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Everything posted by Turbo

  1. 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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  2. 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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  3. 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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  4. 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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  5. 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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  6. 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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  7. 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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  8. 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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  9. Turbo added a post in a topic 1st flt Avid - impressions   

    Thanks for all the tips & info, guys!  Leni's comment re running out of elevator makes me wonder why Dean Wilson didn't start with the low-moment NACA 23012 and just find an acceptable LE shape mod to keep the high CL max while using a 2.5" radius.  My experience as an aerodynamicist holds that undercambering the undersurface can help up near the L.E., but further aft does almost nothing for max CL, while adding undesirable nose-down moment, especially in the aft end, near the T.E.  Oh well!  Maybe the best place for VGs is the H-stab's undersurface.  A flat-plate stab has got to have pathetic aerodynamics.  If only there was an easy way to seal the gap beteen stab & elevator.
     Thanks, Joey for the encouragement.  I will play around with idle mixture a bit more.  It's nice to know that smooth, slow idle is possible!  The internals of the E-box include a torsional softener like the C-box has, right? 
    Looks like I'll be making up new bungees.  Merci to Fred Stork for posting a great way to fab the end loops.  Love your videos flying around France in the springtime!
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  10. Turbo added a post in a topic 1st flt Avid - impressions   

    Again, the flaperons:  I was not too happy to find that the flaperon mixer geometry is such that, at higher flap deflections, the flaperon's down deflection is greater than the other side's up deflection.  Maybe I'm all wet here, but I think there's a simple way to reverse that nasty characteristic.  This would enhance safety and allow greater flap deflections for landing.  Wouldn'it be nice to have flaps we could actually use?  Currently, I'm unclear as to the advantages of these Junkers-style flaperons over the more conventional flaps & ailerons, aside from being easy to build.
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  11. Turbo added a post in a topic 1st flt Avid - impressions   

    I am planning to adjust the port wing twist.  Will use inclinometer as you suggested, for how much.  I have a series of springs of different stiffness to try for the trim.  Right away I'm going to the stiffest.  Ship wanted to dive; not enough decalage. I was only holding about 1/2 " left stick, so it's not too far out of rig.  Rough running was only on the idle jets, less than 3000rpm.  I was close to the Rotax " one turn" setting, so carbs were rich at idle.  I think I'll play with them some more before ordering a clutch, since engine-out glide is a safety consideration.  I wonder if this roughness is a C-box resonance thing.
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  12. Turbo added a post in a topic 1st flt Avid - impressions   

    Oh yeah, the flaperons:  I did not use them.  Previous owner & builder did nothing for trim, and this first flight I was also test-flying a first iteration version.  It didn't work, so I was wthout trim, which makes holding airspeed not-so-easy.  On top of this, I was holding left-stick to stay level, i had a cheezy com system, it was saturday, with traffic, including glider ops.  Hood river is a short, narrow strip, so I may have bit off a bit too much!  But what a nice, smooth, calm day!  Suffice it to say bird & pilot survived!
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  13. Turbo added a post in a topic 1st flt Avid - impressions   

    I've got the Maule tailwheel, which doesn't give the wings much AOA on the ground.  It seems the rough idle is due in part to some kind of engine-prop resonance.  It seemed like it wanted to shake unplugged all my instruments!  Is this a C-box only phenom, or do the other boxes have torsional impulse softening rubbers?  If your opinion is that the clutch is worthwhile and manageable, I reckon I'm in.  The 582 seems a robust little engine, if one obeys the rules:  1. Don't take off before achieving operating water temp,  2. Keep egt between 1000 and 1200 Fahrenheit, 3. Give it a little time to cool back down, and 4. Keep an eagle eye out for coolant leaks.  Seems simple enough.   Leni, thanks for the encouragement.
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  14. Turbo added a post in a topic 1st flt Avid - impressions   

    Thanks for the words of encouragement, JimChuck.  Mine is the C model, with empty weight around 470lbs.  After banging it in trying to land slow, I settled on 55mph for final.  Seems to me a clutch will give a big step between high windmilling drag and positive thrust with the clutch engaged, with no in-between.  Without, and with static idle set at 2300 rpm roughly, idle on final gave me a rough 3000rpm.  After touchdown, it didn't want to slow down!  I'm glad the brakes are weak enough to keep me out of trouble, since I had to use them to slow down.  I expect a clutch would help there, too.  The IVO prop has a sharp leading edge, which I expect would help reduce windmilling drag since the prop's backside would be stalled when windmilling. At least them little airfoils are stalled!  For a bird that small, that prop seems a monster!
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  15. Turbo added a topic in Close Call's and dumb stunts   

