F-117 Nighthawk - 3D Printed (so far not stable)

[BigMacDaddy]

I have occasionally been trying to get an F-117 Nighthawk that I 3D printed to launch. I know that folks have made much larger versions, including boost glider versions but I just wanted to do something small and cheap to launch. I posted about this originally in a 3D printing technique thread since I am letting the software hollow out a solid model and kind of working in negative with internal cutouts that I want to have shells around them.



I tried to do a pop-pod in my first version and it was too difficult to get enough weight in there and to move it out the back (actually ejection charge blew the nose off instead of popping out the pop-pod. So I tried a version with a normal ejecting nose cone and small parachute. Still not stable since I could not put enough weight in there. I have been trying to do this with a mini engine so thinking of going up to an 18mm engine and loading in a bunch more nose weight. Here are videos of the my couple of launch attempts so far (pretty anti-climactic).





Shame I could not get it to launch with the mini engine but do you guys think I can get this stable with enough nose weight and an 18mm engine? Definitely makes for a more invasive engine mount but I think it is not too bad since it is partly hidden by the rear fins.

 

[neil_w]

That’s inherently an extremely difficult shape to make stable.

You can always take the Matt Steele approach and put some flaps on it so it’ll spin on the way up…

 

[BABAR]

Thinking outside the box, what if you made TWO of them, mated them bottom surface to bottom surface?

use only one motor, the second rocket the chute is only held in by the flat side of the mated opposite half, so it just falls out when they separate (unless you make both gliders, which would be really cool, something like this has been done in the past, I know it didn’t end well but I don’t think the CONCEPT WAS AT FAULT.). You could go up to an 18.mm D or go with a 24 mm mount which will give you plenty of power. I might cheat on the scale of the tail V fins and bump them up a bit for more stability, depends on how dedicated you are to perfect scale.

on a grass field you could probably get away with nose blow or streamer recovery, there will be a lot of tumble drag once you blow the nose, in any case you could probably get away with minimal parachute.

launch lug is gonna be tricky, because the two are “mated” you don’t have a convenient place to put the lug, I would run a straw or a MicroMaxx minimum diameter tube INSIDE bird two (the one without the motor), the rear hole won’t matter and the forward hole will likely be invisible due to the paint scheme, but you can do a @neil_w maneuver and blend it with the fiddly bits or something.

with only one bird carrying a motor, your thrust is minimally asymmetric and I doubt will significantly alter flight path. I routinely fly birds with motor mount effectively offset about one body tube diameter without any discernible effect (as always, YMMV.). As I said, adding larger surface are tail V fins will solve your stability problems at a cost of being less than perfect scale, but I doubt it would detract from looks.

as for making them gliders, remember you aren’t looking for contest record flights, and in fact given you have to track two birds, you really don’t want long glides. Also, regarding scale, the real F-117 was pretty much (minus it’s stealth capability) a crappy fighter plane, but it wasn’t designed to go toe to toe with other third or fourth generation fighters, it was designed to sneak in at night, hit a heavily guarded critical target, and get out in one piece. So if you go the Toy Story motto, it’s may not be gliding but it’s ”Falling with Style”, and you have a soft grassy landing field, old say if it goes up with a net straight trajectory (it may corkscrew a bit), separates, and both pieces are recovered intact, chalk it up as a win.

just an idea, Good luck!

 

[lakeroadster]

Thinking outside the box, what if you made TWO of them, mated them bottom surface to bottom surface?

What about making them... inverted...

 

[cwbullet]

Mots nose weight?

 

[Bravo52]

You could also elongate the fuse a little. Most wouldn't know it "just looks a little off" but it would increase the margin for stability. That, added with more nose weight might make it fly...

This plan view has been stretched 20% nose to tail and it doesn't really look too off... Yes, some will be able to tell, but if you get it to fly it will accomplish your goal of "small and cheap to launch"

 

[GrouchoDuke]

Just make sure there are shakas.

