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Jun 08, 2019, 09:52 AM
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Scratch build Sr-71


Hey everyone, I am building a sr-71 from scratch. I'm still in the planning stage and going to need some tips on building a plane like this, don't know what airfoil I should use. never done a scratch build but have done balsa kits.

I am thinking about doing a method similar to what Ramy RC does with his big planes, just to keep it light.
Boeing 787-9 Dreamliner RC airplane build video PART 1 (14 min 45 sec)


this method of foam board and bits of balsa throughout is cheep and light, this needs to be a light plane because I am planning to use two Freewing EDFs (eather 64, 70, or 80mm edf) and am going to make those cones that are on the front of the SR-71's engines and thought that those might block lots of the air that the EDFs need.
Last edited by OldGreg; Jun 08, 2019 at 09:54 AM. Reason: forgot to post link.
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Jun 09, 2019, 11:17 AM
IMO ( In My Opinion ) →
balsa or carbon's Avatar
If scratch building for the first time .... I strongly recommend starting with a simple , proven design . Then work your way up gradually to more complicated builds and self-designed builds . In scratch building , as in life ..... it's best to learn to crawl , then learn to walk , then learn to run .

Here is a simple , proven design :

https://www.rcgroups.com/forums/show...lable%21%21%21

And RC Groups member burkefj sells kits , and may also have plans available for his SR-71 ( photo below ) . Here is his blog page : https://www.rcgroups.com/forums/member.php?u=118173
For the kit , here is his website : https://dynasoarrocketry.com/?page_id=1424
Jun 09, 2019, 12:09 PM
Retired CAD guy
birdofplay's Avatar
Use either ply on CF to strengthen that Forward fuse as it's Thin and easily broken.

PLy lamination worked for me.
Jun 09, 2019, 12:31 PM
Registered User
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cool thanks, do you think it is a good idea to use foam board to make the basic shape of the fuse then use balsa and ply to strengthen it.
Jun 09, 2019, 12:37 PM
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Thread OP
I want to put a few cool things into this build. I have lights from my E-flight cirrus sr22t, also thinking of making a parachute.
Jun 09, 2019, 07:49 PM
Retired CAD guy
birdofplay's Avatar
Can't find any vids from that long ago.
Here's a launch from a field elevation of ~ 4200' in Joseph Oregon.

https://www.rcgroups.com/forums/show...&postcount=275

edit l8r:

I found a vid from 2010.
You have to Dl it .
It's kinda of a Maiden test and had some squirrely moments.

https://www.rcgroups.com/forums/show...26&postcount=1

Site was ~ 30 miles in land from the Pacific coast ,in a valley know for 120"+ annual rainfall.
Last edited by birdofplay; Jun 09, 2019 at 08:02 PM.
Jun 10, 2019, 12:40 AM
Ft Lauderdale
Number_6's Avatar
Quote:
Originally Posted by OldGreg
cool thanks, do you think it is a good idea to use foam board to make the basic shape of the fuse then use balsa and ply to strengthen it.
Hi,

I used foam board, black Elmers, MPF, and spray glued
3 mil film lamination film over it, then used rattle can
flat black paint.....on my F117.

Yes, internal ply support for edf unit, long
cf and fg spars, fiberglass extreme tape,
on and on.

It flew, but kind of heavy, looks great, not
aerobatic.

Ramy built his Dreamliner from foam, but
fiber glassed the whole bird in part 4....added weight.
Messy, smelly job, IMHO.

So, if I build an SR 71 .... I would do it the way I built
this F117.....realizing ahead of time.... that it will
not be a lightweight, not be very aerobatic,.....
and without a real aerodynamic shape if you
roll past 30 degrees it will head straight
for the ground.

Have fun.

F 117 Version 4 80mm EDF Vista View Scratchbuilt (2 min 3 sec)
Last edited by Number_6; Jun 10, 2019 at 01:00 AM.
Jun 10, 2019, 06:08 AM
Registered User
Thread OP
That's pretty cool. I think your right, I need to make sure that i am making this as light as possible. Maybe I should 3d print some parts of the plane such as the cones that cover the engine.
Jun 10, 2019, 06:52 AM
Registered User
1. I wouldn't put any cones in front of the EDF units. Messing with the inlet air will almost certainly degrade their performance.
2. Build technique depends a lot on the size. Larger builds typically require much more effective bracing while keeping the weight within reason.
3. There have been quite a few successful SR-71 scratch builds right here on this site - do a search to get some ideas.
Jun 10, 2019, 07:33 AM
Ft Lauderdale
Number_6's Avatar
Good point, see what others have built ... and were successful.

BTW, many full scale military planes do not fly great at
small scale. Warbirds are trucks of the sky.

If you watch YouTube vids of real SR71, the max AOA was 17 degrees,
if I remember. The real SR broke apart where the fuse meets the
wing...on real crashes. The real SR had tremendous yaw event when
one engine quit said the pilot.

One of the worst airframes to model is the F117, black diamond.

The angled ailerons needed computers.

If you notice my F117, I changed the hinge line and
size of ailerons. And reflex at hand launch was set
about twice normal, then trimmed out at cruise.

Anyway, my experience....don’t be afraid to change
your plans.... which may include larger control
surfaces, extra spars, widen wing span to create
more lift in your model. Go with single, central motor
...yet try to keep the ‘style’.

To me, this is scratch building.....make a bad
design fly.

Last, these crazy designs are not aerobatic,
in my experience. No slow rolls, no 3D, etc.

Have fun.
Jun 10, 2019, 07:53 AM
Registered User
Quote:
Originally Posted by OldGreg
never done a scratch build but have done balsa kits.
Just noticed this sentence above.
SR-71 as your first scratch build will be somewhat challenging (some understatement there, if that wasn't obvious) - both from a build perspective and a flying perspective.
The flying characteristics of a very low aspect ratio plane typically mean a VERY fast roll rate. If you're not a seasoned flier, a stabilizer/flight-controller might be useful here...
Jun 10, 2019, 03:22 PM
Registered User
Thread OP
Thanks for all the advice guys, really appreciate it. I definitely got my work cut out for me. I think that I will get my plans by the weekend and start getting to work.
Jun 10, 2019, 03:28 PM
Registered User
Thread OP
I realize that this is going to be a hard plane to make fly. what do I need to worry about besides the CG, the airfoil, the weight, and strength?
Jun 10, 2019, 05:39 PM
IMO ( In My Opinion ) →
balsa or carbon's Avatar
Quote:
Originally Posted by OldGreg
I realize that this is going to be a hard plane to make fly. what do I need to worry about besides the CG, the airfoil, the weight, and strength?
Seems like you have a lot of "unknowns" . Why not do what I suggested in post #2 : start with a simple & easy proven design , and then work your way up to more complicated & difficult self-designed ? IMO , most ( if not all ) of your "unknowns" would be learned/answered in the process .
Last edited by balsa or carbon; Jun 10, 2019 at 05:52 PM.
Jun 10, 2019, 08:24 PM
Registered User
Thread OP
I think you'r right, this could save me from wasting my time and money.


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