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Foam Board SR-71 Twin 50mm EDF(?)

#1
I have loved the SR-71 Blackbird for as long as I can remember, and have wanted an RC version ever since I started flying. I drew up plans for an twin 80mm version a year ago, but never built is as I was a little to unsure it would fly to invest the time and money into it.

Lockheed_SR-71_Blackbird_2.jpg

However, recently Ben Harber designed and built a B-52 powered by eight homemade EDF units using a gremlin motor, 1.9" props and 3D printed motor mounts. Check out his forum post HERE for more details. After seeing this it got me thinking, would 2 of these be able to power a foam board SR-71?

I took a 3 view of the SR-71 and scaled it to fit Ben's Gremlin EDF's which left a plane roughly 3'-0" long with a wingspan of 1'-7 1/2". At this scale I though that if I built it "geometrically" it would not need any internal structure, and if fit on 2 sheets of foam board it may be possible. After a few hours of drafting I came up with a set of plans.

Preliminary Plans.jpg


Then a few hours of cutting and gluing later...

20181118_223725.jpg 20181118_223847.jpg 20181118_224001.jpg

The plane is surprisingly rigid at this scale. I was torn on weather or not to add rudders, but ultimately opted to make this a bank and yank flyer for now. If during test flights it appears rudders are needed I will add them. The nacelle nose cones are currently removable because I am waiting on motors and ESC's which should arrive tomorrow. I plan to do the initial test flights without them and see how the motors perform. If the cones end up being a permanent fixture during flight I will likely need to cut "cheater holes" in the nacelles to let in extra air (approximately 1 square inch in size). I still need to cut a battery hatch in the plane, but am going to wait till the motors are installed so I know where the battery needs to go and get it cut it in the correct spot.

I was able to do some glide testing with this version to find where the CG should be. It looks to be about 1/4" behind the front of the engine nacelles. An interesting thing I discovered during flight testing is that with this design it has a lifting body effect that causing the plane to stall like it has canards. With the proper CG set I was able to get this glide 30'-40' throwing it horizontally from the ground which has my hopes up that the motors will have enough thrust for it to fly.

All said and done the plane should weight 630 grams. In Ben's testing he said he got 130 grams of thrust per motor. I bought a higher kV version of the motor than he was using (mine are 6,000kV), and I am going to put 5 blade props on it instead of 4 blade. So assuming the motors don't melt I am hoping to get closer to 180 grams of thrust per motor. With that the plane won't have vertical clime capabilities, but will hopefully fly. I am planning to get everything installed and test fly the plane this Friday or Saturday (weather permitting).

20181118_223817.jpg
 

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mrjdstewart

Well-known member
#3
thoughts on using 4 of the 1106's (kinda like they did in the original giant A-10), use a quad PDB and quad esc's, you could double your thrust output. make it a real sr71. just a thought...

it would be more work and electronics but i think you could easily do it, and for the gain in thrust maybe worth it.

very cool no matter what.

me :cool:
 

PoorManRC

Well-known member
#5
I'm of COURSE on board with this Build!!! I know her better than any other Bird. I knew the Designer (well, as an 11 year old!). I'm proud to say that my Dad flew her, 1969-74!! :eek::cool::cool:

Kelly Designed her as a lifting body, but in reality, "she had the gliding capabilities of a ROCK."
~ quote from my Dad, Noel Widdifield and Buz Carpenter

She overcame any Wing Loading issues with sheer Brute FORCE!

You have the Tynes and waistline looking really good. Outstanding for DTFB!! (y)(y)
My humble opinion, I would leave her bank n yank. The twin flying Rudder mechanics would add quite a bit of weight...
 

SquirrelTail

Well-known member
#12
#14
thoughts on using 4 of the 1106's (kinda like they did in the original giant A-10), use a quad PDB and quad esc's, you could double your thrust output. make it a real sr71. just a thought...

it would be more work and electronics but i think you could easily do it, and for the gain in thrust maybe worth it.

very cool no matter what.

me :cool:
Putting 2 units in each nacelle was my Plan B. Unfortunately 1.9" prop options are somewhat limited in pitch availability.

