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Thread OP
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Discussion
48" Stick-and-Tissue-Blenheim IV/Bolingbroke IV - It Flies!!!!
Okay, okay, I'll admit right off the bat that the chances of my completing not one, but TWO build-off entries by the time the contest closes are close to nil, but I figured a little external motivation is probably a good thing. While waiting for my Lancaster parts to be laser cut, I've started sketching out a CAD design for the Bristol Blenheim Mk IV/Bolingbroke Mk IVT.
The Blenheim was one of the obsolescent twin-engined bomber designs that equipped the RAF at the outbreak of WWII, and for a space of two years was one of the very few means by which Britain could strike at Germany. A breathtakingly advanced design when the prototype was rolled out in 1935 (it could outrun all of the first-line fighters of the time), by 1940 it was almost hopelessly outclassed by fighters such as the Bf 109 and 110, and they exacted a terrible toll on attacking Blenheims during the battles for France, Norway, and then throughout '40 and '41 as the RAF tried to keep British spirits up during the Blitz by attacking Germany itself. The bravery of Blenheim crews, who sometimes lost 11 of 12 aircraft participating in a single bombing mission, is almost unimaginable today. Blenheim production was also undertaken in Canada to meet an RCAF requirement for a maritime reconnaissance aircraft, and later to to provide modernish twin-engined aircraft for the British Commonwealth Air Training Plan. Canadian-built Blenheims were designated the Bolingbroke, or "Bolly" for short. I've been inspired by Jeff's (jhspring) interesting conversion of a Balsacraft Blenheim kit to Bolingbroke configuration here: https://www.rcgroups.com/forums/show....php?t=1093606 and I thought I would try a lightweight stick-and-tissue design along the lines of Sky's (glue_sniffer) beautiful Beech D-18 here: https://www.rcgroups.com/forums/showthread.php?t=480576 While I've started tracing the outlines of some three views, I have yet to design any structure, and am a little torn over how big to make it. I'd like to keep wing loading to the 8-10 oz/sq ft range for scale-like performance, but don't want a model much bigger than around 48" span. 1/16 scale yields a tidy 42.5" span model that could easily be powered by a couple of Blue Wonder 1300 kv outrunners I have lying around, but doesn't leave a lot of wing area to carry the inevitable weight. 45" span is 1/15 scale, and could still be powered by the Blue Wonders, and 48" span gives 1/14 scale. I'm leaning toward the 45" size model, but could still opt for 48" if weight calculations force it. Any suggestions? Steve |
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Last edited by Steve85; Oct 31, 2023 at 05:28 PM.
Reason: Build underway!
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Thread OP
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Ha! As much as I'd like to contribute to your build, Jeff, 56" is going to be just a little too big for my shop.
  I've run some numbers and if I want to target 8-10 ozs/sq ft wing loading, the different sizes provide the following target weights: 42.5" span 15-19 ozs 45" span 16.5-21 ozs 48" span 19-24 ozs Looking at glue_sniffer's Beech D-18 at 45" span, he managed to include retracts at an AUW of 20-21 ozs, which works out to around 10 oz/sq ft, so I think the smaller 42.5" option is pretty much ruled out. At 45", I think I may have to work a little too hard to achieve the wing loading I want, so 48" is looking like the right size. At 48", scale props are 7.5" in diameter, and there's plenty of room for 8" ones. The Blue Wonders will pull 65W each on a GWS 8x4 prop, giving 130 W total, or between 85-110 watts per pound for the target weight range, which will be more than enough for scale-like performance. A quick look at Keith Sparks' Parkflyer Plastics site shows he has 3 3/4" cowlings and 9 cylinder radial engines that are close enough to scale dimensions to be used without any qualms. I'll have to vacuum form the distinctive nose glazing and mid-upper turret, but finding ready-made cowlings and engines is a real bonus. Okay, so 48" it is going to be. Time to scale the drawings and start sketching in structure.  Steve |
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There is a full sized under restoration.
