Scaled Jets (F-16)

Discussion in 'Aircraft Design / Aerodynamics / New Technology' started by Acrojet, Jun 6, 2016.

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  1. Jun 8, 2016 #81

    Acrojet

    Acrojet

    Acrojet

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    Funny, I was just thinking that very same thing. Maybe a blending of the two to get the best of both worlds.....
     
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  2. Jun 8, 2016 #82

    Doggzilla

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    The chart you are looking at isnt labeled, it turns out its one of those ones where the bottom 75% is cut off and it makes everything look out of scale.

    [​IMG]

    For a given taper, 40 degree sweep reduces the CL from .86 to .72, about a 16% difference. So even taking out a significant amount of the sweep would only have a relatively minor improvement of a few percent.

    This does not take into account any additional lift by the extended chord at the root, which can have both positive effects on lift and drag. The extended chord of the inner wing can reduce wing drag by a few percent, so its not a total loss. Its something like 5% reduced drag for a 20% increase in chord. The CG is also much less sensitive on a swept wing, the main reason its used on many large subsonic civil aircraft. The transonic issues are usually taken care of by adding carrots to the wing, which also provide room for control equipment.
     
  3. Jun 8, 2016 #83

    Glider

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    One last thing: is Lockheed Martin still suing over the appropriation of "their" F-16 shape? Perhaps someone in Congress stopped this. At the time I was surprised that the shape wasn't the intellectual property of We the People.

    You may also have Department of State ITAR, and Department of Commerce EAR, issues to deal with. Re-export is what I'm thinking of. I am not an expert in this area, and can not provide advice.
     
  4. Jun 8, 2016 #84

    Doggzilla

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    The total reduction in CL for a 40 degree sweep is only about 16 percent, so even a fairly large reduction in sweep would have only a few percent effect on lift.

    The changes in CG for scale jets are also a lot different for jets than prop aircraft, with the internal differences helping to cancel out any change in the CG.

    For instance, the prop aircraft loses a large mass in front of the center of lift, and gains one behind it, very hard to keep any sort of scale there.

    But with a jet, you lose the radar in front, and avionics bay behind the pilot, countering about half the change in CG. The rest of the change in CG can be countered by the lack of afterburner allowing the engine to be mounted much further back, with the hot section of the turbine literally sticking out where the afterburner nozzle would be protruding from. The engine only weighs half of what it does for a Cessna, but its also at nearly twice the moment arm away from the center of lift, so it provides much more of a balancing force despite having less total mass.

    Compared to mounting the engine in the standard CG location, moving it far back solves a lot of issues while allowing you to maintain the look. It fixes the CG, simplifies structural design of the rear fuselage, and nearly eliminates having to design heat resistant fuselage components for a very significant portion of the fuselage.

    It also simplifies the structure because you can attach the split air brakes to the same member that is holding the engine, cancelling out the competing forces of thrust and drag that would usually have to be passed through the fuselage.

    It should also free up some space by moving the upper wall of intake duct a few inches over several feet of the duct, allowing for a noticeable change in total fuel capacity. Possibly an additional 12-16 gallons.
     
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  5. Jun 8, 2016 #85

    Acrojet

    Acrojet

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    Interesting thought.... I can't imagine it would be an issue, but you never know. I'm not selling these to other countries to compete with Lockheed Martin. This is a sport jet that doesn't have 1/4 of the speed, let alone the capability. If I get a letter from Lockheed I will "cease and desist". ;-)
     
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  6. Jun 8, 2016 #86

    Wanttaja

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    Well, good. Sounds like you're not going into this with bright visions of future fortunes washing through your head. If you're (mostly) out to have fun, you'll probably succeed at that, at least....

