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F-117 Nighthawk

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vhhjr

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I have been looking at a F-117 Stealth fighter for some time in about 1/3 scale. If you enlarge the cockpit area about 10% in the vertical a real person will fit. It's a ducted fan, something that few have made successful. However, I think it is possible to make a workable DF installation. There was one at Oshkosh this year. I'm thinking of a center mounted single engine, a Mazda seems a good choice, with belt driven fans. The other option is to use two, 2-cycle engines such a Hirths. They would produce more power and the reliability issue would be offset by there being two of them. The full size aircraft engines produce about 21,000 lbs of thrust. That's about 40% of the gross weight. If you built the scaled down version in the 1200 lb gross weigh range you would need about 500 lbs of thrust. That's on the high end for a Mazda, but in the range. Also, the wing loading of a homebuilt would not be scale and would be much lower. There's always the question about the full size F-117 requiring a computer to fly it. That's true, but there are many examples of large scale RC F-117s that fly just fine without even using gyros. Since the F-117 is all flat panels you can build it any of several ways. Use a tube frame and fabric it like the FMX-4 Facetmobile. Make it from plywood using foam sandwich panels for the flat parts. Or, you could make it from foam and fiberglass or carbon if you have deeper pockets. A possible approach would be to build a large scale RC model of the intended design and instrument it using a system like Eagle Tree offers and off-the-shelf electric ducted fan units. Many homebuilt designers have gone this route. Such a project would certainly be an attention getter. Any thoughts?
 

Doggzilla

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Honestly, I hate imitation aircraft. I like inspired aircraft, but not imitations...

That said, the original prototypes "HAVE BLUE" Were much smaller and had much worse design, and yet they flew... mostly, but crashed probably due to the much high sweep of the wings.

Also remember that drag is exponential, so you can have a full size model... and be powered by a fraction of the power. I think someone had a full size WWII warbird imitation... that had 100hp. The whole structure was lighter, they just kept the same dimensions. It could fly with only 100hp because it doesnt physically require all the weight that a real warplane with 2000hp requires. It was only going to go something like 100mph... so it didnt need to be heavy like the real thing.
 

Toobuilder

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Also remember that the 117 is a truly awful flying machine. Like the radio control flying lawn mowers, mail boxes or dining room tables, it is an aerodynamic freak that flies only because of excessive thrust and huge flight controls.
 

Doggzilla

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Also remember that the 117 is a truly awful flying machine. Like the radio control flying lawn mowers, mail boxes or dining room tables, it is an aerodynamic freak that flies only because of excessive thrust and huge flight controls.
Well, dont hold back my friend!

If its not meant to be stealth, that really leaves a lot of room for improvement, and a decent set of slots and flaps could conceil a better wing...

But yes, the standard design is a bit crap, but it actually performs well for an aircraft which is roughly comparable to an F-5 without afterburners... and 4000lbs of bombs...

So, its not awful... but its no A-4.
 

Jay Kempf

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Also remember that the 117 is a truly awful flying machine. Like the radio control flying lawn mowers, mail boxes or dining room tables, it is an aerodynamic freak that flies only because of excessive thrust and huge flight controls.
Not only that but it was designed right before computers were really able to compute stealth functions on curved shapes. So it was severely faceted which is not very aerodynamic. Just look at the F117 and then the B2 by comparison. One in front of and one behind the rise of analysis tools. No one would design a foldaplane like that again me thinks.
 

Doggzilla

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Not only that but it was designed right before computers were really able to compute stealth functions on curved shapes. So it was severely faceted which is not very aerodynamic. Just look at the F117 and then the B2 by comparison. One in front of and one behind the rise of analysis tools. No one would design a foldaplane like that again me thinks.
Whats crazy is that they understood both, if you can believe that!

