Balsa Basics

by Mike Pirie

Balsa selection has always been important for us glider guiders, but now with the increasing popularity of electric models, it has become even more important to choose wood which will result in the lightweight airframes required. The main considerations when selecting wood, are grain direction, stiffness and weight. The first two are fairly self evident, but the third, the weight, is not always so obvious, and the only sure way of establishing its weight, or density, is to weigh it accurately.

With this purpose in mind, I devised, a few years ago, a table of multiplication factors (shown below) to aid in the calculation of densities for sheets of various sizes. Once the density has been established, I clearly mark the value on to the sheet using a felt tip marker. Balsa densities vary between 6 and 14 lbs/cu ft but heavier and lighter pieces are occasionally encountered. To use the table, simply weigh the balsa sheet in grams, and, using the appropriate multiplication factor from the table, calculate the density. The third column in the table is for an individual square inch and can be used for odd shaped pieces whose area in square inches, is known.

Remember to leave on the weight information on when cutting pieces off the sheet. This way even the small pieces which end up in your scrap box will be marked with the correct weight information shown.

With regards to weighing equipment, digital balances weighing up to 1200g in 1g steps can be obtained from Quicktest, but are rather expensive. A cheaper option would be the postal scales sold by Mike Woodhouse of Free Flight Supplies for £5.75. They weigh up to 100g in 1g steps. Happy building.

Fuselage Jig

by George Whelan

Over the past couple of years I have flown a number of ARTF models from Puffin Models, while these are well made models and good flyers I still like to cut and glue sticks.

As the Winter draws on I decided to set a production line and build an Elektra 7 electric soarer, a Sagitta 600 with ailerons for the slope and a Sagitta 900 model for next years 100S Nationals. These models are traditional construction being entirely built up from balsa, ply and spruce.

Despite having built many of this sort of model I still am not confident in building straight fuzzes no matter how many clamps are applied. The answer is to clamp the fuz to something i.e. a jig.

Looking on the Internet at the available options it was down to the SLEC jig or a build it yourself, many people (Norrie) have used the SLEC for years, but I decided to go down the build your own route. I mulled over a number of ideas including adjustable cradles built onto a tube or box section, however I decided to go for the right angle brackets on a board method.

So to the shed, looking around I came across some good 12mm ply from a chest of draws I demolished, ideal for the brackets. I got some graph paper and drew up a couple of shapes before deciding on a final shape based on the size of wood available, this shape was transferred to the wood giving me 5 pairs out of one piece. Using my band saw and Fostner drills I very quickly cut out the brackets and fettled them up on my sanding disk. I was very pleased with the product and decided to cut up another piece, making one pair extra long for fin alignment.

The brackets would be held onto the board with M5 X 60 screws and wing nuts, this meant that any side to side adjustment would need to be incorporated into the board via slots.

The board is a piece of Conti board with M5 slots routed from side to side at 75mm spacing. This was a learning curve for me, She who must be obeyed bought me an excellent router for Christmas 2 years ago and this was the first time I have tried heavy duty routing, probably about 90% success, from the photo you can see the odd wobble, however the whole thing works a treat, straight fuzzes from now on. I have included some photos, which tell a thousand words. The only addition is a centre line drawn down the board for alignment.

Main lesson learned, use strong clamps for the router guide or it will drag you all around the shed. Wear a mask and goggles as the dust gets everywhere.

Foam Wing Cutting

by George Whelan

Following my presentation on cutting foam wings I thought it would be a good idea to do an online presentation as an aide memoir for anybody who fancies having a go. I have been cutting wings for years and my method has evolved from very simple to complex and back to the current simplified method.

Why Foam

• Repeatability, once you have your templates you can very quickly turn out multiple wings.
• Variability, once you have your templates you can use them to turn out a variety of wing plans.
• Robustness, I believe foam wings can absorb impact energy better than a built up wing, but crash them hard enough and they will break.
• Profile accuracy, while the profiles we cut are at best a representation of any particular profile, this profile is constant across the wingspan.

