Scaled Jets (F-16)

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

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  1. Jun 7, 2016 #61

    BoKu

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    Quoted for posterity below.

    A couple of other points:

    * Paragraphs are your friend. Single monolithic blocks of uninterrupted text are the crazy uncle you avoid at family reunions.

    * They're, their, and there.

    * You are free to debate to your heart's content the degree of obligation and the degree to which you personally undertake it. Nobody is going to legislate it for you, and I am not saying they should. But I will observe that nobody does tandem BASE jumping or wingsuiting.

    Thanks, Bob K.




     
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  2. Jun 7, 2016 #62

    bmcj

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    Auto states the point that I was going to make. CF can be heavier if metal pieces are simply replaced with CF pieces. Designing CF with a "metal mentality" does not take advantage of what CF can do for you. If your core structural design has not been done by someone who is a competent composite aircraft designer, then it has not been optimized for the properties of CF. A trip to a composite designer (perhaps Rutan or Scaled Composites for example) may surprise you.

    By the way, I'm not trying to be negative. I would like to see this project fly. The F-16 is also one of my favorites.
     
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  3. Jun 7, 2016 #63

    BoKu

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    Are you saying that if it is heavier then the best glide angle will be steeper?
     
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  4. Jun 7, 2016 #64

    bmcj

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    Good catch. Heavier weight (for a given design) should give you roughly the same glide ratio, just at a higher speed.
     
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  5. Jun 7, 2016 #65

    StarJar

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    Bob, I don't think you're in a good mood today. As you know there are a thousand points to a snowflake. Common sense is the most powerful trait. The glide angle may be the same, but he was able to perceive the heavier plane is at a disadvantage in engine out situations. Peace.
     
  6. Jun 7, 2016 #66

    Doggzilla

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    This is why the predecessors have twin tails, like the F-15, F-14, and F-18. The F-16 does not require these because it uses belly strakes, which are much smaller but are directly in the airflow at high angles of attack, as the belly sticks out into the airflow. It also simplifies the structure by helping balance aerodynamic loads created by the tail drag. Twin tails both push in one direction, causing a torque on the frame that has to be compensated for. Having stabilizers on belly means the forces are somewhat balanced on the rear of the aircraft, and help cancel out the torque that would otherwise be present from the tail drag, especially at supersonic speeds.

    The air brakes are on the tail are similar. Being split means the force is relatively balanced and creates little torque on the frame for the amount of drag. It greatly reduces the weight of the structure designed to mount them, since the forces are partially equalized by each other.
     
  7. Jun 7, 2016 #67

    StarJar

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    I was recently working on a scale design, and realized it would all work better if I modified the wing planform, which at first was kind of sad, because it would no longer be scale, by the strict definition. After increasing the span by 2 ft., reducing the aspect ratio slightly, and going from 23° sweep to 16° sweep, (this had a swept wing) the design still looked great and to my surprise, still gave off the image of being the exact airplane.
    I would not hesitate to screw with the wing planform if you could get benefits from it.
     
  8. Jun 7, 2016 #68

    Lucrum

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    I designed with that mentality, rather than adhering to a strict scale. I just don't "need" 40 deg LE wing sweep for the speeds I'm anticipating.
    In my mind the mid wing & blended fuselage made the "look" I wanted. More so than wing plan form.
     
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  9. Jun 7, 2016 #69

    Doggzilla

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    I see a lot of people doing this on designs, but its not really necessary. The sweep isnt the real big factor in lift, its the wing loading. For a given wingspan, a swept wing usually has about the same lift. But swept wings have improved static stability. This is why the F-104 is not as stable as swept wing designs of the same wingspan.

    The reason most people associate sweep with poor handling is that most swept wing aircraft have high wing loading, and high density, and therefore have inertial coupling issues.

    One of the important factors to remember is scaling aircraft is that in jets, the removal of the radar and avionics helps counter the increased mass of the pilot compared to the size of the aircraft, helping keep the CG in balance. Extending the fuselage slightly between wing and elevator can cancel out any other differences in CG fairly easily by giving the tail a better moment arm. A small counterweight would also help keep a scale aircraft within CG without having to modify the general layout very much.

