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Old Aug 31, 2012, 06:38 PM
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This is a photo of the retract units and struts I was thinking of using. I actually weighed the tank and strut assemblies and the weight came out to 158 grams. Not very heavy at all. I think I am going to use them. I have some thin carbon fiber (1.5 mm) sheet I will use to make the mounting plates. That should be strong enough. Unless I change my mind again. The retracts are for models up to 3.5 pounds. I think I can make that weight.
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Old Sep 12, 2012, 04:28 PM
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All the templates finally done and fuselage blocks laminated. Time now to cut foam.
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Old Sep 12, 2012, 06:33 PM
ich bauen groß modell flugzeug
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im gonna Say the 4 contra rotator 3 bladers are for a Tu-95 Bear..! anyway Subbed..
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Old Sep 12, 2012, 06:52 PM
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First fuselage section cut. I will have to open and hollow it out later. The "Tiny C" is assisting tonight. Picture is kinda fuzzy though...
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Old Sep 19, 2012, 12:37 PM
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Wing now ready for covering. It has plywood fixed gear mounts and dual servo set up. Not the best core I have ever cut, but, spackle and sanding gets around that.

There was a book that I don't know if is still published called Modeling is Mostly Mounting. I disagree, at least in my case, as modeling is mostly sanding.
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Old Oct 08, 2012, 08:18 PM
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This is the progress so far. Estimated AUW is 1.8 pounds. Final wing loading will hopefully be around 12 to 14 oz per square foot. Power loading about 130 watts per pound. I think that is fairly respectable for a semi-scale foam model. I still have a fair amount of covering to do on the fuselage. I plan to use some pseudo-scale bronze/titanium colored film on the fuselage, but, may change my mind on that. Since there was only one prototype built and that Russia is one of the largest producers of titanium I feel like taking a bit of poetic license. Apparently the SR-71 aircraft were built of Soviet titanium. The purchase of which was undertaken by a "shell" company to hide that fact. I have been up close to a Soviet titanium plane at Monino outside Moscow (Sukhoi T-100) and the covering looks pretty close.
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Old Oct 08, 2012, 08:41 PM
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the CIA apparently did get some soviet titanium alloy for the SR-71, but most was made by US companies after they figured out how to reproduce the alloy (Ti-13V-11Cr-3Al). This is what most of the skin was made of. The engines used a different alloy for the first stage blades (8-1-1 I think). It probably had 6-4 alloy for bulkheads.
Titanium Metals Corp I know made it and probably the old Crucible metals too.
The alloy is hardly ever made anymore, replaced by slightly easier to make alloys.

there is a ukranian company called Unicraft that makes limited runs of resin model airplanes, mostly 1/72 scale. they have a model of this airplance, but not inexpensive

http://www.unicraft.biz/on/tu91/tu91.htm
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Old Nov 11, 2012, 02:02 PM
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Here are some photos of the completed plane. I think it came out OK, but, I see only the flaws. It also has some deviations from scale, so, I guess it is pseudo-scale. I left off the windows in the lower cockpit area figuring they would be a weak spot for ground fire (in the three views they look like air intakes of some type but they are windows). In my universe this would be replaced by a remote viewing camera or FLIR. I also left off the rear view periscope for the same reason. As such; this is more of a Tu-91 M0. I simulated (roughly) titanium skinning for the fuselage with light bronze Solar Kote. The IL-10 used a similar technique whereby the fuselage skin was actually armored plate protecting vital systems (like the engine and crew) from ground fire.

I use various adhesives in construction. There is some epoxy, Bob Smith cyanoacrylate, a fair amount of white Gorilla Glue polyurethane, some no name rubber cement /contact adhesive for foam, and a new product (to me) by Liquid Nails called Perfect Glue. I really like the "Perfect Glue". It is foam safe and remains fairly flexible after it cures. It has a consistency similar to silicone sealant when set. The surface of the foam should be moistened with water prior to joining. The water promotes curing, but, does not cause expansion like water does with polyurethane adhesive. The ingredient list for the Liquid Nails product reads like a foreign language to me. It has silane based chemistry from what I can tell which means it will glue just about anything; even to glass and metal. Isopropyl alcohol will remove the uncured adhesive fairly well. I would recommend gloves when using it.

Taxi testing in the street does confirm my fear that the wheels are likely too small for my flying site. I will go with some larger ones I have on hand. I will also have to change out the axles. No real big deal.

All up weight is 2 lbs 3oz with a 3s 1600 MaH battery. I could probably fly it with a 1300 MaH battery, but, flight times would be pretty short and I would have to have a higher "C" rated battery than the 20 C ones I have on hand. 20 amps out of a 26 amp rated battery just seems to be pushing it IMHO. The ESC's are 18/20 amp HK units and are over-rated for the job. They barely seem to get warm at all in testing. I may down size the props depending on what flight testing shows as I could then go with the smaller battery. Currently power loading is over 100 watts per pound which is more than adequate and also gives me some leeway.

