1:12 Scale SR71 - RC Groups
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Jul 19, 2016, 04:44 PM
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Ianda217's Avatar

1:12 Scale SR71


Considered comments and criticisms invited on my proposed SR71 build.

I have developed plans for a 1/12 scale SR71 to be built fairly conventionally from balsa and lite ply. The model is to be powered by two Wemotec midi pro 90mm fans with alternate motor/battery combinations and to have Turnigy electric retracts. The fuselage will split in front of the wings and the outer wings and engine pods will also be removable so that the model will break down into four sections for transport. Even so, the wing sections will still be nearly 4' x 2' each. I have prepared a parts plan so that all the formers, ribs, etc can be laser cut to make life easier and for a more accurate model.

The model is as accurate as I have been able to establish. There are two areas that are not quite up to scratch. 1) I am not happy with the side profile of the nose and cockpit and might modify this and 2) I have not incorporated the drooping leading edge to the wings.

I have done a budget cost and am looking at around £1,200 plus paint to complete. I have also estimated the total weight at around 15 lbs. A copy of the budget and weight calculation is attached.

The plans can be downloaded in both .dwg and .pdf format from:


Be warned - the plans are enormous. The model is drawn on 2A0 sheets (1680 x 1190mm) although they can actually be printed off on any size paper. The parts plans are drawn onto 900 x 300 sheets to suit laser cutting in the UK.

I have run the model through Motocalc and the prediction from that is pretty good. The anticipated performance will not satisfy the speed freaks but it is looking at around 75 mph which scales out to over mach 1 and is plenty for me.
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Jul 19, 2016, 10:38 PM
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Pretty nice of ya to post plans as you have. Best of luck with the build. Subscribed.

Jul 20, 2016, 11:47 AM
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Ianda217's Avatar

I have to make the plans available so that I can be told where I've gone badly wrong.

Having said that, the plans are up for grabs if anyone wants to use them.
Jul 21, 2016, 01:01 PM
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union1's Avatar
Have you thought about having the SR made from milled Styrofoam?
It might be worthwhile contacting Jim Gordon of Jim's jets for a quote. He does some brilliant 3D cad work and is used to designing large edf's.
Easy chap to work with too!
Jul 21, 2016, 02:09 PM
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No, I had never given it a thought. Now that the thought has been planted in my mind, I would have to say no, mainly because I am a very old fashioned modeller and I enjoy the building much more than the flying. Balsa for me! (Even though I now cheat by getting my parts laser cut whenever possible)
Jul 23, 2016, 03:52 AM
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union1's Avatar
Nothing wrong with good old fashioned balsa bashing!
It was just a thought because I know he was working on one a while ago. I don't know if it's still on the drawing board (pun not intended) but as it's a favourite of mine I'll be keeping both eyes wide open for developments, balsa or foam.
Keep up the good work!
Mar 15, 2017, 08:25 AM
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It's been a long time since I posted on this topic. I have been trying to sort out (mainly) the viability of the power supply and ductwork before I committed myself too far.

With the help of others on the forum, I have now established that the Wemotec fans are a practical solution for this model. I have the weight as closely as I can at around 7.5 kg and two Wemotec midi evo fans with HET 650-68-2000 motors will provide almost 6.5 kg of thrust. The only downside is that I cannot fit a solid inlet spike as this reduces the total thrust to 5 Kg so I'm planning to go ahead using a cruciform profile spike for flying. I will build a dummy solid spike to slip over this for static display. I'm having a bit of trouble with my ESC's overheating but I've got some possible answers for that in the pipeline.

During my absence, I have been modifying and tweaking the plans to make sure that everything fits without any interference. Hopefully, I've got all that right.

I have now purchased virtually everything I need for the build except for the wood and servos but the laser cutting and balsa has been ordered although it might be four weeks before I get them delivered. I've even found a piece of worktop 7' 0" long by 3' 0" wide to build on.
Mar 17, 2017, 01:10 AM
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Big Thanks for you Landa....its a Great Plans....Awesome!!!

Btw.... dwg format not available in the link...please reupload....thanks
Mar 17, 2017, 06:56 AM
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I pulled the .dwg files off the web because they have been much modified (mostly in small ways). Before I put them back on, I would like to make sure that the model is actually buildable. I certainly hope it is because I have now ordered almost all parts and materials for the model so I'm pretty much committed. Watch this thread.
Mar 21, 2017, 05:38 AM
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I’ve had lots of goodies arrive from Hobbyking last week – retracts, oleos, wheels, a couple of slim wing servos, mini power UBEC, and servo extension wires. So, while I wait for the laser cutting to be done, I can get started on sorting out some bits and pieces.

The first thing I needed to do was to modify the oleo legs. I don’t know if it is just me but Hobbyking seem to constantly change their range of items. Looking at their website today, there seem to be a lot of retract units that weren’t there before. The set that I bought and had delivered are no longer listed although they have similar units available. Anyway, my retracts came with 5mm pins as photographed. I needed 120mm long oleo legs for the main gear and a 130mm long leg for the nose gear. Hobbyking did not have oleo legs to match so I had to buy oleo legs with solid 11mm pins and hope that I could modify them to suit the retracts. Fortunately, I could. The before and after photograph below shows the alterations.

The oleo leg is a simple tube with a spring inside it retained at the top by a solid, interference fit alloy plug. I was able to drill the plug for the retract mounting pin, drive it deeper into the leg (thus increasing the pre-load on the spring slightly) and cut off the solid 11mm section at the top. I then drilled and tapped the top of the leg for two m3 grub screws to fit the leg onto the retract pin.

Retracts and wheels are now all ready to install.
Mar 22, 2017, 10:02 AM
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The laser cutting for the model does not include the 1/8” and ¼” birch ply parts. These need to be cut by hand. Not literally, of course – they need to be cut by hand on a bandsaw and disk sander.
That has been today’s job. I now have the three ¼” ply retract mounting plates cut out and fitted with captive nuts, the ¼” ply fan mounting plates cut and fitted with nuts and the 1/8” ply power board cut out.

I have mounted the mini power UBEC, receiver, lipo flight battery and retract door sequencer to the power board, as well as a couple of multi connectors for the retracts and door servos. This board will be fitted upright in the model via a hatch on the underside and the ESCs will be fitted to the underside of the board.
Mar 22, 2017, 10:51 AM
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turbonut's Avatar
Nice work so far...So no scale triple main gear for now?
Mar 22, 2017, 12:28 PM
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Ianda217's Avatar

When you consider the fussing I have done over the scale outline, you would think everything would be spot on but I'm afraid not. I have no engineering facilities (or skills) so it's off the shelf purchases for me. Twin wheels is as close as it gets.

Mar 22, 2017, 04:27 PM
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turbonut's Avatar
I get it..I wouldnt do scale gear until after its totally sorted out..then maybe never as most wont ever call you on it
Mar 23, 2017, 06:17 AM
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I don’t have a building board big enough for this model (who does) but I had a good sized piece of 900mm wide laminated worktop in the wood store. I cut this down to 1550mm long which is enough to build the rear and front sections separately and just long enough to get both sections on and lined up to ensure that the fuselage formers at the join are perfectly aligned.

Two layers of self-adhesive cork tiles and it’s all ready to go.

And now it’s time to really get going. My laser cut parts and all the balsa has arrived. Time to get the marker pen out.


I have jut opened the parcel for a quick look and the laser cutter has cut the old plan I sent them a year ago (to get a quotation) and not the new plan I sent them last week. Lots of pieces have been altered and added and it looks like I'm going to have to get a large part of it recut.

As Rabbie Burns said "The best laid schemes of mice and men gang oft agley"!

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