Simon Chaddock Posted February 6, 2014 Share Posted February 6, 2014 'Stick and tissue' is the staple of most vintage planes as used by many Keil Kraft kits like this one in their scale rubber series, the 20" span Auster Arrow. Built almost entirely from 1/16th balsa it was simple, light and when built as per the instructions it flew quite well. In my bigger Depron planes I have used balsa and Depron as a composite in structures like the main spar. This is a lightweight 'test' spar 6x2mm with 0.8mm thick balsa flanges. 1/3 the weight of a solid balsa equivalent and just as stiff and nearly as strong. The same principle applied to the Auster wing. It is 8x1.5mm. The ribs are 2mm Depron. 1/3 Thicker than 1/16 balsa still only half the weight. The completed RH wing. The leading and trailing edge as well as the wing tip are Depron although the trailing edge is backed by a 2x0.8 balsa strip. The leading edge has extra support from a lattice of 2mm square Depron struts. So a conventional stick and tissue wing but with most of its sticks in Depron. Uncovered it weighs just 1.1g. So the question is do I try the same sort of thing on the rest of the structure? or is it just a waste of time? Edited By Simon Chaddock on 06/02/2014 12:50:50 Quote Link to comment Share on other sites More sharing options...
Devcon1 Posted February 6, 2014 Share Posted February 6, 2014 Hi Simon, Stick with it, I like your sandwich method for the spar I may incorporate that in my current build. John Quote Link to comment Share on other sites More sharing options...
Simon Chaddock Posted February 7, 2014 Author Share Posted February 7, 2014 The fuselage needed more thought. It has to take the significant compression loads from the rubber motor as well as being rigid enough to resist the pull of the tissue. This suggested that at the very least the fuselage longerons would need some balsa. The final method selected was to build the fuselage side as per the plan in 2mm sq Depron but with the inside of the main longerons reinforced with strips of 2x0.8mm balsa. The resulting extra width of the fuselage side does mean that some of the cross pieces have to be slightly shorter. The fuselage thus continues the wing philosophy of only using balsa where it is required. The completed bare fuselage. The only other place using balsa are the 1mm square balsa stringers at the nose. Not only to support the Depron nose former but their extra weight helps to offset that of the long rubber motor. Covered with lightweight tissue and with cockpit glazing added it weighs 3.1g. It is rather more delicate to handle than the all balsa original but it does look like it could end up just half the weight! Undercarriage next. Edited By Simon Chaddock on 07/02/2014 14:54:49 Quote Link to comment Share on other sites More sharing options...
Stephen Jones Posted February 7, 2014 Share Posted February 7, 2014 Simon , I like you're thinking , I still have that plan as i also built one and it flew well , I also thought on the same lines as you , but i read some where that a modeler had done a comparison in weight and strength , and found that the thinner the depron the less of an weight to strength advantage their is . And i have also thought of building two identical models one in balsa and the other in depron so i will be interested in how you go . Cheers Steve . Quote Link to comment Share on other sites More sharing options...
Monz Posted February 7, 2014 Share Posted February 7, 2014 Very interesting Simon. What did you use to attach the tissue? I'm going to try this method for some indoor scale RC conversions of some of the classic rubber jobs. Quote Link to comment Share on other sites More sharing options...
Simon Chaddock Posted February 8, 2014 Author Share Posted February 8, 2014 Stephen I do actually have a kit built Arrow, very old (about 35 years!) purchased just before KK closed down. Complete but without the rubber (long since perished!) it weighs 27.8g. Depron is about 1/5 the weight of normal strip balsa. 2mm square has 1.5 times the cross section area of 1/16 square so despite this it is still well under half the weight per unit length. By substituting the Depron strip in the areas where the full strength of the balsa is not required I hope to show a significant weight saving without sacrificing much strength or rigidity. It should certainly be stronger than if the same weight reduction was achieved by simply using smaller section balsa. In some respects this is a 'construction' exercise for my own curiosity more than a serious attempt to produce something that flies that much better. Quote Link to comment Share on other sites More sharing options...
