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About Turbo

  • Rank
  • Birthday 05/07/1945

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  • ICQ rthrpwll@gmail.com

Profile Information

  • Gender Male
  • Location Columbia Gorge, WA
  • Interests Avid model C, windsurfing, hunting

Turbo's Activity

  1. Turbo added a topic in Avid Model C   

    Engine cooling
    My model c has the belly radiator and no provision for cabin heat.  There apparently have been several arrangements for in-cowl radiators.  The one that I currently favor has one radiator mounted behind the starboard-side motor mount strut.  This allows a  simple radiator mount, along with a very neat setup for cabin heat - just a little valve-box/dump on the firewall.  But man, that rad is darn close to the firewall!  Does anybody have this arrangement, and how well does it work?  What are the specs for the radiator?  It's clear to me that a good in-cowl arrangement will save drag and fuel over the belly rad.  I'm also thinking that instead of a fixed gill to exhaust the hot air out the bottom, a sort -of  variable cowl flap could reduce cruise drag while allowing for lots ot airflow for climb.  Of course it would likely make removing the cowl kind of a pain.  Gotta work on that.  I'm not too keen on the dual-rad-in-nostrils approach.  Vibration, and so many opportunities for leaks.
    • 2 replies
  2. Turbo added a post in a topic Solo bungee wrapping   

    I'm not convinced we all have this "double-bar" config under the seat.  I know the Avid doesn't.  The axe-handle approach won't work for me.
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  3. Turbo added a post in a topic INVERTED TAIL RIBS QUESTION   

    CL of 4 is possible with slats and flaps.  For a single-element airfoil the max is around 2.2 for Reynolds numbers of interest to us.  A former colleague, Bob Liebeck, used variational methods and boundary layer theory to develop a series of airfoils that establish what is possible.  Even worse than the 5-digit NACA airfoils, they have a very nasty stall..  They lose a lot with just a few bug-splats, and are not suitable for use on fabric-covered wings.   
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  4. Turbo added a post in a topic INVERTED TAIL RIBS QUESTION   

    There is an argument that highly cambered wing airfoils like those on the Avid STOL add to induced drag since they, like flaps, add to the wing's nose-down aerodynamic pitching moment, putting the tail in download.   Low-moment sections like NACA 23012 would require less download on the tail, which lessens load on the wing.  Moving the CG aft decreases pitch stability, so any comparisons would have to be made at the same static margin.  Dean Wilson apparently developed the Avid airfoils to use the front tube spar as leading-edge shape.  In my experience, this design compromise comes at a cost; cambered elliptical leading-edge shapes seem to do much better for max CL.  Is this why we see so many VGs on these birds?  BTW, I've seen much worse "airfoils" on other ultralights.  Almost anything reasonable will provide lift.  Even a flat plate can get to CL of 0.7!
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  5. Turbo added a post in a topic Blocking off the Oil Injector 582   

    So maybe those Rotax engineers used a plastic gear to drive the oil injection pump in anticipation of users failing to remove it when reverting to premix mode.  When the oil injection's piston pump eventually seizes due to lack of lubrication, the plastic gear acts as a mechanical fuse, failing, but allowing the engine to continue to run!  Smart, eh?
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  6. Turbo added a post in a topic Leaning the carbs for altitude - on the cheap   

    Fails?  This would be a fully manual system.  Clearly one would be monitoring egt, rpm, and coolant temp, as in normal operation.  Shutting the needle valve brings you back to the basic configuration.  At least with leaning we have some control over egt.  Perhaps I am missing your point here?
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  7. Turbo added a topic in Two Strokes   

