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Old Mar 17, 2016, 03:24 PM
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Build Log
SR-71 Blackbird with twin 120's

Hi everyone. I'm waiting on some parts to come back from the kit cutter so I've started playing around with an old idea I've had for the last few years. I've wanted to build a big SR-71 for a while but I've never really dug too deep into the idea.

I'll laid down a few rough lines based on twin Jetfan 120s and I came out with a wingspan of about 60'' ((1.5m) and a length of about 109'' (2.8m). I have no clue as to how much she will weigh but I'm guessing it could be around 30-35lbs.

The airframe will be molded fiberglass and carbon to keep things as light as possible.

I'll post some pictures as the drawing progresses...

I have a feeling I'm going to have to expend my current workshop...
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Old Mar 17, 2016, 03:29 PM
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My favorite plane of all time! Sub'd
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Old Mar 17, 2016, 03:49 PM
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Do it Right, the first time!
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I have a feeling I'm going to have to expend my current workshop...
Yes sir, that's a big one! You'll need a trailer to transport it.

Some where down the road I have a Y/A F-18 single to do , but it's small compared to yours-- only, Length 68" Span 47"
and that will take up a ton of room in my small shop area.

Good luck on the build-- I'll be watching your progress.
Cooler
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Old Mar 17, 2016, 04:01 PM
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Originally Posted by CoolerByTheLake View Post
Yes sir, that's a big one! You'll need a trailer to transport it.

Some where down the road I have a Y/A F-18 single to do , but it's small compared to yours-- only, Length 68" Span 47"
and that will take up a ton of room in my small shop area.

Good luck on the build-- I'll be watching your progress.
Cooler
Thanks Cooler... I did estimate out a version with twin 90's. I think it ended spanning around 40'' with a length of about 70''. Still a large model. I still have more figuring to do with this one. Either way, it's going to have to come apart for me to haul it around.

I remember your F-18 project. That's still a big jet!
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Old Mar 17, 2016, 05:48 PM
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Originally Posted by funkworks View Post
Hi everyone. I'm waiting on some parts to come back from the kit cutter so I've started playing around with an old idea I've had for the last few years. I've wanted to build a big SR-71 for a while but I've never really dug too deep into the idea.

I'll laid down a few rough lines based on twin Jetfan 120s and I came out with a wingspan of about 60'' ((1.5m) and a length of about 109'' (2.8m). I have no clue as to how much she will weigh but I'm guessing it could be around 30-35lbs.

The airframe will be molded fiberglass and carbon to keep things as light as possible.

I'll post some pictures as the drawing progresses...

I have a feeling I'm going to have to expend my current workshop...
Great project, the bigger the better they fly, if correctly set up. Always great challenge with these type of deltas. Took me a lot of CG corrections to get this monster going right, at 82x122 inch quite a handle on my own to load it in my van, but can manage. Get a big van!
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Old Mar 17, 2016, 06:10 PM
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Great project, the bigger the better they fly, if correctly set up. Always great challenge with these type of deltas. Took me a lot of CG corrections to get this monster going right, at 82x122 inch quite a handle on my own to load it in my van, but can manage. Get a big van!
Very nice! Great job! Yes. I was thinking about the fact it would fly much better if it was larger. I'm going to split the fuse along the bulkhead where the delta wing begins. I'm also going to have the outer wing panels removable just outside the nacelles. I have an SUV so it should fit if I do it that way.

I built a smaller version several years ago that had two .25 size OS glow engines where the spikes are. Flying it was like giving two cats a bubble bath at the same time. I learned a lot from that one.
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Old Mar 17, 2016, 09:53 PM
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Flying it was like giving two cats a bubble bath at the same time. I learned a lot from that one.

I love your description but don't quite get it? What was the issue with the model? I used to fly two twins "C-160s" with those '25 FS, they did purred like pussies. Cheers.
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Old Mar 17, 2016, 10:59 PM
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Flying it was like giving two cats a bubble bath at the same time. I learned a lot from that one.