I'm of COURSE on board with this Build!!! I know her better than any other Bird. I knew the Designer (well, as an 11 year old!). I'm proud to say that my Dad flew her, 1969-74!! :eek::cool::cool:

Kelly Designed her as a lifting body, but in reality, "she had the gliding capabilities of a ROCK."
~ quote from my Dad, Noel Widdifield and Buz Carpenter

She overcame any Wing Loading issues with sheer Brute FORCE!

You have the Tynes and waistline looking really good. Outstanding for DTFB!! (y)(y)
My humble opinion, I would leave her bank n yank. The twin flying Rudder mechanics would add quite a bit of weight...
Especially because if I did do rudders I would want to make them so the entire vertical stabilizer rotated like the actual plane. I do have a plan to do that if needed though.

Another? Just found this and an A-10, two more to add to the build list. By the way, plans?
I want to verify its flight worthiness before I post plans. I also have a few things that need adding/fixing before they are ready. I plan to post them with some build instructions in the hopefully near future assuming it flys.
 

PoorManRC

Well-known member
#15
Especially because if I did do rudders I would want to make them so the entire vertical stabilizer rotated like the actual plane. I do have a plan to do that if needed though.
That was really my whole point.... Getting both flying Tail Fins moving simultaneously is plaguing two others here, also trying to build her.
The Mechanism will intrude into the Nacelles.
 

kilroy07

Well-known member
#16
Awesome build!!

I've been mulling around (in my head) some of the challenges for a twin 65/70mm build... (I hope she'll fly on the 65s I have, but I'll canabilize to get the 70s if I have to.) :(

Were your 80mm plans DTFB?... (If so, I'd love to ask a few technical questions about your design...)
I have to get my entry for the fantasy aircraft build finished first (I honestly haven't done anything since September...):sleep:
But... that'll be the next project on my workbench!

If you'll like to join forces, I'm all in!!
 
#17
Awesome build!!

I've been mulling around (in my head) some of the challenges for a twin 65/70mm build... (I hope she'll fly on the 65s I have, but I'll canabilize to get the 70s if I have to.) :(

Were your 80mm plans DTFB?... (If so, I'd love to ask a few technical questions about your design...)
I have to get my entry for the fantasy aircraft build finished first (I honestly haven't done anything since September...):sleep:
But... that'll be the next project on my workbench!

If you'll like to join forces, I'm all in!!
It is designed to be built out of foamboard. I had a center fuselage spar with ribs along it every few inches for skinning it as that one was designed to be curvy. The wings were typical FT construction with a u spar. The landing gear is really the only thing I had not figured out before I stopped working on it. If you have any questions I would be happy to do the best I can to help.
 
#18
That was really my whole point.... Getting both flying Tail Fins moving simultaneously is plaguing two others here, also trying to build her.
The Mechanism will intrude into the Nacelles.
I agree, thought the mechanism for the stabalizer to rotate on would be easy to keep out of the way, there is just no good place to locate the servo without putting in in the nacelles as you mentioned, or having a very long control wire.

To rotate the stabalizers, since the bottom 1/8 or so of them on the real plane doesn't rotate, I had thought to use a bbq skewer or carbon fiber rod as a shaft that I would glue into the rudder piece, run through the bottom 1/8th piece, then put a small plastic disc or something inside the top of the nacell to keep it from coming off.
 

kilroy07

Well-known member
#19
Yea... it's a bugger. I even contemplated direct drive (embedding the servo horn into the rudder and burying the servo in the base...)
Rudders are #3 on my list of issues to "explore"...

For my fuselage I was thinking a simple box fuselage with ribs and covering with poster board. I hate poster board, but I want to keep the weight down as much as possible.

I think gear are just going to have to be custom fabricated. I haven't found anything pre-made that'll work. (I looked real hard for nose wheels, but most of them pivot...)
 

PoorManRC

Well-known member
#20
To either of you......
If you have the ambition to go Large Scale - you COULD just fit micro Servos in the root of each Rudder!
But, then you'll have the new problem..... POWERING a huge Bird with incredibly high Wing Loading!!! :eek:

Linear Servos may still be the best bet.