    Gracious, no one ever trusted ME with the good airplanes. I had to make do with the <100 HP cheapies. :)

    Ron Wanttaja
     
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  7. Jun 8, 2016 #87

    Acrojet

    Acrojet

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    You my friend are picking up, what I'm laying down.... You get it. Light engine far back, keeps airplane light, helps CG, etc... All working to make this a viable design
     
  8. Jun 8, 2016 #88

    Acrojet

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    Well, if this sucker works out, we'll see if we can't get you out in the little rocket!
    ;-)
     
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  9. Jun 8, 2016 #89

    mcrae0104

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    I do not think that chart means what you think it means. C[SUB]Lmax[/SUB]/outer panel C[SUB]Lmax[/SUB] is not the same as C[SUB]Lmax[/SUB].

    maxresdefault.jpg
     
  10. Jun 8, 2016 #90

    DangerZone

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    It's an interesting question because today the situation is quite contrary to this mentioned method of suing for the F-16 shape. Lockheed Martin seems to be going for the 12k hours resources of the original F-16 airframe, rising it from 8k. Thus such a scaled jet would only be contributing to further sales in the future, both of used F-16 or new airplanes. They also allowed production of a scaled 'improved' F-16 version to Korea, it's called the KAI FA-50 fighter.

    [video=youtube;_W0gxDVaAyE]http://www.youtube.com/watch?v=_W0gxDVaAyE[/video]

    https://en.wikipedia.org/wiki/KAI_T-50_Golden_Eagle

    So, if Lockheed Martin is allowing serial production of an F-16 like aircraft, there is really no concern they would object to a single private jet prototype. If it flies well at airshows, that would be great advertisement for the F-16 once they bump up the resources of the airframe to 12k hours and start selling at a higher price.

    Anyway, can this kind of scale replica aircraft seem threatening to anyone? To me it only produces positive thoughts for the guy who built it, cause it takes a lot of time to build something like that.

    [video=youtube;qQ9dSrrBN28]http://www.youtube.com/watch?v=qQ9dSrrBN28[/video]
     
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  11. Jun 8, 2016 #91

    Doggzilla

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    Woops, but at second look, it actually proves my point further. Change in taper makes a significantly larger effect than reducing sweep. Cutting half the sweep would only have half the effect of an intense taper.
     
  12. Jun 8, 2016 #92

    mcrae0104

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    No, the chart you posted is related to lift distribution, not lift reduction due to sweep. You can find the chart in context here.

    A 40-deg quarter-chord sweep angle reduces C[SUB]Lmax[/SUB] by approximately 23%. (Gudmunssen, General Aviation Aircraft Design, fig. 9-59, "Change in C[SUB]Lmax[/SUB] with Sweep Angle")

    Where did this idea come from? Every source I have does not mention this. Raymer, for instance, says, "Wing sweep is used primarily to reduce the adverse effects of transonic and supersonic flow." (Aircraft Design: a Conceptual Approach; emphasis added)
     
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  13. Jun 8, 2016 #93

    Jay Kempf

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    This thing is the one that worries me about your overall plan. The F16 is designed with that giant inlet so the depth of the cockpit is not so much. 60% might be aggressive. The rest of the airframe might be just about perfect though for monocoque carbon with very few bulkheads. Of all the fighters out there the wing is pretty straightforward with a ring for a spar passthrough as the only issue. Maybe you could use the two place as it has a longer canopy and higher and lay the pilot down almost ala HP18.
     
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  14. Jun 8, 2016 #94

    Tiger Tim

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    I had thought about this too but I think we're picturing the real F-16 at the wrong size. We're used to tight fitting bubble canopies that have just enough room for your head, where jet fighters have enormous bubbles that go down nearly to the pilot's waist. Likewise, the real F-16 probably has a lot under the pilot's butt where the scaled down version can have you practically sitting on the floor. In any case, it would be interesting to see an overlay of the pilot and passenger on his 60% three-view drawing.

    What's been keeping me up at night is how to convert a turbojet (or maybe just gas generator?) into a turbofan. Is there room or any provision to pass the fan shaft down the inside of the turbine shaft or is the solution more... creative? I understand if that's a trade secret for now, it's just been an interesting thought to chew on.
     
