Turbo

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Everything posted by Turbo



  1. Turbo added a topic in Avidfoxflyers General Hangar   

    Seat improvements
    The sling seat forces you into kind-of a fetal position, with absolutely no lumbar support. Others have remarked that the fiberglass seatpan is uncomfortable.  In a KF, there's space for big improvements in comfort, but in an Avid-C or earlier, we're cramped for space.  I am contemplating a plywood seatback stiffener, with a lumbar support cushion.  Any alternative ideas on this out there?
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  2. Turbo added a post in a topic Wider gear using stock legs.   

    The original bungees included with the kit for my bird 26 years ago tested out 1/3 as stiff as the new ones from Acft Spruce!  Anybody who has original bungees should swap them out for new!  Yeah, they reached max stretch at about 30-35 lbs as opposed to over 100 lbs for the new ones!  Hard to believe, huh!  That sub-par stiffness is what you'd get with the hardware-store variety bungees.  Make no mistake! 
    OTOH, my plywood truss-savers probably should be vertical-grain plank.  In my case this is not too critical as I have omitted the "safety cables" as C5 suggested long ago.  Nylon webbing would seem a viable alternative to those cables, but isn't that what the bungee sheaths are in essence anyway?
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  3. Turbo added a post in a topic Wider gear using stock legs.   

    Nylon braid has good stretch and good strength, with strength likely way better than bungee.  I have tested the Airfcraft Spruce 3/8" bungee to over 100 lbs and double original length,  with no failure, but did not have the equipment to take it further.  Transverse load path thru the fuselage tubing, seat truss must be a concern with this new idea.  One would want to look at loads vs column buckling possibility on the squeezed seat truss structure.  Having suffered the indignity of a cable-caused seat truss crimp failure, I heartily agree with the idea of omitting the damn things.  I have quality 1/2" plywood ziptied in the critical triangles.
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  4. Turbo added a post in a topic Starting my Homemade Hackman   

    Nice job, Vance!  My craftmanship is so underdeveloped I was afraid to try to drill my venturis.   I like your carb-leveling plate.
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  5. Turbo added a post in a topic Tire comparison 8.00-8.50's   

    Nice looking bird!  I gotta believe that lateral squishiness is a good thing, as is using the tire as suspension in preference to the landing-gear legs.  I think the lateral motion of the tires as the landing gear legs pivot sideways under load whilst careening down the runway adds to the tendency to swerve and careen more, so is destabilizing.  The guy who sold me my Avid C advised only10 psi in the tires.  I think this makes sense.  Fresh, stiff bungees, or stiff springs help too, as does a wider gear with larger angle of the gear legs out from the fuselage, so the lateral motion of the tires is less.
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  6. Turbo added a post in a topic Starting my Homemade Hackman   

    So has anybody looked at how Cowlove set up his manual leaning system?  It's way simpler than this hacman system, with no drilling or other mods to the carburetors or air cleaner.  It just uses a small, cheap 12 VDC vacuum pump hooked up to a rheostat, with the suction side plumbed into the float-bowl vents.  A small orifice lessens the effect of the vacuum pump.  With the rheostat I can run the vacuum pump at any speed and pressure ratio I want.  I crank it up to kill the engine at idle, instead of stressing my fuel pump by shutting the fuel off then waiting for the lines and float bowl to empty and the engine to stop..  BTW, no change is needed to any of the jets.  Is there any disadvantage to this set-up?. I have not had a chance to use it at altitude yet.  
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  7. Turbo added a post in a topic Fiberglass strut fairing POC   

    Just keep it right side up; the struts are fine in tension.  Woods vary in stiffness, elastic modulus, even within the same species.  Negative G situation can buckle the struts at lower than design (negative) load if the fairing wood is less stiff than what was used in the design.  So there's a potential danger there.  Of course, most of us prefer staying right side up anyway, but severe turbulence is generally unseen.
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  8. Turbo added a post in a topic Wind   

