Gotchas with the Carmichael racer

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

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  1. Jun 15, 2019 #41

    BJC

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    My bet is that he presented a strictly aero concept. The pylon structure with the engine embedded in the wing will be a challenge to keep light.

    Anyone know how the blunt aft fuselage / pod of the Ultra Bat affected the propeller?


    BJC
     
  2. Jun 15, 2019 #42

    bmcj

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    Wait... how did he do that AFTER he died?
     
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  3. Jun 15, 2019 #43

    plncraze

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    When...not after LOL. I meant before.
     
  4. Jun 15, 2019 #44

    Hephaestus

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    Cabane struts enclosing front and rear spars, tie in down to the tube.... swapped to a full vw as the 582 upright just looks wrong.

    Not super elegant but mechanical simplicity - going from bottom of engine to top of pod leaves 120mm +/- exposed...
    carmichael1.png carmichael1-side.png
    Not exactly as Carmichael designed it - but simple, then shroud it in as nice as possible,
     
  5. Jun 15, 2019 #45

    Vigilant1

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    I dunno. There are a lot of parts and intersection drag in that cabane/TE/engine fairing assembly, it's hard to believe it would be better than a conventional solid cowling on the aft end of the pod.. I suppose a small model and a wind tunnel (water tank?) might give you some information.

    Working on that engine would be a nightmare, even something as simple as changing the plugs. Maybe this would work okay for an electric motor.
     
  6. Jun 15, 2019 #46

    Hephaestus

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    Depends how you look at it. I was just eyeballing raising the vw so underside was cleaner. Give 6" to side for maintenance (?)

    Could also go with 2 cabane struts in tandem... Opens up access a bit more.

    I'll have to look at f3d's cfd, I know some basics can be resolved in there. But like ronczs spreadsheet - will it handle laminar flows? I don't know... (That said 126 cruise for Carmichaels design)
     
  7. Jun 16, 2019 #47

    Hephaestus

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    Well if you scale Carmichaels duct/pylon... It works to 3" walls, fairly substantial air intake... Going to see if f3d will play nice now and import a vw motor model, see if I can get a better idea than my rough blocked out aerovee version.

    Yeah I stupidly scaled the fuse in Z to test this idea and everything got wonky. However notice the engine bump down is basically at the same spot the fuselage starts to drop away...
    carmichael2-pylon.png carmichael2-pylon2.png
     
  8. Jun 16, 2019 #48

    BBerson

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    The rule about fuselage max width continue to wing trailing edge mostly applies to low wing. The high wing is not that critical regarding interference drag.
    But if you put your engine on top of the wing thats like a big dive brake.
    No free lunch.
     
  9. Jun 16, 2019 #49

    Hephaestus

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    Well ignoring the vdub bits that should disappear on an aero conversion... Fit would be pretty decent. Pretty close to the original drawing.

    [​IMG]

    carmichael2-pylon3.png carmichael2-pylon4.png carmichael2-pylon5.png
     
  10. Jun 16, 2019 #50

    Vigilant1

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    The single pylon is probably less draggy than the twin cabanes. The pylon and down into the pod would be a pretty good spot for fuel (close to the CG). I just can't get past the access issues: Everything will need a ladder or at least a step, and with the engine surrounded by wing, getting to the front (accessory case?) and sides won't be very convenient. I'm sure seaplanes make it work, but my back hurts just thinking about trying to adjust the valves.
     
  11. Jun 16, 2019 #51

    Hephaestus

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    Crank sits at 6' on short little retracts. So I guess that depends on your height to some extent. If I follow Carmichael's drawings - there's a lower and upper cowl to remove for access and you can see there's a fair bit of room in there for the engine mount and rear spar carry through.

    Step-over height to get in shouldn't be bad, 36" as drawn right now. Going electric retracts on the nosewheel could reduce that as well - might involve a rethink on gear placement.
     
