Turbo

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About Turbo

  • Rank
    Advanced Member
  • Birthday 05/07/1945

Contact Methods

  • 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 post in a topic Spar Stiffener reinforcement   

    Really, with the huge differences in strength and stiffness between plywood and aluminum, these inserts are really spar crimp preventers, not stiffeners.  Their real role is to prevent compression buckling on the top of the spar tube under high G conditions.  If the spar tubing thickness is 0.049", one could use the next tubing size down (1/8" smaller O.D.) or an internal doubler to accomplish the same thing.  But really, all you need is something to immobilize the top of the thinwall spar at this max-stress span location.  -Turbo
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  2. Turbo added a topic in Technical tasks   

    Tool for "easy" bungee install
    The pictured tool greatly facilitated the installation of new bungees on my Avid.  The tool is a wooden gantry that stands on top of the seat tube, is stabilized above by a strap to a structural tube, and supports a block and tackle with 6:1 purchase.  Additionally, a small fairlead and clamcleat are used to quickly secure the line.  For line I used green paracord.  I used parts from my collections of yahtie and windsurf stuff.  In order to grip and stretch the bungee, I used a small camcleat mounted on a bent piece of 1/8" aluminum.   In mounting the bungees I was able to pull each wrap tight with the block & tackle setup.  Vicegrip pliers were particularly useful for clamping already-tensioned bungee cord to the landing gear on the bottom.  They held well and were easy to put on and take off.  For each wrap I tried to stretch the bungee as much as I could, so that in the final wrap the end loop would start off already through the aperture in the bottom of the airplane.  I then simply fed a 12-15" piece of paracord thru the eye of the bungee, tied a simple loop knot in the other end, and attached it to the block.  After lifting with the block & tackle, and maneuvering it onto the hook with a screwdriver, I simply untied the loop and pulled the paracord piece out of the eye.  Done!  Solo! (Jimmie Durante would wag his substantial nose and say "hotcha!".)
    Some other comments are in order.  Out of curiosity I stretch-tested a plain hardware-store bungee (Installed on the plane by a previous owner) for comparison with the Mil-spec Type-1 3/8" bungee cord sold by Aircraft Spruce & Specialty.  I was gobsmacked at the difference!  The Mil Type-1 bungee is about 3X stiffer, and will go up beyond 120 lbs per strand, whereas the hardware store stuff more-or-less stopped stretching around 35 lbs.  The Mil Type-1 stuff will stretch to double its 0-tension length.  Whatever you do, DON'T USE THE HARDWARE-STORE STUFF!.  My calculations indicate that 7 wraps of the Mil Type-1 bungees can handle a 3-G landing at my TOGW of 916lbs, but the outward gear deflection is almost 19 degrees from the 0-G gear position,  and the average bungee cord stretch is more like 5 inches!  This analysis even modeled the inward roll of the contact patch relative to the wheel as the gear splays out.  I was unable to stretch either bungee to failure, but would love to know how much tension they can take!  Online one can find many purveyors of different kinds of bungee.  If you insist on wandering off of the righteous path, I heartily recommend stretch testing, followed by analysis.  Avid's recommendation for the safety cables is to allow 3".bungee deflection.  I'd go to 5", or maybe leave them off like Joey did, just adding a bungee inspection to my pre-flight inspection list.  If you, like I, bought your bird used, I would definitely take a careful look at the LG bungees.  Are they single-flecked (hardware-store) or double-flecked (type-1)?  Is there adequate deflection capability in the safety cables?  (My airplane was a double whammy on these questions, and I needed to have my seat tube repaired after a hard landing.)