    For my TD endorsement training I never had a sesh with much crosswind.  I, like Allen, am a bit intimidated by strong winds & big gusts.  I know what to do, but just have not done it in anything but light winds.  I am comfortable with the forward slip, but the bird doesn't feel graceful with the entry/exit, perhaps due to its adverse yaw characteristics.  Crabbing it in, then transitioning into the slip just before touchdown, like Flightchops Steve does seems out of the question.  It's comforting to get an idea of what the airplane can handle in hands more skilled than my own.  Just this discussion has ramped up my fear threshold a bit.  Thanks, guys!
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  9. Turbo added a post in a topic Pitot tube attachment   

    For what it's worth, the Pitot tube probe head need not be very far below the wing's lower surface.  It just has to be clear of the viscous boundary layer of the wing's lower surface.  This is typically very thin, like a few hundredths of an inch, unless, of course, the flow under there is separated (not too likely!).  It senses the total pressure, i.e. the sum of static pressure and ram, impact, or dynamic pressure.  The total pressure is totally unaffected by the static pressure field around the wing - as long as it's clear of the viscous boundary layer around the wing at that location.  The total pressure is very easy to measure correctly, as the probe can even be misaligned by, say, 10 degrees, and the total pressure error will be less than 1%.  Align the probe tip parallel to the aircraft centerline and parallel with the wing's lower surface at the probe tip location, and you're good to go.  I say all this since a large standoff increases the chances of the probe being damaged.  The IAS is reckoned from the difference between total and static pressure.
    The real airspeed error comes from the static pressure measurement, which is fiendishly difficult to get right.  What do they do on our airplanes?  Just leave the static port on the ASI open to the cockpit pressure, and live with the very systematic error.  We could calibrate out this error if we could get a good static pressure.  Static pressure is influenced by the airplane's presence, so getting a good one involves  putting the static pressure probe sufficiently far away to zero those effects, and somehow nulling out the AOA & yaw effects.
    -Turbo, reired Boeing aerodynamicist and former associate professor of aerospace engineering, Avid C owner
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  10. Turbo added a post in a topic Chemtrails at night   

    Don't 2-strokes count?
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  11. Turbo added a post in a topic Hello from Alberta Canada   

    Welcome, Kent!  Looks like you've got a couple of nice built-up wings there.  Also looks like you're well on your way.  Unless you somehow scare the begeebies out of him in the process, once you take him up, the rest will be history!
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  12. Turbo added a post in a topic Howdy all   

    Yes, we proud de-facto acolytes of Vishnu (Hindu God of preservation & restoration) salute you for taking on the salvation of that bird!  I wonder what engine you have?  It looks air cooled.  The airplane was originally designed for the Cuyuna snowmobile engine, I think.  The majority of us (I think) fly behind the liquid-cooled Rotax 582 engine.  Of course yours, being the A model, has the motor mount as an integral part of the fuselage frame.  In later models it separates at the firewall, and is held on with through bolts.  Looks like you're in for an adventure!
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  13. Turbo added a post in a topic 100 horses in a 4 cylinder with fuel injection. Antique   

    Man, that V4 is compact!  Dunno how one would go about setting it up in a cowling: bring the air in between the V and then out and down?  Plugs come in from the sides!  Very interesting  although, like you say, not too practical!  
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  14. Turbo added a post in a topic Issues flying behind a 2-stroke   

    JimChuk, your point is well taken.  My bird is light and apparently clean, as I can easily top 80 mph at 5000 rpm, at low altitudes.  And I'm not tightly constrained economically, so I can afford the petrol, as well as the other stuff.  I suppose my fixation with minimum power stems from a desire to understand my bird's aerodynamic properties.  Hey, I'm a retired airplane aerodynamicist, what can I say?  Even this simple little putt-putt airplane is an object both magical and fascinating to me.  I also see it as a vehicle for a new class of adventures that I can share with my wife, that are exciting, yet not too strenuous for our old bodies.  This is in addition to serving as a testbed for various minor experiments.  True, I can't turn it into a map-gobbling rocketship, but that's o.k..  Its ability to fly is all the magic it needs, as far as I'm concerned.  It's the magnificent journey, not the boring destination!  I already live in my flavor of paradise.  Your kind words are always appreciated.
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  15. Turbo added a post in a topic Issues flying behind a 2-stroke   

