December 14th 2009

"Cottage Engineering"

Our last meeting of the year, 'A light hearted look at engineering at home', was given by three of our own members, John Rant , Tony Meacock,and Chris Hoskins, who were also members of the U3A Curiosity Club – part of the University of the Third Age.


John opened the meeting by telling us that his background was a microbiologist rather than an engineer. ' The evening was divided up as a 'menu' with different 'courses.

The 'starter' course was called “light fantastic”. John introduced the proceedings with a description of the groups experiments comparing the efficiency of low energy and tungsten light bulbs. The set up was simple and used photographic exposure meter to measure the light output and a simple watt-meter to record the power input. One surprising outcome was that the light output for a clear tungsten bulb was identical to that of a pearl bulb.

As a second 'course' John then talked about rope making and introduced Chris Hoskins who showed a simple rope making jig. Chris Hoskins then gave a short presentation on the history of rope and rope making and indicated that a basic machine had been made (which he said members were welcome to view after the talk was over) to demonstrate how this was done.

As an 'entree' John went on to describe his own work in updating 1950s model aircraft designs. John had tried to reduce the weight of one plane by 40% by making all the spars and ribs as a geodetic structure. This included improved and more rigid spar arrangements in the wings and conversion of a diesel power aircraft to an electric motor. I found it surprising that in spite of the weight of the battery the overall weight was less than the diesel plus it's fuel and also showed an improved thrust. An interesting point is that the charge in the battery has no weight and so the centre of gravity of the aircraft remains the same as the battery discharges. This is more difficult to achieve with an engine which consumes fuel.The power of the electric motors was measured on a thrust measurement device made from electronic kitchen scales.

As the 'main' course Tony discussed the problems with thermals in model flying which can often result in many hours of patient construction sailing off into the wild blue yonder never to be seen again. The traditional way of preventing this is to have a fuse which after it has burnt through allows the tailplane to tilt forward, stall the aircraft and return it to earth safely. John demonstrated his own version made from the clockwork motor from a toy baby walker modified by increasing the inertia of the escapement to convert it into a timer capable of triggering the stall after 3 or 4 minute flight. His practical demonstration was that of a rubber band model made from polystyrene wallpaper, balsa wood and a propeller fashioned from sections cut from a plastic cup. He was so confident of it's performance that he was able to take it from it's box and without any adjustments fly it over the audience in a perfect 20 foot circle in level flight and straight back to his hand. Very impressive!

The 'dessert' course was served by Tony Meacock and was called “Tony's adventures in windy-land” Tony had studied the design of wind tunnels and decided to try and make one. Various problems had to be overcome viz the design of the fan, the flow straightener (made from 1135 straws) he also made a device to measure pressure difference and wind speed. Next it was Tony Meacock's turn to amaze us with his home built wind tunnel. Tony had done a lot of research on the internet and had finally chosen a design using a fan sucking the air through the tunnel. However there were several problems to be overcome before a reasonable working model could be constructed.

At the air input end some form of honeycomb had to be formed to ensure that the air would flow evenly through the tunnel. Hexagonal honeycomb is available commercially but not a price suitable for home constructors. So ¼ inch diameter drinking straws were cut to about 1 inch lengths and glued together to form an approximate honeycomb. Using a circular section rather than a hexagon to form the honeycomb resulted in a 10% loss in area through which the air had to flow but that was considered acceptable. To measure lift and drag strain gauges were needed and were available at £150 each – not an option! Instead kitchen scales purchased from Aldi's at a more modest £7 each were cannibalised to extract the strain gauge which was then connected to the electronics with a lead and the digital display of the scale was retained.

Tony demonstrated lift and drag of various shapes by observing a cotton thread in the air flow. The motor and speed controller were available from Tony's junk box but a fan had to be constructed. In the first iteration a 6 bladed fan was made from sheet aluminium with the the blades folded at about 30 degrees to plane of rotation. Although this worked the blades bent under pressure and were liable to work harden and fail. A stiffer version of the angled section blade fan was built to get the tunnel working and later Tony designed the current curved section blade fan using the wind tunnel itself!

The last problem was the measurement of air velocity and Tony decided to use a pitot tube. Initially this was connected to manometer to measure the air pressure. Unfortunately velocity is proportional to the square root of pressure so a conventional vertical manometer gives a very non linear scale with the lowest velocity reading cramped at the bottom. Tony solved the linearity problem by using a horizontal tube bent in the shape of the square root. It had to be levelled and needed an adjustable reservoir to set the zero pressure point but it has an easily read linear scale of velocity. Tony showed that the results for various shapes including a flat plate, a cylinder and an aerofoil matched published results well. He also gave a dramatic demonstration of wind pressure on a chimney both with and without helical spoilers. Having an interest in sailing his demonstrations had to include a model boat to show both windward sailing and running and the dramatic effects of a gybe.

Past President, David Grint, thanked the team for a very entertaining evening which seemed to go down well with members. The crowd that gathered for discussion with the presenters at the end of the meeting was a testimony to how much the evening had been enjoyed by all. I'm sure that the evening will be remembered as one of the highlights of 2009 and a great way to finish our year.

Jack Gentle (President 1997-1999) December 2009