    1st flt Avid - impressions
    Celebrated Cinco de Mayo 2018 with first flight...yowtza!  Mine, N23PB is STOL taildragger with grey-head 582, 3:1 C-box, swinging 72" 3-bladed IVO.
    I've got to admit all this talk about groundloops had me fearing 1st flight, wondering if I too would immediately fall prey.  The good-news takeaway for me was the realization that the Avid has plenty of rudder authority on landing!  The one-person 25mph stall speed is pure myth.  Stall IAS showed up around 40 mph, with no steady-state stall per se, just a controllable mush.  Engine was smooth (above idle) and powerful, and P-factor is reminiscent of WW2 warbirds with their huge props. I found myself sideways on steep climbout before applying the required left rudder.  Just putting around the sky was great; down to 4000rpm I could hold altitude just fine.  4500 seemed a good cruise rpm.  In 1.8 hrs I burned just 5 gallons!  According to the Rotax data, the engine should be burning 3.3 gph at that rpm. Thermal efficiency is pretty good there as well - just below 30%!  But it goes down with rpm - to just over 20% at 6000 rpm.  I'll post fuel flow & thermal efficiency data in customary units if desired, translated from Rotax data in the metric system.  
    I could not do a graceful takeoff!  The bird ran along the ground, then ballooned up suddenly!  I clearly need practice!  Landing proved way more problematic.  The bird just didn't want to come down, and I executed a number of go-arounds.  The short, narrow runway demanded more of me than I expected: my landings were awful!  I will go elsewhere to practice!  And practice I must!  This bird was demanding, and I fear grace may prove elusive.   Hats off to you guys who fly her well!   
    Shaky Jake has nothing on the 582.  The cabin noise & vibration at idle (below, say 3000 rpm) were horrible.  I am contemplating a clutch to help with that, but need some inputs from those who have gone there.  What does a freewheeling prop do to landing approach?  How are taxiing and landing runout affected?  All-in-all, it was a very exciting day!  - Turbo
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  16. Turbo added a post in a topic Idle jet   

    I have the grey-head 582 with C-box 3:1.  The engine is smooth from about 3k up to wot, but the idle is rough.  On final, what would be 2300 rpm static is now 3000 rpm, and rough.  I have read lots of opinions on how to adjust the idle air screw, from 1 turn open, per the factory, to going leaner.  When I tried leaning it out, the roughness never left, but eventually the engine wouldn't run.   Where else should I be looking?  My engine only has 141 hrs on it.  What's the deal with the clutch?  Is it a centrifugal?  Does the prop not turn at idle?  How does that work out when taxiing?  I always thought the prop also serves as the flywheel.
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  17. Turbo added a post in a topic Compression numbers ?   

    Oh, and on the compression check:  I only got 90 or so on the first pull.  Apparently it's o.k. to do multiple pulls to get that compression number.  After 4 or so pulls I got my final numbers, almost 120 psi.  Maybe that accounts for some of the diffs we see between good-running engines.
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  18. Turbo added a post in a topic Compression numbers ?   

    Egp8111, I'm a bit the newb here, but isn't 6500 static a bit high?  From what I understand, 6200 to 6300 is the rpm range to shoot for statically, so when you're moving thru the air, and the prop is unloaded somewhat, you aren't in danger of exceeding redline.  Now, I may be idiologically mistaken, comrade, but I see the compression test as only a crude indicator, since the engine in operation is turning way faster than even an electric starter can crank it, so any leakage past the rings will be fractionally way less.  I have read that worn-out 2-stroke engines become hard to start.  I'm sure there are folks on this site that have way more 2-stroke experience than I do.  What other symptoms say a 2-stroke engine is ready to suddenly stop running due to wear?  That IS the main concern, right?
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  19. Turbo added a post in a topic Compression numbers ?   

    Comparing both carbs, all the floats seem to be low riders!  Dried and shook them, and none had fuel sloshing inside.  One float bowl had the requisite half-inch level from the rim, the other less.  I am thinking that I just need to "rebuild" the bad carb, and of course clean them both.  Seems like the float-bowl level cutoff valve is the culprit for the bad carb.  Could just have a tiny piece of debris on the seat.  Sorry to have misappropriated the thread!
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  20. Turbo added a post in a topic Compression numbers ?   

    Mine tested at 115psi and 118 psi.  Only one problem.  My bowl floats are sinking!  Holy Moly are they pricy little buggers!  Is there another way out here?  Are the Bing 54 carbs used on snowmachines or something without wings?
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  21. Turbo added a post in a topic Compression numbers ?   

    O.K., so what to do if you have the pull-starter?  Hard to get a consistent compression value if you can't crank at a consistent rpm.  Facing my first conditional with a guy who doesn't have experience with 2-strokes.  Hopefully demonstrating 6200 rpm static full throttle will do, along with appropriate temps.
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  22. Turbo added a post in a topic Folding wings an Avid idea?   

    Nice 2-1/2-cell sesquiplane design.  Spars can be quite minimal as they don't really have to support much bending.  Optimal airfoil thickness for fabric-covered wings (no laminar flow) is around 13%, though, which of course favors monoplanes where you can take advantage of deeper spars.
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  23. Turbo added a post in a topic Curious fawns   

    OOOPS!  Hundreds of MILLIONS of years!
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  24. Turbo added a post in a topic Curious fawns   

    Adorable baby bird.  Those guys put us human fliers to shame though!  Hundreds of years of evolution compared to only 115 for us as fliers.  I am awed with what birds routinely pull off.
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  25. Turbo added a post in a topic Ever wonder what it's like to...   

    Hey Joey,
    Is it true that B-1 drivers always have a dirty windscreen from fuel splatter upon disconnect?  Aerial refueling is so cool, and I'll bet the receivers appreciate that KC-10 with its bigger extension range, envelope, and fuel flow, over the antiquated KC-135!  The big guys just need to keep their T-shaped tailfeathers out of #2's exhaust plume!  The KC-45 will solve that problem (or will it, with the 767's wing engines so close together?).  We, McDonnell Douglas Boeing, had an internal hu-hu with the Boeing Wichita folks, who wanted to warm over the -135 boom for the KC-45.  I got dragged into the early phases of that design activity.  But there was no saving a bad, 1st-gen design!  We almost lost the tanker to Aerbus!  Fortunately, that KC-45 now sport a modified KC-10 boom,  and on some versions 3 (count 'em 3) hose drogues!  3 times the fun!  Anyway, thanks for what you do, and enjoy your "retirement"!
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