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Blackbird fly... blackbird fly, Into the light of the dark black night. Twin 69mm EDF SR-71 Build/Project

JTarmstr

Well-known member
#22
I am building this, definitely. i still have two EDFs left and some servos and stuff. One problem is... i dont have a laser cutter so i will have to figure some way around that.
 

quorneng

Well-known member
#25
Very nice
Any idea what the weight is likely to be and the wing loading?
After all the Blackbird aerodynamics were never intended for low speed flight.
In my experience if you can get the wing loading low enough the plan shape and section of the lifting surfaces becomes far less critical.

I have not built a foam SR72 - yet - but I have done both a scale (including wing sections!) foam XB70 and Concorde.
Following with interest.
 

kilroy07

Well-known member
#27
In my experience if you can get the wing loading low enough the plan shape and section of the lifting surfaces becomes far less critical
No idea on weight yet, I'm using MDF instead of plywood to prototype and it's a tad heaver to begin with.
Once I have the core roughed out I'll put her on the scales.
The idea is a solid core for the fuselage holding the batteries gear and most of the electronics.
Covering the fuselage and main wings with poster board outer wings and leading edge folded foam.
The EDFs I have seem to have quite a bit of trust, so I'm not too worried... yet...
I plan on there being a larger airfoil than "Scale" due to the lower speeds we will see.

Yes we so need a foamboard version of this historic jet
I know... Right?!!

Scale is going to be a retaliative term (I am using Pringles cans after all.) :sneaky:
 

quorneng

Well-known member
#28
How are you going to handle those dreaded intake spikes?
Very much a part of the SR71's appearance yet they block 85%? on the inlet area.
 

Namactual

Well-known member
#29
@kilroy07
Any progress on this bad boy?

Re: intake spikes
Aye, there will definitely be some scale concessions there. You would either have to make them out of a mesh, or you would have to put cheater holes in somewhere.
 

kilroy07

Well-known member
#30
I admit, it’s been on the sidelines mocking me for the last few weeks. I was caught up in the buildurary challenge and getting at least a maiden out of the swordfish II.

That said, I’ve been on the road the past two weeks and getting time for scratching out designs in the room at night. So, hopefully by the end of the month the first mock-up will be ready as #2 of the swordfish.

@Scooter The White Rabbit had a great idea for the spikes and I think I’ll try that approach first.
 
#31
Okay, looking over that vid again. Saving weight and work... One 9 gram metal gear servo pulling/pushing both rear mains into place? Ju-u-ust a thought.
Money for my projects dried up for now. Dead bill caught up to other half, and my work is... erratic. Drawing yes, working no.
 

kilroy07

Well-known member
#32
Saving weight and work... One 9 gram metal gear servo pulling/pushing both rear mains into place? Ju-u-ust a thought.
That's how I have it configured.
You can see in the first mockup picture, didn't appreciate how geometry comes into play so I've got some slight adjustments to make (that's why one gear is lower and operates sooner) :rolleyes:

I've been sidelined myself (although that's due entirely to my own short attention span...) but this month I hope to have first full mock up finished... (finger's crossed)

P.S. Here's a link to the retracts I used...
https://www.aliexpress.com/item/Who...237.html?spm=2114.13010708.0.0.3a994c4dLvI9JI
 
#33
That's how I have it configured.
You can see in the first mockup picture, didn't appreciate how geometry comes into play so I've got some slight adjustments to make (that's why one gear is lower and operates sooner) :rolleyes:

I've been sidelined myself (although that's due entirely to my own short attention span...) but this month I hope to have first full mock up finished... (finger's crossed)

P.S. Here's a link to the retracts I used...
https://www.aliexpress.com/item/Who...237.html?spm=2114.13010708.0.0.3a994c4dLvI9JI
For that price my only question is.... what size are the wheels. Thinking of going retracts on my Guinea... maybe.
 

kilroy07

Well-known member
#34
Alright, alright... Time to get off my keister and get some work done on this bird!!

While I've been on the road for work quite a bit lately, it has given me time to think...
And here's what I've come up with;
IMG_8661[1].JPG

Mind you, this is simply a technology demonstrator... So, it's a great full sized mock up but "this" one is just for construction/glide tests...
Haven't added the ribs yet and I will cover her in posterboard at some point...

It seems a bit smaller than I imagined...
Well, it's not smallish NOW is it?!! :oops:

Any ideas on how to do the nose joint?... I've got it working "okay" now... with some popsicle sticks.. But I am open to other ideas.
the joint is right before the batteries...
 

PsyBorg

Wake up! Time to fly!
#36
I would offset the joint. Make the bottom verticle longer then the top and same on the horizontal.

That way all the force of a nose hit does not transfer all in one spot. Once the skin is on that will help distrubute impact as well.

Tetris baby tetris !!
 

Namactual

Well-known member
#37
One more thing to consider, you might just leave an intended weak point in the nose. Especially on a bird like that with such a long snout. With a lever that long, it would not take much force to tweak the fuse in a bad way. If the nose breaks off, you have a shorter lever on a bad landing.

Just a thought.
 

kilroy07

Well-known member
#38
Yea, I thought about a crumple zone (my Viggen already has a second nose.)
That and battery access is why I placed the joint where I did.

While skinning will help make it much stronger, I might add an arrow shaft (or carbon fiber) down the length which will (I think) both help with strength and provide for the nose to lock in....
 

kilroy07

Well-known member
#39
Update;
I've been on the road quite a bit for business... but that does give on time to doodle on the flights back and forth.
IMG_8806[1].JPG

CAD files have been updated with corrections from full sized mock-up and now just a few 3D printed parts to design/print.

Hope to have a Maiden of the Alpha build next week! (Fingers crossed)
 

kilroy07

Well-known member
#40
So this arrived;
IMG_8828[1].JPG
Just in time to indicate I needed to redesign the tail cones I've been working on (they are really becoming a pain...) :rolleyes:

Still on track for the Alpha maiden next weekend... (fingers still crossed!)