Turbo

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Everything posted by Turbo



  1. Turbo added a post in a topic Disembodied spirit   

    Roger, wilco!   -T
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  2. Turbo added a post in a topic Avid STOL airfoil -some calculations   

    No need.  I think you pegged it with USA35B_mod!
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  3. Turbo added a topic in Avid Model C   

    A few more calculations
    It's hosing down rain, so I thought I'd add a little to the discussion.  I computed the spanwise lift distribution and some sectional Cp distributions at various span stations.  The case is cruise at 3000 ft, 85 mph, at full gross weight of 911 lbf.  Here's a computed spanwise sectional CL distribution of the Avid C STOL wing, and some Cp distributions at some of the outboard span stations.  Note that the wing CL is only 0.4 at this flight condition.  Note how the lower-surface leading-edge -Cp spike grows going outboard.  We are saved past 90% span by the wingtip, whose shape quickly morphs to something very different from that of the basic airfoil.  One can only wonder how high the sectional Cd values get approaching 90%, on the basic wing.  Without airfoil sectional data: Cd vs. CL, etc.  we can only guess.
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  4. Turbo added a topic in "other aircraft your working on"   

    Disembodied spirit
    I once tried to go to the restroom in Mexico.  The label above the door read "Caballeros".  It was at that moment I realized I did not own a caballo, a horse.  I decided to push the rules, instead of watering the bushes behind the building.  So now I am in much the same boat, or- plane,  so-to-speak.  I am no longer the proud owner of an Avid.  I look down, and don't see a shadow anymore.  Nick commented that he knew what he wanted, but that perhaps I didn't.   It was becoming clear that what I really wanted was something faster, longer-legged, and yes, powered by a well-proven 4-stroke.  So aside from it being stuck in SoCal, with me up in WA, with winter running at full steam, I am again an avionero.  This time it's a terrain-gobbling Corvair-powered Sonex.  Here's a pic:
    So what does this portend for my presence on this site?  Probably a rather quick fadeout.  But guys, it's been great!  I wish I could have met more of you in person.  Y'all are the best!  Blues skies and tailwinds, all!     - Turbo  (oh, and yes, it's normally aspirated!)
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  5. Turbo added a post in a topic Avid STOL airfoil -some calculations   

    And that's totally fair.  If only we had 2-D wind-tunnel data for that airfoil, we could know for sure.  As the sectional CL decreases to zero as the tip is approached, It'd be nice to know that the airfoil isn't climbing up the low-CL wall of its drag bucket.  There's likely a reason that planes like the SuperCub use the essentially flat-bottomed USA35B airfoil.
    It's only in this low-speed flight regime that a designer can get away with using one airfoil shape from root to tip.  We sure as hell don't do that in transonic wing design!  It's done for cost reasons on these cheap slow fliers, but to do so successfully, it's important to look at the range of sectional CLs seen at different span stations on the wing in the various phases of flight, and make sure that it never gets too draggy where it counts, like in cruise!  My refined intuition on this (as a retired professional aerodynamicist) says that the undercamber, along with the cylindrical leading edge becomes a liability in cruise, especially outboard near the wingtips.  Then there's the additional nose-down pitching moment, and its effect on tail download and induced drag.  What does the undercamber buy?  A slightly shorter ground run at the AoA set by the gear.  But couldn't the flaperons do this?
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  6. Turbo added a post in a topic Avid STOL airfoil -some calculations   

    Not necessarily.  It's the undersurface that in my opinion never needed to be concave.  It buys almost nothing, but costs in cruise speed.
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  7. Turbo added a post in a topic VGs/ uncambered airfoils and drag   

    FWIW, I found out that the airplane (Avid-C) would flare itself on landing!  Just had to be steely like a fighter pilot, holding 500fpm descent right into the ground!  Trust your airplane, Luke!  Took a while to achieve that level of trust. Playing chicken with Mother Earth, you figure you're gonna lose, a priori, anyway!  
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  8. Turbo added a post in a topic VGs/ uncambered airfoils and drag   