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  3. Turbo added a post in a topic Wing Testing   

    Interesting in that those wood pieces are really not stiffeners, but tube crimp preventers.  The curved steel attach plates for the wing struts likely serve the same function anyway.
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  4. Turbo added a post in a topic Solo bungee wrapping   

    Great idea, Fred!  Unortunately, my nearby friends tend to be little old guys like me.  I have, however found that by far the best tool for holding the tension on wraps already made is a pair of vise grip pliers!  I have an installation tensioning tool that shows promise too, but I need to first validate that my loop terminations will not let go at maximum extension.  If successful, I'll share some pics.
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  5. Turbo added a post in a topic Solo bungee wrapping   

    Hi Fred,
    The type-1 bungee from AS&S is exactly the one you mention, 06-12400.  I am thinking of making a tool to help with the installation.  I know I am no way strong enough to install them unaided.  If the tool is successful I will share it, as you did.  
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  6. Turbo added a post in a topic Solo bungee wrapping   

    Bought type-1 bungees from AS&S, made up 90-in sections with loops on ends.  Marked off the 7 increments.  Tried to stretch the first segment to the mark.  Couldn't do it!  For this old, not-too-strong guy it seems impossible!  Especially with no real way to hold the stretched segment in place.  I am about to punt, and go buy non-type-1 bungee from the hardware store.  It's not nearly as stiff.   Altrnatively, I could put on fewer wraps.  The Canadian's axe-handle trick doesn't work on the Avid.  What have others done here in terms of mechanical advantage & holding?  I think I'll stretch-test the type-1 stuff to see if I can get away with 6 wraps instead of 7. 
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  7. Turbo added a post in a topic L.G. bungee blues   

    I've got to admit the analysis is incomplete, though.  I just got type-1 3/8"bungees from AS&S and they feel significantly stiffer.  I wonder how much tensile preload is built in to those bungees, and at what length ratio do they stop stretching.  Also, it would be reassuring to test my end loops to failure.
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  8. Turbo added a post in a topic Wing Testing   

    Interesting that the brochure for the earlier STOL models have a Vne of 97 mph, and the sandbag test gives a load factor below yield of 5.9 Gs.  If ias is close to eas, and stall is at 40 mph ias,  guess what?  The Vne is really maneuvering speed!  Yes, Virginia, you can go faster in smooth air!
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  9. Turbo added a post in a topic L.G. bungee blues   

    JimChuck, you will likely have to drill some holes along the tops of your seat truss plates for the seat lacing ( maybe you already have!)  I was thinking of putting some high-quality plywood in there, using zip-ties along the outer edges, letting the bungees contain it inboard.  On my bird, the original builder put in split rubber tubing as a buffer on the bottom seat truss bar, but used small hoseclamps to hold the rubber hose sections in place.  This abraded the bungees' outer fabric sheath, and strikes me as a no-no!  I suppose rubber cement is about all one can do, as even zip-ties would likely cause abrasion.
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  10. Turbo added a post in a topic L.G. bungee blues   

    Oops!  In my enginerdly focus I forgot that a loop or two lie on top of others,increasing the preload and likely nulling out my 0.4 inch greater maximum deflection I got over the build manual.  It is important, however, that the cables allow full stretching of the bungees first, before coming into play.  I would consider the 3" deflection allowance as a minimum, and make sure my cable-stops were a little longer.  They were too short on my bird.
    And yes, I did hard-land my bird.  It's in the weld shop getting fixed.  Uggh!
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  11. Turbo added a topic in Technical tasks   

