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BMatthews -> RE: The How To's of designing & scratch building planes. (11/25/2003 5:24 AM)
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A good thread topic Dago. It's been done before but I don't think it's been done in such a way that folks can add small parts until the whole puzzle is in one piece.
Who knows, perhaps this'll be the start of a FAQ on designing.
OK, here's my first bit.
It all starts with an engine, or at least around here it does. From that engine you figure out how big a model you want to build. The size will suggest a maximum weight based on previous models and the expectations for flight performance will be based on that. Unfortunetly there is no way to calculate these first parameters because they are based on the performance of past models and the sizes and weights of those models. We select these first elements by example based on what we are familiar with.
From that basic wing size selection you move to airfoil selection. Not a biggie as we can pretty much say ClarkY, semi symetrical (I'm going to catch it for using THAT term :D ) or fully symetrical and have a fair idea of what these three categories will allow the model to fly like. The actual airfoil may be some high zoot item but it'll still fall into one of these basic families. At least for the most part they will.
Assuming a conventional model you need a tail. How big and how long a tail raises the topic of Horizontal Tail Volume Coefficient which ties in the wing chord and area and the length of the tail moment arm. There's also a Vertical Tail Volume Coefficient as well that factors in the fuselage length and side area, the side area of the wing if dihedraled and the tail moment arm again. Both these values have a "happy number" that goes with them. The optimum number may change slightly based on the airfoil and the type of model but for the most part the successful models of any specific type of model such as Pattern, Sport and Sailplanes all have the optimum values for these factors within a fairly narrow range at least for each category. What's the number you ask? It's been to many years since I saw a graph showing this data and I have no idea at present.
I'm going to stop now with a suggestion. The best way to learn more is to analyze as many models as you can find. Entering the span, area, nose length, tail length, stab area, stab chord, fin area, fin chord, fin span and a few other key elements for many models of differing types into a spreadsheet would allow us to calculate the optimum range of pretty much all these factors.
Nothing says a model has to be a certain shape but it DOES have to comply with the guiding factors that such a spreadsheet would show. After all Ugly Stiks and F22's both fly but they sure don't look like each other.
I remember those articles by Chuck Cunningham. They were very good and covered a lot. Does RCM offer them collected as a book?
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