Tony K
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Everything posted by Tony K
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That is a nice looking aircraft. The smaller version will probably be my next project but (and this is not meant as criticism) the plan appears to be an assembly guide for the cnc parts rather than a working drawing for the scratch builder. I know that all those interlocking tabs and slots make for quick and accurate assembly but for those of us who prefer to spend some time at the bench cutting out the parts, it looks a nightmare. Take parts R12 and S9 for example, not only are they complex shapes but, as one piece parts, the grain direction at the top and bottom appears to be wrong to me. Anyway, as I wrote earlier, it is not my intention to criticise the design. I know that TvM is a well respected designer and builder. I will study the plan carefully and try to simplify and re-draw certain parts.
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I remember when I was about twelve years old (1965), one of these "mini twisters" came across our school sport field.
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Germany, thirty minutes drive from the centre of Cologne.
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Posted by Ben B on 21/06/2012 14:32:29: Posted by Tony K on 21/06/2012 14:08:45: Posted by Erfolg on 21/06/2012 12:15:26: This is unusual, we all seem to agree. We now await BEB's intervention, hopefully not to tell us we are mistaken .... [edited] In the model's frame of reference, the propellor or fan is moving the same amount of air just as quickly when it is standing still as when it is at maximum speed. Are you sure about that? When a plane is moving forward at maximum speed the effective AoA of the prop / fan is very different to that when the plane is stationary. A high pitch prop can easily be stalled when static and only really start "hauling air" when the plane is moving forwards. In those situations they'll be moving very different amounts of air. Ben, in the static case the propellor is moving air rearwards. That air has to be replaced otherwise there would be a vacuum, so air is pulled in from the front. That means that there is an airflow through the propellor disc which, as your diagram illustrates, affects the effective AoA. How much difference is there between the effective AoA when static and when moving forwards? Also, if we take a six inch diameter prop and say the mid point of the propellor blade moves around a three inch diameter circle, at 20000 rpm that point is moving at 180 mph. How much forward speed is needed to make a difference to the effective AoA?
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Posted by Erfolg on 21/06/2012 12:15:26: This is unusual, we all seem to agree. We now await BEB's intervention, hopefully not to tell us we are mistaken Erfolg, may I refer you to Richard Sharman's "Theory and Practise" thread on the EDF board, to which you also contributed, and which unfortunately fizzled out without confirming any theory or practise. Top speed is achieved when thrust equals drag. As far as aircraft are concerned, drag has two components. There is parasite drag, which depends on how slippery the model is, and there is induced drag, which depends on how efficient the lifting surface (wing) is. The interesting thing is that induced drag is higher at low speed (greater aoa) and reduces as the aircraft flies faster whereas parasite drag is lowest at low speed and increases with airspeed. An aircraft designed to fly at high speed will have a relatively much higher induced drag at low speed, hence the need to get the thing moving quickly with a bungee. For prop hanging (which doesn't interest me at all) the thrust must, of course, equal the weight and a vertical climb is possible when thrust is greater than weight. It is true that Newton said F=m*a. You can rearrange that to say a=F/m, acceleration equals force divided by mass. In the equation F=m*a, the m refers to an object with a mass. In the case of moving air (a fluid stream), mass flow is used. So the equation becomes F= mass flow * velocity. In the model's frame of reference, the propellor or fan is moving the same amount of air just as quickly when it is standing still as when it is at maximum speed. I suggest, therefore, that static thrust is a useful indicator of the power units performance.
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Posted by Steve Hargreaves - Moderator on 14/06/2012 11:30:15: 3m from my bath tub puts me in the front bedroom..........I have quite a few sockets in there...... So not really a practical situation for most people then..... In Germany you will often find sockets next to the sink.....for hairdyers & the like..... Here in Germany I don't think I have ever been in a bathroom of any size that doesn't have a socket near the sink. My bathroom is 2m by 2,5m with bog, bath, basin, and shower cubicle. The socket is next to the basin and is used daily by swmbo for her hair dryer. Edited By Tony K on 14/06/2012 11:50:13
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Posted by Richard Harris on 03/06/2012 21:19:41: It may be Greek in origin but its here! , I have just cut some balsa this afternoon but I dont think that is British in origin either still, it brought a bit of enjoyment on a wet and miserable day! Perhaps it is not generally known that the torch relay was an invention of Josef Goebbels' propaganda machine and was first used to open the 1936 Berlin Olympics.
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Posted by Koen Smits on 14/03/2012 18:14:32: We are 3 friends with the same hobby and this is the second time we build the same model. In the winter from 2009 to 2010 we have build 3 Dewoitines D19-C1 from plans out of the german magazine FMT. See pictures. Koen, a bit off topic but good to see the D19 flying, I am currently building one.