    From what I've read, castor oil's film strength can't be beat, even by the best synthetics, but it does eventually morph into a gummy residue.  I understand that certain types of off-road motorcycle racers still use it, since they disassemble at least part of the engine between races anyway.  Smells cool too!
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  16. Turbo added a post in a topic Issues flying behind a 2-stroke   

    I have no intention of operating the engine outside its specified operational parameters.  Any temp wanders outside the limits and it's experiment over.  Calculations so far indicate feasibility if the airframe (excluding prop, as a glider) has a best L/D of just over 8.  I remember being quoted 7:1 for a Cessna 150, but that is with a windmilling prop.  Take that prop out of the picture, and it's likely 10 or more.  I have already demonstrated level flight at low altitude at 4000 rpm, to no ill effect, with the prop pitch where it is.  Close to best L/D the power requirement is low, and the prop doesn't need to provide very much thrust.  In short, this is not lugging.  It's more like loafing.  Without the IFA, I'd need to take off with the pitch somewhat higher than it is now.  This means a longer takeoff roll and a flatter climb, at lower rate, but higher forward speed.  I already do this.  The feasibility can be estimated, and the goal approached incrementally, in recognition of the limitations inherent in my modelling of the prop's behavior with my code.  In the pursuit of cruise economy, I intend to map the tradeoff against climb rate anyway.  I am not proposing to fry my engine.  I'm an old guy, so I do things cautiously.  It's an experimental airplane.  Experimentation is expected, one would think.
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  17. Turbo added a post in a topic Issues flying behind a 2-stroke   

    Tell you what, fellas.  I'll only play this low-power game in the pattern or out in the flatlands, where the consequences of engine failure are mitigated.  No trees in sight.  Just harvested wheat fields.  And like I said, I'll put her "on the bit" once in a while just to keep it clean.  But first I'll see if I theoretically even have a big enough prop to make it work. Just a matter of a few well-thought-out runs thru my prop code.
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  18. Turbo added a post in a topic Issues flying behind a 2-stroke   

    Point taken.  I live in a high-humidity area, so have made the decision not to go with the true synthetic oils, like Amsoil Interceptor, despite their excellent lubrication performance.  My big concern is engine corrosion when the airplane is sitting, and from what I've read, in that regard I'm better off with more conventional "mineral" oils.  Locally, I can get a relatively inexpensive TC-W3 oil that's also rated API-TC, and is hence ashless, so that's what I'm going with.  Chris Bolkan reported good results with a similar oil type.  I'm hopeful that the oil injection system throttling the oil back to 70:1 at idle will spare me some crudding-up there.  
    On cool days I have been able to hold (low) altitude at 4000 rpm, but I see that engine thermal efficiency peak of 34% at 3500 rpm, so am curious as to what I can get away with.  I suppose it's really more of an academic question, not particularly useful operationally.  But you're saying the new synthetics do a better job with rust protection than earlier versions?  Interesting.
    Best of luck with that Hirth!  
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  19. Turbo added a post in a topic Issues flying behind a 2-stroke   

    I've got to believe this is very dependent on the type and size of the engine.  Nobody else has contested the notion that these engines like to operate under load, and that their fuel efficiency is highest, and EGT lowest when loaded.  At 3500 rpm we are out of the gear-train resonance; we're out of that regime by 2500 rpm.  The 582 is liquid cooled, so kept warm by a thermostat, even if engine heat generation is way lower than the radiator can dissipate.  Leni's right that cabin heating may be insufficient for comfort.  Unless one's midrange needles are set up wrong, resulting in the engine running lean, I don't see anything damaging to the engine here, except perhaps in really cold conditions where even the thermostat can't keep the engine above the minimum coolant temp, causing crud to build up in the combustion chambers, and risking seisure.  Except for the very cold OAT case, I see little harm potential here.  I would think this would be very easy on the engine.
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  20. Turbo added a post in a topic Issues flying behind a 2-stroke   