    If a better wing is required, the Japanese F-2 is nearly identical to a F-16, but with a better wing layout.
     
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  10. Jun 8, 2016 #70

    Lucrum

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    No argument about typical jet higher wing loadings. But I was under the impression sweep does affect Cl max in a neg way sweepback.jpg
     
  11. Jun 8, 2016 #71

    radfordc

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    It's true that noone does tandem basejumping/wingsuiting. But...it's not out of consideration of the passenger that it isn't done. Jumpers don't want to die stupidly.
     
  12. Jun 8, 2016 #72

    BJC

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    Then why are they BASE jumping?


    BJC
     
  13. Jun 8, 2016 #73

    Acrojet

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    Thanks Bob

    Have a nice day.
     
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  14. Jun 8, 2016 #74

    Acrojet

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    Thanks for the input.
    It's already been decreased to 38 degrees from 40. It's not much, but it's a start and we're most likely adding another 2 feet of span. For that matter the horizontal stab area/size is slightly larger than the original scale sized so these are all pluses to make it more manageable. ;-)
    To the naked eye, you shouldn't be able to tell..... ;-)
     
    Last edited: Jun 8, 2016
  15. Jun 8, 2016 #75

    Acrojet

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    I believe this is the case. I'm trying to confirm exactly why we are so much lighter than a CF design. Need to do some digging and get a hold of my guy on his numbers......
     
  16. Jun 8, 2016 #76

    Tiger Tim

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    Cool project, Acrojet. Have you thought at all about a single seat variant to save a few more pounds for slightly better performance?
     
  17. Jun 8, 2016 #77

    Acrojet

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    Yes, this is what I mean. With a heavier airplane you're going much faster and you will have less time to assess your options when the #*%^ hits the fan....... Thanks for clarifying.
     
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  18. Jun 8, 2016 #78

    Acrojet

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    Tiger Tim,

    Well, yes actually. One of the reasons I've chosen to do the two-seat design is that I always get asked the question by people who aren't exactly familiar with the BD-5J design is: "does it have two seats." implying of course, that they want a ride. ;-). So I always told myself that if I were ever going to design a jet like this, it would have to have two seats.
    That's kind of a long answer to your question, but it gives you a reason as to why. You can certainly not use the second seat (which I might add is more of a "jump-seat" than a full second cockpit. Similar to what the single seat P-51's use). It ain't comfy, but it will work in a pinch. It will still have dual controls, but the occupant will have to be under 170lbs to even fit.
    You can also not use all 120 gal of fuel. With a max fuel burn at sea level of roughly 40gal/hr. You could put 60 gal and still scoot around for over an hour and those two things will drop your max TO weight to ~1500lbs from 2100. With a 650-700lb thrust motor you're thrust to weight is ~ 1:2. And that's pretty darn good. ;-)

    Better to design for a second seat and more fuel than you'd typically need up front, even if you don't need it initially only to want it later and having to redesign it in when it's already too far down the design and fabrication side of things.
     
  19. Jun 8, 2016 #79

    mcrae0104

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    I don't disagree with the conclusion, but this video is not evidence supporting it.


    • A wing is designed to support air loads; nothing more and nothing less. A wing that can support grown men jumping on it is probably heavier than it needs to be (regardless of its material).
    • Do Mike and Burt weigh the same amount? Did they jump to exactly the same height? Did they land the same distance from the edge support? No, no, and no.
    • Mike jumps on the wing before Burt. In fact, the composite section is already resting on the floor before Burt's feet touch it.

    While it may be good salesmanship, it's far from scientific and completely meaningless.

    jump.jpg
     
    Last edited by a moderator: Mar 20, 2019
  20. Jun 8, 2016 #80

    StarJar

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    My sweep change was for CG issues; the span and AR are more relevant. I also increased the root cord a bit. Those little tweaks added up, and helped a lot. At least on x- plane. (Albeit my design is an ultralight.)
    ....................

    An aluminum fuselage, and a CF wing is also another option.
     
    Last edited: Jun 8, 2016

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