    L.G. bungee blues
    I was in the process of replacing my bungees, and thought I'd do a stretch test with the old ones to see how bad they were, and check out the engineering behnd their use as the landing gear springs.  In my stretch test I used a fish scale and tape measure to determine the spring charactrristics, Force vs. Length.  I zip-tied one end and looped the other around the fish scale's hook, then stretched it with a boat winch.  What resulted was a sideways S-shaped curve, showing what anyone who has used a slingshot knows: that a rubber spring eventually stops stretching at some load, even as load is increased.  The Hook's law spring constant is high at the low-force end, but also becomes much higher at the high-force end, and is lower, but slowly increasing in the midrange.  Weird, but likely related to  the molecular structure of rubber.  
    Knowing that a spring's stiffness is inversely proportional to length, I then applied the stretch data to the loop geometry of the Avid's landing gear.  It's convenient that in re-wrapping the bungees, the force required is totally manageable, due to the 7 wraps used.  Of course this means we are applying a preload.  At this point we are stretching the bungees to 132% of their no-load length.  Of greatest interest to me was the realization that the bungees stretch up to 172% of their no-load length.  It is here that the bungees stop stretching, even as force is increased further.  Half of the bungee-centerline wrap distance is 8.5", and with 90" no-load length and 7 wraps (14 strands) each strand's no-ĺoad length is 6.43".  Stetching stops at 172% of no-load length, so full extension should be reached at 3.4", not the 3" prescribed in the build manual!  The safety cable is only there to prevent total collapse, but if it's shorter than full extension, you risk bending &  crimping the thinwall tube under your seat; the tube the bungees wrap around, if you land too hard.  The bungees wouldn't be able to absorb the energy before the merciless cable comes into play!  That cable applies a point load to the tube, not distributed like that of the bungee.  
    So those are the results of my analysis.  Am I nuts?  I'm sure others in the group have thought thru this as well.
    Making up my new bungees, a special thanks is due to Fred, who showed an elegant approach!
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  12. Turbo added a post in a topic 1st flt Avid - impressions   

    Checked wing incidences with inclinometer & found them to be equal to within 1/10 of a degree, the precision limit of  my tool.  Also found an area on the port aileron where the trailing edge had been bent up, likely hangar rash.  That could explain the roll moment.  Bent it back down as best I could.  We'll see if I got all of the roll moment out.  This may take several iterations.  Turns out I have the older f7 mixer arms.  No matter; Just need more airtime.  If my new, stiffer trim spring works with some flap, I'll be tickled silly!
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  13. Turbo added a post in a topic 1st flt Avid - impressions   

    The Junkers- style flaperons have the advantages of the CP peak reduction of a slotted flap if rigged correctly, and of not inheriting the wing's tired boundary layer, but at our scale tend to develop their own wimpy laminar boundary layers, which limits their max CL,  and contribution to roll authority.  It may be possible to trick them into early transition to the much more robust turbulent boundary layer with an appropriate level of surface roughness, which is likely quite fine.  Teensy VGs could also be effective in this, but it's relatively easy to overkill the problem and make things worse.  In any case, there's only so much even a slotted flap ( in our case as an aileron) can do.  Having studied the kinematics of the mixer mechanism, I was somewhat unimpressed to see greater down deflection than up deflection as flaps are lowered.  I think there is a cure, with a different mixer geometry.  Leni says they overpower the elevator at the higher deflections anyway, so maybe it's a moot point.  Maybe they are just the ticket at 15 degs as-is.  I need more experience flying the bird.
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  14. Turbo added a post in a topic 1st flt Avid - impressions   

    I had the carbs apart and they are clean, including the idle jets.  The floats had sunk, so I had to buy new ones.  Gotta look to see what size those idle jets are, though.  Joey flies out of somehere near Fairfield, which has to be less than 100 ft elevation, so maybe the #50 idle jets is a good call right out of the box.  I too will be flying out of fields at less than 1000 ft elevation.  Totally agree re static idle rpm vs on final - part of why I had a tough time getting the bird down.  Trees at approach end of runway & short strip - bad combo for green TW pilot in unfamiliar bird.  Smooth at 1400 static is my new goal!  Next time The Dalles with 5000 ft, 100 ft wide tarmac!  Just loved putting around the verdant Hood River valley & hills, though.  This is why airplanes have such magic.
     
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  15. Turbo added a post in a topic 1st flt Avid - impressions   

    From y'all's comments, I must have come across as whinging.  My bad.  It may be the curse of being an old design engineer to always think design, and it makes one perhaps a bit more critical.  The Avid is what it is, and hey, I bought it!  I like the little bird, and am looking forward to flying it, and working to improve it where I can, like everyone else on the site.  My first flight was more exciting, almost scary, than I anticipated, but that will change as my mastery of the beast improves.  I have windsurfed the Gorge in winds gusting to 50+ mph, so I know I can do this, especially with all the experience y'all bring to the site, and for which I am grateful.  Obviously what I need is lots more time-in-the-cockpit.
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