Im trying not to be an arrogant tool, but if anybody is interested in how it came about, this is the down low:

TACIT BLUE is the basis for Joint stars, it tested the idea of a stealth observation aircraft that had a low observable radar that looks downwards and was used to spot formations of vehicles. The technology worked, the aerodynamics were used on the B-2, and the radar was placed in a Boeing because it was realized that you didnt have to be behind or even over the lines... the radar had a significant range of hundreds of miles, and so the entire front line could be detected from well behind friendly lines... so no need for stealth.

TACIT BLUE was the "competitor" of HAVE BLUE/F117... but in reality, it was to keep Northrop happy and to test totally unrelated equipment than HB. HAVE BLUE, The F-117 prototype, was started in 1977, while TACIT BLUE was started in 1978, years before the first F-117 was even close to being produced. And dont forget, fly by wire had already been around for some years already... in the CF-105 and Concorde, as well as in an F-8 about 5-6 years earlier...

And as for radar computing, the stellar COBRA DAME had already been in operation for some time, which was/is a massive fixed array radar, and could compute radar returns in real time, so we understood a good deal about how to digitally process radar physics. The system uses a complex algorithm and a massive computer to compute the way the waves react between thousands of elements...

The math was already known fairly well, and had been used on the D-21 and SR-71, as well as tried and failed on the U-2 with some pretty absurd tests... The difference was that the men who designed the F-117 followed the math to its conclusion... Even Kelly Johnson thought they were going to fail epicly... he was stunned by the success...

But thats all besides the point. Im just saying that its anything but impossible to make it fly better, no stealth required, and with modern modeling software to see the pressure gradients on the facets... would make the task vastly easier... After all, it was done by 3 guys who had none of that, and HAD to be stealthy...
 

Vigilant1

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Such a project would certainly be an attention getter. Any thoughts?
Yes. People would say "why would anybody build that?" The F-117 was designed to have a low radar cross section and low IR signature using the technologies available at the time, and many aerodynamic sacrifices were made to attain that goal. It worked pretty well. If you made a small one all you'd have left is the crummy aerodynamics with nothing to show for it. Aside from the airframe aerodynamic compromises, going to any ducted fan is going to significantly reduce available thrust when compared to a prop. And if you go as far as to mimic the F-117's slot-like engine nozzles, your thrust loss will be even greater.

So, count me as "not a fan of the idea." But, if you want to do it for some reason, have a good time.
 

Toobuilder

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Well, dont hold back my friend!

If its not meant to be stealth, that really leaves a lot of room for improvement, and a decent set of slots and flaps could conceil a better wing...

So, its not awful... but its no A-4.
No, as an air vehicle, it is awful.

The fact that it flew AT ALL with the facets is what makes it remarkable. Unfortunately, those facets are the entire "charm" of the airplane, so smoothing them over would make a better flying airplane, but also defeat the purpose of the excercise.
 

Radicaldude1234

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I think you'd have better luck with a pair of TJ-100 jet engines (~490lbf of thrust), or 4x AMT Nike (704lbf of thrust) rather than get a ducted fan to work.

There have been RC F-117 models that have flown without the aid of FBW, though I'd be cautious about changing the nose geometry, as it might increase directional instability.

As an aircraft, though, the F-117 was clearly completely devoted to stealth. With a T/W ratio of 0.4 (which is worse than the B-52, btw, which had a ratio of 0.52...when loaded), it was a pretty substandard combat aircraft. That is, it probably didn't have the acceleration and manueverability needed to defeat surface to air threats if it was detected. It's mission profile most likely involved it cruising at high altitude (40,000ft) at the edge of SAM radar coverage and get close enough to its target to drop its laser guided bomb, though I highly doubt that it was an efficient high altitude cruiser due to the drag necessitated by its stealthy shape. With such low performance and its only redeeming quality (stealth) being obsolescent in the face of newer radars, its no wonder that the USAF retired the Nighthawk...
 

Jay Kempf

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I think you'd have better luck with a pair of TJ-100 jet engines (~490lbf of thrust), or 4x AMT Nike (704lbf of thrust) rather than get a ducted fan to work.

