Former Member Posted February 28, 2018 Author Share Posted February 28, 2018 [This posting has been removed] Quote Link to comment Share on other sites More sharing options...
Nigel R Posted February 28, 2018 Share Posted February 28, 2018 To learn about something, usually you'd want several styles of reference. A "how to". Representing the design patterns and lessons of experience. And a "dictionary". Representing the theory and how to figure out how to make a new design pattern with a good chance of success. Pete's article referenced above comes under "how to". A good place for a first time designer to start. The kind of exploration behind the patterns, as driven here by David, sits in the second. Not immediately necessary if you want to run out a conventional design and have success in doing so, before moving on to more complicated stuff. You need both, really. Quote Link to comment Share on other sites More sharing options...
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Former Member Posted February 28, 2018 Author Share Posted February 28, 2018 [This posting has been removed] Quote Link to comment Share on other sites More sharing options...
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kc Posted February 28, 2018 Share Posted February 28, 2018 Yesterday Nigel said he had " sport/trainer I've just finished at 7 oz/cu ft. " ( probably his RM Trainer?) and I wonder how he arrived at that figure. How do you actually arrive at the real cubic feet for a sports model with curved and tapered surfaces? Surely we need an accurate cubic measurement to make any sense of the calculations? Wing area is much easier to estimate! For example how would one calculate the cube for a WOT4 balsa 52inch? ( to have an example that is more familiar to most of us than an RM Trainer ) ( it would be nice if we could write " oz per CUBIC ft " just to avoid confusion with the usual oz/sq ft figure. And lets keep good old ounces and feet rather than metric! Its so nice to have measurements that mean something ) Quote Link to comment Share on other sites More sharing options...
Martin Harris - Moderator Posted February 28, 2018 Share Posted February 28, 2018 This has been an intriguing thread which starts to explain some of the mysteries of scaling models and why conventional wing loadings increase so much at full size. I'm currently building a (1:6.5 scale) MkV Tempest which was designed for competition back in the days when a 5kg limit applied. I read somewhere that the designer built a MkII version (essentially the same airframe but with a radial engine) which was significantly heavier as the limit had been raised. Apparently it flew "much better"... (Just found the reference): I have been in contact with Ian Bailey and he gave me some good advice and tips to improve the model. My drawings were made 15 years ago.. When the model was designed the FAI weight limit was 5kg and IB used an OS 120FS in the prototype. His later models were fitted with Super Tigre 3000 (30ccm), weighed 7kg and flew much better!! The basic modification Ian made was to add full depth 3mm ply dihedral braces on the leading edge and main wing spar joints and also to the wing centre panel to spread the landing loads. These are not shown on my drawings but will be made. Ian actually test flew a Tempest built by a customer who had a Quadra 42 petrol fitted with an all up weight of 10kg. Ian said that it flew better than his at 7kg!! Although I've been careful with wood selection for the tail components etc, I'm guessing that mine will come out at around the 7kg mark or maybe a little more depending on the amount of scale detailing I end up doing. It might be interesting to calculate the wing cubic loading to see if the theory supports this example. Edited By Martin Harris on 28/02/2018 13:39:23 Quote Link to comment Share on other sites More sharing options...
Former Member Posted February 28, 2018 Author Share Posted February 28, 2018 [This posting has been removed] Quote Link to comment Share on other sites More sharing options...
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Former Member Posted February 28, 2018 Author Share Posted February 28, 2018 [This posting has been removed] Quote Link to comment Share on other sites More sharing options...
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Former Member Posted February 28, 2018 Author Share Posted February 28, 2018 [This posting has been removed] Quote Link to comment Share on other sites More sharing options...
Nigel R Posted February 28, 2018 Share Posted February 28, 2018 kc, its gone through in detail for another model by David just above, but in brief: The RM Trainer (as you correctly guessed) has a wing 52" x 9 1/2" - fairly typical proportions - giving us 494 sq in. To go from sq in to cu in, I can do the long route (which may make more sense at this point?): Square root of 494 sq in, is 22.226 (in) So I now have a "length" measurement, which represents the wing as a whole. To get our 'cubic' figure I do... 22.226 x 22.226 x 22.226 = 10979 (cu in) But we need cu ft to bring things into a sensible range. A cubic foot is 12 x 12 x 12 = 1728. I can now use that to convert cu in to cu ft: 10979 / 1728 = 6.354 cu ft The total weight of the model is 2lb 12oz (all told), or in "just oz" - 44 oz. Therefore, oz per cu ft (OPCF in David's postings), is: 44 / 6.354 = 6.9 oz/cu ft. 7 oz/cu ft (or thereabouts) generally means quite slow and floaty. A nice trainer type wing loading, in other words. The Wot 4 XL comes in at 8 oz/cu ft, so I would imagine that it flies in a very similar manner (albeit a bit more agile, with better power/weight, and slightly less affected by gusty conditions). Quote Link to comment Share on other sites More sharing options...
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Djay Posted February 28, 2018 Share Posted February 28, 2018 Hi Supertigerfan. 3 part series by Peter Miller starting Feb 2009. Darryl Quote Link to comment Share on other sites More sharing options...
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kc Posted February 28, 2018 Share Posted February 28, 2018 The justification for asking daft questions is that it brings out the real explanation and gets down to the real nitty gritty! Thanks to David and also Nigel for their patient explanation. it seems to me that the term cubic feet is misleading in this context if it doesn't represent the actual cu ft of the model plane. Perhaps there could be a different term? Quote Link to comment Share on other sites More sharing options...
Martin Harris - Moderator Posted February 28, 2018 Share Posted February 28, 2018 Thanks David. I'd just sat down to try to work out the figures, went to check some details for the calculations and you'd done it for me! Very interesting that your predictions appear to have worked very well in retrospect for this example. Must get back to the undercarriage doors now - I shan't need to worry too much about their weight! Quote Link to comment Share on other sites More sharing options...
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kc Posted February 28, 2018 Share Posted February 28, 2018 I am quite happy to use standard terms to describe things but I am very unhappy to use incorrect terms. To say cubic feet when we don't mean actual cubic feet seems very unscientific! For example Nigel said his RM trainer was 6.354 cu feet and I am very sure my old RM was ( is!) nothing like that in actual volume. I guess 2 cu feet or less. ( It's a bit cold to go out into the garage to check tonight) I understand that your theory works well but to describe the result in incorrect terms seems just plain misleading and confusing! Cubic wing loading seems nearerr, some term llike feet super would seem better Edited By kc on 28/02/2018 18:10:18 Quote Link to comment Share on other sites More sharing options...
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