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  15. Jun 8, 2016 #95

    Lucrum

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    That's what I always thought wing sweep was for.
     
  16. Jun 8, 2016 #96

    Acrojet

    Acrojet

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    Jay,

    Don't worry. Worrying can stress you out, which can lead to obesity and can give you a heart attack....
    So.... You'd be too big to fit, or worst case, dead! ;-)
    But seriously, as mentioned before, it will be tight, but not any worse than a BD-5 (which I fly and fit into, consiquently)
    The pilot will sit almost on the floor of the cockpit area and his feet will go way up to the front of the pointy nose... Almost.
    I'm staying with the "single place" canopy lines just because they look better, but it will be larger to accommodate both occupants. The canopy might be the only thing that looks noticeably "off-scale" to the naked eye. It will still look good, and the rest of the ac will be almost spot on with a few tweaks here and there. Also a 30degree recline is pretty aggressive already and that will work well.
    Scale gear already to be machined, exactly to scale.... That was a real stumbling block until I got the idea to contact one of the companies that supplies scaled machined gear for the RC community and they were gracious enough to scale-up one of theirs and even powder coat it white for me. It will come with both nose and main gear and include the retract mechanism an associated actuators, which I may make pneumatic along with the air brakes. Cool stuff.

    Peter
     
  17. Jun 8, 2016 #97

    Acrojet

    Acrojet

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    Tim,
    It's not my proprietary info re: the engine, but it is for the developer. I've promised not to divulge too much just yet beyond what I've been told I could share, until it's finally up and running and well into the test phase.
    Let's just say, it's pretty darn simple and it's not exactly a "converted" turbojet, more like adding to an existing turbojet as its core (gas generator). And that was the goal from the start. The "gas generator" (turbojet core) is/was developed from the start to be a turbofan, so it's essentially an "all new" design. Everything is manufactured (design, machining, casting, etc) in-house (low overhead and no board members to answer to) and they even make their own ceramic bearings...
    That's about all I dare say at this point. Just stay tuned.... ;-)

    Peter
     
  18. Jun 8, 2016 #98

    nerobro

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    I've been poking around at this for a few weeks. The F-16 appealed to me too, with it's "relatively short" intake duct, and not needing to merge two airstreams it seemed pretty sane. The A-4 also could have very short ducts, but you're dealing with two holes instead of one, and you're going to be fighting combining boundry layers before the compressor...

    Lets say, i'm excited about your project. I can't wait to see where you go with it. I have this funny feeling that experimental class jet replicas is "a thing" people want.

    Are you planning on carrying a compressed air bottle? Or will you have an onboard compressor? I figure a HPA bottle would be lighter, but would limit the number of actuations.

    IIRC.. The F-16's stability is dependent on airspeed. At "normal" airspeeds, it's mostly unstable, due to a deeply aft CG. Even with the larger elevators. There are AOA limits in place to stop the plane from reaching a point where the control surfaces can't bring it back. At mach, IIRC it's stable.

    As noted, with a forward CG, r/c F-16's fly just great. All the way from 1/4 scale down to 6" long peanut scale gliders.

    This drives me nuts. Silicon valley. Silicon is element 14. Silicone is a polymer used in oils and rubbers. Sillicone valley, if anywhere, would probably be Los Angeles. Silicon valley is 300 miles north of there. <steps off his chair> *coughs* Oh yes. It's "all intents and purposes" not "intensive purposes." Thank you. <wanders back to his IT job.>
     
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  19. Jun 8, 2016 #99

    Acrojet

    Acrojet

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    Well, I'm glad you like the idea. I think you'll see more "scaled jet replicas" and an emerging market in the experimental arena. Let's hope so....

    For right now the plan is to go with a small tank that is kept under pressure by a mini compressor like the SubSonex jet uses...:

    https://youtu.be/y0uB7wUfbZM
     
  20. Jun 8, 2016 #100

    gtae07

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    I'm going to hazard a guess and say aft fan, like the CJ-805-23 or CF700.
     

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