Depending on how this flies I may build a larger one in the 1.8 to 1.9 meter range. I do have plans for a non-scale craft I came up with that would use two smaller counter rotating units. It is something like a miniaturized Tu-95 with cylindrical fuselage and a B-29/Tu-4 glazed nose. I also am working on plans for a Douglas Skyshark that would use one of the smaller AEO motor units with the three blade props. This would also be in the 1 to 1.3 meter wingspan range. At this point I will just have to think on it a bit and make up my mind. Either way I will need to order another motor unit.

Speaking of motors; the propellers on these counter rotating (CR) units must be balanced before use. I damaged the motor mount plate on one by running the supplied propellers without balancing. It settled into a harmonic vibration at certain throttle settings and oscillated badly. Once the props are balanced they work well and provide the power of a twin in a much smaller foot print. I tested the one in this plane on an old Kyosho Cessna model I had and it performed impressively. They also need to be hard mounted with as rigid a mount you can fashion. I used a modified Electrifly type outrunner mount on the Cessna (no cowling). It did not vibrate or oscillate. If you want a nearly turn key solution then that would be the way to go.

Covering the plane with film directly over the foam works OK. It does remind me that I really am not that great at using it. On boxy structures like trainers with few compound curves I can do pretty well. On this shaped foam I was constantly fighting with wrinkles. I either need more practice or I need to go back to my old way of doing it with Silkspan and polyurethane. I think the Silkspan method is more ding resistant.

In summary: the CR motors do work well and nullify torque or "P" force. They are not perfect and need the props to be meticulously balanced to avoid calamity. They also open up possibilities for some fairly unique aircraft. Granted; most of these scale subjects were not successful, but, they are really cool and unusual like the Wyvern or Skyshark. I really enjoyed building this one as it got me back to my favorite thing about modeling: building.
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Old Nov 11, 2012, 03:35 PM
Registerd Beaver
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Australia, WA, Warwick
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Well I reckon it looks fantastic Dr Joy and I'm looking forward to the maiden report.

- Phil
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Old Nov 12, 2012, 09:45 AM
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Thank you Phil. I hope to get it out this week and fly it. I am still going to put larger wheels on it prior to that. Other than that there is nothing else to do to it at this point.

My next build should be a little nicer as it has taken some effort to get back into building my own. I don't think I would build anything to museum quality standards as I do not possess the skills needed. For me; I just wanted to build something you do not see everyday. Three months from the decision to completion isn't too bad. Granted it is not fancy, but, I learned a few things along the way. I learned (again) that I have trouble with balsa dust. I had originally planned to carve the cowling from balsa blocks, but, got such eye irritation even with goggles and respirator mask on that I had to abandon that and use foam,

For the next project I had thought of mildly bashing a Spitfire kit into one of the later variants with contra-rotating props. I have a GWS spitfire kit that I put together from spare parts and I have what appears to be an appropriate motor. At any rate I have too many ideas and I need to make up my mind. I am also having to brush up on my flying skills as I have flown very little over the past year due to family issues. (It is also easy to stay on the computer talking about models instead of actually flying them and building them.) So I have the Slow Stick and P-47 out and plan on taking them to fly this afternoon. I will likely get a friend of mine to maiden the Tu-91 as he is a much better pilot and is out flying just about every day. He has some 30 years or more time on the sticks. I trust him much more than I trust myself. Anyway; enough rambling. Time to change out the wheels.
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Old Nov 12, 2012, 07:08 PM
This is a fine fiddly business
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United States, WA, Marysville
Joined May 2009
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Dr Joy,
Very nice result and no one will ever say, "Ho hum, it's just another Tu 91."
Best,
Robert
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Old Nov 13, 2012, 07:03 AM
Registerd Beaver
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Australia, WA, Warwick
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Yeah I bet it will be just another Tu-91 on the flightline with all the F4U's and P-51's and P-47's and A6M2's, Me109's, Fw190's and Spitfires..bla bla bla....

- Phil
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Old Nov 14, 2012, 12:37 PM
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Thank you for the kind words. I took it out today and taxied it up and down my street. It took off and flew a short distance (in ground effect) before I ran out of room. So; I know it will at least fly. The "flight" today was on an 2S battery. I put a 3S battery in it and the and the acceleration was impressive.The nice thing about it is that you need only input minimal amounts of rudder to keep it straight on the take off roll.

My test pilot is going to be available on Saturday so I will know then what it's performance truly is.I have done a fair job of keeping the weight down, but, I think I could do better. On the next one I will go for thinner fuselage walls. I just left too much foam there.

The wheels will be switched today. Hopefully that will work out. They will be a fair departure from the scale look, but, I don't really care about that as long as it flies well.

For my next project I may build the world's ugliest jet ever proposed, a skyshark or something equally as bizarre.

Scott
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