 

[burkefj]

Can you post a picture where you think your CG needs to be relative to the top view to be stable? The problem is you're trying to use a heavy motor in such a small short model and there's just no moment arm to get enough nose weight far enough forward, the key with these are large but lightweight so you can use a smaller motor but get enough Mass far enough forward to get your CG right. You may need to upsize the vertical stabilizers 50% or so as well as there is a lot of forward lateral surface area given the cockpit shape. In simple models of this aircraft there's really nothing inherently unstable about it. If you treat the top view is a simple Delta and ignore the cutouts of the trailing edge of the wing, an aircraft model would typically be about 45% back from the centerline, if you're just building a rocket something around the 35 to 40% back from the nose point should be plenty fine especially if you upsize the stabilizers. In this picture of my radio control version boosting at nsl this year you can see little dots on the leading edge of the wing at about the 45% point but this is controlled obviously so you need to be forward of that for free flight.

 

[BigMacDaddy]

Can you post a picture where you think your CG needs to be relative to the top view to be stable? The problem is you're trying to use a heavy motor in such a small short model and there's just no moment arm to get enough nose weight far enough forward, the key with these are large but lightweight so you can use a smaller motor but get enough Mass far enough forward to get your CG right. You may need to upsize the vertical stabilizers 50% or so as well as there is a lot of forward lateral surface area given the cockpit shape. In simple models of this aircraft there's really nothing inherently unstable about it. If you treat the top view is a simple Delta and ignore the cutouts of the trailing edge of the wing, an aircraft model would typically be about 45% back from the centerline, if you're just building a rocket something around the 35 to 40% back from the nose point should be plenty fine especially if you upsize the stabilizers. In this picture of my radio control version boosting at nsl this year you can see little dots on the leading edge of the wing at about the 45% point but this is controlled obviously so you need to be forward of that for free flight.

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Thanks - your reply made me think about this. I could not really simulate this in OR so I was trying to push CG as close as I could to the front of the wings to be sure it was in front of CP. I think with the mini engine inside the CG was around 1cm back from front of wings. Maybe I overdid it and could have used less weight and let the CG come back more so that the rocket would have more speed.

 

[BigMacDaddy]

Thinking outside the box, what if you made TWO of them, mated them bottom surface to bottom surface?

Thanks -- I had not thought about doing that with this rocket but it could work. I had thought about building this model that I saw at the air and space museum with 3 gliders mated at the bottoms to create a symmetrical rocket.

 

[Michael L]

They are inherently unstable in real life but thanks to a robust flight control system that can be overcome. I was going to say "I doubt if that could be duplicated for a model" but I am not overly confident that is true.

 

[burkefj]

That's not really correct they are balanced and have lateral stability set such that they are on the edge of stability and need numerical control but if you're just building a standard model and can be slightly conservative with CG and stabilizer size they're no problem at all, I built and found several radio control versions.

They are inherently unstable in real life but thanks to a robust flight control system that can be overcome. I was going to say "I doubt if that could be duplicated for a model" but I am not overly confident that is true.

 

[Michael L]

That's not really correct they are balanced and have lateral stability set such that they are on the edge of stability and need numerical control but if you're just building a standard model and can be slightly conservative with CG and stabilizer size they're no problem at all, I built and found several radio control versions.

I read that here: https://www.f117sfa.org/about-the-f117

Its quadruple redundant fly-by-wire flight control system, critical to flight of this inherently unstable aircraft, is derived from the F-16.

But I've also read that the F-117 is not inherently unstable (not just your post).

 

[burkefj]

There are many radio control models of that aircraft that fly just fine with two fingers on the sticks and no stabilization system at all.

I read that here: https://www.f117sfa.org/about-the-f117



But I've also read that the F-117 is not inherently unstable (not just your post).

 

[BigMacDaddy]

There are many radio control models of that aircraft that fly just fine with two fingers on the sticks and no stabilization system at all.

Do you know roughly where the CP should be on an F-117?

 

[burkefj]

If you look at the photo above that shows the launch photo you can see my CG markings this is the boost CG which is slightly tail heavy and the glide CG is about 3/4 of an inch ahead of that, I'm probably running about 10% stability margin at boost so you can work backwards from there.

Do you know roughly where the CP should be on an F-117?