Gotchas with the Carmichael racer

Discussion in 'Aircraft Design / Aerodynamics / New Technology' started by Hephaestus, Jun 12, 2019.

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  1. Jun 21, 2019 #61

    rotax618

    rotax618

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    9CCD66E6-2957-4B84-9B62-620749D69003.jpeg 0383A1FD-E7C2-4DB8-A576-71DA2BDD42E1.jpeg Seems to me that the least drag is to place the engine below the wing, behind the pod, as With my Boorabee
     
  2. Jun 22, 2019 #62

    Hephaestus

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    But how would one possibly get the boorabee up to a 200kt cruise?

    At lower speed it works, but as you probably know you've already created one of the fastest pod and boom pushers.

    Partly why I started looking harder at the Carmichael design as on paper it should be able to achieve the target speeds.
     
  3. Jun 22, 2019 #63

    rotax618

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    No the Boorabee is 100kt max on 70hp, but its not the drag from the engine that is the inefficiency, the wing section is 13% and the wing is fabric covered. None of the flying surfaces are mass balanced and the tail is cable braced.
    If you want to go fast I suggest you look at the Lesher Teal configuration, it is very difficult to get a smooth low drag airflow around a short pod with a prop behind it, no matter how the engine is cowled.
     
  4. Jun 22, 2019 #64

    BBerson

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    Or Molt Taylor's Mini-imp.
     
  5. Jun 22, 2019 #65

    pictsidhe

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    So, a pusher that is smaller and faster than a Long-EZ? That could be Vari EZ!
     
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  6. Jun 22, 2019 #66

    Hephaestus

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    Lesher teal and mini imp, are slower than my Mooney. Also have that major long driveshaft issue. Varieze is no faster either.

    Like I said first post, single place, goal is ultimately speed, faster the better. Not much interested in the lancair (don't fit) and needing 540 power and fuel burn to do it. Yeah there's something to be said for taking a wittsv or sonerai and adding range tanks. But again don't fit. And with the changes it's a whole new aircraft again so why not blank page

    Carmichaels racer you've got the underslung pod so you can get the cleaner aerodynamics, that's all based around keeping laminar flow. So as long as it's built correctly - should be lowest drag possible.
     
  7. Jun 22, 2019 #67

    Hephaestus

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    *chuckles* kc135 wingtips. Needed something there this was easy ;) carmichael6.png Need to rework pod again to mate with the 250% eppler pylon.
     
  8. Jun 23, 2019 #68

    BBerson

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  9. Jun 23, 2019 #69

    mm4440

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    It is hard to avoid a lot of sanding in the production of laminar airfoils for a one off airplane. Even sailplanes that come out of molds are not perfect and often need hand work to achieve max performance. Getting clean airflow into a high efficiency prop is the other big challenge.
     
  10. Jun 23, 2019 #70

    Hephaestus

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    That I'm unfortunately aware. Watched and helped more than one glider pilot spend season after season trying to get the hump at the spar to disappear from the micrometer measurements.

    Have a theory there for the wings at least. Just need to find the right optical non contact micrometer system.

    Since it's a digital design, and it'll be a physical layup. It's theoretically possible to scan, compare to ideal and use an orbital da sander in place of a cutting spindle. Believe I saw something similar on a video tour of a Lockheed facility once.

    Still assures hours and hours of sanding to perfection.
     
  11. Jun 23, 2019 #71

    Vigilant1

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    A general question: I assume that all this sweat, swearing, and aggravation Is expended while trying to get the airfoil surfaces right relative to each other (i.e. no waviness, bumps, dips, etc) and thereby avoid tripping the boundary layer. There's no attempt to make the actual wing match the chosen perfect "as drawn" airfoil to within 1 Angstrom, right?
     
  12. Jun 23, 2019 #72

    Hephaestus

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    I clearly need more coffee to understand your question. Lol
     
  13. Jun 23, 2019 #73

    Hephaestus

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    Angstrom being a unit used to measure separation if electrons and other subatomic particles. Nope not even going to try.

    But yes, it's a laminar design, there's going to be a metric sh*tton or sanding involved.

    I'm hopeful I can use some of the cnc equipment we already have, add a 4th axis, a DA sanding /polishing head and some scanning hardware and software so it's not years of pure sanding alone. With the z on my MPCNC currently at 400 steps per mm (salvaged a fine thread leadscrew for it when I built it) fair bit of work with guide coats and filling, should be able to do better than i can by hand.
     
  14. Jun 23, 2019 #74

    mm4440

    mm4440

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    I think Bruce said +/- 0.002". How critical varies with the design of the airfoil and the position on the airfoil. Some airfoils include features to trip the boundary layer at a fixed location to minimize drag.
     
  15. Jun 23, 2019 #75

    Hephaestus

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    And using the nlf0115 gives us a bit more as part of the nlf design goals were to be more tolerant of surface contamination than the 23015, which was already more tolerant than Carmichaels original choice (trying to place that one now, where the hell did I put it?)
     
  16. Jun 23, 2019 #76

    Hephaestus

    Hephaestus

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    It's a good thing my new glasses arrived.

    Trying to sit with raymers and start working with some real numbers, already going cross-eyed. Lol.
     

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