    Leaning the carbs for altitude - on the cheap
    Someone mentioned the HACman system for leaning the carbs for altitude.  It seemed like a cool idea.  Digging deeper I learn that there once was an automatic leaning setup, but it is no longer offered.
    Digging even deeper, the explanations seem to be a little obscure, perhaps so we'll want to buy something we could easily cobble together on our own.  I hate to ruin the Hacman's day, but c'mon!  My understanding is that one takes the low pressure from the (initially plugged) tap into the Bing carb's throat, mixes it with another pressure source for some reason taken behind the air cleaner screen, and uses this "mixed" pressure to influence the flow through the jets of the carb via the float bowl.  Basically, you're messing with the float-bowl vent pressure.  Lower float-bowl pressure means less fuel flow through the jets.   Elegant and brilliant, no?  Well it seems to me that the float-bowl by default vents to the local pressure inside the cowling, whatever that is, so any pressure drop thru the air filter element is irrelevant.  Plus, if we only want a leaner mix, couldn't we just admit a teensy amount of air into the carburetor throat?  Take out the throat plugs, install hose barbed fittings, connect the two tygon tubes into a Y-fitting, then route the one tube to your panel, and install a brass needle valve from the hardware store, the other side exposed to the cockpit pressure.  Closed would be full rich, and then at altitude open the valve to admit some air directly into the carb throats to lean the mix to get the correct EGT.  Here, you're not even messing with float-bowl pressure.
    Now I've done no calculations, so it's possible that you wouldn't be able to lean the mix enough with this simple scheme.  In that case, we just use two needle valves, one of which connects to the carb throats as before,  and a second, which connects to engine compartment pressure.  We connect the two pressure sources together with another Y-fitting, the third leg of which then goes back to connect to the float-bowl vents (another Y required).  Now we're doing the HACman thing on the cheap.  We set the needle valve connected to engine compartment pressure to some nominal opening.  We will immobilize it after initial tweaking, maybe mounting it behind the panel where we don't normally even think about it.  We leave the valve connected to the carb throats mounted on the panel.  The panel-mounted needle valve is closed for full rich, and again is opened a bit if we want leaning, making the pressure applied to the float bowls something between engine compartment pressure and carb-throat pressure, depending on by how much we open the needle valve.  We would only have to play with the cabin-pressure needle valve in the beginning, in order to set an appropriate sensitivity for the panel-mounted valve.  Clearly it's not set right if full leaning at 10kft requires only 1/8 of a turn, or 5 turns.  In this case we reach behind the panel and adjust the other valve.  Again, EGT is our guide.
    • 4 replies
  8. Turbo added a post in a topic Model C ?   

    Fred, I must say I thoroughly enjoyed your you-tube videos flying around France!  In one there was a glimpse of your engine.  I wondered what it was!
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  9. Turbo added a post in a topic Trim systems for model C   

    I always liked that model A Ford window-crank trim on the ceiling of my old tri-pacer. Like opening & closing a sunroof!
    Bandit showed a pic of a ridiculously elegant trim setup in another thread.  I think that' the ticket!
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  10. Turbo added a post in a topic New elevator!   

    Gotta say: I like the elegance of Bandit's solution!
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  11. Turbo added a topic in Avid Model C   

    Trim systems for model C
    There appears to be no provision for pitch trim on the model C.  Using the flaperons for trim might work in cruise, where the q is high, as they can be used to effectively set the wing's moment contribution, but using them in the pattern at low speeds would have you trying to increase nose-up moment by further upwards flaperon deflection, which may not be possible.  You'd likely run out of trim capability.  At low speeds you would ostensibly want them deflected downward anyway to augment lift.  You can always slip if you want drag for a steeper descent.  At low speed, with them down you'd be way out of trim!  I figure I'll put a tab (bellcrank) on top of the elevator tube between the seats then connect it to a bungee or spring somewhere aft, the aft end of which connects to a large RC electric actuator.  This way, I'd be able to trim out the stick force for low - speed operations, yet still be able to command pitch changes with the stick.  The remaining question is what spring rate would be appropriate.  Guess I'll have to measure stick moment and play with it.  Any thoughts on alternative approaches?  I'm not too keen on tabbing the elevator, although the electric actuator could be placed out there ahead of the tab, I suppose.  Too ugly!  Then there's the perturbation to mass balance...
    • 2 replies
  12. Turbo added a post in a topic CG Excursion   

    I wonder if that 16.5" aft cg limit isn't set by requirements for the tri- gear version, not pitch stability?
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  13. Turbo added a post in a topic Cg formula   

    I wonder if the 16.5" cg aft limit is set by requirements for the tricycle-gear version, not pitch stability.
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  14. Turbo added a post in a topic Engine mount not on center line   

    A left-turning prop will pull to the right at high aoa, like sitting on the tailwheel, taxiing.  Maybe that's why the engine is offset to starboard.
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  15. Turbo added a post in a topic Different wings   

    Sorry, but dholly is incorrect.  Camber refers to curvature of the airfoil's meanline, midway between upper and lower surfaces.  Undercamber is concavity on the lower surface.  
    Back in my hang glider days we used the 0.058"wall thickness a lot since the tubing sizes step in 1/8" increments, and the next size tubing up or down could be used as external or internal doublers.  The 0.065 wall thickness doesn't quite work. My homemade Rogallo had 2024 t-3 doublers!   A great way to make a wing both strong and light is to step the tubing according to bending stresses, and make the rear spar more flexible than the front so under high loading the wing washes out, driving the airloads inboard.  On the Avids the little wood reinforcement inside the spars at the attach point is for crush/crimp prevention - very smart!
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