Indoor Free Flight

The following is a resume of a presentation given by Rodney O’Neill to the Ulster Aviation Society.

Printed by kind permission of the Ulster Aviation Society

Our speaker at the April meeting was an engineer who has taken part in air tests on the Shorts Belfast, worked as a BBC maintenance technician at the Lisnagarvey Transmitter and toured many exotic parts of the world, providing technical back-up and educational support for the former Shorts Missile Division.  But Rodney O’Neill didn’t come to talk about any of that, interesting as it would have been; he came to tell us about Indoor Duration Model Aircraft.

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He is one of four people from Northern Ireland who take part in this pastime, one of the others being his partner, Dorothy, who was also present.  Both have to build their models from scratch as there is no ready source for the finished article.  They have to be built to very precise standards and, even more importantly, they have to be very light in weight. This means, for example, applying glue down the side of a very fine piece of wire, having already thinned it with acetone so that just enough runs into the joint to bond it, while the excess evaporates!  It means weighing thin sheets of balsa wood to find the lightest, then cutting that on a tiny band saw into ten strips a few millimetres square, subjecting each to a stiffness test, to find, not infrequently, that none is stiff enough for the motor stick and having to repeat the process until a suitable one is found.  So that we could understand all of this, he gave us a quick run through Young’s Modulus!  The wing frames are thinner than a matchstick and the ribs thinner again.  Then the whole wing is covered with the thinnest transparent mylar film imaginable.  Power source?  Believe it or not, elastic bands, again not just any old elastic bands but the rubber bands that are used in the windings of golf balls.  Again, Rodney set out the formula for obtaining the best bands for longest duration. He even produced graphs showing turns against torque, the aim being to wind the band just enough to give the most even discharge of power.  He even attaches them with little home-made rubber O-rings, as these provide another few turns which can add three or four seconds to the duration of a flight.  The propeller blades (variable pitch mechanisms are commercially available) are also hand-made and his wooden jig was passed round.  All of this results in the flimsiest models you will ever set eyes on but the rub is that they weigh in at a few grams, with the lightest being less than half, yes, half a gram.  This is hard to get one’s head around but is brought home by the fact that adding the elastic band to the lightest one more than doubles its weight!  So powered, they can remain airborne for up to nearly 0 minutes.  I’m sure everyone present who examined the models on display were very impressed by the craftsmanship and ingenuity that is so obviously present in them.

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Rodney and Dorothy have competed in the open international competitions in Valencia, Belgrade and Moscow (Idaho, USA) where they put in some very creditable performances. Coincidentally, Dorothy was British national champion in 008 and Rodney in 007 and 008.  Rodney’s presentation was fascinating, given with an enthusiasm that was contagious.  I think the current batch of aeronautical engineers present were quite taken at how something could be so lightly built and yet stay aloft.  I have watched the models fly and find it very relaxing to see the propeller slowly turn as the model leisurely circles a few feet overhead, a bit like watching a butterfly flutter around the garden.