There have been RC F-117 models that have flown without the aid of FBW, though I'd be cautious about changing the nose geometry, as it might increase directional instability.

As an aircraft, though, the F-117 was clearly completely devoted to stealth. With a T/W ratio of 0.4 (which is worse than the B-52, btw, which had a ratio of 0.52...when loaded), it was a pretty substandard combat aircraft. That is, it probably didn't have the acceleration and manueverability needed to defeat surface to air threats if it was detected. It's mission profile most likely involved it cruising at high altitude (40,000ft) at the edge of SAM radar coverage and get close enough to its target to drop its laser guided bomb, though I highly doubt that it was an efficient high altitude cruiser due to the drag necessitated by its stealthy shape. With such low performance and its only redeeming quality (stealth) being obsolescent in the face of newer radars, its no wonder that the USAF retired the Nighthawk...
The footage of the Iraq war confirmed that. In the footage you hear the jet go overhead (sound on video so you know it was already gone) then the anti aircraft fire and missiles would go off. Actually I think the stealth shape was good at higher cruising speeds at the onset of compressibility as it focuses and sheds the pressure waves nicely.
 

Vigilant1

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The plane was the first of its kind and did a pretty good job of operating against the vast majority of threats extant when it was designed. At the time the only options against a developed air defense system were flying in very low or going in with the help of a huge package of standoff and penetration jammers. The F-117 provided another tool in the toolbox. On a strategic level, it demonstrated the potential of stealth technology and Western commitment to it, hastening the end of the Cold War.
 

autoreply

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A faceted (sic?) plane can be made to fly just fine, see Wainfan's plane. But the layout of the facets is pretty critical and copying an aerodynamicists worst nightmare is unlikely to deliver exactly that critical layout that results in a plane that flies reasonably well.

I'd personally just make a scale model of the SR71:gig:
 

Radicaldude1234

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The plane was the first of its kind and did a pretty good job of operating against the vast majority of threats extant when it was designed. At the time the only options against a developed air defense system were flying in very low or going in with the help of a huge package of standoff and penetration jammers. The F-117 provided another tool in the toolbox. On a strategic level, it demonstrated the potential of stealth technology and Western commitment to it, hastening the end of the Cold War.
Well, I never said it wasn't good at its intended role :), just that when you take away the one trick it had, which is what happened as radars became more powerful, you're left with a rather poorly performing airplane that would likely lose a drag race with a B-52. With a T/W ratio worse that most business jets, it would have been in a poor position to avoid missiles kinematically, if it was detected. As its mission profile required the Nighthawk to get pretty close to its high value target (<25 miles), which is most likely to be heavily defended, said poor performance would have given an enemy air defense network more time to react to the intruder.

I think air force planners realized this and this is seen in the next generation of stealth aircraft and munitions, which put additional emphasis on speed (supercruise; F-22/YF-23; less time in lethal air defense envelope) and standoff capabilities (stealthy cruise missiles/glide bombs; ability to remain on fringe of air defense system, where detection is least likely and SAMs are kinematically easier to defeat).

I'd personally just make a scale model of the SR71:gig:
I'll see your Blackbird and raise you an YF-23. Much more aerodynamically advanced than the Nighthawk (made to cruise at high altitude and at high speed), stealthy (thus interesting looking, which is what I think the OP was going for), and I think the cockpit section can more easily be enlarged without the aircraft looking goofy...
 

vhhjr

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I thank you all for the replies. There's a local group here that are considering a Facetmobile build and have Wainfan's blessing and limited assistance. It just seemed like an interesting exercise to go a bit further and look at the F-117. Interestingly, Wainfan considered a ducted fan when he was developing the Facetmobile, but decided on the more proven engine in the nose layout. I am considering a large scale model of the Nighthawk and I doubt that a man carrying version is in the cards.
 
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