Simon Chaddock Posted February 8, 2014 Author Share Posted February 8, 2014 A wire undercarriage added with Depron tyres with plastic centres. The undercarriage is mounted in plastic side plates. At 1.1g the undercarriage is relatively heavy adding 30% to the weight of the fuselage. The wing covered It weighs 1.7g Tail feathers next. Quote Link to comment Share on other sites More sharing options...
Monz Posted February 8, 2014 Share Posted February 8, 2014 What glue are you using to attach the tissue to the depron Simon? Quote Link to comment Share on other sites More sharing options...
Simon Chaddock Posted February 9, 2014 Author Share Posted February 9, 2014 Monz All the Depron and balsa are glued with UHU POR. The tissue is stuck on dry using a rather 'thin' PVA paper glue from ASDA and when dry it is water sprayed to shrink it. Finally it is sprayed with firm hold hair spay to 'set' the tissue. Hair spay is not really water proof so it will have to be a dry weather only flyer! Quote Link to comment Share on other sites More sharing options...
Simon Chaddock Posted February 10, 2014 Author Share Posted February 10, 2014 The tail plane under construction. Only its centre spar (2x0.8mm) is balsa all the rest is 2mm Depron. To stand a reasonable chance of not warping the edges are 4x2mm and is has additional 2x2mm cross bracing. As a result they are only fractionally lighter than the all 1/16sq balsa original. The fin construction is similar. The tail feathers added. Like this and all 'hair sprayed' it now weighs 5.1g. The LH wing Slightly different construction. The trailing edge is not balsa backed but is wider and notched to accept the ribs. The spar shear webs go right to the tip but are only 1mm rather than 1.5mm thick. At 1.1g before covering it is exactly the same weight as the RH wing. Time to mount the wings with their load bearing (but not a lot!) wing struts. Edited By Simon Chaddock on 10/02/2014 00:03:41 Quote Link to comment Share on other sites More sharing options...
Simon Chaddock Posted February 10, 2014 Author Share Posted February 10, 2014 Wings added with Depron/balsa/Depron struts. Now structurally complete less the prop and nose plug it weighs 9.7g The kit built all balsa one in this state weighs 23g although it has been painted so it is reasonable to claim the Depron/balsa version is about half the weight. It is indeed a bit more delicate to handle but then with such a significant weight reduction this is to be expected. With a Depron construction the wings are probably the most successful. Adequately strong and stiff yet using a tiny amount of balsa. Despite having significantly more balsa in the fuselage to handle the compressive loads from the rubber motor the delicacy of the structure to handling is more obvious. The tailplane and fin like the wings use very little balsa but need rather more Depron to resist warping so the overall weight reduction is still there but less significant. So at these small sizes (and even more so at say peanut scale) Depron would seems to provide a practical way to save a degree of weight that would be difficult to achieve by simply reducing the size and density of the balsa used. Not really a major issue but cost of the Depron used is insignificant compared to the cost of balsa! Unfortunately it is likely to be several months before the weather improves to the point where it can be tried out doors! Edited By Simon Chaddock on 10/02/2014 17:49:02 Quote Link to comment Share on other sites More sharing options...
Monz Posted February 10, 2014 Share Posted February 10, 2014 This is really interesting Simon. I come from a free flight background so am definitely going to try this method. The weight saving is going to give you a really scale flight profile. Any test glides yet? Quote Link to comment Share on other sites More sharing options...
Simon Chaddock Posted February 10, 2014 Author Share Posted February 10, 2014 Monz Only indoors just to show that the original KK plastic prop was really too heavy. It now has a lighter one from a toy foam flyer! Hopefully I will be able to use very thin rubber as it won't take much power to fly. Quote Link to comment Share on other sites More sharing options...
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