I love your description but don't quite get it? What was the issue with the model? I used to fly two twins "C-160s" with those '25 FS, they did purred like pussies. Cheers.
Ha. Just meant that it was a handful to fly. The wingloading was terrible and very unstable in both roll and pitch. I did well to bring it back in one piece.
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Old Mar 18, 2016, 06:57 AM
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Funkworks
Whilst the SR71 is/was a most remarkable plane there is always the problem that like most high supersonic planes it has a relatively small area inlet which of course does not suit an EDF.

Just out of curiosity how will you address this problem on yours?
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Old Mar 18, 2016, 11:22 AM
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I always thot a clever way to do this would be with profile inlet cones mounted so that in side view you see the full cone shape/size, and most similar views would look OK.
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Old Mar 18, 2016, 12:29 PM
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Funkworks
Whilst the SR71 is/was a most remarkable plane there is always the problem that like most high supersonic planes it has a relatively small area inlet which of course does not suit an EDF.

Just out of curiosity how will you address this problem on yours?
That's actually been on my mind. I've seen some models where the cones were just deleted but I don't think I'll go that route. Right now, the plan is build the model large enough where I can distribute the fan sweep area around the spike. I will probably make a "scale" spike for static display and a "sport scale" spike that is smaller for flight. I'm actually working on this right now.

I may be able to get away with reducing the fan size to 90mm if I can keep the AUW down. This would allow more room for a larger spike.

Other options ive considered was to have a spike that has the lower half removed or having a spike that is made up of 4 profiles that all intersect along the center that would allow almost complete airflow when viewd head on but give the illusion of a solid structure when viewed at an angle.

ive included a screenshot of what I mean about the latter..
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Old Mar 18, 2016, 06:08 PM
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Originally Posted by funkworks View Post
Ha. Just meant that it was a handful to fly. The wingloading was terrible and very unstable in both roll and pitch. I did well to bring it back in one piece.
What was the size and weight of the model? Why do you think it was unstable in roll and pitch? BTW, was it a prop pusher?
You can always make number of grill cheater openings on intake nacelles, so the fans sucks as much as they need. That's easy fix.
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Old Mar 18, 2016, 06:22 PM
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Originally Posted by jofro View Post
What was the size and weight of the model? Why do you think it was unstable in roll and pitch? BTW, was it a prop pusher?
You can always make number of grill cheater openings on intake nacelles, so the fans sucks as much as they need. That's easy fix.
it was pretty small. maybe about a meter long and heavy. It was a long time ago so I don't remember the exact weight.

I bumped up the size of the model a little from 110 inches to 125 inches and modeled out an inlet based on the FSA of the JetFan 120. Assuming the FSA of the fan is 88sq cm an inlet diameter of about 15 cm allows me to have a fairly scale inlet.
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Old Mar 18, 2016, 08:18 PM
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funkworks
That looks pretty good but assuming the EDF exhaust tube is brought down to the 'normal' 85% FSA it is going to make the exhaust nozzle a good bit smaller than scale even when it was 'petalled down' to its minimum for take off and low speed flight.
http://www.skynet-aviation.com/image...hSpecs_550.jpg
Its the age old problem that for a true turbo jet, unlike a turbo fan, the area of the air in is significantly smaller than the exhaust out so something has to be compromised.
The unfortunate aspect of the SR71 is its exhaust nozzles, like its inlet spikes, are a very noticeable feature, hence my curiosity!
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Old Mar 19, 2016, 09:36 AM
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funkworks
That looks pretty good but assuming the EDF exhaust tube is brought down to the 'normal' 85% FSA it is going to make the exhaust nozzle a good bit smaller than scale even when it was 'petalled down' to its minimum for take off and low speed flight.
http://www.skynet-aviation.com/image...hSpecs_550.jpg
Its the age old problem that for a true turbo jet, unlike a turbo fan, the area of the air in is significantly smaller than the exhaust out so something has to be compromised.
The unfortunate aspect of the SR71 is its exhaust nozzles, like its inlet spikes, are a very noticeable feature, hence my curiosity!
I anticipated the same issue but I think I have a solution. I intend to keep the scale outline exhaust shape and still have the normal 85% FSA outlet inside. It will stop an inch or two short of the scale nozel and the space in between will house the LED afterburner ring.
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