8 E.F.-U.K.
Letters to the Editor Firstly a letter from John Bunting (aj.bunting@btopenworld.com) about Scale Speed, following on from my comments to last issues TLC from your TLO article. I'd like to discuss scale speed, and in particular Bob Boucher's article on the Astroflight website, to which you refer in EF-UK 82. I'm perfectly happy to be shot down if I'm wrong, but I'm not convinced by the idea that the model speed should be derived by multiplying the full-size speed by the square root of the scale. Let's take a one-ninth scale Spitfire as an example. The full-size span is 36 feet, and typical speed 300 mph. The square root of one-ninth is one-third, so the speed of the 4-foot span model, according to Bob, should be 100 mph. At this point I begin to have doubts, which grow stronger when he talks about 'Scale Time', and says in the section headed 'Scale Size Loops' that the square root factor applies here as well, so the model should do a loop in one third of the time taken by the real aircraft. Now, suppose you have a video of a real Spitfire doing aerobatics. You watch it several times, and you have a good idea, possibly assisted by a clock display in the corner of the screen, of how long it takes for the machine to do various manoeuvres. For instance, you may notice that it typically takes about ten seconds to do a loop. Then someone shows you another video, also of a Spitfire. For the first few seconds the machine is in steady flight, with a clear sky background, and you might easily think it's a real aircraft. But then it starts doing aerobatics, and the illusion is shattered, because it does a loop, not in ten seconds, but in about three seconds, so you know immediately that it must be a model. So much for realism. To crown it all, Bob tells us, "These very high power military aircraft are very difficult to model at scale speed". Well, I'm not surprised, if you think the scale speed for a one-tenth scale P-51 should be 135 mph. My conclusion is that this square-root-of-the-scale idea is misconceived. For realism in flight, I think the speed should vary directly as the scale. In other words, the real aircraft and the model should each take the same time to cover a distance equal to their own length; in which case our one-ninth scale Spitfire would fly at about 33 mph, and would do a loop in ten seconds. Not always easy to achieve in practice perhaps, but that's another matter. John raises a very interesting point in his letter and I’m sure it has merit. However, I’d like to consider the issue in more detail. I’m not sure about the 300 mph speed that John gives as typical. For completeness I’m going to compare the figures for stall, manoeuvre and maximum speeds (in mph) for a Spitfire. E.F.-U.K. 9