    Oops! Got nomenclature wrong!  Vs1 is clean stall speed.  That's what I meant in the above post.  Sqrt(2)*Vs1 rounded up to nearest 5 is more accurate min speed-to-fly in the pattern.
    On another note, we all know that CL of the wing sections falls off kinda elliptically going out the span as the wingtip is approached.  So the smart designer chooses an airfoil that is not experiencing drag rise (not climbing the walls of the drag bucket) at zero CL.  This is why the undercambered airfoil is bad.  It's draggy out near the wingtips.
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  9. Turbo added a post in a topic In line fuel filter   

    Geez, whatever you do, don't put a filter upstream of the header tank/gascolator!!  The whole point of a gascolator is to trap any water that might get into your fuel tank via thermal respiration.  A filter upstream of the gascolator will eventually fill with water, and the water's high surface tension will stop the flow of fuel.  This happened to Leni, and I have validated this phenomenon in my lab (garage).  Leni's butt was saved by a powerful aux fuel pump. However, too much fuel pressure can overpower the float-bowl needle valve and disable the fuel-pressure regulation the carb's float bowl is supposed to provide.  One fuel filter, downstream of the header tank, but upstream of the diaphragm fuel pump is all you need.
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  10. Turbo added a post in a topic Getting ready   

    So you weren't planning on making your first flights on instruments after all?
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  11. Turbo added a post in a topic VGs/ uncambered airfoils and drag   

    Nick is right on.  Assuming you won't exceed a 60 degree bank, set your minimum speed for flying the pattern at 1.4 Vs0, and you won't be falling out of the sky.  This is a good number to round up to the nearest 5, and wedge firmly in your noggin.
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  12. Turbo added a post in a topic Getting ready   

    Looking good, Allen!
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  13. Turbo added a post in a topic The value of streamlining   

    Coll blimey mate!  Ugly or what?  You'd think they'd put some serious effort into finding another way, or enginerring the sh--t out of some kind of cowling!  Looks like that radiator even has the dreaded forward-facing flats, if the backside is any indication!  But note: the flow velocities are relatively low there, and don't really ramp up until the exit nozzle contraction, where it goes through sonic.  Typically the fan flow continues accelerating externally over the core cowl to low supersonic.
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  14. Turbo added a post in a topic Avid STOL airfoil -some calculations   

    I reckon the attribution doesn't matter much.  My main point is that the NACA 4412, 4415, 4418 all have quarter-chord moment coefficients around - 0.1 in the attached flow region, whereas the Avid STOL airfoil with its lower-surface concavity is closer to -0.14.  This fact is in contrast to the statement in the quotation., which may have been an off-the-cuff, judgment, but is wrong.  Take out that lower-surface concavity, and the AVID shape falls into line with the NACA 44xx shapes.  By the way, the USA35B used, with a minor mod on the supercub and, as I recall, even my old tri-pacer, with its essentially flat bottom, has a CM in the attached-flow range of - you guessed it - -0.1!
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  15. Turbo added a post in a topic Avid STOL airfoil -some calculations   

    Doug, I have seen this quotation, but believe it was from David Lednicer, who collects airfoils and runs them through Marc Drela's Xfoil, an Euler-with-boundary-layer code, for comparison's sake.  However, the comment saying the 4-digit NACA sections like 4412, 4415 etc would have too much nose-down pitching moment is simply incorrect.  The Avid STOL section is worse in that regard than those 44xx sections.  Its high cruise drag may be in part profile drag, as all that camber will move the drag bucket out to higher CL, so in cruise we may find ourselves partway up the drag bucket's wall on the low-CL side, but it is also likely that the undercamber, which helps you get off of the runway/lake faster, unnecessarily downloads the tail, increasing induced drag.  My experience with using the roughly 10-deg flaperons on landing approach (steeper descent by quite a bit for only 10 degrees) suggests that the induced drag piece is significant.  This is at fairly high landing CL.  There was not much elevator authority left, so the tail was lifting downward like crazy!  Just a straight-line undersurface would cut the wing's nose-down pitching moment by 40%, but it would take a wee bit more runway to get flying.  Once up, climb rate and angle would be unaffected.
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  16. Turbo added a post in a topic Avid STOL airfoil -some calculations   