Taken from the link here in Hamilton: http://www.warplane.com/vintage-airc...x?aircraftId=7 Bristol Bolingbroke Mk. IVT Click on one of the thumbnails above to access the 22 photos of this aircraft. In 1937, the Bristol Aeroplane Co. was developing the Bolingbroke as an improved version of its Blenheim Mk. I, light bomber. As the British Air Ministry was more interested in speeding up the supply of Blenheims to the RAF than with developing an improved type, development of the Bolingbroke was handed over to the RCAF, who urgently needed a modern reconnaissance bomber. The first Bolingbroke flew from Filton, England, in September 1937 and was then shipped to Canada for further testing. Fairchild Aircraft, of Longueuil, Quebec was licenced to manufacture the Bolingbroke, in November 1937 and the first Bolingbroke Mk. I flew in September 1939. An early Bolingbroke was fitted with North American instruments and was designated the Bolingbroke Mk. II. The Bolingbroke Mk. III was a seaplane version, which according to test reports had excellent flying characteristics. However, although 24 seaplanes were ordered, only one was ever built. The most numerous Bolingbroke was the Mk. IV, which had the basic British airframe fitted with Bristol Mercury XV engines, together with North American instruments and equipment. The Mk. IVT was the trainer version. During WW II, Bolingbrokes initially equipped eight RCAF Coastal Command squadrons on anti submarine patrols, off both the Atlantic and Pacific coasts. However, most Bolingbrokes were used as trainers with the bombing and gunnery schools of the BCATP. Bolingbrokes were phased out of RCAF service at the end of the war in 1945. A total of 626 Bristol Bolingbrokes were manufactured in Canada between 1939 and 1943. Another 1,980 Blenheim Mk. IVs similar to the Bolingbroke, were built in the UK for the RAF, during the same time period. The Museum's Bolingbroke Mk IVT is currently being rebuilt from the remains of eight aircraft, salvaged from Manitoba, in the mid 1980s. When the long restoration process is complete, the aircraft will be painted in the colours and markings of RCAF No. 119 City of Hamilton. STATUS: On display AIRWORTHINESS: Under restoration to flying condition TYPE: Bomber SERIAL NUMBER: RCAF 10117 CIVIL REGISTRATION: C-GBLY CURRENT MARKINGS: RCAF 714 |
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Hi Steve – great choice – look forward to following this one!
Charles |
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Hi Steve!
Gotta sub in for the thread!  Really nice size.Here's a link to my links I posted earlier so it's all in one place: https://www.rcgroups.com/forums/show...6&postcount=68 raipe |
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Steve, I don't know how over-the-top you are planning to go with this one, but a really nice guy from Scarborough made me a CD with a whole lot of detail photos and such on it. If you would find it useful, I could burn you a duplicate and send it north with my brother when he visits next month. Just let me know, Jeff
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). PM on the way!|
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Thread OP
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I'm enjoying having both an active build on my table (the Lancaster) and this Blenheim design effort underway at the same time. When I get tired of one, I can switch to the other and recharge. It also allows me to capture needed improvements discovered while building the Lanc and reflect them here in the Blenheim. In the Lanc, I used 1/16" balsa for some of the wing ribs, and have realized now in hindsight that 3/32" would have been a better choice. I'll make the changes to the Lanc plan and cut file, but I can incorporate these changes immediately in the Blenheim.
Steve |
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Last edited by Steve85; Mar 02, 2014 at 12:36 AM.
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Thread OP
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A little more work on the Blenheim wing structure. I'll be using the same Clark Y airfoil, laminated wing tips, carbon-fibre reinforced centre leading edge, I-beam spar and bass trailing edge used on my Lancaster. Wing panels outboard of the nacelles will be removable for ease of transport.
Steve |
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Thread OP
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Well it's over seven months since I last posted on this thread, and lots of water has run under the bridge since then. Scale Build-Off 3 is a distant memory, although there seems to be encouraging interest in a Build-Off 4. Personally, I've had to take several months off from building while dealing with some family medical issues, but these appear now to be largely resolved. Happily, while I couldn't build (and the current state of my Lancaster bears this out) I could still design, so I've been pecking away at my Blenheim when not looking after family. Earlier this summer I thought I had lost interest in the Blenheim design, and conducted a little poll to help me decide what to build this winter. I think ultimately I will design and build a FW-189, but Charles (Spitfire1954) was interested in prototyping the Blenheim, so I've focussed on finishing this design in view of Charles' build later this winter.
The Blenheim design is almost complete, with a cutfile done and the final instructions being annotated on the plan sheet. There remains little to do on the plan at this point, although I'm entertaining a little fantasy of drawing up some 3D views of various assemblies like the nacelle structure in Google SketchUp, but time will tell if I can coax that so far enigmatic piece of software to do my bidding... Steve |
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Thread OP
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While not busy cranking up the Build-Off, I've been experimenting with SketchUp 7 to see if I can produce 3D isometric views of some of the major assemblies for the Blenheim. SketchUp is touted as an intuitive 3D modelling program, and I've found that claim to be true, to an extent. Thankfully there are lots of helpful video tutorials on the net, but perhaps the hardest aspect of learning to get something out of it has been learning to understand how 3D images are represented and manipulated on a 2D computer screen. My brain is still grappling with that interface, and so while I've been able to cobble together a reasonable model of one of my nacelle formers and the motor bearers, I can't seem to get it rotated to face the direction I want. Yet.
Thanks to Geoff Sim (geoff.sim) for helping me get started, explaining about plugins and trying to import one of my formers. Steve |
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