    Maybe that's the problem.  I taught myself to sail before the wide boards showed up.  I even had a speed needle for a while.  Not quite red-shift relativistic fast, but it was still scary fast, and lots of fun.  But that was before I moved to the Gorge and learned what fast really is. 
    Seriously, though, I get the obvious benefits of a wide-track gear,  especially to be able to not damage your wings in a bad groundloop, but I am now convinced that it's the way the LG pivots about their attachment to the fuselage sides, with initially more outward motion of the tires than vertical motion, that is the major contributor to the narrower gear feeling squirrely.  Ironically, this same motion should make a tricycle-gear setup feel more benign and stable, but in either case, hard landings are sure to chew up your tires!
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  21. Turbo added a post in a topic Issues flying behind a 2-stroke   

    C.D., I b'lieve you're onto something!  Based on my prop calculations, and the factory data, with my light, clean little bird, the shaft power required to fly level at low altitude is less than what the engine can produce at 3500 rpm.  Of course, I would need the IFA to pull it off.  And hopefully the prop efficiency won't tank at the higher pitch!  But the words of experience say I'd better not try to do it for long, or my engine will crud up.   Got to put the bit in her teeth at least every so often.  O.k. then!
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  22. Turbo added a post in a topic Controls   

    Interesting that the low-speed, skidding turn (too much rudder) is the dangerous  one.  It must be the wonked fuselage's wake that helps the underpriveleged, higher-AOA Inside wing to let go first.  Accelerated stalls happen above rectilinear stall speed, and turns involve centripital acceleration, and pulling some Gs, as we all know.  This is the classic spin entry that seems to kill so many pilots when maneuvering for landing.  Slips, on the other hand, are relatively safe.  This is good, especially when landing in crosswinds. Cool that you can bank, yet stop the turn with rudder, allowing you to slide sideways without actually pointing in the direction you're going!  Then, you're even safe to stall this way!  Aren't airplanes cool?
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  23. Turbo added a post in a topic Issues flying behind a 2-stroke   

    Maybe it's the combo of the two.  As a swerve initiates, weight shifts to the tire on the outside of the swerve. The gear leg on that side squats in response to the increased load, shoving the tire on that side outward.  Since that tire now is carrying more load, its firmer grip on the pavement makes this really manifest as shoving the airplane in the direction of the initial swerve, exacerbating the swerve.  A wider track lessens the weight transfer.  A suspension that does not shove the tire outboard when it squats might not have this problem at all.  Hmmmmm...  Thoughts, guys?
    The guy who sold me my bird advocated for 10 psi for the Carlisles.  I see that this, in addition to the mercy of a little  lateral squishiness, makes the tires a more significant part of the suspension.  Of course tight new bungees have got to help too.
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  24. Turbo added a post in a topic Issues flying behind a 2-stroke   

    Interesting!  For sure that's a lot more rotational inertia!  Definitely something to consider.
    One thing some folks are saying that mystifies me is the degree to which wider gear stabilizes the airplane on the ground.  Other than less roll to the outside of a ground turn, and less sideways movement of the tires when the gear squats on hard landings, I don't get it at all!  The CG is still behind the mains, so the propensity to groundloop can't be that different.  What physics tidbit am I missing?
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  25. Turbo added a post in a topic Issues flying behind a 2-stroke   

    Thanks, JimChuk.  I guess even an L/D of, say, 1/2 is a whole lot better than zero!  Lots of guys favor those monster tires, bigger than our golf-cart Carlisles!  These have got to be draggier, but even the Carlisles are draggy.  So I guess Leni is saying that he sees little benefit for the IFA prop, outside of floatplane ops.
     
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