R/C <> Full-scale

Discussion in 'Hangar Flying' started by Topaz, Jan 13, 2010.

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  1. Jan 15, 2010 #21

    PTAirco

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    If only we lived on a planet with half the gravity and twice the air density, airplane building would get a lot easier....
     
  2. Jan 15, 2010 #22

    Norman

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    Gravity and density do scale. One mass is fixed so you scale the other one. Viscosity is a PITA but all it really boils down to is that the airfoil needs to be thinner at lower Re and Clmax is lower. At Re above 500,000 viscous effects aren't a big issue. Between 150,000 and 500,000 it's something to be taken into account but a competent engineer can get usable data. Below Re=100,000 viscous effects will screw up your data. Generally if you can carry the model in one hand it's too small for good flight testing
     
  3. Jan 15, 2010 #23

    autoreply

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    Which is probably one of the few things in aerospace that grows with the 4th power of all things we appreciate, volume, payload, speed :roll:
     
  4. Jan 15, 2010 #24

    Mac790

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    Another amazing power to weight ratio, this time in real plane, there is no title for this vid (I don't want to have problems with copyrights), I just uploaded it for you guys:). Any comments for 0.24-0.30, 0.52-0.59, 1.44-1.47?

    YouTube - 11

    Seb
     
  5. Jan 15, 2010 #25

    Dana

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    Other than that the pilot seems to like inverted snap rolls?

    -Dana

    New Yorkers like to boast that if you can survive in New York, you can survive anywhere. But if you can survive anywhere, why live in New York?
     
  6. Jan 15, 2010 #26

    Mac790

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    Wow, sounds like a piece of cake :), but for me 1.44-1.47 looks almost unreal, this plane has really high power to weight ratio.


    Seb
     
  7. Jan 15, 2010 #27

    bmcj

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    Wayne Handley's Raven (before the crash) would loop from the takeoff roll or go from hanging vertically on the prop to a controlled tail-first descent, then power up and climb vertically.

    The Raven was based on the Stephens Acro / Laser and was powered with a turboprop engine.
     
  8. Jan 15, 2010 #28

    Mac790

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    I've seen Raven, this one is nice too.

    http://www.youtube.com/watch?v=swhSLbQHL74&feature=channel

    Seb
     
  9. Jan 15, 2010 #29

    autoreply

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    Isn't that a Lomcevak?
     
  10. Jan 16, 2010 #30

    Dana

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    Yes, though he's not really doing the full tumble you expect of a Lomcevak... which is basically the transition between a climbing outside snap roll and and inside one in the opposite directon (or vice versa).

    -Dana

    And they shall beat their swords into plowshares, for if you hit a man with a plowshare, he'll know he's been hit!
     
  11. Feb 26, 2010 #31

    MicRuler

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    How much HP does it take to make a Cri-Cri hover vertically? :gig:
     
  12. Feb 26, 2010 #32

    lr27

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    The entire wings do NOT pivot on that indoor plane. Only about 40 or 50 percent! I think the rocking at 10 seconds is more from pushing air around than from the weight. Although that guy is very talented, I've seen a few more graceful performances on Youtube. There's an eceptional one from, I think, a competition in Germany. The guy has long ribbons on each wing tip and can generate a nice spiral. The guy here is using a plane with no airfoil, just flat plate. This is a lot easier to make but it gives up on some lift. OTOH, an accomplished flyer of these things told me that the sharp l.e. leads to a more predictable stall break.
    Some somewhat more graceful stuff:



    That 0.17 lbs/ft^2, or perhaps just a little higher, can start to seem kind of high indoors. And that's not even with an aerobatic type, and with a cambered airfoil. You have to think way ahead.

    The models often have 150 percent static thrust to weight. I bet that first full scale guy has a bit less than 1, but manages the momentum very well. That's an advantage that heavier planes have. Another model thing is to use a variable pitch prop and back up in flight. I don't know how they keep it from flipping over, although I think in this video they say he uses thrust vectoring. If you're impatient for the beta pitch, go to 1:53, but the whole thing is worth watching:

    I think his style has gotten a bit more abrupt than it was, but still precise.
     
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  13. Feb 26, 2010 #33

    Norman

    Norman

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    I haven't converted it to hp but the power to weight ratio to hover is somewher between 130 and 150 Watts per pound. Most GA airplanes get by with les than 80 W/lb. The good old J3 Cub is about 45 W/lb
     
  14. Feb 27, 2010 #34

    lr27

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    The power to hover depends on the weight versus prop size and efficiency as well. There are rubber powered model helicopters, after all, and when I was a kid I used to have rubber models that would go straight up. I don't think they were 150 watts per lb. One of the Rotorway helicopters gets by with 70 watts per lb. Of course if you pitch forward and make the prop axis horizontal it doesn't fly so well. ;-) But it shows just how inefficient the little guys are at hovering. Actually, I have a little 7 oz foamy that can almost hover when the battery is fresh with perhaps 90 watts per lb.
     
  15. Feb 27, 2010 #35

    MadRocketScientist

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    I estimate around 80HP or more with the std size props, with larger diameter props this would come down substantially.

    Shannon.
     

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