Turbo

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!

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?

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?

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.

Comments?

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!

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!

Gotta say: I like the elegance of Bandit's solution!

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...

I wonder if that 16.5" aft cg limit isn't set by requirements for the tri- gear version, not pitch stability?

I wonder if the 16.5" cg aft limit is set by requirements for the tricycle-gear version, not pitch stability.

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.

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!

What a great idea to put it inside the fuselage and out of the airstream!  The fabric is transparent to radio signals.  Can't do this on a metal bird.   A metal ground plane is a great idea too.  Checked the correct 1/4-wavelength at 108 mHz at about 70 cm.  Supposedly the optimum for simple dipole antenna.  Drag is fuel flow.  Hopefully aluminized dope isn't opaque!

Yeah, the undercamber likely hurts st high speeds.   (Low CL)  The undercamber typically doesn't do much for max CL anyway -LE shape is a much bigger driver.  In my experience the most fertile place for drag reduction is the engine cooling system.  Good baffeling for the radiators is absolutely key.  "My" bird has the 582 grey head, but the radiator is under the belly.  Ugh!  No easy way to get cabin heat either.  Not too keen on heat muff approach.  The cowling is the same as on the mk4, and there is an asymmetric single-radiator installation that allows a lightweight &simple cabin heat option that doesn't need to be modified for summer.   Problem is - where to find the correct radiator?  Thinking about manual cowl flaps.  Since we're limited to 95mph ias, drag reduction is about lowering fuel burn.

Hi Y'all,  Decided to use moniker "Turbo"; sort of a corruption of Arturo, but I go by Art.  I live in the Columbia Gorge and love to windsurf here.  Many years ago I had a Tri-Pacer, and discovered that going fast wasn't where it's at for me; I just love being up there, so found myself, even on longer trips putting along at 2200 rpm and 85 mph, 1000 AGL.  This realization ruled out rocket-ships like Sonex and Quasar in favor of the low & slow type.  I have not flown for a long time, and likely have rust about 3" thick.  I suppose, however, that one never forgets how to ride a bike.  I am working on regaining my currency and getting that TD endorsement, if the weather would just cooperate!  I am in the process of buying an Avid model C from a chap way down in southern Oregon.  I have been reading many of the blogs on this forum, and it has been very instructive for me.  Thanks Guys! As an Aerodynamics engineer (retired from Boeing) I hope I too can contribute here and there.  The bird I'm buying has no cabin heat and no trim.  I've got some ideas I'd like to bounce off y'all at some point.  In the meantime, keep them props turning!

Hey Y'all, This is Turbo, aka Art.  I love to windsurf in the Columbia Gorge where I live, but there are days when there's no wind.  What to do, what to do?  Years ago I owned a 1957 Tri-Pacer.  It was lots of fun, but on long trips I found myself more interested in flying than actually getting anywhere in particular, so I'd throttle back and tool along at 2200 rpm and 85 mph indicated.  This realization lead me away from rockets like the Sonex or Quasar, and in the direction of the Avid.  I am currently in the process of buying an Avid model C, with some Mk IV features, from a fellow down in southern Oregon.  Since I haven't flown in quite awhile, I am also working on regaining my currency and getting the taildragger endorsement.  I retired from Boeing in 2012, where as an aerodynamicist,  I was one of a very few laminar flow control experts in the company.  I capped my career off with contributions towards achieving low-drag laminar flow on the empennage of the 787-9 and -10 airplanes.  Seeing a technology that I had a hand in developing over decades of R&D finally make its way onto a production airplane was for me the cat's meow!  The Avid I'm buying doesn't have cabin heat or a trim system, and looks like it needs some aerodynamic cleanup.  Unnecessary drag is wasted fuel. I've got lots of ideas to bounce off of the group.   I dream of someday going up when there's enough wind for windsurfing, and flying the Avid backwards!

Oops! Misread!  You said Nosedragger!  

Edmo, Could you explain to me how this last statement makes sense?  The CG of a taildragger must be behind the mains, otherwise we'd have a real tough time keeping her from tipping over onto her nose.

I couldn't agree more.  Well put.  It really is terrifying.

I hope the dems' half-dead reaction to the very upbeat SOTU speech drives home to even their 'true believer' base that they don't really give a rat's ass about the people of this country, to whom they owe their first allegiance.  For them, it's all about power, apparently.  

So how many welders, machinists, mechanics etc. would it take to design something like a Boeing 787 and make it successful economically?  Build it, heck yeah, but design is a completely different endeavor.  Hey, as an engineer myself I have also seen my share of dumb engineers, and face it: we all make mistakes from time to time.  But I also have met and worked with true geniuses and have been awed with their intellectual prowess.  People are people, however, Nlappos makes a good point.  There are instances in which only detailed understanding of the physics, and an ability to accurately model same mathematically are absolutely critical for a design concept to be successful.  It's easy to nip at our heels, but can you really claim to walk in our shoes?  For relatively simple things, sure, no stress, thermodynamic, heat transfer or aerodynamic analyses required.  By the way, the engineering specialist on challenger said don't fly; it was the politicians who overrode him.  

I am amazed that there is seemingly no provision for trim on the Model C.  Using the flaperons works in cruise, I suppose, but at lower airspeeds it appears no trim is possible since the flaperons would be at their TE-up limit.  From various posts I have read, the flaperons' chief effect is in changing the moment contribution of the wing, in part because the CG tends to be well forward of the wing-tail aerodynamic center in these birds. 

Sorry if I appear as a ghost.  I'm new, and am not sure I'm set up properly.   - Turbo