  12. Jun 17, 2019 #52

    Hephaestus

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    Forms suck so much in f3d! 3...4 days of playing intermittently...

    carmichael3.png

    Main gear like a wildcat - right at the back of the pod, mounts off the boom, kicks them wider than taller, wheel axle mounted between a-arms

    [​IMG]

    Yes just working out ideas, see where it goes, need to see if there's an engineering student who wants some summer calculations for beer money ;)
     
  13. Jun 18, 2019 #53

    Hephaestus

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    I found a PDF with the first part of that article (it was 2/2 apparently)
     

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  14. Jun 18, 2019 #54

    Victor Bravo

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    A guy named Doug Crane (Crane's Planes) owns the one intact J-5 Marco that I know of in the USA. I exchanged an e-mail with him years ago. His solution for minimizing drag is that he wants to bury the engine behind the pilot, and run a narrow belt drive or something up the pylon to the propeller.

    This would allow the lowest drag, and get rid of the greatest amount of intersection interference and bad flow and bound vortices and influenza that is possible to remove. At the mechanical expense of having to troubleshoot a tall belt drive.

    If your goal is to achieve the least drag for this configuration, then dealing with the buried engine (and everything that goes with that) is definitely going to give you the least drag for the pod and boom pusher configuration.

    I am a big fan of the J-5 and J-6. A small electric motor in a well shaped fairing behind the pilot, with no engine, fins, exhausts, pots or pans hanging outside... and powered by an engine driven alternator buried in the center section... seems like it could work well enough to justify the complexity.
     
  15. Jun 18, 2019 #55

    BJC

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    Is that project still active? (I hope so.) I have checked it’s status over a number of years, but have seen no progress. Note that the original was well faired, and a belt PSRU moved the propeller up and away from the boom.
    It does look like a long belt. Do you know if he has run it?
    Agree. Cooling of the engine and the surrounding composite structure is going to be problematic.
    Great minds think alike.


    BJC
     
  16. Jun 18, 2019 #56

    Hephaestus

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    Possibly. Not quite sure I'm ready to do a full hybrid electric drive yet. That would open some interesting options however, but create a massive amount of new ones. That said - it could solve some wing issues here too, dual generators with a little battery capacity - two to make take off power, drop back to one for cruise, redundant power available if needed. But how many have been done? Versus cutting a cheque to aerovee and making parts?

    I'm just playing with this version right now. (Call it more CAD practice) I do like the overall shape so far, the drawing definitely made it look worse than reality.

    But I may start to do the math on it, nothing looks really impossible/overly difficult on it right now. Besides getting laminar smoothness - which would be a challenge any which way.
     
  17. Jun 20, 2019 #57

    Hephaestus

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    Question of the day.

    Just caught onto the engine placement on the original drawing.

    [​IMG]
    Bruce drew it considerably farther back than I have. I've reworked w/b a few times... I can't decide if he's drawn as a 2 place or perhaps pilot weight farther forward? I've got cockpit drawn with my weight basically underneath the leading edge.

    Now I'm curious - is it perhaps aerodynamic reasons that he's pushing the engine back so far on an extended pylon? I was more leaning towards keeping it tight to help maintain laminar flow over the rear part of the fuselage.

    Any theories?
     
  18. Jun 21, 2019 #58

    Tiger Tim

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    Without reading up too much on it, I would think Carmichael designed it to be a Formula 1 racer. That would make it single seat, O200 powered, have some set wing area, and tiny. It could just be that the engine is tucked up as close to the spar as it could get.

    I dig those cool clamshell wheel pants.
     
  19. Jun 21, 2019 #59

    Vigilant1

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    I think you may have guessed right about aerodynamics possibly being his rationale. I recall reading that the Vmax Probe went with a prop on the tail because Bruce urged that the prop be as far back from the >wing TE< as possible. So, maybe this was his attempt to do that, but just on a less extreme scale.
     
  20. Jun 21, 2019 #60

    Hephaestus

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    I'm also not designing for tight high g turns, so splitting the rear spar and carrying the load through/around the engine mount seems reasonable.

    Pushing engine back gets a quieter cabin, but more weight to cantilever it off the wing as well, but I could also run support up in the rear of the pylon. ... ... More thinking required.

    Still working details, increased wingspan and area a touch, reworking the pylon again, playing with the engine cowling. Still working main gear mechanism hard to get it to fit tight up top
     

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