draped and shrunk over each 'thread'. Perfecto.

Original Stearman wing covering
16 kb
Underside of original wing covering
14 kb
Full size tape and stitching
19 kb
Cutter ready for use
21 kb
Gearbox to advance platform
25 kb
Cutting arm and spring-loaded platform
26 kb
Cutting heads
16 kb
Cutting heads
39 kb

Top of page
Click on the links to find out more.
Click on the pictures for a larger image and more info.
Click on the links below for other pages.

Home Art of the Possible Absence of Matter Links Updates Email
SITE MAP