    Wilja67, I have only analyzed the Avid STOL airfoil, and don't have coordinates for any KF section.  Is that what you want?
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  17. Turbo added a post in a topic The value of streamlining   

    I wonder where one might find solid bladed wire?  Some biplanes have it.  Lots of road bikes use bladed spokes.
    On the belly radiator - Its drag, or thru-flow pressure drop causes much air to flow around it instead of through.  The lower-momentum air right behind in its wake does help mitigate the drag of those un-faired, circular-cylinder landing gear struts tucked up against the belly, though.  Those who have IVO props must be aware that the blade angle goes to zero approaching the hub.  This is draggy, and certainly doesn't help in producing thrust.  But again, although the inboard roughly 1/3 of radius almost certainly creates drag, not thrust, it does reduce airflow velocities near the fuselage, helping to lessen the drags of other little nasties downstream, like those lovely lower strut attachments/fold hinges, the belly rad, etc..
    Many years ago I was at family day at MacDonnell Douglas, and spotted a little oil radiator standing there naked in the fan duct of a DC-10's engine, downstream of the fan.  I was appalled, but wondered if its lack of a cowling meant that achieving a drag reduction by cowling a radiator is not an easy task, and that just designing something that looks "right" might yield a drag increase or no reduction at all, with a weight penalty to boot.  A-priori intuition can be misleading in the aerodynamics world.  Data talks and theory walks.  There's lots of good stuff in Hoerner, for starters.
     
     
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  18. Turbo added a post in a topic plywood seat truss reinforcement   

    Just my gut feel recommendation, but I wouldn't go below 1/2" if you want any compression resistance.  Also, high-quality multi-layer plywood, perhaps with the face layers in the direction of the expected crushing force, and tightly fitted into the triangle.  Mine are held in place with both the bungees and some zip-ties.  No test data, though!  Does anyone have any crush test data on this subject?  If not, it's a matter of educated guesswork, with no real answer, other than the obvious "Not so thick that it rubs on and chews up the bungees.".
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  19. Turbo added a post in a topic Fuse sidewall gusset materials   

    Just my 2 cents, but I'd sure use something with some compressive strength, like high quality 1/2" plywood, or thicker.  It should be a tight fit between the tubes, inside the triangles.  1/8" won't do much at all, other than buckle!  I have 1/2" ply in my seat truss area, held in place with zip-ties.
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  20. Turbo added a post in a topic Bush gear topic reopened   

    Top of seat truss - big compression forces!  Potential for column failure.  Need to maybe reinforce in two dimensions?
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  21. Turbo added a post in a topic HOw to bend the gear   

    W-W-Wait a minute!  A hard landing should extend the spring, not compress it.  So I don't understand it unless the extension is limited by a metal-to-metal hard stop.  I'm sure that's what you meant.
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  22. Turbo added a post in a topic The value of streamlining   

    Forward-facing flats are aerodynamic sin!
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  23. Turbo added a post in a topic Another engine out from an oil injection failure   

    Gotta love those oils rated both tc-1 and tc-w3.  They are good in either application!  Thanks to Chris B. for that informational tidbit!  Incidentally, I was at a shop that services snowmachines, and the mechanic there said he had seen evidence of scuffing with Amsoil at 100:1.  Maybe the particular case was one in which the cooling wasn't as good as we typically get with our application.  Don't know enough detail here.  
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  24. Turbo added a post in a topic Another engine out from an oil injection failure   

    Hey, all technical progress is made by us nerds!
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  25. Turbo added a post in a topic How do you tall guys do it?   

    At least your headset will provide some protection!  I take it you're still (re)building, and have not sat in the slingseat for two hours straight flying - yet.  It's right about at 2 hrs that my lower legs cramp up!  UGGH!  That's right about time to land and re-fuel anyway.
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