Materials

• Dow Corning Roofmate, Floormate & Wallmate, usually called blue foam, this is a closed cell polyurethane foam which is available in various densities and has good impact resistance. Sheffield Insulation sells this material in packs. There are variations on this foam, I buy a peach coloured version from Inverurie, about £13 a sheet, and you get 10 blocks by 2 wing panels out of a 75mm thick piece, enough to make 5 sets of 100” wings.
• Polystyrene foam, this is blown polystyrene, it does not have the same strength as blue foam, ideally look for virgin foam with a bubble diameter of about 3mm. B&Q sell recycled insulation, be careful, this foam can contain hard lumps which may interfere with the cutting wire.
• There are a number of other foams which are less commonly available ore are quite expensive, pink foam, spyder foam used for surfboards etc.

ALL THESE FOAMS WHILE NOT TOXIC SHOULD BE CUT IN A WELL VENTILATED SPACE

Bows

• Over the years I have used a number of homemade bows, the first was a piece of 2 x 1 hardwood about three feet long, a 6mm hole drilled about 50mm from each end about 5 – 10 degrees from the vertical. A piece of electric fire element wire stretched tightly between the ends of 6mm piano wire inserted into the holes. This bow is quite adequate for the job; the tension is the critical element. My current bow is a variation on this theme made by Feather Cut; this has a metal body and folds into itself for storage and transport.
• I also have a bow made from aluminium tubing with fibreglass limbs; this looks similar to a modern composite archers bow. The beauty of this type is that it maintains its tension when the wire heats up.
  
• There is a bow based on the old carpenters bow saw, this uses a string windlass to tension the wire, the downside of this is it is not self-tensioning and the cutting wire will slacken as it heats up.
• The bow can be hung beneath the cutting board or suspended above it depending on the length of the board and bow.
  
  

I have a variety of bows in different lengths up to about 1 meter long, longer than this the drag on the cutting wire makes the profiles inaccurate. You can still make long wings; you just do it by gluing a number of panels together. I use this method for making multiple taper panels. I used to use a standard car battery charger for powering my bows, this was more than adequate and safe, and they are cheap enough to buy if you don’t already have one. For me using a 240-volt light dimmer switch is scary, take care with anything electric, and remember you will be handling this equipment. Cutting wire is available from the likes of SLEC, you could use a guitar string, and my first source of wire was an electric fire element, just wind the required length of the element. My power supply is a Variac transformer which can be turned up to 275V ac. I have this pegged at a maximum of 24 volts. However I do not use voltage to regulate the cutting speed I use current, hence the dvm. I have one type of wire that cuts well at 2.45 amps and another that cuts well at 4.25 amps. If you use amps to drive the cutting wire then it will be constant regardless of the length of bow.

Templates

• These can be made of a variety of materials, thin ply, tufnol, traffolite, thick card with edges hardened with cyano watch out for the fumes.
• The form can be in the shape of the aerofoil which is stuck to the foam with a couple of nails or pop rivets, or it can be my preferred method of cutting a separate bottom and top profile and again attach with nails or slotted into some 10mm aluminium channel.
  
• If you use nails or rivets to hold the templates lay them together and drill the nail holes through both templates, this will ensure alignment between the top and bottom templates.
• I print off the aerofoil using Compufoil software and stick this on my template material using double-sided tape. Profili is also good cheap software, readily available on the internet.
  
  Remember to allow for the thickness of your cap strips or veneer if you are bagging your wing.
• Wash in can easily be built into the wing just by tilting the template.
  
  

Cutting Board

• I have a range of cutting boards made variously from Conti board, insulation board, and plate glass; the main requirement is for it to be flat.
• If you are cutting tapered wings you can use the two handed method where yourself and your colleague each handle one end of the bow and cut according to stations marked on the template, one person must lead the exercise calling out the marks as they are cut, the other person following. Those of you who saw my demo will remember I used a simple dividing arm. This is a meter stick pivoted at one end and draw wires run from each end of the bow, round a couple of pulleys and attached to the arm. The arm is weighted and when the bow is hot the wire will be drawn through the foam. If you are cutting a tapered wing it is simple to work out the taper ratio and attach the pulling wire to 2 separate points on the arm.
• Using the dividing arm the cutting speed is self-regulating by the weight on the arm, not too much about 0.5 – 0.75 kg is sufficient
  
  
  

Once you have your wing panels you can glue them together, install a joining mechanism and glass and bag them, but that’s another story.

Wire Gauge Table