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On this day in 1943, 7th.January, the inventor Nikola Tesla died. It might not be known that among his many inventions was a radio controlled model boat, demonstrated to an audience in 1898. Tesla was given the patent for his invention and so became the father of RC modelling.
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Well worth a visit if you are ever in the Hamburg area. Buy a ticket in advance because the queues are horrendous.
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Posted by Martin Harris on 07/12/2011 23:27:48: Sure these are diameters and not cross sectional areas? Edited By Martin Harris on 07/12/2011 23:28:45 My mistake, it's a German catalogue and it was late last night when I read it. Querschnitt is cross section. It's for mini aileron servos extended about 40cm (16 inches) so will the 0,25 square mm be OK?
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I need to make some servo extensions but the catalogue I have lists servo cable in 0,08mm, 0,14mm, and 0,25mm diameter. Which size should I use?
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Sorry to jump in on your thread but this was my (last) winter's project. Draught beer and a built in fridge.
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Thanks for the welcome. The model is called PET and was a free plan but I made a few changes. I changed the wing structure from a single 4x12mm to two 5x3mm spars, I made the fuselage a bit wider to get my fingers in, I moved the fin rearwards because I thought it looked better, and I added ailerons. I actually started a semi scale model first but put it aside to build this trainer. I have just ordered a roll of "Oracover" and will soon have to start covering. Another new skill to learn!
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Just returning to the hobby after a break of about forty years. I have a trainer type model under construction based on a free plan from a German magazine. It is 1,2m span and will be electric powered. Progress is a bit slow at the moment because of work and other commitments and having to relearn old skills as well as learning some new ones. I am finding the posts on this forum a real inspiration. TK.
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Interesting thoughts, Erfolg. I have had another look at the test results which caused Richard concern on 28/10. test1: ve=47m/sec, T=6.1N test2: ve=58m/sec. T=10.5N Using my spreadsheet and the equation T=mdot * v average, a thrust of 6,17N requires the ve to be 50 m/s. Bearing in mind the difficulty of measuring ve accurately, that is pretty close. For a thrust of 10,5N, ve should be 65,2 m/s. Not quite as close but acceptable. For Pete's installed static results of 21/10, a thrust of 10,4 N corresponds with a ve of 60,6 m/s against his measured value 60,35 m/s. Close enough to be ignored. Also, using the quoted power consumptions, the efficiency (power out/power in) of both systems is about 0,70 to 0,73. I think these figures are realistic but would be interested in any comments.
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Posted by Erfolg on 19/11/2011 13:19:12: The force comes from the acceleration of the fluid. On the basis that the air at the boundary of the system is at zero and is accelerated through the duct. I hope you mean that the air is accelerated before it enters the duct. As you know, if the cross section is constant and no heat exchange is involved, the mass flow at the inlet is the same as the mass flow at the outlet. Therefore no acceleration. So where are the system boundaries? If the generally accepted equation, F=mdot*dv, does not enable one to calculate the thrust in a parallel duct, how can it be used to calculate the thrust in a tapered duct? So what is dv? It can not be the efflux velocity minus free stream velocity because that would mean that the thrust reduces as the model moves faster. Countering increasing drag with reducing thrust does not make sense. Looking at the units of force or thrust, kg.m/s^2, the mass flow takes care of kg/s so we only need a speed, m/s. Could it be that the "speed" we are looking for is simply the mean velocity of the air between the inlet and outlet? BTW, I agree with your opinion of Newton and I have previously stated my opinion of the Scharnhorst paper. TK. Edited By Tony K on 20/11/2011 11:02:23
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I have a question. Suppose you have a duct with a constant cross section, ie. the inlet area is the same as the outlet area. Put a fan in the duct to generate an airflow which will produce a thrust. The inlet velocity will equal the outlet velocity so the delta velocity (ve-vi) is zero. The equation F=mdot*dv gives you zero thrust. How can that be?
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Quote,"not always easy when the voltage is varying under load. " In a static test the load should be constant. Why does the voltage vary under load? Quote, "Yes, there is a clear velocity gradient across the duct, and some variation due to turbulence no doubt, but the readings are probably not far off truth." Your test results would suggest otherwise. Quote, "Timbo always stresses that this sort of testing is "relative" rather than "absolute" but I am less worried - it's probably good enough for our purposes." Unless we can arrive at an absolute thrust figure, I am not sure how this exercise can achieve anything usefull. Quote, "For dynamic thrust in the air we have no measurements at all." I have previously tried to show how (theoretically) static thrust equals dynamic thrust. At the moment, I am not sure if I can contribute anything more to this discussion. T.K.
