There are essentially two ways of doing this, the Easy Way and the Not So Easy Way but before that . . .
Safety First. DO NOT FLY kites, balloons or anything else with a tethering string, such as this, near to any overhead power cables. To be safe, make sure that the length of line is FAR SHORTER than the distance to the nearest power cable in any direction. This type of buoyant device is best flown in still air which can have slight, short-lived or even prolonged gusts in ANY DIRECTION so BE SAFE. I have found it best to fly away from trees, houses and other structures that the cord can snag against.
The Easy Way is to purchase the Solar Powered Air Ship from Astromedia at http://www.astromedia.de/. These are not expensive, they are quick and easy to put together, all of the maths is done, all of the materials are sourced and packaged for you in the correct amounts from one supplier, they only cost a few German Marks or Euros, you can order them over the Internet and all you have to do is to follow the instructions. No sticking or cutting - if you can tie knots in string, you can make this.
I would recommend that you start with one of these (as I did) so as to get the hang of what is going on - how to handle it, wind conditions and so on. These are quite impressive and you occasionally hear people expressing astonishment. On the left, you can see one about 40 metres up in the air and on the right, one fairly close to the ground. They are around 2.5 metres or so long and you can see the shadow on the ground on the right (it is quite close to the trees). They move quite slowly and people generally don't notice them until you draw attention to them (try pointing into the air and people will look where you are pointing. Just for a joke, try standing still in a street, with a friend and point at some part of the sky - you will soon find that others will stop and try to see what you appear to be looking at and very occassionally, someone else will point and if you are really lucky, you may even get someone to claim to someone else that they can see something). These balloons will go very high and on one occassion we even had birds circling it.
This is for a very simple cylinder (right) made from bin liners from a supermarket. It has the advantage that you can make it any shape or size you want within the laws of physics (as we know them ;-). Make it like the animation on the right or make it slightly flatter and more like a UFO. All you need to do is unroll, cut and tape. This page shows you how.
If you can get that lot together then you can start to calculate how you should build one. To help, I have made a lotus 1-2-3 97 spreadsheet that you can download (screen shot on the right in 1024 x 768 mode) and it will do all of the hard work for you (calculating which variants are more likely to fly than others). This is what you do . . .
So much for the column on the left, now, you can calculate the number of degrees centigrade elevated internal temperature required to make the UFO weigh the same as the air that it is in. Any hotter and it takes off (hopefully providing enough lift for the length of string that you have, hence choose a lightweight string). You can see that the spreadsheet makes life easier as it will calculate the required temperature elevation that your black bin liner will need to generate from the amount of solar radiation falling on its surface for a given configuration of bin liners. To generate output from the model, all you need to do is to put in the number of bin liners that you put side by side and end to end. Try putting a thermometer inside a black bin liner and measuring the increase in temperature that it will generate for where in the world you are when the sun shines on it.
Wide. If you use your bin liner tube without slitting it up the side, you will not have to tape up the side (the model takes this into account by having one less length of tape in that case) but if you do slit the liner up the side, you can stick 2, 3 or more, side by side to make a larger diameter cylinder (less surface area for a given volume). This is the number in the "Wide" row.
High. If you tape the open ended bin liner cylinders end to end, you can increase the enclosed volume. This number is put into the "High" row.
This is where to put the figures on the spreadsheet (only in the Bold Magenta Cells) . . .
This is all about enclosing the largest volume with the smallest surface area so the more "spherical" the shape, the better it will perform, ie. the smaller the required temperature difference for a given lift.
Of course, there is nothing to stop you from making the shape a little more like a flattened truncated cone or from putting partially transparent sections on it (using white swing bin liners) or using different coloured bin liner pieces to make colourful characters or emblems as long at it remains tethered securely.
Some things to consider are:
the Lotus 1-2-3 97 model by clicking here. buoyant.123.
File size 14,688 Bytes